Porphyrin complexes reactivity

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... 

The chemistry of organorhodium and -iridium porphyrin derivatives will be addressed in a separate section. Much of the exciting chemistry of rhodium (and iridium) porphyrins centers around the reactivity of the M(ll) dimers. M(Por) 2-and the M(III) hydrides, M(Por)H. Neither of these species has a counterpart in cobalt porphyrin chemistry, where the Co(ll) porphyrin complex Co(Por) exists as a monomer, and the hydride Co(Por)H has been implicated but never directly observed. This is still the case, although recent developments are providing firmer evidence for the existence of Co(Por)H as a likely intermediate in a variety of reactions. 

Vitamin B12 catalyzed also the dechlorination of tetrachloroethene (PCE) to tri-chloroethene (TCE) and 1,2-dichloroethene (DCE) in the presence of dithiothreitol or Ti(III) citrate [137-141], but zero-valent metals have also been used as bulk electron donors [142, 143]. With vitamin B12, carbon mass recoveries were 81-84% for PCE reduction and 89% for TCE reduction cis-l,2-DCE, ethene, and ethyne were the main products [138, 139]. Using Ni(II) humic acid complexes, TCE reduction was more rapid, leading to ethane and ethene as the primary products [144, 145]. Angst, Schwarzenbach and colleagues [140, 141] have shown that the corrinoid-catalyzed dechlorinations of the DCE isomers and vinyl chloride (VC) to ethene and ethyne were pH-dependent, and showed the reactivity order 1,1-DCE>VC> trans-DCE>cis-DCE. Similar results have been obtained by Lesage and colleagues [146]. Dror and Schlautmann [147, 148] have demonstrated the importance of specific core metals and their solubility for the reactivity of a porphyrin complex. 

It is well known that crystal and electronic structures are interdependent and define the reactivity of chemical substances. In Section 1.4.2, it was noted that copper-porphyrin complex gives cation-radicals with significant reactivity at the molecular periphery. This reactivity appears to be that of nucleophilic attack on this cation-radical, which belongs to n-type. The literature sources note, however, some differences in the reactivity of individual positions. A frequently observed feature in these n-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of porphyrin consistent with a localized structure rather than the delocalized structure usually ascribed to cation-radical. A pseudo Jahn-Teller distortion has been named as a possible cause of this alternation, and it was revealed by X-ray diffraction method (Scheidt 2001). 

Although photochemically induced cleavage of Al—C bonds in the aluminum porphyrin complexes has been exploited in several applications, relatively little is known about the intimate mechanism of this process. Similar reactivity is observed for the organo-gallium and indium porphyrins, and for these elements... 

Aluminum-porphyrin complex lb with an alkoxide ligand also demonstrates the same reactivity as la in the presence of only 0.1 mol.% of 2a. The polymerization rate with lb/2a catalyst system is dependent on the concentration of 2a in the range from 0.025 to 2.5 mol.%, the increase of 2a results in more rapid polymerization. On the other hand, molecular weight and the number of polymer chains are independent of the molar ratio of 2a to... 

As described above, cis olefin is more reactive than trans olefin in the homogeneous model oxidations catalayzed by synthetic iron porphyrin complexes, however, both L29H/H64L and F43H/H64L Mb catalyze the trans olefin more efficiently, presumably due to the presence of the substrate-binding site. 

The indole oxidation has been shown to proceed via the hydroperoxide intermediate 9 (126), but whether this is formed via coordination catalysis, for example, as suggested in Reaction 41 for a phenol substrate (10— 12,13,14) (124), or via Haber-Weiss initiation, poses the same problem encountered in the organometallic type systems. A reactivity trend observed for Reaction 40 using tetraphenyl-porphyrin complexes (Co(II) Cu(II) Ni(II)) is reasonable in that the Co(II) system is known to give 1 1 02-adducts (at least, at low temperatures) but the reactivity trend also was observed for the catalyzed decomposition rate of 9. It is interesting to note that in Reac-... 

The synthesis of a stable Felll-porphyrin complex-alcenethiolate complex, in which the sulphur atom is sterically protected from reactive molecules such as 02 and NO by bulky groups, has been reported (Suzuki et al., 2000). The electronic absorption and infrared spectra indicate that NO coordinates reversibly to the Fein atom of the complex. 

N-substituted iron porphyrins form upon treatment of heme enzymes with many xenobiotics. The formation of these modified hemes is directly related to the mechanism of their enzymatic reactivity. N-alkyl porphyrins may be formed from organometallic iron porphyrin complexes, PFe-R (a-alkyl, o-aryl) or PFe = CR2 (carbene). They are also formed via a branching in the reaction path used in the epoxidation of alkenes. Biomimetic N-alkyl porphyrins are competent catalysts for the epoxidation of olefins, and it has been shown that iron N-alkylporphyrins can form highly oxidized species such as an iron(IV) ferryl, (N-R P)Fe v=0, and porphyrin ir-radicals at the iron(III) or iron(IV) level of metal oxidation. The N-alkylation reaction has been used as a low resolution probe of heme protein active site structure. Modified porphyrins may be used as synthetic catalysts and as models for nonheme and noniron metalloenzymes. 

One motivation for the characterization of the above compounds has been to more fully understand the involvement of such higher valent manganese porphyrin complexes in model systems which imitate the catalytic activity of monooxygenase cytochrome P-450 and related enzymes. The catalytic cycle of cytochrome P-450 appears to involve the binding and reduction of molecular oxygen at a haem centre followed by the ultimate formation of a reactive iron oxo complex which is responsible for oxidation of the substrate. For example, cytochrome P-450 is able to catalyse alkane hydroxylation with great selectivity. 

Usually, high oxidation states of metals are stabilized most effectively when in combination with the most electronegative atoms, fluorine and oxygen. Rather surprisingly, no FeIV or higher oxidation state compounds with fluorine are known. Indeed rather few stable or well-defined higher oxidation state iron compounds are known. Despite this, there can be no doubt about the importance of such compounds. We have already encountered the FeIV and Fev states in reactive intermediates in the reactions of several metalloproteins and of several synthetic porphyrin complexes. 

---