Ferrate complex

Apart from catalysis with well-defined iron complexes a variety of efficient catalytic transformations using cheap and easily available Fe(+2) or Fe(+3) salts or Fe(0)-carbonyls as precatalysts have been pubhshed. These reactions may on first sight not be catalyzed by ferrate complexes (cf. Sect. 1), but as they are performed under reducing conditions ferrate intermediates as catalytically active species cannot be excluded. Although the exact nature of the low-valent catalytic species remains unclear, some of these interesting transformations are discussed in this section. 

Apart from this mechanistic hypothesis, another scenario, with a ferrate complex as intermediate, may be possible. In 1928, Hieber discovered that Fe(CO)5 78 underwent a disproportionation in the presence of ethylenediamine 122 [97-101]. Depending on the reaction temperature, different ferrate complexes were formed that incorporated a [Fe(en)3] cation (en = ethylenediamine) and mono-, di- or trinuclear ferrate anions (Scheme 32) [102-107]. As the reaction discussed above is also performed with amines at high temperatures, these ferrates may well be involved in the catalytic cycle of the carbonylation discussed above. 

A determination of dimethyl sulphoxide by Dizdar and Idjakovic" is based on the fact that it can cause changes in the visible absorption spectra of some metal compounds, especially transition metals, in aqueous solution. In these solutions water and sulphoxide evidently compete for places in the coordination sphere of the metal ions. The authors found the effect to be largest with ammonium ferric sulphate, (NH4)2S04 Fe2(S04)3T2H20, in dilute acid and related the observed increase in absorption at 410 nm with the concentration of dimethyl sulphoxide. Neither sulphide nor sulphone interfered. Toma and coworkers described a method, which may bear a relation to this group displacement in a sphere of coordination. They reacted sulphoxides (also cyanides and carbon monoxide) with excess sodium aquapentacyanoferrate" (the corresponding amminopentacyanoferrate complex was used) with which a 1 1 complex is formed. In the sulphoxide determination they then titrated spectrophotometrically with methylpyrazinium iodide, the cation of which reacts with the unused ferrate" complex to give a deep blue ion combination product (absorption maximum at 658 nm). 

Redox Reactivity of Coordinated Ligands in Pentacyano(L)Ferrate Complexes Jose A. Olabe... 

REDOX REACTIVITY OF COORDINATED LIGANDS IN PENTACYANO(L)FERRATE COMPLEXES... 

When the above factors are put under control, the possibility of changing the ligand L in the pentacyano(L)ferrate complexes adds a further dimension for studying systematic reactivity changes, brought out by the controlled modification of the redox potentials of the Fe(II)-Fe(III) redox couples. In this way, the rates of electron transfer reactions between a series of [Fen(CN)5L]re complexes toward a common oxidant like [Coin(NH3)5(dmso)]3+ showed a variation in agreement with Marcus predictions for outer-sphere electron transfer processes, as demonstrated by linear plots of the rate constants versus the redox potentials (123). 

Cyclic voltammetry was carried out in the presence of penta- and hexacyano-ferrate complexes in order to probe the homogeneity and conductivity of the TRPyPz/CuTSPc films (125), (Fig. 36). When the potentials are scanned from 0.40 to 1.2 V in the presence of [Fe (CN)6] and [Fe CN)5(NH3)] complexes, no electrochemical response was observed at their normal redox potentials (i.e., 0.42 and 0.33 V), respectively. However, a rather sharp and intense anodic peak appears at the onset of the broad oxidation wave, 0.70 V. The current intensity of this electrochemical process is proportional to the square root of the scan rate, as expected for a diffusion-controlled oxidation reaction at the modified electrode surface. The results are consistent with an electrochemical process mediated by the porphyrazine film, which act as a physical barrier for the approach of the cyanoferrate complexes from the glassy carbon electrode surface. 

Allylic alkylations are among the most widely applied catalytic C-C bond formation reactions in organic chemistry. In the 1980s already mononitrosyl ferrate complexes of type 31 were reported to be active in regioselective allylic alkylations [94, 95]. However, this pioneering work suffered from low turnover numbers and the reaction had to be carried out under CO atmosphere. 

Tetracarbonylferrate reacts with phthaloyl dichloride to give dimeric biphthalidene in 23% yield (Mitsudo et aL, 1972). The ferrate complex is a versatile reagent for the synthesis of aldehydes (Watanabe et aL, 1971). Carboxylic acid anhydrides and carboxylic alkylcarbonic anhydrides can be reduced to the corresponding aldehydes (Watanabe et aL, 1973, 1975). 

Organic halides or tosylates containing a carbon-carbon double bond at an appropriate position cyclize with the aid of the ferrate complex to give cyclic ketones (Merour et aL, 1973). 

Two ferrate complexes A and B were used by Fiirstner and co-workers to catalyze the intramolecular [5+2] cycloadditions of alkyne-VCPs to give the corresponding seven-membered products in good to excellent yields with a good substrate scope and diastereoselectivity (see (26)) [59],... 

Due to the low oxidation state of the metal in carbonyliron complexes and ferrates, these species can be applied for the reduction of various carbonyl compounds. Initially, these reagents have been applied in stoichiometric amounts. First examples describe the hydrogenation of a,p-unsaturated carbonyl compounds by carbonyl(hydrido)ferrate complexes to give saturated carbonyl compounds or saturated alcohols. Low valent iron species for the reduction of carbonyl compounds and imines can also be generated in situ from iron(II) chloride and lithium powder in the presence of 4,4 -di-rert-butylbiphenyl. Catalytic versions have been developed subsequently. Thus, pentacarbonyliron functions as a precatalyst for the hydrogenation of aldehydes and ketones in the presence of a tertiary amine as solvent (Scheme 4-322). The catalytically active system probably consists of (tetracarbonyl)(hydrido)ferrate and the protonated amine. ... 

SCHEME 20.23 [5+2] Cycloaddition of tethered alkyne-VCPs catalyzed with ferrate complexes. 

Catalytic Wittig-type and Doyle-Kirmse reactions have been achieved using diazo compounds as carbenoid sources in the presence of ferrate complex Bu4N[Fe(CO)3(NO)] as catalytic species this result highlights for the first time the potential of the electron-rich iron complex for activating diazo compounds to iron carbenoids. 

Finally, Fiirstner showed that the low-valent tetrakis(ethylene) ferrate complex [Li(tmeda)]2[Fe(C2H4)4] is an excellent catalyst for the cross-coupling reaction between alkyl halides and aryl Grignard reagents (Scheme 4.83) [194c]. 

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