On Homogeneous Gold/Palladium Catalytic Systems

Abstract

Two substrates containing an aryl iodide and an allenoate ester were prepared and the goldinduced cycloisomerisation to vinylgold(I) species and their proto-deauration as well as the intramolecular palladium-catalysed cross-coupling reactions were investigated. Switching to catalytic amounts of gold and palladium and stoichiometric amounts of silver did indeed furnish the product of a cycloisomerisation/intramolecular cross-coupling. Control experiments revealed that silver cannot substitute for gold or palladium in these reactions, but a different palladium catalyst in a different oxidation state also afforded the cycloisomerisation/intramolecular crosscoupling products in only slightly reduced yields. By ICP analysis the palladium was shown to contain gold only at the sub-ppm level. This shows how carefully results obtained with such systems have to be interpreted. Then a series of allylic and benzylic o-alkynylbenzoates were investigated in gold-and palladium-catalysed reactions. For esters of benzyl alcohol and cinnamyl alcohol no palladium co-catalyst was needed for the conversion. All reagents were thoroughly checked for palladium traces by ICP analysis in order to thoroughly exclude a gold/palladium cocatalysis. Optimisation of the gold complex, counter ion and solvent showed that gold(I) isonitrile precatalysts and silver triflate as activator in dioxane are suitable to convert a number of substrates with aryl, alkyl and even cyclopropyl substituents. Crossover experiments proved an intermolecular allyl transfer.

Direct observation of two-electron Ag(I)/Ag(III) redox cycles in coupling catalysis

“…Copper and gold have been directly implicated in M(I)/M(III) oxidative addition and reductive elimination processes [18][19][20][21][22][23][24][25] , whereas such chemistry has never been considered for silver. Nevertheless, silver has been reported as an effective catalyst in cross-coupling reactions such as Agcatalysed Sonogashira couplings 26 and Ag-catalysed Ullmanntype C-heteroatom bond-forming reactions 27 .…”

Fundamental Elementary Steps in Gold Chemistry

“…Gold(I) complexes proved to be inert towards the oxidative addition of aryl halides, a crucial stage in many cross coupling reactions, even when an intramolecular carbon-halogen bond is oriented in close proximity to gold in favor for this process (Fig. 3.2) [30].…”

“…The redox-neutral nature of gold is often considered as an advantage: this "isohypsic reactivity" stands in an orthogonal relationship to the one of platinum and palladium, allowing for reaction sequences initiated by gold-catalyzed nucleophilic activation of a π-system, followed by Pd or Ni catalyzed cross-coupling [30,31].…”

Stille Coupling Involving Bulky Groups Feasible with Gold Cocatalyst

“…[4] However, so far only a few Au/Pd bimetallic catalyzed processes have been reported, including the Sonogashira-like cross-coupling, [5] the carbometallation of alkynes, [6] and processes combining cyclization with cross-coupling steps. [7,8] Herein we examine the potential of the Au/Pd pair in a Stille reaction, where tin is the third metal involved in the system, thus providing the nucleophile.…”