Gold trifft Palladium für Kreuzkupplungen

Abstract

Gold und Palladium: eine einzigartige Liaison. Eine Studie der Transmetallierungsfähigkeit von Organogold‐Verbindungen bildet die Grundlage für eine neue Klasse von Kreuzkupplungen. Stabile Zwischenstufen der Gold‐Katalyse führen durch Palladium‐katalysierte Kupplungsreaktionen zu neuen, komplexen Produkten (Beispiel siehe Schema).

On Homogeneous Gold/Palladium Catalytic Systems

“…[15] The failure of the palladium(II) catalyst PdA C H T U N G T R E N N U N G (dppf)Cl 2 in the absence of gold can be explained by the need of reduction of the palladium(II) which is induced by the vinylgold compound forming the bisA C H T U N G T R E N N U N G (vinyl)palladium(II) compound by transmetallation and subsequent reductive elimination. [8] The failure of palladium(0) alone can be explained by the coordinatively saturated neutral palladium(II) species formed after oxidative addition not being electrophilic enough to induce the cyclisation of the allene part, while the cationic palladium(II) spe- Table 1. Influence of the solvent on the catalytic gold/palladium system (2.5 mol% Ph 3 PAuCl/2.5 mol% PdCl 2 A C H T U N G T R E N N U N G (dppf)/ 1.1 equiv.…”

“…[8] Since both individual cycles, the gold-catalysed cycloisomerisation and the palladium-catalysed cross-coupling are fast, the transmetallation must be the critical step. This might be due to the low concentration of the species involved in the catalytic cycles, which makes collisions of the vinylgold species A and the palladium(II) spe-cies B rare, and the competing protodeauration might outrun the desired transmetallation.…”

“…[7] We have recently investigated cross-coupling reactions in detail which were stoichiometric in the vinylgold compound A and the aryl halide ArX but catalytic in palladium. [8] These crosscoupling reactions efficiently deliver vinylarenes 3 and thus prove that an efficient catalytic transmetallation from gold to palladium is possible, but reactions catalytic in both gold and palladium did not give satisfactory results. Now Blum et al have reported a reaction specific to allylic esters 4 as substrates, which is catalytic in both gold and palladium (Scheme 2).…”

Isolable vinylgold intermediates — first access to phantoms of homogeneous gold catalysis

“…Furthermore, these organogold compounds can even be used in cross-coupling chemistry, this adds a new dimension to the organic chemistry of gold (4). Even exactly Hammond's vinylgold complexes could be used, Figure 3 shows the principle of these reactions, which form new C-C bonds.…”

“…The alkyne is activated by the interaction with gold (complex 2) (2), backside-attack of a nucleophile (Nu), either intramolecular or intermolecular, then delivers a vinylgold species 3. From this vinylgold intermediate 3 the gold catalyst is liberated by an electrophile (E), typically a proton (1), seldom a halogen (3) and most recently even palladium (4). During the same step the product 4 is also set free.…”