Over the past decade, ionic liquids (ILs) have received great deal of attention as possible “green” replacement for volatile organic solvent mainly due to their nonmeasurable vapor pressure and good dissolubility for other salts. [1-5] Reaction types successfully performed in ILs include Diels–Alder,  Friedel–Crafts,  olefin hydrogenation,  hydroformylation,  ,  oligomerization,  and Heck and Suzuki coupling reactions.  ,  In addition to solvent, ILs may have multiple functions in catalytic reactions. They may act as catalyst, co-catalyst, support, or ligands for the catalytic process.  In particular, some “unexpected” effects have been observed in affecting the catalytic reaction pathway. For example, the cations/anions in ILs may be involved in the formation of the active species changing the reaction mechanism.  -  Understanding the functions of ILs in the catalytic reaction is of critical importance for deliberately modifying existed reaction system and exploiting new types of synthetic route by using this “green” solvent.
Catalytic oxidation is a class of commercially important reaction. As an environmentally benign oxidant, hydrogen peroxide (H2O2) has been used for several catalytic oxidation. So far, significant improvements on the catalytic performance, in terms of yield and selectivity, have been observed using ILs as the solvent for the H2O2 oxidation reaction.  -  Actually, in many cases, ILs are active participant because the formation of radical species, stabilization of the charged reactive intermediate, and immobilization of the actual catalyst can be strongly affected by the presence of an ionic environment. In comparison with traditional organic solvent, the use of ILs in catalytic oxidation has been regarded as a new means for recycling the catalyst and enhancing the yield and selectivity of the product. Though a great number of catalytic oxidation have been performed in ILs, there are still rare examples which demonstrate how the ILs affect the reaction pathway and the reactivity.
This chapter aims at summarizing the examples that concern the H2O2 oxidation reactions in ILs, in particular the benzene hydroxylation, alcohol oxidation, and olefin oxidation. The effects of ILs on the reaction pathway and the selectivity are discussed, drawing to the conclusion that ILs are offering unique properties as solvent by recycling the catalyst and enhancing the yield and selectivity of the product. What should be pointed out is that the examples are limited as far as possible to those that inform the readers’ understanding of the role of ILs in the H2O2 oxidation reaction. We apologize that some fine work is not covered, and we hope to stimulate more discussions in the future.
2. Benzene hydroxylation
Direct hydroxylation of benzene to phenol with H2O2 has been extensively investigated owing to the reduced reaction steps and environmentally benign byproduct of water when comparing to the commercial cumene process for phenol production. One of the fundamental targets in this intriguing investigation is to enhance the utilization efficiency of H2O2 and the selectivity of phenol. The low efficiency of H2O2 always derives from the fast decomposition of H2O2, and the low product selectivity is mainly originated from the over-oxidation of phenol. Studies have shown that solvents used in the hydroxylation play an important role on enhancing both the H2O2 efficiency and the product selectivity. For example, water was the solvent in the traditional Fenton’s reagent (FeII-H2O2) catalyzed hydroxylation,  whereas, the decomposition of H2O2 was very fast.  The selectivity to phenol was rather poor in the aqueous solution since phenol is more reactive toward oxidation than benzene itself. Acetonitrile and acetic acid were then used as the solvents for most of the catalyzed hydroxylation of benzene,  -  and a biphasic water-acetonitrile (1:1) system was developed to decrease the over-oxidation of phenol.  In Bianchi
In addition to organic solvents, Peng
In our work,  ,  a benzene–triethylammonium acetate ([Et3NH] [CH3COO]) IL biphasic system was constructed for the benzene hydroxylation (Figure 2). The Fenton-like reagent (FeIII–H2O2) existed in the IL phase and most of the phenol was extracted to the benzene layer. The [Et3NH] [CH3COO] IL was found to be stable in the water- and oxygen-rich environment. Benzene acted as both the substrate and the extractant in the hydroxylation reaction. In comparison with the aqueous-IL biphasic system, the continuous extraction of phenol by benzene from [Et3NH] [CH3COO] IL protected phenol from further oxidation by directly avoiding the contact of phenol with the catalyst and oxidant. As a result, moderate yield (20%, based on benzene converted, excluding evaporated) and high selectivity (> 99.5%) of phenol were obtained in the IL-benzene biphasic system.
