Dicobalthexacarbonyl complex

Sugihara et al. in 1997.106 They utilized aqueous ammonium hydroxide as a reaction medium, which provided ammonia as a hard ligand to labilize the CO ligands and therefore enhance the rate of the PKR. The reaction of dicobalthexacarbonyl complexes of enynes and alkynes provided expected cyclopentenones via intramolecular and intermolecular modes respectively (Scheme 4.10). 

The Pauson-Khand reaction (PKR) [96] consists of the synthesis of cyclopen-tenones by reaction of an alkene with a dicobalthexacarbonyl complexed alkyne (Scheme 57) and has recently emerged as one of the methods of choice for the obtainment of five-membered carbocyclic rings [97]. Its unique atom connectivity, which involves the two unsaturated carbons of the reagents and the carbon atom of a carbon monoxide ligand of cobalt usually in a regioselective manner (Scheme 57), has brought to refer to PKR as a [2 -I- 2 + 1] cycloaddition. 

In another conceptually novel [5 + 2]-process, Tanino and co-workers synthesized cycloheptene derivatives by stereoselective [5 + 2]-cycloadditions involving hexacarbonyldicobalt-acetylene complexes as the five-carbon component and enol ethers as the two-carbon component (Schemes 22 and 23).60 61 The role of the dicobalthexacarbonyl complex is to facilitate formation and reaction of the propargyl cation putatively involved as an intermediate in this reaction. The dicobalthexacarbonyl moiety can be removed using various conditions (Scheme 24) to provide alkane 60, alkene 62, and anhydride 63. 

To solve this problem, we modified the steps as show in Figure 6, where the methyl group be introduced after epimerization of the dicobalthexacarbonyl complex 25 which may give a 1 4 mixture of 25 and 26 through our previous study (10). In addition, we have recently developed a new reductive decomplexation method of bis-cobalthexacarbonyl complex, which is applicable to the synthesis of 27 in a single step from bis-cobalthexacarbonyl complex 26... 

Pauson-Khand reactions. This salt Na[CpMo(CO)3] converts alkyne-dicobalthexacarbonyl complexes into mixed Co-Mo analogs which often show higher reactivities in the cyclopentenone synthesis. 

Aldehydes were utilized to react with a number of reagents, e.g., cyclic allylsiloxanes <0410C6874>, dicobalthexacarbonyl complex of dimethyl 2-ethynylcyclopropane-l,l-dicarboxylate <04CC2474> and an acylated bromooxazolidinone <04TL4457> to form complex tetrahydrofuran frameworks. The scheme below is an example to demonstrate the versatility of this approach <04OL3865>. 

After masking of the alkyne function by a dicobalthexacarbonyl complex, RCM was used to cyclize the diene 573 (Scheme 9.15). Then, decomplexation enabled a Diels-Alder reaction with the disiloxydiene 575, ensued by elimination of isobutylene. Two hydroxy groups of the resorcylic macrolide (576) obtained were protected and the TBS-ether was cleaved. Thus, dehydration with concluding deprotection of all alcohol functions furnished aigialomycin D (482), in an overall yield of 8%. 

In 1971 at the University of Strathclyde in Glasgow, Professor Peter Pauson reported on the retro-Diels-Alder reaction of norbomadiene 3 induced by dicobalthexacarbonyl complexes of acetylene or phenylaeetylene 1 to provide dicarbonylcyclopentadienylcobalt complexes 4 in high yield. Almost as an after thought, he mentions In addition to the above products, the reaction of norbomadiene with complexes 1 yields hydrocarbon and ketonic products derived from norbomadiene, acetylene and carbon monoxide. ... 

Initial step is the formation of a dicobalthexacarbonyl-alkyne complex 5 by reaction of alkyne 1 with dicobaltoctacarbonyl 4 with concomitant loss of two molecules of CO. Complex 5 has been shown to be an intermediate by independent synthesis. It is likely that complex 5 coordinates to the alkene 2. Insertion of carbon monoxide then leads to formation of a cyclopentenone complex 6, which decomposes into dicobalthexacarbonyl and cyclopentenone 3 ... 

Products 7a and 7c, with the substituent R a to the carbonyl group, are by far predominantly formed. This regioselectivity is a result of the preferential approach of the alkene 2 to the dicobalthexacarbonyl-alkyne complex 5 from the side opposite to the substituent R of the original alkyne. The actual incorporation of the alkene however is less selective with respect to the orientation of the olefinic substituent R, thus leading to a mixture of isomers 7a and 7c. 

The [2+2+1] cycloaddition of an alkene, an alkyne and carbon monoxide is commonly known as the Pauson-Khand reaction. This transformation has been adopted many times in the synthesis of complex natural products and related compounds, which contain a cyclopentenone moiety, for example, prostaglandins. Two independent reports of this reaction appeared almost simultaneously in late 2002 by Iqbal and co-workers25 and Fisher and co-workers26, respectively. They not only used very similar substrate systems in their studies, but they also reached very similar conclusions Toluene was found to be the preferred solvent in this reaction, even though it is a very poor microwave absorber. A reaction time between 5 and 10 min, using dicob alto ctacar-bonyl or dicobalthexacarbonyl as the carbon monoxide source, and a temperature of 100-120°C resulted in high yields of the products. Fisher and co-workers used 20 mol% Co2(CO)8 and cyclohexylamine as an additive (Scheme 5.12), since this system had been used previously in order to allow a catalytic reaction. Iqbal and co-workers did not use cyclohexylamine, but instead used 1 equiv. of the carbon monoxide (Co2(CO)6) source. In both reports, the products were formed in 40-70% yield. 

Treatment of epoxy-alkyne derivatives 314 with dicobalthexacarbonyl [Co2(CO)6] and a catalytic quantity of Lewis acid, followed by oxidative demetallation of the intermediate cobalt complex 315 affords 2-alkynyl-3-acetoxy tetrahydropyrans 316 in excellent yield. Syn- or tf /7-tetrahydropyrans are obtained by selection of either the anti- or syn- starting epoxides 314 respectively (Scheme 79, Table 10) <1998T823, 2000JOC6761>. 

Complexation of an alkyne to dicobalthexacarbonyl is a well-known way to stabilize carbocationic charges generated in the carbon a to the alkyne. These carbocations react with different nucleophiles. This process, the Nicholas reaction [112], has been used to generate enynes that undergo, in a domino fashion, a PKR. 

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