Sodium Dicarbonylcyclopentadienylferrate

[12152-20-4; 12107-09-4]  · C7H5FeNaO2  · Sodium Dicarbonylcyclopentadienylferrate  · (MW 199.96)

(nucleophilic source of iron;1 precursor to iron alkene complexes;1 precursor to h1-allyliron and h1-propargyliron complexes; precursor to acyliron complexes;2 precursor to iron carbenes;3 with subsequent oxidation will carbonylate alkyl halides or sulfonates4)

Solubility: sol ethereal organic solvents (Et2O, THF).

Preparative Methods: to Mercury(0) (70 mL) stirring under nitrogen in a 500 mL flask with a stopcock in the bottom is slowly added Sodium metal (7.2 g), cut into small pieces, under a strong flow of nitrogen. The resulting amalgam is cooled to room temperature, and THF (100 mL) is added, followed by Bis(dicarbonylcyclopentadienyliron) (35.4 g, 0.1 mol). The mixture is stirred for 40 min, and the mercury is drained through the stopcock. The resulting Na[Fe(CO)2Cp] is used without further purification.5 An alternative preparation using sodium dispersion in THF at reflux has also been reported.6

Handling, Storage, and Precautions: generally prepared and used immediately; it is destroyed by protic solvents.

Organoiron Reagents.

The reagent, NaFp (the [Fe(CO)2Cp] unit is commonly abbreviated Fp), is a highly nucleophilic source of iron which readily substitutes alkyl halides or pseudohalides.7 The reaction occurs with inversion at a chiral center, and substrate reactivity, as well as yields, are as expected for SN2 substitutions (primary > secondary >> tertiary). Benzyl and allyl halides also react easily (eq 1), while propargyl substrates give the expected propargyl-Fp complexes if the remote alkyne terminus is substituted (eq 2)7b and s-bonded iron allenes (by formal SN2) if unsubstituted (eq 3).7c Epoxides react readily but undergo subsequent rearrangement. Acid chlorides or anhydrides are attacked by NaFp to give useful yields of acyl iron complexes (eq 4).8 b-Chloroenones undergo addition-elimination with NaFp to afford b-Fp enones.9

Iron Alkene Complexes.1

Epoxides suffer nucleophilic attack by NaFp, and give cationic iron alkene complexes upon protonation with strong acid. Iodide ion in acetone liberates the free alkene, giving an overall stereospecifically syn reduction of epoxides (eq 5). Pyrolysis of the iron alkoxide intermediate gives the other alkene isomer (eq 6), allowing access to either isomer from one epoxide. Stereospecificity is high for diaryl or dialkyl epoxides, but isomeric mixtures are obtained in the preparation of enones.10

The cationic enone-iron complexes made by the above procedure undergo highly regioselective b-nucleophilic attack. They have been shown to be reactive as Michael acceptors with enolates or silyl enol ethers and may serve as Robinson ring annulation precursors (eq 7).11

The attack of NaFp upon a-bromo acetals, followed by acid- or trityl cation-induced loss of alkoxide, gives cationic Fp-vinyl ether complexes (eq 8).12,13 These compounds react with enolates a to the alkoxy unit, regardless of additional substituents; subsequent treatment with strong acid and iodide ion gives overall vinylation of the enolate.13 The complex of ethyl a-ethoxyacrylate (1) serves in a closely analogous manner in the three step synthesis of a-methylene-g-lactones (eq 9).13b

Alkene Synthesis.

Aliphatic alkyl halides or pseudohalides may be converted into alkenes by a three-step process involving nucleophilic substitution by NaFp, hydride abstraction by Triphenylcarbenium Tetrafluoroborate, and Sodium Iodide-induced decomposition of the iron alkene (eq 10). Hydride abstraction is selective, such that both 1-haloalkenes and 2-haloalkenes normally afford terminal alkenes; 3-haloalkenes normally give (Z)-alkenes as products, but yields are modest.14

h1-Allyliron Complexes.

h1-Allyliron complexes, prepared by the nucleophilic substitution of NaFp on allyl halides or pseudohalides, are nucleophilic allylation reagents for aldehydes, ketones, and acid chlorides in the presence of Lewis acids (eq 11).15

