Ionic solid support

Chapters 1 and 2 have been reorganised and updated in line with recent developments. A new chapter on the Future of Purification has been added. It outlines developments in syntheses on solid supports, combinatorial chemistry as well as the use of ionic liquids for chemical reactions and reactions in fluorous media. These technologies are becoming increasingly useful and popular so much so that many future commercially available substances will most probably be prepared using these procedures. Consequently, a knowledge of their basic principles will be helpful in many purification methods of the future. 

Viewed in conjunction with the solid-like, nonvolatile nature of ionic liquids, it is apparent that TSILs can be thought of as liquid versions of solid-supported reagents. Unlike solid-supported reagents, however, TSILs possess the added advantages of kinetic mobility of the grafted functionality and an enormous operational surface area (Figure 2.3-1). It is this combination of features that makes TSILs an aspect of ionic liquids chemistry that is poised for explosive growth. 

The allcylation of a number of aromatic compounds through the use of a chloroa-luminate(III) ionic liquid on a solid support has been investigated by Holderich and co-workers [87, 88]. Here the allcylation of aromatic compounds such as benzene, toluene, naphthalene, and phenol with dodecene was performed using the ionic liquid [BMIM]C1/A1C13 supported on silica, alumina, and zirconia. With benzene, monoalkylated dodecylbenzenes were obtained (Scheme 5.1-56). 

Scheme 5.1-56 The alkylation of benzene with dodecene with an ionic liquid on a solid support.

The ability of iron(III) chloride genuinely to catalyze Friedel-Crafts acylation reactions has also been recognized by Holderich and co-workers [97]. By immobilizing the ionic liquid [BMIM]Cl/FeCl3 on a solid support, Holderich was able to acetylate mesitylene, anisole, and m-xylene with acetyl chloride in excellent yield. The performance of the iron-based ionic liquid was then compared with that of the corresponding chlorostannate(II) and chloroaluminate(III) ionic liquids. The results are given in Scheme 5.1-67 and Table 5.1-5. As can be seen, the iron catalyst gave superior results to the aluminium- or tin-based catalysts. The reactions were also carried out in the gas phase at between 200 and 300 °C. The acetylation reac-... 

Abstract Current microwave-assisted protocols for reaction on solid-phase and soluble supports are critically reviewed. The compatibility of commercially available polymer supports with the relatively harsh conditions of microwave heating and the possibilities for reaction monitoring are discussed. Instrmnentation available for microwave-assisted solid-phase chemistry is presented. This review also summarizes the recent applications of controlled microwave heating to sohd-phase and SPOT-chemistry, as well as to synthesis on soluble polymers, fluorous phases and functional ionic liquid supports. The presented examples indicate that the combination of microwave dielectric heating with solid- or soluble-polymer supported chemistry techniques provides significant enhancements both at the level of reaction rate and ease of purification compared to conventional procedures. 

In conclusion, many methods are available for the immobihzation of metal complexes with diazaligands. The biphasic liquid methods seem to be more versatile - especially in the case of ionic liquids, where modification of the ligands is not needed. Very good results can also be obtained with solid supports, although the surface effects, which can be positive or negative. 

Phase-transfer catalysis is a special type of catalysis. It is based on the addition of an ionic (sometimes non-ionic like PEG400) catalyst to a two-phase system consisting of a combination of aqueous and organic phases. The ionic species bind with the reactant in one phase, forcing transfer of this reactant to the second (reactive) phase in which the reactant is only sparingly soluble without the phase-transfer catalyst (PTC). Its concentration increases because of the transfer, which results in an increased reaction rate. Quaternary amines are effective PTCs. Specialists involved in process development should pay special attention to the problem of removal of phase-transfer catalysts from effluents and the recovery of the catalysts. Solid PTCs could diminish environmental problems. The problem of using solid supported PTCs seems not to have been successfully solved so far, due to relatively small activity and/or due to poor stability. 

This method is certainly the oldest one described in the literature the first example concerns the ion exchange of [Pt(NH3)4]2+ and the surface of a sulfonated silica.15 Even now, the preparation of many heterogeneous catalysts (i.e., supported metal or oxide particles) involves as the first step the reaction of a coordination complex with the surface of an ionic solid such as alumina,... 

When supported complexes are the catalysts, two types of ionic solid were used zeolites and clays. The structures of these solids (microporous and lamellar respectively) help to improve the stability of the complex catalyst under the reaction conditions by preventing the catalytic species from undergoing dimerization or aggregation, both phenomena which are known to be deactivating. In some cases, the pore walls can tune the selectivity of the reaction by steric effects. The strong similarities of zeolites with the protein portion of natural enzymes was emphasized by Herron.20 The protein protects the active site from side reactions, sieves the substrate molecules, and provides a stereochemically demanding void. Metal complexes have been encapsulated in zeolites, successfully mimicking metalloenzymes for oxidation reactions. Two methods of synthesis of such encapsulated/intercalated complexes have been tested, as follows. 

