Octacarbonyldicobalt

Co2(CO)8

[10210-68-1]  · C8Co2O8  · Octacarbonyldicobalt  · (MW 341.94)

(catalyst for carbonylation and related reactions; major source of other synthetically useful organocobalt compounds)

Alternate Names: di-m-carbonylhexacarbonyldicobalt; cobalt carbonyl.

Physical Data: mp 51-52 °C (dec); subl 40 °C/vac.

Solubility: insol H2O; sol common org solvents.

Form Supplied in: dark orange crystals (commonly stabilized with 5-10% hexane).

Handling, Storage, and Precautions: toxic, moderately air sensitive, and unstable at ambient temperature, slowly releasing carbon monoxide (residual finely divided metal may become pyrophoric); may be stored for long periods at or below 0 °C in tightly closed vessels under inert gas (N2, Ar) or (best) CO; brief handling (including weighing) in air causes no problems; heating in inert solvents yields the less reactive Co4(CO)12; use in a fume hood.

Introduction.

In the crystal, the molecule of this reagent consists of two cobalt atoms linked both directly and by two bridging carbonyls and each carrying three terminal carbonyls (cf. eq 33).1 In solution this form is in equilibrium with unbridged (OC)4Co-Co(CO)4.

Halogen oxidation only leads to a halocarbonyl species in the case of iodine; the unstable and incompletely characterized product, probably ICo(CO)4, has found little use. The anion Co(CO)4-, readily formed by reduction (Sodium Amalgam) or by disproportionation on reaction with bases, including not only pyridine and other nitrogen bases but also such donor solvents as methanol and acetonitrile, is treated separately. (See Sodium Tetracarbonylcobaltate.) However, reactions which utilize Co2(CO)8 as starting material are included here even if the reaction conditions imply that this anion or its conjugate acid, HCo(CO)4 (also formed from Co2(CO)8 + H2), may be the active intermediate. The chemistry of the reagent has been reviewed.2,3

Catalytic Uses.

Hydroformylation and Hydrogenation.

Octacarbonyldicobalt catalyzes a wide range of reactions of carbon monoxide (alone or with hydrogen),2,4-8 of which the best known is probably the hydroformylation (oxo reaction) of alkenes. Depending on reaction conditions, notably CO:H2 ratio and temperature, the products are aldehydes (eq 1) or alcohols (hydroalkylation) (eq 2).

A valuable feature is the preference for straight chain over branched chain products; since HCo(CO)4 is an efficient alkene isomerization catalyst, this is true even for internal alkenes. Much research has been devoted to optimizing the product ratio for specific uses, e.g. by adding phosphines or other catalyst modifiers. While cobalt carbonyl (probably as HCo(CO)4) is an effective catalyst for these and many related reactions and has found extensive industrial use (albeit commonly generated in situ rather than introduced as such), it has largely been superseded by related rhodium catalysts; the greater cost of the latter is offset by much higher activity and hence efficient conversions using small catalyst concentrations under much milder reaction conditions. Alkyl halides and alcohols can replace alkenes in many such reaction.

Whereas, in general, reduction of the formyl group (cf. eqs 1 and 2) requires a higher temperature than hydroformylation, reduction of the C=C double bond can be a major side-reaction of cobalt-catalyzed hydroformylation. Thus, in the reaction of eugenol, which is accompanied by cyclization (eq 3), 40% of the starting material is reduced to dihydroeugenol.9

The Co2(CO)8-catalyzed alkene hydrogenation can be useful, as in the smooth reduction of alkylcinnamaldehydes (eq 4),10 the highly selective hydrogenation of methyl (E,E)-octadeca-9,11-dienoate to methyl elaidate (eq 5),11 and of a range of polycyclic aromatics,12 e.g. naphthacene (eq 6).

