Buonerba Antonio, Noschese Annarita, Grassi Alfonso
Dipartimento di Chimica e Biologia and NANOMATES Research Centre for NANOMAterials and nanoTEchnology at Salerno University, Università degli Studi di Salerno, Via Giovanni Paolo II, 84084 Fisciano (SA) (Italy), Fax: (+39) 089969824.
Chemistry. 2014 Apr 25;20(18):5478-86. doi: 10.1002/chem.201303880. Epub 2014 Mar 18.
The selective aerobic oxidation of cinnamyl alcohol to cinnamaldehyde, as well as direct oxidative esterification of this alcohol with primary and secondary aliphatic alcohols, were achieved with high chemoselectivity by using gold nanoparticles supported in a nanoporous semicrystalline multi-block copolymer matrix, which consisted of syndiotactic polystyrene-co-cis-1,4-polybutadiene. The cascade reaction that leads to the alkyl cinnamates occurs through two oxidation steps: the selective oxidation of cinnamyl alcohol to cinnamaldehyde, followed by oxidation of the hemiacetal that results from the base-catalysed reaction of cinnamaldehyde with an aliphatic alcohol. The rate constants for the two steps were evaluated in the temperature range 10-45 °C. The cinnamyl alcohol oxidation is faster than the oxidative esterification of cinnamaldehyde with methanol, ethanol, 2-propanol, 1-butanol, 1-hexanol or 1-octanol. The rate constants of the latter reaction are pseudo-zero order with respect to the aliphatic alcohol and decrease as the bulkiness of the alcohol is increased. The activation energy (Ea) for the two oxidation steps was calculated for esterification of cinnamyl alcohol with 1-butanol (Ea = 57.8±11.5 and 62.7±16.7 kJ mol(-1) for the first and second step, respectively). The oxidative esterification of cinnamyl alcohol with 2-phenylethanol follows pseudo-first-order kinetics with respect to 2-phenylethanol and is faster than observed for other alcohols because of fast diffusion of the aromatic alcohol in the crystalline phase of the support. The kinetic investigation allowed us to assess the role of the polymer support in the determination of both high activity and selectivity in the title reaction.
通过使用负载在纳米多孔半结晶多嵌段共聚物基质中的金纳米颗粒，实现了肉桂醇选择性有氧氧化为肉桂醛，以及该醇与伯醇和仲醇的直接氧化酯化反应，该基质由间规聚苯乙烯 - 顺 - 1,4 - 聚丁二烯组成，具有高化学选择性。导致肉桂酸烷基酯的级联反应通过两个氧化步骤发生：肉桂醇选择性氧化为肉桂醛，随后是肉桂醛与脂肪醇碱催化反应生成的半缩醛的氧化。在10 - 45 °C温度范围内评估了这两个步骤的速率常数。肉桂醇氧化比肉桂醛与甲醇、乙醇、2 - 丙醇、1 - 丁醇、1 - 己醇或1 - 辛醇的氧化酯化反应更快。后一反应的速率常数相对于脂肪醇为准零级，并且随着醇的体积增大而降低。计算了肉桂醇与1 - 丁醇酯化的两个氧化步骤的活化能（Ea）（第一步和第二步的Ea分别为57.8±11.5和62.7±16.7 kJ mol(-1)）。肉桂醇与2 - 苯乙醇的氧化酯化反应相对于2 - 苯乙醇遵循准一级动力学，并且由于芳族醇在载体结晶相中的快速扩散，比其他醇的反应更快。动力学研究使我们能够评估聚合物载体在确定标题反应中的高活性和选择性方面的作用。
