Iron complexes allylic oxidation

A recently described method for insertion of a carbon monoxide molecule into the monoepoxide of a conjugated diene gives /3-lactones in high yield. This is achieved by reaction of iron pentacarbonyl with the starting vinyloxirane to give the 7r-allyl iron complex (66), which on oxidation with cerium(IV) ammonium nitrate gives the /3-lactone. In some cases, y-Iactone products can also be obtained from this reaction (8lJCS(Pi)270). 

Cyclohexene oxidation in the presence of the molybdenum complex, [C5Hr)Mo(CO)3]2, gave two major products at low conversion VI and VII nearly 1 1 mole ratio, Table V. The ketone, VIII, was formed in very low yield in contrast to oxidations using the iron complex. This reaction is far more selective than the oxidation of cyclohexene in the presence of Mo02(acac)2 reported by Gould and Rado (24). When a cyclohexene solution of V was exposed to [CsHsMk COJs] at 70°C, VI and VII were formed in approximately equimolar amounts (Table VI). These data show that the molybdenum complex efficiently catalyzes the epoxidation of cyclohexene by V before the allylic hydroperoxide decomposes substantially. Reaction 16 represents the predominant course of cyclohexene oxidation in the presence of cyclopentadienyltricarbonyl molybdenum dimer. 

Cationic iron-alkene complexes also participate in an unusual cycloaddition process, wherein electron-deficient alkenes are attacked by nucleophilic o -allylic Fp complexes, generating stabilized carb-anions and cationic alkene-iron complexes. Attack of the carbanion on the alkene forming a five-membered ring completes this process (Scheme 13). Oxidative removal of the iron produces useful organic compounds.19-21... 

Iron carbonyls have been used in stoichiometric and catalytic amounts for a variety of transformations in organic synthesis. For example, the isomerization of 1,4-dienes to 1,3-dienes by formation of tricarbonyl(ri4-l,3-diene)iron complexes and subsequent oxidative demetallation has been applied to the synthesis of 12-prostaglandin PGC2 [10], The photochemically induced double bond isomerization of allyl alcohols to aldehydes [11] and allylamines to enamines [12,13] can be carried out with catalytic amounts of iron carbonyls (see Section 1.4.3). 

Removal of the 0-substituted Fp group can be achieved by conversion into the cationic alkene-Fp complex using Ph3CPF6 and subsequent treatment with iodide, bromide or acetonitrile. Oxidative cleavage with ceric ammonium nitrate in methanol provides the methyl esters via carbon monoxide insertion followed by demetallation. The [3 + 2]-cydoaddition has been successfully applied to the synthesis of hydroazulenes (Scheme 1.11) [34]. This remarkable reaction takes advantage of the specific nucleophilic and electrophilic properties of V-allyl-, cationic t 5-dienyl-, cationic ri2-alkene- and ti4-diene-iron complexes, respectively. 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

Roelfes et al. prepared a non-heme iron(II) complex 26 from pentadentate ligand N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine 25 (Scheme 3.32) [126]. In the presence of H202, complex 26 reacted to a low-spin Fe(III)OOH intermediate, which was cleaved homolytically to an oxo Fe(IV) species and a hydroxy radical. Both species are capable of oxidizing various organic substrates via a radical pathway (Scheme 3.32). Under the catalysis of complex 26, cyclohexene la was oxidized with excess H202 to a mixture of products 2a, 3a and 4a. The TON was found to be solvent dependent, with higher TON in acetonitrile than in acetone (Scheme 3.32). In no case were isolated yields given and, furthermore, the allylic oxidation is limited to cyclohexene la. 

As observed from reaction (6.19) and experimental data [41,120,121], ROOH satisfactorily replaces molecular oxygen and the reducer. When oxidized with hydroperoxides in the presence of iron porphyrin catalysts (cytochrome P-450 analogs), olefins mostly convert to allyl oxidation products, namely unsaturated alcohols and ketones, whereas the quantity of epoxides does not exceed 1% [122], According to current suggestions [121] such behavior of iron porphyrin catalysts is explained by olefin epoxidation with the cata-lyst-ROOH complex by the heterolytical mechanism according to the following equation ... 

Iron complexes can also catalyze allylic amination [31,32]. Enders et al. have demonstrated the nucleophilic addition of various acyclic and cyclic amines to the optically active l-methoxycarbonyl-3-methyl-(T)3-allyl)-tetracarbonyliron cation 49 formed in high yield from reaction of 48 with iron carbonyls. Oxidative removal of the tetracarbonyliron group by reaction with CAN gives 50 with high optical purity and retention of the stereochemistry (Eq. (12)) [31]. The reaction proceeds well for the different amines, and has been used for the synthesis of a compound showing cytotoxic activity against diverse cell lines [31b]. 

A slight variation of the [3-I-2] cycloaddition approach <1977ICA(25)165> has also been employed for the diastereoselective synthesis of the corresponding anti- and ry -iron-substituted 1,2-dithiolane 1-oxides 243 from 237, as a source of electrophilic disulfur monoxide, and 7] -allyl iron complex 242 (Scheme 39). The method was also tested with cyclopentadienyl iron dicarbonyl crotonyl complex 244 when four diastereomers 245-248 were generated in modest yields (35%) <19980M5534>. 

Oxidations. By using Phl=0 (in presence of KBr) as an oxidant, alcohols are oxidized to acids and ketones in water in excellent yields. When catalyzed by either poly(4-vinylpyridine)-supported sodium ruthenate or a (salen)chromium complex chemoselective oxidation of alcohols (e.g., allylic alcohols to alkenoic acids) occurs, which is contrary to the effect of (salen)manganese and (porphyrin)iron complexes (giving epoxy alcohols). ... 

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