Silver-Catalyzed Transformation Of Carbon Dioxide Into Carboxylic Acids And Esters

CO2is the main greenhouse gas which causes global warming. It is also a nontoxic, non-flammable, abundant, inexpensive and renewable carbon resource. As the natural environment deteriorating and the supply of fossil resources becoming increasingly tense, the transformation of CO2into value-added fine chemicals through chemical methods has been a strategical research subject for achieving harmonious ecological environment and socio-economic development. Synthesis of carboxylic acids and esters using CO2as the carboxylative reagent has become major topic with regard to the activation and catalytic transformation of CO2.Among the conversion of CO2into carboxylic acids and esters, the classical reaction is the carboxylation with highly reactive organolithium and Grignard reagents. Although these carboxylation reactions can be easily carried out, their functional group compatibility is very poor. The transition metal-catalyzed reactions using CO2as a carboxylative reagent have gained considerable attention due to their higher catalytic activity and broader substrate scopes. Although some main progresses have been made, the reported transition metal-catalyzed chemical reactions usually encountered rigorous reaction conditions. Therefore, the development of the simple and efficient reaction systems for the transformation of CO2into carboxylic acids and esters is of great significance. Using CO2as a carboxylative reagent, this dissertation focuses on the silver-catalyzed carboxylation of terminal alkynes or organoboronic esters for the synthesis of carboxylic acids and esters:1. A new reaction system in which AgI-catalyzed carboxylation of terminal alkynes with CO2in the presence of Cs2CO3to synthesize alkynyl carboxylic acids was built up. A variety of alkyl and aryl terminal alkynes bearing different substituents can be directly converted into various functionalized alkynyl carboxylic acids in good yields under ligand-free and mild reaction condition (2atm CO2,50℃). Primary mechanistic study revealed that terminal alkynes were firstly activated by the silver(Ⅰ) salt and then deprotonated by Cs2CO3to afford the silver(I) acetylide intermediate, in which CO2can rapidly insert into sp-hybridized carbon-silver bond. The method provided an effective approach for the synthesis of alkynyl carboxylic acids. 2. It is further found that AgI/Cs2CO3was also highly effective and selective catalytic system for the carboxylative coupling of a series of terminal alkynes, CO2and active chloride compounds. Various functionalized2-alkynoates were afforded in good yields. Compared with the previously reported (IPr)CuCl/K2CO3catalytic system, the ligand-free silver(I) catalytic system shows greatly enhanced activity and selectivity at much lower catalyst loading.3. AgOAc/PPh3was established as an efficient catalyst system for the carboxylation of organoboronic esters with CO2. Compared with the previously reported rhodium and copper catalyst system using complex ligands, the silver catalyst system using a simple PPhi ligand could catalyze the reaction of a series of arylboronic esters, alkenylboronic esters and heterocyclic boronic esters with CO2to afford the corresponding carboxylic acids in good yields. It was found that in the presence of PPh3and KO’Bu, a new phenylsilver intermediate was firstly formed by the transmetalation reaction of the silver catalyst with an organoboronic ester, then the insertion of CO2into a sp2-hybridized carbon-silver bond realized the carboxylation of organoboronic esters with CO2The above-mentioned silver-catalyzed transformation of CO2into carboxylic acids and esters featured with mild reaction conditions, high efficiency, high selectivity and atom economy. Furthermore, the silver catalyst systems are simple, accessible, relatively cheap and low toxicity, which offer the possibility for their practically industrial application.