Iron complex olefin oxidation

Various kinds of olefin isomerizations are effectively promoted by Fe(CO)5. However, because the isomerized product is usually released from the iron complex by oxidation, these isomerizations require a stoichiometric amount of pentacarbonyliron as described below (Scheme 10.4, eq (1)) [16-17]. 

From 2, it was concluded that the ferryl complex is the catalytically active species. Observation 1 suggested that 80% of the epoxide product in the aerobic reaction is derived from a carbon-based radical, which is quenched by O2 (autoxidation), and this is known to produce epoxide in reactions with cyclooc-tene (325). Methanol (observation 3) is known to quench radicals. The fact that the diols formed are a mixture of cis and trans products (observation 1 this is very unusual in iron-catalyzed olefin oxidations) suggested that the diol results from the capture of OH radicals by the putative carbon-based radical. 

Most of the olefin complexes examined in this study exhibit an unspectacular reactivity towards molecular oxygen, i. e. either ligand exchange reactions, 0-0-bond activation by highly oxophilic metals Sc, Ti, and V, or even complete absence of any reaction are observed (eg. even Cu(C2H4) is unreactive). However, in the case of the iron complexes extensive oxidation reactions are observed. Indeed, not only olefins attached to an iron cation react effectively with molecular oxygen, even stable molecules like benzene and acetone are rapidly oxidized in the presence of Fe+. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

Abstract In this review, recent developments of iron-catalyzed oxidations of olefins (epoxidation), alkanes, arenes, and alcohols are summarized. Special focus is given on the ligand systems and the catalytic performance of the iron complexes. In addition, the mechanistic involvement of high-valent iron-oxo species is discussed. 

Dioxo-ruthenium porphyrin (19) undergoes epoxidation.69 Alternatively, the complex (19) serves as the catalyst for epoxidation in the presence of pyridine A-oxide derivatives.61 It has been proposed that, under these conditions, a nms-A-oxide-coordinated (TMP)Ru(O) intermediate (20) is generated, and it rapidly epoxidizes olefins prior to its conversion to (19) (Scheme 8).61 In accordance with this proposal, the enantioselectivity of chiral dioxo ruthenium-catalyzed epoxidation is dependent on the oxidant used.55,61 In the iron porphyrin-catalyzed oxidation, an iron porphyrin-iodosylbenzene adduct has also been suggested as the active species.70... 

Diastereoselective intermolecular nitrile oxide—olefin cycloaddition has been used in an enantioselective synthesis of the C(7)-C(24) segment 433 of the 24-membered natural lactone, macrolactin A 434 (471, 472). Two (carbonyl)iron moieties are instrumental for the stereoselective preparation of the C(8)-C(ii) E,Z-diene and the C(i5) and C(24) sp3 stereocenters. Also it is important to note that the (carbonyl)iron complexation serves to protect the C(8)-C(ii) and C(i6)-C(i9) diene groups during the reductive hydrolysis of an isoxazoline ring. 

We studied the oxidation of cyclohexene at 70°C in the presence of cyclopentadienylcarbonyl complexes of several transition metals. As with the acetylacetonates, the metal center was the determining factor in the product distribution. The decomposition of cyclohexenyl hydroperoxide by the metal complexes in cyclohexene gave insight into the nature of the reaction. With iron and molybdenum complexes the product profile from hydroperoxide decomposition paralleled that observed in olefin oxidation. When vanadium complexes were used, this was not the case. Variance in product distribution between the cyclopentadienylcarbonyl metal-promoted oxidations as a function of the metal center were more pronounced than with the acetylacetonates. Results are summarized in Table V. 

Further support for an iron-bound active oxidant comes from the study of the [Fe(II)(Tp3,5 Me2)(bf)] complex [237], This complex reacts with 02 to form a species capable of stereospecifically epoxidizing olefins. For example, epoxida-tion of cA-stilbene gives only cA-stilbene oxide as the product, but trara-stilbene cannot be epoxidized, suggesting that epoxidation occurs at a sterically congested transition state, i.e., near the iron center. More studies are needed to provide insight into the nature of the active oxidant. 

