Enantioselective 1,2-Alkylhydroxylmethylation of Alkynes via Chromium/Cobalt Cocatalysis

Mechanism Switch Between Radical-Polar Crossover and Radical Buffering

Radical-polar crossover (RPC) is a classic concept that bridges one- and two-electron chemistry. It has been widely used in Cr-catalyzed carbonyl addition reactions to clarify the formation of alkyl chromium(III) intermediate and subsequent carbonyl insertion. Herein, we proposed an orthogonal bonding model, the radical buffering scenario, for Cr-catalyzed carbonyl alkylation. This radical bonding model features the radical dissociation from the alkyl chromium(III) complex followed by the Cr(II)-carbonyl-coupled radical addition to form the C─C bond. The mechanism switch between the radical and polar bonding models is affected by the radical stability, radical nucleophilicity, radical size, and the presence of an α-heteroatom or α-π bond. The collaborative computational and experimental studies have verified the reliability of the radical mechanism. More importantly, we demonstrated that this radical buffering scenario possesses a different stereoselectivity control model from that in the RPC scenario. A general enantioselectivity and diastereoselectivity control model derived from the multiple ligand-radical interactions is thus established for CrCl2/bisoxazoline-catalyzed asymmetric radical addition.

Photoredox/Cr-catalyzed enantioselective radical-polar crossover transformation via C-H functionalization

Asymmetric multicomponent reactions that aim to control multiple chiral centers with high selectivity in a single step remain an on-gonging challenge. The realm of enantioselective radical-polar crossover transformation achieved through C-H Functionalization has yet to be fully explored. Herein, we present a successful description of a photoredox/Cr-catalyzed enantioselective three-component (hetero)arylalkylation of 1,3-dienes through C-H functionalization. A diverse array of chiral homoallylic alcohols could be obtained in good to excellent yields, accompanied by outstanding enantioselectivity. The asymmetric radical-polar crossover transformation could build two chiral centers simultaneously and demonstrates broad substrate tolerance, accommodating various drug-derived aldehydes, (hetero)aromatics, and 1,3-diene derivatives. Preliminary mechanistic studies indicate the involvement of a radical intermediate, with the chiral allylic chromium species reacting with various aliphatic and aromatic aldehydes through Zimmerman-Traxler transition states enabled by dual photoredox and chiral chromium catalysis.

Radical Carbonyl Propargylation by Dual Catalysis

Carbonyl propargylation has been established as a valuable tool in the realm of carbon-carbon bond forming reactions. The 1,3-enyne moiety has been recognized as an alternative pronucleophile in the above transformation through an ionic mechanism. Herein, we report for the first time, the radical carbonyl propargylation through dual chromium/photoredox catalysis. A library of valuable homopropargylic alcohols bearing all-carbon quaternary centers could be obtained by a catalytic radical three-component coupling of 1,3-enynes, aldehydes and suitable radical precursors (41 examples). This redox-neutral multi-component reaction occurs under very mild conditions and shows high functional group tolerance. Remarkably, bench-stable, non-toxic, and inexpensive CrCl₃ could be employed as a chromium source. Preliminary mechanistic investigations suggest a radical-polar crossover mechanism, which offers a complementary and novel approach towards the preparation of valuable synthetic architectures from simple chemicals.