Preparation Of Hollow Organic Nanospheres-supported Rovis Carbenes And Its Application In Asymmetrical Cascade Reactions

As an organic catalyst,N-heterocyclic carbene has special application in organic catalytic reactions due to its special catalytic mode of polarity reversal.Versatile Rovis N-heterocyclic carbene is one of the most widely used catalysts in N-heterocyclic carb-ene catalysts.However,due to the huge used catalyst amount?20-30 mol%?,severe ca-talytic environment and difficulty in the effective reuse of catalyst,its application in chemical industry is severely restricted.Resulted from the thin shell,large specific sur-face area,high pore volume and hollow structure,hollow polymeric nanospheres are ex-pected to be a good heterogeneous catalyst carrier to improve mass transfer efficiency.In this paper,a simple and effective hard template method was used to prepare ho-llow organic polymeric nanospheres Poly?St-co-EGDMA-co-Rovis carbene?HNs-12'with many merits such as a diameter of 345 nm,shell thickness of 30 nm and high loaded Rovis N-heterocyclic carbene?0.32 mmol/g,88%of utilization?.By optimizing preparation conditions such as feed ratio and solvent,the morphology-controlled and N-heterocyclic carbine-highly loaded Poly?St-co-EGDMA-co-Rovis carbene?HNs-12'was fabricated.The comparative characterization by N₂ adsorption-desorption,SEM,IR and elemental analysis showed that the hollow nanospheres Poly?St-co-EGDMA-co-Rovis carbene?HNs-12'displayed the larger specific surface area and higher pore vo-lume than Poly?St-co-DVB-co-Rovis carbene?NP-1'without hollow structure,posses-sed mesoporous shell and controllable nanosphere morphology.Under the same optimal catalytic reaction conditions,the two supported Rovis carbene with different structures and morphologies were applied in the Michael/Stetter cascade of salicylaldehyde with diethyl acetyl acetylate.It was found that the hollow mesoporous Poly?St-co-EGDMA-co-Rovis carbene?HNs-12'have the better yields?61–78%?and stereoselectivities?86–95%ee?.The reaction kinetic model was established by monitoring the whole process using HPLC to explore the reasons for the difference in the catalytic performances,which elucidated that the efficiency of mass transfer and exposure rate of catalytic sites played important roles in catalytic performances.The hollow nanosphere Poly?St-co-EGDMA-co-Rovis carbene?HNs-12'can be recovered by simple centrifugation.In the catalytic recycles,the catalytic activity and stereoselectivity of the 4^th-reused catalyst remained unchanged in the Michael/Stetter cascade of salicylaldehyde with diethyl ace-tyl acetylate.