[13463-40-6]  · C5FeO5  · Pentacarbonyliron  · (MW 195.90)

(diene complexation;4 carbonyl insertion;62-64 cycloaddition; reduction96 and deoxygenation)

Physical Data: mp -20 °C; bp 103 °C; fp -15 °C; d 1.490 g cm-3; n 1.5196; X-ray diffraction studies at -100 °C confirm the trigonal bipyramidal structure of Fe(CO)5.2

Solubility: miscible with organic solvents.

Form Supplied in: straw-yellow liquid.

Handling, Storage, and Precautions: Fe(CO)5 is a volatile, flammable, and highly toxic liquid,3 which does not react with air at room temperature. It has a relatively high vapor pressure (21 mmHg at 20 °C). When Fe(CO)5 is exposed to light, Fe2(CO)9 impurity slowly accumulates. The monomeric pentacarbonyl complex can be purified by trap-to-trap distillation in a vacuum system, but for routine use this purification step is usually omitted. Use in a fume hood.

Diene Complexation.

Tricarbonyl(h4-butadiene)iron(0), the first iron-diene complex, was prepared as long ago as 1930 by Reihlen and co-workers, by heating butadiene with Fe(CO)5.4 Subsequently, substituted derivatives were studied by Pettit's group and by others.5 The first complex of a cyclic diene, tricarbonyl(h4-cyclohexadiene)iron(0), became available in 1958.6 The real breakthrough for the development of synthetic methods sprang from pioneering work with substituted complexes in the cyclic (particularly cyclohexadiene) series in the 1960s and 1970s, by Birch,7-10 Lewis,11,12 and co-workers, then of the group of Pearson,13 work which was initiated by the discovery by Fischer and Fischer in 196014 of hydride abstraction from tricarbonyl(h4-cyclohexadiene)iron(0) to yield an h5 iron-complexed cyclohexadienyl cation.

A wide range of methods are now available for the preparation of h4-diene tricarbonyliron complexes, but by far the most common technique remains the direct complexation of free dienes. Fe(CO)5 is less reactive than Fe2(CO)9 or Fe3(CO)12, but under the right conditions it provides an inexpensive and efficient route to iron-alkene complexes, particularly from the 1,4-cyclohexadienes.

1,4-Diene Complexation.

Thermal Complexation.

Because 1,4-cyclohexadienes are readily available from the metal-ammonia reduction (Birch reduction) of the corresponding aromatic compounds,15 they have been extensively studied as substrates for direct 1,4-diene complexation, leading to isomerized 1,3-diene complexes. This methodology suffers, however, from the disadvantage that products are often obtained as mixtures of regioisomers. Fe(CO)5 provides a simple, low-cost source of Fe(CO)4 for initial addition to the p-system of a double bond. By recycling the reagents and the uncomplexed ligand, preparations on a 50-60 g scale can be achieved from a series of overnight thermal reactions. The procedure most frequently used, that of Cais and Maoz,16 modified by Birch,8 involves heating a cyclohexadiene with pentacarbonyliron in boiling di-n-butyl ether. In the extensive literature on the formation of differently functionalized tricarbonyl(1,3-cyclohexadiene)iron complexes from substituted 1,4-cyclohexadienes8,9,11,12,17,20 (including the tricarbonyliron complexes of a number of dihydroanisic esters,18 various cyclohexadiene esters,9,19 and 1-methoxy-2,4-dimethyl-1,4-cyclohexadiene21), at present the most useful and representative intermediates for synthetic applications carry methoxy substituents on the diene ligand. For example, 2,5-dihydroanisole (1) or 4-methyl-2,5-dihydroanisole (2) upon treatment with pentacarbonyliron at elevated temperature afford a mixture of the 1- and 2-OMe substituted complexes (3) and (5) or (4) and (6) (eq 1).7-9,11,12,22

Although 1-methoxy-1,4-cyclohexadiene itself leads to a mixture of the 1- and 2-OMe complexes, bicyclic compounds of substitution type (7) do not give the 2-OMe regioisomer, affording instead a mixture of complexes (8) and (9) in a 1:1 ratio (eq 2).22,23 In a more unusual case, an amide substituent was present on the 1,4-diene.24

