Asymmetric Catalysis with Organic Azides and Diazo Compounds Initiated by Photoinduced Electron Transfer

Electron-acceptor-substituted aryl azides and alpha-diazo carboxylic esters are used as substrates for visible-light activated asymmetric alpha-amination and alpha-alkylation, respectively, of 2-aryl itnidazoles catalyzed by a chiral-at-metal rhodium-based Lewis acid in combination with a photoredox sensitizer. This novel proton- and redox-neutral method provides yields of up to 99% and excellent enantioselectivities of up to >99% ee with broad functional group compatibility. Mechanistic investigations suggest that an intermediate rhodium enolate complex acts as a reductive quencher to initiate a radical process with the aryl azides and alpha-diazo carboxylic esters, serving as precursors for nitrogen and carbon-centered radicals, respectively. This is the first report on using aryl azides and alpha-diazo carboxylic esters as substrates for asymmetric catalysis under photoredox conditions. These reagents have the advantage that molecular nitrogen is the leaving group and sole byproduct in this reaction.