Experimental and theoretical evidences of the influence of hydrogen bonding on the catalytic activity of a series of 2-hydroxy substituted quaternary ammonium salts in the styrene oxide/CO2 coupling reaction

Add time:09/07/2019    Source:sciencedirect.com

Among industrial applications of CO2, the formation of cyclic carbonates from epoxides offers relevant chemical potentialities. Here, various N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium salts (HEA16X) associated with different counter-anions (X−) have been investigated as catalysts of the cycloaddition of carbon dioxide to styrene oxide. Most of these salts were characterized by reasonable catalytic activities in benzonitrile at 120 °C and under 15 bar of CO2 compared to choline chloride and cetyltrimethylammonium bromide. The influence of three parameters has been studied: (i) the polar head group, (ii) the anion (I−, Br−, Cl− for halogens and HCO3−, mesylate (Ms−), triflate (Tf−)) in the HEA16X series, as well as (iii) the chain length.The higher conversion and selectivity obtained with HEA16Br, compared to CTABr, pointed out the beneficial contribution of the hydroxylated polar head. The role of the compensation anion or more exactly of its nucleophilicity on the catalyst activity was also shown. However, unexpected results with HCO3−, affording styrene oxide conversions up to 95% (better than with HEA16Cl), were also obtained.DFT calculations supported the beneficial influence of the 2-hydroxyethyl substituent and the unexpected reactivity of the ammonium hydrogenocarbonate, HEA16HCO3. The reaction profiles allowed to draw some correlations (i) between styrene oxide conversion and the relative energy of the first intermediate and (ii) between the styrene carbonate selectivity and the energy of the first transition state. A new and original mechanism was also proposed for the reaction catalyzed by ammonium hydrogenocarbonate.

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