Hexacarbonyldicobalt 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]. 

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). 

A hitherto unknown type of rearrangement of l-(l-alkynyl)cyclopropanols (321) to cyclopent-2-en-l-ones (322) mediated by octacarbonyldicobalt" and hexacarbonyldicobalt" complexes has been described. A possible pathway for the transformation is outlined in Scheme 101. A -proton transfer accompanied by a metal-mediated Stevens rearrangement, which converts a coordinated dimethylsulfane... 

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)]. 

A metal-stabilized dicarbenium ion complex [Mo2(H2CC2CH2)(CO)4 Cp2]2+ has been fully characterized and its structure determined by X-ray crystallography109 however, the [Co2(H2CC2CH2)(CO),)]2+ dication has not been isolated, possibly because of the poorer ability of Co(CO)3 to stabilize positive charge. A bis(9-fluoreniumyl)ethyne ditetrafluoroborate-ethyne-hexacarbonyldicobalt complex 29 has been reported, but is unstable at room temperature.110 (See Formula 29.)... 

Primary cations give both coupled and H-atom abstraction products. Secondary cations undergo coupling only, whereas tertiary cations with methyl substitutents show only H-atom abstraction and electrophilic coupling. The intramolecular variant of this reaction is particularly useful for the efficient synthesis of medium-sized 1,5-diynes and has been applied to the synthesis of hexacarbonyldicobalt-complexed 1,5-cycloctadiynes 31 and a cyclooct-5-ene-l,5-diyne derivative 32 (Scheme 14).121 The eight-membered rings... 

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,... 

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. 

Generally, reduction of diketones 12 (or in some cases their hexacarbonyldicobalt complexes) yields better stereoselectivities than the related ethylenic diketones 11, especially when R is a sterically demanding group. In addition, the propargylic diols obtained can be easily transformed not only into the saturated... 

The two stereogenic centers of the target compound were concomitantly introduced by a diastereo- and enantio selective crotylation technique employing a chiral crotyl silane. The syn/anti selectivity of this reaction was improved dramatically by converting the propargylic starting material into the corresponding hexacarbonyldicobalt complex. 

Note that tetracobalt dodecacarbonyl is a catalyst precursor for reaction at 150 and 10 atm CO. Only 0.01 equiv of the cobalt cluster is required. Actually catalytic amount of Co2(CO)g of high purity is sufficient to complete the intramolecular P-K reaction, also the hexacarbonyldicobalt complex of 2-methyl-3-butyn-2-ol can be decomposed with triethylsilane to generate an active cobalt carbonyl catalyst. ... 

Exocyclic dienes. Hexacarbonyldicobalt complexes of 1,6- and 1,7-enynes are readily formed. In refluxing toluene these complexes release dienes. 

Cyclization-reairangement. Intramolecular capture of a propargyl cation, which is stabilized by hexacarbonyldicobalt complexation at the triple bond by the double bond of an ally lie alcohol, is followed by rearrangement to give a cyclic ketone. The tricyclic portion of ingenol is thereby created. The present organoaluminum catalyst appears to be the most efficient among those screened. 

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

Preparation of Hexacarbonyldicobalt Complexes of Acetylenes General Procedure ... 

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. ... 

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. ... 

Unusual decomplexation. A highly strained cycloalkyne cannot be produced from its hexacarbonyldicobalt complex on reaction with CAN. Instead, carbonylation at the two nascent sp-hybridized carbon sites is observed. ... 

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. 

Coupling reactions. The hexacarbonyldicobalt complexes of cycloocta-l,5-diynes and cyclooct-3-ene-l,5-diynes have been acquired by reductive cyclization of the dication... 

Unsaturated Ring Ketones.—Acetylene hexacarbonyldicobalt complexes react with ethene, alkenes, and cycloalkenes to provide a general synthetic route to cyclopentenones. Unfortunately most monosubstituted ethenes react non-specifically. 2-Bromoethylallene gives 2-methylcyclopent-2-enone on treatment with Collman s reagent. 

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