[15321-51-4]  · C9Fe2O9  · Nonacarbonyldiiron  · (MW 363.79)

(precursor for iron carbonyl complexes, carbonylation, dehalogenation, deoxygenation, and reduction reactions)

Alternate Names: diiron enneacarbonyl; diiron nonacarbonyl; tri-m-carbonylhexacarbonyldiiron.

Physical Data: mp 100-120 °C (dec); d 2.08 g cm-3; X-ray diffraction study.3

Solubility: insol organic solvents.

Form Supplied in: orange flaky solid.

Preparative Method: formed by exposing Pentacarbonyliron to light (eq 1).1 The complex contains six terminal carbonyl groups and three bridging carbonyl groups.2

Handling, Storage, and Precautions: because of its much lower solubility and volatility, Fe2(CO)9 is less dangerous to handle than Fe(CO)5, though it should be remembered that Fe(CO)5 is often formed in reactions using Fe2(CO)9; when handling Fe2(CO)9, avoid dust formed by this light flaky substance; use in a fume hood; store in a freezer; when fully dried, commercial samples can be pyrophoric.4

Diene Complexation.

Cyclohexadiene Series.

Tricarbonyliron complexes are formed by reaction of Fe(CO)4 with dienes. Fe2(CO)9 is an important starting material for this reaction because it offers a convenient source of Fe(CO)4 generated under much milder conditions than from pentacarbonyliron (eq 2).

Starting from a cisoid 1,3-diene, Fe2(CO)9 can yield, under mild thermal conditions, tricarbonyl(h4-1,3-diene)iron complexes. Cyclohexa-1,4-dienes (readily available from the Birch reduction of substituted benzenes) often require prior conjugation, but conjugated 1,3-dienes (both cyclic and acylic) can be converted into tricarbonyliron complexes directly. Some of the earliest examples5-9 of complexation of 1,3-dienes employed Fe2(CO)9 (eq 3, dates shown in parentheses). (Alternative reagents in the early days were Pentacarbonyliron or Dodecacarbonyltriiron.)

More recently, the use of Fe2(CO)9 has been increasingly valued because, under mild conditions, improved regio- and stereocontrol are possible, and sensitive functionality is less severely affected by the complexation conditions. Regiocontrol is often a problem in the complexation of 1,4-dienes by high-temperature direct reaction with Fe(CO)5. Preparation of the diene complex (1), for example, can be improved by preconjugation of the 1,4-diene with p-Toluenesulfonic Acid10 and by complexation of the resulting equilibrium concentration of the 1,3-diene with Fe2(CO)9 under conditions in which the 1,4-isomer does not react (eq 4). Bis-alkene complexes are formed when rearrangement to bring the double bonds into conjugation is not possible.11

Good diastereoselectivity can also be a significant advantage. The dioxygenation of arenes by Pseudomonas putida provides optically active cis-diol products.12 In the mild complexation conditions using Fe2(CO)9, the chirality of the diol is relayed to tricarbonyliron complexes, providing enantiopure intermediates for use in asymmetric synthesis.13

Cycloheptadiene Series.

The treatment of (2) with Fe2(CO)9 in acetone at 35 °C gives a 1:4 mixture in favor of (4). Enriched samples of (3) or (4) are accessible directly (eq 5), according to the reaction conditions.14

Cycloheptatrienone (tropone) reacts efficiently in 60% yield with Fe2(CO)9 to give a complex (5) in which one double bond remains uncomplexed (eq 6).15 This double bond displays normal reactivity, while the remaining unsaturation is effectively protected. Dinuclear bis-h3-tricarbonyliron complexes can also be formed.16

Complexation of Natural Products.

The Fe(CO)3 moiety can provide a method for diene protection. Steroid interconversions employed tricarbonyl(ergosteryl acetate)iron(0)17 and tricarbonyl(ergosteryl benzoate)iron(0).18 Thus treatment of ergosteryl acetate or benzoate with tricarbonyl(4-methoxybenzylideneacetone)iron(0) (preformed by complexation using Fe2(CO)9, or with Fe2(CO)9 in the presence of 4-methoxybenzylideneacetone19), gives the pure desired steroid complex in good yield (60% and 80%, respectively). Nopadiene has been complexed by reaction with Fe2(CO)9 to form an optically active metal complex.20

Exocyclic Diene Complexation.