Another method for generating an epoxidation catalyst on a solid support is to simply absorb or non-covalendy attach the catalyst to the solid support <06MI493>. Epoxidation of olefin 6 with mCPBA and catalyst 8 provides 7 in quantitative yields and with 89% ee. The immobilization of 8 on silica gel improves the enantioselectivity of the reaction providing 7 with 95% ee. Recycling experiments with silica-8 show a decrease in both yield and the enantiomeric excess for each cycle (45% ee after 4 cycles). This is attributed to a leaching of the catalyst from the silica gel. Two other solid supports, a Mg-Al-Cl-LDH resin (LDH) and a quaternary ammonium resin (Q-resin) were also examined. It was expected that ionic attraction between 8 and the LDH or Q-resin would allow the catalyst to remain immobilized through multiple cycles better than with silica gel. Both of these resins showed improved catalytic properties upon reuse of the catalyst (92-95% ee after 4 cycles). 

In a recent study, the group of Buijsman presented a microwave-mediated preparation of a different N-imidazolium-based ionic analogue of the well-known AMEBA solid support (Scheme 7.93). With this soluble support, a set of various sulfonamides and amides was prepared, and furthermore the use of this novel linker in the synthesis of a potent analogue of the antiplatelet drug tirofiban was presented [106]. 

The term Supported Ionic Liquid Phase (SILP) catalysis has recently been introduced into the literature to describe the heterogenisation of a homogeneous catalyst system by confining an ionic liquid solution of catalytically active complexes on a solid support [68], In comparison to the conventional liquid-liquid biphasic catalysis in organic-ionic liquid mixtures, the concept of SILP-catalysis offers very efficient use of the ionic liquid. Figure 7.10 exemplifies the concept for the Rh-catalysed hydroformylation. 

A rather new concept for biphasic reactions with ionic liquids is the supported ionic liquid phase (SILP) concept [115]. The SILP catalyst consists of a dissolved homogeneous catalyst in ionic liquid, which covers a highly porous support material (Fig. 41.13). Based on the surface area of the solid support and the amount of the ionic liquid medium, an average ionic liquid layer thickness of between 2 and 10 A can be estimated. This means that the mass transfer limitations in the fluid/ionic liquid system are greatly reduced. Furthermore, the amount of ionic liquid required in these systems is very small, and the reaction can be carried in classical fixed-bed reactors. 

DNA adsorption properties were first studied using a variety of solid supports for classical analysis methods including Southern and Northern transfers, dot-blotting, colony hybridization and plaque-lifts [31,32]. Studies of the interactions between nucleic acids and nitrocellulose revealed that molecular weight, finite macromolecular conformation, ionic forces and weaker forces of attraction all play a role. DNA is retained on nitrocellulose only in... 

Various solid-supported perruthenate reagents have been designed for the oxidation of alcohols.Solid-supported NMO has also been used. A number of perruthenate systems employing O2 as the terminal oxidant have also been reported. The use of ionic liquids based upon substituted imidazolium cations as alternative solvent media for the selective oxidation of alcohols to aldehydes and ketones has also been investigated. ... 

In conventional solid-liquid or solid-gas heterogeneous catalytic systems, the catalyst is conveniently separated from the fluid-phase reaction product. When an ionic liquid is used as a phase to isolate a catalyst, the catalyst is fully dispersed and mobile and may be fully involved in the reaction. When a homogeneous catalyst is isolated by anchoring onto the surface of a solid support (e.g., by reaction with OH groups), the result may be a stable catalyst that is not leached into the reactant... 

In-situ IR-spectroscopic characterization of the Friedel-Crafts acylation of benzene in ionic liquids derived from AICI3 and FeCl3 showed that the mechanism of the reaction in ionic liquids was the same as that in 1,2-dichloroethane (128). The immobilization of ferric chloride-containing ionic liquid onto solid supports (e.g., silica and carbon) however failed to catalyze the acylation reaction, because leaching was a serious problem. When the reaction was carried out with gas-phase reactants, catalyst deactivation was observed. 

Often in the literature, the term catalyst immobilization has been used to describe the dispersion and retention of a catalyst in an ionic liquid, although it is generally understood that the catalyst in the ionic liquid is not truly immobilized as it is on a solid support surface the term catalyst immobilization is avoided in this context here. 

The coalescence of atoms into clusters may also be restricted by generating the atoms inside confined volumes of microorganized systems [87] or in porous materials [88]. The ionic precursors are included prior to irradiation. The penetration in depth of ionizing radiation permits the ion reduction in situ, even for opaque materials. The surface of solid supports, adsorbing metal ions, is a strong limit to the diffusion of the nascent atoms formed by irradiation at room temperature, so that quite small clusters can survive. 

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