Hypervalent iron

The composition of I, and possibly its structure, may be deduced by identifying Q. Certain examples from peroxide chemistry will illustrate the scope of the method. The reactions of ferrous(nitriloacetate) and ferrous(ethylenediamine-N,N -diacetate) with hydrogen peroxide are complicated processes.1 A particular scavenger T did indeed divert the reaction at high concentrations of T. The required levels of T were, however, much higher than those that would have been needed to trap the hydroxyl radical, HO. It is thereby ruled out. With this and with spectroscopic evidence, a reactive hypervalent iron complex was suggested as the intermediate. 

Biaglow JE, Kachur AV (1997) The generation of hydroxyl radicals in the reaction of molecular oxygen with polyphosphate complexes of ferrous ion. Radiat Res 148 181-187 Biaglow JE, Field KD, Manevich Y, Tuttle S, Kachur A, Uckun F (1996) Role of guanosine triphosphate in ferric ion-linked Fenton chemistry. Radiat Res 145 554-562 Bielski BHJ (1991) Studies of hypervalent iron. Free Radical Res Commun 12/13 469-477 Bielski BHJ, Allen AO, Schwarz HA (1981) Mechanism of disproportionation of ascorbate radicals. J Am Chem Soc 103 3516-3518... 

Although soluble guanylyl cyclase is commonly considered to be the only primary chemical receptor for NO, heme proteins can react with NO in a variety of oxidation states. For example, NO can complex at near diffusion control with hypervalent iron states formed in Fenton-type reactions (63-65). 

There is now substantial evidence that the metal-oxygen complexes described above do indeed form hypervalent intermediates that catalyze both radical and nonradical oxidations (63, 69-78). Most is known about Fe(lV) and Fe(V) complexes, providing support for the idea that hypervalent iron is at least one catalyst in Fenton reactions (79) analogous complexes have been identified for Cu (73, 80) and... 

Figure 2. Formation of ferryl iron in initiation and catalysis of lipid oxidation Reaction schemes lor formation of hypervalent iron states by FeF and Fe complexes and subsequent reactions leading to radicals that can initiate lipid oxidation. L, metal ligand R, alkyl or acyl group. Fe + sequence (71, 73) Fe sequence (81), adapted.

There is much still to be learned about conditions required for formation of fer-ryl or other hypervalent iron complexes, the actual structure of the complexes under different circumstances, the kinetics and mechanisms by which they react, and the overall consequences to lipid oxidation. The factors that appear to be most important include the following ... 

Current evidence indicates that hypervalent iron complexes—ferryl iron (FelV, Fe02, Fe(IV)=0) or perferryl iron (FeV)—are involved in the catalytic mechanism, but there is stiU controversy over the details of reaction mechanisms and what proportion of heme catalysis it accounts for. Very recently, some very elegant chemistry has elucidated binding and 0—0 bond scission mechanisms and identified heme structural elements critical for oxidation catalysis (143, 144). Paradoxically, although the early theories of heme catalysis have been largely dismissed, they nevertheless are consistent with aspects of hypervalent iron behavior. Ferryl iron chemistry encompasses and explains the most important features noted in early studies (99) ... 

Section II,A From recent two-dimensional NMR studies on bleomycin analogs, a revisited structural model for specificity, binding, and double-strand cleavage was proposed (367). An investigation of the reaction of Fe "-BLM with iodosylbenzene by ES-MS showed that neither hypervalent iron nor activated oxygen was involved but that hjq)ervalent iodide I(III) was the oxidant (368). 

Table I summarizes (a) several systems and their reactive intermediates for reaction with alkanes, (b) the initial product(s), and (c) the kinetic isotope effects [KIE, kc-C6Hi2/ -C6Di2 Although many have suggested that hypervalent iron, ferryl... 

N,/V-bis (2-hydroxybenzyl) ethylenediamine-N,N -diacetic acid (HBED) facilitated Fe(II) oxidation but blocked 02 -induced reduction of Fe(III) and consequently pre-empted production of HO or hypervalent iron through the Haber-Weiss reaction cycle (Samuni et al. 2001). The efficacy of HBED as a 1-electron donor was demonstrated by reduction of the 2,2 -azino-bis(3-ethylbenzothiazoline-6-sul-phonate)-derived nitrogen-centred radical cation (ABTS ), accompanied with a short-lived phenoxy radical. 

