## 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  
#### 2.5.1 The State of the Art  
In the course of organic synthesis, many waste products can form in addition to the desired products since the transformations of the synthesis intermediates are not quantitative and selective processes. These waste products should be regenerated, destroyed, and disposed, which consumes significant energy and places a heavy burden on the environment. Therefore, it is of great importance to develop and use synthetic methodologies that minimize these problems. One of the most general and efficient methodologies that fulfill this requirement is phase-transfer catalysis (PTC). This methodology is applicable to a wide variety of reactions in which inorganic and organic anions react with organic substrates. The methodology consists of the use of a heterogeneous two-phase system, in which one phase is a reservoir of reacting anions or a base for generating the organic anions, while the organic reagents and catalysts are located in the second organic phase. The reacting anions are continuously introduced into the organic phase in the form of ion pairs of lipophilic anions with lipophilic cations supplied by the catalyst [150]. In organic synthesis, it is often necessary to carry out reactions between water-soluble and oil-soluble reagents. [PERSON] found that organic soluble quaternary ammonium or phosphonium cations, Q\({}^{+}\), are suitable agents for the transport of anions from the aqueous phase to the organic phase. They act as powerful reaction accelerators [151]. Phase-transfer catalysts allow a reactant to migrate from one phase to another where the reaction takes place, eliminating the need for costly and unsafe solvents that can dissolve all reactants in one phase and costly raw materials, minimizing the waste issue. Phase-transfer catalysts are an alternative or complement to the existing, more traditional biocatalyzed and transition metal-catalyzed systems, representing a highly attractive method for the preparation of complex and multifunctional organic molecules [152]. The development of metal-free small organic molecules as catalysts has been a landmark advancement in organic synthesis in the recent past [153]. Asymmetric phase-transfer catalysis is based on the use of structurally well-defined chiral, non-racemic catalysts [154]. Chiral phase transfer or ion pair catalysis has become one of the fundamental catalytic principles due to the fact that it enables a wide range of powerful asymmetric reactions. One of the main advantages of all these chiral cations (Q\({}^{+}\)X\({}^{-}\)) phase-transfer catalysts is their unique ability to control the reactivity of a wide range of different prochiral nucleophiles through the formation of chiral ion pairs, which then undergo stereoselective \(\alpha\)-functionalization reactions with various electrophiles (Scheme 9) [155]. Over the last two decades, tremendous efforts have focused on the design of novel high-performance organocatalysts to realize unprecedented reactions, including asymmetric transformations. In this context, the rational design of various types of chiral phase-transfer catalysts and their successful application to a wide variety of asymmetric transformations is considered to have the potential to create privileged structures [156; 157]. In recent decades, chiral phase-transfer catalysts containing proton donor groups have become an extremely important strategy in the development of new catalysts; a large number of enantioselective reactions have been developed using PTC with proton-donating groups (OH, phenol, amide, and urea) [158]. It is worth noting that in addition to chiral cation-based phase-transfer catalysts (like the aforementioned onium and binapthyl-based catalysts), the use of chiral anion-based PTCs has recently also attracted a lot of attention and led to a variety of highly versatile methods for use in asymmetric catalysis.