Highly efficient heterogeneous polymer-supported Sharpless alkene epoxidation catalysts

Abstract

More widespread, and cost effective, use of soluble transition metal complex asymmetric catalysts would be achieved if highly efficient heterogeneous analogues could be developed. We now report the synthesis of branched/crosslinked poly(tartrate ester)s and their use as heterogeneous ligands for the complexation of Ti(O iPr) 4, and use of the resulting insoluble catalysts in the asymmetric epoxidation of a range of trans-allylic alcohols employing t-butylhydroperoxide ( tBHP) as the oxidant. Isolated yields of epoxides of up to 80%, with enantiomeric excesses up to 98% have been achieved. Removal and recovery of the polymer catalyst is by simple filtration at the end of reactions. The influence of the ratio of polymer-ligand–titanium, and the polymer backbone molecular architecture on the activity and enantioselectivity of the catalysts has been assessed. Typically polymer branching ratios of 3–11% and a polymer-ligand:titanium ratio of 2:1 yield the optimum results.