Reactions of porphyrins with nitronium tetrafluoroborate in pyridine

Abstract

Octaethylporphyrin reacts with nitronium tetrafluoroborate in pyridine at 80 °C to afford the meso-nitroporphyrin in 31 % yield after 6 h, whereas the zinc complex reacts very rapidly at 20 °C to afford 63% of the meso-nitroporphyrin after work-up. meso-Tetraphenylporphyrin, however, only reacted slowly with nitronium tetrafluoroborate in pyridine at rather higher temperatures in a sealed tube to afford the β-pyridinium salt of the porphyrin as the major product; again when the zinc complex of tetraphenylporphyrin was used the β-pyridinium salt was formed in 85% yield in a few minutes at 20 °C.

Brief treatment of the tetraphenylporphyrin pyridinium salt with alkali afforded a ring-opened glutaconaldehyde derivative of the β-aminoporphyrin, analogous to the well-known Zincke's compounds which can be obtained from N-arylpyridines with alkali. The structures of both the ring-opened products from the pyridinium porphyrin and from N-(2,4-dinitrophenyl)pyridinium chloride, were assigned by highfield n.m.r. spectroscopy, including decoupling and n.O.e. difference measurements.