Inner-hydrogen tautomerism in some 5,15-unsymmetrically disubstituted and beta-unsubstituted porphyrins.

5,15-Unsymmetrically disubstituted and beta-unsubstituted porphyrins such as 5-R, 15-(3,5-dimethoxyphenyl) porphyrins [where R=2-benzyloxy-1-naphthyl (1), 2-(2-naphthylmethoxy)-1-naphthyl (2), anthryl (3), or 2,4,6-triphenylphenyl (4)] and 5-(2-benzyloxy-1-naphthyl), 10,15,20-tri(3,5-dimethoxyphenyl) porphyrin (1') were synthesized and studied by (1)H-NMR spectroscopy. At room temperature, 1, 2, 3 and 4 showed doubling of the inner-hydrogen resonances with equal intensities, whereas the pyrrolic betaH signals were completely averaged. For 1 and 1', variable-temperature (1)H-NMR experiments were also performed. For 1, the two peaks of the inner hydrogen coalesced at about 313 K. In contrast, the pyrrolic betaH signals were only slightly broadened even at 213 K. On the other hand, 1' showed ordinary singlets of the inner hydrogens at room temperature, and the resonances of both the inner hydrogens and the pyrrolic betaH coalesced at about 233 K. We interpret these results as indicating the existence of two distinct paths, one slow and the other fast, leading to NH tautomerization in 1. We discuss the structures and energies of cis-tautomers as transition intermediates in relation to the two paths of NH tautomerization.