Water-based reactions Heck reaction

Seddon s group described the option of carrying out Heck reactions in ionic liquids that do not completely mix with water. These authors studied different Heck reactions in the triphasic [BMIM][PFg]/water/hexane system [91]. While the [BMIM]2[PdCl4] catalyst used remains in the ionic liquid, the products dissolve in the organic layer, with the salt formed as a by-product of the reaction ([H-base]X) being extracted into the aqueous phase. 

The palladium complex of the dibenzofuran-based water-soluble tertiary phosphine 49 was found catalytically active for the internal Heck reaction of N-aUyl-o-iodoaniline in CH3CN/H2O l/l(Scheme 6.6) [21],... 

Like the Heck reaction, the Suzuki coupling can use water as the solvent. Water-based Suzuki reactions are attractive both for industrial processes and for labs that want to minimize the purchase and disposal of toxic solvents. The following examples show the variety of combinations that can be coupled using Suzuki reactions. 

A further evidence on the acceleration enjoyed by a typical Pd-catalysed reaction, the Heck reaction, in an ionic phase ( V-mcthyl-Y.Y. V.-trioctylammonium chloride or Aliquat 336) is found in a triphasic protocol developed by Tundo and coworkers. 7b.The arylation of electron poor olefins is catalysed by palladium supported on charcoal (Pd/C) and is carried out in the heterogeneous isooctane/Aliquat 336/water system (Figure 27). Under this multiphasic condition, Aliquat 336 forms a third liquid phase between the organic and the aqueous phase that traps the catalyst. The use of phosphines is not necessary. As a matter of fact, Aliquat 336 incorporates the solid-supported catalyst and ensures an efficient mass transfer between the bulk phases resulting in an increase of the reaction rate of an order of magnitude compared to the reaction in the absence of the ionic liquid. A determing role is played by the base while I LN drives the reaction towards the formation of ethyl cinnamate, reaction carried out in the presence of KOH lead to formation of Ullmann dimerisation products. 

Heck reaction in the presence of the aminoethylimidazolium salt 1 requires no additional base. Using the l,l -bis(di-t-butylphosphino)ferrocene ligand Heck reaction is accomplished at room temperature in water to prepare styrenes and cinnamate esters. The same conditions are conducive to performing Suzuki coupling. ... 

The PTC technique has been advantageously applied to alkylationand Pd-catalyzed arylation of alkynes. For the Heck reaction, the proper selection of Pd, base, and PTC allows a reaction in water, aqueous/organic solvent mixture, or strictly anhydrous... 

Table 5.4), prepared from reduction of Pd(II) salts with potassium graphite. The results suggested that this catalyst was not very active. However, some years later Jikei and Kakimoto [73] prepared a more active Pd/CGr based on a smaller crystallite size. In 2002, Kohler et al. [74] studied a variety of Pd/C catalysts with different properhes (Pd dispersion, oxidation state, water content, conditions of catalysts preparation etc.) in the Heck reaction of aryl bromides with olefins (entry 4, Table 5.4). The authors pointed out the hypothesis that the leached Pd from the support is the active species and the solid Pd/C catalyst acts as a reservoir that delivers catalytically active Pd species into solution. All catalysts were obtained by wet impregnation (5% Pd loading). The Heck reaction can also be conducted in ionic liquids through promotion by microwave irradiation. Moreover the reaction of iodobenzene with methylacrylate in NMP was reported to be accelerated by ultrasound [75]. The ionic liquid containing the catalyst system was used five consecutive times with only a slight loss of activity (entry 5, Table 5.4) [76]. Perosa [77] reported the addition of a phase transfer catalyst to an ionic liquid as a method to accelerate the C-C coupling reaction. As far as we know, only by using ionic liquids has Pd on carbon been recovered and reused with success.

The application of ethylene in Heck reactions often shows different activities from other olefins, because of Wacker-type side reactions. It was found, however, that iodo- and acceptor-substituted bromoarenes are cleanly converted in aqueous media to the corresponding styrenes utilizing a palladiium-TPPMS complex [13]. Furthermore, high-purity 0- and p-vinyltoluenes were prepared in a dimethyl-formamide/water mixture with palladium tri(o-tolyl)phosphine complexes [14]. Here, the role of water may be the dissolution of the inorganic base (potassium carbonate) in the organic media. 

Water-soluble macromolecular metal complexes based on terminally functionalized ethylene oxides and ethylene oxide-propylene oxide block copolymers have been used as catalysts for hydroformylation, hydrogenation, Wacker oxidation of imsaturated compounds, hydroxylation of aromatic compounds, oxidation of saturated and alkylaromatic hydrocarbons, metathesis, Heck reaction, and some asymmetric reactions. 

The products were surprisingly similar to those found in typical organic solvents, although the isolated yields were lower (17-54% based on conversion of the iodoben-zene). A group at Clemson University have also reported aqueous Heck reactions at 260 °C and in supercritical water at 400 °C. ... 

The Heck reaction is compatible with water, and water-soluble catalysts have successfully been employed (entry 10). Alkali metal salts (NaHCOj, K2CO3, and KOAc) are effective bases in the smooth reactions of acrylic acid with o-, m-, or p-iodobenzoic acid or p-iodophenol in which water-soluble salts are formed and very high yields are encountered (entry 11). Activated heteroaryl chlorides are good arylpalladium precursors (entry 12) while nonactivated aryl chlorides have to date been considered to be less useful in the Heck reaction. In entry 13, the recent protocol devised by Littke and Fu for arylation with nonactivated chlorobenzenes is shown. ... 

Heck reaction has some important applications in industry because it is one of the effective tools for the formation of new C-C bonds [49]. The traditional Heck reaction is performed with a Pd catalyst with phosphine ligands in the presence of a base under an inert atmosphere. However, the expensive Pd complex is often lost at the end of the reaction, which limits the large-scale application of Heck reactions. On the other hand, phosphine ligands, especially the electron-rich ones, are often toxic, and water and air sensitive. For industrial application, it is important to have good strategies for catalyst-product separation and catalyst recycling. SIL catalyst is one of the promising alternatives for the development of eco-friendly processes. 

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