Catalytic enantioselective copper-boryl additions to alkenes
Catalytic enantioselective copper-boryl additions to alkenes furnish chiral organoboron compounds in an enantioselective manner through the action of an enantiomerically pure copper complex, a boron-containing reagent such as bis(pinacolato)diboron, and a Lewis base. The products of these reactions are valuable reagents for cross-coupling and related transformations that result in the formation of C–C, C–N, C–O, and C–X bonds. Copper is an inexpensive and abundant alternative to other transition metals commonly used for C–B bond formation, such as palladium, platinum, and rhodium. Following addition of copper and boron to adjacent carbons of the alkene, elimination or electrophilic substitution of the copper fragment yields the product and regenerates the catalyst.
Migratory insertion of an alkene into a Cu–B bond in a syn manner was first demonstrated through the use of a stoichiometric amount of copper complex. This observation stimulated the development of catalytic copper-boryl functionalizations of alkenes, which employ an electrophilic component (either internally or externally) to turn over the catalytic cycle. The copper–boron bond is formed via transmetalation following nucleophilic activation of the diboron compound by the Lewis base; stereospecific insertion of the alkene into the Cu–B bond follows. Careful selection of the electrophilic component is important because the copper complex must not engage with the electrophile in preference to the alkene. Enantioselectivity in these reactions can be strongly affected by the concentrations of chiral ligand and the electrophilic component as well as the reactivities of the alkene and electrophile.