Dicobalt-alkyne complex

Transition-metal-promoted cycloaddition is of much interest as a powerful tool for synthesis of carbocyclic stmcture in a single step. Utilization of carbon monoxide as a component of the cycloaddition reaction is now widely known as the Pauson-Khand reaction, which results in cyclopentenone formation starting from an alkyne, an alkene, and carbon monoxide mediated by cobalt catalyst. Although mechanistic understanding is limited, a commonly accepted mechanism is shown in Scheme 4.16. Formation of dicobalt-alkyne complex followed by alkene... 

This review has already indicated numerous applications of dicobalt-alkyne complexes in organic synthesis. Like the Nicholas reaction (see Section II,D), the Pauson-Khand reaction has seen widespread use.175 This reaction is a three-component cycloaddition of alkynes with alkenes and carbon monoxide which occurs in the presence of octacarbonyldicobalt to afford cyclopentenones, as shown in Eq. (16). 

This compound was then allowed to react with COj(CO)g to form the dicobalt-alkyne complex. The Nicolas reaction was then used to form a fused 5,8-ring system. This reaction was followed by an intramolecular PKR to give the polycyclic product in high yield. 

Cobalt.- A number of publications are concerned with the preparation of dicobalt alkyne complexes of the type [Co2(CO)e(p-alkyne)]. These include reports on the selective introduction of the dicobalt hexacarbonyl unit into methotrexate , the preparation of derivatives of cortisol, testosterone and dihydrotestosterone for potential use as tracers in immunoassays , and the... 

Figure 7.1 A dicobalt-alkyne complex 7.2 (R = Ph). Reprinted with permission from Sly, W. C. et a/. J. Am. Chem. Soc. 1959, 81, 18. 1959 American Chemical Society.

Two S/P ligands derived from camphor, CamPHOS and MeCamPHOS were also developed by these authors for the diastereoselective coordination to alkyne-hexacarbonyldicobalt complexes (Scheme 10.68). These two ligands were converted in good yields into their borane-protected forms. The influence of the alkyne group (R) on their coordination to dicobalt-hexacarbonyl-alkyne complexes was evaluated. It was shown that MeCamPHOS ligand provided a... 

Although many metal-alkyne complexes are known, probably the most thoroughly studied are those in which the alkyne is coordinated with a dicobalt unit. When this area was reviewed in 1974, more than 170 examples... 

Only severely sterically hindered alkynes, such as [(OC)9Co3CC=CC Co3(CO)9],6 fail to react with octacarbonyldicobalt. This steric effect is also illustrated by the reactions of [Cp(OC)2FePPh2C=CR]+ (R = H, Me, Ph, p-tolyl, t-Bu) with octacarbonyldicobalt.7 All of the cationic alkyne complexes form hexacarbonylalkyne-dicobalt derivatives except when the substituent is the bulky t-Bu group. 

The chemistry and synthetic utility of cobalt complexed propargyl cations have been demonstrated by Nicholas in an impressive series of papers, and the area was reviewed in 1987.72 More recently, reviews of cluster-stabilized cations73 and propargylium complexes74 have appeared. Two general routes for the synthesis of dicobalt-propargylium complexes have been developed. The most commonly used method is the treatment of an alkynic ether or alkynic alcohol-hexacarbonyldicobalt complex with a Lewis or Bronsted acid [Eq. (7)]. 

Steric differences have been shown to affect the regio-chemistry of cyclization, as the CO inserts in the a-position of the larger alkynyl snbstitnent. However, theoretical stndies on dicobalt hexacarbonyl alkyne complexes have snggested that electronic differences in alkyne substitnents may also exert control over the regiochemistry of cyclizationd Calculations have also provided information on geometries and configurational stabilities of these complexes. 

Dicobalt hexacarbonyl-alkyne complexes are stable to a number of electrophihc reagents, and uncomplexed multiple... 

Cyclopropene can also be used as the aUcene component and affords bicyclo[3.1.0]hexen-2-ones upon reaction with alkyne dicobalt octacarbonyl complexes in the presence of NMO (Scheme 250). Vinyl ethers and vinyl esters serve as ethene equivalents in Pauson-Khand reactions. For example, reaction of vinyl benzoate with complex (169) furnished cyclopentenone (170) (Scheme 251). This reaction was used in a synthesis of (-l-)-taylorine and nortaylorine. Allenes participate in intermolecular Pauson-Khand reactions affording alkylidene-substituted cyclopentenones (Scheme 252). ... 

The total syntheses of (+)-secosyrins 1 and 2 was achieved and their relative and absolute stereochemistry was unambiguously established by C. Mukai and co-workers. To construct the spiro skeleton of these natural products, the intramolecular Nicholas reaction was utilized. The alkyne substrate was first converted to the dicobalt hexacarbonyl complex by treatment with Co2(CO)8 in ether. Exposure of the resulting complex to boron trifluoride etherate at room temperature brought about the ring closure with inversion of configuration at C5 to afford the expected tetrahydrofuran derivative. The minor product was the C5 epimer which was formed only in 15% yield. 

Verdaguer, X., Moyano, A., Pericas, M. A., Riera, A., Alvarez-Larena, A., Piniella, J.-F. Alkyne Dicobalt Carbonyl Complexes with Sulfide Ligands. Synthesis, Crystal Structure, and Dynamic Behavior. Organometallics 1999, 18, 4275-4285. 

Magnus, P. Carter, P. Elliott, J. Lewis, R. Harling, J. Pittema, T. Bauta, W. E. Fortt, S. Synthetic and mechanistic studies on the antitumor antibiotics esperamicin Al and calicheamicin yj- synthesis of 2-ketobicyclo[7.3.1] enediyne and 13-ketocyclo[7.3.1] enediyne cores mediated by 17-2 dicobalt hexacarbonyl alkyne complexes cycloaromatization rate studies. J. Am. Chem. Soc. 1992, 114, 2544-2559. 

Decomplexation. Dicobalt hexacarbonyl complexes of alkynes are efficiently decomposed at room temperature with NaSMe in DMF. 

When an alkyne reacts with [Co2(CO)g], the two C = C —C angles in the formed (n-acetylene)dicobalt hexacarbonyl complex are reduced to a value of =140° [26] therefore, dicobalt hexacarbonyl fragments have been used as protecting groups to allow geometrictilly disfavored cyclization reactions by bending an alkyne moiety [27]. Since the alkyne can be... 

The anti-proliferative effects of some dicobalt hexacarbonyl alkyne complexes derived from aspirin 21, from diphenyl acetylene 22, and from 2-propyn-l-ol have been studied on various cell lines, including... 

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