Effects of substituents on the type II photoreaction of tetrasubstituted cyclopropenes

The triplet sensitized behavior of a number of 3-alkyl subststituted cyclopropenes containing a heteroatom on the side chain has been studied in mechanistitic detail. The triplet state was found to undergo H-atom abstraction by a mechanism analogous to the well-known Norrish type II process of carbonyl compounds. Rate constants for H-abstraction were obtained by plotting $\text{Φ}{}_0\text{Φ}{}_{\mathrm{q}}$trans-stilbene at a constant quencher to cyclopropene ratio. The rate constants for H-abstraction were found to be two orders of magnitude less than that at for related phenyl alkyl ketones and increased as the strength of the C-H bond in the γ-position decreased. The possibility of electron transfer from the heteroatom of the side chain to the π-π^* triplet state is discussed.