Porphyrins reactions with radicals

H. Levanon and P. Neta, Electron Transfer Reactions of Chlorophyll a and Porphyrin Triplets with Radicals in Aqueous Micellar Solutions, J. Phys. Chem., 86 (1982) 4532. 

Iron porphyrins containing vinyl ligands have also been prepared by hydromet-allation of alkynes with Fe(TPP)CI and NaBH4 in toluene/methanol. Reactions with hex-2-yne and hex-3-yne are shown in Scheme 4. with the former giving two isomers. Insertion of an alkyne into an Fe(III) hydride intermediate, Fe(TPP)H, formed from Fe(TPP)Cl with NaBH4, has been proposed for these reactions. " In superficially similar chemistry, Fe(TPP)CI (present in 10 mol%) catalyzes the reduction of alkenes and alkynes with 200 mol% NaBH4 in anaerobic benzene/ethanol. For example, styrene is reduced to 2,3-diphenylbutane and ethylbenzene. Addition of a radical trap decreases the yield of the coupled product, 2,3-diphenylbutane. Both Fe(lll) and Fe(II) alkyls, Fe(TPP)CH(Me)Ph and [Fe(TPP)CH(Me)Ph] , were propo.sed as intermediates, but were not observed directly. ... 

The chemical reactivity of the organoruthenium and -osmium porphyrin complexes varies considerably, with some complexes (M(Por)R2, M(Por)R and Os(OEP)(NO)R) at least moderately air stable, while most are light sensitive and Stability is improved by handling them in the dark. Chemical transformations directly involving the methyl group have been observed for Ru(TTP) NO)Me, which inserts SO2 to form Ru(TTP)(N0) 0S(0)Me and Ru(OEP)Me which undergoes H- atom abstraction reactions with the radical trap TEMPO in benzene solution to yield Ru(OEP)(CO)(TEMPO). Isotope labeling studies indicate that the carbonyl carbon atom is derived from the methyl carbon atom. "" Reaction of... 

Hydroxyl radical is a strong indiscriminate outer-sphere oxidant (generating OH ) and H-atom abstractor (generating H2O) [Huie and Neta, 1999]. Simple Fe porphyrins are known to promote 0-0 bond homolysis in reaction with H2O2 [Watanabe, 2000]. Because of its high reactivity, once generated, "OH probably reacts with the... 

The functionalization of zinc porphyrin complexes has been studied with respect to the variation in properties. The structure and photophysics of octafluorotetraphenylporphyrin zinc complexes were studied.762 Octabromoporphyrin zinc complexes have been synthesized and the effects on the 11 NMR and redox potential of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin were observed.763 The chiral nonplanar porphyrin zinc 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin was synthesized and characterized.764 X-ray structures for cation radical zinc 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin and the iodinated product that results from reaction with iodine and silver(I) have been reported.765 Molecular mechanics calculations, X-ray structures, and resonance Raman spectroscopy compared the distortion due to zinc and other metal incorporation into meso dialkyl-substituted porphyrins. Zinc disfavors ruffling over doming with the total amount of nonplanar distortion reduced relative to smaller metals.766 Resonance Raman spectroscopy has also been used to study the lowest-energy triplet state of zinc tetraphenylporphyrin.767... 

The structure of HRP-I has been identified as an Fe(IV) porphyrin -ir-cation radical by a variety of spectroscopic methods (71-74). The oxidized forms of HRP present differences in their visible absorption spectra (75-77). These distinct spectral characteristics of HRP have made this a very useful redox protein for studying one-electron transfers in alkaloid reactions. An example is illustrated in Fig. 2 where the one-electron oxidation of vindoline is followed by observing the oxidation of native HRP (curve A) with equimolar H202 to HRP-compound I (curve B). Addition of vindoline to the reaction mixture yields the absorption spectrum of HRP-compound II (curve C) (78). This methodology can yield useful information on the stoichiometry and kinetics of electron transfer from an alkaloid substrate to HRP. Several excellent reviews on the properties, mechanism, and oxidation states of peroxidases have been published (79-81). 

FIGURE 3.30. Reaction of iron(0) and iron(I) pophyrins with n-, s-, and r-butyl bromides. The chart shows the various porphyrins and their symbolic designations. iron porphyrins, aromatic anion radical, lines best-fitting parabolas through the aromatic anion radicals data. Dashed lines outer-sphere curves obtained by use of the Morse curve model (Section 3.2.2). Adapted from Figure 4 in reference 47b, with permission from the American Chemical Society. 

This behavior, as well as complementary observations, can be explained on the basis of the reaction mechanism depicted in Scheme 5.3. The main catalytic cycle involves three successive forms of the enzyme in which the iron porphyrin prosthetic group undergoes changes in the iron oxidation state and the coordination sphere. E is a simple iron(III) complex. Upon reaction with hydrogen peroxide, it is converted into a cation radical oxo complex in which iron has a formal oxidation number of 5. This is then reduced by the reduced form of the cosubstrate, here an osmium(II) complex, to give an oxo complex in which iron has a formal oxidation number of 4. 

Analysis of the transition state in terms of energy is certainly a key aspect of the S 2-ET problem. Entropy considerations may, however, bring about additional information, possibly helping us to conceive better the transition between the two mechanisms. It was observed in this connection that, whereas the entropy of activation of both the anthracene anion radical and of the ETIOPFe(O) porphyrin (pp. 99, 100) (which have about the same standard potential) is close to zero in their reaction with s- and t-butyl bromides a definitely negative value, ca. — 20 eu is obtained for the reaction of the porphyrin with n-butyl bromide (Lexa et al., 1988). The same was found for the reaction of two other iron porphyrins, TPPFe(o) and OEP-Fe(i). These activation entropies were estimated from (153), where Z is... 

Once activated, MV-CCP reacts with 1 equiv of H2O2 in a bimolecu-lar reaction, presumably to form compound 0. In YCCP and HRP this species is referred to as compound ES or compound I, respectively, and contains oxyferryl heme and either a porphyrin n -cation radical (HRP) or an amino acid radical (YCCP). However, the presence of an extra reducing equivalent on the second heme in CCP suggests that such an oxidizing radical species close to the active site heme will be very shortlived and readily form compound I (Fig. 10), which is formally Fe(HI) Fe(IV)=0. The bimolecular rate constant for compound I formation is reported to be very close to the diffusion limit (84). 

The radical cations of zinc(II) porphyrins may be prepared from the parent complex ZnL, by reaction with Fem,1187 or by oxidation of a triplet state with Eu111 1188 it photodisproportionates to [ZnL] and [ZnL]2+.1187 1... 

Initially, the products of these reactions suggested radical ions were involved138. In particular, when hexamethyl Dewar-benzene was epoxidized with MCPBA, the nature of the products depended on whether or not the iron(III) porphyrin hemin was added to the reaction mixture1381 . Furthermore, when Z-stilbene was epoxidized with dioxygen, catalyzed by (tetraphenylporphorinato)iron(III) chloride, -stilbene appeared in the reaction mixture139. 

The neutral, high-spin ferric (Fem) resting state reacts with peroxide to form a FeIV porphyrin-cation radical (compound I), which is subsequently reduced to the native resting state by the reaction with a second peroxide. Additional information on iron catalases can be obtained by consulting recent (post-1991) literature [71],... 

Figure 7.3 Methyl radical reactions with iron porphyrins.

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