Alkyl Substituted Beta-Keto Acids: Molecular Structure and Decarboxylation Kinetics in Aqueous Solution and on the Surface of Metal Oxides
Alkyl Substituted Beta-Keto Acids: Molecular Structure and Decarboxylation Kinetics in Aqueous Solution and on the Surface of Metal Oxides
复制标题
烷基取代的β-酮酸：水溶液和金属氧化物表面的分子结构及脱羧化动力学
DOI：
10.1021/acs.jpcc.0c10797
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发表时间：
2021-02-05
影响因子：
3.7
通讯作者：
Brennessel, William W.
中科院分区：
文献类型：
--
作者：
Ignatchenko, Alexey, V;Springer, Morgan E.;Brennessel, William W.
通过有机合成制备了四种β-酮酸，它们被认为是金属氧化物催化的乙酸和异丁酸混合物脱羧交叉酮化反应机理中的中间体，并以结晶状态分离。通过单晶X射线衍射分析了结构。一阶速率常数已被测量和比较的温度范围23-53 ℃的β-酮酸在溶液中的脱羧，以及吸附在金属氧化物催化剂，单斜ZrO 2，和TiO 2，未掺杂和掺杂KOH的表面上。从低到高的速率常数排列的四种酸在溶液中的反应性与由在a位置的烷基的存在引起的C-C键的长度增加相关。首次研究了脱羧酮化反应机理中β-酮酸中间体在金属氧化物催化剂表面的行为。除脱羧反应为主要反应方向外，反缩合反应也为次要反应途径，外推至工业规模操作温度的脱羧反应速率高于催化脱羧酮化反应的总体速率，这表明缩合反应是最慢的步骤。尽管如此，脱羧是两种酸的混合物的脱羧交叉酮化的反应机理的动力学上重要的步骤。脱羧速率常数对酮产物的对称性和所用催化剂的类型的显著依赖性支持了这一结论。也就是说，β-酮酸导致对称酮分解更快的ZrO 2催化剂，而脱羧的其他两种酸导致不对称酮是更快的KOH-TiO 2。这一结果与先前报道的交叉选择性趋势一致，根据该趋势，ZrO 2有利于形成对称酮，而KOH-TiO 2有利于形成不对称酮。对催化剂选择的交叉选择性的这种不寻常的敏感性提出的解释可能涉及通过表面上的一对相邻羧酸酯之间的α质子交换的无规化过程的熵增加，其中一个是烯醇化的。
Four beta-keto acids proposed as intermediates in the mechanism of a metal oxide catalyzed decarboxylative cross-ketonization reaction from a mixture of acetic and isobutyric acids have been prepared by organic synthesis and isolated in a crystalline state. Structures have been analyzed by single-crystal X-ray diffraction. First order rate constants have been measured and compared at the temperature range 23-53 degrees C for the decarboxylation of beta-keto acids in solution as well as adsorbed on the surface of metal oxide catalysts, monoclinic ZrO2, and anatase TiO2, undoped and doped with KOH. The reactivity of the four acids in solution arranged from a low to high rate constant correlates with the increasing length of the C-C bond caused by the presence of alkyl groups at the a position. It is for the first time that the behavior of the beta-keto acid intermediate in the decarboxylative ketonization mechanism has been studied on the surface of metal oxide catalysts. In addition to decarboxylation as the major direction, the retro-condensation reaction is also observed as a minor path. The decarboxylation rate extrapolated to industrial scale operating temperatures is above the global rate of the catalytic decarboxylative ketonization, which points to the condensation as the slowest step. Still, decarboxylation is a kinetically significant step of the reaction mechanism for the decarboxylative cross-ketonization of a mixture of two acids. This conclusion is supported by a remarkable dependence of the decarboxylation rate constant on the symmetry of the ketone product and the type of the catalyst used. Namely, beta-keto acids leading to symmetrical ketones decompose faster with ZrO2 catalysts while decarboxylation of the other two acids leading to unsymmetrical ketones is faster with KOH-TiO2. This result is in agreement with the previously reported trend of the cross-selectivity according to which ZrO2 favors the formation of symmetrical ketones, whereas KOH-TiO2 favors the unsymmetrical ketone. A proposed explanation for this unusual sensitivity of the cross-selectivity to the catalyst choice may involve the entropy increase through the randomization process by the alpha proton exchange between a pair of neighboring carboxylates on the surface, one of which is enolized.