On the Question of Stepwise [4+2] Cycloaddition Reactions and Their Stereochemical Aspects

Even at the end of the twentieth century, the view of the one-step [4+2] cycloaddition (Diels-Alder) reaction mechanism was widely accepted as the only possible one, regardless of the nature of the reaction components. Much has changed in the way these reactions are perceived since then. In particular, multi-step mechanisms with zwitterionic or diradical intermediates have been proposed for a number of processes. This review provided a critical analysis of such cases.

Automated summary

The view of the one-step [4+2] cycloaddition (Diels-Alder) reaction mechanism as the only possible one was widely accepted at the end of the twentieth century, but much has changed since then. Multi-step mechanisms with zwitterionic or diradical intermediates have been proposed for a number of processes, as critically analyzed in this review.