Organic reactions on silica in water.

In nature, many biological processes occur in aqueous environments, and these fascinating in vivo reactions should prompt organic chemists to explore the potential of water as a medium for organic reactions. In addition to scientific interest in aqueous reaction media, social pressure to find environmentally benign alternatives to current organic chemical processes has made these reactions attractive from an ecological point of view. The use of aqueous media in organic reactions offers significant environmental advantages and has attracted a great deal of interest because water is a desirable solvent for the following reasons: low cost, safety, and environmental concerns. Groundbreaking studies of organic reactions in aqueous environments demonstrated that Diels-Alder reactions and Claisen rearrangement of hydrophobic reactants are accelerated in aqueous solutions. In addition to these discoveries, aqueous environments result in reactivity and selectivity that are unique from reactions in organic solvents. As a result, many versatile and efficient organic reactions have been developed. Although many organic reactions are facilitated in aqueous media, some reactions proceed very slowly because the solubility of most organic molecules in pure water is limited. Because solubility is a prerequisite for reactivity, a variety of strategies expanding the scope of water-based organic syntheses have been investigated. Most commonly, organic cosolvents, such as the lower alcohols, acetone, DMF, and acetonitrile, are used to increase the solubility of hydrophobic solutes in water. Some alternative means of achieving aqueous solubility are the use of phase-transfer catalysts and surfactants. In addition to these processes, substrate modifications, such as the addition of a positive or negative charge to an ionizable substrate or the grafting of hydrophilic groups onto insoluble reactants, have been investigated. However, these approaches change the chemical behavior of the substrates and tend to complicate the reactions. Recent studies indicate that water is a promising medium for heterogenized homogeneous catalysis. Although the efficiency of heterogeneous catalysis is generally inferior to that of homogeneous systems, the advantage of immobilized catalysts include their easy recovery from reaction mixtures and their reusability. Catalysts heterogenized through covalent and non-covalent attachment to either inorganic or organic materials, such as silica, layered clays, and polymers, have been successfully employed in aqueous media. The chemical stability of the inorganic supports is important, especially under oxidizing conditions; the mechanical and thermal stability of inorganic supports is often excellent as well. This review will focus on the use of silica as an inorganic support for organic reactions in water. The hydroxy group on the silica surface readily reacts with alkoxyor chlorosubstituted silyl compounds, leading to functionalized silica supports. This manipulation allows not only for the introduction of catalysts to the surface of the support but also for the control of surface hydrophobicity. The proteiform and unique character of silica surfaces have been utilized for organic reactions in aqueous media. This review is organized into three main parts, each devoted to one form of silica that has unique effects on organic reactions in water: heterogenized catalysts on silica (immobilization by covalent binding or adsorption of catalysts on silica), hydrophobic and fluorous reverse-phase silica, and unmodified silica. The first section is subdivided into three heterogenized catalyst systems, as follows: water phase only, water-organic biphasic systems, and water-ionic liquid systems.