Formation of iron carbonyl complexes of (6) illustrates the mild nature of the complexation conditions (eq 7).21

Reaction with Fe2(CO)9 is exo face selective, giving a mixture of the corresponding mono complexes (7) (anti-exo) and (8) (syn-exo). Further complexation of (8) with Fe2(CO)9 occurs in a nonstereoselective fashion to give mixtures of the diiron complexes (anti-exo, syn-endo), (anti-exo, syn-exo), and (anti-endo, syn-exo).

On treatment of (9) with Fe2(CO)9, the endocyclic double bond is first coordinated to yield the exo-Fe(CO)4 complex (10), which then further reacts with Fe2(CO)9 to afford the diiron complex as a major product (eq 8).

Heating promotes deoxygenation to form the substituted tricarbonyl(o-quinodimethide)iron complex (11). Oxidative removal of the Fe(CO)3 moiety in (11) affords the indanone derivative (12) by CO insertion (eq 9).22

Complexes of 7-azanorbornadienes have been examined as sources of nitrenes.23

Complexation of Acyclic Dienes.

Complexation of trans-4-methyl-2,4-pentadienol (13) with Fe2(CO)9 gives the dienol complex (14) as a yellow oil. The product is valuable as a precursor for the cation (15) by reaction with HPF6/Ac2O/Et2O (eq 10).24 Substituted dienes with the (Z) configuration at a methyl-bearing terminus complex less efficiently.25

Fe(CO)3-diene aldehydes have been formed using Fe2(CO)9 in work that culminated in an efficient asymmetric induction using chiral allylborane reagents to give access to metal complexes in high optical purity.26

1-Methoxybutadiene has been complexed photochemically using Fe2(CO)9, instead of the more conventional thermal conditions.27 Complexation of acyclic dienes with Fe2(CO)9 constituted the first step toward trimethylenemethane complexes.28 All details concerning the preparation and spectral properties of tricarbonyl(butadiene)iron complexes are gathered in recent reviews.29

Fe2(CO)9/Ultrasound Method of Complexation.

The reaction of Fe2(CO)9 with sensitive dienes can be promoted by ultrasound (eq 11). Syntheses of a variety of tricarbonyl(h4-diene)iron(0) complexes (16) use the convenient high-yielding sonication method.30

Fe2(CO)9 Used with a Tertiary Amine.

Aliphatic trialkylaminotetracarbonyliron(0) complexes result from direct reaction between R3N and Fe2(CO)9.31 Fair to good yields of (17) can be obtained (eq 12).

The complex (17) transfers Fe(CO)4 to dienes, monoenes (resulting in isomerization), and Fe(CO)5 (to form Fe2(CO)9 and Fe3(CO)12). The R3N unit can also be replaced by other amines and by phosphines. The amine/phosphine exchange was performed with Me3N.Fe(CO)4 at 60 °C in quantitative yield after 0.5 h. The amine/diene exchange reaction has been studied with 1,3-cyclohexadiene in hydrocarbon solution at 52 °C (eq 13).

Other Routes to h4 Complexes.

Alternative access to Fe(CO)3 complexes are afforded by a variety of more unusual starting materials: cyclopropenes, dibromides, dihydrothiophene dioxides, alkynes/CO, allenes, and cyclohexadienones are all discussed below.

Enone Complexation.

Various a,b-unsaturated ketones form moderately stable tricarbonyliron complexes (18),32,37 and offer access to 1,4-diketones (19) by reaction with Grignard reagents, organolithium reagents (eq 14), or organocuprates.33 Trimethylsilyloxybutadienes require phenyl substituents to form stable complexes by reaction with Fe2(CO)9.34 Enone complexes are also useful as transfer reagents to place Fe(CO)3 on diene ligands.32,35 A related procedure employs complexes of a,b-unsaturated imines in the same way.36

Reagents for asymmetric transfer of Fe(CO)3 to dienes have been obtained thermally from (+)-pulegone37,38 and (-)-3b-acetyloxypregna-5,16-dien-20-one38 by reaction with Fe2(CO)9. The chiral enone complexes are used without isolation to provide a direct synthesis of optically active (diene)Fe(CO)3 complexes in up to 40% ee.38,39

Complexes from Alkenediols.

2-Butene-1,4-diols react with Fe2(CO)9 in the presence of Lewis acids under ultrasound conditions to form h3-allyliron complexes.40

Ferralactone Complexes.

From Oxazines.

The first ferralactone complex (21) was obtained during studies on p-allyliron complexes by Heck. The same compounds are also obtained by treatment of oxazines (20) with Fe2(CO)9 (eq 15).41

Formation of b- and a-Lactones from Vinyloxiranes.

