Cobalt salen, with epoxides

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... 

Table 6 Alternating co-polymerization of terminal epoxides with CO2 catalyzed by cobalt-salen complexes...

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. ... 

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. 

Cyclohexene oxide 236 is opened with benzoic acid catalysed by 2.5% of the cobalt salen complex 235 to give the asymmetric half ester 237. Although the ee is only 75% this is improved by recrystallisation to 98% ee. Epoxides on other ring sizes can also be opened enantioselectively. 

Epoxides are a familiar sight in the world of kinetic resolutions. As well as being made by kinetic resolution, racemic epoxides can themselves be the substrates in a kinetic resolution. For example, the use of cobalt-salen complexes - something we shall see again in the dynamic kinetic resolution section - can be used to mediate the formation of enantiomerically pure oxazolidinones.20 Enzymes can be used to react with one enantiomer of epoxide.21 And enzymes are a good way to kinetically resolve compounds and can work under surprising conditions - supercritical C02 for example22... 

Cobalt tetraarylporphyrins with fluorine-containing substituents were active in epoxidation of alkenes using fluorous catalysis in the presence of oxygen and 2-methylpropanal [167,170-171]. Manganese and cobalt complexes of perfluorinated tetraazocyclonone catalyzed allylic oxidation of alkenes with r-BuOOH/Oa [172]. The complex with the salen ligand 57 was active in alkene epoxidation under Mikayama s conditions, and indene was epoxidated at a high stereospecificity [173]. 

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... 

Although the enantioselective intermolecular addition of aliphatic alcohols to meso-epoxides with (salen)metal systems has not been reported, intramolecular asymmetric ring-opening of meso-epoxy alcohols has been demonstrated. By use of monomeric cobalt acetate catalyst 8, several complex cyclic and bicydic products can be accessed in highly enantioenriched form from the readily available meso-epoxy alcohols (Scheme 7.17) [32]. 

The principle cost determinant in typical hydrolytic or phenolic resolutions is the cobalt catalyst, despite the relatively low catalyst loadings used in most cases and the demonstrated recyclability with key substrates. From this standpoint, recently developed oligomeric (salen)Co complexes, discussed earlier in this chapter in the context of the hydrolytic desymmetrization of meso-epoxides (Scheme 7.16), offer significant advantages for kinetic resolutions of racemic terminal epoxides (Table 7.3) [29-31]. For the hydrolytic and phenolic kinetic resolutions, the oligo-... 

Abstract This chapter focuses on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides or oxetanes and carbon dioxide. Emphasis is placed on the use of salen metal complexes, mainly derived from the transition metals chromium and cobalt, in the presence of onium salts as catalysts for the coupling of carbon dioxide with these cyclic ethers. Special considerations are given to the mechanistic pathways involved in these processes for the production of these important polymeric materials. 

The third investigation track demonstrated the immobilization of metal-salen complexes in mesoporous materials and their use in the hydrolytic kinetic resolution of meso and terminal epoxides. The best results were obtained over cobalt-Ja-cobsen catalysts. The catalytic activity of the (S,S)-Co(II)-Jacobsen complex immobilized on Al-MCM-41 was comparable with that of the homogeneous counterpart. Several other immobilization methods are still under investigation. 

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