Stable, Reusable Copper‐Bispidine Catalysts on Agarose With Diffusion‐Limited Turnover: When Immobilization is not Covalent

In this study, we investigated whether copper(II)-bispidine complexes can be heterogenized on epoxy-functionalized agarose beads to provide reusable heterogeneous catalysts. The ligands employed are rigid N-donor systems based on the 3,7-‍diazabi‍cyclo[3.3.1]nonane scaffold, and the corresponding copper complexes are known to catalyze the aziridination of styrene. To enable covalent attachment to the epoxy groups of the beads, hydroxy-functionalized linkers were introduced into the ligand framework. However, immobilization was found to be dominated by noncovalent interactions. Nevertheless, only minimal copper leaching was observed, and the resulting immobilisates were reused over six consecutive reaction cycles. At high catalyst loadings, comparable yields of 17%–21% were obtained for the different complexes, indicating restricted active-site accessibility within the support. Lowering the catalyst loading markedly improved catalytic performance, resulting in average yields of approximately 40% over six cycles. Under these conditions, immobilization could even enhance the performance of otherwise slowly reacting complexes compared with homogeneous catalysis under air, possibly by restricting oxygen access to the active Cu$^I$ species and alleviating mass-transport limitations associated with high catalyst loadings in heterogeneous systems. ... mehrOverall, the results demonstrate that catalyst loading, active-site accessibility, and transport within the agarose matrix are key factors governing the performance of immobilized copper(II)-bispidine catalysts.