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

Abstract

AbstractA Rh(I) complex [κ2(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 Pcy3 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.