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

Vilsmeier formylation of 5-substituted 1,9-diunsubstituted dipyrromethanes afforded 1,9-diformyldipyrromethanes in good yields. Their MacDonald conden sation with tetra-beta -alkyldipyrromethanes produced 5,12,13,17,18-pentasu bstituted porphyrins. A mesoelectron-donating group, presumably acting by d estabilizing the porphyrin at, ground state, directs the nitrations to the mese-carbons. beta -Nitration takes place on porphyrins bearing a meso-elec tron-withdrawing group. Unhindered beta -nitro groups are shown to exert st ronger electronic effects relative to meso-nitro groups by conjugating effe ctively with the porphyrin macrocycle. (C) 2001 Elsevier Science Ltd. All r ights reserved.