Three Phase Microemulsion/Sol–Gel System for Aqueous C–C Coupling of Hydrophobic Substrates

Abstract

Heck, Stille, Suzuki and three‐component coupling reactions with hydrophobic substrates have been carried out in water. The substrates are initially transformed by a general procedure into a microemulsion, which consists of nearly 90 % water with the aid of sodium dodecyl sulfate and either PrOH or BuOH. The surfactant carries the molecules of the substrates to Pd(OAc)2 entrapped within a hydrophobicitized silica sol–gel matrix where the coupling between the substrates is assumed to take place. The products are then returned by the surfactant into the microemulsion from which it can be released. The immobilized palladium catalyst is leach proof and recyclable. It can be used in various coupling processes at least six times without loss of activity. Experiments with D2O have revealed that the water does not take part in the coupling process, but it has an effect on the pore size of the sol–gel matrix, which hosts the palladium catalyst. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

On the Involvement of Palladium Nanoparticles in the Heck and Suzuki Reactions

“…[4] A typical TEM image is shown in Figure 1A. Dark contrast of heavy dense crystalline Pd nanoparticles is seen on the image.…”

Forcing a cis‐Product by Matrix Imprinting: Heck Reaction Catalyzed by Palladium Acetate Entrapped within cis‐Imprinted Sol‐Gel Derived Silicates

“…After removal of the imprint the matrices were used in the Heck coupling reactions that led to the corresponding mono-and dimethylated stilbenes. As noted in our previous paper, [5] both methylated bromobenzenes and methylated styrenes react much slower than the unsubstituted substrates. Nevertheless, with extension of the reaction time, substantial amounts of the cisoriented stilbenes have been formed (see Table 1), and here too the immobilized catalyst could be recovered and recycled after completion of the couplings (4 runs were investigated in each case).…”

“…The specific reaction with which we demonstrate this approach is the palladium acetate-catalyzed Heck coupling [3,4] of a bromoarene and a styrene derivative in an aqueous microemulsion. [5] This reaction has led to hardly any cis products when carried out in solution, [3,4] and to no cis products at all under heterogeneous sol-gel conditions. [5] We found that we could force this reaction to yield considerable amounts of cis-stilbenes -typically cis:trans ratios of 1:1, and up to 8.5:1 -by imprinting the matrix within which the catalyst is embedded with cis-stilbenes.…”

“…[5] This reaction has led to hardly any cis products when carried out in solution, [3,4] and to no cis products at all under heterogeneous sol-gel conditions. [5] We found that we could force this reaction to yield considerable amounts of cis-stilbenes -typically cis:trans ratios of 1:1, and up to 8.5:1 -by imprinting the matrix within which the catalyst is embedded with cis-stilbenes. It should be noted that in the voluminous literature on imprinting of either oxides or polymers since the early 1930s, [6] we could not locate a cisdirecting imprinting study.…”

Suzuki‐Kupplung in dreiphasigen Mikroemulsionssystemen

“…[10] Bereits in einem früheren Beitrag wurden Tensidsysteme als vielseitige Medien in einer HeckReaktion mit einem Palladium-Heterogenkatalysator beschrieben. [11] Da [12] in Gegenwart von Wasser erleichtert wird. Wie in Tabelle 1, Nr.…”