Allyliron complexes

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... 

The overwhehning majority of reported transformations of )] -allyliron complexes either produce cationic jj -alkeneiron complexes or proceed through these intermediates to give other products. This chemistry will be discussed later (Section 4.1.2). 

Cationic phosphine/phosphite -allyliron complexes are more rare. Aside from the previously mentioned report of the preparation of (139a), complex (139b) has been made from the reaction of allyl halides and Fe[P(OMe)3]5. Additionally, complexes with mixed CO/phosphine ligands (139c) have been prepared by the reaction of either (135 X = Br) or [( -allyl)Fe(CO)3]2 with two equivalents of phosphine, followed by NaBPlu addition. ... 

In contrast to almost all other iron allyls, the ferrilactone (155) and ferrilactam (156) allyliron complexes are more stable in their anti configuration. Their stmetnres can be considered to be a distorted octahedron with fac arrangement of CO ligands. In other respects, the stmetnre is typical of other -allyls, and the NMR spectroscopy is characteristic of an anti monosnbstitnted iron aUyl in terms of both chemical shifts and conphng constants. ... 

From 2-methoxyallylic halides a low yield of either a 2-methoxy-substitutcd rf -allyliron complex XXXI or an Fe complexed ketene, XXXII, may be obtained. ... 

Olefin-coupling reactions of Tj -allyliron complexes with a variety of cationic iron-olefin complexes (ethylene, propene, styrene, etc.) were utilized to give cationic bimetallic complexes with cr,7r-hydrocarbon bridges (80,81). The condensation of simple [FpColefin)]" substrates with Fp(allyl) precursors was extended to the reaction with Fp(l,3-butadi-ene)+. Initial attack at C-1 or C-4 leads to the formation of dinuclear complexes with cr-coordinated and 7r-coordinated Fp fragments, which by subsequent intramolecular condensation could give either cyclohexenyl or cyclopentenyl intermediates. Attack at C-2 yields a dinuclear complex incapable of further intramolecular reaction [Eqs. (6-8)]. 

Methyl tetrahydrofuran-3-carboxylates. The allyliron complex reacts with carbonyl compounds to furnish the heterocyclic system which then undergoes carbonylation on treatment with CAN in methanol under CO. 

This reaction is very rapid and produces air-stable, crystalline i -alkyliron complexes in excellent yields from a variety of substituted i/ -allyliron complexes. Other electrophilic olefins, including dichlorodicyanoquinone, dimethyl methylene malonate, and / ,/ -dicyano-o chlorpstyrene undergo similar cycloaddition reactions with // -allylFp complexes. jy vAllyl complexes of j/ -CsHjWCCOyj, j -C5H5Mo(CO)3, i/ -CjHj-Cr(NO)2 and (pyridine)bis(dimethylglyoximato)cobalt react in a similar fashion but are much less thoroughly studied . 

Ill) Iron and Ruthenium. w-Allyliron complexes are not readily prepared directly from olefins by cleavage of an allylic C—H bond, but Jt-allylhydridoiron complexes are intermediates in olefin isomerization catalyzed by Fe(0) carbonyl complexes ". The intermediate 7i-allyliron hydride complexes formed are too reactive to be isolated and undergo a fast reductive elimination to give the isomerized or the starting olefin ... 

Conjugate addition. -jr-Allyliron complexes donate the allyl group to conjugated carbonyl compounds in the presence of MesSil. 

Other reports of (j7 -allyl)Fe(CO)3R complexes are much more scattered and much less systematic. At least three general types of reactions have been observed in more than one case. First, the reaction of (diene)Fe(CO)3 complexes with electrophilic alkenes gives allyliron complexes in two different ways. If the diene in question is acylic, electrophilic attack at C-1 of the diene gives compounds of type (158) (equation 33). In the case of substituted rj -cycloheptatriene-or azepine-Fe(CO)3 complexes, reaction... 

Ethylene sulfite reacts with metal carbonyls such as manganese and chromium to form a complex in which the sulfur atom functions as an electron donor and is directly attached to the metal center (83 and 84) (65CB2248). In contrast, 4-vinyl cyclic sulfite reacts with iron nonacarbonyl to form a ir-allyliron complex 85, with the extrusion of SO2 (90SL224, 90SL331). Some of these reactions are summarized in Scheme 20. 

The procedure given here can be completed easily within a day. Although specific for the preparation of the 2-methyl-1-propene complex it can be adapted readily as an alternative method for the preparation of [Fe( n -C5H5)(CO)2( n -olefin)][BF4] complexes through metallation of an allyl halide or tosylate, followed by protonation of the monohapto-allyliron complex. 

Iron carbonyl complexes containing 77 -alkyl-77 -allyl coordinated hydrocarbon ligands are obtained in several ways. Nucleophilic addition to cationic iron complexes containing 77 -pentadienyl ligands yields (pentenediyl)iron complexes. Oxidatively-induced reductive elimination of these complexes can be utilized as a means to generate 1,2,3-trisubstituted cyclopropanes.The reaction of cationic cycloheptadienyl complexes (Scheme 22) with appropriate nucleophiles also yields the alkyl-allyliron carbonyl complexes. Fe(CO)s also reacts with a- or /3-pincnc in refluxing dioxane (Scheme 22) to produce an alkyl-allyliron complex. Recently, 1,2- and 1,4-disubstituted [(pentadienyl)Fe(CO)3] cations were shown to react with carbon nucleophiles, such as sodium dimethylmalonate, to yield 77 77 -allyl complexes as products. 

