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  • Chemistry
  • Cell & Developmental Biology
  • Inorganic Chemistry
  • American Association for the Advancement of Science (AAAS)  (347)
  • Tehran, Iran  (12)
  • 1
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    Iranian Fisheries Science Research Institute | Tehran, Iran
    In:  http://aquaticcommons.org/id/eprint/25179 | 18721 | 2018-09-03 17:23:17 | 25179 | Iranian Fisheries Science Research Institute
    Publication Date: 2021-07-16
    Description: In this study two species of algae, Sargassum glaucescens and Padina boergeseni that found plenteously in Persian Gulf and Bushehr coast, were collected and hot water extracts of them were lyophilized. F. indicus (11.32±1.20 g), after two weeks adaptation in Shoghab research station were immersed in seawater (39 ppt and 25±1 ○C) containing hot-water extract of each brown algae, S.glaucescens and P.boergeseni, at 100, 300 and 500 mg/l concentration, Survival rate and immunological parameters (total haemocyte count (THC), total plasma protein (TPP), Phagocytic activity, bacterial clearance efficiency and bactericidal activity) were examined. In addition effect of dietary administration of beta 1, 3 1, 6 glucan on prevention of White Spot Disease and immunological parameters of shrimp were investigated. According to results, immersion in seawater containing 300 and 500 mg/l concentration of algal hot-water extract after 2 and 3 hours or oral administration of beta 1,3 1,6 glucan at level of 10 g/kg diet for 14 days significantly enhanced THC, TPP, Phagocytic activity, bacterial clearance efficiency and bactericidal activity. Immersion in seawater containing 100, 300 and 500 mg/l hot-water extract of S.glaucescens after 3 hours, improved the survival rate of WSSV-infected F. indicus.
    Keywords: Biology ; Chemistry ; Iran ; Persian Gulf ; Bushehr Province ; Beta-glucan ; Sargassum ; Padina algae ; WSD ; Fenneropenaeus indicus ; Species ; Algae ; Brown Algae ; S.glaucescens ; P.boergeseni ; Survival rate ; White Spot Disease ; Shrimp
    Repository Name: AquaDocs
    Type: monograph
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  • 2
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    Iranian Fisheries Science Research Institute | Tehran, Iran
    In:  http://aquaticcommons.org/id/eprint/25618 | 18721 | 2018-10-07 16:34:53 | 25618 | Iranian Fisheries Science Research Institute
    Publication Date: 2021-07-16
    Description: Chitin and chitosan are 2 very important products of biopolymer that enjoy high consumption in industry, but their production sources are very limited. In this study, Artemia urmiana cyst shells were obtained from previously collected and stored ones in Iranian Artemia Research Center. 20 kg of Artemia urmiana cyst shells were sampled, cleaned, separated, dried and transferred to Iranian Artemia Research Center Laboratory to extract their Chitin and chitosan. Their chitin and chitosan initially were extracted using optimized common chemical methods. Their properties were compared to 2 other types of Chitin and chitosan obtained from crab and shrimp manufactured by Vietnam and China, respectively. To determine their quality, elemental analysis device, infrared spectrophotometry, x –ray radiography, determination of viscosity , molecular weight, crystallinity percent, color, de stylization measure, empirical and molecular formulas were made. The results showed that the percentage of chitin obtained from Artemia cyst Shells in Chemical method was 28 ± 3 % by weight and efficiency into chitosan (grade steel relief) in this method was 50± 5%. To optimize the extraction procedure and the removal of proteins of chitin by biological practices that were done by sodium hydroxide in the chemical method, it was replaced by the bacterium Bacillus subtilis. And in the bio- phase of chitosan de steelation fungus Aspergillus niger enzyme was replaced instead of sodium hydroxide at high temperatures. The results showed that chitin and chitosan can be extracted from Artemia cyst shell using biological method and their characteristics included as in chitin 49.6% C, 8.2 % N, 7.5 % H, and 34.5 %O. Also the same levels for chitosan were 44.4 %, 8.9, 7.2 and 39.5 %, respectively. Their other quality characteristics were included chitin average molecular weight 4.9×10^6 Dalton, crystallinity percentage of 36.4, viscosity at 20°C 31 centipoise and its color was gray to brown. In the biologic method, the average molecular weight of chitosan, crystallinity percentage, viscosity at 20°C, were 5.1×105 Dalton, 94.5, and 18 centipoises, respectively. Also, its color was pale brown. Chemical structure of extracted chitin and chitosan from the shell of Artemia urmiana cysts were C_7H_12NO_4 and C_6H_11 NO_4c, respectively. The comparison of chitin and chitosan obtained from each chemical and biological method revealed that replacing biological methods instead of chemical methods is possible in achieving these products at suitable condition and better quality. This can eliminate the use of chemicals damaging the environment such as sodium hydroxide and decrease environmental pollution.
    Keywords: Biology ; Chemistry ; Iran ; Artemia urmiana cyst shell ; Chitin ; Chitosan ; Chemical ; Biological methods ; Crab shell ; Shrimp shell ; Biotechnology
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  • 3
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    Iranian Fisheries Science Research Institute | Tehran, Iran
    In:  http://aquaticcommons.org/id/eprint/25642 | 18721 | 2018-10-14 02:40:17 | 25642 | Iranian Fisheries Science Research Institute
    Publication Date: 2021-07-16
    Description: Padina boergesenii is one of the most abundant brown algae distributed in the north of Persian Gulf and Oman Sea. In this study after sampling and preparation of Padina boergesenii by Chroform-Etanol (3-1) solvent and by Methanol has been extract. Separation and purification of the compounds was carried out using thin layer, general and inverse column chromatography, Cephadex and high-performance liquid chromatography (HPLC). Structural elucidation of the constituents was based on the data obtained from H-NMR, 13C-NMR, HSQC, HMBC, DEPT and Cephadex LH-20. The steroids compounds separated from above alga were identified as 22dehydrocholesterol (1), cholesterol (2), fucosterol (3), β-sitosterol (4), stigmasterol (5), ostreasterol (6) and two epimer of hyroxyestrol(7), based on their spectral data and from comparison with those previously reported in the literature.
