Catalysis of Organic..

8Immobilized Metal Complex Catalystsintramolecular cooperation between cobalt catalytic sites. Clarification of the role ofpolymer structure on the observed (improved) catalytic properties continues to be asubject of ongoing investigation.10080ee / %6040200♦ homogeneousΔ 2d2b0 30 60 90 120Reaction Time / minFigure 4 Kinetic plot of the HKR of rac-epichlorohydrin using the homogeneousand poly(styrene) supported Co(III) salen catalysts.A key motivation for developing immobilized metal complexes lies in theirpotential for facile recovery and reuse in subsequent reactions. The recycling of thecopolymer-bound Co(salen) complex 2b (Table 2) was studied by precipitation ofthe catalyst after the HKR of epichlorohydrin by the addition of diethyl ether. Theprecipitated catalyst was reactivated with acetic acid and then reused under identicalconditions to the first run. Whereas the enantioselectivity of the reused catalystessentially did not change after four cycles, the catalytic reactivity decreasedgradually. To evaluate whether the deactivation was due to leaching of catalyst,more racemic epichlorohydrin and water were charged into the pale yellowishorganic phase from the workup of the first catalytic run of precatalyst 2b. About 4%additional epichlorohydrin was consumed in sixty minutes. Further controlexperiments showed that no background reaction was detected in the absence of thecatalyst or in the presence of the non-metallated salen polymer. These resultsindicated that, at least in part, the loss of catalysts during workup was responsible forthe observed deactivation on recycle (45). It is not clear what the role of otherfactors such as potential morphological changes of the polymers have on the long