Kinetics epoxide ring-opening

Enantioselective Epoxidation, Ring-Opening and Kinetic Resolution of Epoxides... 

The Katsuki-Sharpless asymmetric epoxidation of racemic diol ( )-496 (obtained by allylation of ( )-crotonaldehyde) gives, after chromatographic separation, the erythro-epoxidc (+)-497 (33% yield, >95% ee). Its urethane undergoes assisted epoxide ring opening under acidic conditions, providing a 10 1 mixture of araZ mg-carbonate (+)-498 and the nZ 6>-stereomer. Carbonate hydrolysis and subsequent ozonolysis generates D-olivose (Scheme 13.114). Asymmetric epoxidation of the kinetically resolved dienol (—)-496 (72%) leads to (—)-497... 

Conjugated dienes are oxidized to epoxides (or diols, if water is present) with the MTO/H2O2 system [11]. Urea/H202 avoids the subsequent epoxide ring opening. Electron-rich and conjugated dienes are more easily oxidized than electron-poor dienes and dienes with isolated double bonds. According to kinetic measurements complex 3 plays no important role as catalyst in this case. Compound 2 is an active species [11a]. 

Keywords Epoxide, Ring opening, Desymmetrization, Kinetic resolution... 

These fundamental aspects of epoxide ring opening were established by kinetic and isotopic labeling studies2 The dominant role of bond cleavage in acidic hydrolysis is indicated by the increase in rates with additional substitution. Note in particular that the 2,2-dimethyl derivative is much more reactive than the cis and trans disubstituted derivative, as expected for an intermediate with carbocation character. 

Following these results, Darensbourg et al. have continued the research and used other bifunctional Cr(salen) complexes as catalysts for polycarbonate synthesis. They observed that when a monofunctional Cr(salen) complex (5) was used to catalyze the reaction between epoxide and CO2, the product formed was cyclic carbonate. However, when a bifunctional Cr(salen) catalyst (6) was used, 79% selectivity towards the polycarbonate was obtained at 70 °C. The reason for this difference lies in the structure of the bifunctional catalyst, which provides steric hindrance in the epoxide ring-opening process to form the cyclic carbonate. Therefore, it can be inferred that spatial requirements in the active site of the metal catalyst determine the selectivity for the kinetic polymer product over the thermodynamically more stable cyclic carbonate product. 

Highly active oligomeric (salen)Co complexes such as (198) were designed for asymmetric hydrolysis of meso-epoxides and kinetic resolution of terminal epoxides, based on cooperative bimetallic mechanism postulated for epoxide ring-opening reactions (Scheme 16.59) [83, 84]. 

Jacobsen also showed that 2,2-disubstituted epoxides underwent kinetic resolution catalyzed by (salen)Cr-N3 complex 3 under conditions virtually identical to those employed with monosubstituted epoxides (Scheme 7.34) [64]. Several epoxides in this difficult substrate class were obtained with high ees and in good yields, as were the associated ring-opened products. The kinetic resolution of TBS-... 

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