Catalytic properties of [Pd(COOMe)nX2-n(PPh 3)2] (n = 0, 1, 2; X = Cl, NO2, ONO 2, OAc and OTs) in the oxidative carbonylation of MeOH

Article abstract of DOI:10.1021/ic901569w

cis-[Pd(ONO₂)₂(PPh₃)₂] (1) reacts under mild conditions with CO in methanol (MeOH) in the presence of pyridine (py), yielding trans-[Pd(COOMe)(ONO₂)(PPh₃) ₂] (1a). The use of NEt₃ instead of py leads to a mixture of 1a, trans-[Pd(COOMe)₂(PPh₃)₂] (2), and [Pd(CO)(PPh₃)₃]. Pure 2 was prepared by reacting cis-[Pd(OTs)₂(PPh₃)₂] with CO in MeOH and subsequently adding NEt₃. The nitro complex trans-[Pd(COOMe)(NO ₂)(PPh₃)₂] (3a) was prepared by reacting trans-[Pd(COOMe)Cl(PPh₃)₂] with AgNO₂ or with AgOTs and NaNO₂. New syntheses for 1 and trans-[Pd(NO ₂)₂(PPh₃)₂] (3) are also reported. All complexes have been characterized by IR and ¹H and ³¹P{¹H} NMR spectroscopies. Complexes 1 and 2 exchange irreversibly and quantitatively one nitrato with one carbomethoxy ligand, yielding 1a. 2 in CD₂Cl₂ at 40 °C decomposes with the formation of dimethyl carbonate (DMC), whereas under 4 atm of CO, DMC and dimethyl oxalate (DMO) are formed, ca. 12% each; in the presence of PPh ₃ and in the absence of CO, decomposition occurs at 60 °C with the formation of DMC only, suggesting that decarbonylation involves a five-coordinate intermediate or predissociation of a PPh₃ ligand. The oxidative carbonylation of MeOH does not occur when using NaNO₂ or NaNO₃ as the oxidant and 1, 1a, 3, or 3a as the catalyst precursor. On the contrary, when using benzoquinone (BQ) as the oxidant, these complexes, 2, or [Pd(COOMe)_2-nX_n(PPh₃)₂] (X = Cl, OAc, OTs; n = 1, 2) promote selective catalysis to DMO. After catalysis the precursors are transformed into [Pd(BQ)(PPh₃)₂] ₂ H₂BQ, [Pd(CO)(PPh₃)]₃ and [Pd(CO)(PPh₃)₃]. Also the last with BQ gives selective catalysis to DMO. The solid-state structures of 1 CH₂Cl₂ and 1a have been determined by means of single-crystal X-ray diffraction.