Enantioselective construction of C-B axially chiral alkenylborons by nickel-catalyzed radical relayed reductive coupling.

Abstract

The catalytic asymmetric synthesis of axially chiral alkenes remains a daunting challenge due to the lower rotational barrier, especially for longer stereogenic axis (e.g. C-B axis). The asymmetric radical difunctionalization of alkynes represents an efficient strategy for these targets. Key to the success of such transformations lies in aryl-stabilized highly reactive alkenyl radical intermediates, however, it remains an elusive whether a boryl group could play a similar role. Here we report a nickel-catalyzed atroposelective radical relayed reductive coupling reaction of our designed ethynyl-azaborines with simple alkyl and aryl halides through a boron-stabilized vinyl radical intermediate. This transformation enables a straightforward access to the challenging axially chiral alkenylborons bearing a C-B axis in generally high enantioselectivity and excellent stereoselectivity.