New alkoxycarbonyl complexes of palladium(II) and their role in carbonylation reactions carried out in the presence of an alkanol

The new complexes of general formula trans-Pd(COOR)ClL2 (L = PPh3, R = Et, n-Pr, iso-Pr, n-Bu, iso-Bu, sec-Bu, 2-ethoxyethyl, cyclopentyl, cyclohexyl; L = 1,2-diphenylphosphinoethane, R = Et) are prepared by reacting trans-PdCl2L2, suspended in an alkanol, under 10–50 atm of carbon monoxide, at 50–70°C, in presence of a base such as a trialkylamine or a carboxylic acid anion. They have been characterized by IR, 1H and 31P NMR spectroscopy. The complexes with R =Me and Et show two absorption bands centered at ca. 1650 cm−1, which are probably due to conformational isomers with cis and trans geometry. The other show only one band at ca. 1650 cm−1. The 1H NMR and 31P NMR spectra of all the monophosphine complexes show that only one isomer is present in solution. Instead, for the diphosphine ethoxycarbonyl complex the 1H NMR spectra suggest that two isomers are present, in the ratio of ca. 1:1, as confirmed by decoupling experiments. The two isomers may take origin from different orientation of the alkoxycarbonyl ligand with respect to the asymmetrical metal centre, because of hindered rotation around the Pd-C bond due to the partial double bond character. The R group of the alkoxycarbonyl ligand can be exchanged with a different R′ group by reacting the Pd(COOR)Cl(PPh3)2 complex with an excess of R′OH. The reaction is practically quantitative when R is bulkier than R′. The alkoxycarbonyl complexes react with the corresponding alkanol, in the presence of a base such as a trialkylamine, at 90–100°C, under carbon monoxide, yielding Pd(0) carbonyl-phosphine complexes with formation of dimethyl carbonate in an almost quantitative yield. They react also with water giving off CO2 and yielding Pd(0) complexes. The reaction is promoted by an acid of a non-coordinating anion such as HBF4. Instead, the reaction with HCl yields the corresponding dichloride of Pd(II), beside carbon monoxide and the corresponding alkanol. The role of these complexes in catalytic alkoxycarbonylation reactions is discussed.