Toward Understanding the Origin of Positive Effects of Ionic Liquids on Catalysis: Formation of More Reactive Catalysts and Stabilization of Reactive Intermediates and Transition States in Ionic Liquids

Abstract

Over the past decade, ionic liquids have received a great deal of attention as a new means for catalyst immobilization. Large numbers of catalysts having polar or ionic character have been successfully immobilized in ionic liquids, thus allowing their recovery and recycling. However, catalyst immobilization is not the only benefit of ionic liquids in catalysis, of greater importance are the positive effects of ionic liquids on catalytic rates. In this Account, we highlight our work in elucidating the origin of the accelerating effects of ionic liquids in a range of catalytic reactions. Lewis acidic metal triflates often become much more reactive in ionic liquids containing noncoordinating anions as a result of "anion exchange." Consequently, the more electrophilic Lewis acidic species generated in situ accelerate the catalytic reactions dramatically. In some cases, highly reactive intermediates, such as vinyl cations, arenium cations, oxygen radical anions, and so forth, can be stabilized in the presence of ionic liquids, thus increasing the reactivity and selectivity of the reactions. Concerted processes such as S(N)2 and Diels-Alder reactions can also be accelerated through the cooperative activation of both the nucleophile and the electrophile by ionic liquids. In transition metal-catalyzed reactions, certain catalytically active oxidation states can be stabilized in ionic liquids against deactivation to catalytically inactive species. Thus it is clear that gaining an understanding of the origin of these "positive ionic liquid effects" is highly important, not only for predicting the effects of ionic liquids on other organic reactions but also for designing new catalytic reactions. Ionic liquids, by virtue of (typically) having a synthetically accessible carbon backbone, are amenable to tailoring by the organic chemist. Accordingly, their molecular structures can be subtly varied to give "tunable" properties, which can then be used to rationally examine the fundamental reasons that they accelerate catalyzed reactions. Although the origins of enhanced catalytic rates by ionic liquids have been elucidated in many areas, other undiscovered ionic liquid phenomena remain to be unearthed. Developing a better understanding of these modularly tunable liquid salts will foster new discoveries of catalytic reactions that are accelerated by ionic liquids as solvents or additives.

Ionic Liquid Immobilized Organocatalysts for Asymmetric Reactions in Aqueous Media

“…Over the past decades, research in the field of ionic liquid (IL) chemistry has grown phenomenally with the promise of green chemistry, environmental and technological benefits [11][12][13][14][15][16], examples of the main types of ionic liquids are shown in Figure 1. In 1914, Paul Walden reported the first synthesis of room-temperature ionic liquid ethylammonium nitrate [17].…”

Towards Rational Design of Nanoparticle Catalysis in Ionic Liquids

“…The synthetic procedure was straight forward. A hydroxyl-functionalized IL, [C 2 OHmim][Tf 2 N], was chosen as solvent in which both Pd(OAc) 2 and Au(OAc) 3 were dispersed. The decomposition was then conducted at 393 K under vacuum to remove the volatiles generated.…”

“…Stable suspensions of metal NPs can be synthesized in ILs, because the cationic and anionic species act as electrostatic stabilizers. However, NP immobilization and stabilization is not the only benefit of ILs in catalysis, of greater importance are the positive effects of ILs on catalytic rates and selectivity [2]. Indeed, some reactions catalyzed by NPs exhibit unusual, but desired selectivity patterns when performed in ILs, due to the unique microenvironment created by the anions/cations.…”

Ionic Liquids: Applications in Heterocyclic Synthesis

“…After all, the reaction has been accelerated, and the solvent has remained unchanged by the process. In this sense, Lee et al [Lee et al, 2010] proposed some pathway to this role of ionic liquids. The first example shown here is the synthesis of aziridines 3 using ionic liquids from the reaction of imines 1 and EDA (ethyl diazoacetate) 2 (Table 2) [Xia et al, 2003] Table 2, where a catalytic amount (0.1 mmol) of ionic liquid was dissolved in co-organic solvents.…”