Investigations on the directive effects of a single meso-substituent via nitration of 5,12,13,17,18-pentasubstituted porphyrins: syntheses of conjugated β-nitroporphyrins

Vilsmeier formylation of 5-substituted 1,9-diunsubstituted dipyrromethanes afforded 1,9-diformyldipyrromethanes in good yields. Their MacDonald condensation with tetra-β-alkyldipyrromethanes produced 5,12,13,17,18-pentasubstituted porphyrins. A meso-electron-donating group, presumably acting by destabilizing the porphyrin a2u ground state, directs the nitrations to the meso-carbons. β-Nitration takes place on porphyrins bearing a meso-electron-withdrawing group. Unhindered β-nitro groups are shown to exert stronger electronic effects relative to meso-nitro groups by conjugating effectively with the porphyrin macrocycle.