Mechanistic Aspects of Diels‐Alder Reactions: A Critical Survey

Abstract The question of concerted or consecutive bond formation arises in all types of cycloaddition reactions. Stereochemical investigations and studies of regiospecificity as well as intensive kinetic investigations with regard to substituent and solvent effects, and the dependence of the reaction on temperature and pressure permit in many cases mechanistic conclusions concerning the Diels‐Alder reaction. Efforts towards a theoretical interpretation, ab initio and semiempirical calculations, the application of frontier molecular orbital theory (FMO) as well as thermochemical measurements permit a description of the energy hypersurface of these 4+2‐cycloadditions. An attempt is made here to draw a line of distinction between the mechanistic alternatives—one‐step reaction versus two‐step reaction—considering all experimental and theoretical criteria.