Ionic neutral catalyst systems

A number of Suzuki reactions (see Scheme 10.9) have been conducted in ionic liquids using Pd(PPh3)4 as the catalyst at 30 °C [10], Although the catalyst is neutral, the ionic liquid-catalyst solution can be used repeatedly without a decrease in activity. In fact, the catalyst shows a significant increase in activity compared to when it is used in conventional organic solvents. Another attractive feature of the system is that NaHC03 and Na[XB(OH)2] (X = halide) by-products can be removed from the ionic liquid-catalyst phase by washing with water. 

The first investigations of rhodium-catalyzed hydroformylation in room-temperature Hquid molten salts were published by Chauvin et al. in 1995 [6, 67]. The hydroformylation of 1-pentene with the neutral Rh(CO)2(acac)/triarylphosphine catalyst system was carried out as a biphasic reaction with [BMIM][Pp6] as the ionic liquid. 

PCH materials offer new opportunities for the rational design of heterogeneous catalyst systems, because the pore size distributions are in the supermicropore to small mesopore range (14-25A) and chemical functionality (e.g., acidity) can be introduced by adjusting the composition of the layered silicate host. The approach to designing PCH materials is based on the use of intercalated quaternary ammonium cations and neutral amines as co-surfactants to direct the interlamellar hydrolysis and condensation polymerization of neutral inorganic precursor (for example, tetraethylorthosilicate, TEOS) within the galleries of an ionic lamellar solid. 

The nickel and palladium compounds described above are useful in processes for polymerising various olefins, and optionally also for copolymerising olefinic esters, carboxylic acids or other functional olefins with these olefins. When (I) is used as a catalyst, a neutral Lewis acid or a cationic Lewis or Bronsted acid whose counterion is a weakly coordinating anion is also present as part of the catalyst system. The neutral Lewis acid is originally uncharged (i.e. not ionic). Suitable neutral Lewis acids include SbFs, A B and BF3. By a cationic Lewis acid is meant a cation with a positive charge such as Ag+, H+ and Na+. 

The organic solvent-free variant of the l2-catalyzed aziridination was subsequently developed by Komatsu and coworkers by making use of the ionic character of chloramine-T (Scheme 2.34) [53]. As a result of the survey on quaternary ammonium salts as a phase-transfer catalyst, they identified benzyltriethylammounium chloride (BTEAC) as the most effective catalyst among those tested for the acceleration of the aziridination in aqueous media. For example, the aziridination of styrene in the system completed within 1 h to give the aziridine product quantitatively, whereas a similar reaction under their previous MeCN/neutral buffer system required lOh for the completion. The notable feature of this system is the applicability to large-scale synthesis. The preparation and purification of 2-phenylaziridine (23.5 g, 86%, >99% purity) using only decantation and recrystallization processes was demonstrated. 

Chloroaluminate(III) ionic liquid systems are perhaps the best established and have been most extensively studied in the development of low-melting organic ionic liquids with particular emphasis on electrochemical and electrodeposition applications, transition metal coordination chemistry, and in applications as liquid Lewis acid catalysts in organic synthesis. Variable and tunable acidity, from basic through neutral to acidic, allows for some very subtle changes in transition metal coordination chemistry. The melting points of [EMIM]C1/A1C13 mixtures can be as low as -90 °C, and the upper liquid limit almost 300 °C [4, 6]. 

Thiolates, generated in situ by the action of ammonium tetra-thiomolybdate on alkyl halides, thiocyanates, and disulfides, undergo conjugate addition to a, (1-unsaturatcd esters, nitriles, and ketones in water under neutral conditions (Eq. 10. II).29 Conjugate addition of thiols was also carried out in a hydrophobic ionic liquid [bmim]PF6/water-solvent system (2 1) in the absence of any acid catalyst to afford the corresponding Michael adducts in high to quantitative yields with excellent 1,4-selectivity under mild and neutral conditions (Eq. 10.12). The use of ionic liquids helps to avoid the use of either acid or base catalysts... 

This simple example may illustrate that in general the reaction of an organic halide salt [cation]X with an excess of a Lewis-acid MXy can result in new catalytic materials even if other Lewis-acids are applied than AICI3. In contrast, the use of other Lewis-acids to form the ionic liquid of type [cation][MXy+i] + excess MXy (the excess of MXy may be dissolved in the neutral ionic liquid or may form acidic anionic species such as e.g. [M2X2y+i]-) gives access to new combinations of properties (e.g. a liquid, less oxophilic, Lewis-acidic catalyst with defined solubility and acidity properties). In Table 2 other examples of ionic liquids are presented which are formed by the reaction of an organic halide salt with different Lewis-acids. All these systems should be in principle useful acidic catalysts for synthetic organic chemistry even if not all displayed examples have been already discribed in the literature for this application. 

Indeed, it is not intended to discuss recent developments of conventional cationic polymerizations, i. e., polymerizations in which a cat-ionically initiable vinyl compound is attacked by a suitable (usually Friedel-Cr afts halide) catalyst and the growing ion is neutralized by the corresponding MeX type gegen-ion, etc. Rather, this review concerns unusual cationic polymerization systems and mechanisms which have not been discussed in a comprehensive manner. 

The catalyst for hydroformylation is a rhodium(I) hydride species, which is clearly distinct from the species that are active for hydrogenation. The hydrogenation catalysts are cationic Rh(I)+ or neutral Rh(I)Cl species. Carbonylation of alcohols also requires an ionic Rh(I) species, e.g. [Rl CO y-- Often rhodium(I) salts are used as the precursor for hydroformylation catalysts. Under the reaction conditions (H2, CO, ligands, temperature >50°C) these salts are converted to a rhodium hydride complex, although there are several papers that seem to invoke cationic rhodium species as the catalysts. Chlorides have a particularly deleterious effect on the activity (i.e. they are not converted into hydrides under mild conditions) and it has been reported that the addition of bases such as amines has a strong promoting effect on such systems ... 

A substantial increment in the relative lifetime values of copper-impregnated samples is also observed after sodium bicarbonate treatment (Table XI). Thus, the copper-catalyzed system also benefits from the neutralization process. However, the change in relative stability values does not compare as favorably. A small but consistently adverse effect upon relative stability indicates that unlike iron species, ionic copper species can be more effective catalysts at lower acidic concentra-... 

Due to the good solubility of organometallic compounds, ionic liquids have been used as reaction media, replacing traditional molecular solvents, or as the catalyst-supporting phase in a biphasic system. Influences of the ionic liquid on the reaction rate and selectivity can mostly be explained by the reactivity of the anion, which can be noncoordinating or coordinating as well as Lewis-acidic, Lewis-basic or neutral. The cation, in contrast, is considered to be essentially noncoordinating and innocent. 

The efficacy of an iridium/iodide catalyst for methanol carbonylation was discovered by Monsanto at the same time as their development of the process using the rhodium/iodide catalyst [5]. Mechanistic investigations by Forster employing in situ HPIR spectroscopy revealed additional complexity compared to the rhodium system [115]. In particular, the carbonylation rate and catalyst speciation were found to show a more complicated dependence on process variables, and three distinct regimes of catalyst behavior were identified. At relatively low concentrations of Mel, H20, and ionic iodide, a neutral iridium (I) complex [Ir(CO)sI] was found to dominate, and the catalytic reaction was inhibited by increasing the CO partial pressure. Addition of small amounts of a quaternary ammonium iodide salt caused the dominant iridium species to become an Ir(III) methyl complex, [Ir(CO)2l3Me]. Under these conditions, the rate... 

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