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  • Chemistry  (408,469)
  • Inorganic Chemistry  (83,670)
  • LUNAR AND PLANETARY EXPLORATION  (14,408)
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  • 1
    Publication Date: 2019-05-06
    Description: This study aimed to evaluate the nutritional value (proximate composition, fatty acid profiles, vitamins and minerals) contents and also nutritional quality indices (NQI)) of grass carp (Ctenopharyngodon idella) prepared according to common consumer techniques: raw, poached, steamed, microwaved, pan-fried and deep-fried (in olive oil). In comparison to raw fish fillets, when grass carp was cooked there was an increase in protein, lipid and ash contents. Cooking methods had no significant effect on total n-3 fatty acids except for frying fillets. Lowest and highest content of n-3 was shown in deep-fried and pan-fried samples, respectively. Total n-6 fatty acid of cooked samples increased in comparison to raw samples. Na, K, Mg, P and Zn contents of boiled fish fillets significantly decreased. None of cooking methods had a significant effect a vitamin D. However, vitamin A, B_1 and B_3 contents of cooked fish significantly decreased.
    Keywords: Aquaculture ; Biology ; Chemistry
    Repository Name: Aquatic Commons
    Type: Article , PeerReviewed
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  • 2
    Publication Date: 2019-05-06
    Description: Skin cancer has been reported as a contemporary malignant cancer. Here, anti-cancer effects of sea cucumber extract (SCE) from Holothuria arenicola have been examined on melanoma cells and compared with imidazole carboxamide (Dacarbazine) as a chemotherapy medication against melanoma and Hodgkin's lymphoma. MTT assay and morphological analysis were performed to evaluate cytotoxic effects of H. arenicola extract. Also, several methods were exerted to detect cell dying by SCE and imidazole carboxamide. The MTT assay showed that B16F10 cells proliferation was blocked by SCE (IC_50=31µg ml ^-1) and imidazole carboxamide (IC_50=1600 µg ml ^-1) in a dose and time dependent manner. Apoptosis induction yield treatment occurred at IC50 concentration of SCE and imidazole carboxamide using DAPI staining, Acridine orange/Propodium iodide, PI flow cytometry and annexin/PI assay. The caspase colorimetric kit indicated that SCE and imidazole carboxamide could induce apoptosis through an intrinsic pathway. Collectively, our findings suggested that the methanolic SCE has more efficient cytotoxicity efficiency compared to imidazole carboxamide. Therefore, SCE may be considered as a futuristic marine natural product regarding prevention or treatment of melanoma malignancy.
    Keywords: Biology ; Chemistry
    Repository Name: Aquatic Commons
    Type: Article , PeerReviewed
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  • 3
    Publication Date: 2019-05-06
    Description: The present study was conducted to examine different dietary levels of iron (FeSO4) on some haematological parameters of ship sturgeon, Acipenser nudiventris including red blood cells (RBC), white blood cells (WBCs), differential WBCs, hematocrit (Hct), Hemoglobin (Hb), the mean corpuscular hemoglobin (MCH) and the mean corpuscular hemoglobin concentration (MCHC), the mean corpuscular volume (MCV), immunoglobulin (IgM), ferritin, transferrin and plasma Fe^2+. Four experimental treatments and one control group with three replicates were considered for the experiment. The experimental treatments were fish fed experimental diets containing different levels of FeSO_4 i.e. 100, 200, 250, and 300 mg kg. diet^-1. After 60 days, there were significant differences between experimental groups in terms of WBCs, lymphocytes, neutrophils, monocytes, eosinophils, MCH and MCHC, MCV, total plasma protein, IgM, ferritin, transferrin and plasma Fe^2+ (p〈0.05). In the treatment fed 100 mg. iron kg. diet^-1, more levels of IgM and total plasma protein were observed compared to other experimental groups (p〈0.05). The highest values of neutrophils, monocytes and MCHC were found in control fish whereas MCV, MCH and eosinophils values were higher in the treatment fed 200 mg. iron kg. diet^-1 (p〈0.05). Also, more WBCs were observed in 250 mg. iron kg. diet^-1 treatment than in other groups (p〈0.05). The values of lymphocytes, RBC, Hct, Hb, ferritin, transferrin and plasma Fe^2+ were higher in fish fed 300 mg. iron kg. diet^-1 compared to other experimental groups (p〈0.05). In conclusion, our results showed that the supplementation of ship sturgeon diets with high levels of iron enhances the RBC, Hct, Hb, ferritin, transferrin and plasma iron.
