M. Pasqualetto

Bio: M. Pasqualetto is an academic researcher from Ca' Foscari University of Venice. The author has contributed to research in topics: Substrate (chemistry) & Catalysis. The author has an hindex of 1, co-authored 1 publications receiving 13 citations.

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.

Catalytic hyrodechlorination of chlorophenols in aqueous solution under mild conditions

Abstract: Hydrogen-saturated water or sodium borohydride were inefficient sources of active hydrogen species for the palladium-alumina accelerated hydrodechlorination (HDC) and subsequent hydrogenolysis of 4-chlorophenol (4-CP) or 2,6-dichlorophenol (2,6-DCP) in water at ambient temperature and pressure. By contrast, hydrogen gas in the presence of Pd 0 /Al 2 O 3 accelerator mediated both hydrodechlorination and partial hydrogenolysis to cyclohexanone. 4-CP or pentachlorophenol (PCP) was converted quantitatively to cyclohexanone in 1 h and subsequently reduced further to cyclohexanol (∼25 mol%) after 3 h. With ammonium formate, HDC of 4-CP was quantitative but hydrogenolysis to cyclohexanol was sluggish (∼14 mol% after 1 h). Whereas zero-valent iron or magnesium generated only traces (∼3 mol%) of phenol, nickel metal powder (Ni 0 ) and Raney-type nickel (Ni–Al) but not NiCl 2 mediated HDC/hydrogenolysis in the presence of NaBH 4 but not H 2 . In these cases, hydrogenolysis, at somewhat elevated temperatures, generated cyclohexanol (14–35 mol%) and no cyclohexanone was observed. At elevated H 2 pressure (414 kPa) and 333 K over Pd 0 /γ-Al 2 O 3 , hydrogenolysis of PCP to cyclohexanone was complete after 30 min. Further increase in pressure (to 4137 kPa) with N 2 seemed to accelerate the reaction rate for shorter reaction times (10 min) but if CO 2 served to increase P, the rates but not the course of reaction were retarded appreciably consistent with competitive inhibition of catalytic sites by CO 2 .

Harnessing the Power of the Water-Gas Shift Reaction for Organic Synthesis

Abstract: Since its original discovery over a century ago, the water-gas shift reaction (WGSR) has played a crucial role in industrial chemistry, providing a source of H2 to feed fundamental industrial transformations such as the Haber–Bosch synthesis of ammonia. Although the production of hydrogen remains nowadays the major application of the WGSR, the advent of homogeneous catalysis in the 1970s marked the beginning of a synergy between WGSR and organic chemistry. Thus, the reducing power provided by the CO/H2O couple has been exploited in the synthesis of fine chemicals; not only hydrogenation-type reactions, but also catalytic processes that require a reductive step for the turnover of the catalytic cycle. Despite the potential and unique features of the WGSR, its applications in organic synthesis remain largely underdeveloped. The topic will be critically reviewed herein, with the expectation that an increased awareness may stimulate new, creative work in the area.

Transition-Metal-Catalyzed Regioselective Functionalization of Monophosphino-o-Carboranes

Abstract: A facile approach to the synthesis of diaryl- and dialkyl-substituted monophosphino-o-carboranes by rhodium(I)-catalyzed phosphine-directed B3,6 -H activation has been developed for the first time. Upon switching rhodium(I) to palladium(II), C-arylated and B6 -halogenated products were obtained by using tBuOLi and Li2 CO3 as base, respectively. These discoveries provide some simple and efficient approaches to the modification of monophosphino-o-carboranes.