Indirect formation of carboxylic acids via anhydrides in the palladium-catalyzed hydroxycarbonylation of aromatic halides

Article abstract of DOI:10.1021/ja00120a012

The carbonylation of [(Ph₃P)₂Pd₂Ph₂(μ-OH)₂] (2) in the presence of PhI results in the quantitative formation of benzoic anhydride and [(Ph₃P)₂Pd₂(PhCO)₂(μ-I) ₂] (3). This transformation contributes to some extent to the production of benzoic acid from haloarenes, CO, and alkali, catalyzed by [(Ph₃P)₂PdCl₂], the latter being converted to 2 under the reaction conditions. Phenyl- and benzoylpalladium benzoates are likely key intermediates in the indirect formation of the acid, via the anhydride, by the carbonylation of 2. One such complex, [L₂Pd(Ph)(PhCOO)] (5, L = Cy₃P), was isolated from the carbonylation of [(Cy₃P)₂Pd₂Ph₂(μ-OH)₂] (4) in hexane. The related complexes, [L₂Pd₂Ph₂(μ-PhCOO)₂] (6, L = PPh₃; 7,L = PCy₃), were prepared from benzoic acid and hydroxo complexes 2 and 4, respectively. Treatment of 6 and 7 with the corresponding phosphine afforded [L₂Pd(Ph)(PhCOO)] (8, L = PPh₃; 5,L = PCy₃). When 6-8 were reacted with CO in benzene, benzoic anhydride was readily formed, whereas in the case of 5 both carbonylation and reductive elimination were sluggish. Crystallographic data for 6 (1:2 chloroform solvate): space group Pccn, a = 22.499(4) A?, b = 10.992(3) A?, c = 24.514(6) A?, V = 6062.4(23) A?³, Z = 4, R = 0.056, and R_w = 0.038.