Porphins, reduction

It has been recently demonstrated that the simplest of the cobalt porphyrins (Co porphine) adsorbed on a pyrolytic graphite electrode is also an efficient electrocatalyst for reduction of 02 into 1120.376 The catalytic activity was attributed to the spontaneous aggregation of the complex on the electrode surface to produce a structure in which the cobalt-cobalt separation is small enough to bridge and activate 02 molecules. The stability of the catalyst is quite poor and largely improved by using porphyrin rings with mew-substitu-tion.377-380 Flowever, as the size of the mew-substituents increases the four-electron reduction efficiency decreases. 

Manganese nitrosyl porphyrins [215] are considered good models for the iron-nitric oxide analogs, which are relatively unstable but very vital to many biological operations. A six-coordinate manganese nitrosyl porphyrin of the form (por)Mn(NO)(L), where por can be TTP (TTP = tetra(4-methylphenyl)porphine) and L = piperidine, methanol, 1-methyhmidazole, has been prepared [216] in moderate yields by the reductive nitrosylation of the (por)MnCl complex with NO in piperidine. The crystal structures of these compounds give indication of a linear Mn-NO bond [215]. 

The water-soluble Fe porphyrin, 3Na+ [Fe(III)(TPPS)] -12H20 [H2TPPS4- = tetra-anionic form of meso-tetrakis(7r-sulfonatophenyl)porphine], has recently been shown to be an effective catalyst for the electroreduction of nitrite to ammonia [419]. The Fe meso-tetrakis(A -methyl-4-pyridyl) porphyrin and/or the Fe meso-tetrakis (jr -sulfophenyl) porphyrin complex shows a catalytic activity for the reduction of dioxygen in aqueous solutions, leading to hydrogen peroxide [420]. 

Reduction of p-oxo-bis[tetraphenylporphin iron(iii)] with sodium amalgam gives several products sequentially. The first reduction product is the radical-anion [(tpp)Fe ] , which has a spin state of - at 77 K and at 300 K. This is the first reported study of an Fe -porphyrin.Mossbauer data on a range of tetra-aryl-substituted iron-porphines and their p-oxo-bridged derivatives have been reported. A crystallographic study on bis(imidazole) otPy -tetraphenylporphinatoiron(iii) chloride-methanol solvate shows the [FeN ] core to be quasitetragonal. An n.m.r. study has been made of the related... 

Cobalt porphyrins have been some of the most studied catalysts for oxygen reduction, due in part to their strong interaction with molecular oxygen and the ease with which they catalyze the reduction at low potentials. Anson and coworkers [97] performed a study of the simplest of cobalt porphyrins, cobalt porphine, in the hope of gaining a baseline for the observed reactions of other porphyrins. Instead, they observed a very different process from most other monomeric cobalt porphyrins. 

Cobalt porphine promotes the reduction of O2 in two steps, both of which occur at unusually positive potentials the first step results in the formation of water rather than hydrogen peroxide. 

Fig. 1 Basic structure (left side) and the absorption spectra (right side) of some photosensitizers. The absorption spectra of porphyrins (derived from porphins) consists of a Soret band (around 400 nm) and four Q-bands. Upon reduction of one or two double-bonds of the tetra-pyrrole structure or by expanding the number of 7i-electrons (by expanding the ring structure), the outermost Q-band becomes bathochromically shifted and the absorption coefficient increased as indicated on the figure. As described in the text, such chemical modifications are important for improving the therapeutic effect in deeper tissue layers...

The first example of a chiral-auxiliary-induced [2+2] cycloaddition between 02 and oxazolidinone-functionalized enecarbamates, which proceeds with complete diastereoselectivity as a result of steric repulsions, has been reported to afford 57 <02JACS8814>. The optically active enecarbamates bearing Evans chiral auxiliary were photooxygenated at -35 °C with 5,10,15,20-tetrakis(pentafluorophenyl)porphine (TPFPP) as sensitizer and an 800 W sodium lamp as light source. The dioxetanes 57 were obtained exclusively, but they readily decomposed at room temperature to the expected carbonyl products because of their thermally labile nature. The absolute configuration of the dioxetanes 57 was established by reduction to the corresponding diols with L-methionine. 

