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

Abstract

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 = k_app[BnNH₂][cat] ([BnNH₂] = molar concentration of benzylamine, [cat] = molar concentration of [Pd{P(Oquin)₃}Cl₂], k_app = 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.