Dirhodium complex immobilization on modified cellulose for highly selective heterogeneous cyclopropanation reactions

Abstract

A novel, efficient approach for the functionalization of microcrystalline cellulose (MCC) is presented. The as-obtained material allows the immobilization of chiral dirhodium catalysts preserving their enantioselectivity in asymmetric cyclopropanation reactions. As model, microcrystalline cellulose is modified with a polyethylene glycol derived linker, and Rh2(S-DOSP)4 is grafted on the material to produce a heterogeneous catalyst. SEM images at different stages of the immobilization show an unchanging uniform morphology, providing constantly good separation characteristics. The modification of the cellulose material with the polyethylene derived linker and the immobilization process are monitored using DNP enhanced 1H → 13C CP MAS NMR, quantitative 19F MAS NMR, TGA and ICP-OES analysis, confirming the success of the immobilization as well as the stability of bonds between the used linker molecule and the cellulose material. Finally, the evaluation of the produced catalyst is demonstrated in the asymmetric cyclopropanation reaction between styrene and methyl(E)-2-diazo-4-phenylbut-3-enoate showing excellent enantioselectivity with an ee of nearly 90% over a wide temperature range as well as good recyclability characteristics in four consecutive catalysis cycles.