Toward N-peri-Annulated Planar Blatter Radical through aza-Pschorr and Photocyclization

Preparation of the elusive N-peri-annulated planar Blatter radicals was attempted using aza-Pschorr and photocyclization methods. In both methods, substrates containing N–Me and N–Ac groups yielded a zwitterionic heterocycle lacking the N-substituent as the main product, while in one of them a carbazole derivative representing a new heterocyclic system was also obtained. The formation of the zwitterion and the carbazole suggests the formation of the desired planar Blatter radical, which undergoes facile fragmentation through homolysis of the N–R bond. This mechanism is supported by DFT computational results, which also suggest that N-Ar derivatives should be sufficiently stable for isolation. Electronic structures of three planar Blatter radicals annulated with the O, S, and N–Ph groups are compared.