They are reactive with a variety of other cationic species, including strong acids, and this route has been employed for the preparation of more complex alkene-Fp cations.7b Reaction occurs at the end of the allyl unit remote from iron; this fact has been used to convert allyl alcohols to terminal alkenes through the intermediacy of Fp-allyl complexes (eq 12).16

h1-Allyliron and h1-propargyliron complexes, as well the Fp-allene complex, react with electron-deficient alkenes in a stepwise [3 + 2] cycloaddition; the result is a cyclopentane or cyclopentene ring annulation (eq 13). The most reactive substrates include a,b-unsaturated esters or nitriles possessing two electron-withdrawing groups, whereas simple enones will react in the presence of a Lewis acid.7b,17,18 Heteroatom-based double-bonded systems such as isocyanates, hexafluoroacetone, N-sulfonylurethane, and S2O (generated in situ)19 participate in these cycloadditions, but the resulting Fe-C bond usually has not been further manipulated (S2O adduct excepted). Also reactive is the uncomplexed double bond of tropyliumiron tricarbonyls, which ultimately form the fused 5,7 ring systems of hydroazulenes (eq 14).20

Acyliron Complexes.

a,b-Enoyl-Fp complexes, available by NaFp attack on acid chlorides (eq 4), are susceptible to conjugate addition by amine or thiol nucleophiles. The amine adducts may be oxidatively demetalated to give b-lactams (eq 15).21 Allylstannanes react with a,b-enoyl-Fp complexes in a highly regio- and stereoselective [3 + 2] cycloaddition (eq 16).2 The tin residue of the product is invariably trans to the acyliron function, whereas either cis or trans crotylstannanes result in a methyl function trans to the tin unit. Subsequent conversion of the acyliron function to an ester and conversion of the tin unit to a hydroxy group gives moderate overall yields of cyclopentanes which may be quite highly functionalized. The use of dimethoxycarbenium tetrafluoroborate instead of Aluminum Chloride results in conjugate addition without cyclization.2

The chiral acyl-Fe(CO)(PR3)Cp complexes are not generally accessible by phosphine substitution of acyl-Fp complexes (see (S)-Aceto(carbonyl)(cyclopentadienyl)-(triphenylphosphine)iron). The nicotinyl or dihydronicotinyl acyliron substrates do substitute phosphine ligands successfully, however, and complexes such as (2) prepared in this manner have been shown to be useful in the enantioselective reduction of ethyl benzoylformate to ethyl mandelate.22

Iron Carbene Reagents.2

NaFp reacts with a-chloro ethers and a-chloro thioethers to afford a-methoxy- or a-alkylthioalkyliron complexes (3), respectively (eq 17).23-26 Acyliron complexes undergo an O-methylation-reduction sequence which also gives (3; R = OMe) (eq 18). Alternatively, the reaction of NaFp with aldehydes, followed by O-silylation, gives a-trimethylsiloxyalkyliron complexes (3; X = OSiMe3) (eq 19).27 When treated with protic or (more often) Lewis acids, these compounds (3) are converted to cationic iron carbene complexes (4) (eq 20), which in turn react with alkenes to form cyclopropanes.

Addition to the alkene is stereospecifically cis with respect to alkene geometry. The a-methoxyalkyliron and trimethylsiloxyalkyliron complexes are the most reactive and the most stereoselective at any newly formed chiral centers; substituents on the carbene normally enter cis with respect to alkene substituents (&egt;4:1) (eq 21). The a-alkylthioalkyliron complexes (X = SMe, SPh) are less cis stereoselective, but are more suitable for transfer of a methylene group (R = H) (eq 22). For intermolecular cases, yields are best for methyl-, aryl-, or cyclopropyl-substituted carbenes;28 vinyl substituents give lower yields, and n-alkyl- or dialkyl-substituted carbenes, regardless of precursor, do not give synthetically useful yields. Intramolecular cyclopropanations have also been reported, but yields are variable and both C-H insertion and cationic cyclization may compete.29

Iron carbenes with a-bridgehead carbon atoms decompose by alkyl group migration to give bridgehead alkene-iron complexes, provided the alkene strain is below 88 kJ mol-1. The decomplexed alkene was isolated in one case (eq 23).30

Carbonylation.

Substitution of alkyl halides or sulfonates by NaFp, followed by oxidative carbonyl insertion by Br2 in methanol, gives good yields of carbonylation products (eq 24). The use of NaFp allows milder reaction conditions to be used relative to Sodium Tetracarbonylcobaltate, and ethers, esters, and free hydroxyl groups are tolerated.3 Although applied synthetically for primary halides or pseudohalides, secondary substrates give overall inversion of configuration.31 Carbonyl insertion is also induced by the addition of a ligand, most commonly PPh3; this route is the one of choice for the preparation of racemic acetyl(carbonyl)cyclopentadienyl(triphenylphosphine)iron (eq 25).32


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James R. Green

University of Windsor, Ontario, Canada



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