More extensive rearrangement of the position of unsaturation in a,o-dienes also leads to h4-diene complexes.25 In other cases where rearrangement is blocked, homoconjugated dienes can form stable bis(alkene) complexes by thermal reaction with Fe(CO)5.26 Isotetralene offers an unusual case where rearrangement to the 1,3-diene is possible, but a 1,4-diene complex is reported.27


Photolysis offers an alternative for the direct conversion of 1,4-dienes into 1,3-diene complexes. In eq 3 the deuterium labeling reveals that isomerization during complexation of the 1,4-diene precursor proceeds by a 1,3-hydrogen transfer.28

The direct complexation of 1,4-dienes can form specific stereoisomers. Initial p-complexation occurs before the conjugation step, and the hydrogen atom moves only on the face of the ligand carrying the metal.29 Unfortunately, a mixture of regioisomers usually results, which may present separation problems. Loss of OMe can also sometimes occur, particularly from some 1,3-dimethoxy-1,4-cyclohexadienes. In these cases, preconjugation of the diene is essential30 if a dimethoxydiene complex is required. Mixtures of complexes obtained from 1,4-dienes may also be equilibrated31 to thermodynamic ratios of isomeric complexes, which do not correspond to the equilibrium ratios of the free dienes.32,33

1,3-Diene Complexation.

Thermal Complexation.

Low yields and mixed products render some nonconjugated dienes unattractive as precursors for h4-complexes. For many synthetic applications, cleaner routes are required to form a single defined compound in good yield. A good approach employs a conjugation step prior to complexation.30 The 1-NR2 series can usually be conjugated thermally.33 Equilibration of 1-methoxy-1,4-cyclohexadiene with the 1,3-diene (ca. 75%) using potassium in liquid ammonia as catalyst yields only the 1-OMe substitution pattern on the conjugated isomer.34 Other conjugation catalysts include bases,35 Wilkinson's catalyst,36 Cr derivatives,29 and charge-transfer complexation reagents such as dichloromaleic anhydride.37 Frequently, only about 70-80% of the conjugated isomer is present at equilibrium,38 but the complexation product, if formed under mild conditions, corresponds to the conjugated isomer present. This process of preconjugation followed by complexation, using the usual Fe(CO)5 thermal methodology, has led, for example, to an important key intermediate in the synthesis39 of limaspermine. The use of azadiene catalysts improves the thermal complexation reaction.40

Complexation of dienes with several methyl,41 alkyl,42 or methoxy43 groups is straightforward, but with bulky substituents, rearrangements can lead to the formation of mixtures of products,44 and care is also needed in the presence of side-chain leaving groups.45 Trialkylsilyl-substituted dienes complex well. In the reactions of 2-(trimethylsilyl)-1,3-cyclohexadiene derivatives with excess pentacarbonyliron in refluxing di-n-butyl ether,46 (10) gave rise to an 8:2 mixture of (11) and its epimer (12), with coordination to Fe(CO)3 occurring preferentially on the less hindered face of the 1,3-cyclohexadienyl ring (eq 4).

Photochemical Complexation.

The studies of Birch et al. on the thebaine modification using tricarbonyliron complexes47,48 exemplify the possibility of complexation of 1,3-dienes by photolysis, and illustrate the potential for iron complexes to ensure control of skeletal rearrangement. Thus, via the initial step of complexation of the methoxycyclohexadiene ring of thebaine (13), practically quantitative access to northebaine (14) and 14a-substituted thebainone derivatives could be easily obtained with Fe(CO)5 under UV irradiation (eq 5).47

Thermal and Photochemical Reactions in Other Ring Sizes.

Similar complexation reactions can be performed in seven- and eight-membered rings,49 but with cyclopentadiene the reaction takes a different course, producing a dimeric cyclopentadienyl complex.50 Complexation of silyl-substituted cyclopentadienes afforded dinuclear products.51 In more unsaturated systems, competing reduction processes are possible. Cycloheptatriene reacts with pentacarbonyliron to give a mixture of tricarbonyliron complexes of cycloheptatriene and 1,3-cycloheptadiene.52 Substituted cycloheptatrienes53 and cycloheptadienes54 can also be complexed. Thermal complexation of divinylpyrrolidines forms exocyclic diene complexes by rearrangement of the location of unsaturation.55

Complexation Using Fe(CO)5 and Me3NO.