    Keywords: Aquaculture ; Biology ; Chemistry
    Repository Name: Aquatic Commons
    Type: Article , PeerReviewed
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  • 4
    Publication Date: 2019-05-06
    Description: The effects of chitosan (2%) coating combined with green tea extract (GTE) (1%) on the quality of tiger-tooth croaker (Otolithes ruber) fillets including chemical, microbiological, texture, color and sensory properties during 16 days of refrigerated storage was investigated. The color changes were significantly retared, and the texture parameteres and sensory scores were significantly improved in tiger- tooth croaker coated by chitosan, GTE and chitosan combined with GTE, compared with the control. The coincidental lowered rate of increase total volatile base (TVB) content, thiobarbitoric acid (TBA), peroxide value (POV) and free fatty acids (FFA) were obtained in tiger-tooth croaker coated chitosan+GTE. Chitosan in combination with GTE had higher inhibition on microbial growth and yielded the tigertooth croaker with higher likeness, compared with the other treatments. Therefore, fish coated with chitosan+GTE had the lowest losses in quality during refrigerated storage.
    Keywords: Biology ; Chemistry ; Fisheries
    Repository Name: Aquatic Commons
    Type: Article , PeerReviewed
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  • 5
    Publication Date: 2019-05-06
    Description: Hand cream is used for rehydrating and improving skin properties. The skin is the first point of contact with the environment. Thus having healthy skin with proper barrier properties is quite important. The beneficial effects of fish oil on skin make it a favorable ingredient to be used in hand care products. In the present study, the efficacy and stability of various hand cream formulations using 0, 1.0, 2.5 and 5.0% of fish oil were investigated and compared. The appearance, odor, texture, and pH of formulations containing 1 and 2.5% of fish oil remained stable in very good qualities during the storage, whereas the related values to the appearance and texture of formulation containing 5.0% of fish oil at the end of 6 months of storage at 45°C were significantly (p〈0.05) lower than other formulations and previous evaluations of this formulation. Weight loss was not observed in all formulations under study.
    Keywords: Biology ; Chemistry ; Fisheries
    Repository Name: Aquatic Commons
    Type: Article , PeerReviewed
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  • 6
    Publication Date: 2019-05-06
    Description: Research on eliminating organic pollutants in water by using heterogeneous photocatalysts such as nano-TiO_2 abound. However, the question is whether metabolites, resulting from optical dispersion of environmental pollutants, are still toxic to aquatic organisms. Therefore, this study was designed to evaluate the effect of a co-exposure to paraquat and TiO_2-NPs on blood biochemical indices of common carp. Fish were exposed to 0.2 and 0.4 mg L^-1 paraquat with 0.125 mg L^-1 TiO_2-NPs for 21 days under different light conditions, including natural photoperiod (16 L 8h^-1 D) and complete darkness. No significant alterations in the Aspartate aminotransferase (AST), Alanine aminotransferase (ALT), Lactate dehydrogenase (LDH), Alkaline phosphatase (ALP) and Creatine phosphokinase (CPK), activities and total protein, albumin, globulin, glucose, cholesterol and triglyceride levels were observed in fish exposed to TiO_2-NPs and 0.2 mg L^-1 paraquat under normal lighting conditions. However, a significant change in blood biochemical indices in fish exposed to TiO2-NPs with 0.4 mg L^-1 paraquat (16 L 8h^-1 D) and in fish exposed to TiO_2-NPs and paraquat (under darkness). A significant decrease in the activity of Gamma-glutamyltransferase (GGT) and a significant increase in creatinine level were observed in all groups which were exposed to TiO_2-NPs and paraquat. The results of this study indicate that using 0.125 mg L^-1 nano-TiO2 in order to remove paraquat (0.2 mg L^-1) under lighting conditions can minimize the adverse effects of paraquat and its metabolites on blood biochemical indices of fish. So, using nano-TiO_2 (0.125 mg L^-1) to remove paraquat under lighting conditions can significantly reduce its toxic effects.
