Showing papers by "Luigi Toniolo published in 1997"

Palladium catalyzed hydrodechlorination of α-chloroacetophenones by hydrogen transfer from the H2OCO system

Abstract: 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 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, 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.

Hydrogenation of mandelic acid derivatives to the corresponding phenyl acetic acid derivative catalysed by Pd/C. A kinetic study

Abstract: The hydrogenolysis of the c~-C-O bond of mandelic acid derivatives catalysed by 5% Pd/C, in the presence of hydrochloric or sulphuric acids as cocatalysts, was carried out in water or ethanol as solvent, under 35-150 kPa of hydrogen pressure, at 343 K. Typically, the substrate/catalyst/cocatalyst ratio was 200 : 1 : 10. The hydrogenation of the ethyl ester of mandelic acid in ethanol as solvent is much faster, ca. 20 times, than that of the acid in water. The influence of the concentration of the reagents, products and cocatalysts on the initial reaction rate was investigated. Upon increasing the concentration of the ester the rate increases to a plateau. The pressure of hydrogen has little influence. The products inhibit the reaction. The rate steeply increases and reaches a maximum upon increasing hydrochloric acid concentration. From equilibrium constant data, the concentration of protonated ester as a function of the hydrochloric cocatalyst concentration has been estimated. The trend of the concentration of the protonated species parallels the trend of the reaction rate, thus suggesting that the protonated species plays a key role in relation to the catalyst activity. It is suggested that from this species, adsorbed on the catalyst surface, a molecule of water is displaced by a hydride formed upon activation of molecular hydrogen by palladium. Though less effective than hydrochloric acid, sulphuric acid acts also as a cocatalyst. However, in the latter case, the initial hydrogenation rate increases to reach a plateau. In addition, when HC1 is introduced in the reaction after the preactivation step of the catalyst, the hydrogenolysis rate is equal to the rate observed when sulphuric acid is used as cocatalyst. It is suggested that in the first case the possible formation of the superficial PdOxCly may be related to the higher activity of the chlorided catalyst, i~ 1997 Elsevier Science B.V.