Studies Dealing with the Intramolecular Ene Reaction of Cyclopropene Derivatives

Article abstract of DOI:10.1021/ja00292a042

The thermal and triplet-sensitized behavior of a series of 3-(o-alkenylphenyl)-substituted cyclopropenes has been studied.The results obtained indicate that the course of the thermolysis depends on the substituent groups present on the double bond.Thermolysis of the trans-substituted 3-(o-1-propenylphenyl)cyclopropene gives rise to a 2 + 2 cycloadduct.In marked contrast, heating a sample of the cis isomer results in a concerted ene reaction.The geometry for this type of reaction is easily achieved with the Z isomer.The triplet-sensitized reaction of these systems produced a 3:1 mixture of the 2 + 2 cycloadduct as well as the ene product.A related ene reaction was also observed with the 3-(o-(2-methyl-1-propenyl)-phenyl)-substituted cyclopropene.The primary deuterium isotope was determined (k_H/k_D = 3.5) and was found to be consistent with a concerted transition state where the hydrogen transfer is nonlinear.No significant isotope effect was encountered in the sensitized irradiation.The regiospecificity associated with the thermal and triplet-sensitized ene reaction of an unsymmetrically substituted cyclopropene was also studied.The regiochemistry encountered in the sensitized irradiation is attributed to ?-? bridging to give the most stable diradical intermediate.Frontier MO theory provides a nice rationalization for the regiochemistry in the thermal reaction.