Cobalt complexes ethers

Not surprisingly, the cobalt-complexed ether products obtained by the Nicholas reaction can be subjected directly to the Pauson-Khand sequence (Scheme 7).244... 

Quite recently, Ciampolini and coworkers have reported the synthesis of two isomeric mked oxygen-phosphorus macrocycles and the crystal structures of their cobalt complexes. Synthesis of macrocycle 27 was accomplished by condensation of 1,2-bis-(phenylphosphino)ethane dianion with 2,2 -dichlorodiethyl ether in THE. The two isomers of 27 were isolated in 1.5% and 2% yield. The synthesis is formulated in Eq. (6.17), below. 

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

In a different approach three different structurally defined aza-crown ethers were treated with 10 different metal salts in a spatially addressable format in a 96-well microtiter plate, producing 40 catalysts, which were tested in the hydrolysis of /xnitrophenol esters.32 A plate reader was used to assess catalyst activity. A cobalt complex turned out to be the best catalyst. Higher diversity is potentially possible, but this would require an efficient synthetic strategy. This research was extended to include lanthanide-based catalysts in the hydrolysis of phospho-esters of DNA.33... 

The high reactivity of the cobalt-complexed propargylic systems has also allowed for ether oxygens to serve as nucleophiles, which has led to the fashioning of medium-sized ring ethers via ring expansion (Equation (60)), and also... 

The cobalt complexes reported in Fig. 17.20 can be easily produced by mixing 1 equivalent of [ ( 20) ]2 + with 3 equivalents of a bidentate ligand or 2 equivalents of a tridentate ligand under magnetic stirring in refluxing methanol for 2 h. Addition of ethyl ether results in the precipitation and isolation the product that is then usually used without any further purification. 

Co2(CO)8-catalyzed reactions of benzylic acetates with trimethylsilane and CO proceed under mild reaction conditions to give trimethylsilylethers of /3-phenethylalcohol in 43-76% yield. The highest yields are observed for benzyl acetates with electron-donating substituents.111 Secondary alkyl acetates are also good substrates in the reaction system, yielding enol silyl ethers.112 In addition, the cobalt complex is an effective catalyst for siloxymethylation of five-membered cyclic ortho esters, as shown in Eq. (41).113... 

Ring-opening silylformylation has been observed by Murai and coworkers in reactions of cyclic ethers. When the cobalt complex Co2(CO)8 is used as the catalyst in reactions of epoxides, an excess of substrate is required to prevent further reaction of the product siloxy aldehyde.1192 Further investigation led to the discovery of [RhCl(CO)2]2/l-methyl-pyrazole as an effective catalyst combination for the reaction of oxi-ranes119b and oxetanes.J19c For example, oxetane undergoes silylformylation to give 4-(dimethylphenylsiloxy)butanal in 81% yield [Eq. (45)]. 

Propargylation (8,148-149 10,129). In the presence of a Lewis acid, cobalt-complexed propargylic ethers alkylate silyl enol ethers with high -selectivity. Example ... 

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... 

The same radical cyclization can be performed using a more simplified vitamin B12 model, such as Z w(dimethylglyoximato)(pyridine)Cobalt chloride Co3+ Cl -Py complex (B), as shown in eq. 11.3. Treatment of propargyl P-bromoethyl ether (3) with NaBH4 and a catalytic amount of the cobalt complex (B) provides fi-exo-methylene tetrahydrofuran via 5-exo-dig manner [2-7]. 

Pattenden and coworkers studied the cyclization of allyl ort/io-iodophenyl ether 293a (R=H) catalyzed by (salen)cobalt complex 294 (Fig. 70) [317], Dihydro-benzofuran 295a (R=H) was obtained in a moderate 45% yield. 

Table 78 Structural Data for Cobalt(lI)-Ether Complexes...

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)]. 

Cobalt-complexed propargylic aldehydes can be alkylated using both cyclic and acyclic trimethylsilyl enol-ethers and -esters in the presence of a Lewis acid. Boron trifluoride etherate is the most frequently employed Lewis acid. 

The alkyne cobalt complex is purified by silica gel chromatography. To the red solution is added 2 g of silica gel and the mixture is concentrated on a rotary evaporator (Note 13). The silica gel is then placed under vacuum (1 mm) until effervescence ceases (about 10 min). The dried silica gel is placed at the top of a column of silica gel (diameter 3 cm height 9 cm), eluting first with hexane to remove nonpolar by-products and subsequently with 15% ether/hexane. The desired product elutes as a red band. Concentration of the appropriate fractions on a rotary evaporator (Note 13) and under high vacuum (0.7 mm for 10 min) affords 1.4 g (79%) of 5 as a fine red powder (Note 14). 

Another example of tr-allyl ligand transfer has been described recently by Heck 159). Treatment of tr-allyl derivatives of palladium chloride with NaCo(CO)4 in ether gives tr-allyl cobalt complexes. 

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