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

Electron-acceptor-substituted
aryl azides and α-diazo carboxylic
esters are used as substrates for visible-light-activated asymmetric
α-amination and α-alkylation, respectively, of 2-acyl
imidazoles 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 α-diazo carboxylic esters serving as precursors
for nitrogen and carbon-centered radicals, respectively. This is the
first report on using aryl azides and α-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.