Acidic Ionic Liquids as Metal-Free and Recyclable Catalysts for Direct Reduction of Aromatic Allylic Alcohol in Dimethyl Carbonate via Hydrogen Transfer

Abstract

Direct reduction of alcohols, to avoid tedious reaction steps, has drawn broad attention. However, the transformation was mainly catalyzed by transition metals or Lewis acids. Developing a metal-free and recyclable catalytic system is still a challenging task from the viewpoint of green chemistry. Herein, pyrrolidine-based ionic liquids as metal-free and recyclable catalysts for direct reduction of aromatic allylic alcohol were developed using p-methylbenzyl alcohol as the reducing agent in dimethyl carbonate (DMC). DMC not only acted as the green solvent but also could stabilize the reaction intermediates and improve the reactivities. Aromatic allylic alcohols with electron-donating or -withdrawing groups could provide up to 92% yield. The gram-scale experiment was performed smoothly, and the recycle experiment of the ionic liquids could be achieved through simple steps, which was significant for environmental protection and efficient utilization of resources. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy proved that ionic liquids interacted with the substrates and the reducing agent by forming multiple hydrogen bonding interactions. Kinetic isotope effect (KIE) experiments (kH/kD = 2.27) indicated that the C–H bond cleavage might be a rate-determining step, and deuterium-labeled results demonstrated that the hydrogen of the product was derived from the benzylic H of p-methylbenzyl alcohol. Meanwhile, the experiments of two preprepared ether intermediates as the substrates showed that they may be involved during the reaction, and a possible mechanism containing intermediates, hydrogen transfer, and hydrogen bonding effects was proposed.