Ionic tetraalkylphosphonium-based

Besides the advantage of recyclability, reactions in ionic liquids are generally faster and are run under milder conditions than reactions with conventional solvents. Further activation may come from ultrasonic agitation.520 Since the majority of ionic liquids used are imidazolium salts, the effect of these solvents can be at least partly attributed to the in situ formation of carbene complexes (Section 9.6.3.4.10).521 Cross-coupling of ArB(OH)2 can also be efficiently performed in ionic liquids based on long-chain tetraalkylphosphonium salts, in which case aryl bromides and some aryl chlorides can be processed in the presence of the trivial ligand PPh3.522... 

Sesto RED, Corley C, Robertson A, et al. Tetraalkylphosphonium-based ionic hquids. J. Organomet. Chem. 2005. 690, 2536-2542. 

Weng LJ, Liu XQ, Liang YM et al (2007) Effect of tetraalkylphosphonium based ionic liquids as lubricants on the tribological performance of a steel-on-steel system. Tribol Lett 26 11-17... 

Penalber, C., Adamova, G., Plechkova, N.V, Baldelli, S. and Seddon, K.R., Sum-frequency generation spectroscopy of tetraalkylphosphonium-based ionic liquids at the air-liquid interface (to be published). 

Ionic liquid stability is known to be a function of temperature (for details see Section 3.1) but the presence of nucleophiles/bases and the water content also have to be considered. There is no doubt that, under the conditions of a catalytic reaction, temperature stability issues are more complicated than imder the conditions of a TGA experiment. The presence of the catalyst complex, the reactants and impurities in the system may well influence the thermal stability of the ionic liquid. Basic and nucleophilic counter-ions, reactants and metal complexes may not only lead to deprotonation of 1,3-dialkylimidazolium ions (to form carbene moieties that will undergo further consecutive reactions) but will also promote thermal dealkylation of the ionic liquid s cation. If basic reaction conditions are required for the catalysis only tetraalkylphosphonium ions can be recommended as the ionic liquid s cation at this point in time. Tetraalkylphosphonium cations have been recently shown to display reasonabe stability, even under strongly basic conditions [290]. In contrast, all nitrogen-based cations suffer to some extent from either carbene formation, Hofmann elimination or rapid dealkylation (with alkyl transfer onto the nucleophilic anion). 

They have discovered that the use of 1,3-dialkylimidazolium cations led the reactions less efficient than when conventional solvents were used (it may be due to carbene formation [59]). In comparison, tetraalkylphosphonium ionic liquids led the reaction much more efficiently than organic solvents, or with neat reagents. Based on these observations, they have developed the following reaction and commercialized it (Eq. 20.5). 

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