Carbonylation of aromatic aldehydes to phenylacetic acid derivatives catalyzed by a Pd-PPh3-HCl system

Aromatic aldehydes having electron-releasing para substituents, such as a hydroxy group, are carbonylated to phenylacetic acid derivatives in the presence of a Pd-PPh3-HCl catalytic system, at 90–120 °C, 50–100 atm of carbon monoxide, 1–2 h, in the presence of water or an alkanol. PPh3 and HCl play key roles in the catalysis, since in their absence no activity is observed. When a Pd(II) compound, such as PdCl2, Pd(AcO)2, Pd(PPh3)Cl2, [Pd(1,3-η-C3H5)Cl]2, is used as catalyst precursor, partial decomposition to palladium metal occurs. Pd/C is also active, provided that PPh3 and HCl are present. The catalytic system is more active when an excess of PPh3 and HCl is used. Vanillin (4-hydroxy-3-methoxy-benzaldehyde) is carbonylated with 70–75% yields when catalyst and reagents are employed in the ratio Pd(II)/PPh3/HCl/aldehyde/alkanol = 1/6/100/100/800–1000, in a solvent such as dioxane, benzene or dichloroethane ([Pd] = 10−2 M). Higher yields are achieved with primary alkanols, except with MeOH. In t-BuOH only trace amounts of product are obtained because the alkanol subtracts HCl, yielding t-BuCl. The yield increases on increasing the concentration of EtOH added to the solvent and reaches a maximum of ca. 75% when the ratio EtOH/solvent = 1/1, while it is lower (55%) when EtOH alone is used. The yield increases on increasing the carbon monoxide pressure and the concentration of the catalyst, and is almost independent of the aldehyde concentration. In the proposed catalytic cycle it is suggested that a chloride of type ArCH(OR)Cl (R = H, alkyl radical of the alkanol), formed in situ by addition of HCl to the C=O double bond of the aldehyde, oxidatively adds to a Pd(0) species with formation of an intermediate having a Pd-[CH(OR)Ar] moiety, which, upon insertion of carbon monoxide followed by interaction with the alkanol or water, yields a mandelic acid derivative, ArCH(OR)COOR. HCl reacts with this product with formation of the corresponding chloride, ArCHClCOOR, which in turn gives oxidative addition to a Pd(0) species to form an organopalladium(II) intermediate having a Pd-CH(COOR)Ar bond. Successive insertion of carbon monoxide and interaction with water or the alkanol yields a phenylmalonic acid derivative, which, upon decarboxylation, leads to the final product, ArCH2COOR.