Photochemistry of cyclopropene derivatives. Ring-opening reaction of 3-heteroaryl substituted cyclopropenes

Abstract

The photochemical rearrangement of several 3-heteroaryl substituted cyclopropenes 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 transient intermediate so produced can undergo either a 1,3- or a 1,5-sigmatropic hydrogen shift to give the observed products. One strong piece of evidence for carbene intervention is the observation of a 1,3-butadiene derivative as one of the photoproducts obtained from the photolysis of a 3-pyrrolyl substituted cyclopropene. The 1,3-diene is thought to be derived by insertion of the vinylcarbene into the neighbouring methyl group. The regioselectivity of the rearrangement can be accounted for in terms of competitive 1,3-and 1,5-sigmatropic hydrogen migrations.