Palladium catalyzed hydrodechlorination of α-chloroacetophenones by hydrogen transfer from the H2O-CO system

Article abstract of DOI:10.1016/S1381-1169(97)00040-X

PdCl₂(PPh₃)₂, in combination with an extra amount of PPh₃, is an excellent catalyst precursor for the hydrodechlorination of α-chloroacetophenone to acetophenone by hydrogen transfer from the H₂O-CO system. The reaction occurs with concomitant evolution of CO₂. Under typical reaction conditions (50-70°C, 40-80 atm, substrate/Pd/P = 2000/1/50, H₂O/substrate = 8-12/1), the reaction occurs in 70-80% yield in 2 h, using ethanol or dioxane as a solvent ([Pd] = 5.10^-4 mol · l^-1). When the catalyst precursor is employed without adding an additional amount of PPh₃ extensive decomposition to metallic palladium occurs. Also Pd/C is active in promoting the hydrodechlorination reaction. As expected the reaction rate increases upon increasing concentration of catalyst, carbon monoxide pressure and temperature. The yield is slightly influenced by the concentration of the substrate. The effect of the concentration of H₂O is the most significant. In ethanol as a solvent at low concentration of water the reaction rate increases to reach a plateau above 6-7 10^-2 mol · l^-1 of water. On the basis of the fact that it is known that (i) the precursor is reduced to a Pd(0) species by the H₂O-CO system, even in the presence of hydrochloric acid, which is freed during the course of the hydrodechlorination reaction and that (ii) the starting α-chloroacetophenone oxidatively adds to Pd(0) to give Pd(CH₂COPh)Cl(PPh₃)₂ (I) and that (iii) this complex reacts with hydrochloric acid to give acetophenone and PdC1₂(PPh₃)₂ (II), it is proposed thai the hydrodechlorination reaction proceeds via the intermediacy of a species analogous to complex (I) and that (II) is reduced to the Pd(0) complex through the intercation of CO and H₂O with the metal center to give a species having a Pd-(COOH) moiety, which after β-hydride abstraction gives a palladium-hydride species with concomitant evolution of CO₂. The hydride gives off a proton and reduces Pd(II) returning a Pd(0) species back to the catalytic cycle. We found also that complex (I) is reduced to a Pd(0) complex with formation of acetophenone through the action of H₂O and CO. It is proposed that this reaction, which may be at the base of a different catalytic path, occurs via the intermediacy of a species having a H-Pd-(CH₂COPh) which, after reductive elimination of acetophenone give the Pd(0) complex starting a new catalytic cycle. In the case of the Pd/C catalyzed hydrodechlorination it is suggested that H₂O and CO interacts on the surface of the metal to give a hydride and evolution of CO₂ and that this hydride displaces a chloride anion from α-chloroacetophenone absorbed on the catalytic surface to give the hydrodechlorination product.