IRON

1

Iron

Fe

[7439-89-6]

Fe

(MW 55.85)

InChI = 1/Fe
InChIKey = XEEYBQQBJWHFJM-UHFFFAOYAG

(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 Precaution:

moisture sensitive.

General. Iron is used to catalyze a wide variety of organic

transformations. The nature of the iron catalyst depends on the re-
action conditions (solvent, cocatalysts, etc.) employed for a spe-
cific 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 γ-bromo-α,β-

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

R

1

CO

2

R

R

2

Br

O

O

R

1

R

2

iron powder
120–150 °C

(1)

~40–66%

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% Fe

2

+ and 30% Fe

3

+ (via oxidation of Fe

2

+).

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, CO

2

, and FeO (eq 2).

3

CO

2

H

CO

2

2

+

Fe

2

+

H

2

(2)

O

thermolysis

60–75%

+

FeO

+

CO

2

Fe

Table 1

Coupling of allyl chlorides with iron in DMF (150

◦

C, 1 h)

Conversion

Crude

Allyl chloride

(%)

Product

yield (%)

Allyl chloride

65

1,5-Hexadiene

61.5

β

-Methallyl chloride

77

2,5-Dimethyl-1,5-

hexadiene

60

Allyl chloride +β-

methallyl chloride
(1:1)

98

1,5-Hexadiene

21

2-Methyl-1,5-

hexadiene

44.8

2,5-Dimethyl-1,5-

hexadiene

24.1

Reduction. The reduction of nitroaromatics to anilines with

iron powder is carried out in a mixture of methanol and concen-
trated Hydrochloric Acid (eq 3). In the absence of methanol, only
partial reduction is observed.

4

(3)

Cl

CN

NO

2

Cl

CN

NH

2

Fe

MeOH, HCl

89%

The reduction of optically active nitroalkanes to active amines

is accomplished with iron in Acetic Acid. The reaction proceeds
with ≥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, Fe

2

FeO(OAc)

6

py

35

, 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 (µ

3

-oxo) triiron com-

plexes, some of which have been used as a catalyst for the epox-
idation of alkenic alcohol acetates with molecular oxygen (see
µ

µ

µ

3-Oxohexakis(µ

µ

µ

-trimethylacetato)tris(methanol)triiron(III)

Chloride).

6

Iron(0) Catalysts: Reduction of Fe

III

Compounds.

Deprotonation of Aldehydes and Ketones: Preparation of

Silyl Dienol Ethers.

Grignard reagents have been shown

7

to

reduce Fe

III

compounds to air sensitive Fe

0

and it is this re-

duced species that is responsible for catalyzing a number of
useful organic reactions.

8

For example, aldehydes and ketones

are smoothly converted to the corresponding, thermodynami-
cally favored, trimethylsilyl enol ethers upon treatment with Fe

0

,

Chlorotrimethylsilane, and Triethylamine (eq 4). When a sto-
ichiometric amount of Fe

0

catalyst is reacted (no additional

Grignard reagent) with cycloalkenones, the exocyclic through-
conjugated dienol ether is predominantly formed.

9

When a stoi-

chiometric amount of Grignard reagent is reacted with the cyclo-
hexenone in the presence of a catalytic amount of Iron(III) Chlo-

Avoid Skin Contact with All Reagents

2

IRON

ride (Kharasch reagent),

10

the endocyclic through-conjugated

dienol ether is formed. Likewise, when 0.5–1 equiv of Fe

0

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.

(4)

O

OTMS

OTMS

OTMS

+

+

1. stoic. Fe

0

, no MeMgBr

1. MeMgBr

+

cat. FeCl

3

1. stoic. Fe

0

+

MeMgBr

2:96:2

7:1:92

2:96:2

2. TMSCl, Et

3

N

2. TMSCl, Et

3

N

2. TMSCl, Et

3

N

Iron(0) compounds have been proposed to be the active catalyst

in a number of other organic reactions. For example, cross cou-
pling 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., Helv.
Chim. Acta 1970

, 53, 403.

2.

Hall, D. W.; Hurley, E., Jr., Can. J. Chem. 1969, 47, 1238.

3.

(a) Davis, R.; Granito, C.; Schultz, H. P., Org. Synth. 1967, 47, 75.
(b) Davis, R.; Schultz, H. P., J. Org. Chem. 1962, 27, 854.

4.

Koopman, H., Recl. Trav. Chim. Pays-Bas 1961, 80, 1075.

5.

Kornblum, N.; Fishbein, L., J. Am. Chem. Soc. 1955, 77, 6266.

6.

Ito, S.; Inoue, K.; Mastumoto, M., J. Am. Chem. Soc. 1982, 104, 6450.

7.

Krafft, M. E.; Holton, R. A., J. Org. Chem. 1984, 49, 3669 and references
therein.

8.

(a) Felkin, H.; Swierczewski, G., Tetrahedron 1975, 2735. (b) Tamura,
M.; Kochi, J. K., Synthesis 1971, 303. (c) Karasch, M. S.; Lambert, F.
L.; Urry, W. H., J. Org. Chem. 1945, 10, 292, 298. (d) Corey, E. J.;
Yamamoto, H.; Herron, D. K.; Achiwa, K., J. Am. Chem. Soc. 1970,
92

, 6635. (e) Corey, E. J.; Posner, G. H., Tetrahedron Lett. 1970, 315.

(f) Ashby, E. C., Pure Appl. Chem. 1980, 52, 545.

9.

(a) Krafft, M. E.; Holton, R. A., J. Am. Chem. Soc. 1984, 106, 7619.
(b) Also see Ref. 7.

10.

Kharasch, M. S.; Tawney, P. O., J. Am. Chem. Soc. 1941, 63, 2308; 1945,
67

, 128.

11.

(a) Kochi, J. K.; Neumann, S. M., J. Org. Chem. 1975, 40, 599.
(b) Molander, G. A.; Rahn, B. J.; Shubert, D. C.; Bonde, S. E.,
Tetrahedron Lett. 1983

, 24, 5449. (c) Fiandanese, V.; Miccoli, G.; Naso,

F.; Ronzini, L., J. Org. Chem. 1991, 56, 4112. (d) , J.-L.; Julia, M.;
Verpeaux, J.-N., Bull. Soc. Chem. Fr. 1985, 5, 772. (e) Grichey, H.;
Wilkins, G. W. Jr., Tetrahedron Lett. 1976, 723. (f) Molander, G. A.;
Etter, J. B., Tetrahedron Lett. 1984, 25, 3281. (g) Kochi, J. K.; Smith,
R. S., J. Org. Chem. 1976, 41, 502.

Mark W. Zettler

The Dow Chemical Company, Midland, MI, USA

A list of General Abbreviations appears on the front Endpapers