Carbon-Carbon Bond Cleavage at Allylic Positions: Retro-allylation and Deallylation

The development of C-C bond-cleaving transformations is an issue in modern organic chemistry that is as challenging as it is important. Among these transformations, the retroallylation and deallylation of allylic compounds are uniquely intriguing methods for the cleavage of C-C sigma bonds at the allylic position. Retro-allylation is regarded as a prospective method for the generation of highly valuable regio- and stereodefined allylic metal compounds. Because the C-C cleavage proceeds via a favorable six-membered chairlike transition state, the regio- and stereochemical information on the starting homoallylic alcohols can be transferred onto the products. Moreover, retro-allylation can also be achieved using enantioselective C-C cleavage powered by chiral catalysts for the synthesis of enantiomerically enriched compounds. As a result of these attractive features, retro-allylation has wide utility in regio-, stereo-, and enantioselective synthesis. Deallylation is C-C sigma-bond cleavage involving the departure of an allylic fragment and the formation of a relatively stable carbanion or radical, and it proceeds via either oxidative addition to a low-valent metal or an addition/beta-elimination cascade. The removal of the versatile allylic group might seem to be unproductive; however, this unique transformation offers the opportunity of using the allylic group as a protective group for acidic C-H bonds. This Review aims to exhibit the synthetic utility as well as the uniqueness of these two C-C sigma-bond cleavage methods by presenting a wide range of transformations of allylic compounds with the aid of main group metals, transition-metal catalysts, and radical species.

Automated summary

The retroallylation and deallylation of allylic compounds are intriguing methods for cleavage of C-C sigma bonds, generating valuable allylic metal compounds and serving as protective groups for acidic C-H bonds. These transformations offer wide application in regio-, stereo-, and enantioselective synthesis.