Manganese-catalyzed epoxidations of alkenes in bicarbonate solutions
Manganese-catalyzed epoxidations of alkenes in bicarbonate solutions
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DOI：
10.1021/ja025956j
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发表时间：
2002-10-09
影响因子：
15
通讯作者：
Burgess, K
中科院分区：
文献类型：
--
作者：
Lane, BS;Vogt, M;Burgess, K
本文描述了一种通过平行筛选发现和改进的烯烃环氧化方法。它使用过氧化氢作为末端氧化剂，通过催化量 (1.0-0.1 mol %) 的锰 (2+) 盐促进，并且必须使用至少催化量的碳酸氢盐缓冲液来进行。反应中形成过氧单碳酸酯 (HCO4-)，但在没有锰的情况下，在本研究使用的溶剂（即 DMF 和 (BuOH)-Bu-t）中观察到极小的环氧化活性。在类似条件下筛选了 30 多种 d 区和 f 区过渡金属盐的环氧化活性，但发现最好的催化剂是 MnSO4。 EPR 研究表明，Mn2+ 最初在催化反应中被消耗，但在过程结束时可能会再生，此时过氧化氢可能被耗尽。在这些条件下，使用 10 当量的过氧化氢，各种芳基取代的、环状的和三烷基取代的烯烃发生环氧化，但单烷基烯烃则不然。为了改善底物范围并提高过氧化氢的消耗效率，筛选了 68 种不同的化合物，以寻找能够提高环氧化反应相对于过氧化氢竞争性歧化反应速率的添加剂。成功的添加剂是 (BuOH)-Bu-i 系统中的 6 mol% 乙酸钠和 DMF 系统中的 4 mol% 水杨酸。这些添加剂将所需环氧化反应的速率提高了 2-3 倍。在这些添加剂存在下进行的反应需要更少的过氧化氢和更短的反应时间，并且它们提高了从反应性较低的烯烃底物获得的产率。讨论了该反应的可能机制。
This paper describes a method, discovered and refined by parallel screening, for the epoxidation of alkenes. It uses hydrogen peroxide as the terminal oxidant, is promoted by catalytic amounts (1.0-0.1 mol %) of manganese(2+) salts, and must be performed using at least catalytic amounts of bicarbonate buffer. Peroxymonocarbonate, HCO4-, forms in the reaction, but without manganese, minimal epoxidation activity is observed in the solvents used for this research, that is, DMF and (BuOH)-Bu-t. More than 30 d-block and f-block transition metal salts were screened for epoxidation activity under similar conditions, but the best catalyst found was MnSO4. EPR studies show that Mn2+ is initially consumed in the catalytic reaction but is regenerated toward the end of the process when presumably the hydrogen peroxide is spent, A variety of aryl-substituted, cyclic, and trialkyl-substituted alkenes were epoxidized under these conditions using 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 screened to find additives that would enhance the rate of the epoxidation reaction relative to a competing disproportionation of hydrogen peroxide. Successful additives were 6 mol % sodium acetate in the (BuOH)-Bu-i system and 4 mol % salicylic acid in the DMF system. These additives enhanced the rate of the desired epoxidation reaction by 2-3 times, Reactions performed in the presence of these additives require less hydrogen peroxide and shorter reaction times, and they enhance the yields obtained from less reactive alkene substrates. Possible mechanisms for the reaction are discussed.