Choline-based eutectic mixtures as catalysts for effective synthesis of cyclic carbonates from epoxides and CO2

Martina Vagnoni,Chiara Samorì,Paola Galletti

doi:10.1016/j.jcou.2020.101302

Abstract

• Choline salts have been mixed with hydrogen bond donors to form eutectic mixtures. • Choline-based eutectic mixtures catalyze CO 2 conversion to cyclic carbonates. • Carbonatation run in mild condition of temperature (80 °C) and pressure (0.1–0.4 MPa). • Good to very good yields are obtained on various terminal epoxides. • The eutectic catalysts are bio-based, non-toxic, recyclable and easily synthesized. CO 2 is a renewable, abundant and cheap C1-feedstock and its conversion to cyclic carbonates starting from epoxides has been widely explored in the last years. Nevertheless, conducting this reaction under mild and sustainable conditions is still a challenging task. Herein we present the use of choline-salt based eutectic mixtures as catalysts for the reaction of CO 2 with epoxides to give cyclic carbonates. Choline chloride and choline iodide have been coupled with various hydrogen bond donors (HBDs), mainly cheap and bio-based carboxylic acids and polyols, to form two classes of eutectic mixtures. Very good yields were achieved under mild conditions (80 °C in 7–22 h) for various terminal epoxides, with both classes of catalysts. While a pressure of 0.4 MPa of CO 2 is required to obtain appreciable conversions using choline chloride-based mixture, atmospheric pressure of CO 2 (balloon) has been successfully used with choline iodide-based mixtures. Furthermore, the catalysts could be recycled without appreciable loss of the catalytic activity. The improved catalytic performance of both choline-based eutectic mixtures is attributed to the synergistic activity of the halide, responsible for the opening of the epoxy-ring, and the HBD that has a role in the stabilization of the alkoxide intermediate.

Highlights

The possibility to capture and reuse CO2 is a source of inspiration for numerous research studies [1,2]
CO2 con version to cyclic carbonates starting from epoxides has attracted a lot of attention as an alternative pathway to the use of phosgene [4] and cy anates, and a very large number of catalysts and conditions have been proposed to carry out this reaction [3,5,6]; among them, methods that involve the use of DBU-based organocatalysts, [7,8] organometallic catalysts, [9,10,11,12,13] alkali metal salts, [10] metaloxides [14] and phos phonium salts [15] have been recently reported
Büttner et al showed that choline halides are ineffective for the synthesis of cyclic carbonates in solventless conditions (2 h, 90 ◦C, 1 MPa) and an elonga tion of the alkyl chains of the ammonium ion is required for reaching good conversions [39]

Summary

Introduction

The possibility to capture and reuse CO2 is a source of inspiration for numerous research studies [1,2]. CO2 con version to cyclic carbonates starting from epoxides has attracted a lot of attention as an alternative pathway to the use of phosgene [4] and cy anates, and a very large number of catalysts and conditions have been proposed to carry out this reaction [3,5,6]; among them, methods that involve the use of DBU-based organocatalysts, [7,8] organometallic catalysts, [9,10,11,12,13] alkali metal salts, [10] metaloxides [14] and phos phonium salts [15] have been recently reported The challenge for this kind of reaction is to develop methods that use CO2 at low pressure and temperature in sustainable solvents or under solventless conditions [16]; several good results have been achieved at room temperature (rt) or with CO2 atmospheric pressure, especially in the field of organocatalysis [17,18,19,20], and using ionic liquid-type catalysts [21,22,23,24,25,26,27]. Amaral et al obtained good results using Choline Iodide (ChI) when ethanol was used as solvent (6 h, 85 ◦C, 1 MPa) [45]

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