Thermal rearrangement of O-methyleneamino thiocarbamates

Hudson, R. F.; Lawson, Alexander J.; Record, K. A. F.

doi:10.1039/p29740000869

Cited by 13 publications

(9 citation statements)

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“…Additionally, the stabilization of the radical cation site on the exocyclic C S bond should be favoured in relation to that on the C O bond which is unable to compete with the S atom in the ring in the case of thiones. The above statements support the observation that no reverse rearrangement Scheme 1 takes place in the fragmentation of 3,4-disubstituted-1,2,4thiadiazole-5(4H)-ones (11)(12)(13)(14)(15)(16)(17). The results of the thermal rearrangement of many 3,4-disubstituted-1,2,4-oxadiazole-5(4H)-thiones 2 also support the rearrangements of the ionized 1,2,4-oxadiazole-5-thiones.…”

Section: Methodssupporting

confidence: 81%

“…weaker than the other bonds of the ring; (iii) the 1,2-NO bond is cleaved in two other abundant retrocycloadditions (routes I and III in Scheme 1); (iv) in similar acyclic systems, thermal rearrangement has been reported to occur by a homolytic N-O bond fission. 16 If the molecular ions of compounds 1-10 rearrange similarly, then only the bond energies 17 of the N-O D °298 D 631 kJ mol 1 and N-S bonds D °298 D 464 kJ mol 1 are important, i.e. the driving force of the rearrangement is purely thermodynamic.…”

Section: Methodsmentioning

confidence: 99%

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Electron ionization mass spectra of 3,4‐disubstituted‐1,2,4‐oxa(thia)diazole‐5(4H)‐thione(ones). Substituent effects on the mass spectrometric rearrangement of 3‐aryl‐4‐(p‐tolyl)‐1,2,4‐oxadiazole‐5(4H)‐thiones to the corresponding oxo compounds

2001

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The fragmentation behaviour of ten 3,4-disubstituted 1,2,4-oxadiazole-5(4H)-thiones and seven 3,4-disubstituted 1,2,4-thiadiazole-5(4H)-ones studied here confirmed the earlier observations about the partial rearrangement of the former after ionization into the latter before further fragmentation. In the case of eight 3-(substituted phenyl)-4-(p-tolyl)-1,2,4-oxadiazole-5(4H)-thiones the fragmentations reflecting the above-mentioned molecular ion rearrangement show a clear correlation on the substituent sigma values. The electron-withdrawing substituents destabilize the molecular ion, so higher amounts of the rearranged ion [R(1)NCO](+.) are obtained. A good correlation of log[R(1)NCO](+) against sigma was obtained (r = 0.96). Only a satisfactory correlation prevailed for log([R(1)NCO](+)*/[R(1)NCS](+)*) against sigma(r = 0.87).

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Section: Methodssupporting

confidence: 81%

Section: Methodsmentioning

confidence: 99%

Electron ionization mass spectra of 3,4‐disubstituted‐1,2,4‐oxa(thia)diazole‐5(4H)‐thione(ones). Substituent effects on the mass spectrometric rearrangement of 3‐aryl‐4‐(p‐tolyl)‐1,2,4‐oxadiazole‐5(4H)‐thiones to the corresponding oxo compounds

2001

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“…This approach relies on earlier work by Hudson and collaborators, who found that thiocarbonyl derivatives of oximes were thermally and photochemically unstable, and rearranged into the sulfenamide isomers. 26 This decomposition was assumed to involve caged radicals and this was supported by ESR studies, but no attempt was made to incorporate an intramolecular trap to capture the presumed reactive species.…”

Section: Cleavage Of N-o Bondsmentioning

confidence: 99%

“…The second approach also derives from prior observations by Hudson. 26 In a set of ingenious and mechanistically insightful experiments, Weinreb and his co-workers demonstrated that the nitrogen radicals arising from the thermal decomposition of sulfinates and trivalent phosphorus derivatives of oximes and hydroxamic acids can be intercepted by an alkene, and the resulting carbon radical in turn quenched by various radical traps. 27 The richness of this process is showcased by the series of transformations pictured in Scheme 16.…”

Section: Cleavage Of N-o Bondsmentioning

confidence: 99%

Recent progress in the generation and use of nitrogen-centred radicals

2008

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Nitrogen-centred radicals hold much promise as useful synthetic intermediates. Even though their popularity is still extremely limited and very far from matching that of carbon radicals, the recent development of various routes allowing their generation under mild conditions and a better appreciation of their reactivity thanks to the increased availability of absolute rate constants should encourage their use. It is hoped that this tutorial review will help increase the awareness of synthetic chemists and help revive the interest in these forgotten species.