Dehydration or loss of alcohol accompanies hydroalkylation (eq 2) when applied to allylic alcohols (giving tetrahydrofurans rather than 1,4-diols) or to acrylic acids or esters, which give butyrolactones (eq 7) in moderate to excellent yields.13

In more or less related reactions, Co2(CO)8 is a useful catalyst in carboxylation (viz. alkoxycarbonylation) and a wide range of carbonylation reactions. Both catalysis of the water-gas shift reaction14 and of Fischer-Tropsch processes15 have been demonstrated on solid supports and probably involve Co2(CO)8-derived polynuclear carbonyls.16

Carboxylation and Related Reactions.

Although nickel, palladium, and other catalysts have been widely studied for carboxylation reactions and are clearly preferable in the case of alkynes, octacarbonyldicobalt (especially in combination with Pyridine) is an effective catalyst for the conversion of alkenes into acids and esters (eq 8) and differs significantly in selectivity from the nickel-based systems. Thus, whereas styrene gives predominantly the branched esters with both types of catalyst, terminal n-alkenes give predominantly branched esters with nickel catalysts, but straight-chain esters with Co2(CO)8, as in eq 9.4 The related hydrocyanation reaction, on the other hand, gives branched-chain products and, except with the most reactive alkenes, poor yields17 when using Co2(CO)8 and better nickel-based catalysts are available.

The condensation of two alkene molecules with Carbon Monoxide to yield ketones [e.g. ethylene -> 3-pentanone (95%)]4 succeeds well only in special cases, e.g. with methyl acrylate (eq 10).18

Octacarbonyldicobalt-catalyzed homologation of alcohols (eq 11) is difficult to control and of little practical value, while the related carboxylation of methanol to acetic acid requires such extreme conditions that it cannot compete with the technically important rhodium-catalyzed process.

Relatively mild conditions suffice for the regioselective carbonylation (eq 12)19 and carboxylation of epoxides (eq 13).20 The rather poor yields quoted in these examples have been attributed to extensive side-reactions, but specific cases giving excellent results are also known, e.g. with a carbohydrate epoxide (eq 14).21

Carboxylations of halogen compounds include the technically useful conversion of chloroacetate to malonate (eq 15),22 and several patent reports of quantitative conversion of Benzyl Chloride to phenylacetic acid.23 The incorporation of two molecules of carbon monoxide to give a-keto-acids, as exemplified by the reaction of benzyl chloride (eq 16), is remarkably solvent-dependent24 and almost certainly involves reaction of the halide with Co(CO)4-.

A carboxylation which is followed by a Michael addition occurs with high overall efficiency when Acrylonitrile reacts in the presence of an alcohol and pyridine (eq 17).25

Carbonylation.

This term is used here to denote carbon monoxide insertions (with or without cyclization) which do not involve H2 or ROH.

Unlike epoxides, oxetanes react with carbon monoxide in the presence of Co2(CO)8 by ring expansion to g-butyrolactones, rather than ring opening; thietanes (eq 18) as well as azetidines (eqs 19 and 20) behave similarly. Mixtures of cobalt and ruthenium carbonyls (1:1) are reported to give better yields than either catalyst separately.26 The examples shown illustrate the strong dependence of regioselectivity on substitution pattern in the azetidine case.27

Co2(CO)8-catalyzed reactions of a variety of other nitrogen compounds, e.g. azo compounds (eq 21),28 imines (eq 22),29 and amides (eq 23),30 involve CO insertion into N-N and N-H bonds and cyclizations which probably proceed via cyclometalation.

The mechanism of the remarkable formation of furans31 from 3-diethylamino-1-propyne (or -butyne) (eq 24) (R = H or Me) is unknown, but it must involve CO insertion into a C-N bond.

The carbonylation of Diphenyl Diselenide (eq 25) illustrates the general behavior of diaryl diselenides and ditellurides,32 but proceeds more smoothly than most other examples studied.

Reversibility of carbonylation processes is illustrated by decarbonylation of phthalic anhydride by Co2(CO)8 under H2 to benzoic acid,33 and of Diphenylketene to tetraphenylethylene.34

Carbonylation of alkynes is discussed below in connection with the stoichiometric reactions of alkyne-cobalt compounds.

Amidocarbonylation.

