On the mechanism of the hydrogen transfer from H2O-CO to gamma-keto-alpha-hydroxy carboxylic acids to yield gamma-keto acids catalyzed by a PdCl2(PPh(3))(2) precursor in combination with hydrochloric acid
On the mechanism of the hydrogen transfer from H2O-CO to gamma-keto-alpha-hydroxy carboxylic acids to yield gamma-keto acids catalyzed by a PdCl2(PPh(3))(2) precursor in combination with hydrochloric acid
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DOI：
10.1016/1381-1169(95)00145-x
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发表时间：
1996-01-28
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;Toniolo, L
PdCl_2（PPh（3））（2）-HCl催化体系在H_2 O-CO到PhCOCH（2）CHOHCOOH的氢转移反应中具有高活性和高选择性，该反应生成相应的γ-酮酸PhCOCH（2）CH（2）COOH，并伴随CO_2的释放。温度、一氧化碳压力和催化剂浓度的增加对反应速率具有有益的影响，反应速率在底物中似乎是一级的，并且当改变HCl浓度时通过最大值。有人提出，HCl的一个重要功能是产生氯化物PhCOCH（2）CHClCOOH，其与氢化钯相互作用，该氢化钯来源于具有Pd-COOH部分的物质的脱羧，这反过来是由H2O和CO在金属中心上的相互作用产生的。随着H_2O浓度的增加，产率达到最大值，这一趋势归因于这样的事实，即一方面，H_2O有利于Pd-COOH物种的形成，而另一方面，它可能与其他反应分子竞争与金属中心的配位。此外，H2O不利于氯化物的形成。当以相对高的浓度使用时，催化剂前体已经作为钯（0）、Pd-3（CO）（3）（PPh（3））（3）或Pd（CO）（PPh（3））（3）的络合物回收，后者在PPh（3）的存在下。还原为钯（0）仅在H2O存在下发生，并且可能通过Pd-COOH物质的中间作用发生，其在CO2释放后可能通过从氢化物中间体trans-PdHCl（PPh（3））（2）中还原消除HCl得到还原络合物。此外，PhCOCH=CHCOOH与HCl组合（相当于PhCOCH（2）CHClCOOH）与Pd（CO）（PPh（3））（3）反应，得到氢化产物PhCOCH（2）CH（2）COOH和PdCl 2（PPh（3））（2）。基于这些结果，并且已知HCl与Pd（CO）（PPh（3））（3）反应得到氢化物PdHCl（PPh（3））（2），建议通过以下步骤进行催化循环：（i）H2O和CO与前体的金属中心相互作用，产生Pd-COOH物质，（ii）这释放出CO2，形成氢化物，（iii）它与氯化物PhCOCH（2）CHClCOOH相互作用，产生产物PhCOCH（2）CH（2）COOH和钯（II）前体，返回催化循环。
The catalytic system PdCl2(PPh(3))(2)-HCl is highly active and selective in the hydrogen transfer reaction from H2O-CO to PhCOCH(2)CHOHCOOH which yields the corresponding gamma-keto acid PhCOCH(2)CH(2)COOH, with concomitant evolution of CO2. An increase of temperature, pressure of carbon monoxide and catalyst concentration have a beneficial effect on the reaction rate, which appears to be of the first order in the substrate and passes through a maximum when varying the concentration of HCl. It is proposed that one important function of HCl is to give rise to chloride PhCOCH(2)CHClCOOH which interacts with a palladium hydride that takes origin from the decarboxylation of a species having a Pd-COOH moiety, which in rum results from the interaction of H2O and CO on the metal center. The yield passes through a maximum on increasing the concentration of H2O, This trend is attributed to the fact that, on one hand, H2O favors the formation of the Pd-COOH species, while, on the other hand, it may compete with other reacting molecules for coordination to the metal center. Moreover, H2O does not favor the formation of the chloride. When employed in relatively high concentration, the catalyst precursor has been recovered as a complex of palladium(0), Pd-3(CO)(3)(PPh(3))(3) or Pd(CO)(PPh(3))(3), the latter in the presence of PPh(3). The reduction to palladium(0) takes place only in the presence of H2O and is likely to occur via the intermediacy of a Pd-COOH species, which after CO2 evolution gives the reduced complex probably via reductive elimination of HCl from the hydride intermediate trans-PdHCl(PPh(3))(2). Moreover, PhCOCH=CHCOOH in combination with HCl (equivalent to PhCOCH(2)CHClCOOH) reacts with Pd(CO)(PPh(3))(3), to give the hydrogenated product PhCOCH(2)CH(2)COOH and PdCl2(PPh(3))(2). On the basis of these results, and knowing that HCl reacts with Pd(CO)(PPh(3))(3) to give the hydride PdHCl(PPh(3))(2), it is proposed that the catalytic cycle proceeds through the following steps: (i) H2O and CO interact with the metal center of the precursor yielding a Pd-COOH species, (ii) this gives off CO2 with formation of a hydride, (iii) this interacts with chloride PhCOCH(2)CHClCOOH to yield the product PhCOCH(2)CH(2)COOH and the palladium(II) precursor back to the catalytic cycle.