Imandi Venkataramana, Nair Nisanth N
Department of Chemistry, Indian Institute of Technology Kanpur , Kanpur 208016, India.
J Phys Chem B. 2015 Aug 27;119(34):11176-83. doi: 10.1021/acs.jpcb.5b03099. Epub 2015 Apr 30.
The major controversy regarding the mechanism of the Wacker oxidation of alkenes at low [Cl(-)] concerns the mechanism of nucleophilic attack of a solvent water molecule on alkene. Most of the recent mechanistic studies on the Wacker oxidation of ethene have reported that nucleophilic attack occurs by an outer sphere mechanism and not by an inner sphere mechanism. One of the crucial experimental findings in support of the inner sphere mechanism is that isotope scrambling does not take place when deuterated ally alcohol was oxidized under the standard Wacker conditions. In this work, we try to explain these experimental results in the framework of the outer sphere mechanism. We simulated the Wacker oxidation of allyl alcohol using ab initio molecular dynamics (AIMD) techniques in order to probe the detailed mechanism, free-energy profiles, and the rate-determining step. Our simulations show that the mechanism of allyl alcohol oxidation follows outer sphere hydroxypalladation, and the rate-determining step involves Cl(-) ligand isomerization, contradicting the conclusions from the isotope scrambling experiments. However, by carrying out microkinetic modeling based on the free-energy barriers of the elementary steps obtained from our AIMD simulations, we also observe no isotope scrambling. This led us to determine the genesis of the observed absence of isotope scrambling. Most importantly, here we demonstrate that the absence of isotope scrambling is in fact consistent with the outer sphere hydroxypalladation and cannot disprove it.
关于在低[Cl⁻]浓度下烯烃的瓦克氧化反应机理的主要争议在于溶剂水分子对烯烃亲核进攻的机理。最近大多数关于乙烯瓦克氧化反应的机理研究报告称，亲核进攻是通过外层机理而非内层机理发生的。支持内层机理的关键实验发现之一是，在标准瓦克条件下氘代烯丙醇被氧化时不会发生同位素重排。在这项工作中，我们试图在外层机理的框架内解释这些实验结果。我们使用从头算分子动力学（AIMD）技术模拟了烯丙醇的瓦克氧化反应，以探究详细的机理、自由能剖面图以及速率决定步骤。我们的模拟表明，烯丙醇氧化的机理遵循外层羟基钯化，速率决定步骤涉及Cl⁻配体异构化，这与同位素重排实验的结论相矛盾。然而，通过基于我们的AIMD模拟获得的基元步骤的自由能垒进行微观动力学建模，我们也未观察到同位素重排。这使我们确定了所观察到的同位素重排缺失的根源。最重要的是，我们在此证明，同位素重排的缺失实际上与外层羟基钯化是一致的，并且不能反驳它。
