THE SYNTHESIS, PROPERTIES, AND REACTIVITIES OF FREE-BASE-COMPOUND ANDZN(II)-N-METHYL HYDROPORPHYRIN COMPOUND - THE UNEXPECTED SELECTIVITY OF THE DIRECT METHYLATION OF FREE-BASE HYDROPORPHYRIN COMPOUNDS

The free-base and Zn(II) complexes of N-methyl-beta-octaethyl- and mes o-tetratolylchlorin and isobacteriochlorin were synthesized and charac terized. 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 pro duced for each hydroporphyrin free-base. This result was independent o f the electropilic reagent [(MeSPh2][BF4] for meso-tetraaryl compounds and methyl trifluoromethanesulfonate for beta-octaethyl compounds) or the peripheral substituents on the hydroporphyrin. However, the great er basicity of the beta-oetaethyl substituted compounds resulted in th eir isolation as protonated cations. Methylation occurred at a pyrrole ring rather than a pyrroline ring. In chlorins, the pyrrole ring acro ss the macrocycle from the pyrroline ring was methylated to afford the symmetric N-methyl chlorins H(s-N23-MeTTC) and H-2(s-N23-MeOEC)(+). T he selectivity is a result of kinetic rather than thermodynamic factor s. Slow air oxidation of H(N-MeTTiBC) affords the unsymmetric N-methyl chlorin H(u-N22-MeTTC). The bacteriochlorins H-2(TTBC) and H-2(OEBC) were unreactive toward all electrophilic reagents investigated. An alt ernative synthetic approach, reduction of H(N-MeTTP), appears to have a selectivity complementary to direct methylation. It afforded a compl ex mixture of compounds that contained H(u-N22-MeTTC) and one other ye t unidentified N-methyl hydroporphyrin. Free-base N-methyl hydroporphy rins react rapidly and quantitatively with zinc salts to afford Zn(II) complexes. The H-1 NMR spectra were characterized by N-methyl group r esonances that have shifts between 0 and 4 ppm upfield of TMS and decr eased ring current effects as the saturation of the macrocycle increas es. The inequivalence of the two faces of the macrocycle owing to the N-methyl group revealed that the meso-aryl groups undergo restricted r otational motion. The barriers to rotation vary with saturation and me talation but are substantially smaller than in metallo-TTP compounds. Both the oxidations and reductions of free-base N-methyl hydroporphyri n compounds are markedly irreversible. However, the zinc complexes hav e reversible reductions.