    Keywords: Biology ; Chemistry ; Iran ; Oman Sea ; Persian Gulf ; Brown Algae ; Padina boergesenii ; Steroids compounds ; Extraction ; Purification ; Identification ; Amount verification ; Sargassum glaucescens ; Algae ; Abundant ; Sampling ; Chromatography ; Cephadex ; Dehydrocholesterol ; Cholesterol ; Fucosterol ; β-sitosterol ; Stigmasterol ; Ostreasterol
    Repository Name: AquaDocs
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  • 4
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    Iranian Fisheries Science Research Institute | Tehran, Iran
    In:  http://aquaticcommons.org/id/eprint/25792 | 18721 | 2018-10-13 08:54:26 | 25792 | Iranian Fisheries Science Research Institute
    Publication Date: 2021-07-16
    Description: Cheshmehkileh River and adjacent mountainous streams, play a strategic role as a historical axis for anthropogenic civilization, human welfare also habitat and migration pathway of commercial – biologic valuable fishes e.g. Caspian trout, Caspian kuttum, members of Cyprinidae family in south Caspian Sea drainage. Treats such as overfishing of Caspian trout and Red spotted trout stocks in mountainous headwaters, barriers construction and manipulations those are out of river carrying capacity developed by human activities, affected normal function of river as well. Sand mining big factories establishment next to the river, legal and illegal trade of river sediments, direct entry of Tonekabon landfill leakage into the river, development of Rainbow trout farms since 3 decades and huge effluents into the river containing dead fish and types of solids, escapement of cultured Rainbow trouts into the river, … are major minimum factors which needs basic information for integrating inclusively drainage management system. Cheshmehkileh River contains Headwaters of Dohezar (Daryasar & Nusha), Sehezar and Valamroud rivers during 13 monthly sampling phases between September 2009 and October 2010 based on macrozoobenthoses investigations by EPT, EPT/C EPA protocols, measurements of nominated physic-chemical and microbiologic parameters. Probability of Rainbow trouts escapement and invasion, existence, nutrition in Cheshmehkileh environment indeed investigated. Data analysis explained significant differences (P〈0.05) between groups of measured parameters in different sampling stations. Dendogram of clustered analysis based on consolidation of major biologic/ physic-chemical and microbiologic parameters, separated stations No. 1, 3, 2, 4 in one group and remained classified in different groups. Station 8 and 9 similarly separated which expressed general similarities according to Sehezar river environment which were differs in comparison with other stations. Station 11 separated according to its natural quality of water and environment. Similarities between station 10 to Sehezar river stations 8 and 9 expressed general influence of Sehezar River more than Dohezar River in Cheshmehkileh condition especially in station No. 10. High scores of EPT and EPT/C indices in upstream stations 1, 3 and 8 also low score of indices in stations 7, 13 and 6 expressed levels of environment quality between these groups of stations. Maximum average biomass of macroinvertebrates belongs to Trichoptera order in Cheshmehkileh River. Significant decrease of biomass in stations 11, 12 and 13 in comparison with other stations stated environment degradation in mentioned stations relevant to excessive sand mining as well. Pollution resistant groups of invertebrates significantly increased in downstreams against upstream stations. Also disappearing of Plecoptera order in station No. 7, 9, 10 and 13 stated low quality of environment in comparison with upstream stations. Confirmation of effects quality and quantity for point and non-point sources of imported pollutants require specific management considerations in order to present exploitations, pollutants control and emergencies for river monitoring in forthcoming years.
    Keywords: Aquaculture ; Management ; Iran ; Tonekabon ; Cheshmehkileh ; River ; Pollution ; Aquaculture ; EPTC ; Assessment ; Macroinvertebrates ; Chemistry ; Microbiology
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  • 5
    Publication Date: 2021-07-16
    Description: In this study, first, the influence of three precooking methods (steaming, oven-baking and microwave-cooking) on the contents of mineral elements-copper, zinc, iron, calcium and sodium-of silver carp (Hypophthalmichthys molitrix) was evaluated. Determination results of evaluated elements in raw fish and cooked samples were in range 3.05 to 4.19 for copper, 71.45 to 82.85 for zinc, 32.18 to 40.70 for iron, 425.6 to 529.46 and 315.5 to 534.76 for calcium. Results showed different precooking treatments had no significant effects on the amounts of mineral elements such as copper, zinc, calcium and sodium. The Iron content in the samples subjected to microwave cooking increased. With doing multivariate analysis, on comparing the raw and precooked fillets, steam cooking found to be the best precooking method on retain mineral elements. After choosing the best precooking method, for evaluating the influence of different filling media including sunflower oil, soybean oil, olive oil and brine, on the quality of canned silver carp, amounts of mineral elements(copper, zinc, iron, calcium and sodium) and microbial indices(total counts, thermopile count and clostridium) and sensory indices (color, smell, taste and texture) was surveyed. Assessments of microbial indices showed no microbial activity in canned products. After canning amounts of sodium increased in all the treatments. Except for iron, using soybean oil had no significant effect on the contents of other elements. The highest levels of iron and copper were observed in olive oil canned samples sterilized at 130°C. Sensory evaluating showed, kind of filling media had no significant effects on sensory indices such as taste, smell and color of canned samples. The texture of soybean oil canned samples and the quality defects of olive oil and brine canned samples had better condition than other treatments. In the last step, canned silver carp were proceed under three different temperatures (115°C, 120°C and 130°C) with equal lethality value (Fo=7min), then sensory indices and amount of mineral elements were compared. Results showed, the contents of iron, copper, sodium and calcium were changed in soybean oil canned sample. The amounts of copper and sodium in sunflower oil canned sample subjected to different heating regimes showed significant variation. The highest amount of copper was observed in the canned samples subjected to 130°C heating regime. Results showed contents of iron and copper of olive oil canned sample subjected to 120°C and 130°C heating regimes were higher than sample subjected to 115°C heating regime, while the zinc and calcium contents had no variation. In brine canned samples the highest amounts of copper and iron was obtained after sterilization in 120°C. Results of sensory evaluation showed different heating regimes had no significant effects on the indices of taste, smell, and color of products. Doing 130°C heating regime in brine canned samples led to obtain the better tenacity of texture. While this heating regime caused to increase the quality defects of soybean oil canned samples as a result of existence of hard parts of bone.
    Keywords: Chemistry ; Fisheries ; Iran ; Precooking ; Filling media ; Heating regime ; Sterilization ; Silver carp ; Mineral elements ; Hypopthalmichthys molitrix ; Samples ; Sunflower oil ; Soybean oil ; Silver Carp
    Repository Name: AquaDocs
    Type: monograph
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  • 6
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    Iranian Fisheries Science Research Institute | Tehran, Iran
    In:  http://aquaticcommons.org/id/eprint/25118 | 18721 | 2018-08-22 02:17:23 | 25118 | Iranian Fisheries Science Research Institute
    Publication Date: 2021-07-16
    Description: Studies show that thyroxine can play an important role in regulating growth and other physiology activities. Since the direct role of thyroxine in growth metamorphose larval of bony fishes hasn't been proved yet, during the experiment accomplished in Shahid Ansari and Jajrood station, the hormones function in the survival of development of silver carp, grass carp and rainbow trout were studied. In this study accomplished of thyroxine baths with 0.1, 0.5 and 1 ppm (with 3 repeat). The other phase, thyroxine injected to females grass carp, silver carp, rainbow trout and barbell of Caspian Sea with different doses (1, 10 and 100 g/g B.W.). Tehn survival of development of embryo and larval and precent of fertilization were studied. Also, the percent of mortality were compared in two phases in stages of development. Results show that: 1) Trout: a) Phase of thyroxine bath: The number of hatching eggs and survival of larval in 0.5 ppm were increased to other treatment. b) Phase of hormone injection: The survival of larval in treatment of 10 g/g was 8.58% that was meaningful difference to other treatment (P〈0.05). 2) Silver carp: a) Phase of thyroxine bath: The number of hatching eggs and survival of larval in 0.5 ppm were (20%) increased to other treatment (P〈0.5). b) Phase of hormone injection: The survival of larval in treatment of 10 g/g was meaningful difference to other treatment (P〈0.05). 3) Grass carp: a) Phase of thyroxine bath: The number of hatching eggs and survival of larval in 0.5 ppm were 39% increased to other treatment. b) Phase of hormone injection: The survival of larval in treatment of 1 g/g was meaningful difference to other treatment (P〈0.05). 4) Barbel of Caspian Sea: Development stages wasn't determined in this fish. Number of eggs degenerated, number of eggs were the first development stages and growth of numbers increased on ovary.
