A novel dinuclear chiral niobium complex for Lewis acid catalyzed enantioselective reactions: design of a tridentate ligand and elucidation of the catalyst structure.
A novel dinuclear chiral niobium complex for Lewis acid catalyzed enantioselective reactions: design of a tridentate ligand and elucidation of the catalyst structure.
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用于路易斯酸催化对映选择性反应的新型双核手性铌配合物：三齿配体的设计和催化剂结构的阐明。
DOI：
10.1002/anie.200462204
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发表时间：
2005
影响因子：
--
通讯作者：
Y. Yamashita
中科院分区：
文献类型：
--
作者：
S. Kobayashi;K. Arai;H. Shimizu;Yoichi Ihori;H. Ishitani;Y. Yamashita
手性催化剂的开发是有机合成领域的重要课题之一。[1]虽然已经开发了各种类型的催化剂，但开发适用于新的对映选择性反应的新型催化剂，特别是基于不寻常金属和新型配体的催化剂仍然非常重要。本文描述了第一种高对映选择性的铌（V）刘易斯酸催化剂。铌在第5族中位于钒以下，预计具有高刘易斯酸性[2]，只有少数报道手性铌催化剂已用于不对称Diels-Alder反应和氧化。[3，4]这种情况与由相邻的钛[5]和锆的第4族金属络合物催化的许多不对称转化形成显著对比。[6]为了制备用于活化醛或亚胺的有效手性五价铌（V）催化剂，我们设计了三齿配体4a-d（参见等式（1））[7]，其根据常规方法合成）。[8]将配体4a-d与铌醇盐（Nb（OR）5）组合，并用作亚胺1与硅烯醇化物2的曼尼希型反应的催化剂[Eq.①]。[9]对于1a与2a的反应，检查了几个反应参数，结果总结在表1中。发现N-甲基咪唑（NMI）作为附加配体是有效的。[10]在筛选的手性配体中，4c给出最好的对映选择性（表1，条目3）。在甲苯-CH 2Cl 2（1：1）混合溶剂体系（表1，条目6）和在3-或4-分子筛（MS 3或MS 4;表1，条目7和8）存在下，选择性进一步提高。我们还研究了几种铌醇盐。当Nb（OMe）5与配体4c组合时，以86%产率和99%ee获得所需的曼尼希型加合物（表1，条目10）。值得注意的是，使用这种新型手性铌催化剂已经实现了几乎完全的选择性。
Development of chiral catalysts for asymmetric reactions is one of the most important tasks in organic synthesis.[1] While various types of catalysts have been developed, exploitation of novel catalysts, especially those based on unusual metals and novel ligands, applicable to new enantioselective reactions, is still very important. Herein we describe the first highly enantioselective niobium (v) Lewis acid catalyst. Niobium lies below vanadium in Group 5 and is expected to have high Lewis acidity,[2] there are only a few reports of chiral niobium catalysts, which have been used in asymmetric Diels–Alder reactions and oxidations.[3, 4] This situation is in remarkable contrast to many asymmetric transformation catalyzed by the neighboring Group 4 metal complexes of titanium [5] and zirconium.[6] To prepare an efficient chiral pentavalent niobium (v) catalyst for activation of aldehydes or imines, we designed tridentate ligands 4a–d (see Equation (1)),[7] which were synthesized according to conventional methods).[8]Ligands 4a–d were combined with niobium alkoxides (Nb (OR) 5), and were employed as catalysts for the Mannichtype reaction of imines 1 with silicon enolates 2 [Eq.(1)].[9] For the reaction of 1a with 2a, several reaction parameters were examined, and the results are summarized in Table1. It was found that N-methylimidazole (NMI) was effective as an additional ligand.[10] Among the chiral ligands screened, 4c gave the best enantioselectivity (Table1, entry 3). The selectivity was further improved in a toluene–CH2Cl2 (1: 1) mixed solvent system (Table 1, entry 6) and in the presence of 3-or 4-molecular sieves (MS3 or MS 4; Table 1, entries 7 and 8). We also examined several niobium alkoxides. When Nb (OMe) 5 was combined with ligand 4c, the desired Mannich-type adduct was obtained in 86% yield with 99% ee (Table 1, entry 10). It is noteworthy that almost complete selectivity has been achieved using this novel chiral niobium catalyst.