Historical Perspective of the Heck Reaction

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Table 1. Heck reaction of aryl bromides with styrene catalysed by palladium complex 3 aTotal GC yield of all isomers, based on the aryl bromide using decane as internal standard. bTurnover no. (TON) = fraction of products (6 + 7 + 8) × substrate/Pd ratio. performed under argon. However, in air and at a very low palladium concentration, for a catalyst:4-bromobenzaldehyde ratio of 1:100000, the yield was diminished. This phosphorus-free complex with additional potential N-donors is thermally stable under argon, and efficiently catalyses the Heck reaction of aryl bromides with styrene, with good turnover numbers and a good selectivity towards trans-stilbenes. This system efficiently catalyses the Heck reaction of aryl bromides (from electronrich to electron-poor) with styrene under argon, with turnover numbers of up to 42700, at 150ºC after 24 h, and with a selectivity towards trans-stilbenes ranging from 92.0 to 96.4 %. In air, for activated aryl bromides and for a palladium concentration of 1 mM, the yields are essential the same to those obtained when the reaction performed under argon. These phosphine-free catalysts offer the advantage of the successful coupling of aryl halides and the synthesis of biaryls under aerobic conditions [7]. 14
3.3 Palladium nanoparticles as catalysts for Heck reaction A palladium-nanoparticle-cored G-3 dendrimer, characterized by TEM, TGA, absorption, and IR spectroscopies, has approximately 300 Pd atoms in the metallic core and an average diameter of 2.0 nm, to which are attached fourteen G-3 dendrons. Nearly 90% of the metal nanoparticle surface is unpassivated and available for catalysis. The dendrons inhibit metal agglomeration without adversely affecting chemical reactivity. Thus, investigations have shown that Pd-G-3 can efficiently catalyze Heck and Suzuki reactions [8] Synthesis of Pd-G-3, in Which Seven of the Fourteen G-3 Wedges Are Shown (see text) Pd-G-3 was prepared by the Brust reaction (K2PdCl4 was phase transferred into toluene using tetraoctylammonium bromide (TOAB). Fre´chettype dendritic polyaryl ether disulfide3 of generation 3 (G-3S) was then used. The mixture was cooled in ice (0-2 °C) and excess of NaBH4 was added. The Pd-G-3 thus obtained was a black powder, freely soluble in methylene chloride, chloroform, toluene and THF, but insoluble in ether and alcohol. Pd-G-3 was stable for several months both as a powder and as a dilute solution in CH2Cl2. FT-IR and TGA experiments showed that Pd-G-3 incorporated some amount of TOAB and this could not be removed by the standard purification procedure. Figure 5 shows a highresolution TEM image of Pd-G-3 and the corresponding core-size histogram. It can be seen from Figure 1 that the particles exhibit a relatively wide size distribution (1-5 nm). The mean core diameter obtained from the histogram plot was 2.0 nm TEM image and core-size histogram of Pd-G-3. 15
Page 1 and 2: Dimitra Kovala-Demertzi Section of
Page 3 and 4: Aryl and vinyl halides- Aryl and vi
Page 5 and 6: Solvents-Heck reactions are conduct
Page 7 and 8: Oxidative addition is often the rat
Page 9 and 10: Amino-Heck reaction In the amino-He
Page 11 and 12: All of the palladacycles shown in t
Page 13: The monoanionic HSal4Et ligand is c
Page 17: Figure 2. Possible pathways involve

Historical Perspective of the Heck Reaction

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