Iron reaction intermediates

Besides, iron(IV) imides have also been implicated as reaction intermediates in nitrogen atom/group transfer reactions [36]. The first structurally characterized iron-imide complex was reported by Lee and coworkers [37] through the reaction of FeCls with 2 equiv. of LiNH Bu in THF, giving one of the products as a stable site-differentiated cubane having three Fe(lll) and one Fe(IV) centers and the Fe(IV) center has a terminally bonded imido ligand (Scheme 4). 

High-valent iron-imido complexes have also been proposed as reaction intermediates in several reactions of the iron catalysis. Que and coworkers have provided evidence for Fe(IV) imide as a reaction intermediate in the reaction of [(6-PhTPA)-Fe°(CH3CN)2] " with PhI=NTs. Borovik and coworkers have also reported the formation of an amide product involving the generation of a putative iron(IV) imide [36] (Scheme 7). 

Scheme 7 Proposed iron (IV) imido reaction intermediates...

Iron-nitrene/imido complexes are proposed to be the reaction intermediates in nitrogen group transfer reactions. The nitrene group can be transferred to organic substrates. Aziridination and amination are the well-known nitrogen atom/group... 

Support for this conclusion is provided by the hydroperoxide specificity of BP oxidation. The scheme presented in Figure 6 requires that the same oxidizing agent is generated by reaction of h2°2/ peroxy acids, or alkyl hydroperoxides with the peroxidase. Oxidation of any compound by the iron-oxo intermediates should be supported by any hydroperoxide that is reduced by the peroxidase. This is clearly not the case for oxidation of BP by ram seminal vesicle microsomes as the data in Figure 7 illustrate. Quinone formation is supported by fatty acid hydroperoxides but very poorly or not at all by simple alkyl hydroperoxides or H2C>2. The fact that... 

An oxoiron(V) species such as 6 derived from 1 and peroxides is accessible in nonaqueous media (51). The reaction of the tetraphenylphosphonium salt of la with 2 to 5 equivalents of m-chloroperbenzoic acid (mCPBA) at — 60°C in n-butyronitrile produces within about 10 s a bis-iron(IV)- i-oxo dimer followed by an as - yet uncharacterized EPR - silent iron(IV) intermediate. After 15 min, the deep green oxoiron(V) species 6 forms with distinct absorption maxima at 445 nm (e — 5400 Mr1 cm-1) and 630 nm (s — 4200 M-1 cm-1). At —60°C, 6 decays by 10% in 90 min, but it is stable for at least one month at 77 K. Selected spectral data for the oxoiron(V) species are shown in Fig. 13. DFT calculations favor the low-spin (S = 1/2) configuration of the ground state. The calculated Fe-0 bond length of 1.60 A is in excellent agreement with the EXAFS results. The Fe atom is displaced out of the 4-N plane by 0.5 A. 

Such reaction with aromatic hydroperoxides has been studied by Coon and coworkers [54], Instead of hydroperoxides, iodosobenzenes, and iodobezene acetates may participate in Reaction (8) as the oxygen donors [55]. These authors proposed that the reaction of cytochrome P-450 with iodosobenzene proceeds to form an iron-oxo intermediate complex containing only one oxygen atom derived from the substrate. 

The same electrochemical process was also used for the coupling between aldehydes or ketones and activated alkyl halides such as a-chloroesters, -nitriles, and -ketones as well as aya-dichloroesters.334 Electroanalytical studies have shown initial electroreduction of Fe(n) to Fe(i) and subsequent formation of an iron organometallic intermediate (e.g., a 7t-allyliron complex in Equation (27)) before reaction with the corresponding carbonyl compounds.335... 

Fig. 9-6. Adaorption coverage of a reaction intermediate of hydroxo-complezes in anodic dissolution of a metallic iron electrode as a function of electrode potential in acidic sulfate solutions at pH 1.0, 2.0 and 3.0 solution is 0.5 M (Ns2S04 + H2SO4) at room temperature. Oqh adsorption coverage of reaction intermediates FeOH,4 and PeOH Vss = volt referred to the saturated silver-silver chloride electrode. [Prom Tsuru, 1991.]...

Ozawa, S., Fujii, H., and Morishima, 1. (1992). NMR studies of iron(ll) nitrosyl rr-cation radicals of octaethylchlorin and octaethylisobacteriochlorin as modeb for reaction intermediate of nittite teductase. J. Am. Chem. Soc. 114, 1548-1554. 

Locating minima is not always straightforward since a reaction surface is usually complex, and a geometry optimization calculation will only locate minima close to the starting point. It is usually not feasible to systematically explore all possible conformers, so chemical intuition and corroborative evidence from experiment play important roles. A nice example is the identification of the coordination geometry of oxo-iron(IV) intermediate in TauD (22). As mentioned above, during optimization of enzyme active sites, key atoms are sometimes fixed to mimic the constraints that the protein environment exerts on the active site (20). 

Sometimes it happens that incursion of oxoanion reduction in place of hydrogen evolution as the cathodic reaction in the corrosion of iron leads, not to an increased rate of corrosion, but to a drastic retardation. This is because strongly oxidizing conditions (e.g., in concentrated nitric acid) can force the immediate oxidation of iron to iron(III), rather than via the persistent iron(II) intermediate (as described in Sections 16.1 and 16.2), so that an insoluble iron (III) oxide layer forms at once on the anodic and cathodic surfaces alike and the iron becomes passivated (Section 16.3). Michael Faraday s demonstration of this phenomenon is instructive ... 

In laboratory model systems using reduced DOM constituents (Dunnivant et al., 1992), Fe(II) adsorbed to iron (hydr)oxides (Klausen et al., 1995), or zero-valent iron metal (Agrawal and Tratnyek, 1996) as reductants, the nitroso- and particularly the hydroxylamino compounds have been observed as reaction intermediates, but were ultimately also converted to the corresponding amino-compounds. 

Mercury-transition metal bonds have been described for all members of Groups V-VIII of the transition series except, apparently, technetium. They commonly involve a low oxidation state of the transition element and are particularly numerous for the chromium, iron and cobalt families.1 In addition, mercury-titanium bonded species have been postulated as unstable reaction intermediates.2... 

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