Palladium Catalysis in Ionic Liquids

Ionic liquids have been used as solvents for palladium catalyzed reactions in attempts to address some of the common problems associated with the use of palladium catalysts. Specifically, these include decomposition 

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P-P bonds, 67-68 Palladium catalysis in ionic liquids, overview, 256-257... 

Mo, J., Xu, L.J., Ruan, J.W. et al. (2006) Regioselective Heck arylation of unsaturated alcohols by palladium catalysis in ionic liquid. Chem. Commun., 3591-3. 

Enantioselective Catalysis in Ionic Liquids 259 Table 7.10 Palladium catalyzed asymmetric allylic amination in [bdmim][ BF4]. 

In the last few years numerous reports have been published in the field of microwave-promoted aryl halide cyanation, utilizing nickel [71], palladium [72,73] and copper [74,75] catalysis. Even water [75] and ionic liquids [76] have proven useful as solvents in these processes. Srivastava and Collibee have exemplified a swift and dynamic procedure using polymer-supported triphenyl phosphine to enable easy subsequent removal through filtration [72]. As shown in Scheme 19, both bromides and iodides could be activated using palladium catalysis in DMF. Even without optimization of the individual reaction times, the overall process time involving simple filtration and extraction for compound isolation appears to be short. 

Very few examples of the oxidation of olefins to ketones in ionic liquids have been reported. In one case, [C4Ciim][BF4] or [C4Ciim][PF6] were used in the palladium-catalysed oxidation of styrene to acetophenone with H2O2 as oxidant, however the concept of biphasic catalysis was not exploited and no attempts were made to recycle the catalyst.[131 The ionic liquid serves the purpose of a co-catalyst rather than that of a reaction medium. 

The Mizoroki-Heck reaction in liquid imidazolium salts as the solvent is a special case of an in situ system Under the reaction conditions NHC complexes of palladium are formed as the active catalyst from the solvent and the ligand-free palladium precursor. In general, ionic liquids are novel reaction media for homogeneous catalysis. They allow easy separation of product and catalyst after the reaction. ... 

In none of the above cases has a reaction been performed whilst taking the EXAFS data. Hamill et al. [50] have investigated catalysis of the Heck reaction by palladium salts and complexes in room-temperature ionic liquids. On dissolution of palladium ethanoate in [BMIMj and N-butylpyridinium ([BP] ) hexafluorophos-phate and tetrafluoroborate ionic Hquids, and triethyl-hexyl ammonium bis(trifluo-romethanesulfonyl)imide, a gradual change from ethanoate coordination to the formation of palladium metal was observed in the Pd K-edge EXAFS, as shown in Figure 4.1-13. 

Dupont, J., de Souza, R. F., Suarez, P. A. Z. Ionic Liquid (Molten Salt) Phase Organometallic Catalysis. Chem. Rev. 2002, 102, 3667-3691. Amatore, C., Bahsoun, A. A., Jutand, A., Meyer, G., Ntepe, A. N., Ricard, L. Mechanism of the Stille Reaction Catalyzed by Palladium Ligated to Arsine Ligand PhPdl(AsPh3)(DMF) Is the Species Reacting with Vinylstannane in DMF. J. Am. Chem. Soc. 2003,125,4212-4222. 

Another important reaction, the Heck reaction catalyzed by palladium salts and complexes, has been studied as well via EXAFS [191]. The authors of the study highlight the importance of correlating structural data with reaction kinetics. Among others the authors showed that in chloride-based ionic liquids the Heck reaction shows poor reactivity until, at some point during the reaction, Pd nanoparticles form, which initiates the reaction. They pointed out that the correlation between the induction period and the nanoparticle formation was strong, and although the metal may not be the active site for the catalysis [91], the presence of Pd(0) is clearly important. 

Palladium electrodeposition is of special interest for catalysis and for nanotechnology. It was reported that it can be deposited from basic chloroaluminate liquids, in the acidic regime the low solubility of PdCh and passivation phenomena complicate the deposition [52]. However, thick Pd layers are difficult to obtain from basic chloroaluminates. With different melt compositions and special electrochemical techniques at temperatures up to 100 °C Endres et al. succeeded in depositing mirror bright and thick nanocrystalline palladium coatings [10]. Stm et al. have reported that Pd-Au [53] and Pd-Ag [54] alloys can be electrodeposited in a basic l-ethyl-3-methylimidazolium chloride/ tetrafluroborate ionic liquid. 

New concepts or reaction media may also result in serious effects toward catalysis. Thus, the use of ionic liquids on the basis of C -unsubstituted imidazohum salts as solvents for the palladium-catalyzed methoxycarbonylation of iodobenzene leads to protolysis of the Pd-phenyl intermediate and the undesired formation of benzene [41]. Chlorobenzene is also formed in the presence of imidazohum chlorides. This explains why phosphites added did not show modifying properties, because they are... 

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