Mechanisms of Hydride Abstractions by Quinones

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• 作者：Xingwei Guo ; Hendrik Zipse ; Herbert Mayr
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：October 1, 2014
• 年：2014
• 卷：136
• 期：39
• 页码：13863-13873
• 全文大小：696K
• ISSN：1520-5126

文摘

The kinetics of the hydride abstractions by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) from 13 C鈥揌 hydride donors (acyclic 1,4-dienes, cyclohexa-1,4-dienes, dihydropyridines), tributylstannane, triphenylstannane, and five borane complexes (amine鈥揵oranes, carbene鈥揵oranes) have been studied photometrically in dichloromethane solution at 20 掳C. Analysis of the resulting second-order rate constants by the correlation log k₂(20 掳C) = s_N(E + N) ( J. Am. Chem. Soc. 2001, 123, 9500) showed that the hydride abstractions from the C鈥揌 donors on one side and the Sn鈥揌 and B鈥揌 hydride donors on the other follow separate correlations, indicating different mechanisms for the two reaction series. The interpretation that the C鈥揌 donors transfer hydrogen to the carbonyl oxygen of DDQ while Sn鈥揌 and B鈥揌 hydride donors transfer hydride to a cyano-substituted carbon of DDQ is supported by quantum-chemical intrinsic reaction coordinate calculations and isotope labeling experiments of the reactions of D₈-cyclohexa-1,4-diene, Bu₃SnD, and pyridine路BD₃ with 2,5-dichloro-p-benzoquinone. The second-order rate constants of the reactions of tributylstannane with different quinones correlate linearly with the electrophilicity parameters E of the quinones, which have previously been derived from the reactions of quinones with 蟺-nucleophiles. The fact that the reactions of Bu₃SnH with quinones and benzhydrylium ions are on the same log k₂ vs E (electrophilicity) correlation shows that both reaction series proceed by the same mechanism and illustrates the general significance of the reactivity parameters E, N, and s_N for predicting rates of polar organic reactions.