Alkylated BDHC complexes

The alkylated BDHC complexes are very unstable in general and cannot be isolated in pure form from the reaction mixture. A large portion of an alkylated product decomposes during the extraction procedure with organic solvents. Thus, the decomposition behavior of the alkylated BDHC complexes was investigated as they were prepared in aqueous media, after decomposition of the excess NaBH4 with acid in the presence of excess alkyl halide. An alkylated BDHC complex also can be prepared by the reaction of [Co(II)(BDHC)]C104 with an alkyl iodide under alkaline condition (pH 12) the disproportionation reaction yields the Co(III)(BDHC) species and an alkylated BDHC complex. 

Table X. Half Lives for Photolysis of Alkylated BDHC Complexes...

Alkylated cobaloximes yield the corresponding dimeric species of alkyl radicals by photolysis under acidic conditions. But the BDHC complex with a hexyl or benzyl group at its axial site does not yield the corresponding dimeric species by photolysis (dodecane and bibenzyl, respectively). Consequently, the hydrogenation product must be obtained through the formation of a carbanion by heterolytic cleavage of the Co-C bond, followed by its protonation. 

Based on the kinetic data on the alkylation of Co(I)(BDHC) (Table VII), the BDHC complex can be used as a catalyst for selective hydrogenation of primary alkyl halides using sodium hydroborate as the stoichiometric reducing reagent (see Scheme 1). In fact, when... 

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 second-order rate constants for reactions of Co(I)(BDHC) with alkyl halides were determined spectrophotometrically at 400 nm (17). These rate constants are listed in Table VII along with those for Co(I)(corrinoid)(vitamin Bi2s) in methanol at 25°C (35). These data indicate that the SN2 mechanism is operative in the reaction of Co(I)(BDHC) the iodides are more reactive with the cobalt complex than the bromides, and the rate decreases with increasing bulkiness of the alkyl donor. The steric effect is more pronounced for Co(I)(BDHC) than for vitamin B12s, which is confirmed by the rate ratios for... 

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