Ionic-Liquid-Catalyzed Approaches under Metal-Free Conditions

Yanfei Zhao; Buxing Han; Zhimin Liu

doi:10.1021/acs.accounts.1c00251

Ionic-Liquid-Catalyzed Approaches under Metal-Free Conditions

Acc Chem Res. 2021 Jul 30. doi: 10.1021/acs.accounts.1c00251. Online ahead of print.

Authors

Yanfei Zhao^{1

2}, Buxing Han^{1

2}, Zhimin Liu^{1

2}

Affiliations

¹ Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid, Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
² University of Chinese Academy of Sciences, Beijing 100049, China.

PMID: 34327980
DOI: 10.1021/acs.accounts.1c00251

Abstract

ConspectusMetal-free catalysis is a promising protocol to access chemicals without metal contamination. Ionic liquids (ILs) that are entirely composed of organic cations and inorganic/organic anions have emerged as promising alternatives to molecular solvents and metal catalysts due to their unique properties such as structural tunability, the coexistence of multiple interactions among ions (e.g., electrostatic interaction, hydrogen bonding, van de Waals forces, acid/base interactions, hydrophilic/hydrophobic interactions, etc.), unique affinity for a wide range of chemicals, good chemical and thermal stability, and quite low volatility. ILs have shown potential applications in various chemical processes.In this Account, we systematically described our most recent work on IL-catalyzed approaches under metal-free conditions. The first section presents the IL-catalyzed strategies toward the transformation of CO₂ to value-added chemicals, focusing on the CO₂-reactive IL-catalyzed CO₂ transformation to various heterocycles and the IL-catalyzed reductive transformation of CO₂ to chemicals. In these approaches, we designed task-specific ILs that are able to chemically capture and activate CO₂ via forming anion-based carbonate/carbamate or cation-based carboxylate/carbamate intermediates, thus further accomplishing its transformation to a series of heterocycles including quinazoline-2,4(1H,3H)-diones, cyclic carbonates, 2-oxazolidinones, oxazolones, and benzimidazolones under metal-free conditions. For the IL-catalyzed approaches to reducing CO₂ with hydrosilanes to chemicals, we employed ILs capable of activating the Si-H bond in hydrosilanes and the N-H bond in amine substrates via H-bonding, thus achieving the reductive transformation of CO₂ to formamides, benzimidazoles, and benzothiazoles via cooperative catalysis. The second section describes our finding on the IL-catalyzed hydration of the C≡C bond in propargylic alcohols. Azolate anion-based ILs that can chemically capture CO₂ via the formation of carbamates could serve as robust nucleophiles to attack the C≡C bond in propargylic alcohols and then efficiently catalyze the hydration of propargylic alcohols to produce α-hydroxy ketones with the assistance of atmospheric CO₂ gas under metal-free conditions. The third section unveils the cooperative catalysis strategy of hydrogen bond donors and acceptors of ILs for chemical reactions. In the hydrogen-bonding catalysis protocols, cations of the ILs act as H-bond donors and anions, as acceptors, forming H-bonds with the reactant molecules, respectively, in opposite ways, which can cooperatively catalyze the ring-closing C-O/C-O bond metathesis reactions of aliphatic diethers to O-heterocycles, the dehydrative etherification of alcohols to ethers, and direct oxidative esterification of alcohols to esters. We believe that these IL-catalyzed metal-free processes and strategies display promising practical applications, and their commercialization would bring great benefits to the production of the as-afforded value-added chemicals.