Unraveling the Wacker Oxidation Mechanisms

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• 作者：John A. Keith ; Robert J. Nielsen ; Jonas Oxgaard ; William A. Goddard ; III
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：October 17, 2007
• 年：2007
• 卷：129
• 期：41
• 页码：12342 - 12343
• 全文大小：110K
• 年卷期：v.129,no.41(October 17, 2007)
• ISSN：1520-5126

文摘

The mechanisms for the aqueous PdCl₂ mediated olefin oxidation reaction (the Wacker process) have been studied with density functional theory, with special emphasis on determining competitive pathways that explain the product distribution's dependence on reaction conditions. Surprisingly, our results indicate that the previously suggested inner-sphere rate-determining step for this process is incompatible with the experimental observations. We describe three key steps, all with barriers between 22.7 and 23.3 kcal/mol. These results, together with literature experimental data, were used to construct a model that explains the observations in the Wacker process. We find that the rate-determining step under low [Cl^-] conditions is not hydroxypalladation as generally believed, but intermolecular isomerization after a lower-energy water-catalyzed internal nucleophilic attack. The pathway under high [Cl^-] leading to anti-addition aldehyde products is only accessible when CuCl₂ is available to selectively stabilize associative chloride exchange. The controversial switch in mechanisms is caused by both this selective stabilization from CuCl₂, and the prerequisite dissociation of Cl^- prior to internal attack. Finally, we suggest that the previously published rate expression for the Wacker process under high [Cl^-] is incomplete and should be replaced with a two-term expression, featuring one term first-order and one term second- (or higher) order in [CuCl₂].