    Keywords: Biology ; Chemistry ; Environment ; Pollution
    Repository Name: Aquatic Commons
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2019-05-06
    Description: To investigate the effects of carvacrol supplementation in rainbow trout induced by lipopolysaccharide (LPS), relative gene expressions levels of some pro-inflammatory cytokines and apoptosis markers in kidney and liver were measured. Fish with a mean weight of 44.71±1.33 g were studied using four different treatments with three replicates each. Inflammation and apoptosis were performed using LPS of Escherichia coli (25 µg ml ^-1) except in the control group and only carvacrol (100 µg ml ^-1 diet) containing diets were fed to this group (CAR). The last group was the infected fish fed carvacrol supplemented diet (+CAR). Kidney and liver tissues were removed 3 days after to determine the levels of interleukin-1β (IL-1β), interferon gamma (INF-γ), caspase 3 (Cas 3), caspase 8 (Cas 8) using Real-Time PCR analyses. IL-1β expressions of both kidney and liver was significantly decreased (12.9 and 2.14 fold, respectively) in LPS treated cells (p〈0.05). While IFN-γ expression was up regulated in kidney, it had down regulation in liver. LPS decreased both Cas 3 and 8 expressions in kidney but increased in liver. +CAR increased expressions of IL-1β and IFN-γ compared with the control (up to 2-3 fold) in all tissues except caspase gene expressions that were similar in the control. In all tissues, IL-1β and IFN-γ expressions increased in +CAR group, expect IFN-γ in the kidney. However, Cas 3 and 8 expressions including apoptosis was induced by up-down regulations in all tissues compared to LPS-injected fish. The results showed that carvacrol had pro-inflammatory and apoptotic effects especially on liver tissue of LPS-induced inflammation model of Oncorhynchus mykiss.
    Keywords: Aquaculture ; Biology ; Chemistry ; Fisheries
    Repository Name: Aquatic Commons
    Type: Article , PeerReviewed
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  • 8
    Publication Date: 2019-05-06
    Description: The study on physicochemical factors and primary production of the aquatic ecosystems is important to know the characteristics of the environment and capacity of recharge. Mahshahr Creeks are the most important creeks in the Northwest Persian Gulf, Iran. In this study, variations in physicochemical factors, chl-a, and primary production in creeks of Mahshahr, Zangi, and Moussa were evaluated for 2 months of every season of 2014. The following variables were analyzed: nitrate, phosphate, nitrite, silicate, temperature, salinity, DO, pH, conductivity, turbidity, light, chl-a, primary production and phytoplankton abundance. The maximum amount of chl-a (1.6 mg m^3) was observed in Moussa Creek in April while the minimum amount (0.1 mg m ^-3) was seen in Moussa in July. The highest (1.19 gc m^-2 day^-1) and lowest (0.1gc m^-2 day^-1) amount of primary production were observed at the entrance of Moussa Creek in April and in Zangi Creek in November, respectively. The maximum and minimum abundance of phytoplankton was observed in April and October, respectively in all study areas. The results of correlation test between the studied factors revealed that primary production has a significant relationship with nitrite, chl-a, phytoplankton abundance and light. Primary production in these creeks is relatively high in April and is attributed to the uptake of nutrients including nitrate and phosphate from shallow waters entering these creeks, sufficient light and suitable temperature, but in other months the production is low.
    Keywords: Biology ; Chemistry
    Repository Name: Aquatic Commons
    Type: Article , PeerReviewed
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  • 9
    Publication Date: 2019-05-06
    Description: Concentrations of heavy metals (Cd, Ni and Pb) were determined in soft and hard tissues (three separated shell sections) of gastropod Bufonaria echinata as well as surficial sediments collected in October 2015 from two sampling sites located in the sub-littoral zone of Qeshm Island, Persian Gulf. There were significant differences between the sampling sites for concentrations of all the three elements in the shells and sediments. But in terms of the soft tissues, in the case of Ni and Pb significant differences between the sites could be observed. In all the cases, higher levels were observed in the samples from Suza site, which may be mainly due to the proximity of this site to the relevant anthropogenic sources. Comparison of the gained data from this study with the other relevant researches shows that in most cases the levels of the elements in the soft tissues and shells either fell within the range for other world areas or were lower. The observed increasing trends of metals accumulation in the shell sections (from older to younger sections) could be mainly attributed to the gradual increase of relevant anthropogenic pollutants in the study area, especially in Suza pier, during the recent years. Generally, it can be concluded that the shells of B.echinata could be possibly employed as a biomonitoring tool for historic metals contamination in northeastern part of the Persian Gulf.
