Catalyst cobalt-salen

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

In the realm of hydrolytic reactions, Jacobsen has applied his work with chiral salen complexes to advantage for the kinetic resolution of racemic epoxides. For example, the cobalt salen catalyst 59 gave the chiral bromohydrin 61 in excellent ee (>99%) and good yield (74%) from the racemic bromo-epoxide 60. The higher than 50% yield, unusual for a kinetic resolution, is attributed to a bromide-induced dynamic equilibrium with the dibromo alcohol 62, which allows for conversion of unused substrate into the active enantiomer <99JA6086>. Even the recalcitrant 2,2-disubstituted epoxides e.g., 64) succumbed to smooth kinetic resolution upon treatment with... 

Addition of a quaternary ammonium salt to a cobalt-salen catalyst drastically enhances the catalytic performance for the co-polymerization of PO with GO2. Lu and Wang investigated the binary catalyst system that consisted of a cobalt-salen complex and a tetrabutylammonium salt (Table 6). This binary catalyst system is able to promote... 

Enantiomer-differentiating co-polymerization of terminal epoxides is achieved by chiral chromium and cobalt complexes. Jacobsen etal. reported the co-polymerization of 1-hexene oxide with GO2 by using complex 35a. The reaction proceeds with kinetic resolution at 90% conversion, the unreacted epoxide is found to be enriched in the (i )-enantiomer of 90% ee. Detailed information about the resultant polymer, however, is not described. As discussed in the previous section, chiral cobalt-salen complex 34c co-polymerizes PO and GO2 (Table 3). When 34c with /r<3 / j--(li ,2i )-diaminocyclohexane backbone is applied to the co-polymerization, (A)-PO is consumed preferentially over (i )-enantiomer with a of 2.8 to give optically active PPG (Equation (8)). In a similar manner, a binary catalyst system, 34d/Bu4NGl, preferentially consumes (A)-PO over R)-PO with = 2.8-3.5. ... 

Since both nickel(II) and copper(II)(salen) complexes have been found to form asymmetric phase-transfer catalysts, the use of other metal(salen) complexes was investigated. Cobalt(salen) complexes 42a-d provided an opportunity to probe the influence of the oxidation state of the metal on the catalytic activity of the complex [42]. Hence, each of these complexes was prepared and tested as a catalyst for the benzylation of substrate 16a, according to the conditions specified in Scheme 8.18. 

A number of cyclic and sugar-derived halo acetals 273 were subjected to radical 5-exo cyclizations catalyzed by a cobalt salen catalyst 274 with NaB H4 as the stoichiometric reductant but in the presence of air (entry 11) [321, 322]. Under these conditions, bicyclic oxygenated tetrahydrofurans 275a were obtained in 50-84% yield. Diastereomeric isomers 275b were isolated as the minor components. The yields were similar to those obtained with tributyltin hydride. The oxygen concentration proved to be important, since air gave better yields... 

We can conclude that a comparison of the respective catalytic results of these new heterogeneous catalysts and their homogeneous counterparts showed that the entrapment of the organometallic complex was achieved without considerable loss of activity and selectivity. The immobilised catalysts are reusable and do not leach. The oxidation system applies only O2 at RT instead of sodium hypochloride at 0°C. A disadvantage is the use of pivalic aldehyde for oxygen transformation via the corresponding peracid. This results in the formation of pivalic acid which has to be separated from the reaction mixture. The best results so far - 100 % conversion, 96 % selectivity and 91 % de - were achieved with the immobilised Cobalt(salen-5) complex in the epoxidation of (-)-a-pinene. 

Opening an epoxide by an alkoxide moiety can be done intramolecularly, and a new cyclic ether is generated. Ethers of various ring sizes can be produced depending on the length of the tether between the alkoxide unit and the epoxide. Specialized conditions are common, as in the conversion of 116 to 117. Another variant of this transformation used a cobalt-salen catalyst. A specialized version has the alkoxide moiety on the carbon adjacent to the epoxide, leading to the Payne rearrangement, where a 2,3-epoxy alcohol is converted to an isomeric one, by treatment... 

A chiral cobalt-salen complex bearing lil j serves as an active catalyst for the HKR of terminal epoxides [90]. The polymeric salen-Co complex 158 (Scheme 3.46) also showed a high enantioselectivity in the same reaction [91]. 

Week [203] has performed the HKR of several epoxides by means of polymer-supported cobalt salen catalysts containing different coimterions (325/Co-OAc, 325/Co-I, 325/Co-OTs, 325/Co Scheme 137). 325/Co-I and 325/Co-OTs catalysts have shown higher activities than 325/Co-OAc and ee could be up to > 99% with a conversion of 55% after Ih with 325/Co-I for the HKR of epiehlorhydrine. These catalysts can only be recycled once as a result of their decreased solubility after the reoxidation step. 

BimetaUic chiral cobalt salen catalysts containing transition-metal salts have also been demonstrated by Kim et al. [190] to be remarkably efficient and highly enantioselective in hydrolytic KRs of various epoxides. Enantioselectivity of up to 99% ee for the recovered epoxides combined with very high catalytic activity could be reached. Another means for fixing or linking two or more Co(salen) units in dose proximity to decrease the catalyst requirements by making the reaction of... 

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