Indenes, from cyclopropenes

Although cyclopropene formation has been observed to be reversible,150 and its conversion to indene has been achieved thermally,43 the two products are not interconvertible under the photochemical reaction conditions, showing that they are formed by different pathways.45,153 An elegant proof of this is the isolation of different indenes from 116 and 118 (Scheme 59).71... 

Photolysis of pyrazoles such as (64) has also been shown to produce indenes as well as cyclopropenes. The indene is apparently derived from a triplet intermediate, while the cyclopropene is singlet derived 54). In a related example, direct photolysis of dimethyl diazomalonate in an alkyne leads to moderate yields of cyclopropene, whereas sensitized photolysis leads to furans by ring closure of an intermediate diradical (65) 55). 

The first naphtho[a]cyclopropene 68d was also obtained by that route. 3H-indazole was proposed as an intermediate in this reaction 78>. The reaction proceeds equally well from the triplet state of 7. However with acetone light capture from the sensitizer may not have been complete. A trapping of the diradical intermediate 67 to give the known indene 69 by photolysing 7 in dimethyl-acetylene-dicarboxylate (ADC) afforded only a trace of 69. However the yield of 68a was reduced to 40%. Either trapping of 67 was not effective enough or ADC may have acted as quencher. Trapping with furan or cyclopentadiene was also not effective 78a). 

Finally there are a number of papers dealing with photoreactions of diaryl-cyclopropenes and arylindenes, which formally involve migration of a carbon group from the side-chain to the ortho-position of an aromatic ring. An example of the basic reaction is the formation of indenes when 1,3-diphenylcyclopropenes (199) are irradiated directly. Now it is shown that isomeric indenes are formed when the reaction is carried out in the presence of 9,10-dicyanoanthracene, and an electron-transfer mechanism is invoked to account for the change of product. In a separate paper concerned largely with intramolecular photocycloaddition reactions, cyclopropenes (199 R = allyl) are reported to give both types of indene on direct irradiation, via the normally postulated vinylcarbene intermediate. The... 

Acetylenes generally react with carbenes to form cyclopropenes. Whether this is a reaction via a singlet carbene or a two-step process via a biradical cannot be determined from the product, as is the case in the trapping with olefins. Isolation of indenes (116) from the reaction of acetylenes with di-phenylcarbene generated photochemically from the diazo-compound, however, was considered to be evidence for the intermediacy of a triplet carbene. 

Carbenes are considered to be intermediates in the formation of the two indenes (26) and (27) from photolysis of the cyclopropene (28) in benzene. The 4 1 ratio of (26) (27) indicates preferential cleavage of bond (a) over... 

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