[12078-25-0]  · C7H5CoO2  · Dicarbonyl(cyclopentadienyl)cobalt(I)  · (MW 180.05)

(catalyst, especially for alkyne trimerization and cotrimerization;3-5 source of a wide range of cyclopentadienylcobalt complexes and of cyclopentadienones, pyridones,10 etc.)

Alternate Names: cyclopentadienyldicarbonylcobalt; cyclopentadienylcobalt dicarbonyl.

Physical Data: bp 139-140 °C/710 mmHg; 37-38.5 °C/2 mmHg.

Solubility: very sol all common org solvents; insol H2O.

Form Supplied in: dark red liquid; available commercially.

Preparative Methods: synthesized quantitatively from Octacarbonyldicobalt and Cyclopentadiene1 or from Bis(cyclopentadienyl)cobalt and Carbon Monoxide.2

Handling, Storage, and Precautions: somewhat air sensitive; toxic; best stored under CO or inert gas (N2, Ar) at low temperature (<0 °C). Use in a fume hood.

General Remarks.

Many of the reactions in which CpCo(CO)2 acts as catalyst involve dissociation of CO to leave the catalytically active species, regarded as CpCo. As this can readily arise from other CpCoL2 complexes (e.g. L = PR3, P(OR)3, C2H4; L2 = diene) or from Cp2Co, useful reactions of the phosphine complexes are included here and the entries for Bis(cyclopentadienyl)cobalt and (1,5-Cyclooctadiene)(cyclopentadienyl)cobalt(I), the preferred catalyst for pyridine synthesis, should also be consulted.

Catalytic Reactions.


Dicarbonylcyclopentadienylcobalt is the most widely used catalyst for alkyne cyclotrimerization and can effectively catalyze such related processes as pyridine synthesis from two alkyne molecules and one nitrile molecule, cyclohexadiene synthesis from two alkyne molecules and one alkene molecule, and participate in the (usually stoichiometric) formation of cyclopentadienones from two alkyne molecules and CO. It is known that the first step in these processes is the thermally or photochemically induced dissociation of CO and there is strong evidence for intermediate formation of bis(alkyne) complexes (eq 1) which can react with the third substrate molecule to give the above products.

The reaction in eq 1 must be assumed to proceed by coordination of one alkyne, followed by dissociation of the second CO. Although monoalkyne complexes CpCo(CO)(R1C2R2) have not been isolated from such systems, their analogs, CpCo(PPh3)(R1C2R2) are well known. They are accessible, inter alia, by the sequence shown in eqs 2 and 3,3,4 followed by eq 4.5

The products CpCo(PR3)(R1C2R2) also yield bis(alkyne) complexes CpCo(R1C2R2)2 on further treatment with alkyne.6 It is uncertain whether cyclization of these bis(alkyne) complexes to cobaltacyclopentadiene systems (eq 5) precedes or is induced by addition of the third ligand L. When L is a third alkyne this leads to the immediate precursor of the aromatic cyclotrimerization product; it is also the step which determines the regiochemistry of that product. The cobaltacyclopentadienes arising according to eq 5 have been well characterized for L = PPh3, but the compounds with L = alkyne, nitrile, alkene, or CO presumably collapse rapidly to the above-mentioned endproducts. In the cases where these are dienes (eqs 6 and 7), these can remain bound as stable complexes, but arenes and 2-substituted pyridines are too readily displaced by alkyne, regenerati ng the bis(alkyne) complexes CpCo(R1C2R2)2 and making the reactions catalytic. Only very exceptionally (eq 8) has an h4-arene complex been isolated.7

Cyclotrimerization, Including Mixed Cyclotrimerization of Alkynes.

