BMIM-type ionic liquids

Unlike DABCO, hexamethylenetetramine (HMTA) combined with bmim-type ionic liquids is an effective catalyst system for MBH reactions between aromatic aldehydes and acrylonitrile or methyl acrylate, affording the corresponding adducts in moderate to good yields (50-85%) in short reaction times (Scheme 1.118). Additionally, the ionic liquids can be recycled three times without any loss of activity. ... 

In terms of supramolecular chemistry, imidazolium type ionic liquids are interesting because they are excellent C-H hydrogen bond donors. The X-ray crystal structure of bmim Cl shown this property clearly with C-Cl 3.39 A, Figure 13.24. Interestingly, two different polymorphic forms of this material are known depending on whether it is crystallised from the molten ionic liquid for from solution,... 

Nitrones are effective 1,3-dipoles, and they can undergo readily cycloaddition with electron-deficient olefins to produce substituted isoxazolidines. Yadav and coworkers reported that these type of reactions are efficiently promoted in ionic liquid [bmim][PFg] (Fig. 12.14) [10], The method is highly regio- and diastereose-lective, and products are obtained in excellent yields. They also showed that the same results can be obtained by using [bmim][BF ] ionic liquid. 

With the aim of completing this stndy, we analysed the microscopic features of solvent systems resulting from the basis of binary mixtures formed by a protic molecular solvent (MeOH, ethanol (EtOH), propan-l-ol, propan-2-ol or 2-elhoxy-ethanol (EtOEtOH)) and a bmim-based ionic liquid cosolvent composed of different anions [BE ], [PE ], chloride [Cl] and bromide [Br]. The selection of these molecular solvents was done mainly on the basis of their HBD capacity. At the same time, it was conveiuent to discuss the incidence of anion type on the solvation pattern. 

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. 

All the halide exchange reactions mentioned above proceed more or less quantitatively, causing greater or lesser quantities of halide impurities in the final product. The choice of the best procedure to obtain complete exchange depends mainly on the nature of the ionic liquid that is being produced. Unfortunately, there is no general method to obtain a halide-free ionic liquid that can be used for all types of ionic liquid. This is explained in a little more detail for two defined examples the synthesis of [BMIM][(CF3S02)2N] and the synthesis of [EMIM][BF4]. 

As early as 1990, Chauvin and his co-workers from IFP published their first results on the biphasic, Ni-catalyzed dimerization of propene in ionic liquids of the [BMIM]Cl/AlCl3/AlEtCl2 type [4]. In the following years the nickel-catalyzed oligomerization of short-chain alkenes in chloroaluminate melts became one of the most intensively investigated applications of transition metal catalysts in ionic liquids to date. 

A basic ionic liquid, l-methyl-3-butylimidazolium hydroxide ([bmIm]OH) and l-butyl-3-methyl-methylimidazolium tetrafluoroborate ([bmim]BF4), has been introduced as a catalyst and reaction medium for the Markovnikov addition of imidazoles 116 to vinyl esters 115 under mild conditions to give imidazoesters 117 <06JOC3991 06TL1555>. A series of (nitroimidazolyl)succinic esters and diacids were prepared from the Michael-type addition of the nitroimidazole to the a,P-unsaturated ester <06S3859>. 

Additions of the Reformatsky-type reagents to aldehydes can also proceed in ionic solvents (Scheme 108).287 Three ionic liquids have been tested 8-ethyl-l,8-diazbicyclo[5,4,0]-7-undecenium trifluoromethanesulfonate ([EtDBU][OTf]), [bmim][BF4], and [bmim][PF6]. The reactions in the first solvent provided higher yields of alcohols 194 (up to 93%), although results obtained for two other ionic liquids were also comparable with those reported for conventional solvents. 

The reaction was not influenced by the type of ionic liquid, since no significant differences were observed when the reaction was carried out in [BMIM][PF6] or [BMIMJtn-CgHnOSOa]. In Table 7.5 the most relevant results for the hydroformylation of propene with the different rhodium catalyst systems in the ionic liquid [BMIM][PF6] are compiled. 