Moreover, the [Et3NH] [CH3COO] IL exhibited retardation performance for the decomposition of H2O2 and protection performance for the over-oxidation of phenol. From a molecular aspect, the CH3COO- anions of [Et3NH] [CH3COO] IL were found to be coordinated with the Fe ions, forming Fe complexes,
In the benzene hydroxylation, both hydrophobic and hydrophilic ILs have shown the feasibility of acting as solvent for enhancing both the yield and selectivity of phenol when compared with that in aqueous solution (
3. Alcohol oxidation
The partial oxidation of alcohols to aldehydes, or secondary alcohols to corresponding ketones is a fundamental synthetic transformation in organic chemistry and is industrially important.  -  However, this transformation always suffers from drawbacks such as poor conversion and selectivity due to over-oxidation. Stable-free nitroxyl radicals such as TEMPO(2,2,6,6-tetramethylpiperidine-1-oxyl) has recently emerged as a catalyst or co-catalyst to promote the formation of the catalytically active species for selective oxidation of alcohols to aldehydes or ketones where volatile organic solvents such as CH2Cl2 are frequently used.  -  However, the recycling of the quite expensive TEMPO is problematic due to the homogeneous character of the classic organic media. Replacement of organic solvents with ILs or immobilization of TEMPO on ILs provide alternative strategies for solving the above-mentioned problems. On the one hand, TEMPO can be anchored on ILs allowing the recycling of the catalyst; on the other hand, ILs provide advantages for increasing the selectivity of the product by promoting oxidation of alcohols to aldehydes but suppressing over-oxidation of these aldehydes to acids.
The strategy of anchoring TEMPO on ILs has also been applied in other catalytic oxidation reaction. In Fall
In addition to TEMPO, the combination of various kinds of catalysts (
In this part, the anchoring of the actual catalyst on ILs offers opportunity for immobilizing catalyst with the solvent, allowing the recycling of the catalyst, especially some expensive reagent. This “anchoring” can be either chemical coupling or physical confinement. Chemical coupling requires special tailoring or functionalization of the ILs. The functionalized ILs show prospect as both catalyst and solvent for the H2O2 oxidation reaction. The physical confinement of ILs within some solid porous materials has dual effects on the catalytic oxidation: on the one hand, the ILs supply special microenvironment on affecting the reaction pathway; on the other hand, the micropores of the porous material allow the controlling of the selectivity of the product. In addition, we may expect structural modification of the functionalized ILs by deliberately varying the cations/anions to meet the distribution requirement of the substrate or product, which will be of great importance for increasing the yield and selectivity of the product. Beyond that, more synthetic method should be developed to support the actual catalyst in order to shed light on the effective utilization of the expensive reagent in future catalytic oxidation.
4. Olefin oxidation
Recently, significant improvements on the catalytic performance in some transition metal-catalyzed reactions have been observed using ILs as the solvent.  ,  ,  The room-temperature ILs have emerged as environmentally benign reaction media as well as new vehicles for the immobilization of transition metal-based catalysts. Singh
Han and coworkers  synthesized novel Ni2+-containing 1-methyl-3- [(triethoxysilyl)propyl] imidazolium chloride (TMICl) IL immobilized on silica to catalytic oxidation of styrene to benzaldehyde with H2O2 under solvent-free condition (Figure 9). With the aid of the IL, both hydrophobic reactant and the hydrophilic reactant were accessible to the active sites of the catalyst: styrene and H2O2 are miscible with the IL, and the Ni2+ was coordinated by the immobilized IL that allowed both reactants to access to active sites of the catalyst effectively. Under solvent-free condition, the conversion of styrene reached 18.5%and the selectivity to benzaldehyde was as high as 95.9% on the IMM-TMICl-Ni2+ catalyst.
