Cyclopropenes photorearrangements

The elusive diazoalkenes 6 and 14 are unlikely to react with methanol as their basicity should be comparable to that of diphenyldiazomethane. However, since the formation of diazonium ions cannot be rigorously excluded, the protonation of vinylcarbenes was to be confirmed with non-nitrogenous precursors. Vinyl-carbenes are presumedly involved in photorearrangements of cyclopropenes.21 In an attempt to trap the intermediate(s), 30 was irradiated in methanol. The ethers 32 and 35 (60 40) were obtained,22 pointing to the intervention of the al-lylic cation 34 (Scheme 10). Protonation of the vinylcarbene 31 is a likely route to 34. However, 34 could also arise from protonation of photoexcited 30, by way of the cyclopropyl cation 33. The photosolvolysis of alkenes is a well-known reaction which proceeds according to Markovnikov s rule and is, occasionally, associated with skeletal reorganizations.23 Therefore, cyclopropenes are not the substrates of choice for demonstrating the protonation of vinylcarbenes. 

A carbonyl ylide is undoubtedly implicated in the singlet-derived photorearrangement of the epoxydiene (88) to the cyclopropene (89)75 a proposed pathway is outlined in Scheme 4. Carbon-oxygen bond homolysis is preferred, however, on triplet-sensitized irradiation, leading to the formation... 

One of the major areas of study in the photochemistry of heterocycles is the photorearrangement of five-membered heteroaromatic systems. Various mechanisms have been proposed to account for these transformations.123 A ring contraction-ring expansion pathway, via an intermediate cyclopropene, appears to be involved in the rearrangement of derivatives of furan. The isolation of an intermediate (140) of this type together with an allene (141) has been effected on irradiation of 2,4-di-tert-butylfuran (142), as shown in Scheme 9.124 Similar transformations have been reported in perfluorotri-... 

The acetophenone-sensitized photorearrangement of the benzonorbor-nadiene (49) yielded the tricycloheptene (50a) as the exclusive product." Analogous products (50b—d) were also formed by sensitized irradiation of the cyclopropene derivatives (51a—c), respectively. 

Singlet excited state reactions of cyclopropenes proceed with low quantum efficiencies due to thermal return of carbene 193 to starting material as elegantly illustrated with optically active esters 234. On photolysis racemization of 234 occurs four times faster than ring cleavage. The photorearranged furan product results from six-electron cyclization of the intermediate carbene and such species have been invoked to account for the interconversion of variously substituted furans By comparison... 

The importance of cyclopropene as an intermediate in the thermal isomerization of allene to propyne is matched in the photorearrangements of 1,1-diarylallenes, e.g. 260, which give cyclopropenes, e.g. 261, alkynes, e.g. 262, and indenes, e.g. 263, as the primary photoproducts. The proposed mechanism (equation 81) requires two consecutive H shifts to give 262, whilst bond rotation after one such shift provides a less congested diradical which has cyclization paths available to it to give 261 and 263. 

The major product in the photorearrangement of 3-substituted 2-methyl-1,3-diphenyl-cyclopropenes was an indene produced by cleavage of the ring bond which is methyl- rather than phenyl-substituted, apparently by funneling of the excited state of the cyclopropene to the energy surface of the higher lying carbene triplet. ... 

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