Porphyrins and

A major trend in organic synthesis, however, is the move towards complex systems. It may happen that one needs to combine a steroid and a sugar molecule, a porphyrin and a carotenoid, a penicillin and a peptide. Also the specialists in a field have developed reactions and concepts that may, with or without modifications, be applied in other fields. If one needs to protect an amino group in a steroid, it is advisable not only to search the steroid literature but also to look into publications on peptide synthesis. In the synthesis of corrin chromophores with chiral centres, special knowledge of steroid, porphyrin, and alkaloid chemistry has been very helpful (R.B. Woodward, 1967 A. Eschenmoser, 1970). 

Porphyrins and chlorophylls are the most widespread natural pigments. They are associated with the energy-converting processes of respiration and photosynthesis in living organisms, and the synthesis of specific porphyrin derivatives is often motivated by the desire to perform similar processes in the test tube. The structurally and biosynthetically related corrins (e.g. vitamin B,j) catalyze alkylations and rearrangements of carbon skeletons via organocobalt intermediates. The biosyntheses of these chromophores are also of topical interest. 

It is conceivable that related ligands, e.g. dehydrocorrins, could be obtained from pyrrolic units using pathways similar to those used for porphyrins and could be hydrogenated to corrins. This has indeed been achieved (I.D. Dicker, 1971), but it is, of course, impossible to introduce the nine chiral centres of cobyrinic acid by such procedures. 

LB Films of Porphyrins and Phthalocyanines. The porphyrin is one of the most important among biomolecules. The most stable synthetic porphyrin is 5,10,15,20-tetraphenylporphyrin (TPP). Many porphyrin and phthalocyanine (PC) derivatives form good LB films. Both these molecules are important for appHcations such as hole-burning that may allow information storage using multiple frequency devices. In 1937 multilayers were built from chlorophyll (35). 

Chelation itself is sometimes useful in directing the course of synthesis. This is called the template effect (37). The presence of a suitable metal ion facihtates the preparation of the crown ethers, porphyrins, and similar heteroatom macrocycHc compounds. Coordination of the heteroatoms about the metal orients the end groups of the reactants for ring closure. The product is the chelate from which the metal may be removed by a suitable method. In other catalytic effects, reactive centers may be brought into close proximity, charge or bond strain effects may be created, or electron transfers may be made possible. 

In other sections in this chapter, we have referred to a variety of macropolycyclic structures which are more elaborate than the simple three-stranded bicyclic cryptands. This includes bridged double-macrocycles " , in-out bicyclic amines and the macrotricyclic quaternary ammonium salts of Schmidtchen. In addition to these, there are two other types of compounds which deserve special note. The first of these is a stacked twin-ring cryptand, but it is a hybrid molecule rather than a double-cryptand . The species shown below as 20 is a crowned porphyrin, and was designed to provide a pair of metal cation binding sites similar to those which might be available in natural biological systems . 

Fuhrhop, J.-H., Smith, K. M. laboratory Methods in Porphyrin and Metalporphyrin Research. Elsevier Publ., Amsterdam 1975, 243ff. 

In 1985, in the course of their interest in nitroalkane chemistry, Barton and Zard reported the base-catalyzed reaction of nitroalkenes with a-isocyanoacetates leading to pyrrole esters having an ideal substitution pattern for the synthesis of porphyrins and bile... 

The major application of the Knorr pyrrole synthesis is in the construction of porphyrins, and many examples exist,particularly from the work of Lash, who also demonstrated the formation of novel pyrroles, such as Cyanopyrroles are available... 

L. R. Milgrom, in The Colors of Life An Introduction to the Chemistry of Porphyrins and Related Compounds , Oxford Univ. Press, 1997. 

Porphyrin systems therefore obey Hiickel s rule in having An + 2 n = A) TT-electrons in a planar, cyclic, conjugated array. Both major tautomeric forms have delocalization pathways with opposite N-Hs (trails tautomers), as shown in 71a 71b. It is already known (76AHCS1) that tautomers with inner hydrogens adjacent (cis tautomers) are much less stable, playing an important role only in the mechanism of proton transfer in porphyrins and phthalocyanines. 

Porphyrin and its isomers, porphycene, hemiporphycene, corphycene, and iso-porphycene, as high-technological dyes 99CFY767. 

Nonplanar porphyrins and their significance in proteins 98CSR31. 

Solvation effects and coordination properties of porphyrins and metalloporphyrins in solutions 98MI19. 

Synthesis, characterization, and chemistry of core-modified porphyrins and their nickel complexes 97NJC691. 

Macrocyclic effect and specific character of complex formation with rigid macrocyclic ligands such as porphyrins and phthalocyanins 97MI8. 

F. Aromatic and Heteroaromatic Large Rings 1. Porphyrins and Related Compounds... 

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

The cyclotetramerization of suitable monopyrroles is the most simple approach to prepare porphyrins and it has been used in the synthesis of numerous porphyrin structures. But this method can only be used satisfactorily if the pyrroles 1 and 2 bear identical /i-substituents (R = R), otherwise the cyclotetramerization leads to a statistical mixture of four possible constitutional isomers. 

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