    Keywords: Biology ; Chemistry ; Iran ; Caspian Sea ; Thyroxine ; Female ; Survival ; Growth ; Egg ; Larval ; Grass Carp ; Silver Carp ; Thyroxine (T4)
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  • 7
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    Iranian Fisheries Science Research Institute | Tehran, Iran
    In:  http://aquaticcommons.org/id/eprint/25713 | 18721 | 2018-10-10 10:11:15 | 25713 | Iranian Fisheries Science Research Institute
    Publication Date: 2021-07-16
    Description: Chitin nanofibers are prepared from the exoskeletons of shrimp by a simple mechanical treatment after a series of purification steps. The nanofibers have fine nanofibers networks with a uniform width of approximately 10 nm. Grinders and highpressure water jet systems are effective for disintegrating chitin into nanofibers. Acidic conditions are the key factor to facilitate mechanical fibrillation. Ultrafine fibers were successfully fabricated chitosan and fish skinextracted gelatin via electrospinning (ES). Important ES parameters, such as concentration of aqueous acid and fish gelatin solutions, and electric field intensity were examined to investigate the effects on the morphology of the gelatin nanofibers. Due to the poor mechanical properties of the fish gelatin membranes, composite nanofibers made of fish gelatin and poly(L-lactide)(PLLA) were produced with a novel solution. The introduction of PLLA remarkably improved the mechanical properties of the gelatin membranes. With a combination of good biocompatibility and mechanical properties, fish gelatin/PLGA blending non-woven mats are considered to be very promising in fish fillet coating application.. in this study, we fabricated a novel nanofibers composed of fish collagen (FC) and polycaprolactone (PCL) blends by using the electrospinning method. Nanofibers were characterized using a scanning electron microscope (SEM), and it was revealed that the diameter of nanofibers decreases as FC content was increased in the FC/PCL composite nanofibers. Several modifications to the chitin NF surface are achieved, including acetylation, deacetylation and maleylation. The results of this study revealed that: 1–It is possible to produce Nanofibers from chitosan and fish gelatin. 2– Covering and coating of processed fish by nanofibers are applicable and increasing the possibility of shell life for the processed fish. 3– Nanofibers which have been produced from chitosan and fish gelation not only is environmentally friendly but also it will be eatable while has been covered for fish fillets. biocompatible chitosan and gelatin made from fish, fresh fish fillets do not have the ability to cover and packaging, but is edible and used.
    Keywords: Biology ; Chemistry ; Fisheries ; Iran ; Chitin ; Chitosan ; Nanofiber ; Chemical modification ; Fish ; Aquatic ; Maintenance
    Repository Name: AquaDocs
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  • 8
    Publication Date: 2021-07-16
    Description: This study was conducted to determine phosphorous and nitrogen species at the surface sediment in the southern Caspian Sea (Mazandaran Coast, Kelarabad). Surface sediments samples were collected at three stations from autumn of 2011 to winter of 2012. All samples were prepared by digesting and extracting processes and then phosphorous and nitrogen species concentrations determined using spectroscopy instrument. Results of this study showed that annual mean of Loosely-P, Fe-P, Al-P, Bioava.-P, Ca-P, TIP, Rese.-P and TP was obtained as 5.06±0.33, 55.11±2.52, 42.38±3.74, 102.52±5.68, 172.91±7.12, 275±9.12, 333.30±28.52 and 608±52 µg/g.dw, respectively. The results also showed that inorganic phophorous was less than organic phophorous during different seasons, however, mean percentage of residue-P containing organic compounds and non-degradable compounds was more than 50 percent. Percentage of Ca-P was higher than 60, whereas Bioava.-P was less than 40 percent. In addition, Fe-P and Loosely-P attained the maximum and minimum values, respectively, among the bioavailable phophorous. The order of different forms of phosphorous were recorded as Org-P〉Ca-P〉FeP〉Al-P〉Loosely-P. Annual mean of NH4/N, NO2/N, NO3/N, TIN, TON and TN were observed as 4.23±0.50, 0.06±0.01, 0.74±0.12, 5.02±0.53, 2.48±0.63 and 7.53±0.51 µg/g.dw, respectively. Annual percentage of TIN was two folds than TON and concentration of NH4/N was also four times than NO3/N. As a conclusion, the results revealed that main causes of Bioava.-P adsorption and desorption were temperature, Eh and pH. Also, the form of NH4+/N was of a high percentage because of anaerobic condition in the sediments. According to the high ratios of nitrogen/phosphorous of sediments to nitrogen and phosphorous of bottom water, it finds that released of those from the sediments to water will be happened with high rates. Therefore, it is expected that the establishment of fish farming cages should be carried out with more precautionary approaches which not leads to increased algae bloom.
    Keywords: Biology ; Chemistry ; Ecology ; Iran ; Caspian Sea ; Kelarabad ; Mazandaran coast ; Phophorous ; Nitrogen ; Surface sediment ; Determination ; Breeding
    Repository Name: AquaDocs
    Type: monograph
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  • 9
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    Iranian Fisheries Science Research Institute | Tehran, Iran
    In:  http://aquaticcommons.org/id/eprint/25772 | 18721 | 2018-10-13 08:08:05 | 25772 | Iranian Fisheries Science Research Institute
    Publication Date: 2021-07-16
    Description: Seaweed belonged to baseborn marine plants with cell wall containing of valued polysaccharides such as Agar, Alginate, Limen sulfate and Carmine, used as food additives and in different industries. Base on previous studies, more than 1000 MT. of Sargassum seaweed withdraw from Oman Sea in Sistan and Baluchistan coastal line which can be collect, dried and powdered for using as supplement in shrimp feed. In this project, Sargassum ilicifolium collected from 6 coastal areas, rinsed, dried, powdered and measured the nutritional values in laboratory for surveying statistically. According to the high nutritional value of Tis coastal seaweed, this variety seaweed powder, replaced with protein resources (fish meal and Soy and Wheat) of white-leg shrimp feed which was formulated by Havorash feed factory of Boshehr in four treatments (A: as control without any replacement) B: with 5%, C: 10 % and D: 15% seaweed replacement, each with three replicates in order to obtain isonitrogenus 33% CP., and iso-caloric (13% fat and 15% carbohydrate) feed using winfeed software. The weighed milled ingredients were carefully mixed using a laboratory food mixer. The mixtures were primed with 30% hot water to yield a suitable pulp. Wet diets were made into 2 mm pellet size and dried at 40 °C in a drying cabinet and maintained in standard condition which was used for water stability and absorption capacity test of the pellets in sea water, statistically one way- ANOVA. The Tis coastal seaweed with 9.8% CP, 2% lipid and 23% carbohydrate had higher nutritional value compared to the other gathered seaweed. Also amino acid and fatty acid profiles, vitamins and minerals were measured in all seaweed samples each, with three replications. As result, for using Sargassum ilicifolium as vitamins and mineral premixes in white- leg shrimp feed, Zinc, Cobalt and Phenylalanine with dose of 1.1, 06.0 and 4.0 ml. 100% dried seaweed must be added respectively. The water stability of D feed treatment in seawater (98%) and C (97%) had statistical differences with A and B (95% stability) (P〈0.05). Water absorption capacity of feeds after one hour immersion in seawater showed significance difference between D (110%) and three others, C(100%), B(85% and control(80%) (P〈0.05). As final aim of this project, enriched product of this seaweed as minerals and vitamins supplements were handsel joinery in the Second Medicine Plant Festival, 2016 and registered in recording organization to give the final certificate.