Some more examples are given by combining ILs with metal peroxides or polyoxometalates in the catalytic epoxidation of olefins with H2O2. In Yamaguchi
Numerous examples have shown that ILs are offering unique properties in the transition metal-, metal peroxide- or polyoxometalates-catalyzed oxidation of olefins with H2O2. The ILs supply special environment for the generation and stabilization of the active intermediate, or act as support for immobilizing and recycling the actual catalyst, both of which are necessary for performing effective catalytic oxidation. By delicately designing the combination of catalyst, support and ILs, the interactions between the hydrophobic substrate, hydrophilic oxidant, and the active site could be reinforced, intensifying the catalytic efficiency of the oxidation reaction.
Catalytic oxidations have been widely studied in ionic liquids, and much of this interest is centered on the possible use as “green” alternatives to traditionally used volatile organic solvents. This chapter summarizes limited examples that illustrate the applications of ILs in the catalytic oxidation using H2O2 as the oxidant, in particular benzene hydroxylation, alcohol oxidation, and olefin oxidation. We focus our discussion on understanding how the unusual solvent environment provide solute species that affect the reactions occurred in them.
As innocent and non-vaporized solvents, ILs provide good solubility to salts and most of the hydrophobic substrate, endowing them good solvent for the transition metal complexes-, peroxides-, oxides-, polyoxometalates-, or organic molecules-catalyzed oxidation. The miscibility of ILs with water and organic molecules can be elaborately tuned by varying the cations/anions (
The financial support from the National Natural Science Foundationof China (No. 20901053 and 20872102) and PCSIRT (No. IRT0846) are greatly appreciated.
Anastas, P. T.; Warner, J. C., Green Chemistry: Theory and Practice, Oxford University Press: New York, 1998.
Welton, T., Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev.1999, 99, 2071-2084.
Dupont, J.; De Souza, R. F.; Suarez, P. A. Z., Ionic Liquid (Molten Salt) Phase Organometallic Catalysis. Chem. Rev.2002, 102, 3667-3692.
Hernandez, O. R., ToTreat or Not to Treat? Applying Chemical Engineering Tools and a Life Cycle Approach to Assessing the Level of Sustainability of a Clean-up Technology. Green Chem.2004, 6, 395-400.
Clift, R., Sustainable Development and Its Implications for Chemical Engineering. Chem. Eng. Sci.2006, 61, 4179-4187.
Fischer, T.; Sethi, A.; Welton, T.; Woolf, J., Diels-Alder Reactions in Room-temperature Ionic Liquids. Tetrahedron Lett.1999, 40, 793-796.
Boon, J. A.; Levisky, J. A.; L., P. J.; Wilkes, J. S., Friedel-Crafts Reactions in Ambient-Temperature Molten Salts. J. Org. Chem.1986, 51, 480-483.
Brown, R. A.; Pollet, P.; McKoon, E.; Eckert, C. A.; Liotta, C. L.; Jessop, P. G., Asymmetric Hydrogenation and Catalyst Recycling Using Ionic Liquid and Supercritical Carbon Dioxide. J. Am. Chem. Soc.2001, 123, 1254-1255.
Chauvin, Y.; Mussmann, L.; Olivier, H., A Novel Class of Versatile Solvents for Two-Phase Catalysis: Hydrogenation, Isomerization, and Hydroformylation of Alkenes Catalyzed by Rhodium Complexes in Liquid 1,3-Dialkylimidazolium Salts. Angew. Chem., Int. Ed. Engl.1996, 34, 2698-2700.
Favre, F.; Olivier-Bourbigou, H.; Commereuc, D.; Saussine, L., Hydroformylation of 1-Hexene with Rhodium in Non-aqueous Ionic Liquids : How to Design the Solvent and the Ligand to the Reaction. Chem. Commun.2001, 1360-1361.
