Mechanistic studies on the selective oxidative carbonylation of MeOH to dimethyl oxalate catalyzed by [Pd(COOMe)n(TsO)2−n(PPh3)2] (n = 0, 1, 2) using p-benzoquinone as a stoichiometric oxidant

The reactivity of the complexes cis-[Pd(OTs)2(PPh3)2] (I), trans-[Pd(COOMe)(OTs)(PPh3)2] (II) and trans-[(COOMe)2(PPh3)2] (III), regarding the catalytic oxidative carbonylation of MeOH to dimethyl oxalate (DMO) using benzoquinone (BQ) as a stoichiometric oxidant, has been studied in CD2Cl2/MeOH (10/1, v/v) by 1H and 31P{1H} NMR spectroscopy. I reacts with CO and MeOH at 193 K giving II, which is transformed into III upon addition of a base. The same occurs in the presence of BQ. Instead, if the base is added before admission of CO, [Pd(BQ)(PPh3)2] is formed. Starting also from II, complex III is formed only after addition of a base. The base neutralizes TsOH which is formed in the transformation of I to II and III. III is unstable in the presence of 1 equivalent of TsOH and it is transformed into II. At 333 K, under 0.4 MPa of CO, III decomposes with formation of DMO and dimethyl carbonate (DMC) (15%) each), whereas, in the presence of BQ, III is unstable already at 298 K, with formation of only DMO (10%). Catalysis to DMO is observed at 333 K. Thus BQ enhances the reactivity of III and directs the catalysis selectively to DMO. I, II and III have also been used in catalytic experiments in pure MeOH at 298 K, under 0.3 MPa of CO. II and III are active even in the absence of a base (TOF ca. 30 h−1). I is active only after addition of a base. A catalytic cycle is proposed.