DIRECT OBSERVATION OF TAUTOMERIC FORMS OF DEUTEROPORPHYRIN DERIVATIVES BY H-1-NMR SPECTROSCOPY - SUBSTITUENT EFFECTS AND STRUCTURE IMPLICATIONS

A study of the position of tautomeric equilibrium in deuteroporphyrin IX methyl ester (3) and monosubstituted derivatives 4-7 using variable-temperature studies on the N,N-dideuteriated derivatives 8-12 is reported. The kinetic isotope effect on the prototropic exchange process is sufficiently large to allow the convenient observation of the individual tautomers by H-1 NMR spectroscopy. As in the case of 2-substituted 5,10,15,20-tetraphenylporphyrins 1, the position of the tautomeric equilibrium in deuteroporphyrin derivatives is dependent on the substituent pattern on the porphyrin outer periphery. For the isomeric acetylporphyrins, 8-acetyldeuteroporphyrin IX dimethyl ester (9) and 3-acetyldeuteroporphyrin IX dimethyl ester (10), the acetyl group is the major influence in determining tautomer stability, i.e., both porphyrins are essentially one tautomer (> 85% at 298 K) in which the acetyl group is not involved in the aromatic delocalization pathway. The more stable tautomer (59% at 298 K) of 3-(1-hydroxyethyl)deuteroporphyrin IX dimethyl ester (11) is that in which the hydroxyethyl substituent is directly substituted on the aromatic pathway. The vinylporphyrin (12) exists predominantly (56% at 298 K) as the tautomer in which the vinyl group is substituted on the beta-beta pyrrolic bond remote from the aromatic delocalization pathway. Conjugation of the vinyl group in 12 and the carbonyl group in the acetylporphyrins 9 and 10 with the beta-beta pyrrolic double bond probably contributes to the overall stability of the major tautomer of these porphyrins. This work serves to further emphasize that all nonsymmetric free-base porphyrins are necessarily a mixture of two tautomers of different energies which should be taken into account in interpreting the physical and chemical properties of these systems.