A rapidly emerging set of catalytic reactions involves intermediates that contain a copper-substituted stereogenic carbon centre. Here, we demonstrate that an intimate understanding of this distinction provides ways for addressing limitations in reaction scope and explaining why unexpected variations in enantioselectivity often occur. By using catalytic enantioselective Cu–boryl addition to alkenes as the model process, we elucidate several key mechanistic principles. We show that higher electrophile concentration can lead to elevated enantioselectivity. This is because diastereoselective Cu–H elimination may be avoided and/or achiral Cu–boryl intermediates can be converted to allyl–B(pin) rather than add to an alkene. We illustrate that lower alkene amounts and/or higher chiral ligand concentration can minimize the deleterious influence of achiral Cu–alkyl species, resulting in improved enantiomeric ratios. Moreover, and surprisingly, we find that enantioselectivities are higher with the less reactive allylphenyl carbonates as chemoselective copper–hydride elimination is faster with an achiral Cu-alkyl species.

中文翻译：

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基于机理的涉及铜取代的立体碳中心的反应范围和对映选择性的增强

一组迅速出现的催化反应涉及含有铜取代的立体碳中心的中间体。在这里，我们证明了对这种区别的深入了解为解决反应范围的局限性和解释为什么对映选择性经常发生意想不到的变化提供了方法。通过将催化对映选择性的Cu-硼烷基加成烯烃作为模型过程，我们阐明了一些关键的机理。我们表明，较高的亲电试剂浓度可以导致对映选择性的提高。这是因为可以避免非对映选择性Cu-H的消除和/或非手性Cu-硼烷基中间体可以转化为烯丙基-B（pin），而不是添加至烯烃。我们说明，较低的烯烃含量和/或较高的手性配体浓度可以使非手性铜烷基物种的有害影响最小化，从而提高对映体比率。而且，令人惊讶的是，我们发现，反应性较低的烯丙基碳酸酯的对映选择性更高，因为非手性铜烷基化合物的化学选择性氢化铜消除速度更快。