Aqueous conditions Heck reaction

In aqueous DMF, the reaction can be applied to the formation of C-C bonds in a solid-phase synthesis with resin-bound iodobenzoates (Eq. 6.33).80 The reaction proceeds smoothly and leads to moderate to high yield of product under mild conditions. The optimal conditions involve the use of 9 1 mixture of DMF-water. Parsons investigated the viability of the aqueous Heck reactions under superheated conditions.81 A series of aromatic halides were coupled with styrenes under these conditions. The reaction proceeded to approximately the same degree at 400°C as at 260°C. Some 1,2-substituted alkanes can be used as alkene equivalents for the high-temperature Heck-type reaction in water.82... 

Greater durability of the colloidal Pd/C catalysts was also observed in this case. The catalytic activity was found to have declined much less than a conventionally manufactured Pd/C catalyst after recycling both catalysts 25 times under similar conditions. Obviously, the lipophilic (Oct)4NCl surfactant layer prevents the colloid particles from coagulating and being poisoned in the alkaline aqueous reaction medium. Shape-selective hydrocarbon oxidation catalysts have been described, where active Pt colloid particles are present exclusively in the pores of ultramicroscopic tungsten heteropoly compounds [162], Phosphine-free Suzuki and Heck reactions involving iodo-, bromo-or activated chloroatoms were performed catalytically with ammonium salt- or poly(vinylpyrroli-done)-stabilized palladium or palladium nickel colloids (Equation 3.9) [162, 163],... 

Arenediazonium salts reacted with tetramethyltin under very mild conditions in acetonitrile yielding the corresponding toluenes [63] and this reaction could be carried out in aqueous media, as well [64] (Scheme 6.29). Similar to the Heck reactions discussed in 6.1.1, a one-pot procedure could be devised starting from anilines, with no need for the isolation of the intermediate diazonium salts. The pH of the solutions should always be kept below 7 in order to avoid side reactions of the diazonium salts, however, unlike with the Heck reactions, HCl or H2SO4 can also be used. Since organotin compounds are easily hydrolysed in acidic solutions, a careful choice of the actual pH is required to ensure fast and clean reactions. Diaryliodonium salts are hydrolytically stable and also react smoothly with various organotin compounds (Scheme 6.29) [65]. 

Kiji, J., Okano, T. and Hasegawa, T. (1995) Palladium-catalyzed arylation of ethylene (the Heck reaction) under aqueous conditions. J. Mol. Catal. A Chem.,... 

A nonclassical substrate for the Heck reaction is 2,3-epoxycyclohexanone. The reactivity of this molecule under Heck coupling conditions is most likely attributed to its in situ isomerization to 1,2-cyclohexanedione. The 1,2-diketone subsequently reacts with aryl bromides as an olefin via the enol tautomer. Thus, within 5 to 30 min of directed microwave heating of the aqueous PEG mixture, up to 13 different C3-arylations were conducted using less than 0.05 mol % palladium acetate and no phosphine ligand (Scheme 12) [51]. 

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. 

Furthermore, the Heck reaction also takes place under some unconventional conditions, such as superheated water (e.g., 260°C)," aqueous solution, ionic liquid, and even without the presence of ligand other hand, some other transition metals also... 

Figure 15.5 Water-soluble phosphine ligands tested in Mizoroki Heck reactions under aqueous conditions.

It has been found that the Heck reaction can be accomplished under PTC conditions with inorganic carbonates as bases under mild conditions at room temperature. Such conditions can be used in case of substrates, like methyl vinyl ketone, which do not survive the usual conditions of Heck arylation (action of base at high temperature). Subsequently, it has been shown that the Heck reaction can be carried out in water and aqueous organic solvents, catalysed by simple palladium salts in presence of inorganic bases like K2CO3, Na2C03, NaHC03, KOH etc. ... 

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. 

Besides iodo- and bromoderivatives, the Heck reaction has been applied to diazonium and iodonium salts, always in phosphine-free processes. The former are usually processed in alcoholic solutions, sometimes in the presence of water.f The reactions of aryl and alkenyliodonium salts are done under very mild conditions in aqueous solvents,t i t ° which allows one to use this method for selective arylation (alkenyladon) of allylic alcohols (Scheme 34). 

Aqueous conditions are suitable for more complex multistep reactions. Thus, a tandem Heck-Suzuki process has been realized in a heterogeneous system with high content of watert (Scheme 53). 

As conclusion, the high catalytic efficiency of the polymeric palladacycle derived from Kaiser oxime resin similar to that of related unsupported dimeric pal-ladacycles was observed in the Mizoroki-Heck reaction. This polymer showed a good catalytic activity, not only in organic but also in aqueous media, for aryl iodides, bromides, and activated chlorides with high turnover numbers (TONs), rmder aerobic conditions at relatively lower temperature. 

Many transition metal-catalyzed cross-couplings such as the Stille coupling, the Heck reaction, the Suzuki reaction, and the Trost-Tsuji reactions have been carried out in aqueous conditions.In oiu own studies, we recently discovered that various palladium-catalyzed couplings of the above types can be performed in air and water. For example, a highly... 

---