N-methyl derivatives of highly substituted porphyrins - The combined influence of both core and peripheral substitution on the porphyrin conformation

N-Methylation of the peripherally highly substituted octa-ethyltetraphenylporphyrin (1) with either methyl iodide or methyl triflate gave the respective mono-, di-, tri- and tetra-N-methylated derivatives. Use of methyl triflate under carfully controlled conditions often gave the N-methylporphyrins in quantitative yield. The N-methylporphyrins possess both core and peripheral steric strain and are good candidates for the study of the combined conformational effect of such substituent patterns. Introduction of successively more N-methyl groups leads to a stepwise increase in the bathochromic shift of the bands by about 10-15 nm for each substitution. Crystal structure determinations of 11 and 12 showed that these molecules are indeed severely distorted and they exhibit degrees of nonplanarity which significantly exceed those observed for nonmethylated, dodecasubstituted nonplanar porphyrins and for 22,24-dihydroporphyrin dications. After methylation of meso-tetraalkylporphyrins 4-7 with methyl triflate only the respective N-methylated porphyrins 14-17 could be recovered. Use of the extremely ruffled porphyrin 8 led to formation of a 5,21,22,23,24-pentamethylated porphyrin cation with phlorin structure. Additionally, structural analysis of a cis-21,22-dimethyloctaethylporphyrin showed the first example of a porphyrin cation 20 with almost complete localization of the conformational distortion in the porphyrin half bearing the two N-methyl groups.