Cobalt complexes hydroboration

Hydridotris(pyrazolyl)borates, as cobalt complex ligands, 44 274-278 Hydroboration, 16 230-234 chain lenghtening, 16 231-232 role in sythesis, 16 231, 232 Hydrocarbons clusters, 18 57-59... 

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). 

Sodium acetoxytrihydroborate is also effective,while certain titanium, rhodium and cobalt complexes catalyze hydroborations with sodium tetrahydroborate." In the latter case the facial selectivities can be quite different from, even opposite to, those observed with borane-THF. ... 

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]. 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

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