Bis(dicarbonylcyclopentadienyliron)1

[12154-95-9]  · C14H10Fe2O4  · Bis(dicarbonylcyclopentadienyliron)  · (MW 353.92)

(dinuclear metal carbonyl catalyst for a variety of carbon-carbon bond forming reactions;3,7 radical initiator;19 precursor to alkyl- and acyliron complexes useful in organic synthesis20)

Physical Data: mp 194 °C (dec); d 1.77 g cm-3.

Solubility: very sol CHCl3, pyridine, and alcohol; sol CCl4 and CS2; slightly sol pentane; insol H2O.

Form Supplied in: dark reddish-purple solid; purity >97% widely available.

Handling, Storage, and Precautions: the reagent is stable to air, unaffected by H2O, and can be stored for long periods of time. Reputed to be very toxic by inhalation and ingestion; avoid breathing of particles or vapors. Use in a fume hood.

Catalysis.

The catalytic properties of bis(dicarbonylcyclopentadienyliron) (Fp2) are well established.2 The hydroformylation of alkenes at moderate temperatures (125-175 °C) and pressures of H2 and CO (50-100 atm) employing catalytic amounts of Fp2 produces saturated aldehydes.3,4 Aldehyde formation is regioselective, favoring the formation of branched iso-aldehydes over n-aldehydes. The reagent catalyzes isomerization of internal alkenes to terminal alkenes; the products are the iso- and n-aldehydes (eqs 1 and 2).3 Oxo reactions catalyzed by the dinuclear metal carbonyls of Co or Ru give rise to considerable amounts of alcohols and/or higher aldehydes via aldol condensation or hydrogenation of the aldehyde.4 Reactions catalyzed by Fp2 are clean and selective, a result of the considerably weaker acidity and hydrogenating ability of the active metal hydride species.5 The catalytic activity of the reagent is lower than the other dinuclear metal carbonyl species; thus longer reaction times are necessary.3 The range of optimum temperatures is narrow. Best results have been observed at 150 °C.

The reagent catalyzes the reaction of cycloalkanes with CCl4 to give cycloalkyl halides in good yield.6 The simple addition of CCl4 to terminal alkenes at moderate temperatures is catalyzed by Fp2, and results in good yields of 1,1,1,3-tetrachloroalkanes.7 Similar reactions promoted by mononuclear metal carbonyl catalysts give mixtures of telomer homologs,8 which are not formed when Fp2 is employed.7a The reaction is completely regiospecific in that CCl3 is added exclusively at the terminal carbon of the alkene. The reagent also effects the regioselective carbonylation of terminal alkenes in the presence of CCl4 and CO, offering a convenient regioselective route to 2-alkyl-4,4,4-trichlorobutanoyl chlorides (eq 3).7a,9 In alcohol solvents the corresponding ester is obtained in nearly the same yield.7a Whereas the peroxide catalyzed reaction of ethylene, CO, and CCl4 results in a telomeric mixture,10 the Fp2 catalyzed reaction gives only the 1:1 adduct.7a The Fp2 catalyzed reactions of methyl trichloroacetate with terminal alkenes give good yields of 4-alkyl-2,2-dichloro-g-butyrolactones (eq 4).11 The reagent also promotes intramolecular cyclization of allyl trichloroacetate to trichlorinated g-lactones, albeit in lower yield than when CuI salts are employed.12 The reaction of benzylamine with CCl4 at moderate temperatures (120-130 °C), using Fp2 as catalyst, provides a useful route to 2,4,5-triphenylimidazoline derivatives (eq 5).13 While this reaction is promoted by other catalysts,14 Fp2 is the cleanest and most reactive. Perfluoroalkyl-substituted acetals are obtained in excellent yield by the Fp2 catalyzed coupling of perfluoroalkyl iodides, enol ethers, and alcohols (eq 6).15

The reagent catalyzes the cyclotrimerization of disubstituted acetylenes to benzenes at high temperatures.16 Butadiene and isoprene are rapidly and selectively cyclodimerized to 4-vinyl-cyclohexenes by the action of Fp2.17 Aromatic and aliphatic thioketones are desulfurized and converted to substituted ethylenes by the action of Fp2 in refluxing benzene.18

Radical Promoter.

Fp2 is an efficient promoter of radical reactions in the manner of organotin compounds. Irradiation of alkyl halides in the presence of a stoichiometric equivalent of Fp2 leads to C-centered radicals via halogen abstraction by the initially formed 17-electron metal-centered CpFe(CO)2 radical. These carbon radicals can be trapped by alkenes, leading to saturated addition products. Unsaturated addition products can be obtained in the absence of electron-withdrawing substituents on the alkene (eq 7).19 If no alkenes are present, hydrogen abstraction from solvent leads to alkanes in high yield.

Alkyl- and Acyliron Complexes.

Treatment of Fp2 with Sodium Amalgam generates the CpFe(CO)2- anion (Fp-), a powerful nucleophile which reacts with a variety of electrophilic carbon species.20 Alkyl- and acyliron (Fp-R and Fp-COR) complexes are readily available on a large scale and have found extensive use in organic synthesis.21 One-electron oxidants demetalate the alkyl or acyl ligand and lead to a variety of carbonyl compounds,22 e.g. esters,23 acids,23a,24 and amides.23a,25 Decomplexation occurs with a high degree of stereoselectivity (eq 8).26

Treatment of alkyliron complexes with Triphenylcarbenium Tetrafluoroborate results in b-hydride abstraction to give cationic Fp(alkene) complexes.27 Subsequent liberation of the alkene gives products with a high degree of regio- and stereoselectivity. 2-Alkyliron complexes usually give only the terminal alkene, and 3-alkyliron complexes give only the (Z)-alkene (eq 9).28 Cationic Fp(alkene) complexes, prepared from alkyliron complexes,27,28 or other routes,29 react smoothly with lithium enolates and enamines, to give high yields of alkylated products.30 However, the reactions often show poor regioselectivity. Complexes derived from a,b-unsaturated carbonyl compounds are both highly reactive and highly regioselective with a wide variety of nucleophiles.31 Vinyl ether complexes, likewise highly reactive and regioselective, are useful vinyl cation equivalents for the vinylation of enolates (eq 10).31b,32

Electrophilic transition metal alkylidene complexes of the type Cp(CO)2Fe=CRRŽ+ (Fp=CRRŽ+) smoothly effect the cyclopropanation of alkenes.33 These iron-carbene complexes are readily prepared by ionization of a leaving group located at the a-position of the alkyliron complex.34 In particular, the dimethylsulfonium salts offer a chemically mild route to the carbene.34c,d Because of the electrophilic nature of the carbene, cyclopropanation is selective for electron rich alkenes (eq 11).35 High stereoselectivity can be achieved; carbene transfers usually favor higher energy, sterically crowded cis products (eq 12).34d,e,36 Enantioselective cyclopropanations are possible through the use of phosphine substituted iron-carbene complexes possessing a chiral metal center. Optically pure complexes of the type Cp(CO)(PR3)Fe=CRRŽ+ are efficiently prepared from the acyliron complex.37 While enantioselectivities observed for methylene are low (10-38% ee),38 ethylidene transfers to various alkenes occur with high selectivity (70-97% ee) (eq 13).37,39

Related Reagents.

Dicarbonyl(cyclopentadienyl)(isobutene)iron Tetrafluoroborate; Sodium Dicarbonylcyclopentadienylferrate.


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William J. Kelly

Bloomsburg University, PA, USA



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