Retro-ene reaction

What is retro-ene reaction?

Perkins and Cruz likely first reported the retro-ene reaction in 1927 while preparing undecylenic acid from ricinoleic acid, which is the primary component of castor oil.

Retro-ene reaction - general reaction scheme - retro-ene fragmentation - retro-ene decomposition - retro-ene elimination
Retro-ene reaction

This reaction is an intramolecular thermolysis of organic compounds with an unsaturated functional group, which involves transferring a γ-hydrogen to the unsaturated center via a six-membered transition state to produce both ene and enophile. The retro-ene reaction is also known as retro-ene fragmentation, retro-ene decomposition, or retro-ene elimination. It is theoretically a pericyclic reaction but involves a [1,5]-H sigmatropic migration.

The reaction can occur in olefins, 1,6-dienes, allyltrimethylsilanes, acetylenic ethers, allylic and propargylic diazenes, and many other functional groups. The reaction usually occurs through a concerted mechanism and preserves the stereochemical integrity of the product.

Retro-ene reaction - general reaction scheme - retro-ene fragmentation - retro-ene decomposition - retro-ene elimination - acetylenic ethers to yield allenes
Retro-ene reaction (acetylenic ethers to yield allenes)

The retro-ene reaction through a planar transition state geometry is feasible for acetylene, while olefins prefer an envelope-like transition state geometry. The retro-ene reaction does not occur among cyclic alkenes smaller than a seven-membered ring or fused rings where one or more atoms involved in the reaction are part of the fused ring, except for a cyclopropyl ring. The alkyl group at the γ-carbon facilitates the retro-ene reaction for immonium salts by distributing a substantial fraction of positive charge at the transition state. However, olefins with an acetylene group at β-position will undergo a competing Cope rearrangement instead of the retro-ene reaction.

References

Perkins, G. A., and Cruz, A. O. (1927). Synthesis of compounds similar to chaulmoogric acid. II. dl-chaulmoogric acid. Journal of the American Chemical Society, 49(4), 1070-1077
DOI: 10.1021/ja01403a030

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