Colloid coupling reaction

Copper-catalyzed Suzuki cross-coupling reactions using mixed nanocluster catalysts have been studied recently. Copper-based catalysts were shown to be effective as reagents that can present an inexpensive and environmentally friendly alternative to noble metal catalysts. In the hydrogenation of cinnamic acid to corresponding alcohol, the selectivity can be varied by doping Sn with Rh colloid catalysts. A selectivity of 86% was achieved using a colloidal Rh/Sn (Rh/Sn = 1.5 1) catalyst on... 

Bars, J.L., Specht, U., Bradley, J.S., and Blackmond, D.G., A catalytic probe of the surface of colloidal palladium particles using heck coupling reactions, Langmuir, 15, 7621, 1999. 

Modifications of monodisperse colloidal silica, of 10 or 500 nm in diameter, were carried out using trialkoxysilane-terminated polymer in a low polar solvent, such as acetone or 1,2-dimethoxyethane. without coagulation during the coupling reaction (35,37-42). In this modification, the hydrophobic polymer can be efficiently bound to hydrophilic colloidal silica surface. The reaction mechanism of the binding... 

There have been numerous studies in recent years investigating the differences in oxidative coupling reactions of phenols catalyzed by enzymes or mineral colloids. Both mineral colloids and oxidoreductive enzymes contain metals that can act as electron acceptors to catalyze the oxidative transformation of organics hence there are similarities in their reaction products. However, there are differences in the mechanisms by which these catalysts operate. 

There are two main uncertainties associated with this general mechanism. First, there are a number of C-C coupling reactions where there is no direct evidence for the reduction of the Pd(II) precatalyst into a zero-valent palladium species. Second, like the hydrosilylation system, a number of these reactions may involve colloidal palladium. Also, the general catalytic cycle needs to be substantially modified to rationalize the successful use of 7.63 as a precatalyst. 

It is believed that the accelerated reaction rates that are observed in ammonium salts are due to their ability to stabilise nanoparticles, and especially when simple palladium salts are employed under ligand-free conditions, formation of colloidal palladium is highly likely.[7 9] As carbon-carbon coupling reactions are frequently conducted at elevated temperatures, typically between 120-150°C, a wide range of low-melting salts may be... 

The hydrolysis of (EtO)4Si (and the snbseqnent polycondensation of Si OH containing molecnles) has been the snbject of considerable investigation because of its importance in the fabrication of glasses and colloidal silica via the sol gel process (see Sol-Gel Synthesis of Solids). Hydrolysis of the Si-0 bond in alkoxysilanes is also very widely used to attach silicon compounds to surfaces and in coupling reactions. 

Several investigators have attempted to identify the nature of the metal species that catalyze these coupling reactions in solution. Due to the harsh conditions of these reactions and the thermodynamic instability of colloidal particles, it is highly possible and probably likely that multiple catalytic mechanisms occur simultaneously. A few review articles on carbon-carbon bond formation reactions catalyzed with colloidal transition metal nanoparticles are available [7, 11]. 

The first studies that intentionally used colloidal nanocatalysts were reported independently by Beller et al. [50] and Reetz et al. [51] using chemical reduction and electrochemical techniques, respectively, to synthesize colloidal palladium nanoparticles for the Heck reaction. Both Beller and Reetz concluded that the solution-phase catalysis occurred on the surface of the nanoparticle, without confirming that a homogeneous catalytic pathway was nonexistent. Le Bars et al. [52] demonstrated an inverse relationship between the size of Pd nanoparticles and the TOF (normalized to the total number of surface atoms) for the Heck reaction (Fig. 18.4a). After normalizing the rate to the density of defect sites (for each nanoparticle size) (Fig. 18.4b), the TOF for all particle sizes was identical. Colloidal PVP-capped palladium nanoparticles synthesized by ethanol reduction are effective catalysts for Suzuki cross-coupling reactions in aqueous solution [53]. The El-Sayed group reported that the initial rate of reaction increased linearly with the concentration of Pd nanoparticles [53] and the catalytic activity was inversely proportional to the... 

Narayanan R, El-Sayed MA (2004) Effect of colloidal catalysis on the nanoparticle size distribution dendiimer-Pd vs PVP-Pd nanoparticles catalyzing the Suzuki coupling reaction. J Phys Chem B 108 8572... 

Coupling Reaction and Storage as Colloid. Iodide peroxidase or a coupling enzyme catalyzes the coupling reaction at the cell-colloid interface by intramolecular coupling of two iodotyrosyl residues with formation of an iodothyronyl residue. Coupling of DIT residues is favored thus, formation of T4... 

At the opposite of the molecular chemistry described until now, nanoparticles are reminiscent of heterogeneous catalysts. However, these colloid-derived materials have been shown to catalyze efficiently in water coupling reactions which have been previously described in pure homogeneous systems. For instance, poly(N-vi-nyl-2-pyrrolidine)-stabilized palladium nanopartides promote the Suzuki crosscoupling in aqueous media with high yields (see also Section 6.6) [87]. 

Beller et al. have shown for the first time that palladium colloids are effective catalysts for the olefination of aryl bromides (Heck reaction). Reetz et al. have studied Suzuki and Heck reactions catalyzed by preformed palladium clusters and palladium/nickel bimetallic clusters and further progress was achieved by Reetz and Lohmert using propylene carbonate stabilized nanostructured palladium clusters as catalysts in Heck reactions. In addition, the use of nanostructured titanium clusters in McMurry-type coupling reactions has been demonstrated by Reetz et... 

The development of Pd colloids as catalyst for C—C coupling reactions is rather recent [5]. The first example was reported by BeUer et al. in 1996 they used preformed Pd coUoids stabihsed by tetra-octylammonium bromide prepared following the Bonneman procedure in the Heck arylation [6]. The colloidal system was effective for the Heck arylation of styrene or butyl acrylate by activated aryl bromides, but showed only moderate to little activity for deactivated aryl bromides and aryl chlorides. To obtain these results, the authors found that the colloidal pre-catalysts must be added slowly to the reaction mixture to avoid the formation of inactive palladium black at the beginning of the reaction. 

By contrast, the role of colloids in C-C coupling reactions, especially the Heck and Suzuki reaction, is currently the topic of debate [54—57]. Possible mechanistic pathways include the colloidal particles being in equilibrium with mononuclear complexes, which are the active species (Figure 3a), or reaction occurring directly on colloid particle surface atoms (Figure 3b). For hydrosilylation, the importance of platinum colloids has long been recognized [58]. 

Table 1 Examples of C-C coupling reactions catalyzed by palladium colloids stabilized by polymers or surfactants. 

Table 2 Examples of C-C coupling reactions with colloids stabilized by other means than polymers or surfactants and Heck coupling with heterogeneous catalysts. 

---