Zhang Rui, Yu Wing-Yiu, Sun Hong-Zhe, Liu Wei-Sheng, Che Chi-Ming
Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong.
Chemistry. 2002 Jun 3;8(11):2495-507. doi: 10.1002/1521-3765(20020603)8:11<2495::AID-CHEM2495>3.0.CO;2-G.
The mechanism of stoichiometric enantioselective alkene epoxidations by the D4- and D2-symmetric homochiral trans-dioxoruthenium(VI) porphyrins, [RuVI(D4-Por*)O2] (1) and [RuVI(D2-Por*)O2] (2a), in the presence of pyrazole (Hpz) was studied by UV/Vis spectrophotometry and analysis of the organic products. The enantioselectivity of styrene oxidations is more susceptible to steric effects than to substituent electronic effects. Up to 72% ee was achieved for epoxidation of 3-substituted and cis-disubstituted styrenes by employing 1 as the oxidant, whereas entantioselectivities of only 20-40% were obtained in the reactions with 2-substituted and trans-disubstituted styrenes. Complex 2a oxidized 2-substituted styrenes to their epoxides in up to 88% ee. Its reactions with trans-alkenes are more enantioselective (67% ee) than with the cis-alkenes (40% ee). Based on a two-dimensional NOESY NMR study, 2a was found to adopt a more open conformation in benzene than in dichloromethane, which explains the observed solvent-dependent enantioselectivity of its reactions with alkenes. The oxidation of aromatic alkenes by the chiral dioxoruthenium(VI) porphyrins proceeds through the rate-limiting formation of a benzylic radical intermediate; the observed enantioselectivity (eeobs) depends on both the facial selectivity of the first C-O bond formation step and the diastereoselectivity of the subsequent epoxide ring closure. To account for the observed facial selection, "side-on" and "top-on" approach transition state models are examined (see: B. D. Brandes, E. N. Jacobsen, Tetrahedron Lett. 1995, 36, 5123).
通过紫外可见分光光度法和有机产物分析，研究了D4和D2对称的手性反式二氧钌（VI）卟啉[RuVI(D4-Por*)O2]（1）和[RuVI(D2-Por*)O2]（2a）在吡唑（Hpz）存在下进行化学计量对映选择性烯烃环氧化的机理。苯乙烯氧化反应的对映选择性受空间效应的影响大于取代基电子效应。以1作为氧化剂时，3-取代和顺式二取代苯乙烯环氧化反应的对映体过量值（ee）可达72%，而在与2-取代和反式二取代苯乙烯的反应中，对映选择性仅为20-40%。配合物2a将2-取代苯乙烯氧化为环氧化物，对映体过量值高达88%。它与反式烯烃的反应比对顺式烯烃的反应具有更高的对映选择性（ee为67%）。基于二维NOESY核磁共振研究，发现2a在苯中比在二氯甲烷中具有更开放的构象，这解释了观察到的其与烯烃反应的溶剂依赖性对映选择性。手性二氧钌（VI）卟啉对芳香烯烃的氧化反应通过苄基自由基中间体的限速形成过程进行；观察到的对映选择性（eeobs）取决于第一步C-O键形成步骤的面选择性和随后环氧化物闭环的非对映选择性。为了解释观察到的面选择性，研究了“侧基进攻”和“顶部进攻”的过渡态模型（见：B.D.布兰德斯、E.N.雅各布森，《四面体快报》，1995年，36卷，5123页）。
