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)3), are described and its coordination chemistry toward the metal precursor [Pd(COD)Cl2] (COD = 1,5-cyclooctadiene, C8H12) is reported. A new Pd(ii)-P(Oquin)3 metal complex was isolated and fully characterized ([Pd{P(Oquin)3}Cl2]), 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 = kapp[BnNH2][cat] ([BnNH2] = molar concentration of benzylamine, [cat] = molar concentration of [Pd{P(Oquin)3}Cl2], kapp = k[O2]c = 0.756 L mol-1 h-1 = apparent rate constant). In situ NMR tests were performed to gain some insight into the reactivity of the Pd(ii)-P(Oquin)3 complex toward benzylamine.