Ionic liquids nitration

The choice of the anion ultimately intended to be an element of the ionic liquid is of particular importance. Perhaps more than any other single factor, it appears that the anion of the ionic liquid exercises a significant degree of control over the molecular solvents (water, ether, etc.) with which the IL will form two-phase systems. Nitrate salts, for example, are typically water-miscible while those of hexaflu-orophosphate are not those of tetrafluoroborate may or may not be, depending on the nature of the cation. Certain anions such as hexafluorophosphate are subject to hydrolysis at higher temperatures, while those such as bis(trifluoromethane)sulfonamide are not, but are extremely expensive. Additionally, the cation of the salt used to perform any anion metathesis is important. While salts of potassium, sodium, and silver are routinely used for this purpose, the use of ammonium salts in acetone is frequently the most convenient and least expensive approach. 

Other methods of nitration that Laali investigated were with isoamyl nitrate in combination with a Bronsted or Lewis acid in several ionic liquids, with [EMIM][OTf] giving the best yields (69 %, 1.0 1.0 o p ratio). In the ionic liquid [HNEt( Pr)2] [CE3CO2] (m.p. = 92-93 °C), toluene was nitrated with a mixture of [NH4][N03] and trifluoroacetic acid (TEAH) (Scheme 5.1-37). This gave ammonium trifluoroacetate [NH4][TEA] as a by-product, which could be removed from the reaction vessel by distillation (sublimation). 

Scheme 5.1-37 Aromatic nitration reactions in ionic liquids.

Thanks to their special properties and potential advantages, ionic liquids may be interesting solvents for biocatalytic reactions to solve some of the problems discussed above. After initial trials more than 15 years ago, in which ethylammonium nitrate was used in salt/water mixtures [29], results from the use of ionic liquids as pure solvent, as co-solvent, or for biphasic systems have recently been reported. The reaction systems are summarized in Tables 8.3-1 and 8.3-2, below. Table 8.3-1 compiles all biocatalytic systems except lipases, which are shown separately in 8.3-2. Some of the entries are discussed in more detail below. 

Ethylammonium nitrate, CH CH,NH NO , was the first ionic liquid to be discovered. Its melting point of 12°C was reported in 1914 and it has since been used as a nonpolluting solvent for organic reactions and for facilitating the folding of proteins. 

The anion in an ionic liquid can be varied, too. Many ionic liquids contain relatively simple inorganic anions, such as nitrate (NO3 ), tetrafluoroborate (BF4 ), or hexafluorophosphate (PFg ). Anions that are more exotic also are possible, such as the two shown below. Varying the anion provides another way of tuning the properties of an ionic liquid to match a desired application. 

A number of new conditions and catalysts have been used for the synthesis of quinazolinones 50 from anthranilic acids, amines and ortho esters, including bismuth trifluoroacetate with an ionic liquid <06TL3561>, lanthanum nitrate or tosic acid under solvent-free conditions at room temperature <06TL4381> and Nafion-H <06SL2507>. 

Using day supported ammonium nitrate (dayan), selective deprotection of methoxyphenyl methyl (MPM) ether has been achieved recently using microwave irradiation in solvent-free conditions (Scheme 6.15) [56]. The same reagent has been used for the cleavage of tetrahydropyranyl (THP) ethers. A similar selective preparation and deavage of THP ethers has been achieved under microwave irradiation catalyzed by iodine [57] or neat reaction in an ionic liquid [28],... 

Example 2, using ionic liquid ethylammonium nitrate (EAN) as solvent ... 

According to their miscibility with water, ionic liquids are also frequently classified as hydrophilic or hydrophobic. The hydrophilic ionic liquids are typically salts composed of halide, acetate, nitrate, trifluoroacetate, and, in some cases, tetrafluoroborate anions, in particular their salts with [AMIM] having short alkyl chains, as these ionic liquids are totally miscible with water. The ionic liquids composed of PF and (CF3S02)2N with [AMIM] are immiscible with water in bulk, and are therefore referred to as hydrophobic ionic liquids. The ionic liquids consisting of BF4 and CFsSOi" ions with [AMIM] can be totally miscible or immiscible depending on the substituents on the cation, and they are therefore sometimes called tunable ionic liquids (22). A recent review covers relevant properties of some ionic liquids for catalysis (42,43). 

The development of ionic liquids dates to 1914. The first research efforts involved the synthesis of ethylammonium nitrate. Hurley and Wier at the Rice Institute in Texas, 1948, developed the first ionic liquids with chloro-aluminate ions as bath solutions for electroplating aluminum. These liquids were studied primarily for their applications as electrolytes in electrochemistry technologies such as electroplating, batteries and alloy preparations. 

Domanska, U., Lugowska, K., and Pernak, Phase equilibria of didecyldi-methylammonium nitrate ionic liquid with water and organic solvents, /. Chem. Thermodyn., 39, 729, 2007. 

Dai, S., Ju, Y.H., and Barnes, C.E., Solvent extraction of strontium nitrate by a crown ether using room-temperature ionic liquids, /. Chem. Soc., Dalton Trans., 1201,1999. 

The Diels-Alder reaction is an important and widely used reaction in organic synthesis (Sauer and Sustmann, 1980), and in the chemical industry (Griffiths and Previdoli, 1993). Rate enhancement of this reaction has been achieved by the use of solvents such as water, surfactants, very high pressure, lithium amides, alkylammonium nitrate salts, and macrocyclic hosts (Sherman et ak, 1998). Diels-Alder reactions can be ran in neutral ionic liquids (such as 1-butyl-3-methylimidazolium trifluoromethanesulfo-nate, l-butyl-3-methylimidazolium hexafluorophophate, l-butyl-3-methylimidazolium tetrafluoroborate, and l-butyl-3-methylimidazolium lactate). Rate enhancements and selectivities are similar to those of reactions performed in lithium perchlorate-diethyl ether mixtures. 

Their utility has been demonstrated in various Friedel-Crafts reactions,44-47 halogen addition,48 49 electrophilic nitration of aromatics,50 and various hydrogenation processes51-53 including the Ru-catalyzed hydrogenation of C02 to /V,/V-dipropyl formamide in supercritical C02 under biphasic conditions 54 The use of in situ IR spectroscopy allowed Horvath and coworkers to demonstrate that the same substrate-catalyst and product-catalyst complexes are produced in Friedel-Crafts acetylation of benzene in ionic liquids as in CH2C12 55... 

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