Suzuki Coupling Reactions in Neat Water as the Solvent: Where in the Biphasic Reaction Mixture Do the Catalytic Reaction Steps Occur?

Abstract

Many reports on water-compatible palladium catalysts have appeared in the recent literature. For hydrophobic substrates, mixtures with pure water are biphasic, and it is widely not regarded that the elusive locality of the catalytic process (in water, the organic layer, or at the phase boundary) has an important impact on the mechanism and efficiency of the reaction. In the present work, for the first time systematic variation of reaction parameters has been performed for Suzuki coupling experiments with chloro- and bromoarenes in pure water. The investigations are not only aimed at the factors influencing the catalytic activity, but also at the effects that may occur particularly in water/organic biphasic media, and on the question as to in which of the two liquid phases the reaction takes place. These investigations have revealed that dilution of the base (in the aqueous layer) and the Pd species (in the organic layer) are detrimental to the reaction, and that phase-transfer processes play a major role in the overall mechanism. A series of experiments with variation of parameters like precatalyst hydrophilicity, organic and water phase volume, additives, stirring rate, base concentration, and so forth, indicate that for the systems under study the reaction occurs in the organic layer. The water phase needs to be present to dissolve and provide polar reactants, and re-absorb side products. The results encourage to pay more regard to the question of phase locality of coupling reactions in water in general.

The mechanism of a self-assembled Pd(ferrocenylimine)–Si compound-catalysed Suzuki coupling reaction

“…47 The reduction of Pd II to Pd 0 by amines was also observed, specifically in ligand-free systems. 48 To obtain more detailed speciation information, the catalytic films were analysed by XPS ( Fig.…”

Viral templated palladium nanocatalysts for dichromate reduction

“…In particular, the solvent ratio played an importantr ole in the selectivityo ft he Suzuki reaction, where ah igherw ater/ acetonitrile ratio significantly facilitated the cross-coupling pathway,a nd suppressed the homo-coupling reaction (Figure 7c), presumably due to the enhanced solubility and accessibilityo fb ase to activate the iodoanisolea nd boronate species required for the transmetalation step in the reaction mechanism. [33,34] The reactant contentr atio study (Figure 7d) showed that highera romatic boronic acidc ontentt han iodoanisolei nt he reactant mixture enhanced the completion of the cross-coupling reaction, while acertain amount of aromatic boronic acidf ollowed the homocoupling pathway. [34] The Pd loading applied fore ach batch reaction is 0.15 mol %b ased on the based on aryl halide, which is lower than in other heterogeneous catalyst systems; for example, 0.3 mol %f or Pd/C, [35] 2.5 mol %f or Pd/zeolite, [36] and 0.5 mol %for Pd/Chitosan.…”

Palladium-Catalyzed Suzuki Reactions in Water with No Added Ligand: Effects of Reaction Scale, Temperature, pH of Aqueous Phase, and Substrate Structure

“…However, the limited solubility of the halogenated substrates and ligands in water often leads to slow rate and low yields. To overcome this limitation, alternative methodologies using water-soluble ligands, [52][53][54][55][56] organic co-solvents, [25][26][27][28][29][30][31][32] phase-transfer catalysts 33,[48][49][50][51] and inorganic salt promoters, 34,35 and heating with microwave or ultrasound were reported. [36][37][38][39][40] In 1989, Bumagin et al 57 were the first to report a Pd(OAc) 2 catalyzed Suzuki reaction of phenylboronic acid with aryl iodides containing a -OH or -COOH substituents; Na 2 CO 3 was employed as the base in neat water under an argon atmosphere.…”