Photochemical Studies of Cyclopropenes and Cyclopentadienes. Mechanistic and Exploratory Organic Photochemistry

Article abstract of DOI:10.1021/jo00132a017

The photochemistry of two vinylcyclopropenes and one cyclopentadiene was investigated.Thus, 3-phenyl-3-(1-phenylvinyl)cyclopropene, 3-phenyl-3-(2,2-diphenylvinyl)cyclopropene, and 2,5,5-triphenylcyclopentadiene were studied.The vinylcyclopropenes were designed with the vinyl moieties being the low-energy chromophores in contrast to previously studied examples where the cyclopropene ? bond is lower in energy.As with previous vinylcyclopropenes, irradiation led to cyclopentadienes and indenes. 2,5,5-Triphenylcyclopentadiene was the main photoproduct of the irradiation of the (diphenylvinyl)cyclopropene. 3-(2,2-Diphenylvinyl)indene was a lesser product that was encountered. 1,2-Diphenylcyclopentadiene, 3-(1-phenylvinyl)indene, and 3,4-diphenyl-1,2,4-pentatriene were formed from direct photolysis of the styryl cyclopropene.Interestingly, the corresponding sensitized irradiation led exclusively to 1,2-diphenylcyclopentadiene.Quantum efficiencies were determined for these reactions.Direct irradiation of 2,5,5-triphenylcyclopentadiene led to novel ring contraction to afford the (diphenylvinyl)cyclopropene.Additionally, phenyl migration was observed, leading to formation of 1,4,5-triphenylcyclopentadiene.Sensitized reaction of 2,5,5-triphenylcyclopentadiene led only to the phenyl migration product.Again quantum yields were determined.The (diphenylvinyl)cyclopropene was labeled in order to ascertain the skeletal change in the rearrangement.Similary, labeling studies were carried out with 2,5,5-triphenylcyclopentadiene, thus allowing delineation of the fate of each carbon.Additionally, studies were carried out independently to generate the 3,5,5-triphenylpentadienyl carbene.The (diphenylvinyl)cyclopropene was the major product along with 2,5,5-triphenylcyclopentadiene and 3-(2,2-diphenylvinyl)indene.