Heck solvent choice

The choice of an ionic liquid was shown to be critical in experiments with [NBuJBr (TBAB, m.p. 110°C) as a catalyst carrier to isolate a cyclometallated complex homogeneous catalyst, tra .s-di(ri-acetato)-bis[o-(di-o-tolylphosphino) benzyl] dipalladium (II) (Scheme 26), which was used for the Heck reaction of styrene with aryl bromides and electron-deficient aryl chlorides. The [NBu4]Br displayed excellent stability for the reaction. The recycling of 1 mol% of palladium in [NBu4]Br after the reaction of bromobenzene with styrene was achieved by distillation of the reactants and products from the solvent and catalyst in vacuo. Sodium bromide, a stoichiometric salt byproduct, was left in the solvent-catalyst system. High catalytic activity was maintained even after the formation of visible palladium black after a fourth run and after the catalyst phase had turned more viscous after the sixth run. The decomposition of the catalyst and the formation of palladium... 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

The standard Mizoroki-Heck reaction is the substitution of a vinylic hydrogen by an alkenyl or aryl group catalysed by palladium(O) complexes (Scheme 11.1). Since its discovery in 1968 by Heck [1-3], this elaboration of substituted alkenes by direct C-C bond formation at the vinylic carbon centre has evolved into a synthetic transformation whose potential has only recently been exploited in the key steps of many total syntheses (Chapter 16) [4]. This recent exploitation has been due to a better understanding of the proper choice of reactants, solvent, base, additives, catalyst precursor and ligands necessary for optimal reaction conditions. 

The Heck reaction can be accomplished under phase-transfer conditions with inorganic carbonates as bases under very mild conditions even at room temperature. The reactions were carried out in a liquid-liquid system composed of aqueous solution of base and organic reagents without organic solvent. Later, this method was developed into a common-purpose Jeffery-Larock protocol. It has been shown that, depending on substrates and base, the phase-transfer Heck reaction can be accomplished in either an aqueous liquid-liquid or a nonaqueous solid-liquid system,t THi i though (he actual choice between these two techniques is often a matter of taste. 

An interesting set of conditions are the Jeffrey conditions, using a polar solvent and a quaternary ammonium salt (Table 5.1). Detailed investigations have shown that the correct choice of ammonium salt and addition of a small amount of water can be critical. Under the right conditions, many Heck reactions can mn at or near room temperature. The main effect is due to the cation of the phase-transfer catalyst, not the anion, and the effect was most marked when inorganic bases were used. The phase-transfer catalyst may facilitate the final step of the Heck mechanism. 

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