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“…6 We were interested to see if iminyl radicals formed in the homolytic fragmentation pathway could be captured by an internal olefin.…”

mentioning

confidence: 99%

Generation and Capture of Iminyl Radicals from Ketoxime Xanthates

Gagosz

Zard

1999

Synlett

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Irradiation of ketoxime O-(S-methyl xanthates) containing a g,d-double bound leads to a dihydropyrrole through cyclisation of an intermediate iminyl radical in a radical chain reaction. The last propagation step involves transfer of a dithiocarbonate group and various external radical traps can be incorporated into the medium, allowing access to a variety of substituted dihydropyrroles.Iminyl radicals have been recognised during the past few years as useful intermediates for the synthesis of nitrogen containing heterocycles. 1,2 Thus, intramolecular cyclisation of an iminyl radical onto an olefinic moiety allows the construction of functionalised dihydropyrroles which can be further elaborated into more complicated structures such as alkaloids. 3 However, many of the reported procedures are based on the use of tributyltin hydride leading to well known difficulties with purification and toxicity. Recently, the groups of Narasaka and Weinreb have developed two new procedures for the mild generation and cyclisation of iminyl radicals respectively from O-2,4-dinitrophenyl oximes and O-2,6-dimethylbenzenesulfinyl oximes. 4,5 As part of our own work in this area, we report a new method using ketoxime O-(S-methyl xanthates) as a source of iminyl radicals.In 1974, Hudson proposed that the rearrangement of oxime thionocarbamates to oxime thiocarbamates could occur via two different pathways : one being a formal [1,3] sigmatropic process, the other occurring via a pair of caged radicals. 6 We were interested to see if iminyl radicals formed in the homolytic fragmentation pathway could be captured by an internal olefin.For this study, O-(S-methyl xanthates) of g,d-unsaturated ketoximes were chosen as substrates. These are easily obtained by successive treatment of the ketoximes with NaH, CS 2 and MeI. 7 Despite their relative instability, they can be purified by flash chromatography and manipulated with reasonable ease. In initial experiments, heating compound 2a in 1,2-dichloroethane at 90°C produced a small amount of the desired cyclised product 3a (14%) and the oxime thiocarbonate rearrangement product 4a which was isolated in 84% yield. Modifications of the experimental conditions led to an increase in the formation of the cyclised product; the highest yield (54%) was obtained when the reaction mixture was rapidly plunged into an oil bath preheated at 100°C (Scheme 1).The reason for this behaviour is still unclear but it may be that sudden heating produces enough radicals to start the chain (to give 3a, see Scheme 2) before an extensive onset of the rearrangement leading to 4a. Incorporation of an initiator (AIBN or lauroyl peroxide) has not produced clear cut results. Scheme 1Nevertheless, from these observations it seems that the rearrangement process leading to 4a, which in fact can occur by three different pathways (Scheme 2), is favored under thermal conditions. Furthermore, considering the final structure of the cyclised product, which contains the dithiocarbonate function, it is reasonable that the cyclisation mo...

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Thermal rearrangement of O-methyleneamino thiocarbamates

Cited by 13 publications

References 0 publications

Electron ionization mass spectra of 3,4‐disubstituted‐1,2,4‐oxa(thia)diazole‐5(4H)‐thione(ones). Substituent effects on the mass spectrometric rearrangement of 3‐aryl‐4‐(p‐tolyl)‐1,2,4‐oxadiazole‐5(4H)‐thiones to the corresponding oxo compounds

Electron ionization mass spectra of 3,4‐disubstituted‐1,2,4‐oxa(thia)diazole‐5(4H)‐thione(ones). Substituent effects on the mass spectrometric rearrangement of 3‐aryl‐4‐(p‐tolyl)‐1,2,4‐oxadiazole‐5(4H)‐thiones to the corresponding oxo compounds

Recent progress in the generation and use of nitrogen-centred radicals

Generation and Capture of Iminyl Radicals from Ketoxime Xanthates

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