Formation of Olefins by Eliminative Dimerization and Eliminative Cross‐Coupling of Carbenoids: A Stereochemical Exercise

Two carbenoids combine to generate an olefin by a mechanism involving formation of an ate complex, 1,2‐metalate rearrangement, and β‐elimination. As each stage of this eliminative coupling is stereospecific, the overall stereochemical outcome can be understood and, in principle fully controlled, providing that the absolute stereochemical configurations of the reacting carbenoid species are defined. In contrast to traditional alkene syntheses, the eliminative cross‐coupling of carbenoids offers a connective approach to olefins capable of precisely targeting a given isomer regardless of the nature of the features distinguishing the isomers. The formation of olefins by the eliminative dimerization and eliminative cross‐coupling of carbenoids is reviewed with a range of illustrative examples, including the reactions of α‐lithiated haloalkanes, epoxides, and carbamates. An emphasis is placed on stereochemical analysis and methods to generate sp3‐hybridized carbenoids in stereodefined form are surveyed. Mutual elimination of electrofugal (M) and nucleofugal (X) leaving groups from pairs of reacting carbenoids results in the generation of a C−C double bond. This eliminative coupling process is inherently regio‐ and stereospecific and offers a programmable synthesis of olefins. Homo‐ and cross‐coupling variants are reviewed and prospects for further development are highlighted.