Synthesis of chiral boranes via asymmetric insertion of carbenes into B-H bonds catalyzed by the rhodium(I) diene complex

Asymmetric insertion of arydiazoacetates into B-H bonds of NHC-BH2R adducts gives rare compounds with chiral boron centers. The reaction is catalyzed by the rhodium(I) complex with the chiral diene ligand tBu2-TFB, which can be conveniently synthesized by diastereoselective coordination of the racemic diene with (S-Salox)Rh(CO)2. The target boranes were obtained typically in 75−90% yields with 90-95% ee and 2:1-5:1 dr.