The Heck reaction has been the subject of intense investigation in the past decade. Many new types of catalysts have been developed in addition to the existing palladium/phosphine complexes. Prominent among these are palladacycles, pincers, several types of heterogeneous palladium catalysts, colloids and ligand-free palladium, usually in the form of Pd(OAc)2. Most of the newer types function only at higher temperatures, typically between 120 and 160 degrees C. It has been shown that irrespective of the catalyst precursor, none of these catalysts are stable at these high temperatures. They all have a tendency to form soluble palladium(0) colloids or nanoparticles, certainly with less reactive substrates such as aryl bromides or chlorides. The Heck reaction takes place by attack of the arylating agent on the palladium atoms in the outer rim of the nanoparticles. This leads to formation of monomeric or dimeric anionic palladium complexes that undergo the usual steps of the Heck mechanism as described by Amatore and Jutand.

as the base in methanol in a closed vessel (120 °C). 4 The original procedure reported by Heck is also ligand- Although P-ligands stabilise palladium and influence its catalytic activity, the simplest and cheapest Pd-catalysts are of course ligand-free systems, specifically when used in such low amounts that the term homeopathic is justified. Such an experimental protocol would constitute a significant advancement, especially if generally applicable. Recently, such ligand-free Heck reactions in organic solvents at extremely low Pd-loadings have been described. 11,12 They work astonishingly well with aryl bromides, as in the reaction of styrene (1) with bromobenzene (2b). Here we focus on this synthetically important development and also address mechanistic questions. Formation and catalytic activity of Pd-colloidsOur interest in two seemingly unrelated projects carried out in Mülheim, namely Heck reactions and Pd-colloids † led to the realization that the two areas are related. It had been known for a long time that tetraalkylammonium halides stabilize nano-sized transition metal colloids. 13 The surfactants prevent undesired agglomeration by forming a monomolecular layer around the metal core, as demonstrated for Pd-colloids using transmission electron microscopy (TEM) and scanning tunnel microscopy (STM). 14 Pd-colloids of this kind can be prepared size-selectively in the range of 1-5 nm in organic solvents or in water by reducing PdCl 2 chemically or electrochemically in the presence of a tetraalkyl ammonium salt. 14 In an extension of this work it was discovered that an external reducing agent is not necessary if a palladium salt such as PdCl 2 , Pd(OAc) 2 or Pd(NO 3 ) 2 is gently warmed in THF in the presence of excess tetraalkylammonium carboxylates (R 4 N + RACO 2 2 ) which function as a reducing and stabilizing agent (Fig. 1). 15 We suspected that similar Pd-colloids are formed during Heck reactions under Jeffery conditions. Indeed, preformed Pdcolloids of this kind were shown to be Heck catalysts. 16 Moreover, we demonstrated that Pd(OAc) 2 (which in pure form is known to decompose at 100°C with formation of Pd-black) can be used as a precursor for solvent-stabilized Pd-colloids simply by heating in a polar solvent such as propylene carbonate, 17 the 8-10 nm sized Pd(0)-nanoparticles showing no tendency to agglomerate (no undesired Pd-black) even at 120 °C for days! Other polar solvents such as NMP are also capable of stabilizing Pd-colloids in the absence of surfactants, polymers or ligands, 17 which normally need to be added as stabilizers. The exact nature of the stabilizing effect of polar solvents (complexation?) is currently unknown. These nanomaterials also display catalytic activity in Heck reactions, although the conditions and amount of catalyst used were not optimized. 17 These observations suggest, but do not prove, that phosphine-free Heck reactions involve nano-sized Pd-colloids. In order to put these speculations on a more sound basis we applied in situ transmission electron microscopy (...

[reaction: see text] Ligand-free Pd(OAc)(2) can be used as a catalyst in the Heck reaction of aryl bromides as long as the amount of catalyst is kept between 0.01 and 0.1 mol %. At higher concentrations palladium black forms and the reaction stops. The actual catalyst is monomeric. Palladacycles merely serve as a source of ligand-free palladium in Heck reactions of aryl bromides.

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