Studies Dealing with Cycloaddition and Sigmatropic Reactions of Cyclopropenyl-Substituted Indenes

Article abstract of DOI:10.1021/jo00015a033

The thermal and photochemical reactions of a number of substituted cyclopropenylindenes have been studied.The thermolysis of theses systems resulted in a series of 3,3-sigmatropic rearrangements.The product distribution parallels the expected relative order of thermodynamic stabilities of the indenes.The major product obtained upon extended heating corresponds to the isomer in which the double bonds are in conjugation with the phenyl groups on both the indene and cyclopropene rings.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 cyclopropapentalene.The formation of the products can be rationalized in terms of two competing pathways.One path involves ?-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.Attack of the vinylcarbene carbon on the neighboring double bond produces the cyclopropapentalene ring skeleton.The sensitized photolysis was found to give intramolecular <2+2>-cycloadducts.The observed regiospecificity of the reaction is understandable in terms of formation of the most stable diradical intermediate.