Porphyrins, preparation

Chiral porphyrins, prepared in different ways84,85 (chiral units attached to preformed porphyrins84, chiral substituents introduced during the synthesis of porphyrins86 or chiral porphyrins synthesized without the introduction of chiral groups69,87-90), proved to be effective as asymmetric epoxidation catalysts. 

With the exception of the texaphyrins of Sessler et al. [59, 60, 65, 95] vide supra) the sapphyrin macrocycle and its heteroatom analogues (c.f. Scheme 36) are perhaps the best studied of the expanded porphyrins prepared to date. Sapphyrin was the first example of an expanded porphyrin to be reported, being discovered... 

The largest vinylogous porphyrin prepared to date was first reported by Knubel and Franck in 1988. It is the 34 Ji-electron octavinylogous [34]porphyrin-... 

The capped porphyrins prepared by Baldwin et al. [56, 57] are other model systems designed to test the consequences of steric hindrance on CO binding (Scheme 4). These compounds were reported to discriminate against dioxygen in favor of carbon monoxide [62, 119-121]. The CO affinity of the capped porphyrins differs by less than a factor of three from that of unprotected iron(II) tetraphenylporphyrin, while the dioxygen affinity is more than a factor of 100 lower. Kinetic studies of CO binding show that the CO dissociation rate constants are very similar to those of unprotected hemes. Recently, the X-ray crystal structure of a carbonylated complex of the smallest capped porphyrin was obtained [122]. The CO ligand is reported to deviate 7° and 4° from the heme normal, respectively, for each independent molecule present in the unit cell. 

Kitaoka and coworkers provided a new methodology for porphyrin preparation with an acidic IL (Fig. 12.43) [29]. The acidic IL phase separated with dichloromethane becomes quite instrumental for reducing the amount of the halogenated solvents used in porphyrin preparation. 

The metalloporphyrins as macrocyclic compounds have a few sites for specific and universal solvation and are able to axial coordination of some ligands. At the present time chemical modification of macrocycle is a main way of increasing of selectivity of molecular complex formation. The data obtained earlier [1,2] show that the selectivity may be increased due to specific %-% interactions of the metalloporphyrins with aromatic molecules. Aromatic molecules coplanar to the macrocycle will rise geometrical requirements to axial coordinating ligands. In particular, the results of the thermodynamic study of the axial coordination of n-propylamine by zinc(II) porphyrins in benzene have demonstrated the formation of the complexes of the metalloporphyrin containing both w-propylamine and benzene [2], The aim of this work is to study the molecular complexes of zinc (II) porphyrins prepared by slow crystallization from saturated solutions in benzene, w-propylamine and mixed solvent benzene - -propylamine. 

Franck et al. [109] suggested a strategy for the synthesis of C-glycosyl benzaldehyde derivatives to be used in porphyrin preparations. They started by condensing 2,3,4,6-tetra-0-acetyl-l-thio-j6-D-glucopyranose with a,a -dibromo-p-xylene to afford thioglucoside 159 (Scheme 29). The latter, by... 

Abstract The metal-driven construction of multi-porphyrin assemblies, which exploits the formation of coordination bonds between exocyclic donor site(s) on the porphyrins and metal centers, has recently allowed the design and preparation of sophisticated supra-molecular architectures whose complexity and function begin to approach the properties of naturally occurring systems. Within this framework, meso-pyridyl/phenyl porphyrins (PyPs), or strictly related chromophores, can provide geometrically well-defined connections to as many as four metal centers by coordination of the pyridyl peripheral groups. Several discrete assemblies of various nuclearities, in which the pyridylporphyrins are connected through external coordination compounds, have been constructed in recent years. In this review, we summarize recent work in this field from our and other laboratories. The photophysical properties of some ruthenium-mediated assemblies of porphyrins prepared by our group are also described. 

Sessler. J.L. Weghorn. S. Lynch, V. Johnson. M.R. Turcasarin. the largest expanded porphyrin prepared to date. Angew. Chem. Int. Ed. Engl. 1994.33 (14), 1509-1512. Sessler, J.L. Gebauer, A. Weghorn. S.J. Expanded... 

The introduction of all l groups at the porphyrin meso positions induces ruffled deformation [9,10,27,28]. A typical example is meso-tetra(te/t-butyl)porphyrin prepared by Smith and coworkers [27,51]. The meso carbon atoms deviate from the mean porphyrin plane upward and downward alternately. The average deviation of the meso carbons in the zinc complex Zn(T BuP) reaches as much as 0.899 A [28], Thus, the iron complexes of this porphyrin such as Fe(T BuP)X and [Fe(T BuP)L2] should be the best model to elucidate the effect of ruffled porphyrin ring on the electronic structure of the iron(lll) complexes. However, the synthesis of Fe(T BuP)X has been unsuccessful until now. For this reason, meso-tetraisopropylporphyrin complexes Fe(T PrP)X, where the meso te/t-butyl groups are replaced by the less bullqr isopropyl groups, have been examined [52,53]. 

