CLEAVAGE VS REARRANGEMENT RATIOS AND SECONDARY DEUTERIUM KINETIC ISOTOPE EFFECTS IN THE THERMOLYSIS OF CONFORMATIONALLY FIXED CISOID VINYLCYCLOBUTANES

The first-order rate constants for the thermally induced retro 2 + 2 cleavage and rearrangement of 5-methylenespiro[3.5]nonane occur in a 1:2 ratio. In contrast, a 2:1 ratio of these rate constants is usual from conformationally unbiased vinylcyclobutanes. Comparison of the activation parameters suggests that the rearrangement in unbiased systems results from an unfavorable entropy of activation which suggests that the rearrangement is concerted. For the rearrangement of 5-methylenespiro[3.4]octane, the secondary deuterium kinetic isotope effect at the exo-methylene carbon (k(H)/k(D)2) is 1.086 +/- 0.023. This is also consistent with concert in the rearrangement where exo-methylene rotation contributes to the reaction coordinate. The secondary KIE for cleavage of 5-methylenespiro[3.4]octane is 1.025 +/- 0.027.