Review Organic Reactions in Ionic Liquids

Mini-Review Organic Reactions in Ionic Liquids 

Ionic liquids are organic salts that are liquid at or near room temperature. It has been found recently that such liquids can be useful solvents for organic reactions. Often, the organic products can be removed from the ionic liquid by extraction with, e.g., ether, without resorting to an aqueous workup. This can be particularly useful when a precious metal catalyst is used in the reaction. The catalyst often remains in the ionic liquid, so that the catalyst solution can be directly reu.sed. 

Roberta Bernini of the Univ. of Tuscia in Viterbo illustrated (Tetrahedron Lett. 44 8991, 2003) the power of this approach with the MeReO,-catalyzed oxidation of 1 to 2 in [bmim]BF. The product could be extracted with ether and the catalyst-containing ionic liquid recharged with substrate and H O, for four cycles before the conversion and yield started to drop off. This drop off may be due to the accumulation of water in the ionic liquid, which could be removed by distillation. 

Alternative purification protocols are available. Zhaolin Sun of Lanzhou University reports (Tetrahedron Lett. 45 2681,2004) that the ionic liquid TISC was specifically designed to promote Beckmann rearrangement. TISC is not soluble in water, so the product caprolactam was easily removed from the ionic liquid by extraction with water. 

The counterion of the ionic liquid can be tuned to achieve one desired reactivity or another. Martyn Earle of Queen s University, Belfast has observed (Organic Lett. 6 707,2004) that the reaction of toluene can be directed toward any of the three products 6, 7, or 8, depending on the ionic liquid used. 

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Mini-Review Organic Reactions in Ionic Liquids Adventures in Polycyclic Ring Construction... 

Several reviews have been published within the year which are of general relevance to the photoreactions of aromatic compounds. The subjects of these reviews include photochemistry in ionic liquids and in isotropic and anisotropic media, organic synthesis utilizing photoinduced electron-transfer reactions," heteroatom-directed photoarylation processes, photochromism, and photochemical molecular devices. Reviews more directly pertinent to the sections in the present chapter include those of the photoisomerization of five-membered heteroaromatic azoles, the photocycloaddition of benzene derivatives to alkenes, Diels-Alder additions of anthracenes, advances in the synthesis of polycyclic aromatic compounds, diarylethene-based photochromic switches, the photo-Fries rearrangement, and the application of Diels-Alder trapping of photogenerated o-xylenols to the synthesis of novel compounds. " A number of chapters in the two recently published handbooks of photochemistry and photobiology and in the revised edition of the text on photochromism are also pertinent to the current subject matter. 

Since then, the process has been extended to a wide variety of lactones of different size and to several lipases, as recently reviewed [93-96]. Interestingly, large-membered lactones, which are very difficult to polymerize by usual anionic and coordination polymerizations due to the low ring strain, are successfully polymerized by enzymes. Among the different lipases available, that fi om Candida antarctica (lipase CA, CALB or Novozym 435) is the most widely used due to its high activity. An alcohol can purposely be added to the reaction medium to initiate the polymerization instead of water. The polymerization can be carried out in bulk, in organic solvents, in water, and in ionic liquids. Interestingly, Kobayashi and coworkers reported in 2001 the ROP of lactones by lipase CA in supercritical CO2... 

An overview of the reactions over zeolites and related materials employed in the fields of refining, petrochemistry, and commodity chemicals reviewed the role of carbocations in these reactions.15 An overview appeared of the discovery of reactive intermediates, including carbocations, and associated concepts in physical organic chemistry.16 The mechanisms of action of two families of carcinogens of botanical origin were reviewed.17 The flavanoids are converted to DNA-reactive species via an o-quinone, with subsequent isomerization to a quinone methide. Alkenylbenzenes such as safrole are activated to a-sulfatoxy esters, whose SnI ionization produces benzylic cations that alkylate DNA. A number of substrates (trifluoroacetates, mesylates, and triflates) known to undergo the SnI reaction in typical solvolysis solvents were studied in ionic liquids several lines of evidence indicate that they also react here via ionization to give carbocationic intermediates.18... 

Ionic liquids offer a number of potential advantages over organic solvents from a green perspective. Loss of solvent by evaporation is effectively zero. Reactions may be more selective in an ionic liquid, thereby reducing separation costs both from an economic and environmental perspective. For catalytic reactions as well as increasing selectivity, an ionic liquid may stabilize a catalyst and so increase lifetime and turnover number. The use of ionic liquids in catalysis has been the subject of several reviews in recent years,and organometallic chemistry in ionic liquids is reviewed in Volume 1. 

From reviewing much of the literature it is easy to conclude that ionic liquids are excellent solvents for catalysts and reagents but not for products, which is obviously not the case. Whilst some products can be decanted from the liquid and others can be recovered by distillation, there are many useful reactions in which removal of the product (or residual reactants) from the ionic liquid is challenging. Extraction with an organic solvent, or even water, would reduce the overall eco-efficiency. Initial... 

The use of ionic liquids (ILs) to replace organic or aqueous solvents in biocatalysis processes has recently gained much attention and great progress has been accomplished in this area lipase-catalyzed reactions in an IL solvent system have now been established and several examples of biotransformation in this novel reaction medium have also been reported. Recent developments in the application of ILs as solvents in enzymatic reactions are reviewed. 

In microwave-assisted synthesis, a homogeneous mixture is preferred to obtain a uniform heating pattern. For this reason, silica gel is used for solvent-free (open-vessel) reactions or, in sealed containers, dipolar solvents of the DMSO type. Welton (1999), in a review, recommends ionic liquids as novel alternatives to the dipolar solvents. Ionic liquids are environmentally friendly and recyclable. They have excellent dielectric properties and absorb microwave irradiation in a very effective manner. They exhibit a very low vapor pressure that is not seriously enhanced during microwave heating. This makes the process not so dangerous as compared to conventional dipolar solvents. The polar participants of organic ion-radical reactions are perfectly soluble in polar ionic liquids. 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

In a recent review, some positive attributes of ionic liquids in biocatalysis were discussed 273). An example was given, which compares the enzymatic performance of Pseudomonas cepacia lipase (PCL)-catalyzed reactions as a function of the solvent polarity in both organic and ionic solvents, as shown in Fig. 17. The PCL shows no activity in organic solvents in the polarity range of the ionic liquids, but it is active in the ionic liquids. 

Room temperature ionic liquids continue to attract interest by both fundamental and applied researchers. Several general review articles have been published in recent years that describe not only their physical properties but also discuss how these physical properties can be applied for solvents used in separations and as replacement for organic solvents for homogeneously-catalyzed reactions. In this review, we focus our attention on those physical... 

In this chapter, the recent advances in amidocarbonylations, cyclohydrocarbonylations, aminocarbonylations, cascade carbonylative cyclizations, carbonylative ring-expansion reactions, thiocarbonylations, and related reactions are reviewed and the scope and mechanisms of these reactions are discussed. It is clear that these carbonylation reactions play important roles in synthetic organic chemistry as well as organometallic chemistry. Some of the reactions have already been used in industrial processes and many others have high potential to become commercial processes in the future. The use of microwave irradiation and substitutes of carbon monoxide has made carbonylation processes suitable for combinatorial chemistry and laboratory syntheses without using carbon monoxide gas. The use of non-conventional reaction media such as SCCO2 and ionic liquids makes product separation and catalyst recovery/reuse easier. Thus, these processes can be operated in an environmentally friendly manner. Judging from the innovative developments in various carbonylations in the last decade, it is easy to anticipate that newer and creative advances will be made in the next decade in carbonylation reactions and processes. 

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