Bronsted Acid-Catalyzed Cyclopropanation of Indoles Using α-Aryl-α-diazoacetates

Metal-free cyclopropanation of N-protected indoles with alpha-aryl-alpha-diazoacetates proceeded by using a catalytic amount of triflic imide as a Bronsted acid catalyst. The use of a catalytic amount of methyl phenylacetate improved the efficiency of the Bronsted acid-catalyzed cyclopropanation. A reaction mechanism including C-protonation of diazoacetates with HNTf2, followed by formal [3+2] cycloaddition with indoles and 1,3-rearrangement was proposed for the cyclopropanation.

Automated summary

Metal-free cyclopropanation of N-protected indoles with alpha-aryl-alpha-diazoacetates was successfully achieved using triflic imide as a Bronsted acid catalyst. The addition of a catalytic amount of methyl phenylacetate improved the efficiency of the reaction. A proposed reaction mechanism involving C-protonation of diazoacetates with HNTf2, followed by formal [3+2] cycloaddition with indoles and 1,3-rearrangement, explains the success of the cyclopropanation reaction.