Iodide-based ionic liquids

Rhodium catalyzed carbonylations of olefins and methanol can be operated in the absence of an alkyl iodide or hydrogen iodide if the carbonylation is operated in the presence of iodide-based ionic liquids. In this chapter, we will describe the historical development of these non-alkyl halide containing processes beginning with the carbonylation of ethylene to propionic acid in which the omission of alkyl hahde led to an improvement in the selectivity. We will further describe extension of the nonalkyl halide based carbonylation to the carbonylation of MeOH (producing acetic acid) in both a batch and continuous mode of operation. In the continuous mode, the best ionic liquids for carbonylation of MeOH were based on pyridinium and polyalkylated pyridinium iodide derivatives. Removing the highly toxic alkyl halide represents safer, potentially lower cost, process with less complex product purification. 

It appeared that, we needed to limit or omit the ethyl iodide if we were going to operate the ethylene carbonylation in ionic liquids. Unfortunately, the previous literature indicated that EtI or HI (which are interconvertible) represented a critical catalyst component. Therefore, it was surprising when we found that, in iodide based ionic liquids, the Rh catalyzed carbonylation of ethylene to propionic acid was still operable at acceptable rates in the absence of ethyl iodide, as shown in Table 37.2. Further, we not only achieved acceptable rates when omitting the ethyl iodide, we also achieved the desired reduction in the levels of ethyl propionate. More importantly, when the reaction products were analyzed, there was no detectable ethyl iodide formed in situ. However, we should note that we now observed traces of ethanol which were normally undetectable in the earlier Ed containing experiments. 

The rhodium catalyzed carbonylation of ethylene and methanol can be conducted in the absence of added alkyl halide if the reactions are conducted in iodide based ionic liquids or molten salts. In the case of ethylene carbonylation, the imidazolium iodides appeared to perform best and operating in the absence of ethyl iodide gave improved selectivities relative to processes using ethyl iodide and ionic hquids. In the case of... 

Nitriles are versatile and important components of a range of dyes, natural products, and pharmaceuticals. Aryl nitriles can be synthesized from aryl halides by direct reaction between aryl halides and copper cyanide, known as the Rosemund von Braun reaction [36]. These cyanation reactions can have several disadvantages, in particular the long reaction times required. Ren and coworkers showed that 1,3-dialkylimidazolium halide-based ionic liquids can be used as solvents in the Rosemund von Braun reaction [37]. Complete conversion, based on GC-MS analysis, was achieved after 24 h at 90 °C. When using microwave irradiation and ionic liquid as a solvent, Leadbeater and coworkers showed the reaction times could be reduced to between 3 and 10 min [38]. Under the optimized reaction conditions, 2 equiv. CuCN and 1 equiv. aryl halide were rapidly heated to 200 °C in [i-PrMIM]Br as solvent. Representative results are collected in Table 7.3. The microwave method works as well as the conventional method for a range of aryl iodide and aryl bro-... 

Useful ligands for Mizoroki-Heck reactions stem from the family of imidazolium-based ionic liquids which have a 2-pyridyl residue at C-2 [27]. For example, reactions of such compounds with palladium(II) chloride gave palladium(II) ionic liquid complexes like 1 (Scheme 15.1). Mizoroki-Heck reactions of various aryl iodides or bromides, with acrylic esters or styrene in the presence of these catalysts were achieved in yields up to 94%. The catalyst was recycled efficiently. In most applications the organic compound was... 

The low volatility of ionic liquids and the easy separation of catalysts (which usually remain in these polar media) have made ionic liquids an interesting alternative to typically used organic solvents. Rather unsatisfactory results have been obtained in both copper-mediated [36] and copper-free [37] Sonogashira reactions, with aryl iodides being the only aromatic electrophiles coupled at reaction temperatures between 60 and 80 °C. It should further be noted that imidazolium-based ionic liquids are not necessarily innocent solvents, but can be deprotonated in the presence of bases to generate N-heterocycUc carbenes (NHCs). 

This reactivity was found not to be confined to Pd -complexes. Indeed, addition of methyl iodide to a solution of a [Ni°(NHC)2] complex at very low temperature initially yielded the desired [Ni°I(Me)(NHC)2]-lype complex, which then rapidly decomposed to form a C2-methylated imidazolium salt (Fig. 2). Studies on Ni-NHC catalysed alkene dimer-isation also illustrated the formation of C2-allq lated imidazolium salts. However, since these reactions were performed in imidazolium-based ionic liquids, rapid reformation of the active catalyst via oxidative... 

In a very unique approach cyano substituted triazolate based ionic liquids were prep>ared by exchanging the anions in common ionic liquids such as [emim][l], [empyrr][I] or [epy][T with silver 4,5-dicyano-triazolate in water. Here the triazolate ring serves as a counter-anion with delocalized negative charge, and results in a ditninished cation-anion interaction due to its decreased charge density. Hence, there is a decrease in the viscosity of the resulting ionic liquids as compared to those with iodide counter-ions (Kitaoka, Nobuoka et al. 2010). 

More interesting was the elemental analysis of the residue. Whereas a 2 1 AcOH [DMEpy]l should have contained 33% iodine, the elemental analysis indicated the residue contained only 0.7% iodine. This clearly indicated that we no longer had an iodide salt, but more likely had an acetate salt, most likely a 2 1 mixture of AcOH [DMEpy] [OAc]. (The formation of a 2 1 salt would be typical of our experience with ionic liquids. In practice they normally tenaciously retain ca. 2 mol AcOH/mol of ionic liquid, a phenomena we noted in om earlier reports. (3) Closer comparison of the salt obtained and low levels of Mel detected in the effluent indicated that the amount of [DMEpy] [OAc] generated closely matched the total Mel (ca. 90-95% yield of Mel based on [DMEpy][OAc].) Further, the elemental analysis was unable to detect any Rh in the effluent, so we could conclude that there was no aspiration occurring. This clearly indicated that our ionic liquid loss was due to metathesis of the ionic liquid from the iodide to the acetate salt, likely due to reaction (23) which likely sublimed overhead. In principle, the miniscule amount of Mel and ionic liquid could be returned to the reactor to maintain the process. 

In SILP carbonylation we have introduced a new methanol carbonylation SILP Monsanto catalyst, which is different from present catalytic alcohol carbonylation technologies, by using an ionic liquid as reaction medium and by offering an efficient use of the dispersed ionic liquid-based rhodium-iodide complex catalyst phase. In perspective the introduced fixed-bed SILP carbonylation process design requires a smaller reactor size than existing technology in order to obtain the same productivity, which makes the SILP carbonylation concept potentially interesting for technical applications. 

Room-temperature ionic liquids are attractive due to their chemical and thermal stability, negligible vapor pressure, high ionic conductivity, and ample electrochemical window. Their properties can be varied by a rational choice of the cations and of the anions and can represent an important iodide source for an I /I3 -based electrolyte (Fig. 17.12). 

Gratzel Cell with an Ionic Liquid Based on Iodide... 

The second synthetic approach involved the reaction of 1-imidazole, chloroetha-nol, and sodium ethoxide to form (2-hydroxyethyl)imidazole. Treatment of this with dicyclohexylcarbodiimide (DCC) to form the N,N -dicyclohexyl isourea followed by reaction with hydroxybenzaldehyde and quaternization with an alkyl iodide gave the iodide-based supported benzaldehyde. Anion exchange then was performed to generate the desired supported ionic liquids (Scheme 7.14). 

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