Cobalt, complexes acetylene

Other examples of the uses of enolate derivatives of thioesters for highly diastereoselecave aldol reactions were reported by Gennari [373] and Hanaoka et al. [374. 375]. The latter reported the reaction between a chromium-complexed benzaldehyde (1) and the titanium enolate of a thioester (2) as the first step of sequences directed towards the synthesis of (+)-goniofufurone [374] and the taxol C13 side-chain [375]. They also used cobalt-complexed acetylenic aldehydes for the selective formation of syn-aldols [376]. 

A very attractive extension of cobalt complexed acetylene... 

Dinuclear cobalt acetylene complexes [Co2(RCCR)(CO)6] in which the alkyne molecule constitutes a bridge are very stable. In these compounds, as in other similar complexes, acetylenes are 4e ligands. In complexes of the type [Co4(RCCR)(CO)io], the alkyne molecule is bonded to all cobalt atoms. In dimeric cobalt complexes, acetylene molecules may undergo exchange reactions ... 

Bj-fEj-Methoxyallyldiisopinocampheylborane 16 was synthesized via the reduction of methoxy-3-(phenylthio)propene 145 with potassium naphthalenide under Hoffmann et al. conditions, followed by the treatment of the resulting anion with Ipc2BOMe. Ganesh and Nicholas demonstrated the use of this reagent for the synthesis of anti-P-alkoxyhomoallylic alcohols anti-1% via the alkoxyallylbo-ration of cobalt-complexed acetylenic aldehydes 77 (Scheme 25.16). 

Ganesh, P Nichols, K. M. 1993. Reactions of cobalt-complexed acetylenic aldehydes with chiral (Y-alkoxyallyl)boranes Enantioselective synthesis of 3,4-dioxy 1,5-enynes. J. Org. Ghem. 58 5587-5588. 

The Auger depth profile obtained from a plasma polymerized acetylene film that was reacted with the same model rubber compound referred to earlier for 65 min is shown in Fig. 39 [45]. The sulfur profile is especially interesting, demonstrating a peak very near the surface, another peak just below the surface, and a third peak near the interface between the primer film and the substrate. Interestingly, the peak at the surface seems to be related to a peak in the zinc concentration while the peak just below the surface seems to be related to a peak in the cobalt concentration. These observations probably indicate the formation of zinc and cobalt complexes that are responsible for the insertion of polysulfidic pendant groups into the model rubber compound and the plasma polymer. Since zinc is located on the surface while cobalt is somewhat below the surface, it is likely that the cobalt complexes were formed first and zinc complexes were mostly formed in the later stages of the reaction, after the cobalt had been consumed. 

Organocobalt complexes catalyze the cyelocotrimerization of acetylenes and nitriles, which affords pyridine and benzene derivatives (100). (Cyclo-pentadienyl)cobalt complexes such as CoCp(COD) favor pyridine formation (100), and modification of the Cp ligand has considerable influence on the activity of the catalyst and the chemo- and regioselectivity of the catalytic process (101). 

The cobalt mediated homo Diels-Alder reaction of norbomadiene (560) with phenyl acetylene (568a), affording a phenyl substituted deltacyclene, demonstrated the potential of low-valent cobalt complexes as catalysts332. Lautens and coworkers327 extended the scope of this reaction and were able to synthesize a wide range of substituted deltacyclenes from alkynes 568 (equation 164, Table 33). The low-valent cobalt or cobalt(O) species to be used was prepared in situ by reduction of Co(acac)3 with Et2AlCl. Monosubstituted... 

For a decade or so [CoH(CN)5] was another acclaimed catalyst for the selective hydrogenation of dienes to monoenes [2] and due to the exclusive solubility of this cobalt complex in water the studies were made either in biphasic systems or in homogeneous aqueous solutions using water soluble substrates, such as salts of sorbic add (2,4-hexadienoic acid). In the late nineteen-sixties olefin-metal and alkyl-metal complexes were observed in hydrogenation and hydration reactions of olefins and acetylenes with simple Rii(III)- and Ru(II)-chloride salts in aqueous hydrochloric acid [3,4]. No significance, however, was attributed to the water-solubility of these catalysts, and a new impetus had to come to trigger research specifically into water soluble organometallic catalysts. 

Fig. 15. The oloso distorted octahedral structure of the cobalt carbonyl acetylene complex Co4(CO)ioC2Et2 (50).

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

While there is no direct evidence for this, they have succeeded in isolating an analogous cobalt carbonyl-acetylene complex (Sternberg et al., 43). 

The [Con(bipy)2 ]2+ species has also been reported to activate hydrogen peroxide and ter -butyl hydroperoxide for the selective ketonization of methylenic carbons, the oxidation of alcohols and aldehydes, and the dioxygenation of aryl olefins and acetylenes (36). Later reports (37), however, while confirming that the cobalt complexes did indeed cata-... 

