The free-base and Zn(II) complexes of N-methyl-β-octaethyl- and meso-tetratolylchlorin and isobacteriochlorin were synthesized and characterized. Direct methylation of free-base hydroporphyrin compounds was unexpectedly selective. Only one of the several possible regioisomers that could result from alkylation of the inequivalent N atoms was produced for each hydroporphyrin free-base. This result was independent of the electrophilic reagent ([MeSPh2][BF4] for meso-tetraaryl compounds and methyl trifluoromethanesulfonate for β-octaethyl compounds) or the peripheral substituents on the hydroporphyrin. However, the greater basicity of the β-octaethyl substituted compounds resulted in their isolation as protonated cations. Methylation occurred at a pyrrole ring rather than a pyrroline ring. In chlorins, the pyrrole ring across the macrocycle from the pyrroline ring was methylated to afford the symmetric N-methyl chlorins H(s-N23-MeTTC) and H2(s-N23-MeOEC)+. The selectivity is a result of kinetic rather than thermodynamic factors. Slow air oxidation of H(N-MeTTiBC) affords the unsymmetric N-methyl chlorin H(u-N22-MeTTC). The bacteriochlorins H2(TTBC) and H2(OEBC) were unreactive toward all electrophilic reagents investigated. An alternative synthetic approach, reduction of H(N-MeTTP), appears to have a selectivity complementary to direct methylation. It afforded a complex mixture of compounds that contained H(u-N22-MeTTC) and one other yet unidentified N-methyl hydroporphyrin. Free-base N-methyl hydroporphyrins react rapidly and quantitatively with zinc salts to afford Zn(II) complexes. The 1H NMR spectra were characterized by N-methyl group resonances that have shifts between 0 and 4 ppm upfield of TMS and decreased ring current effects as the saturation of the macrocycle increases. The inequivalence of the two faces of the macrocycle owing to the N-methyl group revealed that the meso-aryl groups undergo restricted rotational motion. The barriers to rotation vary with saturation and metalation but are substantially smaller than in metallo-TTP compounds. Both the oxidations and reductions of free-base N-methyl hydroporphyrin compounds are markedly irreversible. However, the zinc complexes have reversible reductions.

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