Mechanism of palladium-catalyzed decarboxylative cross-coupling between cyanoacetate salts and aryl halides |
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Authors: | YuanYe Jiang Yao Fu Lei Liu |
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Affiliation: | 1. Department of Chemistry, University of Science and Technology of China, Hefei, 230026, China 2. Department of Chemistry, Tsinghua University, Beijing, 100084, China
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Abstract: | Recently we reported Pd-catalyzed decarboxylative cross-coupling of cyanoacetate salts with aryl halides and triflates. This reaction shows good functional group tolerance and is useful for the synthesis of ??-aryl nitriles. To elucidate the mechanism for this reaction, we now carry out a density functional theory study on the cross-coupling of potassium cyanoacetate with bromobenzene. Our results show that the decarboxylation transition state involving the interaction of Pd with the ??-carbon atom has a very high energy barrier of +34.5 kcal/mol and therefore, must be excluded. Decarboxylation of free ion (or tight-ion-pair) also causes a high energy increase and should be ruled out. Thus the most favored decarboxylation mechanism corresponds to a transition state in which Pd interacts with the cyano nitrogen. The energy profile of the whole catalytic cycle shows that decarboxylation is the rate-determining step. The total energy barrier is +27.5 kcal/mol, which is comprised of two parts, i.e. the energy barrier for decarboxylation and the energy cost for transmetallation. |
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