Cobalt-Catalyzed Asymmetric Reductive Addition to Carbonyls and Related Polarized Unsaturated Bonds

Conspectus The catalytic asymmetric construction of carbon–carbon bonds through the addition of carbon-based fragments to carbonyl compounds represents a fundamental transformation in organic synthesis, providing direct access to enantioenriched alcohols and amines that are ubiquitous structural motifs in pharmaceuticals, natural products, and functional molecules. Classical approaches typically rely on stoichiometric organometallic reagents, such as Grignard and organolithium reagents, which suffer from intrinsic limitations including air and moisture sensitivity, restricted functional group compatibility, and the requirement for preformed nucleophilic reagents. These drawbacks often increase synthetic complexity and reduce overall efficiency. Therefore, the development of catalytic asymmetric addition strategies that directly utilize stable and readily available precursors, while avoiding the preparation of sensitive organometallic intermediates, represents a longstanding and highly desirable goal. In this context, reductive coupling of organic halides as latent carbon nucleophiles offers an attractive alternative strategy. The seminal Nozaki–Hiyama–Kishi (NHK) reaction established the feasibility of directly employing organic halides in carbonyl additions; however, its reliance on chromium catalysis and stoichiometric metal reductants limits its practicality and broader application. Despite substantial advances, the development of general catalytic asymmetric methods capable of controlling radical-mediated carbon–carbon bond formation, particularly for constructing sterically congested tetrasubstituted stereocenters and contiguous stereogenic centers, remains a formidable challenge. The pursuit of sustainable and efficient catalytic processes has stimulated growing interest in earth-abundant first-row transition metals, among which cobalt has emerged as a particularly promising catalyst because of its rich single-electron redox chemistry, versatile coordination behavior, and intrinsic ability to mediate radical transformations. In this Account, we summarize our recent efforts in exploiting cobalt catalysis for the asymmetric reductive addition reactions of carbonyl-derived electrophiles and related polarized unsaturated systems with organic halides. This strategy, herein termed cobalt-catalyzed asymmetric reductive addition (CARA), utilizes readily available metal reductants under mild conditions and enables stereocontrolled radical addition processes directed by chiral cobalt complexes. Through this platform, we have developed modular approaches for the synthesis of diverse enantioenriched molecules, including amino acids, alcohols, amides, and sulfinamides, directly from readily available precursors. The development of this catalytic platform is discussed from three perspectives according to the reaction partners: (1) asymmetric reductive addition of simple organohalides for the construction of sterically demanding tetrasubstituted stereocenters; (2) enantioconvergent radical additions of racemic alkyl halides for the formation of contiguous stereogenic centers; and (3) reductive additions to other polarized unsaturated bonds, including isocyanates and sulfinylamines. Through extensive mechanistic investigations and DFT calculations, we have established that these transformations share a common radical addition pathway that is mechanistically distinct from the classical NHK reaction. We anticipate that this Account will stimulate further exploration of catalytic asymmetric reductive additions beyond those of conventional organometallic reagents. Continued development of cobalt-based radical platforms, together with advances in catalyst design and mechanistic understanding, may provide new solutions to longstanding challenges in stereoselective synthesis.