N-methylated tetraphenylporphins

Abstract

Methylation of meso-tetraphenylporphin affords N-methyl, trans-N_aN_b-dimethyl, and trans,trans-N_aN_bN_c-trimethyl derivatives. The structures of the products were assigned from n.m.r. data, and that of the N_aN_b-dimethylporphin was confirmed by ethylation to form the unsymmetrical N_aN_b-dimethyl-N_c-ethyl derivative. The N_aN_bN_c-trimethylporphin decomposed on heating to give N_aN_b-dimethyl- and N-methyl-tetraphenylporphins. In none of our experiments was either of the isomeric cis- and trans-N_aN_c-dimethylporphins found, in contrast to earlier findings in the octa-alkylporphin series. The results are discussed in relation to the effects of the meso-phenyl and N-methyl groups on distortion of the aromatic nucleus.