Aqueous Systems without Additional Organic Solvent

Lewis acids as water-stable catalysts have been developed. Metal salts, such as rare earth metal triflates, can be used in aldol reactions of aldehydes with silyl enolates in aqueous media. These salts can be recovered after the reactions and reused. Furthermore, surfactant-aided Lewis acid catalysis, which can be used for aldol reactions in water without using any organic solvents, has been also developed. These reaction systems have been applied successfully to catalytic asymmetric aldol reactions in aqueous media. In addition, the surfactant-aided Lewis acid catalysis for Mannich-type reactions in water has been disclosed. These investigations are expected to contribute to the decrease of the use of harmful organic solvents in chemical processes, leading to environmentally friendly green chemistry. 

Ultrasound in combination with an organic solvent facilitates the formation of binary systems with an aqueous electrolyte, thereby increasing the current intensity Figure 8.14B shows this effect on the sono-voltammogram of A/,A/,A/, /V -tetramethyl-p-phenylenediamine (TMPD) in 0.1 M aqueous KCI with and without the addition of 40% v/v heptane [156]. The increased current in the acoustically emulsified media was ascribed to enhanced transport of electroactive material in heptane droplets towards the electrode surface, and related to the analyte solubility in the organic phase. The ratio of the current increase to the volume fraction of organic solvent ([Pg.286]

The rather classic catalyst palladium on activated carbon has been applied by Sun and Sowa et al. [183] and Heidenreich et al. [158] without additional ligands. This simple system was able to convert (mainly activated) aryl chlorides in mixtures of water and an organic solvent (DMA and NMP, respectively). Lysen et al. were able to convert aryl chlorides in pure water and without addition of any ligand [184,185]. Suzuki reactions using Pd/C in an aqueous medium were also reported by Arcadi et al. (in the presence of surfactants) [186] and the group of Leadbeater, who applied microwave techniques [187]. Microwave (as weU as ultrasound) conditions were also employed by Cravotto and Palmisano et al. [188] The substrate scope of Suzuki-type reactions in the presence of Pd/C was extended to halopyri-dines and haloquinoUnes by Tagata and Nishida [189]. 

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