The reaction of aldehydes with primary amides and carbon monoxide in the presence of cobalt carbonyl (eq 26) provides a valuable alternative to the Strecker reaction as a route to N-acyl-a-amino acids.8,34-36 Its convenience is enhanced by the possibility of generating the aldehyde in situ by isomerization of an allylic alcohol for which a cocatalyst [PdCl2(PPh3)2, Carbonylhydridotris(triphenylphosphine)rhodium(I), or Nonacarbonyldiiron] is advantageous,37 (e.g. eq 27), or by hydroformylation of an alkene (e.g. by the reactions of eq 28 which can be combined to give the N-acetylamino acid in 83% yield8,38), or of an alcohol (e.g. eq 29). In the last example39 the product is isolated (as the methyl ester after Diazomethane treatment) in 73% total yield as a mixture of threo and erythro isomers.

Other Octacarbonyldicobalt-Catalyzed Reactions.

A retro-Diels-Alder reaction catalyzed by Co2(CO)8 was observed in which a barrelene derivative loses a C2H2 fragment (eq 30).40 An analogous cleavage accompanies the cyclopentenone synthesis (see below) when norbornadiene reacts in certain solvents with alkynehexacarbonyldicobalts, as shown by the formation of dicarbonylcyclopentadienylcobalt,41 whereas a Diels-Alder addition catalyzed by a cobalt carbonyl species is involved when the same reaction is applied to cyclohexadiene.42

Efficient and appreciably regioselective ring-opening addition of Cyanotrimethylsilane to tetrahydrofurans is illustrated by the example shown (eq 31).43 The same reagent undergoes Co2(CO)8 (or Dicarbonyl(cyclopentadienyl)cobalt(I)) catalyzed addition to alkynes to give pyrrole derivatives (eq 32).44

Octacarbonyldicobalt shares with many other metal carbonyls and other organometallics the ability to catalyze the cyclotrimerization of alkynes45 and related condensations, e.g. of two alkyne and one nitrile molecule to give pyridines. Derived compounds including Hg[Co(CO)4]2, Co(CO)3NO (obtained from Co2(CO)8 with NO or other nitrosating agents) and the alkyne complexes (RC2R)Co2(CO)6 (see below) all share this activity, but none of these compare in efficiency with the preferred CpCoL2 catalysts.

The few examples of catalytic cyclopentenone formation from alkyne, alkene, and CO are discussed below, as is the bifurylidenedione synthesis from alkyne and CO. It should also be noted that Co4(CO)12, obtained by heating the dinuclear carbonyl, can probably replace the latter in most of the above catalytic reactions, but without obvious advantage.

Stoichiometric Reactions.

Reactions with Dienes.

Conjugated dienes and norbornadiene displace two terminal carbonyl groups from first one and then both metal atoms of the octacarbonyl (eq 33).46

Either of these products can be oxidized to [(diene)Co(CO)3]+ salts,47 whose synthetic potential arises from their reactivity towards nucleophiles.47b,48 Whether 1- or 2-substituted butadiene ligands react highly regioselectively at the 4-position (e.g. eq 34),47b or at both C-1 and C-4, depends both on the substituent and on the nucleophile, but attack at C-2 has only been found in the reaction of the butadiene complex with Lithium Diisopropylamide.48b

Greater synthetic interest attaches to the cycloaddition of dienolates of b-diketones or b-keto esters, illustrated for the cyclohexadiene complex (eq 35), which leads, after Tetra-n-butylammonium Fluoride treatment of an intermediate (not isolated), to dihydrofuran derivatives.

Cyclopentadiene does not yield an isolable diene complex when reacting with Co2(CO)8, yielding instead Dicarbonyl(cyclopentadienyl)cobalt(I).

Reactions with Alkynes.

In contrast to dienes (see above), alkynes replace the two bridging carbonyls of Co2(CO)8 producing stable, readily isolated complexes in reactions which are normally rapid at room temperature (eq 36).49 The alkyne moiety lies at right angles to the Co-Co bond, so that the alkyne carbons form a tetrahedron with the two cobalt atoms. If the alkyne is terminal (R2 = H), these products (hereafter drawn in the simplified form of eq 37) react with acids (e.g. Sulfuric Acid in MeOH) to give the trinuclear compounds RCH2C[Co3(CO)9],50 which also form directly from the alkyne with Co2(CO)8 at 85 °C.49b (For further elaboration of such trinuclear complexes see Sodium Tetracarbonylcobaltate, from which a wider range of such compounds is most conveniently prepared.)

