Ionic Liquids-catalyzed Reductive Functionalization Of Carbon Dioxide With Amines

Due to its abundant,nontoxic and renewable characteristics,CO₂ as C₁ resource has been applied in the field of synthetic chemistry to produce high value-added energy-related products and chemicals.The strategy of reductive functionalization of CO₂ has attracted great attention since it was proposed in 2012.On the one hand,CO₂can be reduced to the levels of formic acid,formaldehyde and methanol through 2-,4-and 6-electron reduction,respetively;on the other hand,the functionalization could construct new C–N/C–O/C–C bonds.As a result,choosing different nucleophilic reagents and controling CO₂ to different reduction levels will result in the formation of various products,which greatly broadens the potential application range of CO₂and is an attractive new method.ILs?ionic liquids?are ideal green catalysts and solvents,because of their advantages like stable,environmentally friendly,recyclable and designable property.Therefore,designing specifically functionalized ILs for the chemical transformation of CO₂ is considered as a green and sustainable strategy.In this paper,a series of task-specific ILs were designed and prepared for selectively 2-,4-and 6-electron reductive functionalization of CO₂ to obtain formamide,aminal and methylamine products,respectively.The main contents are divided as follows:We have designed and prepared the task-specific ILs such as imidazolium-based and DBU-based ILs and so on for exploring the N-formylation of secondary amines withCO₂.Itwasfoundthat[DBUC₁₂]Br?1-dodecyl-2,3,4,5,7,8,9,10-octahydropyrido[1,2-a][1,3]diazepin-1-ium bromide?showed high efficiency to afford formamides with up to 99%yield employing PhSiH₃as the reductant,under the conditions of 0.5 MPa CO₂,room temperature and without any organic solvents or additives.Moreover,the catalyst could be easily recycled over5 times without significant loss of its activity.Notably,DFT?Density Functional Theory?calculations and control experiments suggest the activation of hydrosilane is crucial in comparison with the substrate amine activation.And we found that bromide anion of ILs could activate hydrosilane but the cations with active sites may inhibit the nucleophilicity of anion.The acetate-based ILs e.g.[ⁿBu₄N]OAc?tetra-n-butylammonium acetate?was successfully developed as a highly efficient catalyst for reductive functionalization of CO₂ with secondary or primary amines and phenylsilane to selectively afford aminals and methylamines with up to 99%yield.Through subtly tuning the CO₂ amount,the challenging C⁰ species aminals were obtained exclusively under 1 equivalent CO₂ per amine;increasing CO₂ amount to 20 equivalents,aminals will be reduced to methylamines i.e.6-electron reductive products of CO₂ with amines.This is the first IL catalysis for 4-and 6-electron reduction of CO₂ with amines.Notably,PMHS?polymethylhydrosiloxane?,a cheap,byproduct of the silicone industry,also worked well as a reductant for the[ⁿBu₄N]OAc-catalyzed CO₂ 4-electron reductive functionalization to aminals in the presence of various secondary amines,and we therefore realized upgrading usage of carbon and silicon wastes to valuable chemicals via ILs catalysis.Furthermore,DFT computations were also performed thoroughly,and an alternative pathway was rationally proposed and also expounded the essence of being kinetically favorable with 4-electron reduction and thermodynamically favorable with 6-electron reduction.