Photorearrangement studies of 3-heteroaryl-substituted cyclopropenes

Abstract

The photochemical rearrangement of several 3-heteroaryl-substituted cyclopropenes containing a methyl group on the heterocyclic ring has been studied. The rearrangements are derived from the π-π* singlet state of the cyclopropene. Ring-opening occurs to give a vinylcarbene intermediate which undergoes a subsequent electrocyclization. The resulting product was found to undergo a sigmatropic 1,5-methyl shift on being heated in mesitylene at 200 °C to give a cyclopenta[b]furan or thiophene. Yet another reaction resulting from the vinylcarbene is insertion into the adjacent methyl group to give a buta-1,3-diene derivative. The product distribution is dependent on the stereochemistry of the vinylcarbene. Irradiation of 1,2-diphenyl-3-heteroaryl-substituted cyclopropenes also give products corresponding to a 1,2-heteroaryl substituent shift. A mechanism involving π-π* bridging of the excited cyclopropene with the heteroaromatic ring followed by diradical cleavage is proposed to account for the formation of the rearranged cyclopropenes.