    Keywords: Biology ; Chemistry ; Pollution
    Repository Name: Aquatic Commons
    Type: Article , PeerReviewed
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  • 10
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    UNSAM EDITA
    Publication Date: 2019-02-10
    Description: Este libro contiene las presentaciones de las conferencias invitadas de expertos y las realizadas en formato “poster” por los estudiantes participantes en el Workshop “Aguas + Humedales”, realizado en San Martín entre el 17 y el 22 de octubre de 2016 en la Universidad Nacional de San Martín, San Martín, Argentina. Con el objeto de brindar a los estudiantes de posgrado un panorama amplio del problema del agua y su proyección hacia el futuro, el material que aquí se presenta se ocupa de temas tales como el agua como fuente segura para consumo humano y su vida cotidiana, como recurso fundamental para la actividad agropecuaria, como fuente de alimentos, como fluido de proceso y de transporte de calor, como vía de comunicación y transporte. Se plantean interrogantes sobre la disponibilidad del recurso, sobre la intervención humana en el ciclo hidrológico y en el clima. También se analizan problemas relacionados con tecnologías para el tratamiento de aguas, tanto para consumo humano como tratamiento de efluentes de diverso tipo. Se presta especial atención a la dimensión social del tema: derecho y economía del agua, estudios vinculados con las decisiones sobre el uso del recurso y el desarrollo de legislaciones y regulaciones.
    Keywords: Agriculture ; Aquaculture ; Biology ; Chemistry ; Conservation ; Earth Sciences ; Ecology ; Engineering ; Environment ; Fisheries ; Limnology ; Policies ; Pollution
    Repository Name: Aquatic Commons
    Type: Book , PeerReviewed
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  • 11
    Publication Date: 2018-11-15
    Description: In this study, the removal of congo red and erythrosine as anionic dyes were investigated by using the cucurbit[6]uril nonporous as a new adsorbent. In the proposed mythology, the batch removal method was applied. In order to obtain maximum removal efficiency, the effects of acidity of solution, adsorbent dosage and contacting time were investigated and optimized. Adsorption experimental data were analyzed using Langmuir and Freundlich isotherm models and the best-fitted data was obtained with the Langmuir model. The maximum adsorption capacity was 128.2 mg g^-1 and 95.9 mg g^-1 for congo red and erythrosine, respectively. This method was successfully applied for the removal of congo red and erythrosine dyes from Karron and Bahmanshir water river and Persian Gulf water samples.
    Keywords: Biology ; Chemistry ; Pollution
    Repository Name: Aquatic Commons
    Type: Article , PeerReviewed
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  • 12
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    Massachusetts Institute of Technology and Woods Hole Oceanographic Institution
    Publication Date: 2018-11-02
    Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2018
    Description: Many chemical constituents are removed from the ocean by attachment to settling particles, a process referred to as “scavenging.” Radioisotopes of thorium, a highly particle-reactive element, have been used extensively to study scavenging in the ocean. However, this process is complicated by the highly variable chemical composition and concentration of particles in oceanic waters. This thesis focuses on understanding the cycling of thorium as affected by particle concentration and particle composition in the North Atlantic. This objective is addressed using (i) the distributions 228,230,234Th, their radioactive parents, particle composition, and bulk particle concentration, as measured or estimated along the GEOTRACES North Atlantic Transect (GA03) and (ii) a model for the reversible exchange of thorium with particles. Model parameters are either estimated by inversion (chapter 2-4), or prescribed in order to simulate 230Th in a circulation model (chapter 5). The major findings of this thesis follow. In chapters 2 and 3, I find that the rate parameters of the reversible exchange model show systematic variations along GA03. In particular, 𝑘1, the apparent first-order rate "constant" of Th adsorption onto particles, generally presents maxima in the mesopelagic zone and minima below. A positive correlation between 𝑘1 and bulk particle concentration is found, consistent with the notion that the specific rate at which a metal in solution attaches to particles increases with the number of surface sites available for adsorption. In chapter 4, I show that Mn (oxyhydr)oxides and biogenic particles most strongly influence 𝑘1 west of the Mauritanian upwelling, but that biogenic particles dominate 𝑘1 in this region. In chapter 5, I find that dissolved 230Th data are best represented by a model that assumes enhanced values of 𝑘1 near the seafloor. Collectively, my findings suggest that spatial variations in Th radioisotope activities observed in the North Atlantic reflect at least partly variations in the rate at which Th is removed from the water column.