Bicyclo[4.1.1] or [3.2.1]octenones and cyclopropanes have resulted from decomplexation of the iron tricarbonyl group from the alkyl-allyliron tricarbonyl complex, using oxidative (i.e., GO atmosphere) or carbonylative methods for the bicyclooctenones and ceric ammonium nitrate (GAN) for the cyclopropanes. Photolysis of analogous tricarbonyl iron complexes leads to monoolefmic hydrocarbons or aldehydes. The kinetics of GO substitution in reactions of 77 -cyclopropenyl complexes of iron is also reported. " A number of comprehensive reviews have appeared since 1992, illustrating the chemistry of ry -allyliron complexes. 

The reactions of various iron carbonyl complexes, such as Fe(GO)4(NMe3), with allene compounds under photo-lytic conditions, yield chelated 77 -allyliron complexes. Two brief reviews discussing the chemistry and application to organic synthesis of these (7r-allyl)tricarbonyl iron lactone complexes have appeared recently. Reaction of the iron lactone complexes with trimethyloxonium tetrafluoroborate yields the carbene complex 23 in good yields. Treatment of the cationic carbene complex with triphenylphosphine results in substitution at the terminal end of the allyl ligand of the trimethylenemethane complex 24. 

The chemistry of ri -organoiron complexes has been reviewed short time after their discovery. Their most prominent examples are T -allyliron complexes. 

Scheme 4-18. Formation of ri -allyliron complexes by alkylation of the dicarbonyl(cyclopentadienyl)ferrates.

Epoxides are also suitable electrophiles in this reaction. A second way to gain access to neutral ri -allyliron complexes is the deprotonation of cationic ri -alkene-Fp complexes in the allylic position. The reaction is stereospecific. Heteroatoms in the allylic position lead to (Z)-olefms, whereas alkyl groups give ( )-configured double bonds (Scheme 4-19). " "" ... 

Ethyl 3,3-dicyanoacrylate (150 mg, 1 mmol) dissolved in dichloromethane (3 mL) is added to a solution of the ri -allyliron complex (369 mg, 1 mmol) in the same solvent (7 mL). The reaction mixture is then stirred at room temperature for 20 h. After removal of the solvent under reduced pressure, the crude product is purified by chromatography with Florisil. The residue is applied to the column as a benzene solution and is eluted with petroleum ether (40-60 °C)/diethyl ether (1 1) to give the (Ti -alkene)iron complex mp 148.5-149 °C (decomp.) 341 mg (70%). ... 

The first ri -allyliron complex was obtained by Emerson and Pettit treating ri -butadiene(tricarbonyl)iron with Bronsted acid such as tetrafluoroboric acid to obtain the coordinatively unsaturated cationic T -allyl(tricarbonyl)iron complex. In the presence of carbon monoxide, tricarbonyl(T -diene)iron complexes can be protonated to give tetracarbonyl(Ti -aIlyl)iron complexes (Scheme 4-77). ... 

Scheme 4-77. Protonation of (T -diene)iron complexes to form cationic Ti -allyliron complexes.

Another option to gain access to T -allyliron complexes is the removal of a leaving group from substituted neutral (T] -alkene)iron complexes. This procedure starts from allylic alcohols, allylic ethers, or carboxylates, which are complexed with nonacarbonyldiiron and subsequently treated with tetrafluoroboric acid to aflbrd the cationic Ti -allyl(tetracarbonyI)iron complexes (Scheme 4-78). The reaction is stereoselective with respect to the geometry of the starting allyl alcohols. ( )-AlIyl alcohols give anti products, whereas syn complexes are formed from (Z)-allyl alcohols. The alkene complexes can also be generated in situ to afford the q -allyliron complexes after treatment with acid. ... 

Scheme 4-79. T -Allyliron complexes by displacement of allylic leaving groups by [Fe(CO)3NO] .

Similarly, the reaction of 3-halo-1-alkenes with tetrabutylammonium tricarbonyl-(nitrosyl)ferrate (TBAFe) gives T -allyl(tricarbonyl)(nitrosyl)iron complexes. Treatment of Y-bromo-a,p-unsaturated esters or amides with potassium tricarbonyl-(nitrosyl)ferrate provides planar chiral T] -allyl(dicarbonyl)(nitrosyl)iron complexes. Enantiopure amides as starting materials lead to a low diastereoselectivity, whereas no diastereoselectivity is observed for the chiral esters. The diastereoisomers of the amide-substituted allyliron complexes can be conveniently separated by column chromatography (Scheme 4-80). ... 

Scheme 4-80. Diastereomeric Tj -allyliron complexes by reaction of potassium tricarbonyl(nitrosyl)ferrate with optically pure a,P-unsaturated amides.

Scheme 4-81. Tj -Allyliron complexes from reaction of vinyloxiranes with tetrabutylammonium tricarbonyl(nitrosyl)ferrate.

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