Reaction of a conjugated diene with Fe(CO)5 in the presence of Trimethylamine N-Oxide resulted in the formation of tricarbonyl(1,3-diene)iron complexes. The use of amine oxides, preferably Me3NO, permits fast reactions even at low temperatures. Reduction of amine oxides by Fe(CO)5 has been described by Alper et al.56 and, in the case of trimethylamine N-oxide, the intermediate compound trimethylaminotetracarbonyliron was isolated (eq 6).57,58

The reverse reaction, namely the known disengagement of the ligand from the (diene)Fe(CO)3 complex, presents no problem because of the lower reaction temperature and the smaller molar ratio of Me3NO:Fe(CO)5 employed in the complexation procedure.57 Trimethylaminotetracarbonyliron is attractive as a mild reagent but further generalization is needed.

Acyclic Series.

The first complex in the acyclic series was prepared from butadiene by the thermal method.4 Heating isoprene and pentacarbonyliron at high temperature, however, is inefficient due to competitive Diels-Alder dimerization. Despite the formation of some bis(diene) iron carbonyl complexes on prolonged irradiation, the photochemical method is superior in this case. Complexation of acyclic dienes by Fe(CO)3 is limited to those that can adopt a cisoid conformation, with the syn substitution pattern normally preferred. 2,4-Hexadienoic acid, for example, can be conveniently complexed by a photolytic procedure.59 Trialkylsilyl-substituted dienes have also been complexed.60

Isomerization of Alkenes.

In some cases the positional rearrangement of alkenes affords useful products which are not 1,3-diene complexes. The isomerization of allyl ethers in the presence of Fe(CO)5 under UV irradiation has also been reported.61

Ring-Opening Reactions with Carbonyl Insertion.

Cyclopropane rings.

Synthesis of (diene)Fe(CO)3 complexes, and of cyclohexenones, lactones, ketones, or sulfones, can follow from the opening of vinylcyclopropanes with Fe(CO)5 under either thermal or photochemical conditions. Reactions of the vinylcyclopropane (15) with Fe(CO)5 under thermal conditions resulted in ring opening with concomitant hydrogen shift to form the diene complex (16) (eq 7).62,63,64

The reaction of (16) under photochemical conditions led to (17) by ring opening and carbonyl insertion. Conjugation may occur, forming the cyclohexenone (18) (eq 7).65 The expansion of the carbon skeleton of terpene hydrocarbons via carbonylation offers an interesting means to prepare optically active strained ketones (eq 8).66

Regiocontrol, however, is a limitation. In the reaction between (+)-2-carene (19) and Fe(CO)5, five compounds are isolated. A recent study67 improved the reaction with (+)-2-carene and extended the method to (+)-3-carene.

Epoxide Rings.

The synthesis of alcohols, b-lactones, and bicyclic derivatives occurs via a ferralactone complex. Irradiation of a,b-unsaturated epoxides in the presence of Fe(CO)5 yields ferralactone complexes by ring opening and CO insertion (eq 9). For example, the vinyloxirane (20) gives the lactone (21).68

While the photochemical reaction69 proceeds stereospecifically, the thermal process between Fe(CO)5 and vinyl epoxides gives mixtures of diastereoisomers. Subsequent treatment of ferralactones with Cerium(IV) Ammonium Nitrate results predominantly in the formation of b-lactones, with a few exceptions where d-lactones are obtained.70

A thermal treatment of the ferralactone displaces one molecule of CO. The a,b-unsaturated epoxide (22) forms the lactone (23) after irradiation in the presence of Fe(CO)5, but subsequent heating leads to the formation of the (hydroxycyclohexadiene)Fe(CO)3 complex (24) (eq 10).67

UV irradiation of 9-oxabicyclo[6.1.0]nona-2,4,6-triene (25) (the monoepoxide of cyclooctatetraene) in the presence of Fe(CO)5 promotes an overall 1,2 to 1,4 skeletal rearrangement via the lactone (26) and the Fe(CO)3 complex (27) to form the Fe(CO)3-Fe(CO)4 complex (28). Subsequent decomposition of the bis(iron) complex (28) gave the previously unknown 9-oxabicyclo[4.2.1]nona-2,4,7-triene (29) (eq 11).71

Carbonylative Ring Expansion.

When either a- or b-pinene (30) or (31) is heated neat with an equimolar amount of Fe(CO)5 under an initial pressure of 30 psi CO at 160 °C, two ketones (32) and (33) are obtained, formed by insertion of CO into the cyclobutane ring (eq 12).72 Both ketones are optically active.

