INDIRECT FORMATION OF CARBOXYLIC-ACIDS VIA ANHYDRIDES IN THE PALLADIUM-CATALYZED HYDROXYCARBONYLATION OF AROMATIC HALIDES

The carbonylation of [(Ph(3)P)(2)Pd(2)Ph(2)(mu-OH)(2)] (2) in the presence of PhI results in the quantitative formation of benzoic anhydride and [(Ph(3)P)(2)Pd-2(PhCO)(2)(mu-I)(2)] (3). This transformation contributes to some extent to the production of benzoic acid from haloarenes, CO, and alkali, catalyzed by [(Ph(3)P)(2)PdCl2], 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, [L(2)Pd(Ph)(PhCOO)] (5, L Cy(3)P), was isolated from the carbonylation of [(Cy(3)P)(2)Pd(2)Ph(2)(mu-OH)(2)] (4) in hexane. The related complexes, [L(2)Pd(2)Ph(2)(mu-PhCOO)(2)] (6, L = PPh(3); 7, L = PCy(3)), were prepared from benzoic acid and hydroxo complexes 2 and 4, respectively. Treatment of 6 and 7 with the corresponding phosphine afforded [L(2)pd(ph)(phCOO)] (8, L = PPh(3); 5, L = PCy(3)). 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 Peen, a = 22.499(4) Angstrom, b = 10.992(3) Angstrom, c = 24.514(6) Angstrom, V = 6062.4(23) Angstrom(3), Z = 4, R = 0.056, and R(w) = 0.038.