Finally, phthalocyanine iron catalysts were also used for the oxidation of alcohols to yield corresponding carbonyl compounds with nonbenign hypervalent iodine oxidants [147]. 

In recent years, hypervalent iodines have been employed to oxidize various organic substrates. H5l06 is also capable of oxidizing sulfides to sulfoxides in pyridine [160], However, it has been established that the oxidation of sulfides in the presence of catalytic amounts of FeCl3 (3 mol%) takes place in shorter reaction times than that without the catalyst, indicating the catalytic effect of this iron salt (Scheme 3.51) [ 161 ]. 

The bleomycins (Fig. 12.6) are a family of glycopeptide-derived antibiotics which are used in the treatment of various tumors. They bind iron in the blood and form an active hypervalent oxo-iron species. The two-dimensional structure is well known but no crystal structures of bleomycin or its metal complexes have been reported. The MM2 force field was modified and extended by modeling of the crystal structures of the cobalt complexes of two bleomycin analogues in order to develop a force field for... 

Reduction of dinitrobenzothiadiazoles 280 with iron dust in acetic acid gave diamines 281 (Scheme 39). The reaction of diamines 281 with selenium dioxide gave [l,2,5]selenadiazo[4,5-c]-2,l,3-benzothiazole derivatives containing a hypervalent sulfur atom 82 and 83 in 40% and 82% yields, respectively <1997T10169>. 

Oxidative Coupling Reactions with Hypervalent Iodine Reagents 484 Other Reagents for the Oxidative Coupling Reaction 495 Iron(III) 495... 

The oxidation of thiols follows a completely different course as compared with the oxidation of alcohols, because the capacity of the sulfur atom to form hypervalent compounds allows it to become the site of oxidation. Thiols are readily oxidised to disulfides by mild oxidants such as atmospheric oxygen, halogens or iron(III) salts (Scheme 6). This type of reaction is unique to thiols and is not undergone by alcohols, it is a consequence of the lower bond strength of the S-H as compared with the O-H bond, so that thiols are oxidised at the weaker S-H bonds, whereas alcohols are preferentially oxidised at the weaker C-H bonds (Scheme 7). The mechanism of oxidation of thiols may be either radical or polar or both (Scheme 6). The polar mechanism probably involves transient sulfenic acid intermediates like (7) and (8). In contrast, thiols react with more powerful oxidants, like potassium permanganate, concentrated nitric acid or hydrogen peroxide, to yield the corresponding sulfonic acids (10). This oxidation probably proceeds via the relatively unstable sulfenic (7) and sulfinic acids (9), which are too susceptible to further oxidation to be isolated (Scheme 8). 

Hypervalent iodine species were demonstrated to have a pronounced catalytic effect on the metalloporphyrin-mediated oxygenations of aromatic hydrocarbons [93]. In particular, the oxidation of anthracene (114) to anthraquinone (115) with Oxone readily occurs at room temperature in aqueous acetonitrile in the presence of 5-20 mol% of iodobenzene and 5 mol% of a water-soluble iron(llI)-porphyrin complex (116) (Scheme 4.57) [93]. 2-ferf-Butylanthracene and phenanthrene also can be oxygenated under similar conditions in the presence of 50 mol% of iodobenzene. The oxidation of styrene in the presence of 20 mol% of iodobenzene leads to a mixture of products of epoxidation and cleavage of the double bond. Partially hydrogenated aromatic hydrocarbons (e.g., 9,10-dihydroanthracene, 1,2,3,4-tetrahydronaphthalene... 

Although the majority of direct arylations have been catalyzed by palladium, rhodium, and ruthenium, some additional studies have also focused on direct arylations catalyzed by first-row metals, such as iron and copper. For example, an iron-catalyzed direct arylation reaction between arylzinc reagents and 2-arylpyridine derivatives has been reported (Equation 19.146). Several direct couplings of heteroarenes with aryl halides (Equation 19.147) or hypervalent iodine reagents ° catalyzed by copper halides have also been reported. 

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