Iron

Fe

[7439-89-6]  · Fe  · Iron  · (MW 55.85)

(efficient catalyst for a wide variety of organic transformations)

Physical Data: mp 1535 °C; d 7.860 g cm-3.

Solubility: insol water, alkalis, alcohols, ethers; sol acid (attacked or dissolved).

Form Supplied in: chip, foil, filings, powder, wire.

Analysis of Reagent Purity: atomic absorption methods can be used.

Handling, Storage, and Precautions: moisture sensitive.

General.

Iron is used to catalyze a wide variety of organic transformations. The nature of the iron catalyst depends on the reaction conditions (solvent, cocatalysts, etc.) employed for a specific transformation. The reaction descriptions included in this entry will be limited to those utilizing iron powder and to some reactions in which iron(0) is generated in situ, serving as the active catalyst during the reaction.

Iron Powder.

Formation of Lactones.

The lactonization of g-bromo-a,b-unsaturated carboxylic methyl esters and acids is promoted by heating the substrate in the presence of iron powder (eq 1). Iron apparently facilitates the elimination of methyl bromide and rotation about the double bond.1

Preparation of Nonconjugated Dienes.

Nonconjugated dienes are conveniently prepared from allylic halides via an iron-promoted coupling reaction in DMF (Table 1).2 Excellent yields are obtained in many cases and some catalytic effect of added halide salts is also observed. The iron salts in the DMF residue were analyzed as 70% Fe2+ and 30% Fe3+ (via oxidation of Fe2+). Similar types of reaction are observed with a variety of transition metal catalysts, including Pd- and Ni-based catalysts.

Preparation of Ketones from Carboxylic Acids.

Carboxylic acids react with hydrogen-reduced iron powder to give an iron dicarboxylate intermediate which, upon thermolysis, decomposes to give the condensed ketone, CO2, and FeO (eq 2).3

Reduction.

The reduction of nitroaromatics to anilines with iron powder is carried out in a mixture of methanol and concentrated Hydrochloric Acid (eq 3). In the absence of methanol, only partial reduction is observed.4

The reduction of optically active nitroalkanes to active amines is accomplished with iron in Acetic Acid. The reaction proceeds with &egt;82% optical purity. Other reducing agents, such as Lithium Aluminum Hydride, gave a completely racemic mixture of amines.5

Iron Cluster Compounds.

Gif Catalyst-Air Oxidation of Saturated Hydrocarbons.

The Gif catalyst, Fe2FeO(OAc)6py3.5, is a triiron cluster compound that is generated from the reaction of iron dust, acetic acid, and pyridine. The use of this and a reducing agent in acetic acid and pyridine promotes the air oxidation of saturated hydrocarbons. It is a selective reagent in that least hindered, secondary positions are preferentially attacked, leading to the formation of ketones. It does not epoxidize simple alkenes.

The Gif catalyst is but one of a number of (m3-oxo) triiron complexes, some of which have been used as a catalyst for the epoxidation of alkenic alcohol acetates with molecular oxygen (see m3-Oxohexakis(m-trimethylacetato)tris(methanol)triiron(III) Chloride).6

Iron(0) Catalysts: Reduction of FeIII Compounds.

Deprotonation of Aldehydes and Ketones: Preparation of Silyl Dienol Ethers.

Grignard reagents have been shown7 to reduce FeIII compounds to air sensitive Fe0 and it is this reduced species that is responsible for catalyzing a number of useful organic reactions.8 For example, aldehydes and ketones are smoothly converted to the corresponding, thermodynamically favored, trimethylsilyl enol ethers upon treatment with Fe0, Chlorotrimethylsilane, and Triethylamine (eq 4). When a stoichiometric amount of Fe0 catalyst is reacted (no additional Grignard reagent) with cycloalkenones, the exocyclic through-conjugated dienol ether is predominantly formed.9 When a stoichiometric amount of Grignard reagent is reacted with the cyclohexenone in the presence of a catalytic amount of Iron(III) Chloride (Kharasch reagent),10 the endocyclic through-conjugated dienol ether is formed. Likewise, when 0.5-1 equiv of Fe0 catalyst and 1 equiv of Grignard reagent is reacted with the same cyclohexenone (oxygen excluded), the endocyclic through-conjugated dienol ether is again the predominant dienol ether formed.

Iron(0) compounds have been proposed to be the active catalyst in a number of other organic reactions. For example, cross coupling reactions between Grignard reagents and alkyl, vinyl, allyl, and phenyl halides are catalyzed by various iron(III) complexes, such as Tris(dibenzoylmethide)iron(III).11 It is proposed that the reaction between the Grignard reagent and the iron(III) compound produces the active iron(0) catalyst.


1. Loffler, A.; Norris, F.; Taub, W.; Svanholt, K. L.; Dreiding, A. S. HCA 1970, 53, 403.
2. Hall, D. W.; Hurley, E., Jr. CJC 1969, 47, 1238.
3. (a) Davis, R.; Granito, C.; Schultz, H. P. OS 1967, 47, 75. (b) Davis, R.; Schultz, H. P. JOC 1962, 27, 854.
4. Koopman, H. RTC 1961, 80, 1075.
5. Kornblum, N.; Fishbein, L. JACS 1955, 77, 6266.
6. Ito, S.; Inoue, K.; Mastumoto, M. JACS 1982, 104, 6450.
7. Krafft, M. E.; Holton, R. A. JOC 1984, 49, 3669 and references therein.
8. (a) Felkin, H.; Swierczewski, G. T 1975, 2735. (b) Tamura, M.; Kochi, J. K. S 1971, 303. (c) Karasch, M. S.; Lambert, F. L.; Urry, W. H. JOC 1945, 10, 292, 298. (d) Corey, E. J.; Yamamoto, H.; Herron, D. K.; Achiwa, K. JACS 1970, 92, 6635. (e) Corey, E. J.; Posner, G. H. TL 1970, 315. (f) Ashby, E. C. PAC 1980, 52, 545.
9. (a) Krafft, M. E.; Holton, R. A. JACS 1984, 106, 7619. (b) Also see Ref. 7.
10. Kharasch, M. S.; Tawney, P. O. JACS 1941, 63, 2308; 1945, 67, 128.
11. (a) Kochi, J. K.; Neumann, S. M. JOC 1975, 40, 599. (b) Molander, G. A.; Rahn, B. J.; Shubert, D. C.; Bonde, S. E. TL 1983, 24, 5449. (c) Fiandanese, V.; Miccoli, G.; Naso, F.; Ronzini, L. JOC 1991, 56, 4112. (d) Fabre, J.-L.; Julia, M.; Verpeaux, J.-N. BSF 1985, 5, 772. (e) Grichey, H.; Wilkins, G. W. Jr. TL 1976, 723. (f) Molander, G. A.; Etter, J. B. TL 1984, 25, 3281. (g) Kochi, J. K.; Smith, R. S. JOC 1976, 41, 502.

Mark W. Zettler

The Dow Chemical Company, Midland, MI, USA



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