Chiral pyridinium-based ionic liquids

Pernak, J. and Feder-Kubis, J., Chiral pyridinium-based ionic liquids, containing the (lR,2S,5R)-(-)-menthyl group. Tetrahedron Asymmetry, 17,1728,2006. 

Chiral pyridinium-based ionic liquids can be prepared by N-alkylation of pyridines with chloromethyl ()-menthyl ether <2006TA1728>. A room temperature ionic liquid brominating agent is obtained when A- -pcntylpyridinium bromide is reacted with bromine <2004SL1318>. 

Also the use of moisture stable ionic liquids as solvents in the Diels-Alder reaction has been carried out, and in all examples an enhanced reaction rate was observed [182,183]. The application of pyridinium-based ionic liquids allowed the utilization of isoprene as diene [184]. The chiral ionic liquid [bmim][L-lactate] was used as a solvent and accelerated the reaction of cyclopentadiene and ethyl acrylate, however, no enantiomeric excess was observed [183]. In addition several amino acid based ionic liquids have been recently tested in the Diels-Alder reaction. Similar exo. endo ratios were found but the product was obtained as racemate. The ionic liquids were prepared by the addition of equimolar amounts of HNO3 to the amino acids [185]. Furthermore, an enantiopure imidazolium salt incorporating a camphor motive was tested in the Diels-Alder reaction. No enantiomeric excess was found [186]. 

Chiral enamine, dienamine, and trienamine intermediates have been shown to catalyse asymmetric Diels-Alder reactions via HOMO activation. These Diels-Alder reactions generally occur with high chemo-, regio-, and stereo-selectivities. ° The effect of the viscosity of pyridinium-based ionic liquids, with tetrafluoroborate and bis(trifluorosulfonimide) anions, on the kinetics of intra- and inter-molecular Diels-Alder reactions has been investigated. Results show that the intramolecular cycloaddition reaction is less susceptible to viscosity variations than the intermolecular reaction. 

Truong and Vo-Thanh (2010) developed an efficient method for the synthesis of functionalized chiral ammonium imidazolium and pyridinium based ionic liquids derived from (IR, 2S) ephedrine by solvent-free asymmetric Michael addition under microwave irradiation with good yields in very short reaction time. 

Recently, Ni and group [66] introduced a new type of chiral ionic liquid based on pyridinium cation having a chiral moiety tethered to a urea unit. The synthesis of salt involves a reaction of 2-aminomethyl pyridine with chiral 2-isocyanate-3-methylbutyrate and then heating in the presence of alkyl halide to form salt (Scheme 17.18). In total, nine chiral pyridinium salts were synthesized with varying amino acids. Currently, the authors are using these salts for asymmetric induction in organic transformation. 

Scheme 17.18 Synthesis of chiral ionic liquid based on pyridinium cation having a chiral moiety tethered to a urea unit...

---