    Keywords: Aquaculture ; Chemistry ; Iran ; Sargassum illicifolium ; Sargassum illicifolium ; Mineral and vitamins supplements ; Natural binder ; Litopenaeus vannamei ; Seaweed ; Vitamin ; White leg shrimp
    Repository Name: AquaDocs
    Type: monograph
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  • 10
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    Iranian Fisheries Science Research Institute | Tehran, Iran
    In:  http://aquaticcommons.org/id/eprint/25580 | 18721 | 2018-10-07 10:35:48 | 25580 | Iranian Fisheries Science Research Institute
    Publication Date: 2021-07-16
    Description: By-products consist of more than half of the total weight of fish. Most part of these wastes made from connective tissue proteins that provide good potential to produce gelatin. Nowadays, consumers are more health conscious and have responded to the call for a diet that contains low calorie and low fat. This work involved the characterization of a low-fat spread made of fish gelatin as gelling agent, emulsifier and fat replacer. Six treatments with Fish gelatin (FG) to pectin ratio 1:1 and 2:1, with pectin concentration of 1, 2 and 3%, were produced and evaluated for quality changes and storage-life at refrigerated storage. The results showed that prepared emulsions were physically stable and no phase separation observed during 8 wks. The pH of treatments was 4.5 to 4.9. The moisture content of treatments varied from 68.9% for T1 (FG/pectin ratio 1:1) to 65.4% for T6 (FG/pectin ratio 6:3). The Max. lipid of samples was measured 28.4%. Max. and min. protein of samples were 3.8% for T6 and 1.6% for T2 (FG/pectin ratio 2:1), respectively. No significant differences (p≥0.05) were observed between ash content of all treatments and control (commercial margarin). The acidity of treatments increased during storage and were significantly different from production day (p≤0.05). Peroxide value (PV) of treatments increased significantly at refrigerated storage. No significant differences were observed between PV of treatments during different wks of sampling. The results of texture profile analysis showed that firmness, compressibility and adhesiveness properties were significantly increase with higher substitution of FG with pectin. Color, Aroma, Taste and texture properties of prepared samples were found to significantly lower than control, according to sensory evaluation. Significant increases were observed in mold/yeast and psychrophilic count of treatments during storage. The macroscopic growth of molds was observed on all samples in 8th wks. No coliform growth was observed in all treatments at any time. Results suggest that low-fat spread properties were significantly influenced by different ratios of fish gelatin and pectin incorporated. It seems that these results can provide new opportunities to develop market with introducing novel products to response consumer demands. Some quality defects observed in this study can be improved by using suitable machinery at industrial scale.
    Keywords: Biology ; Chemistry ; Iran ; Fish gelatin ; Low-fat spread ; Fish wastes ; Quality characterictics ; Texture profile ; Shelf-life ; Refrigerated temperature ; Cultured Carp ; Diet ; Lipid
    Repository Name: AquaDocs
    Type: monograph
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  • 11
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-02-16
    Keywords: Chemistry
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 12
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-02-16
    Description: The comment and response concerning the report of oxidation of methane to methanol by water (Reports, 5 May 2017, p. 523) do not fully capture the implications of thermodynamic limitations. A nonisothermal process in which each cycle requires a large temperature swing and permits only substoichiometric methane conversion surely could not be carried out on any practical scale.
    Keywords: Chemistry
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 13
    Publication Date: 2018-02-16
    Description: Labinger argues that stepwise reaction of methane with water to produce methanol and hydrogen will never be commercially feasible because of its substoichiometric basis with respect to the active site and the requirement of a large temperature swing. This comment is not touching any new ground, beyond describing the thermodynamic feasibility, thermal cycling, and the role of water as discussed previously. Most important, it does not have a solid numerical basis.
    Keywords: Chemistry
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 14
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-07-27
    Keywords: Chemistry
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 15
    Publication Date: 2018-07-27
    Description: Vinyl carbocations have been the subject of extensive experimental and theoretical studies over the past five decades. Despite this long history in chemistry, the utility of vinyl cations in chemical synthesis has been limited, with most reactivity studies focusing on solvolysis reactions or intramolecular processes. Here we report synthetic and mechanistic studies of vinyl cations generated through silylium–weakly coordinating anion catalysis. We find that these reactive intermediates undergo mild intermolecular carbon-carbon bond–forming reactions, including carbon-hydrogen (C–H) insertion into unactivated sp 3 C–H bonds and reductive Friedel-Crafts reactions with arenes. Moreover, we conducted computational studies of these alkane C–H functionalization reactions and discovered that they proceed through nonclassical, ambimodal transition structures. This reaction manifold provides a framework for the catalytic functionalization of hydrocarbons using simple ketone derivatives.
    Keywords: Chemistry
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 16
    Publication Date: 2018-06-22
    Description: It is commonly assumed that recognition and discrimination of chirality, both in nature and in artificial systems, depend solely on spatial effects. However, recent studies have suggested that charge redistribution in chiral molecules manifests an enantiospecific preference in electron spin orientation. We therefore reasoned that the induced spin polarization may affect enantiorecognition through exchange interactions. Here we show experimentally that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization. The interaction is not controlled by the magnetic field per se, but rather by the electron spin orientations, and opens prospects for a distinct approach to enantiomeric separations.
    Keywords: Chemistry
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 17
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-06-29
    Keywords: Chemistry
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  • 18
    Publication Date: 2018-12-14
    Description: Theory has established the importance of geometric phase (GP) effects in the adiabatic dynamics of molecular systems with a conical intersection connecting the ground- and excited-state potential energy surfaces, but direct observation of their manifestation in chemical reactions remains a major challenge. Here, we report a high-resolution crossed molecular beams study of the H + HD -〉 H 2 + D reaction at a collision energy slightly above the conical intersection. Velocity map ion imaging revealed fast angular oscillations in product quantum state–resolved differential cross sections in the forward scattering direction for H 2 products at specific rovibrational levels. The experimental results agree with adiabatic quantum dynamical calculations only when the GP effect is included.
    Keywords: Chemistry
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  • 19
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-17
    Keywords: Chemistry
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  • 20
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-17
    Keywords: Chemistry
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  • 21
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-17
    Description: The chemistry of the carbonyl group is essential to modern organic synthesis. The preparation of substituted, enantioenriched 1,3- or 1,5-dicarbonyls is well developed, as their disconnection naturally follows from the intrinsic polarity of the carbonyl group. By contrast, a general enantioselective access to quaternary stereocenters in acyclic 1,4-dicarbonyl systems remains an unresolved problem, despite the tremendous importance of 2,3-substituted 1,4-dicarbonyl motifs in natural products and drug scaffolds. Here we present a broad enantioselective and stereodivergent strategy to access acyclic, polysubstituted 1,4-dicarbonyls via acid-catalyzed [3,3]-sulfonium rearrangement starting from vinyl sulfoxides and ynamides. The stereochemistry at sulfur governs the absolute sense of chiral induction, whereas the double bond geometry dictates the relative configuration of the final products.
    Keywords: Chemistry
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  • 22
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-17
    Keywords: Chemistry
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  • 23
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-17
    Description: With the recent soaring production of natural gas, the use of methane and other light hydrocarbon feedstocks as starting materials in synthetic transformations is becoming increasingly economically attractive, although it remains chemically challenging. We report the development of photocatalytic C–H amination, alkylation, and arylation of methane, ethane, and higher alkanes under visible light irradiation at ambient temperature. High catalytic efficiency (turnover numbers up to 2900 for methane and 9700 for ethane) and selectivity were achieved using abundant, inexpensive cerium salts as photocatalysts. Ligand-to-metal charge transfer excitation generated alkoxy radicals from simple alcohols that in turn acted as hydrogen atom transfer catalysts. The mixed-phase gas/liquid reaction was adapted to continuous flow, enabling the efficient use of gaseous feedstocks in scalable photocatalytic transformations.