There are macrocyclic ligands of biochemical interest that contain a modified porphine skeleton in varying degrees of reduction compared to the porphine nucleus one such species is tetrahydrocorrin. The reaction between (1,2-... 

The polarographic and controlled potential reduction of porphines (LXXII) has been investigated, and it was shown that porphine was reduced polarographically in two two-electron steps [372]. Reduction of /w o-porphyhyrinedimethyl ester yielded a product that was easily reoxidized by oxygen. From the UV spectrum of the reduced solution, which was reduced in deuterium oxide, it could be shown that the reduction yielded a phlorine (LXXIII) rather than a chlorine (LXXIV). Reduction of a chlorine yielded a chlorine/phlorine. ... 

Chang and coworkers, using extended Huckel MO calculations provided an explanation for the easier oxidations and harder reductions as the porphyrin macrocycle is progressively reduced [93]. The calculated energy levels for the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of porphine, chlorin, iBC and BC complexes of Zn(II) are shown in Fig. 7. As the... 

P.C. Wilkins and R.G. Wilkins, Rapid Pulse Radiolytic Reduction of Iron(III) Complexes of Tetrakis(4-sulfonatophenyl)porphine Anion and Tetrakis(N-methylpyrid-4-yl)porphine Cation, Inorg. Chem., 25 (1986) 1908. 

The tetraarylporphyrin ligand was bonded to macroreticular polystyrene through carboxamide and sulfonamide linkages. The aminopoly-styrene used for the preparation of these materials was obtained from unfunctionalized polystyrene by nitration, followed by reduction of the nitro groups with stannous chloride. This aminopolystyrene was then coupled with the tetra(p-chlorosulfophenyl)porphine and the tetra(p-chlorocarbonyl)-phenylporphine obtained from the respective adds or acid salts to give the sulfonamide-linked and the carboxamide-linked... 

This hypothesis finds support from observations of the effect of the naturally occurring quinones, juglone and lawsone, and an iron porphin (Table 7.8) on the reduction of nitrobenzenes (Fig. 7.4). Note that the electron carriers are present in micromolar (juglone, 10-150 ulM lawsone, 3-250 pJW, and porphyrin, 2-40 iM) rather than the 5-vaM concentrations of the reducing compounds. A cysteine-cystine system was used with the iron porphin that was precipitated by H2S. In the presence of H2S, the Eh(W) in the H2S system was —0.192 V and < — 0.380 V in the cysteine system. Reactions were carried out in the pH range of... 

Polyhalogenated methanes and ethanes are also reduced in both H2S (1 mM)/ juglone (200 jlM) and the iron porphyrin (2-50 piM) cysteine (5 mM) systems. " The reduction for some alkyl halides in the presence of the iron porphin is indicated by a second-order rate constant, since the rate is proportional to the concentration of both the alkyl halide and the porphyrin (Table 7.9). A half-life at a concentration of 10 pM iron porphyrin is included. 

Kinetic resolution of another allylic alcohol in the Sharpless epoxidation was also employed in the synthesis of (Jl)-lO and fully oxygenated AB building block 35 (Scheme 6) [47]. Epoxidation of alcohol 34 [48] followed by LAH reduction and then oxidation with the Fetizon reagent [41,49] furnished hydroxyketone (Jl)-lO. Ketahzation of the carbonyl group and subsequent stereoselective benzyUc hydroxylation, using iodosobenzene in the presence of [ 5,10,15,20-tetrakis(pentafluorophenyl)-2 lH,23H-porphine ] iron(lll) chlo-... 

Hydroporphyrins. H. are derived from completely unsaturated porphyrins by reduction of the double bonds in the porphine skeleton. The spectrum of naturally occurring H. ranges from the dihydro derivatives chlorin and phlorin(s) through the tetrahydro derivatives bacteriochlorin and isobacteriochlorins, the hexahydro derivatives porphyrinogens and precor-rins to the more highly reduced corphins and factor F430. 

The structural skeleton of I. (tetrahydroporphyrins) is formally derived from porphine by reduction of two peripheral double bonds in neighboring pyrrole rings of the porphyrin skeleton. I. are constitutional isomers of the bacteriochlorins, from which their name is derived. Natural members of this structural type are heme d, siroheme, and sirohydrochlorin. 

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