Simple cyclotrimerizations can be achieved efficiently with a wide range of metal carbonyl8 and other complexes. With unsymmetrical alkynes, different catalysts may lead to widely different regioselection. Thus whereas CpCo(CO)2 converts diphenyl-1,3-butadiyne chiefly to the unsymmetrical trimer (eq 9), use of Dicarbonylbis(triphenylphosphine)nickel(0) gives exclusively the symmetrical isomer (83%).9

However, the special value of CpCo(CO)2 with respect to trimerization arises from its ability to promote controlled mixed trimerization reactions when one alkyne molecule carries bulky substituents, e.g. trimethylsilyl, and cannot undergo self-trimerization, apparently for steric reasons. It is then possible to employ that bulky alkyne in excess, e.g. in reactions with dialkynes, so as to suppress their trimerization (eq 10) in favor of the mixed condensation (eq 11). Surprisingly, when a polymer-supported form of the catalyst was substituted, only the reaction in eq 10 was observed (e.g. with n = 3). With CpCo(CO)2 itself, many high-yielding examples of reactions of the type in eq 11, have been studied and reviewed.10

Examples include the remarkably efficient formation of benzocyclobutenes from 1,2,4,5-tetraethynylbenzene (eq 12) and the reaction of an enediyne designed as a steroid synthon (eq 13); in the latter case the intermediate benzocyclobutene is not isolated but ring opens as shown to allow an intramolecular Diels-Alder reaction. The product was converted to (±)-estrone in 80% yield via selective protodesilylation at the 2-position with Trifluoroacetic Acid followed by replacement of the second silyl group by OH using Lead(IV) Trifluoroacetate. The alternative protodesilylation under basic conditions (using Potassium t-Butoxide) or replacement of SiMe3 by halogens adds to the versatility of such silyl-substituted products in synthesis. Approaches to other natural products are exemplified for protoberberine-type isoquinoline alkaloids (eq 14). This example also illustrates the highly regioselective formation of the more sterically demanding arene as a result of the initial coupling of the least hindered alkynic carbons.10

Pyridine Synthesis.

Dicarbonylcyclopentadienylcobalt-catalyzed reactions have been reviewed;10 see (1,5-Cyclooctadiene)(cyclopentadienyl)cobalt(I), the more frequently used catalyst, for a fuller discussion. As in cyclotrimerization (cf. e.g. eq 14), high regioselectivity is often observed; an example is shown in eq 15.10

Pyridones result from alkynes and alkyl isocyanates, but with low regioselectivity.10 This problem is avoided in condensations using o-alkynyl isocyanates (eq 16).11

Other Catalytic Reactions.

Dicarbonylcyclopentadienylcobalt and Octacarbonyldicobalt are alternative catalysts for pyrrole synthesis from alkynes and Cyanotrimethylsilane (eq 17).12 The known addition of Phenyl Isocyanide to the complex CpCo(PPh3)(PhC2Ph)2 (eq 18)13 may provide a model for this reaction, which may be initiated by isomerization of Me3SiCN to Me3SiNC and continue as in eq 19.

Stoichiometric Reactions.

Cyclohexadiene Formation.

As noted above, highly substituted cyclohexadienes can be obtained (eq 6) by treating the isolated cobaltacyclopentadiene complexes with a range of terminal alkenes or dimethyl maleate; subsequent oxidation using cerium(IV) salts liberates the free dienes.14,15 The complex formed from MeN(CMe2C2Me)2 and CpCo(PPh3)2 has similarly been added to diethyl fumarate.16 Cyclohexadiene complexes are obtained more commonly and conveniently, without isolation of cobaltacyclopentadienes. Thus tolan or dimethyl acetylenedicarboxylate reacts with norbornene and CpCo(CO)2 according to eq 20 to yield mixtures of two stereoisomeric complexes.17

Alkynes or diynes have been added in similar fashion to uracil derivatives (eqs 21 and 22).18

Even the 2,3-bond of N-acylindoles can function as the ene component in such condensations (eq 23).19 The free tetracyclic ligand was liberated from the resultant complex in excellent yield by Copper(II) Chloride or Iron(III) Chloride oxidation and the aromatized ligand was obtained when CeIV was used. Cyclization of enediynes10 (eq 24) has been followed by liberation of the organic ligands using oxidation with CuCl2/Et3N and by other transformations.20 An efficient approach to steriods by enediyne cyclization is shown in eq 25.21 The free organic ligand is liberated from the resultant cobalt complex in 81% yield by oxidation.

