Cobalt complexes carbonyl compound hydrogenation

Nishinaga and co-workers isolated a series of stable cobalt(III)-alkyl peroxide complexes such as (170) and (171) in high yields from the reaction of the pentacoordinated Co"-Schiff base complex with the corresponding phenol and 02 in CH2C12. Complex (170 R=Bu ) has been characterized by an X-ray structure. These alkyl peroxide complexes presumably result from the homolytic addition of the superoxo complex Co111—02 to the phenoxide radical obtained by hydrogen abstraction from the phenolic substrate by the CoUI-superoxo complex. The quinone product results from / -hydride elimination from the alkyl peroxide complex (172)561,56,565,566 The quinol (169) produced by equation (245) has been shown to result from the reduction of the CoIU-alkyl peroxide complex (170) by the solvent alcohol which is transformed into the corresponding carbonyl compound (equation 248).561... 

Sodium borohydride (160) was found to serve as a hydrogen donor in the asymmetric reduction of the presence of an a,pi-unsaturated ester or amide 162 catalyzed by a cobalt-Semicorrin 161 complex, which gave the corresponding saturated carbonyl compound 163 with 94-97% ee [93]. The [i-hydrogen in the products was confirmed to come from sodium borohydride, indicating the formation of a metal enolate intermediate via conjugate addition of cobalt-hydride species (Scheme 2.17). 

Further results on asymmetric hydrogenations of activated carbonyl compounds catalyzed by bis(dimethylglyoximato) cobalt (Il)-chiral amine complexes have been reported (55,56). Some chiral reductive dimerizations were observed (55). 

Several studies on the hydrogenation of carbonyl compounds using cobalt complexes have been reported, including the hydrogenation of benzil in the presence of bis(dimethylglyoximato)co-balt(II)-chiral amine complexes. The highest ee (78%) was obtained when quinine was used as the chiral base (equation 17). Cyano cobalt complexes have also been reported to be effective for the hydrogenations of ketones. ... 

Other cobalt catalysts that have been used for enantioselective C—C double bond reduction of a,/ -unsaturated carbonyl compounds are vitamin B12 in combination with zinc/acetic acid (up to 33% ee)S7 and bis(dimcthylglyoximato)cobalt(II) (12) with chiral amines as cocatalysts88 and molecular hydrogen as reducing agent. The best results have been obtained with complex 12 and quinine or (2.S )-A -[(/ )-1 -phenethyl]-2-quinuclidinecarboxaniide (hydrogenation of 1,2-diphenyl-2-propen-l-one 49% ee A, /V -dimethyl-5-benzylidenehydantoin 79% ee). 

Alkenes can be hydroformylated " by treatment with carbon monoxide and hydrogen over a catalyst. The most common catalysts are cobalt carbonyls (see below for a description of the mechanism) and rhodium complexes, " but other transition metal compounds have also been used. Cobalt catalysts are less active than the rhodium type, and catalysts of other metals are generally less active. " Commercially, this is called the 0x0 process, but it can be carried out in the laboratory in an ordinary hydrogenation apparatus. The order of reactivity is straight-chain terminal alkenes > straight-chain internal alkenes > branched-chain alkenes. With terminal alkenes, for example, the aldehyde unit is formed on both the primary and secondary carbon, but proper choice of catalyst and additive leads to selectivity for the secondary product " or primary... 

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