Synthesis and immobilization of triazolium based ionic liquids as recyclable organocatalysts for the transfer hydrogenation of ketones.
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The aim of the study was to bridge the gap between homogeneous and heterogeneous catalysis by combining the advantages offered by single phase homogeneous catalysts with the ease of separation of heterogeneous catalysts while minimizing the disadvantages of both systems. A common method used is catalyst immobilization on solid supports. Hence, triazoles and corresponding salts have been synthesized by adopting the versatile, green and regioselective Cu(Ι) catalyzed cycloaddition reaction of organic azides and terminal alkynes. The triazolium salts were characterized by various spectroscopic techniques and then tested for activity in the transfer hydrogenation of ketones. Selected triazoles were then immobilized onto straight chain polyethylene glycols (PEGs) of various chain lengths. The immobilization was achieved by the tosylation of PEGs to yield PEG ditosylates followed by N-alkylation which created binding O-N covalent bonds. Characterization by NMR and mass spectrometry confirmed successful immobilization. Salt metathesis was done using sodium iodide in dichloromethane that yielded brick red colored ionic liquids. The formed ionic liquids were then tested for activity in the transfer hydrogenation of acetophenone with isopropanol as a hydrogen donor and solvent. The effects of polymer chain length, electronic effect of the triazolium moiety, reaction temperature and time on the catalysis were investigated. Results indicate that the immobilized triazoles bearing tosylate counter ions were inactive as catalysts. However, the results indicate that for optimum reactivity, PEG600 was the ideal chain length for the immobilized systems. The ionic liquids were then tested for transfer hydrogenation of different substituted ketones. The catalyst system was easily recovered by the addition of diethyl ether after the reaction followed by simple decantation. The immobilized catalysts were recycled three times with a percentage conversion of 82% being observed in the third cycle.