Transition Metal/Ionic Liquid Synergistically Activated Carbon Dioxide Conversion Under Atmospheric Pressure

The carbon atom coordinates two atoms of oxygen via covalent bonding forming a well-known molecules as carbon dioxide(CO2).Almost all the living thing on the earth are closely correlation to CO2 molecule,especially humankind.Currently,the CO2 level produced by human activities is up to 5.5 Gt per year,which has surpassed natural carbon cycle capacity,threatening human sustainable development.From the view of chemical synthesis,CO2 is a superior carbon source for its nontoxic,nonflammable,inexpensive,ubiquitous and renewable properties.The utilization CO2 to preparation fine chemicals is a promising green synthetic route.However,the utilization of CO2 remains a challenging due to its highly symmetrical structure and the highest state of carbon atom.Therefore,exploring high-efficiency catalysts and establishing valuable reaction pathways are absolutely urgent to research development.Carbamates and ?-hydroxy ketones are important chemicals in many natural products and synthetic drugs.Moreover,there are widely used in the fields of medicine and organic synthesis.However,the traditional synthesis of carbamates is based on highly toxic phosgene.While the most effective catalytic system for the synthesis of ?-hydroxy ketones is Hg(II)/H2SO4,which has highly toxic and corrosive properties.Herein,we reported two strategies utilized CO2 for the synthesis of carbamates and ?-hydroxy ketones.Further,compared with the traditional methods,these two routes showed the advantages of green,simple steps,high atomic economy and regioselectivity etc.The research contents are including:(1)Firstly,we reported an ionic liquid based Ag Br/1-ethyl-3-methylimidazolium acetate(Ag Br/[C2C1im][OAc])catalytic system for the one-pot synthesis of ?-oxopropylcarbamates through the three-component reaction of propargylic alcohols,secondary amines and CO2 at atmospheric pressure and mild temperature.A series of substrates with various substituent groups could be successfully converted into the target compounds in moderate to excellent yields.Notably,the Ag Br/[C2C1im][OAc] system exhibited robust constancy with low Ag loading(1 mol%)and it could be reused at least five times without any significant loss of catalytic activity.To the best of our knowledge,this is the first reported catalytic system which could be successfully recycled for this three-component coupling reaction.Furthermore,the investigation of reaction mechanism revealed that secondary amines not only as substrates,but also synergistically activated propargylic alcohols and CO2 molecule with OAc-.Notably,further investigation revealed that bis-NHC-Ag complexes existed in our catalytic system which was the key of catalytic system exhibited excellent catalytic activity and recyclability.(2)Based on the above research,silver salt was replaced by low-priced and stable zinc salts.Then we developed the Zn O/[C2C1im][OAc] system for the CO2-promoted hydration of propargylic alcohols.And a variety of terminal and internal propargylic alcohols could be successfully converted into the corresponding ?-hydroxy ketones.This catalytic system also exhibited robust constancy and it could be reused more than 16 times on the premise of yields over than 80%,which is the highest level of all reported catalytic system.Furthermore,the investigation of reaction mechanism revealed that CO2 played a crucial role in this reaction.It not only has an important effect on the formation of product,but also could react with imidazolium cation to form NHC-CO2 adducts,which was the precursor of the bis-NHC-Zn complexes.And the formation of bis-NHC-Zn complexes played a positive role in catalytic activity and recyclability.