Synthesis, characterization and Pd(ii)-coordination chemistry of the ligand tris(quinolin-8-yl)phosphite. Application in the catalytic aerobic oxidation of amines.

The synthesis and characterization of the ligand tris(quinolin-8-yl)phosphite, (P(Oquin)), are described and its coordination chemistry toward the metal precursor [Pd(COD)Cl] (COD = 1,5-cyclooctadiene, CH) is reported. A new Pd(ii)-P(Oquin) metal complex was isolated and fully characterized ([Pd{P(Oquin)}Cl]), and its X-ray diffraction analysis shows that the phosphite ligand coordinates as a bidentate P-N chelate. This complex is an efficient catalyst for the solvent-free mild oxidative coupling of primary amines to imines using air as an oxidant, obtaining moderate to good yields (up to 99%) and turnover numbers (TONs up to 230). This catalyst can be recovered from the reaction mixture and reused in a subsequent run without a significant loss of activity. Kinetic measurements of the oxidation of benzylamine suggest that the rate law is r = k[BnNH][cat] ([BnNH] = molar concentration of benzylamine, [cat] = molar concentration of [Pd{P(Oquin)}Cl], k = k[O] = 0.756 L mol h = apparent rate constant). In situ NMR tests were performed to gain some insight into the reactivity of the Pd(ii)-P(Oquin) complex toward benzylamine.