Chemical-promoted oxidative polymerization of modified-cobalt salen complexes, efficient catalysts for the dynamic HKR

Abstract

FeCl₃ and NOBF₄ have been successfully tested as chemical oxidants to promote the polymerization of thiophene- or pyrrole-modified chiral cobalt salen complexes. The iron oxidation procedure particularly delivered an insoluble polymeric species in nearly quantitative yield that was tested for its propensity to promote the dynamic hydrolytic kinetic resolution of epibromohydrin. This catalyst allowed the synthesis of the expected diol in high yield and up to 85% ee. As an insoluble powder, it was either efficiently recycled in a batch reactor via simple filtration or dispersed in a silica support and reused in a flow procedure for up to five cycles. A copolymerization strategy delivered an even more reliable polymerized complex that was repetitively used without showing any loss in activity or in enantioselectivity for five reuses in this transformation.