Oxidative carbonylation of ethene catalyzed by Pd(II)–PPh3 complexes in MeOH using benzoquinone as stoichiometric oxidant

The complexes [Pd(COOMe)nX2−n(PPh3)2] (n = 0, 1, 2; X = TsO, OAc, ONO2, Cl, Br), [Pd(SO4)(PPh3)2], [PdCl2(PPh3)]2 and PdX2 (X = Cl, Br, I) catalyze the oxidative ethene carbonylation in MeOH using benzoquinone (BQ) as stoichiometric oxidant. The main products dimethyl succinate (DMS) and dimethyl oxalate (DMO) are formed together with minor amounts of methyl propanoate and dimethyl carbonate. The formation of DMS unambiguously proves that ethene inserts into a Pd–COOMe bond. The influence of the CO/ethene ratio at constant total pressure and of the BQ/Pd ratio on the product distribution has been studied. Model reactions of a Pd-hydride with BQ, of trans-[Pd(COOMe)(TsO)(PPh3)2] with ethene in the presence of BQ and of trans-[Pd(COOMe)2(PPh3)2] with BQ have been studied by 31P{1H} NMR. BQ consumes the Pd–hydride and directs the catalysis toward a Pd–COOMe initiator leading to DMS. In the catalysis to DMO, BQ is likely to favour the formation of a Pd–(COOMe)2 species having the two carbomethoxy ligands in vicinal position such to favour the elimination of the product. The proposed catalytic cycles for the formation of the products are discussed.

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► Pd2+/benzoquinone in MeOH catalyze the ethene carbonylation to dimethyl succinate.
► Other products are dimethyl-carbonate, -oxalate and methyl propanoate.
► In the absence of benzoquinone catalysis yields methyl propanoate.
► Benzoquinone switches catalysis from the Pd–H mechanism to the Pd–COOMe one.