Mechanistic Insights into the Enantioselective Epoxidation of Olefins by Bioinspired Manganese Complexes: Role of Carboxylic Acid and Nature of Active Oxidant
Mechanistic Insights into the Enantioselective Epoxidation of Olefins by Bioinspired Manganese Complexes: Role of Carboxylic Acid and Nature of Active Oxidant
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仿生锰配合物对烯烃对映选择性环氧化的机理见解：羧酸的作用和活性氧化剂的性质
DOI：
10.1021/acscatal.8b00874
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发表时间：
2018-05-01
期刊：
影响因子：
12.9
通讯作者：
Sun, Wei
中科院分区：
文献类型：
--
作者：
Du, Junyi;Miao, Chengxia;Sun, Wei
具有非卟啉四齿N4配体的生物启发的锰和铁络合物在过氧化氢（H2O2）的不对称氧化反应中是高效的催化剂，其中羧酸被用作必要的添加剂以提高产物产率和立体选择性、区域选择性和对映选择性。金属催化剂应该具有两个氧化剂的顺式结合位点（例如，H2O2）和羧酸，以通过"羧酸辅助"机制生成作为活性氧化剂的高价金属-氧代物质。本论文以非血红素锰催化剂（如1个四齿N_4配体（L-N_4）和2个五齿N_5配体（L-N_5））为催化剂，以H_2O_2、烷基过氧化氢等氧化剂为氧化剂，研究了羧酸在锰配合物催化烯烃不对称环氧化反应中的作用和反应中间体的性质，和碘酰苯（PhIO）。正如预期的那样，具有两个顺式结合位点的1是一个有效的催化剂，无论在羧酸的存在下的氧化剂。相比之下，2只有一个结合位点是不是一个有效的催化剂在H2O2和烷基过氧化氢的反应，即使在羧酸的存在下。然而，出乎意料地，2在羧酸存在下通过PhIO的烯烃的不对称环氧化中被证明是有效的催化剂。后者的结果表明，锰配合物，它不能结合羧酸作为辅助配体，可以提供一个高的对映体选择性，可能是通过第二球的羧酸和推定的锰氧氧化剂的氧代基团之间的配位作用。我们还提供了间接证据支持，高价Mn（V）-氧物种是催化不对称环氧化反应中的活性氧化剂。
Bioinspired manganese and iron complexes bearing nonporphyrinic tetradentate N4 ligands are highly efficient catalysts in asymmetric oxidation reactions by hydrogen peroxide (H2O2), in which carboxylic acid is employed as an essential additive to improve product yields and stereo-, regio-, and enantioselectivities. The metal catalysts should possess two cis-binding sites for oxidant (e.g., H2O2) and carboxylic acid to generate high-valent metal-oxo species as active oxidants via a "carboxylic acid-assisted" mechanism. In the present study, we have investigated the role(s) of carboxylic acid and the nature of reactive intermediate(s) in the manganese complex-catalyzed enantioselective epoxidation of olefins, by employing non-heme manganese catalysts, such as 1 bearing a tetradentate N4 ligand (L-N4) and 2 bearing a pentadentate N5 ligand (L-N5), and using various oxidants, such as H2O2, alkyl hydroperoxides, and iodosylbenzene (PhIO). As expected, 1 possessing two cis-binding sites is an effective catalyst irrespective of the oxidants in the presence of carboxylic acid. In contrast, 2 possessing only one binding site is not an effective catalyst in the reactions of H2O2 and alkyl hydroperoxides even in the presence of carboxylic acid. However, unexpectedly, 2 turns out to be an effective catalyst in the asymmetric epoxidation of olefins by PhIO in the presence of carboxylic acid. The latter result indicates that a manganese complex, which cannot bind carboxylic acid as an auxiliary ligand, can afford a high enantioselectivity, probably through a second-sphere coordination interaction between carboxylic acid and the oxo group of a presumed manganese-oxo oxidant. We have also provided indirect evidence supporting that a high-valent Mn(V)-oxo species is the active oxidant in the catalytic asymmetric epoxidation reactions.