    Keywords: Chemistry
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  • 24
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-24
    Keywords: Chemistry
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  • 25
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-24
    Keywords: Chemistry
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  • 26
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-31
    Keywords: Chemistry
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  • 27
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-31
    Description: Intrigued by the potential of nanoscale machines, scientists have long attempted to control molecular motion. We monitored the individual 0.7-nanometer steps of a single molecular hopper as it moved in an electric field along a track in a nanopore controlled by a chemical ratchet. The hopper demonstrated characteristics desired in a moving molecule: defined start and end points, processivity, no chemical fuel requirement, directional motion, and external control. The hopper was readily functionalized to carry cargos. For example, a DNA molecule could be ratcheted along the track in either direction, a prerequisite for nanopore sequencing.
    Keywords: Chemistry
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  • 28
    Publication Date: 2018-09-21
    Description: Phosphorothioate nucleotides have emerged as powerful pharmacological substitutes of their native phosphodiester analogs with important translational applications in antisense oligonucleotide (ASO) therapeutics and cyclic dinucleotide (CDN) synthesis. Stereocontrolled installation of this chiral motif has long been hampered by the systemic use of phosphorus(III) [P(III)]–based reagent systems as the sole practical means of oligonucleotide assembly. A fundamentally different approach is described herein: the invention of a P(V)-based reagent platform for programmable, traceless, diastereoselective phosphorus-sulfur incorporation. The power of this reagent system is demonstrated through the robust and stereocontrolled synthesis of various nucleotidic architectures, including ASOs and CDNs, via an efficient, inexpensive, and operationally simple protocol.
    Keywords: Chemistry
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  • 29
    Publication Date: 2018-09-21
    Description: Here we report an anomalous porous molecular crystal built of C–H···N-bonded double-layered roof-floor components and wall components of a segregatively interdigitated architecture. This complicated porous structure consists of only one type of fully aromatic multijoint molecule carrying three identical dipyridylphenyl wedges. Despite its high symmetry, this molecule accomplishes difficult tasks by using two of its three wedges for roof-floor formation and using its other wedge for wall formation. Although a C–H···N bond is extremely labile, the porous crystal maintains its porosity until thermal breakdown of the C–H···N bonds at 202°C occurs, affording a nonporous polymorph. Though this nonporous crystal survives even at 325°C, it can retrieve the parent porosity under acetonitrile vapor. These findings show how one can translate simplicity into ultrahigh complexity.
    Keywords: Chemistry
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  • 30
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-09-28
    Keywords: Chemistry
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  • 31
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-09-28
    Keywords: Chemistry
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  • 32
    Publication Date: 2018-09-28
    Description: Some of the simplest and most powerful carbon-carbon bond forming strategies take advantage of readily accessible ubiquitous motifs: carbonyls and olefins. Here we report a fundamentally distinct mode of reactivity between carbonyls and olefins that differs from established acid-catalyzed carbonyl-ene, Prins, and carbonyl-olefin metathesis reaction paths. A range of epsilon, zeta-unsaturated ketones undergo Brønsted acid–catalyzed intramolecular cyclization to provide tetrahydrofluorene products via the formation of two new carbon-carbon bonds. Theoretical calculations and accompanying mechanistic studies suggest that this carbocyclization reaction proceeds through the intermediacy of a transient oxetane formed by oxygen atom transfer. The complex polycyclic frameworks in this product class appear as common substructures in organic materials, bioactive natural products, and recently developed pharmaceuticals.
    Keywords: Chemistry
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  • 33
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-09-28
    Description: Alkene aminoarylation with a single, bifunctional reagent is a concise synthetic strategy. We report a catalytic protocol for the addition of arylsulfonylacetamides across electron-rich alkenes with complete anti-Markovnikov regioselectivity and excellent diastereoselectivity to provide 2,2-diarylethylamines. In this process, single-electron alkene oxidation enables carbon-nitrogen bond formation to provide a key benzylic radical poised for a Smiles-Truce 1,5-aryl shift. This reaction is redox-neutral, exhibits broad functional group compatibility, and occurs at room temperature with loss of sulfur dioxide. As this process is driven by visible light, uses readily available starting materials, and demonstrates convergent synthesis, it is well suited for use in a variety of synthetic endeavors.
    Keywords: Chemistry
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  • 34
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-10-05
    Keywords: Chemistry
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  • 35
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-10-05
    Keywords: Chemistry
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  • 36
    Publication Date: 2018-10-05
    Description: Photocatalysis based on optically active, "plasmonic" metal nanoparticles has emerged as a promising approach to facilitate light-driven chemical conversions under far milder conditions than thermal catalysis. However, an understanding of the relation between thermal and electronic excitations has been lacking. We report the substantial light-induced reduction of the thermal activation barrier for ammonia decomposition on a plasmonic photocatalyst. We introduce the concept of a light-dependent activation barrier to account for the effect of light illumination on electronic and thermal excitations in a single unified picture. This framework provides insight into the specific role of hot carriers in plasmon-mediated photochemistry, which is critically important for designing energy-efficient plasmonic photocatalysts.
    Keywords: Chemistry
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  • 37
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-10-12
    Keywords: Chemistry
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  • 38
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-10-12
    Keywords: Chemistry
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  • 39
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-10-12
    Description: Single-electron reduction of a carbonyl to a ketyl enables access to a polarity-reversed platform of reactivity for this cornerstone functional group. However, the synthetic utility of the ketyl radical is hindered by the strong reductants necessary for its generation, which also limit its reactivity to net reductive mechanisms. We report a strategy for net redox-neutral generation and reaction of ketyl radicals. The in situ conversion of aldehydes to α-acetoxy iodides lowers their reduction potential by more than 1 volt, allowing for milder access to the corresponding ketyl radicals and an oxidative termination event. Upon subjecting these iodides to a dimanganese decacarbonyl precatalyst and visible light irradiation, an atom transfer radical addition (ATRA) mechanism affords a broad scope of vinyl iodide products with high Z -selectivity.
    Keywords: Chemistry
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  • 40
    Publication Date: 2018-10-12
    Description: Reactions that form a product with the same reactive functionality as that of one of the starting compounds frequently end in oligomerization. As a salient example, selective aldol coupling of the smallest, though arguably most useful, enolizable aldehyde, acetaldehyde, with just one partner substrate has proven to be extremely challenging. Here, we report a highly enantioselective Mukaiyama aldol reaction with the simple triethylsilyl (TES) and tert -butyldimethylsilyl (TBS) enolates of acetaldehyde and various aliphatic and aromatic acceptor aldehydes. The reaction is catalyzed by recently developed, strongly acidic imidodiphosphorimidates (IDPi), which, like enzymes, display a confined active site but, like small-molecule catalysts, have a broad substrate scope. The process is scalable, fast, efficient (0.5 to 1.5 mole % catalyst loading), and greatly simplifies access to highly valuable silylated acetaldehyde aldols.
    Keywords: Chemistry
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  • 41
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-12-21
    Keywords: Chemistry
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  • 42
    Publication Date: 2018-12-21
    Description: Single-molecule magnets (SMMs) containing only one metal center may represent the lower size limit for molecule-based magnetic information storage materials. Their current drawback is that all SMMs require liquid-helium cooling to show magnetic memory effects. We now report a chemical strategy to access the dysprosium metallocene cation [(Cp i Pr5 )Dy(Cp*)] + (Cp i Pr5 , penta-iso-propylcyclopentadienyl; Cp *, pentamethylcyclopentadienyl), which displays magnetic hysteresis above liquid-nitrogen temperatures. An effective energy barrier to reversal of the magnetization of U eff = 1541 wave number is also measured. The magnetic blocking temperature of T B = 80 kelvin for this cation overcomes an essential barrier toward the development of nanomagnet devices that function at practical temperatures.