Reactions of the (RC2H)Co2(CO)6 complexes with carbon monoxide under pressure give lactone complexes51a,b (reducible to g-butyrolactones51c) by insertion of two CO groups into the Co-C bonds (eq 37); the further stepwise insertion of CO and alkyne provides the mechanism for the efficient Co2(CO)8-catalyzed synthesis of 2,2-bifurylidene-5,5-diones (eq 38).52 Under relatively mild conditions and with a 1:1 ratio of C2H2:CO, the extended bifurandiones of eq 39 also become significant products.53

The lactone complexes react with tertiary propynylamines at room temperature in moderate to good yield to give unsaturated aminolactones (eq 40),54 while bifurandiones incorporating the propynyl group and a solvent-derived hydrogen atom result when quaternary propynylammonium salts are employed under similar conditions (eq 41).54

Without CO pressure, the monoalkyne complexes react on warming with an excess of alkyne to incorporate two more molecules of the latter, giving flyover complexes (eq 42).55 These cyclize on oxidation (by Br2) or on heating (typically to 150-170 °C, but in refluxing toluene in the case of the flyover complex from 3 moles of PhC2COMe55b), yielding benzene derivatives. This reaction sequence represents a significantly different mechanism for alkyne cyclotrimerization under Co2(CO)8 catalysis compared, for example, to the dicarbonylcyclopentadienylcobalt-catalyzed process. The possibility of employing two different alkynes (eq 42) should permit synthesis of a wider range of benzene derivatives and, indeed, the use of alkynes with bulky R2 groups (e.g. R2 = t-Bu) provides a route to benzene derivatives with two such groups in adjacent positions (e.g. 1,2-di-t-butyl- and 1,2,4,5-tetra-t-butylbenzenes from the flyovers with R1 = H or t-Bu and R2 = t-Bu).56 However, since the initial complex, (R1C2H)Co2(CO)6, also undergoes alkyne exchange with the added alkyne, yields are severely limited and product mixtures are inevitably formed.56

Intermediates of the reaction of eq 42 are not normally observed, but some examples of complexes formed from octacarbonyldicobalt and two alkyne moieties are known, e.g. from cyclooctyne, giving a product which promotes the cyclotrimerization of cyclooctyne (eq 43),57 and also from MeN(CMe2C2H)2.58 The product of the latter reaction has been treated with phenylacetylene,58 forming an arene system (eq 44). Thus, both of these examples show that this type of bis(alkyne)cobalt complex can also be on the cyclotrimerization pathway.

Alkyne Protection, Distortion, and Altered Substituent Reactivity.

The hexacarbonyldicobalt moiety of the bridged alkyne complexes (R1C2R2)Co2(CO)6 can serve as a valuable protecting group which allows one to perform on other parts of the organic ligand reactions which would not be possible with the free alkyne. At the same time, complexation reduces the bond order from triple to approximately that of a double bond, with corresponding change in bond angles. Hence substituents are in much more favorable locations for many cyclization reactions. Some of the most elegant synthetic applications use both of these features in combination with the a-cation stabilization discussed in the next subsection.

The first demonstration of the stabilizing effect59 involved Friedel-Crafts acetylation of the tolane complex to give regioselectively the 4-acetyl and 4,4-diacetyl derivatives and the smooth liberation of the free alkynes by cerium(IV) oxidation. Open-chain enyne complexes (even with the double and triple bonds conjugated) were shown60 to undergo smooth acid-catalyzed hydration and hydroboration/Hydrogen Peroxide oxidation (e.g. eq 45), as well as reduction of the double bond (by N2H2) and again oxidative decomplexation, while the 5,6-double bond of complexed D5-17-ethynylandrostene-3,17-diol was reduced by hydroboration followed by acetic acid treatment.60 The protective effect has also been utilized to prevent alkyne-to-allene isomerization during oxidation of an allylic alcohol function.61

