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首页> 外文期刊>Chemistry: A European journal >Analysis of Stereochemical Convergence in Asymmetric Pd-Catalysed Allylic Alkylation Reactions Complicated by Halide and Memory Effects
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Analysis of Stereochemical Convergence in Asymmetric Pd-Catalysed Allylic Alkylation Reactions Complicated by Halide and Memory Effects

机译：不对称钯催化的烯丙基烷基化反应中卤化物的立体化学趋同分析及记忆效应

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摘要

A quantitative two-term description of memory effects arising in Pd-catalysed allylic alkylation reactions formally proceeding through 'meso'-type pi-allylpalladium intermediates is presented. The utility of this description (stereochemical convergence (sc) and global enantiomeric excess (ee_g)) is demonstrated by application to a series of Pd-catalysed allylic alkylation reactions involving racemic cyclopentenyl esters. Analysis of such reactions, by employing a range of enantiomerically pure monophosphine ligands, reinforces the conclusion that selectivities (enantiomeric excess (ee) values) obtained under standard 'benchmark' type conditions may be very misleading when powerful memory effects are operative. However, by comparisonof sc and ee for a given ligand/solvent combination under a range of related conditions, one may predict the limting ('latent') selectivity that will be obtained when the memory effect is negated. This technique is exemplified with one particular ligand (4b, 'MAP') for which a number of strategies were employed to find conditions that negate the memory effect and reveal the limiting selectivity of the ligand. These conditions give a higher limiting global selectivity than that obtainable by using standard diastereoisomer equilibration methods such as added halide. Thus, the analysis of sc versus ee_g also allows subtle changes in selectivity to be discerned. The difference in limiting selectivity (chloride versus non-chloride conditions) is proposed to arise through the nucleophilic attack of neutral monodentate versus cationic bidentate MAP (4b) or MOP (4a) complexes.
机译：定量描述了在Pd催化的烯丙基烷基化反应中通过“介孔”型pi-烯丙基钯中间体正式进行的记忆效应的定量描述。通过将其应用于一系列涉及外消旋环戊烯基酯的Pd催化的烯丙基烷基化反应，证明了该描述的效用（立体化学收敛（sc）和整体对映体过量（ee_g））。通过使用一系列对映体纯的单膦配体对此类反应进行分析，可得出结论，即在有效的记忆效应有效的情况下，在标准“基准”类型条件下获得的选择性（对映体过量（ee）值）可能会产生很大的误导性。但是，通过在一系列相关条件下比较给定配体/溶剂组合的sc和ee，可以预测在消除记忆效应时将获得的变（“潜伏”）选择性。该技术以一种特定的配体（4b，“ MAP”）为例进行了说明，针对该配体采用了多种策略来寻找消除记忆效应并揭示该配体的有限选择性的条件。这些条件提供了比使用标准非对映异构体平衡方法（例如添加的卤化物）可获得的更高的极限总选择性。因此，对sc与ee_g的分析也可以看出选择性的细微变化。提议通过中性单齿与阳离子双齿MAP（4b）或MOP（4a）配合物的亲核攻击产生极限选择性（氯化物与非氯化物条件）的差异。

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