Ionic liquids also

The size of the cation in the chloroaluminate ionic liquids also appears to have an impact on the viscosity. For ionic liquids with the same anion(s) and compositions, the trend is for greater viscosity with larger cation size (Table 3.2-2). An additional contributing factor to the effect of the cation on viscosity is the asymmetry of the alkyl substitution. Highly asymmetric substitution has been identified as important for obtaining low viscosities [17]. 

Ionic liquids also showed a catalytic activity for the cyclocondensation of a-tosyloxyketones with 2-aminopyridine [210], the nucleophilic substitution... 

The use of gaseous oxygen as an oxidant in ionic liquids also appears to be limited by its low solubility, for example, in [BMIM]PFg for the oxidation of aromatic aldehydes to give carboxylic acids 219). Hydrogen peroxide and organo-peroxide, with their higher solubilities, have been used efficiently for enzymatic oxidation 220). 

The plausible mechanism of the reaction is shown in Fig. 25. The reaction probably proceeds through the activation of imine (formed in situ from the o-hydroxy benzaldehyde and the aromatic amine) by the catalyst followed by the addition and subsequent cyclization of the enol ether, resulting in the formation of the fused acetal. Ionic liquids are stable enough with amines and water and also effectively activate the imines to undergo cyclization. The recovered ionic liquid can be re-used with gradual decrease in the efficiency of the method. The hydro-phobic nature of the ionic liquid also helps in recovery of the catalyst. 

This structural change is suppressed by the addition of tetrahydrothiophene (THT)19b. It prevents the formation of polymethylene zinc, i.e. (—CH2Zn—) . Without THT, a solution of 3 in THF yields polymethylene zinc at 60 °C. Monomeric bis(iodozincio)methane (3) is much more active for methylenation as compared to polymethylene zinc. As shown in Table 3 (entry 3), the addition of THT to the reaction mixture at 60 °C improved the yield of the alkene dramatically. Practically, however, its stinking property makes the experimental procedure in large scale uncomfortable. Fortunately, an ionic Uquid, l-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), plays the same role. Ionic liquid also stabilizes the monomeric structure of 3 even at 60 °C and maintains it during the reaction at the same temperature. The method can be applied to various ketones as shown in Scheme 14.4... 

Molten salts or ionic liquids (also referred to as fused salts by some authors) were among the very first media to be employed for electrochemistry. In fact, Sir Humphrey Davy describes electrochemical experiments with molten caustic potash (KOH) and caustic soda (NaOH) [1] as early as 1802 A wide variety of single molten salts and molten salt mixtures have been used as solvents for electroanalytical chemistry. These melts run the gamut from those that are liquid well below room temperature to those melting at more than 2000°C. The former present relatively few experimental challenges, whereas the latter can present enormous difficulties. For example, commercially available Teflon- and Kel-F-shrouded disk electrodes and Pyrex glass cells may be perfectly adequate for electrochemical measurements in ambient temperature melts such as the room-temperature chloroaluminates, but completely inadequate for use with molten sodium fluoroaluminate or cryolite (mp = 1010°C), which is the primary solvent used in the Hall-Heroult process for aluminum electrowinning. 

At present, ionic liquids, also known as room-temperature ionic liquids, nonaqueous ionic liquids, molten salts, liquid organic salts, and fused salts, are considered to be the new generation of solvents. In chemical abstracts, they can be found under the headings ionic liquid or liquids ionic. Publications on ionic liquids are increasing in number. 

Ionic liquids show interesting properties with respect to their miscibility with other liquids. They can be broadly divided into hydrophilic and hydrophobic species, which are either miscible or largely immiscible with water. The aqueous miscibility of ionic liquids can be understood by applying the well-known Hofmeister [63] scale of hydrophobicity to their component ions [64], The aqueous miscibility of ionic liquids also depends strongly on temperature, a fact that has been exploited in novel chemical processes [65]. Mixtures of ILs and organic phases also show interesting properties [66, 67], though we will not discuss them in detail in this chapter. 

Electron-rich phenols react with isoprene in ionic liquids and with added Sc(OTf)3 to give 2,2-dimethylchromans <07SL3050>. Ionic liquids also promote the one-pot synthesis of 4-arylchromans from benzaldehydes, phenols and allyl bromide which involves sequential Barbier allylation, Friedel-Crafts alkylation and an intramolecular hydroalkoxylation <07SL1357>. 

Figure 18 shows that the addition of water to any ([C4mim][CH3S03] + ROH) mixture suppresses the formation of ester, and increasing amounts of ionic liquid also decreases the yield, irrespective of the ionic liquid/water composition. Due to these superimposing effects, a possible (albeit small) water scavenging effect of this particular anion is difficult to extrapolate, and further studies with different anions are required. 

