Retro-Diels–Alder reaction

In practice, this reaction generally requires some special structural features in order to proceed at temperatures of synthetic relevance.

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.

Additionally, conducting the rDA in the presence of a scavenging diene or dienophile has led to the capture of many transient reactive species.

[6] It was postulated that at high temperatures, isomerization of kinetic endo adducts to more thermodynamically stable exo products occurred via an rDA/DA sequence.

Evidence for the latter was provided by the reaction below—none of the "head-to-head" isomer was obtained, suggesting a fully intramolecular isomerization process.

[8] A few rDA reactions occur spontaneously at room temperature because of the high reactivity or volatility of the emitted dienophile.

The relative tendencies of a variety of dienes and dienophiles to form via rDA are described below: Diene: furan, pyrrole > benzene > naphthalene > fulvene > cyclopentadiene > anthracene > butadiene Dienophile: N2 > CO2 > naphthalene > benzene, nitriles > methacrylate > maleimides > cyclopentadiene, imines, alkenes > alkynes Because the Diels–Alder reaction exchanges two π bonds for two σ bonds, it is intrinsically thermodynamically favored in the forward direction.

Flash vacuum pyrolysis of Diels–Alder adducts synthesized by independent means can provide extremely reactive, short-lived dienophiles (which can then be captured by a unique diene).