Palladium catalyzed hydrodechlorination of alpha-chloroacetophenones by hydrogen transfer from the H2O-CO system
Palladium catalyzed hydrodechlorination of alpha-chloroacetophenones by hydrogen transfer from the H2O-CO system
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DOI：
10.1016/s1381-1169(97)00040-x
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发表时间：
1997-10-31
影响因子：
--
通讯作者：
Toniolo, L
中科院分区：
文献类型：
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作者：
Cavinato, G;Pasqualetto, M;Toniolo, L
PdCl2(PPh_3)(2)与PPh_3相结合，是催化α-氯苯乙酮在H2O-CO体系中氢转移加氢脱氯制苯乙酮的优良催化剂前体。该反应伴随着二氧化碳的释放而发生。在典型反应条件(50~70℃，40~80atm，底物/Pd/P=2000/1/50，H2O/底物=8~12/1)下，以乙醇或二氧六环为溶剂([Pd]=5.10(-4)mol1(-1))，反应时间为2 h，产率为70-80%。当使用催化剂前体而不添加额外数量的PPh3时，金属钯发生了广泛的分解。Pd/C对催化加氢脱氯反应具有促进作用。如预期的那样，随着催化剂浓度、一氧化碳压力和温度的增加，反应速率增加。底物浓度对产率影响不大。其中，H2O浓度的影响最为显著。以乙醇为溶剂，水的浓度较低时，反应速率增加，在6-7.10(-2)摩尔·L(-1)以上达到平台期。根据已知的事实：(I)前体被H2O-CO体系还原为Pd(0)物种，即使在氢化脱氯反应过程中被释放的盐酸存在，以及(Ii)起始的α-氯苯乙酮与Pd(0)氧化加成得到Pd(CH2COPh)Cl(PPh3)(2)(I)和(Iii)该络合物与盐酸反应得到苯乙酮和PdCl2(PPh3)(2)(Ii)，提出了加氢脱氯反应是通过类似络合物(I)的中间体进行的，(II)通过CO和H2O与金属中心的相互作用还原为Pd(0)络合物，得到一个具有Pd-(COOH)部分的物种，该物种经过β-氢化物抽提后得到一个钯-氢化物物种，并伴随着CO2的析出。氢化物释放质子并还原Pd(II)，使Pd(0)物种返回催化循环。我们还发现，在H2O和CO的作用下，络合物(I)被还原为Pd(0)络合物，生成苯乙酮。结果表明，该反应可能发生在不同的催化途径的基础上，通过具有H-Pd-(CH2COPh)的物种的中间体发生，该物种在苯乙酮还原消除后使Pd(0)配合物开始新的催化循环。在Pd/C催化加氢脱氯的情况下，认为H2O和CO在金属表面相互作用生成氢化物并放出CO2，该氢化物取代吸附在催化剂表面的α-氯苯乙酮中的氯离子而得到加氢脱氯产物。(C)1997年爱思唯尔科学公司。
PdCl2(PPh3)(2), in combination with an extra amount of PPh3, is an excellent catalyst precursor for the hydrodechlorination of alpha-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 degrees 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 1(-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 alpha-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 beta-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 alpha-chloroacetophenone absorbed on the catalytic surface to give the hydrodechlorination product. (C) 1997 Elsevier Science B.V.