The sequence of steps for cyclohexadiene formation can be altered as in eq 26 followed by eq 27.22,23 Again CuCl2 proves to be an efficient oxidizing agent to liberate the free cyclohexadiene with only a small amount of aromatization, whereas the use of cerium(IV) leads to extensive dehydrogenation.23 Loss of PPh3 from the cobaltacyclopentene intermediate (eq 27) is assisted in this case by coordination of an ester carbonyl. The same intermediates also add carbon monoxide or isocyanides, yielding cyclopentenones or aminocyclopentadienes, respectively (eq 28).23

Pyran Synthesis.

Under photochemical conditions, ketones (or aldehydes) can take the place of alkenes and add to diynes to yield pyrans. Examples include the addition of acetone to 1,7-octadiyne (eq 29) with the more reactive maleic anhydride substituted catalyst and the tricyclization of a diynone (eq 30). In the latter case a ring-opened product accompanies the pyran.24 In other examples an ynone is condensed with a silylalkyne (eq 31).24

Formation of Cyclopentadienone Complexes and Competing Reactions.

(Cyclopentadienone)cyclopentadienylcobalt derivatives are major products of the reaction of CpCo(CO)2 with internal alkynes and terminal alkynes, RC2H, which have bulky groups, R. Irradiation was used to obtain the earliest examples (eq 32, R = Me or Ph)25 and remains the method of choice in most cases, but thermal activation has also been widely used. Three other types of product are frequently produced under thermal conditions: (cyclobutadiene)cyclopentadienylcobalt, cyclopentadienyl(p-quinone)cobalt, and (cyclopentadienyl)maleylcobalt derivatives. Formation of the cyclobutadiene complexes has been reviewed.26 CO-free precursors, e.g. (1,5-Cyclooctadiene)(cyclopentadienyl)cobalt(I), are preferable for this purpose since they cannot yield the other products, but CpCo(CO)2 can also give excellent yields as in the examples shown in eqs 33 and 34.27,28 The latter example contrasts with the low-temperature photochemical reaction of the same precursors (eq 35).29 Liberation of the free dienone from the product by CeIV oxidation as well as partial or complete displacement of SiMe3 by Br or H30 emphasizes the synthetic utility of such reactions.

The reaction of 2-butyne under thermal conditions (eq 36)31 shows that quinone complexes are formed more quickly than cyclopentadienone complexes and perhaps decarbonylate to the latter under the reaction conditions. Similarly at 100 °C, 3-hexyne yields the tetraethylquinone complex but at 110-130 °C it yields the tetraethylcyclopentadienone complex as the major product.32 In a series of alkynes, MeC2R, both the electronegativity and size of R affects the product ratios.33 The maleyl complexes appear only as minor products (eq 36) and hence cannot be considered to be synthetically significant.

Tolerance of highly polar substituents is illustrated by the reaction of di-t-butoxyacetylene (eq 37).34 The ionic catalyst, [K(crown)][CpCo(CO)CN], obtained from [K(crown)]CN and CpCo(CO)2, has been used35 in the reaction with tolan, but although the conditions were fairly mild (reflux in butyronitrile), the modest yield (43%) of the tetraphenylcyclopentadienone complex does not encourage use of this procedure.

In contrast to its efficient incorporation into cyclobutabenzenes (see above), Me3SiC2(CH2)2C2SiMe3 gives a very complex mixture of (largely polynuclear) cyclobutadiene and cyclopentadienone complexes in the absence of added alkyne.28 Presumably the monomeric cyclobutenocyclopentadienone is too strained, whereas a derivative with four methyl substituents on the cyclobutene ring is formed as the sole product according to eq 38.36

When the terminal alkyne Me3SiC2H is used, the 2,5-disubstituted cyclopentadienone complex is the major product (eq 39)29 (a 76% yield is reported37 under only marginally different conditions) with CpCo(CO)2, but the corresponding 2,4-disubstituted compound predominates in the product from (C5Me5)Co(CO)2.29

In a reaction closely analogous to the CO insertion, cobaltacyclopentadienes react with isocyanides to give iminocyclopentadiene complexes, e.g. according to eq 40.38

Other Products from Cobaltacyclopentadienes.

