Transition state structure for peracid epoxidation. Secondary deuterium isotope effects

Kinetic deuterium isotope effects were measured for the epoxidation of p-phenylstyrene (3) and three deuterated derivatives (α-d (4); β,β-d2 (5); α,β,beta- d3 (6)) with m-chloroperbenzoic acid in 1,2-dicloroethane at 00. The isotope effects (k/sub H//k/sub D/) observed for 4,5, and 6 are 0.99, 0.82, and 0.82, respectively; for p-nitrostyrene and its α-d analog k/sub H//k/sub D/ at 250 was 0.98. The peracid isotope effect for epoxidation of 3 with m-ClC6H4COOOH(D) at 00 was k/sub H//k/ sub D/ = 1.17. These data suggest that in the transition state for epoxidation there is substantial C/sub beta/--O bond formation, but negligible change in the bonding or hybridization at C/sub alpha/ of the olefin. Epoxidation mechanisms suggested previously cannot accommodate these findings, thus an alternate transition state structure has been proposed. It is characterized by (1) sp2 geometry and partial positive charge at C/sub alpha/ of the olefin, (2) extensive C/sub beta/--O bond formation but with retention of some π bonding between C/sub alpha/ and C/sub beta/, (3) a nearly equal amount of partial negative charge on the peracid carboxylate, and (4) little change in the bonding of the peracid proton