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  • Organic Chemistry  (63)
  • Photolysis  (7)
  • Rearrangements  (4)
  • Wiley-Blackwell  (72)
  • 1
    Electronic Resource
    Electronic Resource
    Synthese und Photolyse von 1,4,4-Trialkyl -4,5-dihydro-5-imino -1H-1,2,3-triazolen.  -  Regio- und diastereoselektive Bildung von (Z)-Aziridiniminen (1990)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 123 (1990), S. 2195-2202 
    Details
    ISSN: 0009-2940
    Keywords: [3 + 2] Cycloaddition ; [3 + 2] Cycloreversion ; 5H-Tetrazoles, 5-alkylidene-1,4-dihydro- ; Aziridinimines ; 1,2,3-Triazoles, 1,4,4-trialkyl-4,5-dihydro-5-imino- ; 1,2,3,4,6,7,8-Heptaazaspiro[4.4]nona-2,7-dienes ; Photolysis ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis and Photolysis of 1,4,4-Trialkyl-4,5-dihydro-5-imino-1H-1,2,3-triazoles.  -  Regio- and Diastereoselective Formation of (Z)-AziridiniminesThe novel 5-iminodihydro-1,2,3-triazoles 14 are synthesized from the 5-isopropylidenedihydrotetrazole 11 through a ring transformation involving cycloaddition and cycloreversion of alkyl azides 12. Thus, [3 + 21 cycloaddition of 11 and 12 pro- duces high yields of the spiro compounds 13. On thermolysis at 120- 130°C, the latter undergo clean [3 + 2] cycloreversion into methyl a i d e (12a) and the iminodihydro-1,2,3-triazoles 14, which exist as mixtures of diastereomers, e.g. (E)-/(Z)-14a = 955, or as a single stereoisomer (E)-14c. When toluene solutions of 14 are irradiated (≥ 1 280 mm) at - 60°C, the configuration of the products is under kinetic control. Irradiation of 14a affords a quantitative yield of the aziridinimines (E)-and (Z)-15a (11:89) besides molecular nitrogen. In order to allow the distinction between the least-motion and the non-least-motion path of product formation, the photolysis of iminodihydro-1,2,3-triazoles is studied which are labelled at N-1 by a deuterated methyl group (14b) or a neopentyl group (14c). Neither photolysis (λ ≥280 nm) nor heating to 120 to 130°C induces any scrambling of the alkyl groups attached to N-1 and the exocyclic nitrogen atom of 14b and c. The major product from (E)-/(Z)-14b (84%) is formed by the non-least-motion path leading diastereoselectively to (Z)-15 b, while the least-motion products (E)- and (Z)-17b (16%) exhibit a diastereo-meric ratio of 3: 1. Very similar results are obtained on photolysis of (E)-14c, except that the least-motion product (15%) arises only in a single configuration [(E)-l7c]. In the non-least-motion products (Z)-15b, c, the geometry of the Me—N=-C—N???—R group, fixed by the ring in 14b, c, is completely retained. Therefore, the (Z) diastereoselectivity is interpreted in terms of a one-bond cleavage leading to diazenyl-1,3-diazaallyl diradicals 22 having lifetimes too short for changes of their configuration which is determined by the structure of the precursor.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19901231118
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  • 2
    Electronic Resource
    Electronic Resource
    Photolysis of Tetraalkyl-1-pyrazoline-4-thiones. - Diastereoselective Formation of (E)/(Z)-Alkylidenethiiranes from cis- and trans-Tetraalkyl-1-pyrazoline-4-thiones (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 1747-1755 
    Details
    ISSN: 0009-2940
    Keywords: 4H-Pyrazole-4-thiones, 3,3,5,5-tetraalkyl-3,5-dihydro ; Thiiranes, 3,3-dialkyl-2-alkylidene ; 1,3-Pentadiene-3-thiol, 2,4-dimethyl ; Photolysis ; Extrusion of molecular nitrogen ; (E)/(Z) Stereoselectivity ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The tetraalkyl-1-pyrazolin-4-ones 5 react with hydrazine to afford the hydrazones 6 which are transformed into the thiones 7 in high yields by treatment with disulphur dichloride in the presence of triethylamine. Selective excitation of the azo chromophor of 7a with 350-nm light gives rise to the isopropylidenethiirane 8, besides molecular nitrogen, in a very clean reaction; at almost quantitative conversions, less than 2% of byproducts are observed, and about 10% of 8 have isomerized to the pentadienethiol 9. Photolysis of the stereoisomers cis- and trans-7b yields mixtures of the alkylidenethiiranes (E)-and (Z)-10 with (E)/(Z) ratios of 35:65 and 49:51. The results are interpreted in terms of diastereomeric bis-orthogonal (cis- and trans-21) and mono-orthogonal thioxyallyl diradicals (E)- and (Z)-23 which cyclize to furnish (E)- and (Z)-10. There seems to be a qualitative resemblance between the photochemical and thermal stabilities in the series of 4-substituted tetramethyl-1-pyrazolines, viz. 7a 〈 12 〈 5a, 13, similar to that suggested by Engel for 2,3-diazabicyclo[2.2.2]oct-2-ene derivatives.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911240813