    Keywords: Chemistry
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  • 43
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-10-19
    Keywords: Chemistry
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  • 44
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-10-19
    Description: The development of highly reactive and stereoselective catalytic systems is required not only to improve existing synthetic methods but also to invent distinct chemical reactions. Herein, a homogenized combination of nickel-based Lewis acid–surfactant-combined catalysts and single-walled carbon nanotubes is shown to exhibit substantial activity in water. In addition to the enhanced reactivity, stereoselective performance and long-term stability were demonstrated in asymmetric conjugate addition reactions of aldoximes to furnish chiral nitrones in high yields with excellent selectivities. The practical and straightforward application of the designed catalysts in water provides an expedient, environmentally benign, and highly efficient pathway to access optically active compounds.
    Keywords: Chemistry
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  • 45
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-11-02
    Keywords: Chemistry
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  • 46
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-12-14
    Keywords: Chemistry
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  • 47
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-10-26
    Keywords: Chemistry
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  • 48
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-11-02
    Description: Exploration of intermediates that enable chemoselective cycloaddition reactions and expeditious construction of fused- or bridged-ring systems is a continuous challenge for organic synthesis. As an intermediate of interest, the oxyallyl cation has been harnessed to synthesize architectures containing seven-membered rings via (4+3) cycloaddition. However, its potential to access five-membered skeletons is underdeveloped, largely due to the thermally forbidden (3+2) pathway. Here, the combination of a tailored precursor and a Pd(0) catalyst generates a Pd-oxyallyl intermediate that cyclizes with conjugated dienes to produce a diverse array of tetrahydrofuran skeletons. The cycloaddition overrides conventional (4+3) selectivity by proceeding through a stepwise pathway involving a Pd-allyl transfer and ring closure sequence. Subsequent treatment of the (3+2) adducts with a palladium catalyst converts the heterocycles to the carbocyclic cyclopentanones.
    Keywords: Chemistry
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  • 49
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-11-16
    Keywords: Chemistry
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  • 50
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-10
    Keywords: Chemistry
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  • 51
    Publication Date: 2018-08-10
    Description: Understanding the practical limitations of chemical reactions is critically important for efficiently planning the synthesis of compounds in pharmaceutical, agrochemical, and specialty chemical research and development. However, literature reports of the scope of new reactions are often cursory and biased toward successful results, severely limiting the ability to predict reaction outcomes for untested substrates. We herein illustrate strategies for carrying out large-scale surveys of chemical reactivity by using a material-sparing nanomole-scale automated synthesis platform with greatly expanded synthetic scope combined with ultrahigh-throughput matrix-assisted laser desorption/ionization–time-of-flight mass spectrometry (MALDI-TOF MS).
    Keywords: Chemistry
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  • 52
    Publication Date: 2018-08-24
    Description: Researchers around the world have observed the formation of molecularly ordered structures of unknown origin on the surface of titanium dioxide (TiO 2 ) photocatalysts exposed to air and solution. Using a combination of atomic-scale microscopy and spectroscopy, we show that TiO 2 selectively adsorbs atmospheric carboxylic acids that are typically present in parts-per-billion concentrations while effectively repelling other adsorbates, such as alcohols, that are present in much higher concentrations. The high affinity of the surface for carboxylic acids is attributed to their bidentate binding. These self-assembled monolayers have the unusual property of being both hydrophobic and highly water-soluble, which may contribute to the self-cleaning properties of TiO 2 . This finding is relevant to TiO 2 photocatalysis, because the self-assembled carboxylate monolayers block the undercoordinated surface cation sites typically implicated in photocatalysis.
    Keywords: Chemistry
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  • 53
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-31
    Keywords: Chemistry
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  • 54
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-31
    Keywords: Chemistry
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  • 55
    Publication Date: 2018-08-31
    Description: The alkaline earth metals calcium (Ca), strontium (Sr), and barium (Ba) typically engage in chemical bonding as classical main-group elements through their n s and n p valence orbitals, where n is the principal quantum number. Here we report the isolation and spectroscopic characterization of eight-coordinate carbonyl complexes M(CO) 8 (where M = Ca, Sr, or Ba) in a low-temperature neon matrix. Analysis of the electronic structure of these cubic O h -symmetric complexes reveals that the metal–carbon monoxide (CO) bonds arise mainly from [M(d )] -〉 (CO) 8 backdonation, which explains the strong observed red shift of the C-O stretching frequencies. The corresponding radical cation complexes were also prepared in gas phase and characterized by mass-selected infrared photodissociation spectroscopy, confirming adherence to the 18-electron rule more conventionally associated with transition metal chemistry.
    Keywords: Chemistry
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  • 56
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-09-07
    Keywords: Chemistry
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  • 57
    Publication Date: 2018-09-07
    Description: Mystery surrounds the transition from gas-phase hydrocarbon precursors to terrestrial soot and interstellar dust, which are carbonaceous particles formed under similar conditions. Although polycyclic aromatic hydrocarbons (PAHs) are known precursors to high-temperature carbonaceous-particle formation, the molecular pathways that initiate particle formation are unknown. We present experimental and theoretical evidence for rapid molecular clustering–reaction pathways involving radicals with extended conjugation. These radicals react with other hydrocarbon species to form covalently bound complexes that promote further growth and clustering by regenerating resonance-stabilized radicals through low-barrier hydrogen-abstraction and hydrogen-ejection reactions. Such radical–chain reaction pathways may lead to covalently bound clusters of PAHs and other hydrocarbons that would otherwise be too small to condense at high temperatures, thus providing the key mechanistic steps for rapid particle formation and surface growth by hydrocarbon chemisorption.
    Keywords: Chemistry
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  • 58
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-09-14
    Keywords: Chemistry
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  • 59
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-04-27
    Keywords: Chemistry
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  • 60
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-04-27
    Keywords: Chemistry
    Print ISSN: 0036-8075
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  • 61
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-04-27
    Description: Basic heteroarenes are a ubiquitous feature of pharmaceuticals and bioactive molecules, and Minisci-type additions of radical nucleophiles are a leading method for their elaboration. Despite many Minisci-type protocols that result in the formation of stereocenters, exerting control over the absolute stereochemistry at these centers remains an unmet challenge. We report a process for addition of prochiral radicals, generated from amino acid derivatives, to pyridines and quinolines. Our method offers excellent control of both enantioselectivity and regioselectivity. An enantiopure chiral Brønsted acid catalyst serves both to activate the substrate and induce asymmetry, while an iridium photocatalyst mediates the required electron transfer processes. We anticipate that this method will expedite access to enantioenriched small-molecule building blocks bearing versatile basic heterocycles.
