Transition-State Structure Variation in the Diels-Alder Reaction from Secondary Deuterium Kinetic Isotope Effects: The Reaction of Nearly Symmetrical Dienes and Dienophiles Is Nearly Synchronous

Article abstract of DOI:10.1021/ja00207a013

Secondary deuterium kinetic isotope effects (KIEs) in the reaction of acrylonitrile, fumaronitrile, vinylidene cyanide, dimethyl fumarate and maleate, and methyl trans-β-cyanoacrylate with isoprene were determined by competition with isoprene-d₀ (d₀), 4,4-dideuterioisoprene (4,4-d₂), and 1,1,4,4-tetradeuterioisoprene (d₄).The d₄ experiment gives the KIE for 1,1-dideuterioisoprene from the KIEs for 4,4-d₂.The KIEs for bond making to the β site of acrylonitrile and vinylidene cyanide are much more inverse than those at the α bond making site but are less than the maximum value expected, which indicates an early, unsymmetrical, concerted transition state.The effects wit fumaronitrile are the same for both 1,1- and 4,4-dideuterioisoprene, indicating equivalent effects at both sites and not bonding to the preferred C-1 site of isoprene most of the time.The KIEs with methyl trans-β-cyanoacrylate are inverse at both bond-making sites to the dienophile and both nearly one-third of the maximum value expected.The results with nearly symmetrical addends are not consistent with a two-step reaction, particularly one with the second step being rate determining, or with a concerted reaction with a highly unsymmetrical transition state, but rather with a concerted pathway that is nearly synchronous.The KIEs with methyl fumarate and maleate suggest an unsymmetrical transition state, but they are within experimental error of a symmetrical transition state.