Porphyrins carbenes

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. ... 

The most significant and widely studied reactivity of the ruthenium and osmium porphyrin carbene complexes is their role in catalyzing both the decomposition of diazoesters to produce alkenes and the cyclopropanation of alkenes by diazoesters. Ethyl diazoacetate is used to prepare the carbene complex 0s(TTP)(=CHC02Et)... 

While metalloporphyrin carbene complexes are well established for ruthenium and osmium, they are less well known for rhodium. Cationic rhodium porphyrin carbene intermediates were implicated in a report by Callot et al. in w- hich... 

Rh(Por)l (Por = OEP. TPP, TMP) also acts as a catalyst for the insertion of carbene fragments into the O—H bonds of alcohols, again using ethyl diazoacetate as the carbene source. A rhodium porphyrin carbene intermediate was proposed in the reaction, which is more effective for primary than secondary or tertiary alcohols, and with the bulky TMP ligand providing the most selectivity. ... 

Scheme 1 Schematic structures of iron meio-tetraarylporphyrins and preparation of Fe-porphyrin carbene complexes...

Recently synthesized iron porphyrin-carbene complexes (136) may be considered as carbon analogues of the oxo-iron(IV) porphyrin (HRP-II and CAT-II) species.476 The one-electron oxidation of [Fe(TPP)(C=CR2)] (R = aryl) by CuCl2 affords the stable complex [Fe(TPP)(C=CR2)] Cl (TPP = wieso-tetra-p-tolyl porphyrin and R = p-ClC6H4) which has a visible spectrum very similar to that of CAT-I. X-Ray analysis of this complex has revealed a structure (137) in which the vinylidene moiety has inserted between the iron atom and one pyrrole ring of the porphyrin and it is suggested that this may constitute a model, i.e. structure (138), for the active site of CAT-I as an alternative to the Fe117 cation radical model.477... 

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... 

The four-coordinate sqnare planar iron(n) porphyrins discussed above are not only of great valne in heme protein model chemistry, but also in chemical applications, since they undergo a wealth of ligand addition reactions. For example it has been shown that TPPFe complexes are active catalysts for important carbon transfer reactions in organic chemistry and are found to catalyze the stereoselective cyclopropanation of aUcenes, olefin formation from diazoalkanes, and the efficient and selective olefination of aldehydes and other carbonyl compounds. The active species in these carbon transfer reactions are presumably iron porphyrin carbene complexes. " It was also found that ferrous hemin anchored to Ti02 thin films reduce organic halides, which can pose serious health problems and are of considerable environmental concern because of their prevalence in groundwater. ... 

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. 

Synthesis and structure of ruthenium and osmium porphyrin carbene complexes and their role in the metal-mediated cyclopropanation of alkenes 02CCR(231)151. 

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) ... 

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. 

Despite the report of a large number of iron porphyrin carbene complexes in the literature, there are extremely few iron porphyrins bearing non hetero atom-stabilized carbenes [53,60]. Recently, Che and coworkers re-... 

Recently, FT-IR and theoretical analysis of iron porphyrin carbene complexes revealed a possible paramagnetic intermediate carbene state during the reaction of ethyl diazoacetate with Fe(TPP). Focusing on the C = O stretching frequency in the carbene iron complex, a decrease of the value is observed, in comparison with diamagnetic carbene complexes since the radical on the C(carbene) delocalizes over the a-carbonyl group [78]. All isolated axial iron carbene complexes are, however, diamagnetic. 

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