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  • 3
    Electronic Resource
    Electronic Resource
    Retention of Configuration and E/Z Stereoselectivity in the Photolysis of cis- and trans-Tetraalkyl-4-methylene-1-pyrazolines (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 1619-1626 
    Details
    ISSN: 0009-2940
    Keywords: 4H-Pyrazoles, 3,3,5,5-tetraalkyl-3,5-dihydro-4-methylene- ; Cyclopropanes, 2,2,3,3-tetraalkyl-1-methylene- and 2,2-dialkyl-1-alkylidene- ; Photolysis ; Extrusion of molecular nitrogen ; Configuration, retention of ; E/Z Stereoselectivity ; Trimethylenemethanes, bis-orthogonal and mono-orthogonal ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The isomeric 4-methylene-1-pyrazolines cis- and trans-2 are synthesized by a Wittig reaction from the pyrazolin-4-ones cis- and trans-1, respectively. Direct irradiation with 350-nm light of cis-2 affords the alkylidenecyclopropanes cis-8, (E)- and (Z)-9 (64: 17: 19) besides molecular nitrogen and small amounts of the 4-methylenepyrazolidine cis-7 as a result of photoreduction. Under the same conditions, trans-2 is converted into trans-8 as well as (E)- and (Z)-9 (64: 14: 22). The configurations of cis- and trans-8 have been established by means of a 1,3-dipolar cycloaddition with mesityl nitrile oxide which furnishes two cycloadducts from cis-8, e.g. cis-14A and B, but only one cycloadduct from trans-8, viz. trans-14. The configurations of (E)- and (Z)-9 have been assigned on the basis of homoallylic 1H,1H coupling constants and nuclear Overhauser experiments. The results demonstrate that, on the least-motion paths leading to cis- and trans-8, the configuration of the ring substituents in cis- and trans-2 is completely retained. Of the non-least-motion products (E)- and (Z)-9 the latter is favoured regardless of the configuration of the precursors cis- and trans-2 though the extent of (Z) selectivity depends on that configuration. The results are interpreted in terms of bis-orthogonal trimethylenemethane diradicals as primary intermediates from which irreversible paths bifurcate. Cyclization yields cis-8 (from cis-2) or trans-8 (from trans-2) and 90°-bond rotations afford two diastereomeric mono-orthogonal trimethylenemethane diradicals [(E)- and (Z)-18] which eventually cyclize to the corresponding non-least-motion products (E)-9 and (Z)-9.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911240724
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  • 4
    Electronic Resource
    Electronic Resource
    Photoreduction on Irradiation of 3,3,5,5-Tetramethyl-4-methylene-1-pyrazoline.  -  An Overlooked Example of a Reaction Competing with the Extrusion of Molecular Nitrogen from Reluctant Azoalkanes (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 1613-1618 
    Details
    ISSN: 0009-2940
    Keywords: 4H-Pyrazoles, 3,5-dihydro-3,3,5,5-tetramethyl- ; 1-Pyrazoline, 4-methylene- ; 1H-Pyrazole, 2,3,4,5-tetrahydro-3,3,5,5-tetramethyl-4-methylene- ; Photolysis ; Photoreduction ; Extrusion of molecular nitrogen ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The photolysis of the 4-methylene-1-pyrazoline 1 is reinvestigated under a variety of experimental conditions. Direct irradiation (350-nm light) affords the alkylidenecyclopropanes 4 and 5 as predominant products, besides molecular nitrogen, and in a constant ratio of (55 ± 1): (45 ∓ 1), which is not altered in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) or piperylene nor in [D6]acetone solution. When oxygen is excluded, an additional compound is formed whose proportion depends on the capability of the solvent to act as hydrogen donor. In the presence of three equivalents of DABCO, it eventually becomes the major product. The 4-methylenepyrazolidine structure 13 of the novel photoproduct is established by NMR spectroscopy, a comparison with an authentic sample, obtained in quantitative yield by zinc reduction of 1, and reoxidation to 1 as well. An extensive investigation to detect any traces of the 4-isopropylidene-2-pyrazoline 6 in the irradiated solutions has failed. This result is at variance with recent disclosures reporting evidence for a turnaround photoisomerization of 1 via diazenylallyl diradicals. The contribution of photoreduction to the decay of the first excited singlet state of reluctant azo compounds is emphasized.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911240723