Dehalogenation Reactions.

Coupling of gem-Dihalides.

Pentacarbonyliron effects dehalogenative coupling of dichloro- or dibromodiphenylmethane and benzylic gem-dihalides to form alkenic compounds in good yields (eq 13).73

a-Halo Ketones.

Treatment of a-halo ketones (34) with Fe(CO)5 was found by Alper to result in formation of coupled 1,4-diketones (35) and reduced monoketones (36) (eq 14).74

a,a-Dibromo Ketones and a,a,a,a-Tetrabromo Ketones.

Although dehalogenation combined with carbon-carbon bond formation is more commonly performed with Nonacarbonyldiiron, Fe(CO)5 may also be used. The use of a tribromo ketone, followed by a separate reductive dehalogenation step, is illustrated for the synthesis of carbocamphenilone (eq 15), a compound of interest as a skewed glyoxal model.75

The reaction between the allylic bromide (37) and Fe(CO)5 constitutes an alternative method of preparation of tricarbonyl(1,3-cyclohexadiene)iron (38) (eq 16).76 Allylic alcohols react in a similar way (eq 17) under neutral conditions,77 but in the presence of acid (HBF4), cationic h3 iron carbonyl complexes are produced.78

Benzohydroxamoyl Chlorides.

Benzohydroxamoyl chlorides (39) are converted into nitriles (40) when refluxed with Fe(CO)5 in THF (eq 18).79

Dehydration and Dehydrosulfuration Reactions.

Certain amides and thioamides, on treatment with Fe(CO)5, yield nitriles or imines depending on the degree of substitution present (eq 19).80 Dithioesters, on the other hand, form trinuclear iron carbonyl complexes.81

Reaction with Oximes; Deprotection of Ketones.

Regeneration of protected ketones from their oximes can be achieved by refluxing the oxime in di-n-butyl ether with one equiv of Fe(CO)5 and a catalytic amount of a Lewis acid (eq 20).82

Other Reactions Occurring with Carbonyl Insertion.

Carbonylation of Benzyl Halides.

Benzyl halides can be converted into phenylacetic acids by reaction with Fe(CO)5.83 Phase-transfer catalysis is useful in this reaction.84

Diazonium Salts.

Carboxylic acids or ketones are obtained from the reaction of diazonium salts with Fe(CO)5, depending on the reaction conditions.85


Nucleophiles such as amines (eq 21)86 and organolithium reagents (eq 22)87 are also found to react with Fe(CO)5 to give products of CO insertion. Cyclopentadienide, on the other hand, undergoes an unusual tricarbonylation reaction to form a cyclopentadienyl complex with an iron-acyl bridge connecting the metal and the cyclopentadienyl ring (eq 23).88 The product was isolated by protecting the two OH groups. An aryllithium reagent, in which the aromatic ring is bound to a tricarbonylchromium group, also follows a different reaction path, and bimetallic tetracarbonyliron ferrate derivatives are isolated.89

An arylmethylmalonate derivative, with an ortho iodine substituent on the aromatic ring, undergoes a carbonylative cyclization by oxidative addition to the aryl iodide and reaction at the nucleophilic center of the malonate group.90

Formylation and Acylation of Pyridine.

Direct formylation and acylation at the b-position of pyridine is possible by reaction with phenyllithium and then with Fe(CO)5. 2-Phenylpyridine is also formed in these reactions.91

Formation of N,N-Disubstituted Ureas.

The substituted ureas are prepared in about 50-95% yield by reaction of aryl or alkyl nitro compounds with the MgBr derivative of an amine in the presence of Fe(CO)5 (eq 24).92

Formation of Carboxylic Esters93,95 and Ketones.94

Grignard reagents can also be converted into esters and ketones by Fe(CO)5 (to supply CO). A cathodic ester synthesis from alcohols and alkyl halides uses carbon monoxide (at 1 atm) and Fe(CO)5 as a catalyst.95

Deoxygenation Reactions and Reductions.

Deoxygenation of N-O Bonds.

Deoxygenation of amine oxides has been discussed above (see eq 6); pyridine N-oxides provide a further illustration (eq 25).96

Nitroso Compounds.

Similar treatment of nitrosobenzene resulted in a 75% yield of azobenzene. Aromatic N-nitroso amines have been converted into the secondary amines in 85-92% yields.97 Under photolytic conditions, azoxyarenes are produced.98 The choice of solvent is important. Dialkylnitrosoamines afford ureas (eq 26).97

Aromatic Nitro Compounds.

