Porphyrins, hydroxy

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

The only difference between the active oxidizing and a ferric porphyrin hydroxide complex is two electrons (scheme 4). Indeed, the electrochemical oxidation of hydroxy ferric tetra-mesitylporphyrin shows two reversible one-electron oxidations (40), and, in principle, use of water and an electrode should allow development of a system capable of catalytically oxidizing hydrocarbons. 

It is essential to characterize the reactant species in solution. One of the problems, for example, in interpreting the rate law for oxidation by Ce(IV) or Co(III) arises from the difficulties in characterizing these species in aqueous solution, particularly the extent of formation of hydroxy or polymeric species. We used the catalyzed decomposition of HjOj by an Fe(III) macrocycle as an example of the initial rate approach (Sec. 1.2.1). With certain conditions, the iron complex dimerizes and this would have to be allowed for, since it transpires that the dimer is catalytically inactive. In a different approach, the problems of limited solubility, dimerization and aging of iron(III) and (Il)-hemin in aqueous solution can be avoided by intercalating the porphyrin in a micelle. Kinetic study is then eased. 

Similarly, a series of hydroxy-terminated poly(ether) dendrimers, 35, with a single carborane nucleus at their core, were observed to have water solubilities comparable to that of chloroacetic acid, or D,L-valine. Again this is in direct contrast to the starting carborane nucleus which is insoluble in aqueous solutions and permitted the use of 35 in neutron capture therapy. Similar effects have also been observed with dendrimers containing calixarenes [66] and porphyrins [67] as the central units. 

Fig. 7. NMR of the hematoporphyrin model (A), its acetate (B), and the purified but methylated diporphyrin 7 (C). The two a H can be seen clearly at 6.1 ppm (adjacent to MeO) and at 7.6 ppm (adjacent to the ring-connecting ester). The a-H next to the methoxy group has a chemical shift very similar to that of the hydroxy porphyrin as verified by monomeric porphyrins.

Reaction of 175 with Cgg yields a hydroxy-functionalized fullerene that can be further derivatized. This hydroxy-fullerene was coupled with a porphyrine unit via a polyethyleneglycol-Hnker. This linker can be arranged similarly to a crown-ether to complex metal cations. Complexation is used to tune the distance between the porphyrin imit and the Cgg-moiety and thus tune the donor-acceptor properties of this porphyrin-fuUerene hybrid [177]. 

An example of side-chain fimctionalizahon is the attachment of tetraphenyl-porphyrin carboxylic acid to a fullerene bound steroid (214) via standard EDCl coupling with the hydroxy group of the steroid (Scheme 4.36) [230]. 

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

The most common methods of attaching substituents to porphyrins utilize ortho-phenyl-substituted derivatives of 5,10,15,20-tetraphenyl-21 H,23//-por-phine (TPP). While (2-aminophenyl) TPP derivatives have been extensively used in the preparation of biomimetic systems,5 the corresponding (2-hy-droxyphenyl) TPP derivatives have only recently been utilized.6 One of the limitations to the use of hydroxy-substituted systems has been the synthesis of 5,10,15,20-tetrakis(2,6-dihydroxyphenyl)-21//,23//-porphine 3. 

The X-ray crystal structure of [Pv TPP(OH)2 ]OH 2H2 O has been determined.32 The porphyrin skeleton is distorted S4 symmetrically to attain short P—distance ( 1.90 A). The hydroxy groups are strongly intermolecularly hydrogen bonded, and the phosphorus atom is displaced from the N4 plane by 0.096 A. The antimony(V) ion is centred in the porphyrin plane in [Sb(OEP)(OH)2]+ with a somewhat large Sb—distance of 2.065(6) A.31 The structure of [Bi(OEP)]NO, is of interest, since the four-coordinate BiUI ion protrudes from the porphyrin N4 plane by 1.09 A with a very long Bi—Npor distance of 2.32 A.31... 

For the synthesis of the porphyrin the formate ester of 3j3-hydroxy-5-cholenic acid 179 was coupled via amide bonds to the a,/ ,a,j -atropisomer of meso-tetrakis(o-aminophenyl)-porphyrin 170, using the mixed anhydride... 

Oxo-metal species participate in a wide range of biological and chemical oxidation reactions. Representative oxidizing enzyme, cytochrome P-450, which carries iron(III)-porphyrin complex as its active site, catalyzes various O-atom transfer reactions such as epoxidation, hydroxy-lation of C-H bond, and oxidation of sulfides. These reactions have been proven to proceed through cationic oxoiron(IV)-porphyrin species, which are generated by the oxidation of Fe(III) complex with molecular oxygen. This conversion from Fe(III) to 0=Fe(IV) species is a... 

Amino-l,2,4-benzotriazine 1,2-dioxide Thin layer chromatography Thymidine-5 -phosphate iV,iV,iV, iV -Telramclhylphcnylcncdiamine Tetranitromethane 3,5,4 -Trihydroxy-frans-stilbene 2-Amino-2-hydroxy-l, 3-propanediol Time-resolved light-scattering p-[meso-5-5,10,15,20-Tetra(pyridyl)porphyrin]tetra kis[ (ns-(bipyridine) chloride ruthenium(II)]... 

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