Dullius, J. E. L.; Suarez, P. A. Z.; Einloft, S.; de Souza, R. F.; Dupont, J., Selective Catalytic Hydrodimerization of 1,3-Butadiene by Palladium Compounds Dissolved in Ionic Liquids. Organometallics1998, 17 (5), 815-819.
Kaufmann, D. E.; Nouroozian, M.; Henze, H., Molten Salts as an Efficient Medium for Palladium Catalyzed C-C Coupling Reactions. Synlett.1996, 1091-1092.
Matthews, C. J.; Smith, P. J.; Welton, T., Palladium Catalysed Suzuki Cross-coupling Reactions in Ambient Temperature Ionic Liquids. Chem. Commun.2000, 1249-1250.
Parvulescu, V. I.; Hardacre, C., Catalysis in Ionic Liquids. Chem. Rev.2007, 107, 2615-2665.
Hallett, J. P.; Welton, T., Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chem. Rev.2011, 111, 3508-3576.
Olivier-Bourbigou, H.; Magna, L.; Morvan, D., Ionic Liquids and Catalysis: Recent Progress from Knowledge to Applications. Appl. Catal. A2010, 373, 1-56.
Lee, J. W.; Shin, J. Y.; Chun, Y. S.; Jang, H. B.; Song, C. E.; Lee, S., 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. Acc. Chem. Res.2010, 43 (7), 985-994.
Stark, A., Ionic Liquid Structure-Induced Effects on Organic Reactions. Top Curr. Chem.2009, 290, 41-81.
Wasserscheid, P.; Keim, W., Ionic Liquids-New "solution" for Transition Metal Catalysis. Angew Chem, Int. Ed. 2000, 39, 3772-3789.
Sheldon, R., Catalytic Reactions in Ionic Liquids. Chem. Commun.2001, 2399-2407.
Gharnati, L.; Doering, M.; Arnold, U., Catalytic Oxidation with Hydrogen Peroxide in Ionic Liquids. Curr. Org. Synth. 2009, 6(4), 342-361.
Betz, D.; Altmann, P.; Cokoja, M.; Herrmann, W. A.; Kuehn, F. E., Recent Advances in Oxidation Catalysis Using Ionic Liquids as Solvents. Coord. Chem. Rev.2011, 255 (13-14), 1518-1540.
Muzart, J., Ionic Liquids as Solvents for Catalyzed Oxidations of Organic Compounds. Adv. Synth. Catal.2006, 348, 275-295.
Walling, C., Intermediates in the Reactions of Fenton Type Reagents. Acc. Chem. Res.1998, 31, 155-157.
Hu, X. K.; Zhu, L. F.; Guo, B.; Liu, Q. Y.; Li, G. Y.; Hu, C. W., Hydroxylation of Benzene to Phenol via Hydrogen Peroxide in Hydrophilic Triethylammonium Acetate Ionic Liquid. Chem. Res. Chin. Univ.2011, 27 (3), 503-507.
Zhang, J.; Tang, Y.; Li, G. Y.; Hu, C. W., Room Temperature Direct Oxidation of Benzene to Phenol Using Hydrogen Peroxide in the Presence of Vanadium-substituted Heteropolymolybdates. Appl. Catal. A2005, 278, 251-261.
Zhong, Y. K.; Li, G. Y.; Zhu, L. F.; Yan, Y.; Wu, G.; Hu, C. W., Low Temperature Hydroxylation of Benzene to Phenol by Hydrogen Peroxide over Fe/activated Carbon Catalyst. J. Mol. Catal. A2007, 272, 169-173.
Jian, M.; Zhu, L. F.; Wang, J. Y.; Zhang, J.; Li, G. Y.; Hu, C. W., Sodium Metavanadate Catalyzed Direct Hydroxylation of Benzene to Phenol with Hydrogen Peroxide in Acetonitrile Medium. J. Mol. Catal. A2006, 253, 1-7.
