Direct Visible-Light-Excited Asymmetric Lewis Acid Catalysis of Intermolecular [2+2] Photocycloadditions

A reaction
design is reported in which a substrate-bound chiral
Lewis acid complex absorbs visible light and generates an excited
state that directly reacts with a cosubstrate in a highly stereocontrolled
fashion. Specifically, a chiral rhodium complex catalyzes visible-light-activated
intermolecular [2+2] cycloadditions, providing a wide range of cyclobutanes
with up to >99% ee and up to >20:1 d.r. Noteworthy is the ability
to create vicinal all-carbon-quaternary stereocenters including spiro
centers in an intermolecular fashion.