Ionic Suzuki reaction

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. 

BMIM]BF4 was applied to a Suzuki reaction. The active catalyst was a trico-ordinated [Pd(PPh3)2(Ar)][X] complex that formed after oxidative addition of aryl halide to [Pd(0)(PPh3)4] 211). The hydrophobic ionic liquid does not compete with the unsaturated organic substrate for the electrophilic active metal center. 

Scheme 5. Ionic liquid mediated solid-phase Suzuki reaction of immobilized 4-iodophenol.

The formation of NHC complexes is clearly going to be important to the palladium chemistry seen in ionic liquids. The formation of such complexes has been seen to be advantageous (as in the Suzuki reaction) or disadvantageous (as in the telomerization of butadiene with methanol), depending on the particular system being investigated. What is yet to be seen is if the ionic liquid can be manipulated to generate NHC s with particular useful properties. 

Wong, H., Pink, C., Ferreira, F., and Livingston, A. (2006) Recovery and reuse of ionic liquids and palladium catalyst for Suzuki reactions using organic solvent nanofiltration. Green Chem., 8, 373-379. 

Different conditions (including additives and solvent) for the reaction have been reported,often focusing on the palladium catalyst itself," or the ligand." Catalysts have been developed for deactivated aryl chlorides," and nickel catalysts have been used." Modifications to the basic procedure include tethering the aryl triflate or the boronic acid to a polymer, allowing a polymer-supported Suzuki reaction. Polymer-bound palladium complexes have also been used." " The reaction has been done neat on alumina," and on alumina with microwave irradiation." Suzuki coupling has also been done in ionic liquids," in supercritical... 

In each case, the mechanism involves generation of an aryl radical from a covalent azo compound. In acid solution, diazonium salts are ionic and their reactions are polar. When they cleave, the product is an aryl cation (see p. 856). However, in neutral or basic solution, diazonium ions are converted to covalent compounds, and these cleave to give free radicals (Ar and Z"). Note that radical reactions are presented in Chapter 14, but the coupling of an aromatic ring with an aromatic compound containing a leaving group prompted its placement here. Note the similarity to the Suzuki reaction in 13-12. 

Due to the pronounced tolerance of the Suzuki reaction towards additional functional groups in the monomers, precursor strategies as well as so called direct routes can be applied for polyelectrolyte synthesis. However, the latter possibility, where the ionic functionalities are already present in the monomers, was rejected. The reason is too difficult determination of molecular information by means of ionic polymers. Therefore the decision was to apply precursor strategies (Scheme 1). Here, the Pd-catalyzed polycondensation process of monomers A leads to a non-ionic PPP precursor B which can be readily characterized. Then, using sufficiently efficient and selective macro-molecular substitution reactions, precursor B can be transformed into well-defined PPP polyelectrolytes D, if appropriate via an activated intermediate C. 

Scheme 7.17. Ionic liquid-supported Suzuki reactions.

Welton and co-workers reported a detailed study of the Suzuki reaction performed in imidazolium ionic liquids using palladium phosphine based catalysts, and found that mixed phosphine-NHC palladium complexes of the formula [(IBuMe)Pd(PPh3)2X] were formed. All catalytic conditions leading to formation of these complexes were successful in affecting the Suzuki reaction, and those conditions in which they could not be detected showed no conversion. This strongly suggests that [(NHC)Pd] complexes are relevant to reactions run in imidazolium-based ionic liquids. 

Ionic liquids are being researched extensively in the fields of hydrogenation, oxidation,oligomerization, hydroformylation, Heck and Suzuki reactions, Trost-... 

To date a number of reactions have been carried out in ionic liquids [for examples, see Dell Anna et al. J Chem Soc, Chem Commun 434 2002 Nara, Harjani and Salunkhe Tetrahedron Lett 43 1127 2002 Semeril et al. J Chem Soc Chem Commun 146 2002 Buijsman, van Vuuren and Sterrenburg Org Lett 3 3785 2007]. These include Diels-Alder reactions, transition-metal mediated catalysis, e.g. Heck and Suzuki coupling reactions, and olefin metathesis reactions. An example of ionic liquid acceleration of reactions carried out on solid phase is given by Revell and Ganesan [Org Lett 4 3071 2002]. 

Scheme 5.2-18 Pd-catalyzed Suzuki cross-coupling reaction in a [BMIM][BF4] ionic liquid.

Sulfonamido-l,3,4-oxadiazoles 141 Sulfonyloximes 147 Supports, functional, ionic liquid 115 Suzuki couplings 21,122 Suzuki-Miyaura reaction 164... 

Miao, W. Chan, T.H. (2003) Exploration of Ionic Liquids as Soluble Supports for Organic Synthesis. Demonstration with a Suzuki Coupling Reaction. Organic Letters, 5, 5003-5005. 

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