Acylcyclopropenes

H-Pyr azoles readily undergo photochemically induced elimination of nitrogen to yield the corresponding cyclopropenes, often by way of detectable vinyl diazo intermediates. l-Aryl-3-methyl-2-phenyl-l-diazobut-2-enes have, in fact, been prepared in this way from the appropriate pyrazoles.345 The vinyl diazo compounds 416, obtained by irradiation of the 3//-pyrazoles 417, were further converted to cyclopropenes 418 via vinylmethylene intermediates 419 by irradiation at 10°C.346 1-Acylcyclopropenes have been... 

While the photolysis of (164) can lead to reasonable yields of acylcyclopropenes, no cyclopropenes are detected from the corresponding reactions of (165) however, if the photolysis of (165, R = Me, R = H) is carried out in the presence of furan, a 2-oxa-bicyclohexane is isolated. This is apparently derived by addition of the carbene (166) to the 2,3-bond. The oxabicycle in turn rearranges to a single triene (167) 128). 

Valence isomer formation is a feature also ot the photochemistry of naphthalenes (3.33) and anthracenes for naphthalenes, as for benzenes, the extent of steric crowding helps to determine which tvpe of valence isomer predominates, since there is more severe interaction in the hicyclnhexadiene products than in the benzvalene products. Amongst five-membered heteroaromatic compounds there are many known ring photoisomerizations that involve conversion of a 2-substituted to a 3-substitutud system (e.g. 3.34). In some cases non-aromatic products can be isolated, such as hicyt lo[2 1. OJpentene analogues from thiophenes (3,35), or acylcyclopropenes from furans (3. Jb) related species may be... 

Facile atmospheric oxidation of 3-acylcyclopropenes, e.g. 240, results in the corresponding a,j5-unsaturated carbonyl compound. Pathways involving oxabicyclobut-anes, oxetanes or cyclopropyl radicals account for the observed product but no distinction between them is presently possible. 

The formation of furans from 3-acylcyclopropenes is catalyzed by copper(I) but not copper(II) salts, and can also be brought about photochemically. With an optically active cycloprop-2-enecarboxylate, racemization in a photochemical reaction occurred 2.5 times faster than furan formation, supporting the intermediacy of a singlet vinylcarbene in equilibrium with the cyclo-propene. ... 

Reduction of 3-acylcyclopropenes using tributyltin hydride and 2,2 -azobisisobutyronitrile gave furans 8 together with ring-opened products 9 and... 

Addition of amines to 3-acylcyclopropenes gave pyrroles 1 and 2 via the formation and ring expansion of a 3- iminomethyl)cyclopropene. In another case, the imine 3 rearranged in the presence of a catalyst to give 4. ... 

Wilsey S, Bearpark M, Bemardi F, Ohvucci M, Robb M. The role of degenerate biradicals in the photorearrangement of acylcyclopropenes to furans. J Am Chem Soc. 1996 118 4469 479. 

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