Retrograde Diels–Alder Reactions

Abstract

The retro Diels-Alder (Rda) reaction is a π 2 s + σ 2 s + σ 2 s electrocyclic process that, as the name implies, is the reverse of the familiar Diels–Alder cycloaddition reaction. A kinetic study of the prototypical Rda reaction, conversion of cyclohexene to 1,3-butadiene and ethylene (equation 1), has been studied in the temperature range of 541–629 °C. 1 Some early observations of Rda reactions were made in 1906 by Albrecht, who reported that the DA adducts of cyclopentadiene with benzoquinone dissociate at their melting points, 2 and in 1929 by Diels and Alder, who reported the dissociation of a furan-maleic anhydride adduct at its melting point. 3 Alder used the reaction as a test to differentiate between acetylenedicarboxylic ester adducts of cyclopentadiene and of cyclohexadiene; 4 heating the latter adduct eliminates ethylene, while heating the former yields only the starting diene and dienophile (the Alder-Rickert rule). Perhaps the earliest use of the Rda reaction in a synthetic mode was in the preparation of acetylenedicarbonyl chloride, 5 in which an anthracene adduct was used as a removable acetylene protecting group. Use of the Rda reaction in achieving selective synthetic conversions has been reviewed several times to date.