Catalysis of Organic..

Jones et al. 7In the HKR of rac-epichlorohydrin, the Co(III) norbornene polymers 1a-dwere dissolved in a mixture of methylene chloride, reactant and chlorobenzene as aninternal standard, followed by the addition of 0.7 equivalents of water to start theresolution. The addition of some methylene chloride as a solvent was necessarybecause the copolymers were not fully soluble in the epoxide. The reaction kineticswere studied via chiral GC-analysis. Using either the homopolymer 1d or the twocopolymers 1a and 1b, the (R) epoxide was fully converted after five hours to itscorresponding diol, leaving pure (S) epoxide in the reaction mixture in above 99%enantiomeric excess. After this time period, 55% of the racemic epoxide wasconverted, meaning that all of the unwanted (R) enantiomer was consumed whileonly 5% of the desired epoxide was converted. This selectivity is similar to theoriginal Jacobsen Co(III)OAc salen catalyst (53% conversion, >99% ee undersolvent-free conditions). The epoxidation rates with 1a and 1b were slightly higherthan the ones using 1d. This observation may seem to go against the hypothesis thatthe reaction involves a bimetallic mechanism for the HKR, however, we suggest thatthe result may be a consequence of the higher backbone flexibility of the copolymersin comparison to the sterically more congested homopolymer. It is clear, however,that the density of the salen complexes in the copolymers is also important, as furtherdilution of the salen-moieties along the polymer backbone via use of 1c resulted in adramatic drop of the activity, with only 43% conversion and 80% ee after fivehours). This result suggests that the extreme dilution of the catalytic moieties alongthe polymer backbone results in decreased reaction rates due to the low probabilityof two complexes being in close proximity, a prerequisite for the bimetallic catalyticpathway.While the poly(norbornene) system represents an easy to manipulate polymericsupport as it can be prepared via a living polymerization and the polymerizationprocess is tolerant to a wide array of functional groups, it also represents a relativelyexpensive way of immobilizing a catalyst. To this end, we pursued poly(styrene)systems as well. The conversions and enantiomeric excesses (ee) of the substrate inthe HKR of rac-epichlorohydrin using the styrene polymers 2b, 2d and thehomogeneous Co salen complex were again monitored by GC analysis. Thecatalytic data are presented in a kinetic plot of ee vs. reaction time in Figure 4. Allthe poly(styrene)-supported catalysts are highly reactive and enantioselective for theHKR of epichlorohydrin. As shown in Table 2, the copolymer-supported catalysts2b and 2c showed the most desired catalytic performances. The remainingepichlorohydrin was determined to have enantiomeric excesses higher than 99%within one hour with a conversion of 54%. In comparison, the homogeneous Co(III)salen catalyst gave 93% ee in 49% conversion under the same reaction conditionsand it took 1.5 h for it to reach >99% ee. It is noteworthy that the copolymersupportedcatalysts 2a-c in general exhibited improved reactivity andenantioselectivity compared with the homopolymer 2d. We attributed thisobservation to the greater complex mobility in the copolymer-bound salen catalysts.Dilution of the salen moieties in the poly(styrene) main-chain might make thecatalytic sites more accessible to the substrate. In addition, the copolymers mighthave more flexible polymer backbones that would increase the possibility of