Cycloaddition Reactions of Strained Ring Systems. Photochemistry of 1-Phenyl-2-carbomethoxy-3,3-dimethylcyclopropene

Article abstract of DOI:10.1021/jo00197a004

The sensitized irradiation of 1-phenyl-2-carbomethoxy-3,3-dimethylcyclopropene produced two novel photodimers.The minor product can be explained in terms of an initial bond formation to give a diradical intermediate.Collapse of the diradical furnishes a tricyclohexane which undergoes a subsequent cycloreversion to give a 1,4-cyclohexadiene derivative.The major photodimer is derived by cyclopropyl ring opening of the initially produced diradical followed by cyclization to give a bicyclo<1.1.0>butane derivative.Direct irradiation of 1-phenyl-2-carbomethoxy-3,3-dimethylcyclopropene afforded a mixture of 1-carbomethoxy-3,3-dimethyl-1-phenylallene, 1-carbomethoxy-3-methyl-2-phenylbutadiene, and 2-carbomethoxy-3-methyl-1-phenylbutadiene.The formation of the three products can be rationalized in terms of a vinylcarbene intermediate which inserts into the adjacent methyl group.The product distribution favors cleavage of the carbomethoxy substituted ?-bond of the cyclopropene ring.This regioselectivity can be attributed to a funneling of the excited singlet state of the cyclopropene to the energy surface of the higher lying carbene state.The photochemical and thermal behavior of several hydroxyalkyl substituted cyclopropenes derived from 1-phenyl-2-carbomethoxy-3,3-dimethylcyclopropene was also studied and the results obtained were compared to the reactions in the carbomethoxy series.