An oxidative addition of vinyloxiranes (22) to iron(0) complexes provides the most usual synthesis of ferralactone complexes (21) (eq 16).42,43

Efficient preparations are possible using Fe2(CO)9 according to two different methods (in THF, or in lower polarity solvents with ultrasound) to produce the ferralactone complexes in moderate to excellent yields.44 Removal of the metal to form d-lactones has been successfully applied to total syntheses of parasorbic acid (23), the carpenter bee pheromone (24), and malyngolide (25).45 Epoxide-derived ferralactone complexes have provided key intermediates in syntheses of routiennocin,46 avermectin B1a,47 and (-)-valilactone.48

Demetalation of ferralactone complexes with Cerium(IV) Ammonium Nitrate produces b-lactones (at low temperature) and/or d-lactones (at high temperature, under a high pressure of carbon monoxide).

Formation of b-Lactams.

The synthesis and oxidative demetallation of ferralactam complexes affords a route to b-lactams.42 The difficulty of preparation of the monoaziridines limits the scope of direct ring opening, but ferralactam complexes are more readily obtained from ferralactones by reaction with an amine and a mild Lewis acid catalyst.49 This methodology has been successfully applied to the synthesis of (+)-thienamycin (28),50 via the key intermediates (26) and (27).

Complexes from Electrophilic Cyclopropenes.

Methyl- and phenyl-substituted cyclopropenes have been shown to react with iron carbonyls by ring opening and carbonylation to yield tricarbonyl(h3,h1-allylcarbonyl)iron complexes (eq 17).51

Franck-Neumann, starting from the easily accessible electrophilic gem-dimethylcyclopropenes (29), could obtain the carbonyliron adducts (30) and (31) in excellent yields.52,53 The reaction of Fe2(CO)9 with the isomers of Feist's ester (32) leads first to the formation of the corresponding tetracarbonyliron alkene complexes (33) (eq 18).54 Reaction of naphthalenocyclopropene with Fe2(CO)9 has also been studied.55

The products from cyclopropene ring opening have the advantage of easy conversion into tricarbonyl(h4-diene)iron complexes (34), its regioisomer and (35), by thermal loss of carbon monoxide, or photochemically without CO evolution (eqs 19 and 20).52,56

Opening of Cyclopropane Rings.

Ring enlargement of compounds such as bicyclo[4.1.0]hept-2-ene (36) or its derivatives (+)-2-carene (37) or (+)-3-carene (38) by carbonylation results in the formation of bicycloheptenones or cycloheptadiene complexes, depending on the reaction conditions.57

Under mild reaction conditions, (+)-2-carene affords the optically active (h1,h3) complex (39) (eq 21).

Metal and Lewis acid induced carbonylative ring enlargement of chiral and prochiral cyclohexadienes can give access to bicyclo[3.2.1]octanes.58

Dehalogenation Reactions.

Formation of Quinonedimethide Complexes.

Formation of a stable quinonedimethide complex (40) can be achieved by dehalogenation.59 The product (40) decomposes to benzocyclobutene at 500 °C (eq 22).

In the bis(bromomethyl)naphthalene series, a s-bonded Fe(CO)4 product has been reported.60

Formation of Cyclobutadiene Complexes.

Cyclobutadiene can be prepared as the stable complex tricarbonyl(cyclobutadiene)iron(0) (41), by reaction of 3,4-dichlorocyclobutene with Fe2(CO)9 (eq 23).61

Dehalogenation of a,a-dihaloalkenes and Ketones.

The preparation (eq 24) of the iron carbonyl complex of trimethylenemethane (42)62 and some Fe2(CO)9-mediated rearrangements of dibromo ketones63 constitute the pioneering work in this area. Noyori has extended the dehalogenation reaction of a-halo ketones to a,a-dibromo ketones and a,a,a,a-tetrabromo ketones which can be dehalogenated with Fe(CO)5 or Fe2(CO)9.64

Iron-stabilized oxallyl cations (generated in situ (eq 25) from a,a-dibromo ketones and Fe2(CO)9) react with alkenes. Noyori used this [3 + 2] cycloaddition reaction to produce cyclopentanone or cyclopentanone derivatives, as illustrated by a single-step synthesis of (±)-a-cuparenone (43) (eq 26).65 The reaction of a,a-dibromo ketones with enamines and Fe2(CO)9 yields substituted cyclopentenones in 50-100% yield (eq 27), as illustrated by the reaction with the a-morpholinostyrene (44).66

