A. Ulman, J. Manassen
Oct 29, 1975
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Journal
Journal of the American Chemical Society
Abstract
A new and general synthesis of tetraphenylporphyrin analogs is described. The tetraphenyl-21,23-dithiaporphyrin was prepared in a pure form. Its electronic spectrum, as well as preliminary X-ray measurements, suggests that the central sulfur and nitrogen atoms are somewhat out of the plane of the molecule, facilitating attainment of a smaller angle between the phenyl groups and the porphin plane than is found in tetraphenylporphyrin. The chemical shift of the @-pyrrole hydrogens as found in the ' H NMR spectrum at room temperature is almost identical with that found for tetraphenylporphyrin at -80'. Protonation is seen to occur at the nitrogen atoms and the optical spectrum of the conjugate acid is qualitatively similar to that of the conjugate acid of tetraphenylporphyrin. The porphin skeleton plays an important role in many biological and catalytic systems.'.* Tetraphenylporphyrin is a thermally stable ligand, which can be synthesized according to established methods. We have shown before that its metal complexes are catalysts for oxidative dehydrogenation3 and symmetry forbidden reactions." The great advantage of using these molecules as catalysts is that their structure can be modified in a controlled way and correlations between catalytic activity and structure can be followed. Catalytic activity has been shown to be dependent on the central metal atoms,3 as well as on substituents in the p-phenyl position^,^ and correlations with physical properties of the catalyst in solution could be found. An attractive modification in the molecule would be to change the immediate environment around the central metal atom, which means to replace the nitrogen atoms by other heteroatoms. In this paper we report the synthesis of tetraphenylporphyrin (TPP) with two opposite NH groups being exchanged for sulfur. In the future we shall report the synthesis of molecules containing other heteroatoms and the catalytic activity of these systems. A general synthesis (Scheme I) for tetraphenylporphyrin has been given by Adler and coworkers,6 and the conditions for optimal yield and purity have been studied.' The only example of a dithiaporphin in the literature is the synthesis by Broadhurst and Grigg,8 who used a [3 + 11 cyclization (Scheme 11). This approach was not found to be suitable for the synthesis of the tetraphenyl derivative, however, and therefore we worked out another synthetic scheme, which appeared to be applicable not only for the disulfur compound, but for other heteroatoms as well. The Synthesis. To arrive a t a good synthetic method, it is best to consider the mechanism of the ordinary acid-catalyzed reaction between benzaldehyde and pyrrole first, and to see whether this reaction can be modified to suit our purpose. The first step is commonly assumed to be the acid-catalyzed addition of pyrrole to benzaldehyde.