Regioselective Rhodium-Catalyzed 1,2-Hydroboration of Pyridines and Quinolines Enabled by the Tris(8-quinolinyl)phosphite Ligand

A Rh(I) complex [& kappa;(2)(P,N)-{P(Oquin)(3)}RhCl(PPh3)] (1) bearing the P,N ligand tris(8-quinolinyl)phosphite, P(Oquin)(3), has been synthesized and structurally characterized. The molecular structure of complex 1 shows that P(Oquin)(3) acts as a bidentate P,N chelate ligand. Reactivity studies of 1 reveal that the triphenylphosphine ligand can be replaced by Pcy(3) or removed upon oxidation with concomitant coordination of a second 8-quinolyl unit of P(Oquin)(3). In addition, the Rh(III) complex [RhCl2{OP(Oquin)(2)}] (3), resulting from treating 1 with either wet CDCl3 or, sequentially, with HCl and water, was identified by X-ray diffraction analysis. Complex 1 catalyzes the 1,2-regioselective hydroboration of pyridines and quinolines, affording N-boryl-1,2-dihydropyridines (1,2-BDHP) and N-boryl-1,2-hydroquinolines (1,2-BDHQ) in high yield (up to >95 %) with turnover numbers (TONs) of up to 130. The system tolerates a variety of substrates of different electronic and steric nature. In comparison with other transition-metal-based hydroboration catalysts, this system is efficient at a low catalyst loading without the requirement of base or other additives.