Regioselective pyridination of m-benziporphyrin

Reaction of tetraaryl-m-benziporphyrin with pyridine and silver tetrafluoroborate yields 22-pyridiniumyl-m-benziporphyrin as the only substitution product. This compound is further rearranged to a fused m-benziphlorin containing a 4a-azafluorene fragment. A mechanism involving a high-valent silver complex is proposed for the pyridination reaction.