Regioselectivity in aqueous palladium catalysed hydroxycarbonylation of styrene: a catalytic and mechanistic study
作者:Adriana Ionescu、G?bor Laurenczy、Ola F. Wendt
DOI:10.1039/b607331j
日期:——
Regioselectivity control was studied in palladium catalysed hydroxycarbonylation of styrene in neat water with water-soluble phosphines, mostly trisulfonated triphenylphosphine, TPPTS, but also N-bis(N′,N′-diethyl-2-aminoethyl)-4-aminomethylphenyl-diphenylphosphine, N3P. The factor giving the highest changes in regioselectivity in the TPPTS system, under similar reaction conditions, is the temperature. In the N3P case, only a minor variation in the n/i ratio as a function of temperature is observed. Insitu normal- and high-pressure NMR experiments were performed to obtain further information about the catalytic cycle and the reaction intermediates. Two palladium hydride intermediates, a palladium η3-benzylic complex and both the branched and linear palladium acyl complexes were identified in the HP NMR experiments. The hydroxycarbonylation in water using styrene as a substrate operates using a hydride mechanism for pathways leading to both linear and branched product. Insertion of styrene in the palladium–hydride bond gives an η3-benzyl compound. A high CO pressure gives a kinetic preference for the iso-acyl in the next step. In the TPPTS system, at moderate temperatures, the hydrolysis of the iso-acyl is faster than its conversion to the thermodynamically more stable n-acyl. A low n/i therefore requires high pressures and reasonably low temperatures. The N3P ligand always favours the linear product since isomerisation of the iso-acyl to the n-acyl in this system is fast under all conditions investigated.
研究了钯催化苯乙烯在纯水中与水溶性膦(主要是三硫化三苯基膦,TPPTS)发生羟基羰基化反应的区域选择性控制,也包括 N-双(N′,N′-二乙基-2-氨基乙基)-4-氨基甲基苯基-二苯基膦,N3P。在类似的反应条件下,TPPTS 系统中区域选择性变化最大的因素是温度。在 N3P 的情况下,只观察到 n/i 比率随温度的微小变化。为了进一步了解催化循环和反应中间产物,我们进行了现场常压和高压核磁共振实验。高压核磁共振实验确定了两种钯氢化物中间体、一种钯 η3-苄基络合物以及支链和线性钯酰基络合物。以苯乙烯为底物在水中进行的羟基羰基化反应是通过氢化物机制进行的,这种机制可产生线性和支化产物。苯乙烯插入钯氢键后会产生一种 η3-苄基化合物。在下一步中,高 CO 压力会使异酰基产生动力学偏好。在 TPPTS 体系中,在中等温度下,异酰基的水解快于其向热力学上更稳定的正酰基的转化。因此,低 n/i 需要高压和合理的低温。N3P 配体总是有利于线性产物,因为在所有研究条件下,该体系中异酰基向正酰基的异构化速度都很快。