The Design and Synthesis of Functionalized Chiral Ionic Liquids and Their Applications in Asymmetric Organocatalysis
|Keywords||Functionalized chiral ionic liquids Combinatorial synthesis Aldol reaction Michael addition reaction α-Alkylation reaction|
Functionalized ionic liquids are receiving growing attention in the fields of catalysis, electrochemistry, energy and material science due to their unique structures, physical and chemical properties and their green credentials as reusable homogeneous supports and dual solvent-catalysts. Recently, chiral ionic liquids (CILs) have emerged as an important kind of functionalized ionic liquids. A number of CILs have been synthesized and applied as chiral resolution reagents, chiral solvents or asymmetric catalysts. As the unique properties of ILs become well-established, more promising applications of CILs are ensured, and the bottleneck now is in the development and synthesis of new kinds of CILs. Meanwhile, the potentials of FCILs as asymmetric catalysts still remain largely underdeveloped at present and the current successes have been primarily confined to the reactions like Michael additions and aldol reactions. Accordingly, my research has mainly focused on the synthesis of new FCILs and potential applications on asymmetric catalysis.(1) By taking advantage of the exploration of FCILs as a new type of asymmetric organocatalysts in selected chiral transformations and "click" reactions, a combinatorial synthetic route of FCILs was developed:The ring-opening reaction of cyclic sulfates, sulfamidates as well as sultones with N-alkyl imidazoles or pyridine was introduced as the key reaction.The so-formed zwitterions could be easily converted to FCILs by simple removing the sulfuric group. Based on this protocol, tens of room temperature FCILs with structure-diversities were synthesized. The resulted FCIL library includes the chiral cation type ionic liquids, chiral anion type ionic liquids and bis-chiral/bis-functionalized ionic liquids, which significantly facilities our further research on asymmetric catalysis and other functional applications.(2) A new type of non-covalently/bifunctional catalysts for asymmetric aldol reactions was developed:Bifunctional or multifunctional catalysis is a prevalent catalytic motif in nature enzymatic catalysis and now in organocatalysis. To achieve an effective bifunctional catalysis, non-covalent strategy would be much more desirable in assembling functional groups when comparing with tedious covalent synthesis. In this regard, the ion pair structure of FCILs provides a versatile platform to introduce different functional groups and their combinations. The feasibility of this strategy was demonstrated in asymmetric aldol reactions using bis-functional chiral ionic liquids.(3) The first desymmetric Michael addition was realized using FCIL type catalyst:It was found that the combination of FCILs and suitable acidic protic additives could efficiently promote the desymmetric Michael addition between 4-substituted cyclohexanones and nitro-olefins. Theγ-nitro ketones bearing three chiral centers could be obtained with high yields, good diastereoselectivities and perfect enantioselectivities. Meanwhile, the catalyst could be reused for 4 times without losing activity. As far as we know, this is the first organocatalytic desymmetric Michael additions.(4) The Asymmetricα-alkylation reaction of carbonyl compounds, particularly ketones, was realized using FCILs as catalysts:The asymmetricα-alkylation reaction of carbonyl compounds has long been recognized as a powerful tool in modern organic synthetic chemistry. Despite of notable breakthroughs in the development of asymmetric intermolecularα-alkylations of aldehydes, there are essentially no reports on the Asymmetricα-alkylations of ketones. We have developed the first asymmetric catalytic directα-alkylation of cyclic ketones catalyzed by FCILs. An unprecedented desymmetricα-alkylation of 4-substituted cyclohexanones was developed with up to 99% yield,>99:1 dr and up to 87% ee. Similarly, the reactions of 3-substituted cyclohexanones gave 2,5-cis-substituted products with up to 80% yield,>99:1 dr and 84% ee. The limitation and scopes were further demonstrated with different donors such as acyclic ketones, aldehydes and different carbocation acceptors.