The effect of distortion is clearly revealed in the macrocyclization of the but-2-yne-1,5-diol complex with diphenyldichlorosilane (eq 46),62 by elegant syntheses of cycloene-diynes related to calicheamicin (e.g. eq 47),63 10- and 11-membered ring lactones (e.g. eq 48),64 but most strikingly in the construction of the cyclooctadecanonayne system by oxidative coupling of 1,3,5-hexatriyne, complexed to phosphine-substituted cobalt carbonyl (eq 49).65

Steric effects of the hexacarbonyldicobalt moiety may be responsible for altered reactivity, notably enhanced stereoselectivity in reactions of adjacent substituents, e.g. formyl groups in aldol condensations.66 Thus, condensation of trimethylsilylpropynal with the silyl enol ether of cyclopentanone gives a 90% yield of the aldol product (eq 50) as a 40:60 erythro:threo mixture, whereas reaction of the Co2(CO)6-complexed aldehyde followed by cerium(IV) oxidation gives the same total yield, but in an 87:13 diastereomeric ratio.66a

Cobalt-Complexed Propargyl Cations and Their Reactions.

The stability and synthetic utility of hexacarbonyldicobalt complexed cations was first reported in 1971.60,67 They are readily formed by protonation of propargyl alcohol complexes or addition of electrophiles (protons, or alkyl or acyl cations) to vinylacetylene complexes. They can be isolated in pure form, usually by precipitation as hexafluorophosphates or tetrafluoroborates, but are more commonly generated in situ and used directly. Most of the early work is due to Nicholas, who has comprehensively reviewed the work up to 1986.68 A detailed description of the procedure for generating the 1-methyl-2-propynyl complex and for its reaction with trimethylsilyloxycyclohexene (eq 51) has been given.69

More recent studies of the utility of the complexed cations in synthesis have paid particular attention to stereoselectivity in their reaction with silyl enolates70 and enol borates,71 e.g. eq 52; in this case the same products result, but with only 2.5:1 stereoselection when using the triethylsilyl enol ether and Boron Trifluoride Etherate catalysis.

Replacement of one carbonyl group by phosphine (thus creating chirality at cobalt) has been shown to permit enantioselective addition of nucleophiles to the cations.72 Regioselectivity of aromatic substitution by the cations has been studied in resorcinol derivatives73 and examples of aromatic substitution extended to include indoles.74

An alternative approach to the antitumor active enediyne systems (cf. eq 47) using Nicholas cations has been extensively studied75 and applied to the dynemycin core structure (eq 53).75c Another example of the use of such cations in forming strained rings is the preparation of a cobalt complex of a trithia-crown ether (eq 54), which has been shown to complex CuI and AgI.76

Cyclopentenone Synthesis.

Synthesis of cyclopentenones from alkynes + alkenes + CO has become a widely used process and is variously known as the Khand reaction or the Pauson-Khand reaction. Although inter alia examples promoted by Pentacarbonyliron, Hexacarbonylmolybdenum, tetracarbonylbis(cyclopentadienyl)ditungsten or -molybdenum, and carbene(pentacarbonyl)chromium have been described, octacarbonyldicobalt is the generally used reactant. The method has been extensively reviewed.52a,77

The general reaction (eq 55) is most commonly conducted by preparing the intermediate alkyne complex (R1C2R2)Co2(CO)6, with or without isolation, before adding the alkene component, but this is not necessary. Under a CO atmosphere, regeneration of this complex from added free alkyne occurs and in at least one case78 efficient catalytic synthesis has been achieved (eq 56). Under conditions of complete conversion of 1-heptyne, the product was obtained in 47-49% yield and contained only 1-2% of the regioisomer, 3-pentylcyclopent-2-en-1-one.

