Selective and Divergent Synthesis of Naphthalene- and Phenanthrene-Fused Azahelicenes by Turning Rearrangement On or Off

The Scholl reaction has been used to synthesize a variety of polycyclic aromatic hydrocarbons, where 1,2-aryl shifts have sometimes occurred to yield unique rearrangement products. However, such 1,2-aryl shifts are often uncontrollable, and the selective and divergent synthesis with or without rearrangement is desired. Here, we achieved the control of the rearrangement in the Scholl reaction of carbazoles by changing the N-substituents. The Scholl reaction of 3,6-bis{2-(2-naphthyl)phenyl}carbazoles and 3,6-bis{2-(9-phenanthrenyl)phenyl}carbazoles with an N-benzyl group gave multiple azahelicenes via double rearrangement, while those with an N-benzoyl group gave aza[9]helicene and quadruple [4]helicene in the former and latter cases, respectively. The reaction mechanisms on the divergent reaction pathways were investigated by DFT calculations, which well supported the experimental results.