    Keywords: Chemistry
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  • 62
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-05-25
    Keywords: Chemistry
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    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 63
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-05-25
    Keywords: Chemistry
    Print ISSN: 0036-8075
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    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 64
    Publication Date: 2018-10-26
    Description: Primary amines are essential constituents of biologically active molecules and versatile intermediates in the synthesis of drugs and agrochemicals. However, their preparation from easily accessible alkenes remains challenging. Here, we report a general strategy to access primary amines from alkenes through an operationally simple iron-catalyzed aminochlorination reaction. A stable hydroxylamine derivative and benign sodium chloride act as the respective nitrogen and chlorine sources. The reaction proceeds at room temperature under air; tolerates a large scope of aliphatic and conjugated alkenes, including densely functionalized substrates; and provides excellent anti-Markovnikov regioselectivity with respect to the amino group. The reactivity of the 2-chloroalkylamine products, an understudied class of amphoteric molecules, enables facile access to linear or branched aliphatic amines, aziridines, aminonitriles, azido amines, and homoallylic amines.
    Keywords: Chemistry
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  • 65
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-11-09
    Keywords: Chemistry
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  • 66
    Publication Date: 2018-11-09
    Description: Despite the enormous potential for the use of stereospecific cross-coupling reactions to rationally manipulate the three-dimensional structure of organic molecules, the factors that control the transfer of stereochemistry in these reactions remain poorly understood. Here we report a mechanistic and synthetic investigation into the use of enantioenriched alkylboron nucleophiles in stereospecific Pd-catalyzed Suzuki cross-coupling reactions. By developing a suite of molecular descriptors of phosphine ligands, we could apply predictive statistical models to select or design distinct ligands that respectively promoted stereoinvertive and stereoretentive cross-coupling reactions. Stereodefined branched structures were thereby accessed through the predictable manipulation of absolute stereochemistry, and a general model for the mechanism of alkylboron transmetallation was proposed.
    Keywords: Chemistry
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  • 67
    Publication Date: 2018-11-16
    Description: Heterobiaryls composed of pyridine and diazine rings are key components of pharmaceuticals and are often central to pharmacological function. We present an alternative approach to metal-catalyzed cross-coupling to make heterobiaryls using contractive phosphorus C–C couplings, also termed phosphorus ligand coupling reactions. The process starts by regioselective phosphorus substitution of the C–H bonds para to nitrogen in two successive heterocycles; ligand coupling is then triggered via acidic alcohol solutions to form the heterobiaryl bond. Mechanistic studies imply that ligand coupling is an asynchronous process involving migration of one heterocycle to the ipso position of the other around a central pentacoordinate P(V) atom. The strategy can be applied to complex drug-like molecules containing multiple reactive sites and polar functional groups, and also enables convergent coupling of drug fragments and late-stage heteroarylation of pharmaceuticals.
    Keywords: Chemistry
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  • 68
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-07-06
    Keywords: Chemistry
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  • 69
    Publication Date: 2018-07-06
    Description: Conical intersections play a critical role in excited-state dynamics of polyatomic molecules because they govern the reaction pathways of many nonadiabatic processes. However, ultrafast probes have lacked sufficient spatial resolution to image wave-packet trajectories through these intersections directly. Here, we present the simultaneous experimental characterization of one-photon and two-photon excitation channels in isolated CF 3 I molecules using ultrafast gas-phase electron diffraction. In the two-photon channel, we have mapped out the real-space trajectories of a coherent nuclear wave packet, which bifurcates onto two potential energy surfaces when passing through a conical intersection. In the one-photon channel, we have resolved excitation of both the umbrella and the breathing vibrational modes in the CF 3 fragment in multiple nuclear dimensions. These findings benchmark and validate ab initio nonadiabatic dynamics calculations.
    Keywords: Chemistry
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  • 70
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-07-13
    Keywords: Chemistry
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  • 71
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-06-22
    Keywords: Chemistry
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  • 72
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-04-13
    Keywords: Chemistry
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  • 73
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-05-04
    Keywords: Chemistry
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  • 74
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-05-11
    Keywords: Chemistry
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  • 75
    Publication Date: 2018-05-11
    Description: Common anionic nucleophiles such as those derived from inorganic salts have not been used for enantioselective catalysis because of their insolubility. Here, we report that merging hydrogen bonding and phase-transfer catalysis provides an effective mode of activation for nucleophiles that are insoluble in organic solvents. This catalytic manifold relies on hydrogen bonding complexation to render nucleophiles soluble and reactive, while simultaneously inducing asymmetry in the ensuing transformation. We demonstrate the concept using a chiral bis-urea catalyst to form a tridentate hydrogen bonding complex with fluoride from its cesium salt, thereby enabling highly efficient enantioselective ring opening of episulfonium ion. This fluorination method is synthetically valuable considering the scarcity of alternative protocols and points the way to wider application of the catalytic approach with diverse anionic nucleophiles.
    Keywords: Chemistry
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  • 76
    Publication Date: 2018-05-18
    Description: Carbon dioxide (CO 2 ) electroreduction could provide a useful source of ethylene, but low conversion efficiency, low production rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alkaline electrolyte reduces CO 2 to ethylene with 70% faradaic efficiency at a potential of –0.55 volts versus a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO 2 reduction and carbon monoxide (CO)–CO coupling activation energy barriers; as a result, onset of ethylene evolution at –0.165 volts versus an RHE in 10 molar potassium hydroxide occurs almost simultaneously with CO production. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between separate hydrophobic and conductive supports, providing constant ethylene selectivity for an initial 150 operating hours.
    Keywords: Chemistry
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  • 77
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-05-25
    Keywords: Chemistry
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  • 78
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-03-23
    Description: White phosphorus, generated in the legacy thermal process for phosphate rock upgrading, has long been the key industrial intermediate for the synthesis of phosphorus-containing chemicals, including herbicides, flame-retardants, catalyst ligands, battery electrolytes, pharmaceuticals, and detergents. In contrast, phosphate fertilizers are made on a much larger scale from phosphoric acid, obtained by treating phosphate rock with sulfuric acid. Dehydration of phosphoric acid using sodium chloride gives trimetaphosphate, and here we report that trichlorosilane, primarily used for the production of high-purity silicon, reduces trimetaphosphate to the previously unknown bis(trichlorosilyl)phosphide anion. This anion offers an entry point to value-added organophosphorus chemicals such as primary and secondary alkyl phosphines, and thus to organophosphinates, and can also be used to prepare phosphine gas and the hexafluorophosphate anion, all previously available only downstream from white phosphorus.
    Keywords: Chemistry
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  • 79
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-04-06
    Keywords: Chemistry
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  • 80
    Publication Date: 2018-04-06
    Description: Cross-coupling chemistry is widely applied to carbon-carbon bond formation in the synthesis of medicines, agrochemicals, and other functional materials. Recently, single-electron–induced variants of this reaction class have proven particularly useful in the formation of C(sp 2 )–C(sp 3 ) linkages, although certain compound classes have remained a challenge. Here, we report the use of sulfones to activate the alkyl coupling partner in nickel-catalyzed radical cross-coupling with aryl zinc reagents. This method’s tolerance of fluoroalkyl substituents proved particularly advantageous for the streamlined preparation of pharmaceutically oriented fluorinated scaffolds that previously required multiple steps, toxic reagents, and nonmodular retrosynthetic blueprints. Five specific sulfone reagents facilitate the rapid assembly of a vast set of compounds, many of which contain challenging fluorination patterns.
    Keywords: Chemistry
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  • 81
    Publication Date: 2018-05-04
    Description: Plasmon-induced chemical reactions of molecules adsorbed on metal nanostructures are attracting increased attention for photocatalytic reactions. However, the mechanism remains controversial because of the difficulty of direct observation of the chemical reactions in the plasmonic field, which is strongly localized near the metal surface. We used a scanning tunneling microscope (STM) to achieve real-space and real-time observation of a plasmon-induced chemical reaction at the single-molecule level. A single dimethyl disulfide molecule on silver and copper surfaces was dissociated by the optically excited plasmon at the STM junction. The STM study combined with theoretical calculations shows that this plasmon-induced chemical reaction occurred by a direct intramolecular excitation mechanism.
