Iron complexes carbenes

Alkoxy(carbene)iron(0) and amino(carbene)iron(0) complexes usually react with alkynes to give rj4-pyrone iron complexes and furans, respectively [54]. Nevertheless the chemoselective formation of naphthols was reported for alkoxy(carbene)iron(0) complexes with the electron-poor alkyne dimethyl... 

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. ... 

Table 1 C NMR data for iron porphyrin carbene complexes...

Interestingly, the cyclopropanation of styrenes with EDA catalyzed by the half sandwich iron complex [CpFe(CO)2(THF)] BF4 afforded cyclopropanes in good yields and with ds-selectivity cisitrans = 80 20) [62]. With phenyldiazomethane as a carbene source, excellent cA-selectivity (92-100%) was achieved (Scheme 15) [63]. 

Cycloaddition of aUcynes catalysed by transition metals is one of the most efficient and valuable ways to prepare benzene and pyridine systems [12], Among the possible catalytic systems able to catalyse this reaction, cobalt and iron complexes containing NHCs as ligands have shown high catalytic activity in the intramolecular cyclotrimerisation of triynes 36 (Scheme 5.10) [13]. The reaction was catalysed with low loading of a combination of zinc powder and CoC or FeClj with two or three equivalents of IPr carbene, respectively. 

In contrast to the reaction of an i72-CS2-rhodium complex with dimethyl acetylenedicarboxylate which gives rise to a metallocycle,186 the iron complexes 103 are converted by activated acetylenes into air-sensitive carbene complexes 104. Decomposition of the latter in air provides an unusual synthetic route to substituted tetrathiofulvene derivatives (Scheme 121).187... 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

Many of the syntheses we have seen within this review depend on the carbonylation of a vinylcarbene complex for the generation of the vinylketene species. The ease of this carbonylation process is controlled, to some degree, by the identity of the metal. The electronic characteristics of the metal will clearly have a great effect on the strength of the metal-carbon double bond, and as such this could be a regulating factor in the carbene-ketene transformation. It is interesting to note the comparative reactivity of a (vinylcarbene)chromium species with its iron analogue The former is a fairly stable species, whereas the latter has been shown to carbonylate readily to form the appropriate (vinylketene)iron complex. 

The molybdenum complex 1, a typical high-valent Schrock-type carbene, efficiently catalyzes the self-metathesis of styrene. On the other hand, the cationic iron complex 3 does not induce metathesis but stoichiometrically cyclopropanates styrene. The tungsten complex 2, again a Fischer-type carbene complex, mediates... 

The reaction of alkoxy(aryl)carbene iron complexes with two equivalents of an isonitrile leads to the formation of azetidin-2-ylidene complexes [197]. For other reactions of Fischer-type carbene complexes with isonitriles see [198]. 

Acid-catalyzed dealkoxylation is particularly suitable for the preparation of highly reactive, cationic iron(IV) carbene complexes, which can be used for the cyclopropanation of alkenes [438] (Figure 3.11). Several reagents can be used to catalyze alkoxide abstraction these include tetrafluoroboric acid [457-459], trifluoroacetic acid [443,460], gaseous hydrogen chloride [452,461], trityl salts [434], or trimethylsilyl triflate [24,104,434,441,442,460], In the case of oxidizing acids (e.g. trityl salts) hydride abstraction can compete efficiently with alkoxide abstraction and lead to the formation of alkoxycarbene complexes [178,462] (see Section 2.1.7). 

Fig. 3.14. Decomposition reactions of cationic iron(IV) carbene complexes [447,459].

Because a-alkoxyalkyl iron complexes are thermally unstable [467] they cannot be stored for long periods of time. More suitable carbene precursors are the corresponding a-(dimethylsulfonium)alkyl complexes, which can be stored indefinitely under ambient conditions [468-473], These complexes are prepared by S-alkylation of a-(methylthio)alkyl complexes, which can be prepared by alkylation of metallates with a-halothioethers, by addition of C-nucleophiles to (alkylthio)carbene complexes, or by addition of thiols to carbene complexes. 

Fig. 3.16. Generation of reactive, cationic iron(lV) carbene complexes from a-(phenyl-thio)alkyl complexes [470,482].

Because electrophilic carbene complexes can cyclopropanate alkenes under mild reaction conditions (Table 3.1) [438,618-620], these complexes can serve as stoichiometric reagents for the cyclopropanation of organic compounds. Thoroughly investigated carbene complexes for this purpose are neutral complexes of the type (C0)5M=CR2 (M Cr, Mo, W) and cationic iron(IV) carbene complexes. The mechanism of cyclopropanation by electrophilic carbene complexes has been discussed in Section 1.3. 

Fig. 3.33. Stoichiometric, intramolecular cyclopropanations with iron(IV) carbene complexes [477,624],...

Few examples of C-H insertions have been reported for carbene complexes without electron-withdrawing groups attached to the carbene carbon atom [696]. Most of these are C-H insertions of cationic iron(IV) carbene complexes. 

Fig. 3.38. Possible mechanism for the insertion of electrophilic iron(IV) carbene complexes into aliphatic C-H bonds.

The intermediacy of a metallacyclobutene is proposed upon reaction of the diphenylcy-clopropenone dimer spirolactone with CpCo(CO)2, ultimately yielding a >j4-vinylketene complex (equation 23 l)295a. Unlike the analogous iron complex (Section IV.B.2.a), no vinyl carbene complex was observed, and hence formation of the metallacyclobutene seems to be more likely. 

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