Bianchi, D.; Bertoli, M.; Tassinari, R.; Ricci, M.; Vignola, R., Ligand Effect on the Iron-catalysed Biphasic Oxidation of Aromatic Hydrocarbons by Hydrogen Peroxide J. Mol. Catal. A2003, 204, 419-424.
Bianchi, D.; Balducci, L.; Bortolo, R.; D' Aloisio, R.; Ricci, M.; Span, G.; Tassinari, R.; Tonini, C.; Ungarellia, R., Oxidation of Benzene to Phenol with Hydrogen Peroxide Catalyzed by a Modified Titanium Silicalite (TS-1B). Adv. Synth. Catal.2007, 349, 979-986.
Peng, J. J.; Shi, F.; Gu, Y. L.; Deng, Y. Q., Highly Selective and Green Aqueous-Ionic Liquid Biphasic Hydroxylation of Benzene to Phenol with Hydrogen Peroxide. Green Chem.2003, 5 (2), 224-226.
Hu, X. K.; Zhu, L. F.; Wang, X. Q.; Guo, B.; Xu, J. Q.; Li, G. Y.; Hu, C. W., Active Species Formed in a Fenton-Like System in the Medium of Triethylammonium Acetate Ionic Liquid for Hydroxylation of Benzene to Phenol. J. Mol. Catal. A2011, 342-343, 41-49.
Hu, X. K., Study on Hydroxylation of Benzene in Triethylammonium Acetate Ionic Liquid. Chinese Doctoral Dissertation2011.
Ley, S. V.; Madin, A., in: Trost, B. M.; Flemming, I. (Eds.), Comprehensive Organic Synthesis, Vol. 7, 305-327, Pergamon Press, Oxford, 1991.
Hudlick, M., Oxidations in Organic Chemistry, American Chemical Society: Washington, DC, 1990.
Sheldon, R. A.; Kochi, J. K., Metal Catalyzed Oxidationd of Organic Compounds, Academic Press, New York, 1984.
Bobbitt, J. M.; Br °uckner, C., Organic Reactions, John-Wiley & Sons, New York, 2009.
Anelli, P. L.; Biffi, C.; Montanari, F.; Quici, S., J. Org. Chem. 1987, 52, 2559-2562.
Fall, A.; Sene, M.; Gaye, M.; Gomez, G.; Fall, Y., Ionic Liquid-Supported TEMPO as Catalyst in the Oxidation of Alcohols to Aldehydes and Ketones. Tetrahedron Lett.2010, 51 (34), 4501-4504.
Hoover, J. M.; Stahl, S. S., Highly Practical Copper(I)/TEMPO Catalyst System for Chemoselective Aerobic Oxidation of Primary Alcohols. J. Am. Chem. Soc.2011, 133 (42), 16901-16910.
Ma, S. M.; Liu, J. X.; Li, S. H.; Chen, B.; Cheng, J. J.; Kuang, J. Q.; Liu, Y.; Wan, B. Q.; Wang, Y. L.; Ye, J. T.; Yu, Q.; Yuan, W. M.; Yu, S. C., Development of a General and Practical Iron Nitrate/TEMPO-Catalyzed Aerobic Oxidation of Alcohols to Aldehydes/Ketones: Catalysis with Table Salt. Adv. Synth. Catal.2011, 353 (6), 1005-1017.
Hoover, J. M.; Steves, J. E.; Stahl, S. S., Copper(I)/TEMPO-catalyzed aerobic oxidation of primary alcohols to aldehydes with ambient air. Nature Protoc.2012, 7 (6), 1161-1166.
Gheorghe, A.; Chinnusamy, T.; Cuevas-Yanez, E.; Hilgers, P.; Reiser, O., Combination of Perfluoroalkyl and Triazole Moieties: A New Recovery Strategy for TEMPO. Org. Lett. 2008, 10 (19), 4171-4174.