A number of the catalytic systems we have thus far discussed all afford high conversion of oxidant into epoxide and diol products however, their utility as catalysts is limited because of the requirement for a large excess of substrate. There has been some effort focused on developing nonheme iron complexes to be used as practical catalysts for synthesis, emphasizing conversion of substrate to product(s). Jacobsen and coworkers explored the catalytic activity of 6 in the presence of added acetic acid and found that olefins could be converted into epoxides in high yield with 3 mol% catalyst and 30 mol% FlOAc (Table 18.2) [40]. The added acetic acid is clearly important, as reactions imder similar reaction conditions but without HOAc afforded a lower yield and selectivity for epoxide [41]. 

The discovery of iron complexes that can catalyze olefin czs-dihydroxylation led Que and coworkers to explore the possibility of developing asymmetric dihydroxylation catalysts. Toward this end, the optically active variants of complexes 11 [(1R,2R)-BPMCN] and 14 [(1S,2S)- and (lP-2P)-6-Me2BPMCN] were synthesized [35]. In the oxidation of frans-2-heptene under conditions of limiting oxidant, 1R,2R-11 was foimd to catalyze the formation of only a minimal amount of diol with a slight enantiomeric excess (ee) of 29%. However, 1P-2P-14 and 1S,2S-14 favored the formation of diol (epoxide/diol = 1 3.5) with ees of 80%. These first examples of iron-catalyzed asymmetric ds-dihydroxylation demonstrate the possibility of developing iron-based asymmetric catalysts that may be used as alternatives to currently used osmium-based chemistry [45]. 

FIGURE 18.4 Mechanistic landscape for olefin oxidation by H2O2 catalyzed by nonheme iron complexes. Sources of oxygen atoms include H2O2 (unfilled circle) and H2O (filled circle). 

A key observation supporting the Fe(III)-Fe(V) mechanistic scheme presented above was the low-temperature trapping of iron(III)-peroxo species for 6, 9, 11, and 12 [46,49,54,57,58]. Since the olefin oxidations catalyzed by these complexes are highly stereoselective, it is unlikely that Fe "-OOH species proposed to form in the course of catalysis would imdergo 0-0 homolysis to produce HO. Instead, it has been argued that the Fe -OOH species must undergo 0-0 heterolytic... 

As indicated by the scheme in Fig. 18.4, the mechanistic landscape for bio-inspired olefin oxidation catalysis by nonheme iron complexes is increasing in complexity. The evidence available to date supports the involvement of the Fe(III)/Fe(V) cycle for 6 and 9 and related complexes having tetradentate ligands with CIS-labile sites and no a substituents on the pyridines. Much less is known... 

Zeolites are well suited for the preparation of encapsulated complexes by virtue of the large supercages. Metallo-phthalocyanines encaged in zeolites have been proposed as enzyme mimics [7,8 Zeolite-encapsulated iron phthalocyanine catalysts have been used in hydrocarbon oxidations it was found that the resistance of the zeolite-encaged complexes against oxidative destruction by far exceeded that of free iron phthaTocyanines [9,10]. In the present work, zeolite-encaged phthalocyanine catalysts were studied in the triple catalytic oxidation of olefins. 

In the last few years, several non-heme iron complexes have been identified as functional models for non-heme iron dioxygenases (85-88). These model complexes are able to catalyze the cis-dihydroxylation of olefins as well as the epoxidation of olefins using H2O2 as the primary oxidant. Table V presents the results of olefin oxidation by some representative mononuclear and dinuclear non-heme iron complexes in combination with H2O2. 

Catalysis of Olefin Oxidation by Some Representative Mononuclear and Dinuclear Non-Heme Iron Complexes in Using H2O2 as Oxidant... 

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