Isotope Effects and the Mechanism of Epoxidation of Cyclohexenone with tert-Butyl Hydroperoxide

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• 作者：Chad F. Christian ; Tetsuya Takeya ; Michael J. Szymanski ; Daniel A. Singleton
• 刊名：Journal of Organic Chemistry
• 出版年：2007
• 出版时间：August 3, 2007
• 年：2007
• 卷：72
• 期：16
• 页码：6183 - 6189
• 全文大小：172K
• 年卷期：v.72,no.16(August 3, 2007)
• ISSN：1520-6904

文摘

The mechanism of the epoxidation of 2-cyclohexen-1-one with tert-butyl hydroperoxide mediated byDBU was studied by a combination of experimental kinetic isotope effects (KIEs) and theoreticalcalculations. A large ¹²C/¹³C (7be10001.gif">/7be10002.gif">) isotope effect of 1.032 was observed at the C₃ () position ofcyclohexenone, while a much smaller ¹²C/¹³C isotope effect of 1.010 was observed at the C₂ () position.Qualitatively, these results are indicative of nucleophilic addition to the enone being the rate-limitingstep. Theoretical calculations support this interpretation. Transition structures for the addition step leadto predicted isotope effects that approximate the experimental values, while the predicted isotope effectsfor the ring-closure step are not consistent with the experimental values. The calculations correctly favora rate-limiting addition step but suggest that the barriers for the addition and ring-closure steps are crudelysimilar in energy. The stereochemistry of these epoxidations is predicted to be governed by a preferencefor an initial axial addition, and the role of this preference in experimental diastereoselectivity observationsis discussed.