Catalytic reductive cyclization of 2-nitrobiphenyls using phenyl formate as CO surrogate: A robust synthesis of 9H-carbazoles

A palladium/phenanthroline catalyzed reductive cyclization of o-nitrobiphenyls to 9H-carbazoles operated by in situ released CO is described. In the present method, the presence of phenyl formate is essential to avoid the use of high-pressure equipment and allows to perform the reaction in a single thick-walled glass tube. The identity of the base, which catalyzes the formate decomposition, is crucial to the selectivity of the reaction. Stability of the catalyst is influenced by the presence of chloride anions. Carbazole yields are most often higher than those previously reported using pressurized CO. The optimized catalytic system features excellent stability and moisture and air tolerance allowing to perform the reaction with low catalyst loadings.