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

Abstract

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 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.

Recent Advances on Synthetic Methodology Merging C–H Functionalization and C–C Cleavage

“…The scission of C–C bonds in acyclic carbon skeletons is a challenging topic since these processes lack the release of the strain present in small carbocyclic structures as a driving force. Nevertheless, this fact has not precluded the development of synthetic methods where unstrained C–C bonds are cleaved [ 22 , 118 , 119 , 120 ]. Furthermore, in some cases these processes have been engaged with the functionalization of C–H bonds either intra- or intermolecularly.…”

“…As happens with the cyclopropyl and cyclobutyl alcohol derivatives commented in the previous section, the OH group of unstrained alcohols has served as a useful handle to induce the scission of the C–C bond through transition-metal catalysis [ 118 , 119 ].…”

Isomerization of a cis‐(2‐Borylalkenyl)Gold Complex via a Retro‐1,2‐Metalate Shift: Cleavage of a C−C/C−Si Bond trans to a C−Au Bond

“…A widely used retro‐allylation process, which is closely related to the β ‐carbon elimination, involves a cleavage of C β −C bonds to furnish allylmetal species via a six‐membered‐ring transition state (Scheme 1 b). [4] It is known that metallacyclopentadiene is in equilibrium with metallacyclopropene and alkyne via C−C bond cleavage at the β ‐position (Scheme 1 c). [5] The β ‐carbon elimination has been directly observed for Rh‐iminyl and (η 6 ‐indenyl)Ru complexes (Scheme 1 d).…”

Enantioselective palladium-catalyzed C(sp2)–C(sp2) σ bond activation of cyclopropenones by merging desymmetrization and (3 + 2) spiroannulation with cyclic 1,3-diketones

“…Accordingly, the construction and breaking of carbon-carbon bonds is one of the most important research topics in modern synthetic chemistry and has been attracted much attentions by chemists in the past decades. [1][2][3][4][5][6][7][8][9][10][11] Despite numerous new reaction strategies have been developed, the improvement of adaptability of catalyst or reaction systems through carbon-carbon bond activation for the enantioselective construction of chiral molecules with complex structures are still not an easy task. In addition, carbon-carbon bond activation of small rings for the synthesis and functionalization of cyclic compounds is intrinsically challenging to a variety of synthetic chemistry and pharmaceutical application.…”