Synthesis of Indoles by Reductive Cyclization of Nitro Compounds Using Formate Esters as CO Surrogates

Alkyl and aryl formate esters were evaluated as CO sources in the Pd- Pd/Ru-catalyzed reductive cyclization of 2-nitrostyrenes to give indoles. Whereas use alkyl formates requires presence a ruthenium catalyst such Ru3(CO)12, reaction with phenyl can be performed by using Pd/phenanthroline complex alone. Phenyl was found most effective source desired products obtained excellent yields, often higher than those previously reported pressurized CO. The tolerates many functional groups, including sensitive ones like free aldehydic group or pendant pyrrole. Detailed experiments kinetic studies allow conclude that activation is base-catalyzed metal doesn't play role decarbonylation step. reactions single thick-walled glass tube little 0.2 mol-% palladium even on 2 g scale. same protocol extended other nitro compounds, affording different heterocycles.