New alkoxycarbonyl complexes of palladium(II) and their role in carbonylation reactions carried out in the presenceof 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 = Ph2PCH2CH2PPh2, R = Et) are prepd. by reacting trans-PdCl2L2, suspended in an alkanol under 10-50 atm CO at 50-70°, in the presence of a base such as a trialkylamine or a carboxylic acid anion. They were characterized by IR, 1H and 31P NMR spectroscopy. The complexes with R = Me and Et show 2 absorption bands centered at ca. 1650 cm-1, which are probably due to conformational isomers with cis and trans geometry. The others show only 1 band at ca. 1650 cm-1. The 1H NMR and 31P NMR spectra of all the monophosphine complexes show that only 1 isomer is present in soln. Instead, for the diphosphine ethoxycarbonyl complex the 1H NMR spectra suggest that 2 isomers are present in ca. 1:1 ratio, as confirmed by decoupling expts. The 2 isomers may originate from different orientation of the alkoxycarbonyl ligand with respect to the asym. metal center, 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 quant. 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° under CO, yielding Pd(0) carbonyl-phosphine complexes with formation of (MeO)2CO almost quant. They also react 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 CO and the corresponding alkanol. The role of these complexes in catalytic alkoxycarbonylation reactions is discussed.