The Heck Reaction in Ionic Liquids:  A Multiphasic Catalyst System

Abstract

Heck coupling of aryl halides or benzoic anhydride with alkenes can be performed with excellent yields in room-temperature ionic liquids, which provide a medium that dissolves the palladium catalyst and allows the product and byproducts to be easily separated. Consequently, the catalyst and ionic liquid can be recycled and reused.The Heck reaction is of major importance in synthetic organic chemistry, as a carbon-carbon bond-forming reaction. 1 It is often used to functionalize aromatic rings and as an alternate to the Friedel-Crafts reaction. 2 The Heck reaction usually involves the interaction of an aromatic halide 3 or anhydride 4 with an alkene, in the presence of a palladium catalyst at typically a 1-2 mol % concentration, to give an aryl alkene. The reaction normally requires a base to be present, 5,6 particularly in the reactions of aryl halides (but not anhydrides). 4 A major problem with the Heck reaction is that the palladium catalyst is often lost at the end of the reaction. 6 Hence, a process for recycling the catalyst system is of importance.Room-temperature ionic liquids have been used to great effect as solvents for a number of reactions, for example, Friedel-Crafts reactions, 7 isomerizations of fatty acid derivatives, 8 dimerization reactions of alkenes, 9 Diels-Alder reactions, 10 and hydrogenation reactions. 11 Ionic liquids such as 1-butyl-3-methylimidazolium hexafluorophosphate 12 ([bmim][PF 6 ]) have a particularly useful set of properties, being virtually insoluble in water and alkanes but readily dissolving many transition metal catalysts. 13 Such biphasic †

Palladate salts from ionic liquids as catalysts in the Heck reaction

“…General procedure for catalytic Heck coupling in BmimPF 6 . The reaction was performed according to the protocol described by Carmichael et al 8 A 50 mL flask containing a stirring bar was loaded with palladium catalyst (10 µmol) and ionic liquid BmimPF 6 (0.550 g, 0.4 cm 3 ) and stirred at room temperature during 5 min. Then the reagents iodobenzene (1 mmol), styrene (1.2 mmol) and triethylamine (1.5 mmol) were added and the flask quickly placed under reflux in an oil bath, and the homogeneous mixture stirred and heated for 1, 3 or 6 hours at 100°C.…”

“…ILs are of special interest due to their specific properties: they present good chemical and thermal stability, low vapour pressure, wide liquid range, good solubility for a wide range of organic and inorganic reagents and they allow the facile separation of the products and recycling of the ILs containing the catalysts. 8,9 In addition to this, the use of ILs in the Heck reaction has opened new insights and perspectives: 10 (i) ILs containing imidazolium cations stabilize ligandfree precursors of catalysts (free of η 2 donor ligands such as phosphines), consequently catalysts like palladium acetate proved to be efficient even at low concentrations and have been widely used; 8,11,12 (ii) When performed in ILs, the reaction rate was significantly increased. [13][14][15] In certain cases, this enabled the regioselectivity of the products obtained to be controlled; 16 (iii)…”

Selective chemical reactions in supercritical carbon dioxide, water, and ionic liquids

“…Seddon and co-workers (207) were among the first groups to study the ILs and show their advantages in terms of recyclability, yield, and control over selectivity 8,9 (208Á 218). Other reactions such as Suzuki cross-coupling, (219Á223) TrostÁTsuji carbonÁcarbon coupling (224,225), and hydroarylation (226) were also extensively studied in ILs media.…”

Heck reaction catalyzed by Pd/C, in a triphasic—organic/Aliquat 336/aqueous—solvent system

“…[14][15][16][17][18][19][20] Multiphase conditions. The effect of A336 on the Heck coupling was then investigated in the multiphase system.…”