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 retro-allylation and deallylation of allylic compounds are uniquely intriguing methods for the cleavage of C-C σ 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 σ-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/β-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 σ-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.