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

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.

Mechanistic aspects of the synthesis of seven‐membered internal nitronates via stepwise [4 + 3] cycloaddition involving conjugated nitroalkenes: Molecular Electron Density Theory computational study

“…Moreover, it exhibits zwitterionic nature (GEDT = 1.3 e). Zwitterionic intermediates of similar type have been recently detected within the reaction paths of some cycloaddition processes of different type 52,53 . The cyclisation of Ia proceeds via the second transition state TS2a .…”

The Participation of 3,3,3-Trichloro-1-nitroprop-1-ene in the [3 + 2] Cycloaddition Reaction with Selected Nitrile N-Oxides in the Light of the Experimental and MEDT Quantum Chemical Study

“…The Berny method was used in optimizations [69,70]. A similar theory level has been commonly used for the mechanistic research aspects of cycloaddition reactions [71][72][73][74][75] The TSs were characterized through frequency analysis, presenting only one imaginary frequency. The intrinsic reaction coordinate (IRC) paths [76] were computed to find the unique connection between the TSs and the minimum stationary points using the second order González-Schlegel integration method [77,78].…”

Asymmetric [4 + 2] cycloaddition synthesis of 4H-chromene derivatives facilitated by group-assisted-purification (GAP) chemistry

“…Based on our investigation and previous reports, 12,34,36,54 especially the recent review regarding stepwise [4 + 2] cycloaddition reactions, 55 a plausible mechanism for the reaction is represented in Scheme 5 . The sequence is initiated by deprotonation of salicyl N -phosphonyl imine 1 by cesium carbonate as a non-nucleophilic base to form intermediate A.…”