The proton magnetic resonance spectra of porphyrins. Part V. Syntheses and spectra of some meso-methylated porphyrins

Abstract

A number of new meso-methyl- and related meso-unsubstituted porphyrins have been synthesised including 2,3-dithyl-γ-meso,1,4,5,8-pentamethylporphin. Comparison of the proton magnetic resonance spectra of the two series confirms our earlier suggestion that the meso-methyl group exerts a pronounced steric effect if the two neighbouring β-positions are occupied, and that there is also an overall decrease in ring current (ca. 3%). These and other new results show that the precise position and multiplicity of the meso and NH-proton resonances of meso-unsubstituted porphyrins in trifluoroacetic acid are not dependent only on the nature of the two neighbouring β-substituents, but are also affected to a lesser extent by the other substituents.