N-methylation and electrophilic substitution reactions of octa-alkyl-porphins, octaethylchlorin, and metalloporphins

Abstract

Methylation of octaethylporphin with methyl iodide yields the 21,22-dimethylporphin iodide as well as the 21-monomethyl derivative. The structure of the salt (trans arrangement of methyl groups) is assigned from n.m.r. data as well as evidence for chirality provided by partial resolution of the corresponding D-camphorsulphonate. Methylation of octaethylporphin with methyl fluorosulphonate gives the 21,23-dimethylporphin fluorosulphonate, probably through the intermediacy of the 21,22,23-trimethylporphin salt, suggesting that the 21- and 23-methyl substituents are cis-oriented. Methylation of Octaethylchlorin gives the 23-monomethyl derivative (methyl fluorosulphonate) or the 22,23,24-trimethylchlorin iodide (methyl iodide). Electrophilic meso-substitution, exemplified by deuteriation, formylation, and nitration, takes place more readily with the metal complexes of porphins than with the free porphins; the first direct meso-methylation of a metalloporphin (Pd^II complex) is reported.