Catalysis in Novel Reaction Media

Sheldon, R.A., Arends, l.W.C.E. and Hanefeld, U. (2007) Catalysis in novel reaction media, in Green Chemistry and Catalysis (eds R.A. Sheldon, 1. Arends and U. Hanefeld), Wiley-VCH Verlag, Weinheim. 

The Mizoroki-Heck reaction in liquid imidazolium salts as the solvent is a special case of an in situ system Under the reaction conditions NHC complexes of palladium are formed as the active catalyst from the solvent and the ligand-free palladium precursor. In general, ionic liquids are novel reaction media for homogeneous catalysis. They allow easy separation of product and catalyst after the reaction. ... 

Indeed, these reactions proceed at 25 °C in ethanol-aqueous media in the absence of transition metal catalysts. The ease with which P-H bonds in primary phosphines can be converted to P-C bonds, as shown in Schemes 9 and 10, demonstrates the importance of primary phosphines in the design and development of novel organophosphorus compounds. In particular, functionalized hydroxymethyl phosphines have become ubiquitous in the development of water-soluble transition metal/organometallic compounds for potential applications in biphasic aqueous-organic catalysis and also in transition metal based pharmaceutical development [53-62]. Extensive investigations on the coordination chemistry of hydroxymethyl phosphines have demonstrated unique stereospe-cific and kinetic propensity of this class of water-soluble phosphines [53-62]. Representative examples outlined in Fig. 4, depict bidentate and multidentate coordination modes and the unique kinetic propensity to stabilize various oxidation states of metal centers, such as Re( V), Rh(III), Pt(II) and Au(I), in aqueous media [53 - 62]. Therefore, the importance of functionalized primary phosphines in the development of multidentate water-soluble phosphines cannot be overemphasized. 

Substrate selectivity and stereoselectivity of enzyme catalysis are known to be influenced by the reaction media. As has been mentioned in the preceding section, a profound feature of the behaviour of enzymes in an anhydrous organic medium is the conformational stability, which leads to enhanced thermal stability and the ligand memory property. These features of enzymes have been exploited to impart novel catalytic characteristics that are absent in the native biopolymers. 

The focus of this chapter is the use of both the spherical polyelectrolyte brashes and microgel particles as carrier systems for novel metal nanoparticles, which can be used for catalysis in aqueous media, that is, under very mild conditions. Thus, the composite systems of metallic nanoparticles and polymeric carrier particles allows us to do green chemistry and conduct chemical reactions in a very efficient way. This approach can open new possibilities for catalytic application of metal composite particles in different reactions and represents a typical example of mesotechnology Nanoscopic objects with catalytic properties are judiciously combined with polymeric carriers to serve for a given, well-defined purpose. 

Ionic Hquids (ILs) are low melting salts (<100 °C) and represent a promising solvent class, for example, for homogeneous two-phase catalysis [7-9] and extractions [10-12]. These and other appHcations of ILs have been reviewed in a number of papers [7, 13-16]. One of the main reasons that ILs have gained interest both in academia and in industry is that they have an extremely low vapor pressure [17, 18], which makes them attractive as alternative reaction media for homogeneous (two-phase) and heterogeneous catalysis [19, 20]. This paper focuses on the concept of a solid catalyst with ionic liquid layer (SCILL) as a novel method to improve the selectivity of heterogeneous catalysts. 

The progress of ruthenium-catalyzed cyclotrimerizations is expected to continue with further investigations on novel catalysts with chiral nonracemic ligands, the exploitation of other substrates and reaction media, and applications in underdeveloped areas. As a result, ruthenium-based catalysis will be indispensable for organic synthesis, pharmaceutical research, and materials science. 

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