Photochemistry of cyclopropene derivatives. 20. Deuterium isotope effects in the triplet-induced photochemistry of tetrasubstituted cyclopropenes (original) (raw)

1981, Journal of the American Chemical Society

The photochemical behavior of a number of 3-(o-alkylphenyl)-substituted cyclopropenes which contain a benzylic hydrogen in the y-position of the side chain has been studied in mechanistic detail. The results obtained indicate that the products of the direct and triplet-sensitized photolysis are completely different. The singlet states of these systems react by u-bond cleavage of the ring to give products which are explicable in terms of the chemistry of vinyl carbenes. The triplet state, generated by sensitization techniques, undergoes hydrogen atom abstraction by a mechanism analogous to the well-known Norrish type I1 process of carbonyl compounds. Rate constants for hydrogen abstraction were obtained by plotting @,,/&J~ vs. trans-stilbene at a constant quencher to cyclopropene ratio. In contrast to the symmetrically substituted 1,2-diphenylcyclopropenes, the quantum efficiency of hydrogen abstraction of the 1,3 isomers was found to depend on the concentration of starting material. The primary deuterium isotope effect encountered with the symmetrical 1,2-diphenylcyclopropene systems is significantly larger than any previously reported value for hydrogen transfer to an excited state (kH/kD a. 20/1). A substantial tunnel effect is proposed to rationalize the results. In contrast to the results obtained with the symmetrical cyclopropenes, a much smaller effect on the quantum efficiency was observed with the unsymmetrical systems (kH/kD = 3.3/1). Of known photochemical processes, hydrogen abstraction has been surely one of the most intensively investigated reactions. Most studies have centered on the photochemistry of the carbonyl group. These include the photoreductionZ of ketones in solvents with abstractable hydrogens and the type I1 reaction of ketones possessing y-hydr~gens.~ The reactivity of the carbonyl group with respect to hydrogen abstraction depends dramatically on the configuration of the lowest lying triplet state.4 The reactivity of n,r* triplets approximates that of alkoxy radical^,^,^ whereas hydrogen abstraction by A,A* triplets is not observed or occurs a t significantly lower rate^.^-^ The higher unpaired electron density on oxygen appears responsible for the greater reactivity in the former state. In contrast to carbonyl compounds, examples of hydrogen abstraction in the direct and sensitized photolysis of olefins are less common. Nevertheless, a number of reports have appeared in the literature which show that the excited r,r* state of certain olefins have the ability to abstract hydr~gen.'"~ Thus, both (1) Photochemical Transformations of Small Ring Compounds.