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Mechanistic Insight into the Rhodium-Catalyzed O–H Insertion Reaction: A DFT Study

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posted on 27.05.2014, 00:00 by Zhi-Zhong Xie, Wen-Juan Liao, Jun Cao, Li-Ping Guo, Francis Verpoort, Weihai Fang
A DFT study on the reaction of diazoacetate with primary allyl alcohol mediated by dirhodium catalyst has been carried out in detail. Calculations indicate that the major O–H insertion product can be obtained via either a [1,3]-proton shift of the free enol or a [1,2]-proton shift of the free oxonium ylide, which are regulated by the orientation of the ester group. In the case of a [1,3]-proton shift the reaction begins with the nucleophilic attack of the alcohol at the carbenoid, generating a metal-associated oxonium ylide followed by a [1,4]-proton shift to the adjacent carbonyl oxygen atom of the ester group, resulting in a metal-associated enol. Subsequently, its decomposition liberates a free enol intermediate. The whole process requires an overall barrier of 4.2 kcal/mol and is exergonic by 6.4 kcal/mol. The [1,3]-proton shift of the enol also readily provides the final O–H insertion product, which has a barrier of 11.7 kcal/mol using a three-alcohol cluster as catalyst. For the free oxonium ylide pathway, formation of an alternative metal-associated oxonium ylide is also straightforward, having an overall barrier of 4.5 kcal/mol. In the presence of extra alcohol molecules, the decomposition of the metal-associated oxonium ylide can generate an alcohol-stabilized free oxonium ylide (endergonic by only 4.1 kcal/mol). Afterward, it undergoes a [1,2]-proton shift, resulting in the O–H insertion product, which requires an energy barrier of 4.7 kcal/mol. In comparison, the competitive [2,3]-sigmatropic rearrangement for the metal-associated oxonium ylides is not sensitive to the orientation of the ester, which has a similar activation free energy around 14.0 kcal/mol. Accordingly, it is always disfavored over the O–H insertion, which kinetically agrees well with the experimental observations, in which traces of [2,3]-sigmatropic rearrangement product were obtained for the primary allyl alcohol.