Porphyrins similar ring compounds

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 . 

M = Eu(III), Y(III), Lu(III)] 8-16%. These protonated compounds were isolated as the sole product, in contrast to the similar reactions with TBPPH2, where the non-protonated analogues Lnnl(Pc)(TBPP) (TBPP, 5,10,15,20-tetrakis[(4- ert-butyl)phenyl]-porphyrin) were obtained instead [106]. These results clearly show that the inversion of a pyrrole ring in N-confused porphyrins can stabilize the protonated complexes. 

To study the possibility of electronic effects on the photodissociation localized to the porphyrin ring, we examined -CO, a compound similar to 1 -CO except for the substitution of ethyl groups in place of vinyl groups on the outside of the porphyrin ring. Earlier bmolecular kinetic studies (11,15) had shown that this change in the electron distribution of the porphyrin had no effect on the reaction of CO with the heme. Our results also indicate that this perturbation has no apparent effect on the picosecond photodissociation. 

A number of model compounds have been synthesized which have Fe(ll)-porphyrin rings carrying a side chain with histidine arranged to be able to coordinate with the metal on one side. Several of these substances show promise as oxygen carriers with properties similar to myoglobin. 

Mitchell and Valero (1982, 1983) studied VO-phthalocyanine (VO-PC) and a Schiff base complex, VO-salen, as model vanadium compounds. The VO-PC provides a metal coordination environment comprised of four nitrogens, similar to the porphyrin. The benzo rings at the /3-pyrrolic positions contribute to enhanced aromaticity in the metal ligand. 

The structures of the biologically active forms of B12 were solved relatively recently (1961) (78) and were shown to contain a cobalt atom surrounded by a corrin ring as shown in Fig. 16 (80). The crystal structure also showed a cobalt-carbon a bond which was quite surprising since the few compounds with cobalt-carbon a- bonds known at that time were quite unstable (79). The corrin ring is similar to the porphyrin ring, but its greater saturation imports less rigidity than the porphyrin. Corrinoids with the axial 5,6-dimethylbenzimidazole substituent are called cobalamins. Vitamin B12 with Co(III) and CN in the top axial position is... 

The positions of the ring methyl and mesoproton resonances for cyano-ferriporphin, MesoCN, and ProtoCN in different solvents are shown in Fig. 26. For all the three compounds the resonance positions show similar solvent dependences. This indicates that the solvent effects come probably from different solvation of the porphyrin ring, rather than from inter-... 

The porphyrin motif, four pyrrole rings linked by carbon atoms to give a planar cyclic compound, is found in many biomolecules. It is present in haemoglobin and myoglobin and its derivatives are also at the core of vitamin B12, hydrogenases, cytochrome c, the chlorophyll photosystem and similar essential proteins. 

Different compounds which complex the fluoride anion were recently developed. Sapphyrine (1) is a porphyrin with 5 pyrrole rings, and when it is doubly protonated it can exclusively complex F in its cavity68. Similarly, Seppelt prepared the thermally stable salt 2 with a bulky cation where the fluoride can be completely desolvated, thus increasing its efficiency as a nucleophile and a base69. 

Finally, the iron compound haem, part of the haemoglobin molecule we use to carry oxygen around in our bloodstream. It contains the aromatic porphyrin ring system with its eighteen elec-trons arranged in annulene style. Chlorophyll, mentioned earlier in this chapter, has a similar aromatic ring system. 

Scheme 9.17). The macrocycle (A) of Scheme 9.1 is the dpp-incorporating 30-membered ring 8 used earlier for making catenanes (see Protocol 5) it is sufficiently small to prevent release of the bis-porphyrin dumbbell. The preparation of 22+ takes place in seven steps.1819 One of the starting materials is 4-lithiotoluene 13, which may be obtained by direct interaction of an excess of freshly cut lithium with 4-bromotoluene 12 in ether under argon at reflux (Scheme 9.10).12 The resulting organolithium compound is titrated by the double titration method described by Gilman et alP The experimental procedure is very similar to that described in Protocol 1.

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