PALLADIUM-CATALYZED HYDRODECHLORINATION OF ALPHA-CHLOROACETOPHENONES BY HYDROGEN-TRANSFER FROM THE H2O-CO SYSTEM

PdCl2(PPh3)(2), in combination with an extra amount of PPh3, is an exc ellent catalyst precursor for the hydrodechlorination of alpha-chloroa cetophenone to acetophenone by hydrogen transfer from the H2O-CO syste m. The reaction occurs with concomitant evolution of CO2. Under typica l reaction conditions (50-70 degrees C, 40-80 atm, substrate/Pd/P = 20 00/1/50, 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 1(- 1)). When the catalyst precursor is employed without adding an additio nal amount of PPh3 extensive decomposition to metallic palladium occur s. 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 sl ightly influenced by the concentration of the substrate. The effect of the concentration of H2O is the most significant. In ethanol as a sol vent at low concentration of water the reaction rate increases to reac h a plateau above 6-7.10(-2) mol.l(-1) of water. On the basis of the f act that it is known that (i) the precursor is reduced to a Pd(0) spec ies by the H2O-CO system, even in the presence of hydrochloric acid, w hich is freed during the course of the hydrodechlorination reaction an d that (ii) the starting alpha-chloroacetophenone oxidatively adds to Pd(0) to give Pd(CH2COPh)Cl(PPh3)(2) (I) and that (iii) this complex r eacts with hydrochloric acid to give acetophenone and PdCl2(PPh3)(2) ( II), it is proposed that 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, whi ch after beta-hydride abstraction gives a palladium-hydride species wi th concomitant evolution of CO2. The hydride gives off a proton and re duces 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 format ion of acetophenone through the action of H2O and CO. It is proposed t hat this reaction, which may be at the base of a different catalytic p ath, occurs via the intermediacy of a species having a H-Pd-(CH2COPh) which, after reductive elimination of acetophenone give the Pd(0) comp lex starting a new catalytic cycle. In the case of the Pd/C catalyzed hydrodechlorination it is suggested that H2O and CO interacts on the s urface of the metal to give a hydride and evolution of CO2 and that th is hydride displaces a chloride anion from alpha-chloroacetophenone ab sorbed on the catalytic surface to give the hydrodechlorination produc t. (C) 1997 Elsevier Science B.V.