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首页> 外文期刊>Chemistry: A European journal >Asymmetric allylic substitution catalyzed by C-1-symmetrical complexes of molybdenum: Structural requirements of the ligand and the stereochemical course of the reaction
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Asymmetric allylic substitution catalyzed by C-1-symmetrical complexes of molybdenum: Structural requirements of the ligand and the stereochemical course of the reaction

机译：钼的C-1-对称配合物催化的不对称烯丙基取代：配体的结构要求和反应的立体化学过程

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Application of new chiral ligands (R)-(-)-12a and (S)-(+)-12c (VALDY), derived from amino acids, to the title reaction, involving cinnamyl (linear) and isocinnamyl (branched) type substrates (4 and 5 -> 6), led to excellent regio- and enantioselectivities (> 30:1, <= 98% ee), showing that ligands with a single chiral center are capable of high asymmetric induction. The structural requirements of the ligand and the mechanism are discussed. The application of single enantiomers of deuterium-labeled substrates (both linear 38c and branched 37c) and analysis of the products (41-43) by H-2{H-1} NMR spectroscopy in a chiral liquid crystal matrix allowed the stereochemical pathways of the reaction to be distinguished. With ligand (S)-(+)-12c, the matched enantiomer of branched substrate was found to be (S)-5, which was converted into (R)-6 with very high regio- and stereoselectivity via a process that involves net retention of stereochemistry. The mismatched enantiomer of the branched substrate was found to be (R)-5, which was also converted into (R)-6, that is, with apparent net inversion, but at a lower rate and with lower overall enantioselectivity. This latter feature, which may be termed a "memory effect", reduced the global enantioselectivity in the reaction of the racemic substrate (+/-)-5. The stereochemical pathway of the mismatched manifold has been shown also to be one of net retention, the apparent inversion occurring through equilibration via an Mo-allyl intermediate prior to nucleophilic attack. Incomplete equilibration leads to the memory effect and thus to lower enantioselectivity. Analysis of the mismatched manifold over the course of the reaction revealed that the memory effect is progressively attenuated with the nascent global selectivity increasing substantially as the reaction proceeds. The origin of this effect is suggested to be the depletion of CO sources in the reaction mixture, which attenuates turnover rate and thus facilitates greater equilibrium. The linear substrate was also converted into the branched product with net syn stereochemistry, as shown by isotopic labeling. An analogous process operates in the generation of small quantities of linear product from branched substrate.
机译：衍生自氨基酸的新手性配体（R）-（-）-12a和（S）-（+）-12c（VALDY）在标题反应中的应用，涉及肉桂基（线性）和异肉桂基（支链）型底物（4和5-> 6），导致极好的区域和对映选择性（> 30：1，<= 98％ee），表明具有单个手性中心的配体能够进行高度不对称诱导。讨论了配体的结构要求和机理。氘标记底物的单一对映体（线性38c和支链37c）的应用以及通过H-2 {H-1} NMR光谱在手性液晶基质中分析产物（41-43）允许立体化学途径为反应明显。使用配体（S）-（+）-12c，发现支链底物的匹配对映体为（S）-5，通过涉及网状结构的过程将其以非常高的区域选择性和立体选择性转化为（R）-6保留立体化学。发现支链底物的错配对映异构体为（R）-5，其也被转化为（R）-6，即具有明显的净反转，但是速率较低且总对映选择性较低。后一特征，可以称为“记忆效应”，降低了外消旋底物（+/-）-5的反应中的整体对映选择性。错配歧管的立体化学途径也已显示为净保留之一，在亲核攻击之前，通过Mo-烯丙基中间体的平衡发生了明显的反转。平衡不完全会导致记忆效应，从而降低对映选择性。在反应过程中对不匹配歧管的分析表明，随着反应的进行，新生初的整体选择性显着增加，从而记忆效应逐渐减弱。据认为，这种作用的起因是反应混合物中CO源的消耗，这削弱了周转率并因此促进了更大的平衡。线性底物也通过净同位立体化学转化为分支产物，如同位素标记所示。类似的过程用于从支化的基底中产生少量的线性产物。

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