Studies dealing with cycloaddition and sigmatropic reactions of cyclopropenyl-substituted indenes

The thermal and photochemical reactions of a number of substituted cyclopropenylindenes have been studied. The thermolysis of these systems resulted in a series of 3,3-sigmatropic rearrangements. Direct irradiation of 1-methyl-1-(1-methyl-2,3-diphenyl-2-cyclopropen-1-yl)-3-phenylindene afforded a mixture of unsymmetrical cyclopropenes together with a cyclopropa [cd] pentalene. The formation of the products can be rationalized in terms of two competing pathways. One path involves σ-bond cleavage to produce a diradical pair which undergoes recombination to give the rearranged cyclopropene. The alternate pathway proceeds by ring opening of the singlet state of the cyclopropene to a vinylcarbene intermediate