Octacarbonyldicobalt reactions with alkynes

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... 

Two syntheses of racemic furanether B (18.8) were described by N.E. ScHORE et al. (755), (755). Both feature an octacarbonyldicobalt catalysed cycloaddition of 8-oxabicyclo 3.2.1 oct-6-ene derivatives with alkynes (Pauson-Khand reaction) as a key step. The bicyclic ketone 26.71, prepared by [4+3] cycloannulation from 2-methyfuran and tetrabromoacetone, was used as starting material. 

Two reports have appeared on the reaction of octacarbonyldicobalt with alkynes. In the reaction with oct-l-yne, oct-2-yne, or diphenylacetylene (R C2R ) in toluene AtoIis was found to fit the rate law... 

A complexation-initiated reaction was realized for the first time as depicted below. Thus, the octacarbonyldicobalt complex of furan is subjected to silica gel and gives rise to the adduct with a seven-membered ring owing to bending of the triple bond to a structure with an angle of around 140° when the alkyne is allowed to react with Co2(CO)8 at room temperature . 

The reaction of octacarbonyldicobalt with various alkynes has long been known to give alkyne-bridged compounds of the type (p.rf-RC CR )Co2(CO)6 (112, 113). However, under more drastic conditions, these... 

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. 

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). 

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... 

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]. 

Alkynes and Allenes.—Ethynyl steroids (239) readily form a stable hexacarbonyl-dicobalt complex (240) on treatment with octacarbonyldicobalt. The complex serves to protect the triple bond during reactions elsewhere (c.g. olefin reduction with di-imide, or hydroboronation). The ethynyl group is regenerated by treating the complex with iron(iii) nitrate in ethanol. ... 

In an analogy to the well-known octacarbonyldicobalt-mediated Pauson-Khand reaction, the formal [2+2+1] cycloaddition of alkyne, alkene, and carbon monoxide can be promoted by pentacarbonyliron. However, the iron-mediated [2+2+1] cycloaddition of two alkynes with carbon monoxide has attracted much more attention. When heated in glyme at 140 °C in the presence of stoichiometric amounts of penta-carbonyliron, substituted trimethylsilylacetylenes afford tricarbonyliron complexed cyclopentadienones in moderate to good yields (Scheme 4-3). ... 

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