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  • 5
    Electronic Resource
    Electronic Resource
    Retention of Configuration in Two Photochemical Reactions: Formation of Cyclopropanimines by Extrusion of Molecular Nitrogen from Tetraalkyl-4-imino-1-pyrazolines and [2 + 1] Cycloreversion of Cyclopropanimines to Isocyanides and Alkenes (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 2545-2554 
    Details
    ISSN: 0009-2940
    Keywords: 4H-Pyrazoles, 3,3,5,5-tetraalkyl-4-imino-3,5-dihydro ; Cyclopropanes, 2,2,3,3-tetraalkyl-1-(alkylimino) ; Alkanamines, 2,2,3,3-tetraalkylcyclopropylidene ; Photolysis ; Extrusion of molecular nitrogen ; Photoreduction ; Configuration, retention of ; Azatrimethylenemethanes ; [2 + 1] Cycloreversion, stereospecific, photochemical ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 1-pyrazolin-4-ones 7 and 9 and the pyrazolidin-4-one 13 are condensed with alkanamines 8 to produce the imines 3,10 and 12 in high yields. Direct irradiation of 3 with 350-nm light at 90°C in deuterated hydrocarbon solvents affords the cyclo-propanimines 4 in almost quantitative yields besides molecular nitrogen and small amounts of the imines 12 as a result of photoreduction. In [D6]acetone, the cyclopropanimine 4b isomerizes in part of the α,β-unsaturated imine 14. Direct irradiation of 3 with the unfiltered light of the high-pressure mercury lamp results in quantitative [2 + 1] cycloreversion of the primary photoproducts 4 into the alkene 6 and an isocyanide 5. At low temperature (10°C), photolysis of 3 occurs much more slowly giving rise to photoextrusion of nitrogen (→4) and photoreduction (→12) to about the same extent. - Photolysis of the stereochemically labelled iminopyrazolines cis- and trans-10 (d.e. 99%) at 90°C produces the cyclopropanimines cis- and trans-16 (d.e. 94%) with high stereospecificity. The configuration of cis- and trans-16 is established by a comparison with the corresponding methylenecyclopropanes cis- and trans-19 and the quantitative and completely stereospecific [2 + 1] cycloreversion into methyl isocyanide (5a) and the 3,4- dimethyl-3-hexenes (Z)- and (E)-17 on irradiation with the unfiltered light of the mercury arc. The necessity of thermal activation for efficient nitrogen extrusion from the 1(n,π*) state of 3 and 10 is indicative of a considerable energy barrier towards the transition into a dissociative state. At low temperature, hydrogen abstraction from the solvent or other molecules becomes important for the deactivation of the 1(n,π*) state, in addition to decay and fluorescence. The stereospecific formation of cis- and trans-16 is interpreted in terms of diastereomeric bis-orthogonal azatrimethylenemethane diradicals as intermediates which retain the configuration on cyclization. The minor non-stereospecific path may involve mono-orthogonal azatrimethylenemethane diradicals. Thus, mechanisms that involve the same types of diradical intermediates can rationalize the photolysis of the iminopyrazolines 3, 10 and of the methylenetriazoline 1 as well. The [2 + 1] cycloreversion of cis- and trans-16 into the alkenes (Z)- and (E)-17 and methyl isocyanide (5a) demonstrates for the first time that such photoreactions can be entirely stereospecific.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911241125
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  • 6
    Electronic Resource
    Electronic Resource
    Ring Expansion of Heterocyclic Ketene N,X-Acetals and 2-Alkylidenedihydroindoles with Methanesulphonyl Azide by [3 + 2] Cycloaddition and Subsequent Extrusion of Molecular Nitrogen (1996)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 1541-1549 
    Details
    ISSN: 0947-3440
    Keywords: [3 + 2] Cycloaddition ; Ketene N,X-acetals, cyclic ; Azides, electrophilic ; Amidines, cyclic N-sulphonyl- ; Ring expansion of heterocycles ; 1,2-Shift of carbon, nitrogen, sulphur, or selenium ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Methanesulphonyl azide (2) reacts readily with isolated cyclic ketene N,X-acetals of type 1, viz. 10a, d, 15b, d, and 22a-c, or those that are generated in situ by deprotonation of the corresponding 2-alkylbenzazolium tetrafluoroborates, i.e. 14a → 15a, 14b → 15b, and 18 → 19. Ring-expanded products are formed by extrusion of molecular nitrogen from intermediate labile [3 + 2] cycloadducts 3 with concomitant 1,2-shift of N (route A1 → 12, 24) or X (route A2 → 16, 20, 21). In addition, 3 may undergo [3 + 2] cycloreversion into N-sulphonylimine 5 and diazoalkane 6 (route B → 13, 17, 25). The configurations of the cyclic N-sulphonylamidines 16b and 21b, the N-sulphonylimine 24 and the N-sulphonylamine 27 are elucidated by means of X-ray diffraction analyses. The ratio of the (useful) ring-expansion reactions vs. the unwanted formation of 5 + 6 is hardly influenced by the solvent employed and temperature of the experiment but strongly by the nature of the potential migrating atom and the substituents at the α-carbon atom.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.199619961008
