Multicomponent cascade strategy of CO2 fixation for synthesis of dimethyl carbonate and α-hydroxy ketone

Much attention was paid to carbon dioxide (CO2) utilized as green C-1 building block in organic synthesis due to its good features such as abundance, nontoxicity and renewability. However. the efficiency of many CO2-involved transformations is very low due to the thermodynamic limitation, and the trouble hinders the industrial process. Dimethyl carbonate (DMC), being labeled as a green chemical, is widely applied in fuel additives, lithium battery electrolyte, and substitutes carbon monoxide and phosgene as carbonyl source in organic synthesis. Traditional routes to DMC were "two-step" indirectcd methods including urea/diols-methanol and epoxide/CO2-methanol protocols through the cyclic carbonate intermediate. Nevertheless, the processes are complicated. During the past decade years, more and more attention was paid to direct synthesis of DMC from CO2 and methanol because of its very simple process and easily available raw materials. However, the efficiency is very low due to the thermodynamic limitation. The removal of water generated in the process can effectively promote the reaction and increase the yield of DMC. Generally, the use of at least quantitative dehydrating agent was chosen along with the produce of much waste, and meanwhile side reactions was easily introduced which reduces the DMC selectivity. Till now. the field still faces great challenge. To solve the thermodynamic problem in the reaction of CO2 and methanol, here, a novel favorable multicomponent cascade strategy namely three-component reaction of propargylic alcohols, methanol and CO2 for the synthesis of DMC was proposed. In this scheme, the potential "water molecule" was transferred into propargylic alcohol with the avoidance of dehydration process. In the same time, the concurrent alpha-hydroxyl ketone traditionally synthsized through the hydration reaction of propargylic alcohol was also obtained. Herein, an efficient and atom-economical Zn(II)-catalyzed three-component reaction was developed, in which CO2 was used as a dicarbonylation reagent with 100% atom economy. The influence of various parameters including metal species, co-catalysts, temperature, pressure, raw material ratio and solvents was carefully examined. Because the coupling reaction involved several steps in different types, the comprehensive effect was also displayed. After the detailed study, 37.3% DMC and 75.7% alpha-hydroxyl ketone were obtained under the optimal conditions (ZnI2 20mo1%, DBU 50mo1%, DMF as solvent, 6 MPa CO2, 140 degrees C, 10 h). Moreover. wide propargylic alcohol scope of the three-component reaction was demonstrated. In this work, the stable intermediate including alpha-methylene cyclic carbonate and chain carbonate was carefully analyzed and quantified. Finally, the plausible catalytic reaction mechanism was proposed. Based on the literature reports and our work, the compatible functional metal catalyst being able to effectively activate CO2 and alcohol substrate was concluded. This project establishes robust catalytic schemes of CO2 conversion and initiates new routes to dimethyl carbonate and alpha-hydroxyl ketones. These theories will provide new thoughts for the further study on CO2 utilization.