A general method for catalytic enantioselective borylation and silylation of ketimines remains elusive due to their extenuated electrophilicity, the difficulty to discriminate the two non-hydrogen substituents on carbonyl groups stereoselectively, and the lability of products. Herein, based on a series of newly developed chiral NHCs, such a general method is developed. In the presence of a 1 mol % copper-YC-NHC3 complex, the borylation of ketimines works smoothly to provide a broad range of α-amino tertiary boronates in moderate to high yields with high enantioselectivity. The silylation of ketimines is accomplished with high enantioselectivity in the presence of a 5 mol % copper-YC-NHC11 complex. In light of the calculated percent buried volumes with CuCl-YC-NHC1 and CuCl-YC-NHC11 complexes using SambVca software, YC-NHCs are found to possess fewer crowded chiral pockets around the Cu(I)-metal center than common NHCs. On the basis of DFT calculations and a classic quadrant model, the confined steric environment leaves only the first quadrant accessible for substrate approach, directing nucleophilic attack from the Re face of the ketimine while rendering the competing Si face pathway kinetically disfavored. Finally, the synthetic utilities of both the borylation and the silylation products are showcased by several transformations.
Cai et al. (Thu,) studied this question.