Optical UV-Vis absorbance A composite thin film of zinc(II) tetra-4-(2,4-di-tert-amylphenoxy) phthalocyanine and copper(II) tetrakis (p-tert-butylphenyl) porphyrin prepared via spin coat-ing on quartz substrate Alcohols Saturated vapors 1... 

Liddell P A, Sumida J P, Macpherson A N, Noss L, Seely G R, Clark K N, Moore A L, Moore T A and Gust D 1994 Preparation and photophysical studies of porphyrin-Cgg dyads Photochem. Photobiol. 60 537-41... 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

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. 

There are, however, some crown type compounds which contain no structural feature except the thiophene subunit, and these deserve some comment here. This is especially true since one of these compounds was prepared very early in the history of crown compounds. Ahmed and Meth-Cohn were interested in sulfur analogs of the porphyrin ring system and prepared compound 7 in 1969 by the method shown in Eq. 

Ono and Lash have been the two pioneers in applying the BZ reaction to the synthesis of pyrroles and, particularly, with applications to the synthesis of novel fused and other porphyrins. Although the concept was recognized by Barton and Zard, Ono and Lash independently discovered the conversion of 2-pyrrolecarbo ylates, prepared by the BZ reaction, into porphyrins by what is now a standard protocol (1. LiAlfL 2. 3. 

Ibers used the Paal-Knorr furan synthesis to prepare a key intermediate for the synthesis of novel porphyrin-like aromatic macrocycles. Bis yrolyljfuran 27 was available in good yield via the acid catalyzed condensation of diketone 26. ... 

Benzyl isocyanoacetate is a nsefnl reagent for the preparadon of benzyl 5-nnsnbsdnited pyrrole-2-carboxylates, which are widely used In the synthesis of porphyrins." Ono and coworkers have prepared pyrroles snbsdnited with various snbsdnients at the fi-posidons."" Because the reqidsite fi-nitro acetates for nitroalkenesi are readily available by the Henry... 

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. 

The enzymatic conversion of a-(aminomethyl)pyrroles is also used by nature to produce porphyrinogens like uroporphyrinogen III (see introduction, compound 8), which is the key building block in the biosynthesis of all known porphinoid natural products. This biomimetic method is a powerful tool for the synthesis of different porphyrins, e.g. for the preparation of JV,Af, V ,Ar"-tetramethylporphyrin-2,3,7,8,12,13,17,18-octaacetic acid dibromide 12.36... 

The synthesis of porphyrins from dipyrrylmethenes was first developed by Fischer42 and his collaborators. Different variants of this method are available. For the preparation of centrosym-metric porphyrins 7, the self-condensation of 5-bromo-5 -methyldipyrrylmethene hydrobromides or perbromides 6 in organic acid melts like succinic acid, tartaric acid or formic acid at temperatures up to 200 °C can be used. 

Syntheses of highly symmetric porphyrins from monopyrrolcs (sec Section 1.1.1.1.) and of less symmetric porphyrins from dipyrroles (see Section 1.1.1.2.) are the most common methods for the preparation of these macrotetracyclcs. However, in their pioneering studies of porphyrins and porphyrin derivatives Woodward45 and Johnson46 48 introduced the completely different... 

The cyclization of 1,19-dideoxybilene-i-dicarboxylic acid esters has been widely used for the synthesis of porphyrins. In this case, the use of tert-butyl esters which can be hydrolyzed with trifluoroacetic acid prior to the cyclization step is necessary. The cyclization step also requires trifluoroacetic acid and orthoformates. However, attempts to prepare porphyrins with /f-acceptor substituents can give rise to problems with side products and yields. 

The bromo substituent in l-bromo-19-meLhyl-l,l9-dideoxybiladienes- c is not essential for porphyrin formation. When 1-methylbiladiene-ac dihydrobromide or the 1,19-dimethyl-biladienc-ac are heated in refluxing methanol or dimethylformamide in the presence of cop-per(II) salts, the porphyrin copper complexes 13 are formed by oxidative cyclization. The free porphyrins can then be obtained by removal of the copper with acid. A wide range of porphyrins 13 can be prepared by this method. However, a restriction is the accessibility of the starting material with special substitution patterns. 

The synthesis of porphyrins from bilanones is free of all symmetry restraints.77 The oxo function is necessary to stabilize the bilane system by its electron-withdrawing effect. The synthesis of porphyrins from the parent bilane without the oxo function and with /3-substituents is possible,54 but the method gives rise to preparative problems due to the sensitivity of these compounds to oxidation, to electrophiles and to acids. Nature circumvents these problems in the cellular environment by exclusion of oxygen, when porphyrinogens, the precursors of porphyrins, are produced from bilanes55 in the course of their biosynthesis. 

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