Oxidative carbonylation of alkanols catalyzed by Pd(II)-phosphine complexes

The complexes trans–[Pd(COOR)(H2O)(PPh3)2](TsO) (R = Me, Et, nPr, iPr, nBu, iBu, secBu) and trans–[Pd(COOR)nX2–n(PPh3)] (R = Me, n = 1, 2; X = ONO2, ONO, OTs, Br. R = iPr, n = 1, X = Cl, Br) have been synthesised and characterised. Most of them have been used as catalyst precursors for the oxidative carbonylation of MeOH, selective to dimethyloxalate, using p–benzoquinone (BQ) as an oxidant. BQ changes properties of the reaction centre. When using iPrOH in place of MeOH, high activity and selectivity toward diisopropyloxalate have been achieved using trans–[PdX2(PAr3)2] with strongly coordinating X (Br), in presence of a base (NEt3 or PAr3–BQ adduct) and of an excess of LiBr. The slow step of the catalysis (RDS) might be the reoxidation of Pd(0), which is formed in the product–forming step. Using [PdX2(P-P)] the best performance has been achieved with weakly coordinating X (TsO) and using P-P of relatively wide bite angle but with cis–geometry. The RDS is related to the nature of P-P.