Conformational Isomerization as a Key Selectivity-Determining Step

Abstract

AbstractOrganic reactions take place in one or more elementary processes, and their product selectivity is determined by which elementary processes are involved in a reaction and how they occur. Among the elementary processes, bond-forming and cleaving processes are typically the only steps drawn in the proposed organic reaction mechanisms1and are generally considered to be more important than other processes such as conformational isomerization, which have rarely been recognized as key steps such as selectivity-determining steps. We report herein on an example where a conformational isomerization process, propeller-like alkene rotation,2,3is considered to determine the selectivity over the reaction pathways in a catalytic reaction. The transition state with the highest energy barrier in some palladium chain walking events was indicated by DFT calculations to correspond to alkene rotation in an alkene hydride palladium species, rather than bond-cleaving b-hydride elimination or bond-forming migratory insertion, even when there is only one transition state between an alkyl complex and an alkene hydride complex. It was also suggested both theoretically and experimentally that the chain walking over internal carbons in alkyl chains proceeds via cis alkene intermediates, rather than thermodynamically more stable trans alkene intermediates, due to their relative ease of undergoing alkene rotation.