The reaction with dienes can be carried out either in benzene at 60-80 °C using a,a-dibromo ketone, diene, and Fe2(CO)9 (1:large excess:1.2), in benzene at room temperature with irradiation (same ratio of reagents), or in benzene at 80-120 °C using the dibromide and tricarbonyl(h4-diene)iron(0) complex (1:1.3). Formation of (45) (eq 28) provides a typical example.67

Noyori et al. have reported a general synthesis of tropane alkaloids from a,a-dibromo ketones.68 Reaction of tetrabromoacetone, N-methoxycarbonylpyrrole, and Fe2(CO)9 (3:1:1.5 mol ratio) in benzene (50 °C, N2) produces two isomeric cycloadducts in a 2:1 mixture, which can be used in the preparation of the alcohol (46), a key intermediate in the synthesis of scopine and other tropane alkaloids.69 A more recent example gives access to the bicyclo[5.2.0]nonene skeleton.70

Reactions with Sulfur Compounds.

Desulfurization of Episulfides.

The reaction of cyclohexene episulfide with Fe3(CO)12, reported by King71 to produce cyclohexene, has been improved:72 2-butene episulfides, e.g. (47) (R1 = R2 = Me, R3 = R4 = H), yield alkenes by treatment with Fe2(CO)9 in refluxing benzene (eq 29). The reaction occurs with retention of the stereochemistry in yields superior to 80%.

Tetramethylallene episulfide, on the other hand, affords a thioallyl Fe(CO)3 complex.73

Reaction with Thioketones.

The reaction of thiobenzophenone derivatives (48) and similar compounds with Fe2(CO)9 has been studied by Alper.74 It has been shown to result in the formation of ortho-metalated complexes (49) in reasonable to high yields (eq 30). Treatment of the complexes (49) with Mercury(II) Acetate effects ortho-mercuration.75 Thioketene complexes have also been examined.76 Complexes of type (49) offer a new route to lactones.74

Reaction with 2,5-Dihydrothiophene 1,1-Dioxide.

2,5-Dihydrothiophene 1,1-dioxides are known to be converted into 1,3-dienes after thermal displacement of sulfur dioxide.77 Reaction in situ with Fe2(CO)9 offers a general preparation of highly functionalized tricarbonyl(h4-buta-1,3-diene)iron(0) complexes (50) (eq 31).78

Reactions with Alkynes.

Fe2(CO)9 reacts with acetylene by carbonylation/cyclotrimerization leading to the formation of tricarbonyl(tropone)iron(0) (51) (eq 32). (For comparison with direct complexation of tropone, see eq 6.) This complex is obtained in low yield (28%) with Fe2(CO)9, but this represents a significant improvement on results obtained with Fe(CO)5.79

Hexyne is carbonylated by Fe2(CO)9. An intermediate can be isolated.80 Alkylthioalkynes are not desulfurized but form iron complexes.81

Reaction with Acid Chlorides.

The reaction of an acid chloride with one equivalent of Fe2(CO)9 affords symmetrical ketones.82

Formation of 1,2,4-Triazines.

1,2,4-Triazines, e.g. (54), are formed regioselectively (eq 33) by cocyclization of adiponitrile (52) and a nitrile (53) in the presence of Fe2(CO)9.83

Deprotection and Deoxygenation Reactions of Oximes and Isobenzofurans.

Oximes, oximic ethers, or oxime O-acetates can be converted into the corresponding ketones by reaction with Fe2(CO)9 in methanol at 60 °C.84 Deoxygenation of 7-oxabicyclo[2.2.1]dienes with Fe2(CO)9 affords aromatic products.85 This method is particularly recommended for deoxygenation of (55) in the formation of (56) (eq 34).86

Reaction with Electrophilic Allenes.

Formation of tricarbonyl(1,3-butadiene)iron(0) complexes from allenes (e.g. 57) is possible via tricarbonyl(trimethylenemethane)iron(0) intermediates (eq 35).87

Cyclohexadienone Reductions with Fe2(CO)9 and Water.

Tricarbonyl(cyclohexadienol)iron(0) complexes are formed by selective monohydrogenation of cyclohexadienones (58) by reaction with Fe2(CO)9 and water, under unusually mild conditions (eq 36).88

These reactions have been examined with a variety of substrates.88,89 Regio- and stereoselectivity depends on the pH of the reaction mixture.

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Sylvie Samson & G. Richard Stephenson

University of East Anglia, Norwich, UK

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