    Description: This work was supported by the US National Science Foundation. Two US NSF grants have supported the research in this thesis (OCE-1232578 and OCE-155644).
    Keywords: Thorium ; Chemistry
    Repository Name: Woods Hole Open Access Server
    Type: Thesis
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  • 13
    Publication Date: 2019-06-05
    Description: In this study alginate was extracted for the first time from few species of Phaeophycota (Cystoseira indica, Padina tetrastromatica and Jolyna laminarioides) whereas Spatoglossum variabile and Sargassum tenerrimum, has been reported earlier from Karachi coast. Chemical bonding of extracted alginate was examined by using FTIR spectroscopy. Furthermore in pre-extraction step seaweeds were treated with diluted sulfuric acid solution for two different time intervals, all the selected alginophytes showed increased yield of alginate at 12 hrs treatment as compare to 5 hrs treatment.
    Keywords: Biology ; Chemistry
    Repository Name: Aquatic Commons
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  • 14
    Publication Date: 2019-06-05
    Description: In this research a comparative study was made between biodegradable films obtained from two different indigenous red seaweeds collected from Karachi coast. Gracilaria corticata and Melanothamnus afaqhussainii were collected from the Buleji coast of Karachi, Pakistan (24o50’N, 66o48’E). Agar extracted from both the seaweeds was used and incorporated as raw material for the formulation of biodegradable films. Sorbitol was used as plasticizer to increase the tensile strength and elasticity of the films. The results showed that the solubility of M. afaqhussainii was higher than G. corticata. The soil burial test showed that the biodegradability percent of M. afaqhussainii (60%) was higher as compared to G. corticata (40%). The tensile strength and elongation break of G. corticata (9.79±1.21 MPa and 28.72±5.13%) was stronger than the M. afaqhussainii (3.98±1.08 MPa and 16.32±3.80). Results of current study showed that the G. corticata has stronger gelling property as compared to M. afaqhussainii.
    Keywords: Biology ; Chemistry
    Repository Name: Aquatic Commons
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  • 15
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-17
    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
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-24
    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: 2018-08-31
    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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  • 18
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-09-21
    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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  • 19
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-09-21
    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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  • 20
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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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  • 21
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-10
    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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  • 22
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-17
    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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  • 23
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-17
    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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  • 24
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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
    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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  • 25
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-07-13
    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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  • 26
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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
    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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  • 27
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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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  • 28
    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
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    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 29
    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
    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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  • 30
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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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  • 31
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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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  • 32
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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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  • 33
    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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  • 34
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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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  • 35
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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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    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 36
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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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  • 37
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    American Association for the Advancement of Science (AAAS)
    In: Science
    Publication Date: 2018-08-24
    Description: Lithium-oxygen (Li-O 2 ) batteries have attracted much attention owing to the high theoretical energy density afforded by the two-electron reduction of O 2 to lithium peroxide (Li 2 O 2 ). We report an inorganic-electrolyte Li-O 2 cell that cycles at an elevated temperature via highly reversible four-electron redox to form crystalline lithium oxide (Li 2 O). It relies on a bifunctional metal oxide host that catalyzes O–O bond cleavage on discharge, yielding a high capacity of 11 milliampere-hours per square centimeter, and O 2 evolution on charge with very low overpotential. Online mass spectrometry and chemical quantification confirm that oxidation of Li 2 O involves transfer of exactly 4 e – /O 2 . This work shows that Li-O 2 electrochemistry is not intrinsically limited once problems of electrolyte, superoxide, and cathode host are overcome and that coulombic efficiency close to 100% can be achieved.
    Keywords: Chemistry
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  • 38
    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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  • 39
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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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  • 40
    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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  • 41
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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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  • 42
    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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  • 43
    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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  • 44
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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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  • 45
    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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  • 46
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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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  • 47
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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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  • 48
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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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  • 49
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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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  • 50
    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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  • 51
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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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  • 52
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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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  • 53
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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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  • 54
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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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  • 55
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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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  • 56
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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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  • 57
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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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  • 58
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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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  • 59
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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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  • 60
    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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  • 61
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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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  • 62
    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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  • 63
    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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  • 64
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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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  • 65
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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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  • 66
    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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