C–C Bond Formation via the Addition or Redistribution of Hydrogen: Chemical Synthesis Beyond Premetalated Reagents or Metallic Reductants

Stoichiometric organometallic reagents are used in countless C–C bond-forming processes but are often hazardous, display poor functional group compatibility, and require multistep preparation from the parent platform chemicals. Zerovalent metals are also used to mediate reductive C–C bond formation, but their use at scale poses similar safety and engineering concerns. The formation of C-nucleophiles as transient catalytic intermediates via hydrogen transfer from abundant low-molecular-weight reductants (H2, Me2CHOH, HCO2H) offers a more ideal alternative to preformed carbanions or zerovalent metals in reductive C–C bond formation. Related hydrogen autotransfer processes where reactants serve as redox pairs en route to products of C–C coupling also are desirable, as they avoid both sacrificial reductants and premetalated reagents. In this Review, hydrogen-mediated variants of classical reactions that exploit carbanions or zerovalent metals are described.