Fluorinated porphyrins

Figure 6.13. Fluorinated porphyrin ligand used in the cobalt catalysed aerobic oxidation of alkenes.[50]...

The synthetic paths of fluorinated porphyrins (fluorine atoms or fluoroalkyl chains) are numerous and depend on the fluorination site (periphery or meso position). These syntheses are performed through the use of fluoroalkylated pyrroles, through DAST fluorination, or through direct perfluoroalkylation. Some examples are given next. 

Fig. 47 (a) Examples of polyphilic molecules with star shaped molecular topologies and (b) their mesophase morphologies [295]. (c) Janus-type porphyrin 177 [277] and (d) modes of self assembly of the completely RF-substituted porphyrin 156 left, for structure see Fig. 42, Colortho G -29 °C Colortho199 °C Iso) and the partly fluorinated porphyrin 177 right, ColTeJp2mg, Cr -22 °C Co n,Jp2mg 163 °C Iso) Colortho = orthorhombic columnar 3D phases (b) Reproduced with permission [295], copyright 2008, The Royal Society of Chemistry (RSC) (d) reproduced with permission [277], copyright 2011, American Chemical Society (ACS)...

As with cobaloximes, substituents on the equatorial ligand have only a moderate effect on the value of Cc for the complexes in Table 3. The same is true for substituents on cobalt porphyrins, 1 and 45—51 (Table 4). For tetrakis(pentafluoroethylphenyl)-porphyrin—Co11 the substituent effect is not clear. The fluorinated porphyrin works moderately for the polymerization of MMA in supercritical C02 with chain-transfer constant Cc = 550 at 60 °C.126 Unfortunately, no data on the chain-transfer constant in bulk polymerization are available, so that it is not clear whether this reduced value of Cc is the result of solvent or the presence of a strong EWG such as pentafluorophenyl in the porphyrin macrocycle. Similar experiments with 9c (Table 2) led to Cc = 378 000, which is 20 times higher than in bulk MMA or in organic solvents.30 We may conclude at this point that additional experiments are required with different catalysts to allow us to make reliable conclusions. 

Porphyrins can be conveniently assembled by condensation of simple pyrroles with aldehydes under Lewis acid catalysis (Lindsey conditions). Accordingly, 3-fluoro- and 3,4-difluoropyrroles have received much attention as precursors of fluorinated porphyrin derivatives. For example, in work on the synthesis of partially [3-fluorinated 5,10,15,20-tetraphenylporphyrins and derivatives, previously unreported 3-fluoro-1//-pyrrole (52) was required (see Fig. 3.24)." ... 

Barnes and coworkers had prepared A-triisopropylsilyl-3-fluoropyrrole 53 from the corresponding bromo derivative but did not report the isolation of the N-unsubstituted parent 52. However, 53 could be desilylated to produce 52, which was used in situ to produce partially p-fluorinated porphyrins and derivatives (Fig. 3.24). Attempts to isolate and purify 52 either from the desilylated crude product or through deprotection of the derived Boc-protected intermediate 54 met with limited success, although sufficient material for characterization was produced... 

There have been several reports of the synthesis of p-fluorinated porphyrins by construction of the macrocyclic ring using fluorinated pyrrole precursors. For... 

Partially fluorinated porphyrins were made by a similar Lindsey cyclization. Condensation of benzaldehyde with mixtures of 3,4-difluoropyrrole and pyrrole gave mixtures of di-, tetra-, and hexafluoro me.so-tetraphenylporphyrin with fluorine atoms in pairs per pyrrole. On the other hand, condensation of 3-fluoropyrrole with benzaldehyde gave a mixture of tetrafluoro-we o-tetraphenylporphyrin with one fluorine atom per pyrrole." These partially [3-fluorinated porphyrins were used to study the differential effects of fluorine substituents on spectral properties and core deformations of the porphyrin system. 

Suzuki, A. Tomizawa, T. Hayashi, T. Mizutani, T. Ogoshi, H. Synthesis of ring-fluorinated porphyrins and reconstituted myoglobins with their iron complexes. Bull. Chem. Soc. Jpn. 1996, 69, 2923-2933. 

Because of their great triplet quantmn yields, it was postulated that these fluorinated porphyrin derivatives can act as potent photosensitizers in their potential medicinal applications [10]. 

In one approach we synthesized a compound with a covalently linked bridge across the porphyrin carrying the phenyl thiol group, and with two cyclodextrins on the other two positions. In the other approach we synthesized a phenylthiol derivative with two linkers to binding groups, and used it with our original fluorinated porphyrin. It bound to two cyclodextrins on one face of the porphyrin, while the substrate bound to the other two cyclodextrins. This is much like the enzyme in which the thiol comes from a cysteine of the protein, not from a covalent link to the porphyrin. In both cases we saw that we could now use hydrogen peroxide as the oxidant for our selective C-6 hydroxylation. Thus a major problem in the previous work - the use of an expensive oxidant - was solved. 

The electronic structures of the porphyrins were compared through the analysis of the H NMR spectra of their dimethyl ester derivatives (Fig. 4). The fluorinated porphyrins exhibited typical spectral patterns similar to those of protoporphyrin and mesoporphyrin. In general, the shifts of the signals are affected by the ring current of the porphyrin macrocycle and the electronic inductive effect of nearby substituents and the ring-current shift is also modulated by the electronic contribution of the substituents. Since four meso protons, namely, protons bound to carbons at the 5, 10, 15, and 20 positions occupy the same orientation relative to the porphyrin macrocycle, in-plane asymmetry of the haem electronic structure is reflected directly... 

Novel nanoscale discotic liquid crystalline porphyrins 14 with partial alkyl chain perfluorination has been designed and synthesized in our group which exhibits exceptionally enhanced tendency to spontaneously self-assemble into homeotropically ordered nanostructures [107]. The defect-free homeotropically aligned fluorinated porphyrin thin films were fabricated and characterized. In the films thinner than 10 pm in glass cells, it shows strong tendency towards homeotropic... 

Alternatively fluorinated porphyrin 474 was synthesized by oxidation of 4-fluoropyrrole 473 with KgFeCCN). The target tetrafluoroporphyrin 474 was prepared in poor yield, however [23]. It should be noted, that due to the high... 

Trifluoromethylpyrroles were also used in the fluorinated porphyrins synthesis [121], Thus, porphyrin 475 was prepared in three steps starting from pyrrole 373a. At first step reduction of carboxyethyl group was performed. Next, intermolecular alkylation takes place to form CH2-bridged precursor of porphyrin under PTSA catalysis. The last step is aromatization under treatment with chloranile. 

Fluorinated Porphyrins and Corroles Synthesis, Electrochemistry, and Applications... 

The Ogoshi and Suzuki groups published the first confirmed report of partially p-fluorinated porphyrins in the mid-1980s. The Ogoshi group performed a tetrameri-zation of the fluorine snbstitnted pyrrole (1) with potassium ferricyanide. 

In contrast to the relatively longstanding interest in p-fluorinated porphyrins, research on the preparation and study of p-fluorinated corroles is quite new. Corroles have been shown to have distinctive coordination chemistry and lower oxidation potentials compared to porphyrins, but the coordination environment offered by corroles can chelate transition metal ions with an avidity similar to that of porphyrins. [1, 32]. Figure 3 shows the structural differences among p-octaflnorinated porphyrins, p-octafluorinated corroles, and p-octaflnorinated (with meso fluorinated aryl rings) Hangman corroles. 

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