Non-aerobic and One-pot Synthesis of Carbazoles from Cyclohexanones and Arylhydrazines

The reaction of cyclohexanones with phenylhydrazines proceeded under an ethylene atmosphere in the presence of a catalytic amount of Pd/C and equimolar amount of p-toluenesulfonic acid monohydrate to afford a variety of substituted carbazoles in good to high yields. The present reaction was carried out under completely non aerobic conditions, which is in contrast with the previously reported aerobic system. This protocol was also applied to the synthesis of symmetrical carbazoles using hydrazine monohydrate in place of phenylhydrazines. The reaction will proceed in a manner similar to Fischer indole synthesis, involving a [3,3]-sigmatropic rearrangement and advancing with 1,2,3,4-tetrahydrocarbazole as an intermediate.