Ionic coupling reactions

The dianion [Ru5C(CO)i4]2- 5 also undergoes an ionic coupling reaction with the monocation [Rh(T 4-CgHi2-l,5)2]+ in dichloromethane to afford the monoanionic cluster [Ru5RhC(CO)i4(7 4-CsH 2-l, 5)] 236 in 64% yield.134... 

Ionic coupling reactions have also proved to be useful for the preparation of carbide, phosphine, " and imino clusters of osmium and rhodium. 

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]. 

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. 

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

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

A copper-free Sonogashira coupling reaction in ionic liquids and its application to a microflow system for efficient catalyst recycling, Org. Lett. 4, 10 (2002) 1691-1694. 

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. 

Carbon-Carbon Cross-Coupling Reactions Catalyzed by Palladium Nanoparticles in Ionic Liquids... 

As carriers for proteins and enzymes biocompatible reactive microgels must be synthesized which are soluble in the serum at 37 °C. Moreover they should be hydrophilic enough that no ionic monomers are needed but they should not be soluble in water. An inert comonomer should serve as a spacer as well as a reactive solvent that may dissolve solid comonomers. The coupling reaction should be possible under mild reaction conditions. 

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When Heck reactions and other C-C coupling reactions are carried out in imidazolium-based ionic liquids, the base can react with the acidic proton on the... 

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