Design And Application Of Green Catalysis Systems Based On N-heterocyclic Carbenes Or Magnetite Nanoparticles

Novel strategies which facilitate the separation, recovery and reuse of heterogeneous magnetically recoverable iron oxide nanoparticle catalysts and homogeneous NHC catalysts have been developed. These strategies are inherently environmentally, ecologically benign, and coincident with the principles of green chemistry(1) A novel strategy that facilitates the separation, recovery and reuse of the NHC catalyst IMes in transesterification reactions has been developed. Primary alcohols underwent transesterification reactions with esters in good yields in the presence of homogeneous organocatalyst IMes which was generated in situ from IMes·HCl. The mechanism for this NHC-catalyzed transesterification reaction was discussed.(2) N-heterocyclic carbene catalyst IPr was generated in situ from its CO2 adduct, IPr·CO2 (IPr=1,3-bis-(2,6-diisopropylphenyl) imidazol-2-ylidene). NMR spectroscopy and CS2 trapping reactions provided evidence that thermolysis of IPr-CO2 generated IPr tracelessly. We evaluated the activity of this IPr·CO2 as precatalysts for transesterification reactions and lactide ring-opening polymerization. The scope of this in situ generated IPr catalyzed transesterification reaction was demonstrated and the yield was in good to excellent. The conversion of lactide in the IPr catalyzed stereoselective polymerization was high and the polydispersity of polylactide was low.(3) A robust and magnetically recoverable Fe3O4 nanoparticle catalyzed three-component coupling of aldehyde, alkyne, and amine (A3-coupling) has been demonstrated. A diverse range of propargylamines were obtained in moderate to high yield under mild conditions in air. The separation and reuse of the magnetic Fe3O4 nanoparticles were very simple, effective and economical.(4) A robust, stable, and magnetically recoverable Fe3O4 nanoparticle catalyzed cross-dehydrogenative coupling (CDC) between Csp3-H bond and Csp3-H bond using molecular oxygen as oxidant under mild conditions has been demonstrated. A diverse range of 1,2,3,4-tetrahydroisoquinoline derivatives were obtained in good to excellent yield. Recovery of the catalyst by decantation of the reaction mixture in the presence of an external magnet is easy, effective, simple, and economical. In addition, the use of Fe3O4 nanoparticles as catalysts is also more environmentally friendly and safer than other transition-metal catalysts.(5) An Fe3O4 nanoparticle supported copper (?) pybox catalyst which exhibits excellent reactivity in asymmetric aldehyde-alkyne-amine coupling, yielding products with good enantioselectivity has been developed. The catalyst can be synthesized from readily available starting materials or directly on the solid phase easily and remains intact throughout the reactions. Recovery of the catalyst by decantation of the reaction mixture in the presence of an external magnet is easy and efficient. The catalyst was recycled over six times in reactions without any obvious loss in activity and enantioselectivity. Several optically active propargylamines were obtained in excellent yields.