Indoles are formed99 by Fe(CO)5 catalyzed deoxygenation of o-nitrostyrenes with CO (eq 27).

Deoxygenation of Epoxides.

Fe(CO)5 in N,N-dimethylacetamide or tetramethylurea deoxygenates epoxides (2 h, 145 °C). The reaction is not stereospecific: trans-stilbene oxide is converted into both trans- (56%) and cis-stilbene (22%).100 Epoxides of 1-alkenes are converted mainly into mixtures of internal alkenes.

Stabilization of Reactive or Unstable Species.

The complex (42), formed from the reaction between the dibromoxylene (41) and Fe2(CO)9 or Na2Fe(CO)4, reacts with Fe(CO)5 to yield the mixture of stereoisomers (43) and the complex (44) (eq 28).101 With pentacarbonyliron, however, (41) is taken directly on to an indanone by carbonylative cyclization (see above).102

A pentafulvalene has been trapped as a diiron complex using Fe(CO)5.103 Thiophene dioxide has also been stabilized as its tricarbonyliron complex.104 The irradiation of a-pyrone (45) in the presence of Fe(CO)5 gives cyclobutadiene stabilized by complexation with the Fe(CO)3 moiety (46), together with the (a-pyrone)Fe(CO)3 complex (47) (eq 29).105

Reactions with Alkynes.


Two alkynes can be combined to form a cyclobutadiene ligand. The reaction has been reported for a dialkylamino trimethylsilyl-substituted alkyne, and forms a diaminocyclobutadiene complex.106

Dimerization with Insertion of One Molecule of CO.

A number of tricarbonyliron complexes of substituted cyclopentadienones have been prepared from alkynes. The reaction was first described by Jones et al.107 for Fe(CO)5 and Ph-C&tbond;CH in the presence of Ni(CO)4. A number of other alkynes (PhC2Me, PhC2SiMe3, Me3SiC2R, BrC6H4C2H, CF3C2CF3, PhC2Ph and a,o-diynes) give cyclopentadienone complexes (eq 30).108

Cyclization of an alkyne to an alkene with carbonyl insertion (analogous to the Pauson-Khand reaction) has also been achieved using pentacarbonyliron.109

Dimerization with Insertion of Two Molecules of CO.

By manipulating reaction conditions, formation of benzoquinones can be achieved (eq 31).110

Reaction with Allenes.

A diallene, hexatetraene (eq 32), reacts with Fe(CO)5 with insertion of carbon monoxide to form 2,5-dimethylenecyclopent-3-enone.111 Allenyl ketones, on the other hand, form dinuclear complexes.112

Formation of Naphthoquinones via Iron Metallocycles.

Benzocyclobutenedione (48) forms the iron complex (49) by irradiation with Fe(CO)5.113 Complex (49) reacts with a wide variety of alkynes to give naphthoquinones, in yields usually >70% (eq 33).

Trimerization of Benzonitriles.

Benzonitrile is trimerized in good yield by heating with Fe(CO)5 for several hours (eq 34).114

Formation of Carbodiimides.

Carbodiimides are obtained in ca. 50% yield from the reaction of azides with isocyanides catalyzed by Fe(CO)5.115

Anionic Iron Carbonyl Reagents.

The reagent [HFe(CO)4]- can be generated in situ from pentacarbonyliron, allowing straightforward, regiocontrolled, hydroxycarboxylation of acrylic acid by Fe(CO)5, Ca(OH)2, and H2O/i-PrOH under 1 atm of CO.116

Formation of Iron Carbene Complexes.

Fe(CO)5 is the starting material for the preparation of the tetracarbonyl(ethoxyphenylmethylidene)iron(0) (50) (eq 35).117

Iron carbene complexes react with alkynes under CO atmosphere to form (a-pyrone)Fe(CO)3 complexes.118

Modified Corey-Winter Alkene Synthesis.

Daub et al.119 have obtained satisfactory yields by using Fe(CO)5 in an iron-mediated version (eq 36) of the reaction of thionocarbonates with Trimethyl Phosphite, which gives poor yields in the case of thermally labile alkenes.

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Sylvie Samson & G. Richard Stephenson

University of East Anglia, Norwich, UK

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