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

Radomir Jasiński

doi:10.3390/sym13101911

Abstract

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.

Highlights

The history of the [4+2] cycloaddition reaction began in 1928, when Otto Diels and his student, Kurt Alder, published the work in which they described the cycloaddition of cyclopentadiene (1) to quinone (2) (Scheme 1) [1]
The vast majority of the [4+2] cycloaddition reactions described in the literature with the participation of nitroalkenes as diene heteroanalogues take place under catalytic conditions [70,71,72]
[4+2] and [2+2] cycloadducts (75 and 76 respectively) (Scheme 20). The presence of the latter cycloadduct leads to the conclusion that a zwitterionic intermediate must be formed in the course of addends interactions, because [2+2] cycloadditions do not occur as one-step processes

Summary

Introduction

The history of the [4+2] cycloaddition reaction began in 1928, when Otto Diels and his student, Kurt Alder, published the work in which they described the cycloaddition of cyclopentadiene (1) to quinone (2) (Scheme 1) [1]. The two-step mechanism may suggest the loss of stereoconfiguration due to addends (Scheme 3), and the presence of acyclic products with a conformation suggesting that they were transformed from the originally formed zwitterions and/or diradicals. The presence of this first group of intermediates in the reaction mixture can be proved independently by spectral analysis using NMR or UV/VIS techniques [25]. Some [4+2] cycloaddition reaction cases can be found, in which the presence of zwitterionic/diradical intermediate can be proposed based on research using the above-mentioned methods These studies are very diverse in nature and practical value

Diels-Alder Reactions

Hetero Diels-Alder Reactions Involving Heteroanalogs of Dienes

Hetero Diels-Alder Reactions Involving Heteroanalogs of Dienophiles

Conclusions