## 2 Topical Research Areas Related to Ionic Liquids and the Contribution of Spiroammonium Compounds
### The Use of Metal-Free Small Organic Molecules as Phase-Transfer Catalysts
#### 2.5.2 Azoniaspiro Compounds as Phase-Transfer Catalysts
The reaction, as shown in Scheme 13, uses the PTCs **a**,**b**, as discussed in the previous reference [165]. Phase transfer reactions are mostly carried out in two or three-phase systems in which generally aqueous and nonpolar solvent mixtures are used. Here, to obtain a chiral Michael adduct, a simple and easy-to-implement solid-liquid biphasic system was used. All of the reaction compounds, including chiral PTCs, were loaded into toluene and stirred at a temperature close to r.t. The reaction mixture was filtered after the completion of the reaction, and the filtrate was concentrated under a vacuum to obtain a crude product as a residue, which was purified via recrystallization from methanol to obtain pure Michael adducts. It has been observed that PTCs **a** and **b** produce good enantioselectivity at 86%; at the same time, the yield of the products was high, amounting to 95%. (B) The asymmetric aza-Michael reaction (aza-MR) is an important synthetic tool for forming new C-N bond(s) for the development of new libraries of bioactive nitrogen compounds. The asymmetric conjugate amination of _tert_-butylbenzyloxycarbamate to \(\beta\)-nitrostyrene in the presence of chiral bi-functional tetraalkylammonium bromide as the chiral PTC was performed in a water-rich biphasic solvent. The reaction proceeded with high ee values of up to 95%, and very good yields (\(\approx\)99%) were achieved in all cases (Scheme 14) [166]. * _The catalytic asymmetric 1,6-conjugate addition_ A binaphthyl-based phase-transfer catalyst has been used in the reaction of asymmetric 1,6-conjugate addition to in situ generated _para_-quinone methides (_p_-QM), with 2,6-dialkyl-4-[phenyl(tosyl)methyl]phenol as substrates and tritylthiol as a nucleophile (Scheme 15) [167]. \(p\)-QM [168] has strong dipolar characteristics and a thermodynamic driving force that promotes rearomatization. This reaction provides a convenient route to optically active \(\alpha\)-substituted benzylthioethers, forming the basis of the structures of bioactive molecules [169]. Using 2,6-dimethyl derivatives as an example, the conditions for the PTC reaction have been worked out. The reaction proceeds smoothly under the conditions developed by the authors in the presence of a saturated aqueous solution of Na\({}_{2}\)CO\({}_{3}\) in CH\({}_{2}\)Cl\({}_{2}\) at r.t. It has been found that quaternary ammonium salts, such as tetrabutylammonium bromide (TBAB), could smoothly promote the reaction of 2,6-methyl-4-[phenyl(tosyl)methyl]phenol with tritylthiol (Table 4, entry 1). A comparative study of the catalytic behavior of various chiral quaternary ammonium salts has shown that tetrabutylammonium bromide (TBAB) smoothly promotes the reaction of 2,6-dimethyl-4-[phenyl(tosyl)methyl]phenol with tritylithiol, yielding the product with a quantity of 90% but in a racemic form. A study of the catalytic behavior of bifunctional chiral quaternary ammonium salts **A** revealed an improvement in yield (98%) and enantioselectivity (90% ee, entry 2) compared to TBAB. However, the reaction with chiral ammonium salt **B** leads to a high product yield that exists in the racemic form (90%, entry 3). It has been found that the best solvent for catalyst **A** is methyl _tert_-butyl ether (MTBE), which provides optimal performance in terms of yield and enantioselectivity (entries 5-7). Finally, a gram-scale reaction was carried out to evaluate the synthetic potential of the process. High enantioselectivity persists with a slight decrease in catalyst yield (entry 7). The generality of this reaction was further investigated using optimal conditions found by examining 17 structurally diverse starting samples of 2,6-disubstituted-4-[phenyl(tosyl)methyl]phenol. It is noteworthy that the desired products were obtained in excellent quantities with high enantioselectivity in a short time at r.t. using equivalent amounts of reagents at a 1 mol% loading of bifunctional chiral catalyst **A**.