Organocatalytic iminium-assisted asymmetric B(sp²)-to-B(sp³) transformation

Tetracoordinate boron compounds are widely used as dyes in medicinal research and material sciences. Current methods for accessing such compounds with a stereogenic boron center rely on chiral substrate-induced diastereoselective reactions or metal-catalyzed desymmetrization of pre-formed tetracoordinate boron molecules. Directly constructing a tetracoordinate B(sp³) center in a catalytic enantioselective manner remains challenging and underdeveloped, as controlled asymmetric reactions on a flat B(sp²) center are virtually unknown. Here, we address this challenge by leveraging the inherent structural features of commonly used boron compounds. In our approach, the amino-thiourea organocatalyst activates the phenol oxygen of the salicylaldehyde as an effective nucleophile for enantioselective addition to the B(sp²) center of a B,N-heterocyclic substrate. Subsequent iminium exchange involving the amine moiety of the B,N-heterocycle furnishes the tetracoordinate B(sp³) products with excellent optical purities. Our study adopted the B(sp²)-to-B(sp³) transformation strategy for the enantioselective synthesis of tetracoordinate boron molecules.