Photochemistry of Cyclopropene Derivatives. 20. Deuterium Isotope Effects in the Triplet-Induced Photochemistry of Tetrasubstituted Cyclopropenes

Article abstract of DOI:10.1021/ja00401a025

The photochemical behavior of a number of 3-(o-alkylphenyl)-substituted cyclopropenes which contain a benzylic hydrogen in the γ-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 ?-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 mechanism analoqous to the well-known Norrish type II process of carbonyl compounds.Rate constants for hydrogen abstraction were obtained by plotting Φ_o/Φ_q 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 (k_H/k_D ca. 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 (k_H/k_D = 3.3/1).