Another type of chiral rhodium complex [Rh-MeDuPHOS] was also immobilized in [BMIM][PF6] and used in the enantiomeric hydrogenation of related enamides [95] (Fig. 41.5). Geresh et al. focused their research on the stabilization of the air-sensitive catalyst in the air-stable ionic liquid, so that the complex was protected from attack by atmospheric oxygen and recycling was possible. 

Indium-mediated Barbier-type coupling between carbonyl compounds and allyl halides has been revealed to proceed effectively in diverse reaction media. Even under solvent-free conditions, allylation works well, although no reaction is observed with benzyl bromide and a-halo carbonyl compounds.59 Various aldehydes react with allyl bromide mediated by indium in liquid carbon dioxide to give homoallylic alcohols (Scheme 1). In contrast to the corresponding neat allylation, the liquid C02-mediated reaction can allylate solid aldehydes successfully.60 Indium-mediated allylations of carbonyl compounds with allyl bromide proceed in room temperature ionic liquids. In [bmim][BF4] and [bmim][PF6] (bmin l-butyl-3-methylimidazolium), the desired homoallylic alcohols are formed with good levels of conversion.61 Homoallyllic alcohols are also prepared by the reaction of resin-bound aldehydes (Equation (l)).62... 

Fig. 12 Top ionic liquids of the type [Bmim]4[Ln(NCS)7(H20)] with Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, and Yb. The color is due to the metallate anion. Bottom molecular structure of the anionic unit [La(NCS)7(H20)]4 showing the hydrogen bonding from coordinated water molecules to the isothiocyanate anions. The [Bmim] cations were omitted for clarity. Image adapted from [67]. Image copyright American Chemical Society (2006). For color image see online version...

Abstract This chapter presents the design and analysis of the microscopic features of binary solvent systems formed by ionic liquids, particularly room temperature ionic liqnids with molecular solvents. Protic ionic liquids, ethylammonium nitrate and l-n-butyl-3-methylmidazohum (bmim)-based ILs, were selected considering the differences in their hydrogen-bond donor acidity. The molecular solvents chosen were aprotic polar (acetonitrile, dimethylsulphoxide and MA(-dimethylformide) and protic (different alcohols). The empirical solvatochromic parameters n, a and P were employed in order to analyse the behaviour of each binary solvent system. The study focuses on the identification of solvent mixtures of relevant solvating properties to propose them as new solvents . Kinetic study of aromatic nucleophilic substitution reactions carried out in this type of solvent systems is also presented. On the other hand, this is considered as a new approach on protic ionic liquids. Ethylammonium nitrate can act as both Bronsted acid and/or nucleophile. Two reactions (aromatic nucleophilic substitution and nncleophilic addition to aromatic aldehydes) were considered as model reactions. 

In general, for the proposed new solvents , it can be observed that the dipolarity/ polarisability and basicity are similar to those of AN and DMF. The acidity is quite different being highly increased, particularly in the case of AN. In relation to the protic solvent + imidazolium-based ILs binary solvent mixtures, the compositions of the mixtures are dependent on both the nature of the protic solvent as well as on the anion type of the ionic liquid. With ethanol and 2-ethoxyethanol as molecular solvents, the IL displays large solubility, particularly when the anions are [BF ] or [Cl]. The ILs [bmim][PF ] and [bmim][Br] exhibit the lowest solubility, so their mixtures were explored at the smallest range of compositions. 

In 2003, Chen et al. studied the effect of different types of ionic liquids in the oxidation of alcohols and 1,2-diols with IBX. The DMSO is not protic solvent found that [bmim][Cl] is the better choice because it can readily dissolve IBX in the presence of small amounts of water at room temperature to form a homogeneous... 

Use of TEMPO immobilized on ionic liquid was reported by Ruijun et al. [51], They used three TEMPO catalysts (Scheme 14.48) for the oxidation of alcohols in the presence of two types of ionic liquids H O and ([bmim][BF ]-HjO and [bmim] [PF,]-H,0). 

From the results obtained for the oxidation of 1-phenylethanol to acetophenone, they have found that the system using IL-NHPI (10 mol%)-Co(OAc)2-02 in the ionic liquid [bmim][PFJ was reusable, hi the same system, various types of carbinols were transformed into the corresponding aldehydes and/or ketones in good yields. 

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