Retro-Diels–Alder reaction

The retro-Diels–Alder reaction (rDA reaction) is the reverse of the Diels–Alder (DA) reaction, a [4+2] cycloelimination. It involves the formation of a diene and dienophile from a cyclohexene. It can be accomplished spontaneously with heat, or with acid or base mediation.^[1][2]

In principle, it becomes thermodynamically favorable for the Diels–Alder reactions to proceed in the reverse direction if the temperature is high enough. In practice, this reaction generally requires some special structural features in order to proceed at temperatures of synthetic relevance. For instance, the cleavage of cyclohexene to give butadiene and ethene has been observed, but only at temperatures exceeding 800 K.^[3] With an appropriate driving force, however, the Diels–Alder reaction proceeds in reverse under relatively mild conditions, providing diene and dienophile from starting cyclohexene derivatives. As early as 1929, this process was known and applied to the detection of cyclohexadienes, which released ethylene and aromatic compounds after reacting with acetylenes through a Diels–Alder/retro-Diels–Alder sequence.^[4] Since then, a variety of substrates have been subject to the rDA, yielding many different dienes and dienophiles. Additionally, conducting the rDA in the presence of a scavenging diene or dienophile has led to the capture of many transient reactive species.^[5]