Enantioselective Lewis Acid Catalyzed ortho Photocycloaddition of Olefins to Phenanthrene-9-carboxaldehydes

Visible-light irradiation (lambda = 457 nm) enabled the enantioselective ortho photocycloaddition of olefins to phenanthrene-9-carboxaldehydes (15 examples, 46-93% yield, 82-98% ee). A chiral oxazaborolidine Lewis acid (20 mol %) was employed as the catalyst. It operates by coordination to the aldehyde inducing a bathochromic absorption shift beyond the n pi* absorption of the uncomplexed aldehyde. At long wavelengths the Lewis acid complex is exclusively excited; within the complex, one enantiotopic face of the aromatic aldehyde is efficiently shielded. Lewis acid coordination also alters the type selectivity and the simple diastereoselectivity of the photocycloaddition.