Manganese-Catalyzed Epoxidations of Alkenes in Bicarbonate Solutions

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• 作者：Benjamin S. Lane ; Matthew Vogt ; Victoria J. DeRose ; and Kevin Burgess
• 刊名：Journal of the American Chemical Society
• 出版年：2002
• 出版时间：October 9, 2002
• 年：2002
• 卷：124
• 期：40
• 页码：11946 - 11954
• 全文大小：405K
• 年卷期：v.124,no.40(October 9, 2002)
• ISSN：1520-5126

文摘

This paper describes a method, discovered and refined by parallel screening, for the epoxidationof alkenes. It uses hydrogen peroxide as the terminal oxidant, is promoted by catalytic amounts (1.0-0.1mol %) of manganese(2+) salts, and must be performed using at least catalytic amounts of bicarbonatebuffer. Peroxymonocarbonate, HCO₄^-, forms in the reaction, but without manganese, minimal epoxidationactivity is observed in the solvents used for this research, that is, DMF and ^tBuOH. More than 30 d-blockand f-block transition metal salts were screened for epoxidation activity under similar conditions, but thebest catalyst found was MnSO₄. EPR studies show that Mn²⁺ is initially consumed in the catalytic reactionbut is regenerated toward the end of the process when presumably the hydrogen peroxide is spent. Avariety of aryl-substituted, cyclic, and trialkyl-substituted alkenes were epoxidized under these conditionsusing 10 equiv of hydrogen peroxide, but monoalkyl-alkenes were not. To improve the substrate scope,and to increase the efficiency of hydrogen peroxide consumption, 68 diverse compounds were screenedto find additives that would enhance the rate of the epoxidation reaction relative to a competingdisproportionation of hydrogen peroxide. Successful additives were 6 mol % sodium acetate in the ^tBuOHsystem and 4 mol % salicylic acid in the DMF system. These additives enhanced the rate of the desiredepoxidation reaction by 2-3 times. Reactions performed in the presence of these additives require lesshydrogen peroxide and shorter reaction times, and they enhance the yields obtained from less reactivealkene substrates. Possible mechanisms for the reaction are discussed.