Mechanistic studies of a reaction promoted by the [YLi3[tris(binaphthoxide)]] complex: are three 1,1'-bi-2-naphthol units in a rare-earth-alkali-metal heterobimetallic complex necessary?
Mechanistic studies of a reaction promoted by the [YLi3[tris(binaphthoxide)]] complex: are three 1,1'-bi-2-naphthol units in a rare-earth-alkali-metal heterobimetallic complex necessary?
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DOI：
10.1002/anie.200454202
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发表时间：
2004-08
影响因子：
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通讯作者：
Noriyuki Yamagiwa;S. Matsunaga;M. Shibasaki
中科院分区：
文献类型：
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作者：
Noriyuki Yamagiwa;S. Matsunaga;M. Shibasaki
自20世纪90年代初以来，[1a]我们报道了一系列能够实现各种催化不对称反应的稀土-碱-金属异双金属配合物。[1]这些配合物的结构经X射线晶体分析、质谱学和核磁共振谱确定，由一个稀土金属(RE)、三个1，1‘-2-双萘酚(联醇)和三个碱金属(M)部分组成(图1)。揭示了含水和无水异双金属配合物在固相结构上的差异。[2]他们还报道了关于异双金属配合物溶液相结构的初步结果。最近，萨尔瓦多和他的同事报告了Yb碱金属异双金属络合物在溶液相中的详细光谱分析，提出了Yb/M/binol=1：3：3结构是否真的是一个活性物种或只是预催化剂的问题(图2)。[3]他们报告Yb/K/binol=1：3：3的异双金属络合物中的二醇是不稳定的(交换光谱实验证实)，并提出只要使用Yb金属，活性物种可以通过一个二醇-Li2单元的解离产生。此外，在某些不对称反应中，以RE/binol/M为1：3：3的比例制备的催化剂效果最好。Yb/K/binol=1：1：3在硝基Mannich反应中效果最好，[4]，SC/Li/binol=1：1：2在Strecker类型反应中效果最好。对稀土-碱-金属异双金属络合物活性物种组成的机理研究将有助于双金属多功能不对称催化剂的设计和在未来研究中的应用。尽管最近Apinall等人[2]和萨尔瓦多及其同事[3]对异双金属配合物进行了详细的光谱分析，揭示了许多新的有趣的性质，但它们在不对称反应催化循环中的行为还没有得到广泛的研究。为了阐明有争议的活性物种的结构，基于实际催化不对称反应的研究以及对异双金属络合物的光谱分析是必不可少的。在这里，我们报告了我们在催化不对称反应机理研究的基础上所做的工作，包括各种动力学研究和核磁共振光谱分析。对[YLi3{tris(Binphthoxide)}]配合物(图1，Li3[Y(Binol)3]，YLB，1)和YLB(1)催化的O-甲基羟胺的不对称1，4-加成反应的机理研究表明，1，4-加成反应的活性物种需要YLB中的所有三个二元醇单元，尽管在反应条件(±208C)下YLB络合物很容易发生配体交换。我们选择1催化的不对称1，4-O-甲基羟胺(3)加成作为机理研究的目标(方案1)。报道了YLb的固相和溶液相结构的详细光谱数据(1)，[2]这个反应是
Since the early 1990s,[1a] we have reported series of rare-earth–alkali-metal heterobimetallic complexes that enable various catalytic asymmetric reactions.[1] These complexes, whose structures were determined by X-ray crystallographic analysis, mass spectrometry, and NMR spectroscopy, consist of one rare-earth metal (RE), three 1, 1’-2-bi-naphtholate (binol), and three alkali metal (M) parts (Figure1).[1b, c] Subsequently, independent studies by Aspinall etal. revealed differences in the aqua and anhydrous heterobimetallic complexes in the solid-phase structures.[2] They also reported preliminary results on the solution-phase structure of heterobimetallic complexes. More recently, Salvadori and co-workers reported a detailed spectroscopic analysis of Yb–alkalimetal heterobimetallic complexes in solution phase, raising the question as to whether the Yb/M/binol= 1: 3: 3 structure is really an active species or just a precatalyst (Figure 2).[3] They reported that binol in the Yb/K/binol= 1: 3: 3 heterobimetallic complex is labile (confirmed by an exchange spectroscopy experiment) and proposed that the active species could be generated by dissociation of one binol–Li2 unit, as long as Yb metal was used. Furthermore, the best results in some asymmetric reactions were observed with catalysts prepared from RE/binol/M in a ratio different to 1: 3: 3. Yb/K/binol= 1: 1: 3 worked best in a nitro-Mannich-type reaction,[4] and Sc/Li/binol= 1: 1: 2 in a Strecker-type reaction.[5] The active species in these reactions has not yet been determined. Mechanistic studies to clarify the composition of the active species of the rare-earth–alkali-metal heterobimetallic complex would facilitate the design and application of bimetallic multifunctional asymmetric catalysis in future research. Although recent detailed spectroscopic analysis of the heterobimetallic complexes by Aspinall et al.[2] and Salvadori and co-workers [3] revealed various new and interesting properties, their behavior during catalytic cycles of asymmetric reactions have not been studied extensively. To elucidate the controversial structure of the active species, investigations based on actual catalytic asymmetric reactions as well as spectroscopic analysis of the heterobimetallic complex are essential. Herein, we report our efforts to determine the active species on the basis of mechanistic studies of a catalytic asymmetric reaction, including various kinetics studies and NMR spectroscopic analysis. Mechanistic studies of the [YLi3 {tris (binaphthoxide)}] complex (Figure 1, Li3 [Y (binol) 3], YLB, 1) and asymmetric 1, 4-addition reaction of O-methylhydroxylamine catalyzed by YLB (1) suggested that the active species of the 1, 4-addition reaction requires all three binol units in YLB, although ligand exchange of the YLB complex occurs easily under the reaction conditions (À208C).We selected an asymmetric 1, 4-addition of O-methylhydroxylamine (3) catalyzed by 1 as a target for the mechanistic studies (Scheme 1).[6] Because Aspinall et al. reported detailed spectroscopic data for both the solid-phase and solution-phase structures of YLB (1),[2] this reaction was most