MECHANISTIC AND EXPLORATORY ORGANIC-PHOTOCHEMISTRY - CYCLOPROPENE PHOTO-CHEMICAL STUDIES .119.

Our investigation of cyclopropene photochemistry was continued with the aim of exploring the nature and generality of the observed photochemistry as well as the reaction mechanisms utilized. Two cyclopropenes were studied. 1-Methyl-2, 3-diphenyl-3-isobutenylcyclopropene and 1, 2-diphenyl-3-methyl-3-isobutenylcyclopropene were found to afford cyclopentadienes and indenes in accord with previous investigations. The former on direct irradiation afforded 2, 3-diphenyl-1, 5, 5-trimethyl-1, 3-cyclopentadiene as the major photoproduct; in addition, there were formed 1, 3-diphenyl-2, 5, 5-trimethyl-1, 3-cyclopentadiene, 1-methyl-2-phenyl-3-isobutenylindene, 1-phenyl-2-methyl-3-isobutenylindene, and 1, 2-diphenyl-3-methyl-3-isobutenylcyclopropene. In contrast, the sensitized photolysis of the 1-methylcyclopropene led to the 2, 3-diphenylcyclopentadiene and 1, 2-diphenyl-3, 5, 5-trimethyl-1, 3-cyclopentadiene. In the case of the 3-methylcyclopropene direct irradiation yielded 1, 2-diphenyl-3, 5, 5-trimethyl-1, 3-cyclopentadiene as the major product and lesser amounts of the 1-methylcyclopropene. Sensitized photolysis gave the 1, 2-diphenylcyclopentadiene as well as the 2, 3-diphenylcyclopentadiene. The preferential formation of 2, 3-diphenyl-1, 5, 5-trimethyl-1, 3-cyclopentadiene in the direct irradiation of the 1-methylcyclopropene as well as the overwhelming preference for this product in the triplet runs is discussed as evidence favoring a diradical rather than a carbene mechanism. The interconversions of the different vinylcyclopropenes are understood as an incipient di-π-methane rearrangement involving a diradical mechanism. Another approach to studying the mechanism involved independent generation of that carbene which would have to be the intermediate leading to the major cyclopropene photoproduct, if one assumes a carbene mechanism. 3, 4-Diphenyl-6-methyl-3, 5-heptadien-2-one tosylhydrazone and 3, 4-diphenyl-5-methyl-3-isobutenyl-3H-pyrazole were utilized as carbene precursors, with photochemical generation of S1 carbenes. The product distribution proved different from that obtained from the cyclopropene photochemistry. © 1979, American Chemical Society. All rights reserved.