It appears difficult to propose a unified mechanism to explain all experimental observations of the cyclotrimerization of acetylene. The most common pathway, studied mainly with cobalt complexes,72 73 involves a metallacyclopentadiene intermediate ... 

The thermolysis of cobalt complex 116 in the presence of copper powder at 190 °C gave 10-membered cyclic acetylene 117 (Scheme 4) <2002AGE1181>. The reaction was iterated to lead to belt-like macrocycles. The same procedure was also utilized to synthesize twofold CpCo-capped bis(cyclopentadieno)superphane <20000M1578>. 

The synthetic plan in Figure 6 has been realized as full steps shown in Figure 7. C-Glycosidation of 18 with phenylthiotrimethylsilyl-acetylene and boron trifluoride etherate followed by treatment with biscobaltoctacarbonyl gave the biscobalthexacarbonyl complex 25 in 92% yield. Epimerization of the cobalt complex 25 was achieved with trifluoromethane sulfonic acid in... 

Cocyclization of acetylene with isocyanides gives interesting new cyclic compounds 103, 116). The reaction patterns are generally similar to the cocyclization wdth carbon monoxide which is already known 103, 117). Low-valent nickel, palladium, or cobalt complexes are active in the following reactions 102, 103) for which intervention of acetylene complexes has been suggested ... 

Yamazaki et al. (91, 119) and Bonnemann el at. (120) have recently reported catalytic syntheses of substituted pyridines from acetylenes and nitriles. Various cobalt complexes serve as active catalysts, in particular, CpCo(PPh3)2 (91) or Co(C8Hi2) (C8Hi3) (120). Similar reactions of acetylenes with CS2 or RNCS also give new heterocycles (91) ... 

The reaction of a Co(I) nucleophile with an appropriate alkyl donor is used most frequently for the formation of a Co-C bond, which also can be formed readily by addition of a Co(I) complex to an acetylenic compound or an electron-deficient olefin (5). The nu-cleophilicity of Co(I) in Co(I)(BDHC) is expected to be similar to that in the corrinoid complex, as indicated by their redox potentials. The formation of Co-C a-bond is the attractive criterion for vitamin Bi2 models. Sodium hydroborate (NaBH4) was used for the reduction of Co(III)(CN)2(BDHC) in tetrahydrofuran-water (1 1 or 2 1 v/v). The univalent cobalt complex thus obtained, Co(I)(BDHC), was converted readily to an organometallic derivative in which the axial position of cobalt was alkylated on treatment with an alkyl iodide or bromide. As expected for organo-cobalt derivatives, the resulting alkylated complexes were photolabile (17). 

The novel cobalt complex came about as a result of the intramolecular coordination of a double bond, present in one of the R groups on the acetylene, to one of the cobalt atoms - taking the place of a CO ligand (67 - 68). They found that the new pentacarbonyl complex could be readily formed in CDC13 at room temperature from the hexacarbonyl dicobalt precursor. Attempts to use the pentacarbonyl complex as a substrate in the PK reaction led to no formation of cyclopentenone product. It was proposed that this is due to the alkene occupying a pseudo-equatorial site -alkene insertion is thought to occur from the axial position (see section 2.4). 

Stable, isolable metallacycles are also obtained from reaction of complexes that serve as sources of the CpCo fragment (e.g. CpCo(PPh3)2) and alkynes. Upon carbonylation diese typically give high yields of cobalt-complexed cyclopentadienones. Direct reaction of CpCo(CO)2 with alkynes is similarly useful. The cycloaddition of di(t-butoxy)acetylene upon photolysis with CpCo(CO)2 is an example (Scheme 5). In all these systems the final complexes lack coordinated CO, and therefore amine oxides are not suitable reagents for liberating the stable cyclopentadienones. Tetra(t-butoxy)cyclopentadienone is accessible on a preparative scale via controlled electrochemical oxidation. Other oxidants such as Cr have been used as well in other systems. 

Kira, K, Hamajima, A, Isobe, M, Synthesis of the BCD-ring of ciguatoxin IB using an acetylene cobalt complex and vinylsilane strategy. Tetrahedron, 58, 1875-1888, 2002. 

In general, the acetylenic triple bond is highly reactive toward hydrogenation, hydroboration, and hydration in the presence of acid catalyst. Protection of a triple bond in disubstituted acetylenic compounds is possible by complex formation with octacarbonyl dicobalt [Co2(CO)g Eq. (64) 163]. The cobalt complex that forms at ordinary temperatures is stable to reduction reactions (diborane, diimides, Grignards) and to high-temperature catalytic reactions with carbon dioxide. Regeneration of the triple bond is accomplished with ferric nitrate [164], ammonium ceric nitrate [165] or trimethylamine oxide [166]. 

The configuration of the acetylene group was inverted through an acetylene-cobalt complex prepared by treatment of the acetylene with Co2(CO)g (O Scheme 31) [52]. Upon treatment of the complex with acid, epimerization occurred via a propargylic cation intermediate stabilized by the cobalt complex to afford the thermodynamically more stable -C-glycoside as the... 

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