Research Article pubs.acs.org/journal/ascecg
Triethylamine Hydroiodide as a Simple Yet Effective Bifunctional Catalyst for CO2 Fixation Reactions with Epoxides under Mild Conditions Yusuke Kumatabara, Megumi Okada, and Seiji Shirakawa* Department of Environmental Science, Graduate School of Fisheries and Environmental Sciences, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan S Supporting Information *
ABSTRACT: An efficient synthesis of cyclic carbonates from epoxides and CO2 under mild reaction conditions was achieved via the use of triethylamine hydroiodide as a simple yet effective bifunctional catalyst. The importance of the bifunctional feature of the catalyst was clearly demonstrated in the present work via control experiments and 1H NMR studies. The scalability of this catalytic system was also demonstrated.
KEYWORDS: CO2 fixation, Bifunctional catalysts, Ammonium salts, Epoxides, Cyclic carbonates
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INTRODUCTION Carbon dioxide (CO2) is considered an undesired industrial waste due to the greenhouse effect, and reduction of CO2 emissions is indispensable for the creation of a sustainable society.1−3 However, from a different point of view, CO2 is recognized as an ideal C1 feedstock because it is abundant, nontoxic, and inexpensive. On the basis of this perspective, the ability to transform CO2 into useful compounds has drawn attention in the field of green and sustainable chemistry.4−9 Among these transformations, the synthesis of cyclic carbonates 2 from epoxides 1 and CO2 has been studied most extensively due to their high utility in industrial processes.10−17 Cyclic carbonates 2 are widely used in industry as green solvents, electrolysis for lithium ion batteries, monomers for polymer synthesis, and intermediates for fine chemicals and pharmaceuticals.3−18 A wide variety of metal catalysts and organocatalysts have been developed to promote the fixation of CO2 using epoxides 1, but high temperature (>100 °C) and pressure (>10 atm) are often required to produce cyclic carbonates 2 in a satisfactory yield. Therefore, the development of a catalyst that can be effective for this reaction under mild conditions is an attractive goal.19−26 To achieve this desirable task, bifunctional quaternary ammonium iodide catalysts possessing a hydrogen-bonding donor group, such as hydroxy or carboxy groups, have recently been developed (Scheme 1).27−38 These bifunctional quaternary ammonium iodides efficiently promote CO2 fixation with epoxides 1 under relatively mild conditions, and the importance of the activation of epoxides 1 by the hydrogen-bonding donor group was clearly demonstrated in these reports. The design of these effective bifunctional quaternary ammonium salt catalysts prompted our interest in © 2017 American Chemical Society
Scheme 1. CO2 Fixation with Epoxides 1 Catalyzed by Bifunctional Ammonium Iodide Catalysts
the catalytic ability of triethylamine hydroiodide as a simple, readily available, and economical catalyst.39−43 The hydrogen on the nitrogen atom of triethylamine hydroiodide is known to possess sufficient acidity to activate epoxides 1 (see pKa in Scheme 1),44 which is followed by an iodide anion nucleophile Received: May 16, 2017 Revised: June 2, 2017 Published: June 13, 2017 7295
DOI: 10.1021/acssuschemeng.7b01535 ACS Sustainable Chem. Eng. 2017, 5, 7295−7301
Research Article
ACS Sustainable Chemistry & Engineering
known to be effective catalysts for this reaction,45−49 were also compared with trialkylamine hydroiodides. Tetraethylammonium and tetrabutylammonium iodides showed low reactivities under the mild reaction conditions (
