Alkyne Cobalt Complexes

Scheme 27 RCM of substrates bearing alkyne cobalt complexes [125c]...

In organometallic systems in particular, clusters with bridging alkylthiolates are well known, as exemplified by Co2(Cp)2(/i-SCH3)2, which undergoes reactions with alkyne-cobalt complexes to yield trinuclear clusters.170... 

The reaction using 11a as a substrate in the presence of several oxides as additives revealed that addition of tributylphosphine oxide, hexamethylphos-phoric triamide, and dimethyl sulfoxide all accelerate the reaction considerably. Furthermore, when about 10 molar amounts of N-methylmorpholine M-oxide (NMO) is added to the alkyne-cobalt complex 12b in THF,the reaction proceeds even at room temperature and cyclopentenone 13 b is obtained in 37% yield accompanied by another rearranged product, the methylenecyclobutanone 35, obtained in 23% yield as a mixture of ( )-and (Z)-isomers (Scheme 14). These facts indicate that dissociation of the carbonyl ligand of the alkyne-cobalt complex 12 is the rate-determining step in this rearrangement. This is also supported by the fact that under a CO atmosphere in refluxing THF the reaction is completely suppressed. 

In spite of its simplicity and conceptual clearances, the original protocol has suffered from many intrinsic problems in a practical sense. For example, the reaction with the alkyne-cobalt complexes provided low chemical yields and required harsh reaction conditions. In addition, it was also difficult to extract the obtained product from the sticky metallic residue. Those problems can be accounted for based on the widely accepted mechanism given in Scheme 1, which, as proposed by Magnus," is supported by many theoretical studies. ... 

Among various candidates, 1,2-dimethoxyethane (DME) in toluene was found to be the best promoter providing the cycloaddition products in high yields, but required a higher pressure of CO (7 atm). Water was less efficient under the same conditions, but provided the comparable yield at higher concentrations. Nevertheless, use of DME as the solvent instead is detrimental to the reaction under the conditions. Once again, one should notice that there must be a competition between the demetallation from the alkyne-cobalt complex and the catalytic cycle for the PKR products. 

Alkynes too have found application in the synthesis of oxepanes. The A/B ring system (45) of ciguatoxin has been formed in a stereoselective manner by cyclisation of the alkyne cobalt complex (44) with boron trifluoride followed by reductive removal of the chromium with... 

The alkyne cobalt complex is purified by silica gel chromatography. To the red solution is added 2 g of silica gel and the mixture is concentrated on a rotary evaporator (Note 13). The silica gel is then placed under vacuum (1 mm) until effervescence ceases (about 10 min). The dried silica gel is placed at the top of a column of silica gel (diameter 3 cm height 9 cm), eluting first with hexane to remove nonpolar by-products and subsequently with 15% ether/hexane. The desired product elutes as a red band. Concentration of the appropriate fractions on a rotary evaporator (Note 13) and under high vacuum (0.7 mm for 10 min) affords 1.4 g (79%) of 5 as a fine red powder (Note 14). 

Scheme 14 Removal of Propargyl Ester Protection via an Alkyne-Cobalt Complex...

The total synthesis of the sesquiterpene (+)-taylorione was achieved in the laboratory of J.G. Donkervoort who used the modified Pauson-Khand reaction to prepare the five-membered ring of the natural product. The preformed alkyne-cobalt complex was exposed to excess triethylamine-A/-oxide, which oxidized off two CO ligands to free up a coordination site for the ethylene. The optimum pressure of the ethylene gas had to be at 25 atm, and the reaction was conducted in an autoclave. 

Reactions of alkyne-cobalt complexes. The hexacarbonyldicobalt complex of a propargylic alcohol containing a juxtaposed benzyloxy group suffers elimination of the propargylic hydroxyl on treatment with BFj OEtj. An intramolecular hydride transfer is involved. Complexes of vic-diols give deconjugated alkynones. ... 

Upon heating, loss of a third equivalent of carbon monoxide opens a coordination site to provide 1-2 and allows cobalt to complex the alkene as in 1-3. The alkene inserts into the least hindered cobalt-carbon bond to form the cobaltacycle 1-4. Carbon monoxide inserts into the new cobalt-carbon bond to give 1-5. Next, acyl migration from cobalt to carbon forms the final carbon-carbon bond as in 1-6. Finally, a formal reductive elimination of the spectator cobalt releases the product cyclopentenone P and cobalt-hexacarbonyl. The cobalt is then ready for another catalytic cycle. In practice the cobalt is most often used stoichiometrically. The alkyne cobalt complex is a stable species that can be purified and isolated by silica gel chromatography. Conditions for catalytic turnover of the cobalt have been reported. 

There are few examples where the absolute stereochemistry is determined in the PKR. Verdaguer has demonstrated the use of a bidentate phosphorous-sulphur ligand to provide an enantiopure cyclopentenone in for the synthesis of the HIV drug (-)-abacavir. The achiral alkyne cobalt complex is generated first in 1 h at room temperature. Heating with enantiopure R,S-ligand 102 provides a separable mixture of diastereomeric... 

The chirality generated by SADH was used in a subtler way by Hayashi et al. [34f] in their synthesis of the complex antitumoral agent (-l-)-fostriecin (Scheme 2.14). Propane-1,3-diol was converted in a sequence of seven steps into 1,3-enyne 71, SADH of 71 gave rise to chiral diol 72 in good yield and enantiomeric excess. A sequence of four steps transformed 72 into the alkyne-cobalt complex schematically... 

Gibson et al. have used polymer-bound cobalt complexes for immobilizing alkynes [27]. Scheme 5 demonstrates the indirect loading approach, meaning that the alkyne-cobalt complex is formed prior to the attachment onto soUd phase. The alcohol moiety of 7a/7b, formed as a mixture, can then be subjected to a munber of transformations, and the alkyne is subsequently released from the polymer by aerial oxidation. The direct loading approach is also described, where the cobalt complex is formed on soUd phase before the alkyne complexation step. This approach gives a somewhat lower substrate loading, but the subsequent steps proceed with almost the same efficiency in both cases. 

Intermediates of the reaction of eq 42 are not normally observed, but some examples of complexes formed from octacar-bonyldicobalt and two alkyne moieties are known, e.g. from cy-clooctyne, giving a product which promotes the cyclotrimeriza-tion of cyclooctyne (eq 43), and also from MeN(CMe2C2H)2. The product of the latter reaction has been treated with phenylacetylene, 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. 

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

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