Organocatalytic Iminium-Assisted Asymmetric B(sp²)-to-B(sp³) Transformation

Tetracoordinate boron compounds are widely used as dyes in medicinal research and material sciences1. Current methods for accessing such compounds with a stereogenic boron center rely on chiral substrate-induced diastereoselective reactions2 or metal-catalyzed desymmetrization of pre-formed tetracoordinate boron molecules3-9. 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, which often contain an iminium or iminium-like moiety10-12. In our approach, an iminium, formed between a salicylaldehyde substrate and an amino-thiourea organic catalyst, 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 represents the first B(sp²)-to-B(sp³) transformation strategy for the enantioselective synthesis of tetracoordinate boron molecules. It introduces new concepts and logical frameworks for controlling and modulating asymmetric bond formation with boron atoms. We expect our study to break new ground in construction of chiral tetracoordinate boron molecules with diverse structures and functions.