Especially, the eco-friendly ionic liquids have obtained extensive attention in organic synthesis with the merits provided as above. The ionic liquids as the unusual green solvents are applied extensively in various organic synthesis reactions, such as Friedel-Crafts reactions, oxidation reactions, reduction reactions, addition reactions, C-C formation reactions, nucleophilic substitution reactions, esterifications, rearrangements, hydroformylations, and nitration reactions [7-14]. Besides, the ionic liquids also have applications in the extraction separation, the electrochemistry, and preparation of nanostructured materials, the production of clean fuel, environmental science, and biocatalysis. This chapter would present in detail the application of the ionic liquids as the unusual green solvents (also as dual green solvent and catalyst) for the alkylation and acylation. 

With respect to the influence of the anion and cation interactions, at first glance, the hydrogen bond basicity is controlled by the anions. However, this is a general trend because the hydrogen-bond acceptor ability of the ionic liquid also depends on the nature of cation. Thus, the overall ability of the IL to form a hydrogen bond with a molecule of solute comes from an antagonistic relationship between its constituent ions. This fact may be desalbed in terms of two competing equilibria. 

A selective oxidation of alcohols at room temperature in ionic liquid was reported by Han and coworkers [24]. First, three different ionic liquids were tested as the solvents for this reaction. Among ([bmim][PF ], [bmim][BF ], [bmimJiCFjCOj]) solvents, the reaction proceeded rapidly in BF -type ionic liquid, slowly in PF -type ionic liquid, and no reaction occurred in CF CO -type ionic liquid. Also, common organic solvents showed poor activity. Later, various types of oxidants such as t-BOOH, NaClO, and were tested that only NaClO gave good results (Scheme 14.25). After optimization of the reaction conditions, oxidation of different alcohols was examined. Results showed that aU primary aromatic alcohols gave excellent yields in short times, but oxidation of both secondary aromatic and aliphatic alcohols could not be completed. Most importantly, in this reaction, [bmim] [BF ] was used as both catalyst and solvent. The catalytic system could be recycled and reused for five runs without any significant loss of the catalytic activity. 

The use of ionic liquids (also called molten or fused salts) as reaction media is a relatively new area, although molten conditions have been well established in industrial processes (e.g. the Downs process. Figure 10.1) for many years. While some molten salts are hot as the term suggests, others operate at ambient temperatures and the term ionic Uquids is more appropriate. This section provides only a brief introduction to an area which has implications for green chemistry (see Box 8.3). 

Refractive indices for a number of ionic liquids have been reported recently [30]. Increasing the number, length and branching of alkyl chains on the cations increases the refractive index, as does introducing functionality into the chain. Changing the anion of the ionic liquid also affects the refractive index, perhaps with less polarizable anions giving lower values. 

Another report on Heck reactions in ionic liquids also confirms that, in Bu4NBr as an ionic liquid, [Pd(OAc)2] rapidly reacts to form catalytically active colloidal particles [35]. The authors point out that it is unclear whether the palladium particles are the actual active species or whether they are a precursor to the latter. 

In March 2003, the use was reported of ionic liquids in a commercial process by BASF. The reaction of phenylchlorophosphines and ethanol forms alkoxyphenylphosphines 109 along with HCl, which is scavenged by A-methylimidazole (Scheme 48). The [hmim][Cl] salt 110 formed is an ionic liquid with a melting point of 75°C this leads to a biphasic mixture, which can be more easily stirred than a solution containing a suspension. This ionic liquid also acts as catalyst for this process of biphasic acid scavenging utilizing ionic liquids... 

Newly developed ionic liquids also have hydrophobic properties that allow handling in air. 

Ionic liquids, also known as room temperature molten salts (RTMS), recently attracted much attention as nonflammable electrolytes, however, the effect as a... 

It is not necessary to use preformed and isolated palladium carbene complexes they can also be generated in situ [31]. When Pd(OAc)2 is dissolved in the presence of a base in [BMIm]Br, styrene was efficiently coupled with iodobenzene affording stilbene (complete conversion and 99% selectivity). The authors were able to isolate the thus-formed palladium carbene complex, derived from the ionic liquid solvent. The counter ion of the ionic liquid also plays an important role. Mizoroki-Heck reactions proceeded much faster in [BMIm]Br than in [BMIm]BF4. In the latter, precipitation of palladium black was encountered. The fact is explained by the necessity of the presence of a halide ion for the stabilization of the carbene-palladium complex [32]. 

---