Ligand-free Heck reactions using low Pd-loading

Abstract

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 (...

Triphasic Liquid Systems: Generation and Segregation of Catalytically Active Pd Nanoparticles in an Ammonium‐Based Catalyst‐Philic Phase.

“…2 We feel this may not be a problem since we have demonstrated that Pd is not present in catalytically significant amounts in the organic phase. Another issue is the tendency of the Pd particles to aggregate.…”

“…With the aim to generate and immobilize Pd-black in the third TOMAC phase, a suitable palladium precursor was sought, by testing PdCl 2 , Pd(OAc) 2 , Pd(PPh 3 ) 4 , and PdCl 2 (BzCN) 2 . Preliminary screening tests carried out with commercial A336 had indicated that Pd(PPh 3 ) 4 was the best precursor: it formed Pdblack which resided in the A336 phase.…”

Catalytic Applications of Metal Nanoparticles in Imidazolium Ionic Liquids

“…[88] Well-defined and in-situ-prepared Pd 0 nanoparticles in ionic liquids have been reported to promote CÀC coupling reactions such as Heck and Suzuki processes (Table 1, entries 6-9). In most cases, however, the Pd 0 nanoparticles are simple reservoirs of homogeneous Pd 0 catalytically active species akin to those observed with phosphane-free palladi- [89][90][91] such as simple Pd II compounds [92] or palladacycles. [93,94] It is highly probable that in these cases the Heck coupling reaction proceeds through the oxidative addition of the aryl halide on the nanoparticles surface, and the oxidized Pd species thus formed are detached from the surface and enter into the main catalytic cycle.…”

Multiphasic Heterogeneous Catalysis Mediated by Catalyst‐Philic Liquid Phases

“…Metal nanoparticles, and in particular Pd-nanoparticles (also called 'Pd-colloids'), are stabilized by the presence-among others-of ammonium salts, [39][40][41][42][43] and surfactants, 44 by PEGs, 45 polysiloxane, 46 and by organic thiol monolayers. 47 The highly polar ethylene glycol was shown to be a catalystphilic phase earlier for SAPC.…”