Such high regioselectivity is typical of terminal alkynes, and the less reactive internal alkynes show only slightly lower selectivity in forming the products with the bulkier group (R1) in the 2-position. Only strong electronic effects may override this sterically determined preference (e.g. see eq 63 below). Regioselectivity with respect to the alkene is much more variable. Simple terminal alkenes show little or no selectivity when the alkyne is also terminal, but significant preference for 5-substitution (R3 in eq 55) when the alkyne is internal. More efficient regioselection may be achieved with functional substituents, notably electron-donor groups in the homoallylic position, which favor formation of the 5-substituted product.79 Thus, the example shown in eq 57 is the key step in a formal synthesis of PGA2;80 it also illustrates the good yields obtainable by the simple thermal reaction in the case of alkenes with donor substituents.

Three more general techniques have greatly improved the earlier yields. Discussion of the solid-phase adsorption method is included in more recent reviews.77a-d It probably remains the method of choice in selected cases and sometimes has results different from those of other methods. For example, the predominant reduction to cyclopentanone in the intramolecular reaction of N-acetyl-N-allylpropargylamine (eq 58)81 and the reductive ether cleavage when related ethers react on alumina (eq 59),82 is avoided on silica, in the presence of oxygen, with formation of the expected bicyclic products.

This solid-phase technique has made possible efficient syntheses of bi- and polycyclic systems from precursors frequently generated via Nicholas cations, (e.g. eq 60)83 and has been used for intermolecular reactions with methylenecyclopropane (eq 61) and methylenecyclobutane to give spirocyclic cyclopentenone derivatives. (1) is the major product when the alkyne is terminal (R2 = H), but only (2) was isolated when EtC2Et or Me3SiC2Me were used.84

The more recent and now widely used improvement involves the use of amine oxides, usually Trimethylamine N-Oxide85a or N-Methylmorpholine N-Oxide.86 Alternatively, while requiring slightly higher temperature and longer reaction time, DMSO (or other sulfoxides) can give equally good yields. A range of other polar solvents (e.g. MeOH, MeCN, HCO2Et) also exert a distinct, but smaller, promoting effect.87

N-Methylmorpholine N-oxide, preferably as monohydrate,79b as well as Me3NO not only induces cyclopentenone formation under very mild conditions (0-20 °C) and in frequently excellent yields, but also removes some earlier limitations. The reductive processes (eqs 58 and 59) do not occur when these promoters or DMSO are used under oxygen.85,87 Hence two efficient syntheses of (-)-kainic acid88 have employed cyclizations of allylpropargylamine derivatives as the key step; in one of these (eq 62), as well as in a synthesis of the dendrobine skeleton,89 use of a chiral precursor was shown to result in enantiospecific bicyclization.88a

Intramolecular cyclizations of enynes using catalytic amounts of octacarbonyldicobalt (3 mol %) succeed in the presence of biphenyl phosphite (10 mol %) undr 3 atm of CO, typically in DMF at 120 °C.85b

Whereas in earlier work the (uncatalyzed) reaction of alkenes bearing electron-withdrawing groups with the alkynecobalt complexes gave conjugated dienes as the only or major products,77 intramolecular cyclizations involving such groups have now been accomplished by both the solid-state method90 and using N-oxide promotion.91 Moreover, electron-deficient alkynes have been successfully employed in both inter- and intramolecular cases. The use of ethyl 2-butynoate (eq 63) illustrates the preferred orientation in this case.92

Further examples of the efficiency of the N-oxide method are the formation of bicyclic lactols (e.g. eq 64), utilized in a synthesis of (±)-loganin,93 and the quantitative cyclization (eq 65) which is the key step in a synthesis of decarboxydemethylquadrone.94

Finally, reference is made to the remarkably facile cyclization which alkynyl Fischer carbene complexes can undergo95 under conditions normally leading only to alkyne-cobalt complex formation (eq 66) (M = Cr or W; R = Ph or Et).

Related Reagents.

Dicarbonyl(cyclopentadienyl)cobalt(I); Octacarbonyldicobalt-Diethyl(methyl)silane-Carbon Monoxide; Sodium Tetracarbonylcobaltate.


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Peter L. Pauson

University of Strathclyde, Glasgow, UK



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