    Keywords: Chemistry
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  • 82
    Publication Date: 2018-05-25
    Description: Identifying catalyst activation modes that exploit one-electron chemistry and overcome associated deactivation pathways will be transformative for developing first-row transition metal catalysts with performance equal or, ideally, superior to precious metals. Here we describe a zinc-activation method compatible with high-throughput reaction discovery that identified scores of cobalt-phosphine combinations for the asymmetric hydrogenation of functionalized alkenes. An optimized catalyst prepared from ( R , R )-Ph-BPE {Ph-BPE, 1,2-bis[(2 R ,5 R )-2,5-diphenylphospholano]ethane} and cobalt chloride [CoCl 2 ·6H 2 O] exhibited high activity and enantioselectivity in protic media and enabled the asymmetric synthesis of the epilepsy medication levetiracetam at 200-gram scale with 0.08 mole % catalyst loading. Stoichiometric studies established that the cobalt (II) catalyst precursor ( R , R )-Ph-BPECoCl 2 underwent ligand displacement by methanol, and zinc promoted facile one-electron reduction to cobalt (I), which more stably bound the phosphine.
    Keywords: Chemistry
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  • 83
    Publication Date: 2018-06-01
    Description: Transition metal–catalyzed arene functionalization has been widely used for molecular synthesis over the past century. In this arena, copper catalysis has long been considered a privileged platform due to the propensity of high-valent copper to undergo reductive elimination with a wide variety of coupling fragments. However, the sluggish nature of oxidative addition has limited copper’s capacity to broadly facilitate haloarene coupling protocols. Here, we demonstrate that this copper oxidative addition problem can be overcome with an aryl radical–capture mechanism, wherein the aryl radical is generated through a silyl radical halogen abstraction. This strategy was applied to a general trifluoromethylation of aryl bromides through dual copper-photoredox catalysis. Mechanistic studies support the formation of an open-shell aryl species.
    Keywords: Chemistry
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  • 84
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-05-18
    Keywords: Chemistry
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  • 85
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-06-01
    Keywords: Chemistry
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  • 86
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-03-23
    Keywords: Chemistry
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  • 87
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-03-23
    Keywords: Chemistry
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  • 88
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-03-30
    Keywords: Chemistry
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  • 89
    Publication Date: 2018-03-30
    Description: The activation of olefins for asymmetric chemical synthesis traditionally relies on transition metal catalysts. In contrast, biological enzymes with Brønsted acidic sites of appropriate strength can protonate olefins and thereby generate carbocations that ultimately react to form natural products. Although chemists have recently designed chiral Brønsted acid catalysts to activate imines and carbonyl compounds, mimicking these enzymes to protonate simple olefins that then engage in asymmetric catalytic reactions has remained a substantial synthetic challenge. Here, we show that a class of confined and strong chiral Brønsted acids enables the catalytic asymmetric intramolecular hydroalkoxylation of unbiased olefins. The methodology gives rapid access to biologically active 1,1-disubstituted tetrahydrofurans, including (–)-Boivinianin A.
    Keywords: Chemistry
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  • 90
    Publication Date: 2018-04-13
    Description: Machine learning methods are becoming integral to scientific inquiry in numerous disciplines. We demonstrated that machine learning can be used to predict the performance of a synthetic reaction in multidimensional chemical space using data obtained via high-throughput experimentation. We created scripts to compute and extract atomic, molecular, and vibrational descriptors for the components of a palladium-catalyzed Buchwald-Hartwig cross-coupling of aryl halides with 4-methylaniline in the presence of various potentially inhibitory additives. Using these descriptors as inputs and reaction yield as output, we showed that a random forest algorithm provides significantly improved predictive performance over linear regression analysis. The random forest model was also successfully applied to sparse training sets and out-of-sample prediction, suggesting its value in facilitating adoption of synthetic methodology.
    Keywords: Chemistry
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  • 91
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-06-22
    Keywords: Chemistry
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  • 92
    Publication Date: 2018-06-29
    Description: Chiral amines are widely used as catalysts in asymmetric synthesis to activate carbonyl groups for α-functionalization. Carbonyl catalysis reverses that strategy by using a carbonyl group to activate a primary amine. Inspired by biological carbonyl catalysis, which is exemplified by reactions of pyridoxal-dependent enzymes, we developed an N-quaternized pyridoxal catalyst for the asymmetric Mannich reaction of glycinate with aryl N -diphenylphosphinyl imines. The catalyst exhibits high activity and stereoselectivity, likely enabled by enzyme-like cooperative bifunctional activation of the substrates. Our work demonstrates the catalytic utility of the pyridoxal moiety in asymmetric catalysis.
    Keywords: Chemistry
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  • 93
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-07-13
    Keywords: Chemistry
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  • 94
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-07-13
    Description: Deconstructive functionalizations involving scission of carbon-carbon double bonds are well established. In contrast, unstrained C(sp 3 )–C(sp 3 ) bond cleavage and functionalization have less precedent. Here we report the use of deconstructive fluorination to access mono- and difluorinated amine derivatives by C(sp 3 )–C(sp 3 ) bond cleavage in saturated nitrogen heterocycles such as piperidines and pyrrolidines. Silver-mediated ring-opening fluorination using Selectfluor highlights a strategy for cyclic amine functionalization and late-stage skeletal diversification, establishing cyclic amines as synthons for amino alkyl radicals and providing synthetic routes to valuable building blocks.
    Keywords: Chemistry
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  • 95
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-07-20
    Keywords: Chemistry
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  • 96
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-07-06
    Keywords: Chemistry
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  • 97
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-07-06
    Description: Transformation of simple precursors into structurally complex cyclobutanes, present in many biologically important natural products and pharmaceuticals, is of considerable interest in medicinal chemistry. Starting from 1,3-enynes and ethylene, both exceptionally inexpensive starting materials, we report a cobalt-catalyzed route to vinylcyclobutenes, as well as the further enantioselective addition of ethylene to these products to form complex cyclobutanes with all-carbon quaternary centers. These reactions can proceed in discrete stages or in a tandem fashion to achieve three highly selective carbon-carbon bond formations in one pot using a single chiral cobalt catalyst.
    Keywords: Chemistry
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  • 98
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-07-27
    Keywords: Chemistry
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  • 99
    Publication Date: 2018-02-23
    Description: Currently, the only compounds known to support fixation and functionalization of dinitrogen (N 2 ) under nonmatrix conditions are based on metals. Here we present the observation of N 2 binding and reduction by a nonmetal, specifically a dicoordinate borylene. Depending on the reaction conditions under which potassium graphite is introduced as a reductant, N 2 binding to two borylene units results in either neutral (B 2 N 2 ) or dianionic ([B 2 N 2 ] 2– ) products that can be interconverted by respective exposure to further reductant or to air. The 15 N isotopologues of the neutral and dianionic molecules were prepared with 15 N-labeled dinitrogen, allowing observation of the nitrogen nuclei by 15 N nuclear magnetic resonance spectroscopy. Protonation of the dianionic compound with distilled water furnishes a diradical product with a central hydrazido B 2 N 2 H 2 unit. All three products were characterized spectroscopically and crystallographically.
    Keywords: Chemistry
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-02-23
    Keywords: Chemistry
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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