Ionic Stille reaction

Despite the clear importance of RA, its behavior is still not properly understood. This can be attributed to a very complex combination of process thermodynamics and kinetics, with intricate reaction schemes including ionic species, reaction rates varying over a wide range, and complex mass transfer and reaction coupling. As compared to distillation, RA is a fully rate-controlled process, and it dehnitely occurs far from the equilibrium state. Therefore, practitioners and theoreticians are highly interested in establishing a proper rate-based description of this process. 

Scheme 7. Ionic liquid mediated Stille reactions.

Even though none of the reactants or products in this reaction is ionic, the reaction is still assumed to involve a transfer of electrons from carbon to oxygen. To explain this, we must introduce the concept of oxidation states. 

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. 

For the Stille reaction, on-line ESI( + )-MS(/MS) monitoring allowed interception and characterization of (a) the actual catalytically active species Pd(Ph3)2, (b) the oxidative addition product 60a as the corresponding ionic species 60b, and (c) the transmetalation intermediate 62a and two products of this process 63a and 64. Gas phase reductive elimination (for 65 ) was observed. Therefore, for the first time, most of the major intermediates of a Stille reaction were intercepted, isolated, and characterized. Using ESI(-)-MS, the counteranion I was the single species detected. 

Solvents used for the Stille coupling include hydrocarbons, organic chlorides, ethereal solvents, highly polar solvents and even water, but most of the reactions are conducted in THF, DMF or NMP. The reaction was also adapted to the fluor-ous biphasic catalysis [19]. Stille reactions were also performed in ionic liquids [20], supercritical CO2 [21] and in aqueous medium [22] by using water soluble ligands. 

While certain TSILs have been developed to pull metals into the IL phase, others have been developed to keep metals in an IL phase. The use of metal complexes dissolved in IL for catalytic reactions has been one of the most fruitful areas of IL research to date. LLowever, these systems still have a tendency to leach dissolved catalyst into the co-solvents used to extract the product of the reaction from the ionic liquid. Consequently, Wasserscheid et al. have pioneered the use of TSILs based upon the dissolution into a conventional IL of metal complexes that incorporate charged phosphine ligands in their stmctures [16-18]. These metal complex ions become an integral part of the ionic medium, and remain there when the reaction products arising from their use are extracted into a co-solvent. Certain of the charged phosphine ions that form the basis of this chemistry (e.g., P(m-C6H4S03 Na )3) are commercially available, while others may be prepared by established phosphine synthetic procedures. 

However, a number of limitations are still evident when tetrafluorohorate and hexafluorophosphate ionic liquids are used in homogeneous catalysis. The major aspect is that these anions are still relatively sensitive to hydrolysis. The tendency to anion hydrolysis is of course much less pronounced than that of the chloroalu-minate melts, hut it still occurs and this has major consequences for their use in transition metal catalysis. For example, the [PF ] anion of l-hutyl-3-methylimida-2olium ([BMIM]) hexafluorophosphate was found (in the author s laboratories) to hydrolyze completely after addition of excess water when the sample was kept for 8 h at 100 °C. Gaseous HF and phosphoric acid were formed. Under the same conditions, only small amounts of the tetrafluorohorate ion of [BMlMjjBFJ was converted into HF and boric acid [10]. The hydrolytic formation of HF from the anion of the ionic liquid under the reaction conditions causes the following problems with... 

It is important to note that benzene does not behave like a typical cyclic olefin in that the benzene ring undergoes ionic substitution rather than addition reactions the ring also resists hydrogenation and is chemically more inert. Despite this, it is still a common practice to represent benzene with three double bonds as if it were 2,4,6-cyclohexatriene,... 

The reviews by Rondestvedt (1960, 1976) are outdated so far as the mechanism of the Meerwein reaction is concerned. This statement is substantiated by Rondestvedt s own comment in his 1976 review (p. 226) in which he states that the generally accepted mechanism involves the aryl radical. .., though the manner of its formation and its subsequent reaction are still controversial . Meerwein et al., in their original paper (1939), expressed the opinion that the reaction is ionic in nature. A radical mechanism was first proposed by Koelsch in 1943 (see also Koelsch and Bockelheide, 1944). He received immediate support from Bergmann et al. (1944) and Bergmann and Weizmann (1944), in spite of the fact that Koelsch s claim was based on rather uncertain and vague arguments. 

In some cases, the Q ions have such a low solubility in water that virtually all remain in the organic phase. ° In such cases, the exchange of ions (equilibrium 3) takes place across the interface. Still another mechanism the interfacial mechanism) can operate where OH extracts a proton from an organic substrate. In this mechanism, the OH ions remain in the aqueous phase and the substrate in the organic phase the deprotonation takes place at the interface. Thermal stability of the quaternary ammonium salt is a problem, limiting the use of some catalysts. The trialkylacyl ammonium halide 95 is thermally stable, however, even at high reaction temperatures." The use of molten quaternary ammonium salts as ionic reaction media for substitution reactions has also been reported. " " ... 

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