Mechanism-based enhancement of scope and enantioselectivity for reactions involving a copper-substituted stereogenic carbon centre

A rapidly emerging set of catalytic reactions involves intermediates that contain a copper-substituted stereogenic carbon centre. Here, we demonstrate that 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 have been able to 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.