12 Reaction mechanisms Part (iii) Pericyclic reactions

Abstract

Reports describing clever experimental studies and advanced computational studies continue to reveal the mechanistic complexity of seemingly simple pericyclic reactions. Some important examples are highlighted here. Experimental and computational evidence for the first two-step, no-intermediate reaction mechanism for the ene reaction of singlet oxygen with a simple alkene is presented by Singleton, Foote, Houk and co-workers. A new type of kinetic isotope effect due to dynamics is described by Singleton et al., discovered in their studies of the ene reaction involving singlet oxygen. Baldwin and Burrell report a study on the complicated thermal behavior of optically active trans and cis-1-(E)-propenyl-2-methylcyclobutanes.

The following papers are relevant to several categories of pericyclic reactions. Otto and Engberts reviewed the importance of hydrophobic interactions on chemical reactivity, and include a discussion of Diels–Alder reactions, 1,3-dipolar cycloadditions, and Claisen rearrangements. Also, Houk and co-workers published a benchmarking study comparing experimental and computational data for eleven hydrocarbon pericyclic reactions. The authors' goals are: to establish reliable experimental data for comparative purposes; to determine what level of theory is necessary to calculate activation enthalpies with high accuracy; and to determine which density functional methods give reasonable results for pericyclic reactions. The most reliable activation enthalpies and heats of reaction are obtained using CASPT2 and CBS-QB3 methods. Some density functional methods also give very good activation enthalpies. All computational methods studied gave reasonable transition state geometries, with the exceptions of those for Cope rearrangement and cyclopentadiene dimerization.