Retro-Diels-Alder

The retro-Diels–Alder reaction (rDA) is the microscopic reverse of the Diels–Alder reaction—the formation of a diene and dienophile from a cyclohexene. It can be accomplished spontaneously with heat, or with acid or base mediation.

With an appropriate driving force, the Diels–Alder reaction proceeds in reverse, 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. 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.

(1)

The retro-Diels–Alder reaction proper is the microscopic reverse of the Diels–Alder reaction: a concerted (but not necessarily synchronous), pericyclic, single-step process. Evidence for the retro-Diels–Alder reaction was provided by the observation of endo-exo isomerization of Diels–Alder adducts. It was postulated that at high temperatures, isomerization of kinetic endo adducts to more thermodynamically stable exo products occurred via an rDA/DA sequence. However, such isomerization may take place via a completely intramolecular, [3,3]-sigmatropic (Cope) process. 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.

(2)

Like the Diels–Alder reaction, the rDA preserves configuration in the diene and dienophile. Much less is known about the relative rates of reversion of endo and exo adducts, and studies have pointed to no correlation between relative configuration in the cyclohexene starting material and reversion rate.

A few rDA reactions occur spontaneously at room temperature because of the high reactivity or volatility of the emitted dienophile. Most, however, require additional thermal or chemical activation. 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

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