In addition to the above alkene and CO insertions, isolated cobaltacyclopentadienes have been shown to react with diazo compounds to yield cyclopentadiene complexes and hence substituted cyclopentadienes (e.g. eq 41),39 and similarly with azides to yield pyrroles (eq 42).40 Acid azides, RCON3, react similarly to give the corresponding N-acylpyrroles while tosyl azide reacts according to eq 43.40

Reaction with sulfur yields thiophenes (eq 44).41 Replacement of triphenylphosphine by phosphites also results in ring formation, leading to cyclic 1-alkoxyphosphole oxide complexes (as mixtures of two stereoisomers) with loss of alkyl groups (eq 45).42

Oxidation of the tetraphenyl- or dimethyldiphenylcobaltacyclopentadiene complex with ground-state oxygen at 70 °C or singlet oxygen at room temperature yields dibenzoyldiphenylethylene or 2,3-dibenzoyl-2-butene, respectively, via the corresponding CpCo complex (eq 46).43

Insertion Reactions.

Cyclobutenediones react smoothly when heated with CpCo(CO)2 to yield cobaltacyclopentenediones (eqs 47 and 48). Both the cyclopentadienyl complex and its pentamethyl derivative have been used with benzocyclobutenedione.44 As shown in eq 48, despite the high temperature required the latter reaction proceeds quantitatively. The product complexes react photochemically with alkynes to give complexes of benzoquinones from which the free quinone may be liberated by cerium(IV) oxidation (eq 49).44

The related insertion into cyclobutenones has been effected using indenylbis(triphenylphosphine)cobalt (eq 50), initially yielding ketene complexes which add alkynes to give phenols.45

Another four-membered ring is expanded when CpCo(CO)2 adds to a disilacyclobutene, e.g. in the reaction sequence shown in eq 51. The initially formed complex reacts photochemically with butadiene or its derivatives, at low temperature merely replacing the CO ligand, but at 60 °C giving disilacyclooctadienes.46

Miscellaneous Reactions.

Azoarenes react with CpCo(CO)2 as exemplified for azobenzene (eq 52). The complexes formed may be carbonylated to yield N-arylbenzimidazolones (eq 53).47 Most diazoalkanes react according to eq 54 yielding dinuclear, methylene-bridged complexes,48 but diethyl diazomalonate yields a cobaltafuranone complex (eq 55).49

The products of the reaction in eq 55 and other cobaltacycles, formed inter alia from alkyne complexes (eq 56)50 or from the isocyanide-phosphine complexes, C5H5Co(PMe3)(RNC),51 have not been utilized in further reactions which would constitute synthetically useful transformations of the organic fragments.

Dicarbonylcyclopentadienylcobalt is the source (via reaction with 3,4-dichlorocyclobutene or with photopyrone) of the cyclobutadiene complex (C5H5)Co(C4H4) whose substitution reactions provide potential routes to many cyclobutadiene derivatives; however, it is not known to have any advantages in this respect over the more commonly used Tricarbonyl(cyclobutadiene)iron.26

Dicarbonylcyclopentadienylcobalt is reduced by Sodium metal to the salt of a dinuclear radical anion (eq 57).52 Alkyl halides convert this anion to dialkyl complexes (eq 58) which react with carbon monoxide, yielding ketones and regenerating CpCo(CO)2 (eq 59).53 Use of dihalides in this sequence leads to cyclic ketones; thus the complex obtained from 1,3-diiodopropane affords cyclobutanone (eq 60), but the same intermediate can be used to generate cyclopropane on treatment with iodine (eq 60).54 In the case of the product from dibromo-o-xylene, C6H4(CH2Br)2, the initial complex disproportionates at 22 °C and the product is carbonylated (eq 61), whereas direct carbonylation of the intermediate leads to dimeric hydrocarbon products (eq 62).55

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Peter L. Pauson

University of Strathclyde, Glasgow, UK

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