High temperature Heck type 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... 

Because of their convenient preparation from palladium(II) salts and stable NHC-precursors (vide supra), paUadium(ll) complexes were first examined as potential catalysts for Heck-type reactions. Due to the high thermal stability, temperatures up to 150°C can be used to activate even less reactive substrates, like, e.g., aryl chlorides. Inunobilization of such catalysts has been shown recently (vide infra) ... 

Scheme 1.7 Formation of Pd-carbene complexes by reaction between palladium acetate and imidazolium salts followed by decomplexation at high temperature and formation of catalytically active PdNPs for Heck-type reactions [33e,i,jj.

While this ligand-free copper-catalysed Mizoroki-Heck-type reaction required relatively high temperatures of 150 °C [19], the use of DABCO (9) as ligand allowed for significantly milder reaction conditions [20]. Thereby, satisfying isolated yields were even achieved for orf/jo-substituted electron-rich aryl iodides and alkenyl bromides (Scheme 10.4). However, aryl bromides, particularly electron-rich ones, were converted only sluggishly. 

Chiral complexes 27 were tested in oxidative Heck-type reactions of boronic acids with acyclic alkenes (Equation (9.2)). Product yields were modest however, enantioselectivities were excellent (9(C98% ee). Other examples of functionalised benzimidazol-2-ylidene Pd complexes include 28 and 29 (Figure 9.6), which required high temperatures and long reaction times to afford reasonable conversions. ... 

In 2004, Molander et al. developed another type of chiral sulfur-containing ligands for the intermolecular Heck reaction. Thus, their corresponding novel cyclopropane-based phosphorus/sulfur palladium complexes proved to be active as catalysts for the reaction between phenyltriflate and dihydrofuran, providing at high temperature a mixture of the expected product and its iso-merised analogue (Scheme 7.7). The major isomer C was obtained with a maximum enantioseleetivity of 63% ee. 

Water-soluble phosphine ligands containing m-guanidinium moieties were synthesized and applied to aqueous Heck coupling reactions.149 High temperature appears beneficial for Heck-type coupling of simple alkenes in water.150... 

More recently, there has been an emphasis on designing direct arylation reactions that occur under milder conditions and lower temperatures. Since the initial findings concerning the use of Pd(II) catalysts in oxidative Heck type transformations and directed palladations, it has been shown that utilization of these highly electron deficient Pd(II) catalysts can enhance the rate of the key electrophilic... 

Alkenyliodonium salts have been used as highly reactive reagents for Heck-type olefination [39, 971], Sonogashira-type coupling with alkynes [965, 972] and similar other palladium-catalyzed cross-coupling reactions [966, 973, 974]. In a specific example, (Z)-p-fiuoro-a,p-unsaturated esters 721 were stereoselec-tively synthesized from (Z)-2-fluoro-l-alkenyliodonium salts 720 by the Pd-catalyzed methoxycarbonylation reaction (Scheme 3.288) [974]. This reaction proceeds at room temperature and is compatible with various functional groups on the substrate. 

Recently, a wide range of organic reactions have been promoted by microwave irradiation," but in the field of Heck chemistry only a limited number of papers have appeared. " " "" Two types of microwave heating equipment have been used, a multimode reactor or a monomode reactor.The latter is more expensive but allows the placement of the reaction mixture at a fixed position of much higher continuous electric field strength than can be obtained in a multimode reactor." This is particularly important with Pd-catalyzed reactions since the reaction mixture must be heated to a high temperature in a reproducible and homogeneous fashion. 

In type 1 and type 2 Mizoroki-Heck reactions, high temperature is required when SRPCs are involved as palladium sources. In type 2 reactions, the need to speed up the oxidative addition step is more important when the palladium catalyst has no particular activating ligands. In this case, a lower rate of oxidative addition would result in the build-up of unsupported palladium(O) with concomitant deactivation. 

As type 2 systems are maintained by rather high levels of soluble palladium concentrations in the reaction media, palladium from any support will be depleted rapidly. Recycling of a supported catalyst will thus be difficult. Aside from stable palladium complexes, there are a few supported systems which deliver reasonable type 2 performance. One of the most active systems was introduced by Molndr and Papp [105] with palladium(II) absorbed by ion-exchange onto montmorillonite clay. The precatalyst operates at high temperatures (150-160 °C) in the presence of NaaCOs in NMP and delivers high yields of Mizoroki-Heck products with, for example, phenyl bromide, 8 and 4-chloroacetophenone (47) at 0.001-0.1 mol% catalyst loading (47 48, Scheme 2.10). With more reactive substrates (e.g. phenyl bromide) this precatalyst survives two or three reuses, which shows that palladium is extracted from the support lattice at a sparing rate. 

Typed Mizoroki-Heck reactions require mild conditions. Partial or full dissociation of bidentate ligands is not expected to require very high temperatures (cf. discussion of various SRPC), at which type 4 precatalysts would simply turn into yet another SRPC. Indeed, Shaw and Perera [149] showed that (L)PdCl2 complexes with various bidentate phosphines (including dppf, dppe and dppp) make useful precatalysts for Mizoroki-Heck... 

An early platinum-catalysed Mizoroki-Heck-type arylation of an alkene with an organic halide was accomplished with [Pt(COD)Cl2] (54) as metal precursor and PPhs (35) as ligand (Scheme 10.17) [43]. Relatively high reaction temperatures, an inorganic base and... 

On the contrary, for layered highly efficient arylations of styrenes and methyl acrylate (1) were accomplished with the inorganic base NaOAc (Scheme 10.19) [45]. As for the previously reported platinum-based catalysts, high reaction temperatures were mandatory. The reusable catalyst was found to be further limited to the use of iodoarenes as electrophiles, since bromoarenes gave no Mizoroki-Heck-type coupling at all. 

Apart from the catalytic systems based on Pd/phosphines typically used in Mizoroki-Heck reactions, many other types of new palladium catalysts have been developed over the last decade. Avoiding the use of the phosphine ligands is a great advantage as they usually cannot be recovered and they frequently hamper the isolation and purification of the final product. One viable alternative is the use of ligand-free palladium catalysts usually in the form of Pd(OAc)2. At the high temperatures required for Mizoroki-Heck reactions, most ligand-free palladium complexes are unstable and have a tendency to form soluble Pd(0) nanoparticles [32]. The question arises as to the role played by the Pd nanopartides formed and whether... 

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