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  • 7
    Electronic Resource
    Electronic Resource
    Zwitterions as Intermediates in 1,3-Dipolar Cycloadditions of Electrophilic Azides to 2-Alkylidenetetrahydroimidazoles and 2-AlkylidenedihydrobenzimidazolesDedicated to Professor Günter Szeimies on the occasion of his 60th birthday. (1996)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 1551-1558 
    Details
    ISSN: 0947-3440
    Keywords: Ketene N,N-acetals, cyclic ; Imidazole, derivatives of ; Benzimidazole, derivatives of ; Azides, electrophilic ; Cycloaddition, 1,3-dipolar, nonconcerted ; Zwitterions ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cyclic ketene N,N-acetals derived from imidazolidine (4a, c) and 2,3-dihydrobenzimidazole (6a-c) add methanesulphonyl azide (2a) or picryl azide (2f) to afford the zwitterions 5 and 7, respectively. The structure of 7d is elucidated by X-ray crystallography. Reversibility of formation and thermal stability of the N-sulphonyl zwitterions depend on the substitution pattern at the carbon atom to which the triazenide moiety is attached: In the case of a pair of geminal methyl groups (5a, 7a) formation is irreversible and decomposition by cyclisation and subsequent reactions occurs above -20°C, while in presence of a single alkyl group (7c, d) these processes require heating to 80°C and are accompanied by partial reversion to 2a and ketene N,N-acetals (6b, c). Cyclisation of the zwitterions yields intermediate spirocyclic [3 + 2] cycloadducts, which may undergo [3 + 2] cycloreversion into N-sulphonylimine 13 and diazo compound 14 or extrude molecular nitrogen to furnish ring-expanded 2-(sulphonylimino)piperazine derivatives (9, 11).
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.199619961009
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  • 8
    Electronic Resource
    Electronic Resource
    An Attempted Synthesis of 2,6-Bis(phenylazo)barbaralane (1997)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 1733-1738 
    Details
    ISSN: 0947-3440
    Keywords: Azo compounds ; Cyclizations ; Hydrazones ; Michael additions ; Nitrogen heterocycles ; Oxidations ; Polycycles ; Rearrangements ; Strained molecules ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Treatment of the unsaturated ketone 6 with phenylhydrazine fails to afford the unsaturated phenylhydrazone 3 but gives the tricyclic phenylhydrazoketone exo-8, which is oxidised with iron(III) chloride to yield the phenylazoketone exo-9. The configuration of exo-9 is elucidated with the help of an X-ray diffraction analysis. The unsaturated phenylazo compound 14 is obtained from Meerwein's diketone 10 in two ways. Iodination of the phenylhydrazone 11 in the presence of pyridine followed by elimination of pyridinium iodide from the bispyridinium salt 13 with aqueous sodium hydroxide in dimethyl sulphoxide furnishes 14 in low yield. A somewhat better yield is more conveniently achieved when the dibromodiketones exo,exo- and exo,endo-12 are allowed to react with phenylhydrazine in the presence of pyridine. Bromination of 14 with N-bromosuccinimide in cyclohexane as solvent affords the labile dibromide 15. Both reagents employed, viz. the zinc/copper couple in refluxing tetrahydrofuran and butyllithium at low temperature, fail to convert 15 into the hoped-for bis(phenylazo)barbaralane 1 but instead give rise to the formation of the isomer 17, either by cyclisation of the intermediate anion 16 or by rearrangement of 1.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.199719970815
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  • 9
    Electronic Resource
    Electronic Resource
    Tetra(2,6-dichlorophenyl)porphyrin  -  a superior sensitiser for the singlet-oxygen ene reaction (Schenck reaction) (1995)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 1495-1501 
    Details
    ISSN: 0947-3440
