TPP analog

High-temperature solution decomposition of Fe(TPP)N3 (TPP = tetraphenyl-porphyrin) in the absence of added base or oxygen has been examined. The only reaction product observed is formulated as (TPP)FeNFe(TPP), analogous to [Fe(TPP)]20.258 The structure of Fe(l-methylimidazole)2(PPIX),CH30H,H20 (PPIX = protoporphyrin IX) has been reported. The porphyrin has a distinctly ruffled inner core. Magnetic resonance studies of the autoreduction of ferric porphyrins in the presence of piperidine or cyanide demonstrate that reduction to iron(ii) is accompanied by one-electron oxidation of the substrate. ... 

The electrochemistry of [Th(Por)(OH)2]3 (Por=OEP,TPP) is of particular interest as they contain three redox active metalloporphyrin units (Kadish et al. 1988). The cyclic voltammogram of the OEP complex recorded in THE at -72 C shows three reversible one-electron reduction couples at -1.49, -1.70, and -1.87 V vs. SCE. As the temperature rises to room temperature, the third reduction becomes irreversible, and it has been shown that it involves a one-electron transfer followed by a fast chemical reaction (probably dissociation) and an additional one or more electron reduction (an electrochemical ECE-type mechanism). The UV-Vis spectrum of the electroreduced species [Th(OEP)(OH)2]3 shows absorption bands at 411, 456, and 799 nm, and its ESR spectrum displays a signal at g=2.003, both of which are characteristic of a porphyrin ti radical anion. Further one-electron reduction doubles the molar absorptivities of the absorption bands at 456 and 799 nm, indicating that the second reduction is also based on porphyrin. The TPP analog [Th(TPP)(OH)2]3 also exhibits three reversible one-electron reductions at -1.13, -1.27, and -1.36V at -55 C, which are shifted by 360-510mV relative to the respective processes for [Th(OEP)(OH)2]3 at —72 C. Three additional irreversible reductions at —1.76, -2.00, and —2.10 V are also observed for this complex when the potential is scanned to -2.20 V which may be due to the formation of dianions localized on each of the three porphyrin units. Spectroelectrochemical data also indicate that the initial three reductions occur at porphyrin based orbitals. 

Using a series of TPP-analogs, Schellenberger was able to obtain important information on the nature of the binding and on the coenzymatic activity. The TPP-derivatives which we have synthesized are shown in Figure 9. All analogs except 2 -ethyl-TPP are enzymatically inactive and competitive inhibitors to the native co-... 

Table III summarizes the and Ki values for the various TPP analogs. The most potent inhibitor tested was oxy-TPP, which has an even higher affinity to the apo-enzyme than TPP. A more detailed interpretation of the different Ki values will be given later.

Activation parameters for the reaction of NO with metMb, Eq. (15), were determined in this laboratory and in collaboration with van Eldik and Stochel (Table II) (23). Comparison of these activation parameters with those determined for reactions of NO with the water soluble ferri-heme complexes Fem(TPPS)(H20)2 and Feni(TMPS)(H20)2 (Table II) demonstrate that the latter compounds represent reasonable models for the kinetics for the analogous reaction with metMb. For example, the kon step would appear to be defined largely by the H20 lability of metMb(H20), although it is clear that the diffusion through protein channels, the distal residues and the proximal histidine binding to the Fe(III) center must all influence the NO binding kinetics (23,24). These properties may indeed be reflected in the lower AS values for both the on and off reactions on metMb. In a related study, Cao et al. recently... 

Literature reports of NO disproportionation reactions with Fe(II) porphyrins contain many mutually inconsistent observations. Although facile NO disproportionation is promoted by Ru(II) and Os(II) (88) porphyrins to yield N20 and the respective M(Por)(NO)(ONO) complexes, the reactivity appears to be quite different with analogous Fe(II) complexes. Ferrous porphyrins such as Fen(TPP) undergo NO addition in ambient temperature solution to give the relatively stable... 

This reaction has been reexamined using optical, IR and NMR spectroscopic methods to probe NO reactions with Fe(TPP)(NO) and the more soluble Fe(TmTP)(NO) (92). These studies confirmed the formation of Fe(Por)(NO)2 in toluene-dg at low temperature (Eq. (43)). NMR line shape analysis was used to calculate K43 = 23 M-1 at 253 K (3100 M-1 at 179 K, AH° = —28kJmol 1) (92). The failure of the Fen(Por) complexes to promote NO disproportionation, in contrast to the behavior of the respective Ru(II) and Os(II) analogs, may find its origin partly in the relatively low stability of the dinitrosyl intermediate (K52 estimated to be 2.8 M-1 at 298 K) and unfavorable kinetics of subsequent reaction of this species with NO. 

Electrochemistry. Compounds 15a [Fen(pz)pzEts] 15b [Fen(l-MeIm)2Ets] exhibited two reversible oxidations at 0.21 and -0.06 V (vs Ag/AgCl), which were attributed to the 2 —> 3 and 3 —> 4 metal-centered oxidations. These values are 400 mV more positive than those reported for the analogous Fe TPP, suggesting that the pz stabilizes the lower oxidation state, Fe(II) better than does the porphyrin (63). This stabilization of a basic oxidation state for the pz may be attributable to the lower energy n orbitals of the pz making it a better n acceptor than the porphyrin. 

The banded coherency spectrum has an analogous definition (Yeung and Pope 1993) where the energy spectra are replaced with the spectra integrated over a finite wavenumber band. From the spectral time-evolution equations, it is easily shown that, in the absence of mean scalar gradients, the time evolution of the coherency spectrum is governed by 1 dPap Tap 1 Taa 1 Tpp ... 

The authors ascribed the ds-stilbene preference to steric interference between the phenyl groups of tranx-stilbene and the phenyl groups on the [Fe -0(TPP+ )]+ intermediate. The Groves research group also gathered the first spectroscopic indications of a synthetic, compound I analog. The [Fe =0(TMP+ )]+ complex (TMP equals the dianion of mexo-tetramesityl-phenylporphyrin) was prepared by the m-CPBA (3-chloroperoxybenzoic acid) oxidation of [Fe -0(TMP)C1]. (See Figure 7.21.)... 

Itoh and coworkers (13) have reported RR spectra for radical cations of metallo-TPP s, all of which form a2u radicals, and have noted that the observed downshift of V4 is contrary to the orbital symmetry argument. In the case of TPP s, there appears to be no band which shifts up on radical formation, analogous to V2 of the OEP a2u radicals. 

The unstable species [RuX4-Bu -TPP)(NR) is prepared by the oxidation of [Ru (4-Bu -TPP)(NR)] (Section 5.6.6.2) using Ag+ or Ce. This complex can transfer the imido group to PMe2Ph at a rate 60 times faster than that its Ru analog. 

The analogous adducts of artemether Ic and fenozan 4 were also obtained and characterized in each case porphyrin adducts were isolated in 20-30% yield. When artemisinin was treated with manganese(II) TPP in the presence of cyclohexene, Robert and Meu-nier also failed to observe any of the corresponding epoxide. The authors suggest that the Dewar benzene rearrangement and the oxidation reactions observed by Posner... 

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