Carbene complexes with iron porphyrins

It has been shown that these carbenes can insert into Fe-Npor bonds to form vinylidine (A(-alkyl-iron porphyrin metallacycle (N-C-C-Fe)) complexes, " which can then go on to form c/s-bridged vinylidines of the porphyrin, with loss of iron. Carbene equivalents of iodosylbenzene can also react with iron porphyrins to give five-membered Fe-O-C-C-N metallocycles. Iron porphyrin carbenes can also react with amines to produce the isocyanide complexes (equation 58) ... 

The chemical shifts of the carbene complex vary from 210 to 385 ppm in the iron series. These large differences have been interpreted by taking into account the more electrophilic nature of the carbene atom due to a possible stabilization of a positive charge by an alkyl group in some cases [102]. An increase of the chemical shift is also noted in going from the pentacoordi-nate complex to the hexacoordinate adduct. For example, the C chemical shift of the diphenyl carbene increases from 358.9 to 385.4 ppm with iron porphyrins [61]. 

The catalytic production of olefins, diethyl maleate and fumarate, from ethyl diazoacetate has been reported with osmium [ 149] and ruthenium [ 128] porphyrins. Despite the periodic relationship of ruthenium to iron and osmium and the syntheses of different carbene complexes of ruthenium porphyrins, developed by Collman et al. [125-127], it is only very recently that cyclopropanation [135,171] and ethyl diazoacetate insertion into heteroatom bond reactions [172] were observed using ruthenium porphyrins as catalysts. The details of the catalytic reaction of diazo esters with simple olefins catalyzed with ruthenium porphyrins have been reported [173]. Product yields. 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

Diazoalkanes are u.seful is precursors to ruthenium and osmium alkylidene porphyrin complexes, and have also been investigated in iron porphyrin chemistry. In an attempt to prepare iron porphyrin carbene complexes containing an oxygen atom on the /(-carbon atom of the carbene, the reaction of the diazoketone PhC(0)C(Ni)CH3 with Fe(TpCIPP) was undertaken. A low spin, diamagnetic carbene complex formulated as Fe(TpCIPP)(=C(CH3)C(0)Ph) was identified by U V-visible and fI NMR spectroscopy and elemental analysis. Addition of CF3CO2H to this rapidly produced the protonated N-alkyl porphyrin, and Bit oxidation in the presence of sodium dithionitc gave the iron(II) N-alkyl porphyrin, both reactions evidence for Fe-to-N migration processes. ... 

Decomposition of diazo compounds by iron porphyrins is a convenient method for the synthesis of non-heteroatom carbene-iron porphyrins [22]. Reaction of [Fe(F2o-TPP)] [F20-TPP = meso-tetrakis(pentafluorophenyl)porphyrinato dianion] with diazo compounds N2C(Ph)R (R = Ph, C02Et, C02CH2CH=CH2) under an inert atmosphere afforded complexes [Fe(F2o-TPP)C(Ph)R] in 65-70% yields (Scheme 2). Like the halocarbene complex [Fe(TPP)(CCl2)], [Fe(F2o-TPP)CPh2] reacted with Melm to afford six-coordinate species [(MeIm)Fe(F2o-TPP)CPh2] in 65% isolated yield. 

Iron(II) porphyrins react readily with haloalkanes in the presence of reducing agents, e.g., excess iron powder, to give chlorocarbene complexes 15,32). With the insecticide DDT [2,2-bis(4-chlorophenyl)-1,1,1 -trichloro-ethane, (C1C6H4)2CHCC13], the reaction proceeds one step further, by elimination of HC1 from the carbene complex, to give diarylvinylidene complexes 33) ... 

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. 

The first carbene complex of an iron porphyrin was prepared and structurally characterized by Mansuy and co-workers (12, 13) through the reaction of carbon tetrachloride with (TPP)Fe11 under reductive... 

Several iron porphyrin carbenes are known1110 that are obtained by the general reaction of the chloro iron(III) complex with RCX3 (X = Cl, Br) in the presence of iron powder or sodium dithionite as in equation (101). The air sensitivity of the product depends on the nature11111112 of R. Formally these complexes could be described as iron(II) species. They are diamagnetic, and can bond an additional axial ligand. The structure of the dichlorocarbene complex [Fe(TPP)(CCl2)(H20)] has been determined by X-ray methods,1113 which showed a short Fe—C distance of 1.83 A. This complex is reactive and, for instance, with primary amines a coordinated... 

Photolysis of hydroxy Fischer carbene complexes (96) (Scheme 20) in the presence of alcohols under several atmospheres of carbon monoxide gives low to moderate yields of a-hydoxy esters (97). It is proposed that the reactions proceed via ketenes formed from the liberated or complexed carbenes and CO. In some cases, acetals formed via thermal decomposition of the carbenes are the major products. Photolysis of iron porphyrin carbene complexes results in cleavage of the iron-carbon double bond, producing a four coordinate iron(II) porphyrin and the free carbene. The carbenes can be trapped in high yield with a variety of alkenes. 

The links between formation of an iron-alkyl complex and irreversible destruction of the heme moiety have not been forged, but model studies with diaryl- and carbethoxy-substituted carbene complexes suggest that the halogenated carbenes may shift to form a bond with a nitrogen of the porphyrin. The resulting A -haloaIkyl adduct are likely to undergo water-dependent hydrolysis and might therefore not be detected by the methods used to isolate other A -alkyl porphyrins. However, the formation of alternative reactive species that attack the protein or the heme cannot be ruled out. 

These derivatives (Type B) are at least formally prepared by the insertion of a fragment into a M-N bond to yield a new M-X-N unit. Such species have been suggested as possible intermediates in the insertion of an oxygen atom into a G-H bond by cytochrome P-450. Several of these derivatives have formally had a carbene fragment inserted into a M-N bond. Such derivatives include a Ni derivative and a cobalt(III) species that has undergone two such insertion reactions. Other species represent the formal reaction of a vinylidene with iron(III) (two different crystalline forms). This iron derivative has an intermediate-spin state. Other complexes result from the insertion of a nitrene or an oxene This last derivative can also be considered to be a porphyrin N-oxide derivative and the structure of a free base species of a porphyrin N-oxide has also been reported". Appropriate stereochemical parameters for the members of this class are found in Table IX. 

Mansuy and coworkers [32] reported the first isolated metalloporphyrin-carbene complex in 1977 and it was an iron porphyrin carbene complex. It was suggested by the authors that iron carbene complexes could be involved in the metabohsm of xenobiotics. Thus, the five-coordinated (TPP)Fe(CCl2) was prepared as a purple red solid by treatment of (TPP)FeCl with CCI4 in the presence of an excess of iron powder (90% yield) [32]. It must be emphasized that the possible formation of carbene complexes after the reduction of iron deuteroporphyrin in CCI4 was initially proposed by Brault and coworkers [79]. An original method of reduction by aqueous sodium dithionite solution was described but, under these conditions, it was not possible to isolate any complex although the reaction leads to a compound of unusually good stability towards air. 

Iron-porphyrin model reactions have been reported for several steps of metabolic oxidation of sydnones by cytochrome P450 [52,96]. Thus, diazoketones react with iron(II) porphyrins to give iron carbene complexes and then AT-alkylporphyrins after a one electron oxidation. In this case, a migration of the carbene moiety to the pyrrole nitrogen atom is observed (Scheme 10) [97]. Similar results were obtained with PhCH2CHN2 [53]. 

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