Construction of pseudo-heterochiral and homochiral di-μ-oxotitanium(Schiff base) dimers and enantioselective epoxidation using aqueous hydrogen peroxide
Construction of pseudo-heterochiral and homochiral di-μ-oxotitanium(Schiff base) dimers and enantioselective epoxidation using aqueous hydrogen peroxide
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DOI：
10.1002/anie.200501318
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发表时间：
2005-01-01
影响因子：
16.6
通讯作者：
Katsuki, T
中科院分区：
文献类型：
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作者：
Matsumoto, K;Sawada, Y;Katsuki, T
光学活性金属配合物催化的不对称合成在过去的半个世纪取得了显著的进展。[1]单金属配合物是目前发展起来的大部分不对称反应的催化剂。特别是在过去的二十年里，已经发现手性金属salen配合物导致多样和优异的不对称催化。[2]手性Salen配体可以由手性二胺和手性和/或适当取代的水杨醛在单一步骤中合成，并与各种金属离子形成络合物。此外，由于存在两个亚甲基碳原子，金属salen络合物在构象上是灵活的，并且它们可以采用三种不同的构型（反式、顺式-α和顺式-β）。金属salen配合物通常采用反式构型，但在双齿配体存在下，它们可以容易地转化为相应的顺式-β配合物。[2，3]因此，已经合成了各种手性金属salen络合物，并用作广泛的不对称反应的催化剂。大多数使用的金属salen络合物是单体的，但是对于某些反应，取决于它们的机理，二聚或低聚金属salen络合物已被证明是比相应的单体络合物更优越的上级催化剂。[4]然而，这种二聚或低聚金属salen络合物作为催化剂的用途受到限制，主要是因为它们的合成涉及费力的路线，除了通过自组装制备的金属salen络合物。因此，可以通过用水处理单体[Ti（salen）]络合物自发制备的二-μ-氧代钛（salen）络合物[5]引起了我们的注意。[6]此外，与通常采用反式构型的单体[Ti（salen）]配合物不同，二-μ-氧代配合物的每个{Ti（salen）}单元均采用顺式-β构型。与反式异构体相反，顺式-β异构体是手性的并且以对映体形式存在（D或L;图1）。因此，对于二-μ-氧代钛（salen）二聚体，存在六种可能的异构体
Remarkable advances in asymmetric synthesis using optically active metal complexes as catalysts have been achieved in the last half century.[1] Monometallic complexes are used as the catalysts in most of the asymmetric reactions developed so far. Particularly in the last two decades, it has been revealed that chiral metallosalen complexes result in diverse and excellent asymmetric catalysis.[2] Chiral salen ligands can be synthesized in a single step from chiral diamine and chiral and/or appropriately substituted salicylaldehydes and form complexes with a variety of metal ions. Moreover, metallosalen complexes are conformationally flexible as a result of the presence of two methylene carbon atoms and they can adopt three different configurations (trans, cis-α, and cis-β). Metallosalen complexes usually adopt trans configurations, but they can be readily transformed in the presence of a bidentate ligand into the corresponding cis-β complexes.[2, 3] Thus, various chiral metallosalen complexes have been synthesized and used as catalysts for a wide range of asymmetric reactions. Most of the metallosalen complexes used are monomeric, but for some reactions, depending on their mechanisms, dimeric or oligomeric metallosalen complexes have proven to be superior catalysts to the corresponding monomeric ones.[4] Nevertheless, the use of such dimeric or oligomeric metallosalen complexes as catalysts has been limited mainly because their synthesis involves laborious routes, except for metallosalen complexes prepared by selfassembly. Thus, di-μ-oxotitanium (salen) complexes that can be prepared spontaneously by treatment of monomeric [Ti (salen)] complexes with water,[5] attracted our attention.[6] Furthermore, in contrast to the usual monomeric [Ti (salen)] complexes that adopt trans configurations, each {Ti (salen)} unit of the di-μ-oxo complexes take a cis-β configuration. In contrast to the trans isomer, the cis-β isomer is chiral and exists in enantiomeric forms (D or L; Figure 1). Thus, there are six possible isomers for the di-μ-oxotitanium (salen) dimer