    Keywords: Singlet-oxygen ene reaction ; Allyl hydroperoxides ; α, β-unsaturated ketones from ; Tetraarylporphyrins as singlet-oxygen sensitisers, persistence of ; 1,2-Carbonyl transposition with concomitant dehydrogenation ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The very useful singlet-oxygen ene reaction of reluctant alkenes, e.g. 1, 5, 7, 8, and, in particular, the mono(allyl hydroperoxides) 10 and 11, is handicapped by the instability of the sensitiser TPP. A comparative study of TPP, TPFPP, and TDCPP in the singlet-oxygen ene reaction of these alkenes shows that 1) the persistence increases in the order TPP 〈 TPFPP 〈 TDCPP, rendering TDCPP the sensitiser of choice for the generation of singlet-oxygen in non-polar solvents, 2) the persistence of the tetraarylporphyrins decreases in the presence of an alkene, 3) this decrease strongly depends on the nature of the alkene, being most pronounced in the case of cyclohexene. These results are interpreted in terms of unknown substrate-derived species which induce oxidative destruction of the tetraarylporphyrins. Alternatively, abstraction of allylic hydrogen atoms from the alkenes by the excited sensitisers may give rise to the observed substrate-dependent photobleaching. - Because the singlet-oxygen ene reaction is the key step of a 1,2-carbonyl transposition with concomitant dehydrogenation, the scope and usefulness of this sequence are distinctly improved.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.1995199508204
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  • 10
    Electronic Resource
    Electronic Resource
    A Novel Synthesis and the Autoxidation of 2,4,6,8-TetraphenylbarbaralaneDedicated to Professor Waldemar Adam on the occasion of his 60th birthday. (1997)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 685-698 
    Details
    ISSN: 0947-3440
    Keywords: Autoxidation ; Copolymerisations ; Cyclizations ; Diradicals ; Epoxidations ; Liquid Chromatography ; Oxygen ; Peroxides ; Polycycles ; Radical Reactions ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Addition to the diphenyldiketone 11 of the reagent prepared from rigorously dried cerium(III) chloride and phenyllithium yields the tetraphenyldiol endo,endo-13 after extended periods of time. The configuration of this diol, which was previously assigned the (incorrect) configuration exo,exo-13, is established by an X-ray diffraction analysis. Dehydration of endo,endo-13 with sulphuric acid in acetic acid affords a mixture of the oxatwistane 16 and the tetraphenyldiene 17 (3:1). Only the latter is obtained from endo,endo-13 by the action of boron trifluoride-diethyl ether in dichloromethane solution. While attempts at allylic bromination of 17 with N-bromosuccinimide in conventional solvents inevitably lead to complete decomposition, use of cyclohexane as solvent allows to obtain solutions of allylic dibromides that can be cyclised with the zinc-copper couple to afford tetraphenylbarbaralane 9 in 58% yield based on 17. Thus, 9 is now available from the diphenyldiketone 11 in only three steps with an overall yield of 42%. - The autoxidation of 9, studied in various solvents, yields mixtures of products of which the epoxyendoperoxides 20 and 21, the unsaturated ketone 22, and a labile, probably oligomeric or polymeric product C are isolated in pure form. The structures 21 and 22 are elucidated by X-ray diffraction analysis and independent synthesis, respectively. On warming, 20 rearranges into 21. Traces of acid convert the unknown autoxidation product A instantaneously into the unsaturated ketone 22. - Rate studies show that the autoxidation of 9 starts after a short initiation period. 2,6-Di-tert-butyl-4-methylphenol exhibits powerful inhibitory effects. These results demonstrate the free radical nature of the autoxidation of 9. - The results are interpreted in terms of a mechanistic scheme involving initiation of the radical chain by addition of triplet oxygen to 9 to generate the diradical 28 followed by a radical chain 1:1 copolymerisation of 9 and oxygen to produce diradical 29. Eventually, endo addition of oxygen to 29 gives rise to the formation of the epoxyendoperoxides 20 and 21. Intramolecular hydrogen abstraction from the methylene group (C9) is accompanied by cleavage of a bridgehead bond (C1-C2) and the adjacent O-O bond to afford the extremely labile product A (perhaps 32 or 33) which undergoes acid-catalysed elimination to furnish ketone 22.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.199719970409
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