Pericyclic Reactions

Abstract

Chapter 3 is dedicated to pericyclic reactions and selected rearrangement reactions, and to reactions involving transient intermediates, such as radicals, diradicals or carbocations. The class of electrocyclic ring-opening reactions is first used to illustrate the application of FMO theory arguments and the classification of reactions as orbital symmetry allowed or forbidden. After a short section on biradical intermediates in cyclization reactions of enediynes and eneyne-allenes, we next analyze cycloaddition reactions with the goal of classifying reactions as concerted or stepwise, as synchronous or asynchronous, and as antarafacial or suprafacial. Aside from (4+2) Diels–Alder reactions, this also includes a larger selection of (3+2) 1,3-dipolar cycloaddition reactions with dipoles of either the allyl or the propargyl type, and a short note on cheletropic reactions. That the chemistry of carbenes extends far beyond the limits of this latter reaction type is illustrated with reactions catalyzed by N-heterocyclic carbenes. The next large section is then dedicated to sigmatropic rearrangement reactions, where we use FMO theory to understand the bond-making/bond-breaking events in rearrangements of the [3,3]- and the [1,2]-variants. This also includes a comparison with closely related reactions, such as the Hock rearrangement and the Baeyer–Villiger oxidation. The stereochemical requirements of [1,5]- and [1,7]-hydrogen transfer reactions are examined next, followed by those for Wittig rearrangements of the [2,3]- and [1,2]-types. The chapter concludes with a short note on ene reactions.