Rhodium(I)-Catalyzed Asymmetric Alkyl Carbene B-H Bond Insertion: Enantioselective Synthesis of Versatile Chiral Alkylboranes

Recent advances in transition metal-asymmetric carbene B-H insertion reactions provide a straightforward and powerful protocol to access chiral organoboron compounds. However, the related reaction involving linear alkyl carbenes has not been successfully developed. Apart from the difficulty of controlling the enantioselectivity, another major challenge is the high propensity of the alkyl metal carbene to undergo a beta-hydride migration to form undesired alkenes. Herein, we report our development of an efficient alkyl carbene B-H insertion reaction using rhodium(I)/diene complexes as the catalysts. This simple catalytic system not only reduces the formation of alkene byproduct but also achieves high enantioselectivity of the carbene B-H insertion. This method facilitates easy asymmetric access to a wide variety of structurally diverse alkylboranes in high yields, and their further synthetic application and transformation have also been described. Mechanistic studies show that the beta-hydride migration is less favorable than the carbene insertion pathway under the rhodium(I)/diene catalytic system and that the B-H bond insertion is the rate-limiting step. [GRAPHICS]