Hexacarbonyldicobalt-alkyne complex

Hexacarbonyldicobalt complexes of alkynes have served as substrates in a variety of olefin metathesis reactions. There are several reasons for complex-ing an alkyne functionality prior to the metathesis step [ 125] (a) the alkyne may chelate the ruthenium center, leading to inhibition of the catalytically active species [125d] (b) the alkyne may participate in the metathesis reaction, giving undesired enyne metathesis products [125f] (c) the linear structure of the alkyne may prevent cyclization reactions due to steric reasons [125a-d] and (d) the hexacarbonylcobalt moiety can be used for further transformations [125c,f]. 

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

Alkyne hexacarbonyldicobalt complexes. The complexes of alkynes with Co2(CO)84 react with 2,5-dihydrofuranes to give 3-oxabicyclo[3.3.0]-7-octene-6-ones in high yield (equation I) when the reaction is conducted under a CO atmosphere. 

There are several examples known where methylenecyclopropanes have been employed as cosubstrates in reactions of the Khand—Pauson type. Generally, this cobalt-mediated reaction is performed with hexacarbonyldicobalt complexes of alkynes, preferably strained alkenes such as norbornene or cyclobutene, and carbon monoxide to produce cyclopent-2-enones. Stoichiometric amounts of cobalt (in the form of the alkyne complexes) are normally needed. Methylenecyclopropane (1) can be employed as the strained alkene and reacts with a variety of alkynes to yield spiro-fused products. Silica gel or zeolites function as promoters of the reaction, giving rise to improved yields. ... 

Treatment of the yellow-brown octacarbonyldicobalt, Co2(CO)g, with an acetylene in ethereal or hydrocarbon solvent at room temperature under an inert atmosphere generates over several hours time the red-violet hexacarbonyldicobalt-alkyne complex, Co2(CO)s al-kyne. These species may be isolated and purified under inert atmosphere conditions at room temperature and readily characterized by the usual spectroscopic techniques [89]. Reaction of this complex with an olefin occurs at temperatures ranging from about 60 to 120 °C over a period of hours to days Eq. (36) depicts one of the best examples [1, 90]. 

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... 

An important procedure for the synthesis of cyclopentenones is the so-called Pauson-Khand reaction, which constitutes a formal [2 + 2 + 1] cycloaddition of an alkene, an alkyne, and carbon monoxide. Due to the increase in structural diversity of the available starting materials, the reaction has become an attractive target for scientific investigations [1-8]. The first successful example was reported by Pauson, Khand et al [9] in 1973 for the conversion of norbornene with the phenylacetylene-hexacarbonyldicobalt complex to give the corresponding cyclopentenone in 45% yield (Eq. 1). 

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)]. 

Reaction of hexacarbonyldicobalt complexes of ene-ynes or hetero-ene-ynes with (biscyclooctene)(tricarbonyl)iron results in formation of [FeCo2 (p.-alkyne)(CO)9] (Scheme 15).142 In both cases the free double bond in the vicinity of the Co2C2 unit facilitates the incorporation of the incoming Fe(CO)3 fragment. 

The formation of alkyne-hexacarbonyldicobalt complexes from the direct reaction of alkynes with octacarbonyldicobalt is represented by Eq. (1). Alkynes that undergo reaction according to the equation include... 

Although Eq. (1) represents the most general synthesis of the complexes Co2(CO)g(RC2R ), several other reactions involving alkynes and/or carbonyl-cobalt compounds are known to give alkyne-hexacarbonyldicobalt complexes. These include the reactions of alkynes with Hg[Co(CO)j2 (2J, 48, 54, 57, 73), ClHgCo(CO)4 (84), Co (CO) (31,... 

Decomplexation. Alkyne-hexacarbonyldicobalt complexes are decomposed by TBAF in THF (4 examples, 81-99%). 

Reductive decomplexation/ Although the common practice for removal of the hexacarbonyldicobalt residue from alkyne complexes involves mild oxidants, it is also possible to convert the complexes to free (Z)-alkenes with BUjSnH. If triethylsilane is used the decomplexation is followed by in situ hydrosilylation. 

The hnker is stable to a great variety of reaction conditions. In the example illustrated in Scheme 46, the alkynes are loaded indirectly onto the phosphine-modified polymer as their hexacarbonyldicobalt(O) complexes, but they can also be attached to a Co-coated polymer. Instead of traceless release of the alkynes after modification, they can also be apphed to Pauson-Khand reactions to form cyclo-pentenones [119]. 

The alkyne-Co2(CO)6 complexes 1 are the binuclear cluster complexes of the acetylenic derivatives with the hexacarbonyldicobalt moiety. These complexes can be readily prepared by treatment of alkynes with commercially available octacarbonyldicobalt [Co2(CO)g] and can regenerate the parent triple bond functionality under some mild oxidation conditions. Two synthetically very useful reactions have so far been developed by taking advantage of the characteristic properties of the alkyne-Co2(CO)6 complexes 1 one is so-called Nicholas reaction" and the other is so-called Pauson-Khand reaction (Scheme 1). The alkyne-Co2(CO)6 complexes 1 possessing a hydroxyl group or its equivalent at carbon p- to alkyne moiety (propargyl alcohol derivatives) could easily... 

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. ... 

The synthesis of fragment B will be done according to Scheme 3. The known hexacarbonyldicobalt complexed alkyne (12) will be prepared according to the work done by Krafft and co workers. First, 3-butyn-l-ol (13) will be converted to its rerr-butyldimethylsilyl (TBS) ether by treating a mixture of (13) with TBS-Cl in triethylamine and DMAP. Treatment of the protected alkynol with 5ec-butyl lithium in THF followed by low temperature quench with ethyl chloroformate yields the alkynoate (14). The alkynoate is then complexed with dicobaltoctacarbonyl in petroleum ether to yield (12) in excellent yield. 

Although the above mechanism has been universally accepted, apart from the initially formed (/r-alkyne)hexacarbonyldicobalt, only one of the proposed intermediates along this pathway has been synthesized and characterized as stable entities. In 2009, Brusey et al. [136] reported the syntheses. X-ray crystallographic structural characterizations, and a variable temperature NMR investigation of a series of ( 7 -alkene)-(/r-aIkyne)pentacarbonyldicobalt complexes, which represented the first isolated examples of the first step in the proposed mechanism of the PK reaction. Fortunately, density functional methods now can make accurate predictions of structures and energetics in organometallic systems. Therefore, a series of density... 

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