Liu, R.; Liang, X.; Dong, C.; Hu, X., Transition-Metal-Free: A Highly Efficient Catalytic Aerobic Alcohol Oxidation Process. J. Am. Chem. Soc.2004, 126, 4112-4113.
Wang, S. S.; Popovic, Z.; Wu, H. H.; Liu, Y., A Homogeneous Mixture Composed of Vanadate, Acid, and TEMPO Functionalized Ionic Liquids for Alcohol Oxidation by H2O2. ChemCatChem2011, 3 (7), 1208-1213.
Jiang, N.; Ragauskas, A. J., TEMPO-Catalyzed Oxidation of Benzylic Alcohols to Aldehydes with the H2O2/HBr/Ionic Liquid [bmim] PF6 System. Tetrahedron Lett.2005, 46 (19), 3323-3326.
Karimi, B.; Badreh, E., SBA-15-Functionalized TEMPO Confined Ionic Liquid: an Efficient Catalyst System for Transition-Metal-Free Aerobic Oxidation of Alcohols with Improved Selectivity. Org. Biomol. Chem.2011, 9 (11), 4194-4198.
Chhikara, B. S.; Chandra, R.; Tandon, V., Oxidation of Alcohols with Hydrogen Peroxide Catalyzed by a New Imidazolium Ion Based Phosphotungstate Complex in Ionic Liquid. J. Catal.2005, 230 (2), 436-439.
Bianchini, G.; Crucianelli, M.; De Angelis, F.; Neri, V.; Saladino, R., Highly Efficient C-H Insertion Reactions of Hydrogen Peroxide Catalyzed by Homogeneous and Heterogeneous Methyltrioxorhenium Systems in Ionic Liquids. Tetrahedron Lett.2005, 46 (14), 2427-2432.
Chen, L.; Zhou, T.; Chen, L.; Ye, Y.; Qi, Z.; Freund, H.; Sundmacher, K., Selective Oxidation of Cyclohexanol to Cyclohexanone in the Ionic Liquid 1-Octyl-3-Methylimidazolium Chloride. Chem. Commun.2011, 47 (33), 9354-9356.
Usui, Y.; Sato, K., A Green Method of Adipic Acid Synthesis: Organic Solvent- and Halide-Free Oxidation of Cycloalkanones with 30% Hydrogen Peroxide. Green Chem. 2003, 5, 373-375.
Singh, P. P.; Ambika; Chauhan, S. M. S., Chemoselective Epoxidation of Electron Rich and Electron Deficient Olefins Catalyzed by Meso-Tetraarylporphyrin Iron(III) Chlorides in Imidazolium Ionic Liquids. New J. Chem.2012, 36 (3), 650-655.
Liu, G.; Hou, M. Q.; Song, J. Y.; Zhang, Z. F.; Wu, T. B.; Han, B. X., Ni2+-Containing Ionic Liquid Immobilized on Silica: Effective Catalyst for Styrene Oxidation with H2O2 at Solvent-Free Condition. J. Mol. Catal. A 2010, 316 (1-2), 90-94.
Yamaguchi, K.; Yoshida, C.; Uchida, S.; Mizuno, N., Peroxotungstate immobilized on ionic liquid-modified silica as a heterogeneous epoxidation catalyst with hydrogen peroxide. J. Am. Chem. Soc. 2005, 127 (2), 530-531.
Berardi, S.; Bonchio, M.; Carraro, M.; Conte, V.; Sartorel, A.; Scorrano, G., Fast Catalytic Epoxidation with H2O2 and [gamma-SiW10O36(PhPO)2]4 in Ionic Liquids under Microwave Irradiation. J. Org. Chem. 2007, 72 (23), 8954-8957.
Liu, L. L.; Chen, C. C.; Hu, X. F.; Mohamood, T.; Ma, W. H.; Lin, J.; Zhao, J. C., A Role of Ionic Liquid as an Activator for Efficient Olefin Epoxidation Catalyzed by Polyoxometalate. New J. Chem. 2008, 32 (2), 283-289.