Designing Br(?)Nsted Acid Ionic Liquid To A Heterogeneous Catalyst Under Solvent-Free Conditions

Most organic reactions occur in a liquid phase. The solvent not only allows a better contact between reactants, stabilizes or destabilizes intermediates and/or transition states but also determines the choice of work up procedures and recycling or disposal strategies. However, the use of hazardous and toxic organic solvents in chemical laboratories and the chemical industry is considered a very important problem for the health and safety of workers and environmental pollution. In order to avoid the use of hazardous organic solvent, a lot of research efforts have been paid to the development of green solvents in the past two decades.Acid catalysts were extensively used in organic synthesis. However, most of the reactions have to be performed in organic solvents that are toxic and sometimes easy to detonate. Our aim in this thesis is to develop a novel catalytic material that integrates the functions of both catalyst and a solvent, with which some reactions can be performed under solvent-free conditions. In order to realize this goal, we turn to one of the advantages of ionic liquids, tunable physicochemical property. Particularly, Br(?)nsted acid ionic liquids have been widely used in organic synthesis because of their excellent catalytic performance. A typical example is Forbes’s ionic liquid (please see J. Am. Chem. Soc.2002,124,5962). However, these ionic liquids are very expensive, and in order to facilitate their recycling, most of the reactions have to be performed under solvent-free conditions. In these cases, a mixture composed of substrate, product and ionic liquid played a role of solvent. However, because the carbocation intermediate that was generated during the reaction may not be stabilized sufficiently in this mixture, the catalytic activity of the IL was, sometimes, not truly displayed.In this thesis, a sulfonyl-containing ammonium-based Br(?)nsted acid ionic liquid was prepared and used as a liquid heterogeneous catalyst for organic reactions. A unique macroscopic phase heterogeneity of the ionic liquid in the reaction system not only ensures an excellent catalytic activity of the ionic liquid catalyst but also avoids the use of organic reaction solvents. The catalyst system is applicable for a wide range of reactions, including nucleophilic substitution of benzhydrol and phenylacetytlene, cyclotrimerization of acetophenone and condensation of aromatic aldehydes and1,1-diarylethylenes.With the aid of this sulfonyl-containing ammonium-based Br(?)nsted acid ionic liquid, a direct dehydrative coupling method for the synthesis of3-vinylindoles by using easily available indoles and simple ketones as substrates was developed. The salient features of this protocol are high synthetic efficiency, metal-and solvent-free system, recyclable catalyst, mild conditions and easy product isolation. With the ionic liquid catalyst, a hitherto unreported straightforward method for the construction of indolo[3,2-b]carbazole skeleton was also developed by using2-hydroxymethylindole and acetophenone as starting materials.This sulfonyl-containing ammonium-based Br(?)nsted acid ionic liquid was also used as an effective catalyst in the reductive alkylation of indole with cyclic ketone. The reaction could be performed in the absence of external reductant. Water generated in the initial stage of the reaction played a key role in rendering the reductive coupling possible. The reaction proceeds most likely in a radical way. An important intermediate of some commercially available drugs, indole functionalized by cyclohexyl and COOH, was also synthesize by means of the developed system through a one-pot method.By using this sufone-containing Br(?)nsted acid ionic liquid as catalyst, various densely substituted chroman derivatives were synthesized through hitherto unreported three-component reactions of aromatic aldehydes and1,1-diarylethylenes. The representative reactions involves (ⅰ) condensation of benzaldehyde,2-naphthol and1,1-diphenylethylene and (ⅱ) selective assembly of salicylaldehyde, indole and1,1-diphenylethylene.The reactions were performed under solvent-free conditions, and the only by-product is water. The Br(?)nsted acid ionic liquid could be recovered and reused without significant loss of its activity.