Palladium nano-sized clusters

The nanoporous hypercrosslinked polystyrene proves to be the material of choice for the preparation of nanocomposites (Chapter 17) by formation of nano-sized clusters of catalyticaUy active metals, for example, platinum or palladium, or of magnetic nano-crystals of iron oxides. Being confined in the nano-cage, the clusters do not agglomerate so that the catalysts remain stable and highly active in any liquid media. 

The palladium complex Pd(MeCN)2Cl2 is actually decomposed under the reaction conditions to form catalytically active nano-sized palladium clusters. However, in the case of less reactive aryl halides bearing electron-donating groups, e.g. 4-bromo toluene, addition of 10 mol% of P(0/-Pr)3 as stabilizing ligand for zero-valent palladium was necessary to rich high conversions. 

Other suitable reaction solvents are aqueous tetrahydrofiiran, 1,2-diraethoxyethane or acetonitrile [28]. The same result was obtained by the method G, whereas 4-nitro-chlorobenzene was also coupled, in almost quantitative yield, within 2 h at 100 °C, or 87 h at room temperature. However, in the presence of tetra-n-butylammonium bromide (5 mol%), a soluble source of bromide anions, the SM reactions of aryl bromides have been effected in ethanol at room temperature in the presence of palladium(II) acetate or chloride (2 mol%) and potassium phosphate (2 eq.) as the base, even under exposure to air [55]. Palladium salts are reduced in situ with arylboronic acids to form catalytically active nano-sized palladium clusters (2-5 nm). The latter are stabilized by adsorbtion of one-layer bromide ions at the surface of each palladium-particle. Otherwise, the unstable nano-sized palladium-clusters are aggregated to the micro-sized catalytically inactive palladium black. In this manner, 2-bromonaphthalene (268) was reacted with 2-methoxyphenylboronic acid (269) to fiimish the biaryl 270 in 98% yield [55], respectively. Scheme 17. 

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