Asymmetric ion-pairing catalysis in chemistry is a type of

asymmetric catalysis Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IUPAC as "a chemical reaction (or reaction sequence) in which one or more new elements of chirality are formed in a substrate molecul ...

taking place specifically with charged intermediates or charged

reagents In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

. In one type of catalysis

ion-pairing In chemistry, ion association is a chemical reaction whereby ions of opposite electric charge come together in solution to form a distinct chemical entity. Ion associates are classified, according to the number of ions that associate with each ot ...

exists with a charged and chiral catalyst. The charged catalyst can be cationic or anionic. Catalysis by anionic catalysts is also called asymmetric counteranion-directed catalysis. In the other variation of asymmetric ion-pairing catalysis called anion or cation binding, the chiral catalyst is neutral but binds in a noncovalent way to an intermediate ion pair. Asymmetric ion-pairing catalysis is distinct from other covalent types of catalysis such as

Lewis acid catalysis In Lewis acid catalysis of organic reactions, a metal-based Lewis acid acts as an electron pair acceptor to increase the reactivity of a substrate. Common Lewis acid catalysts are based on main group metals such as aluminum, boron, silicon, and t ...

and Bronsted acid catalysis. It is one of several strategies in

enantioselective synthesis Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IUPAC as "a chemical reaction (or reaction sequence) in which one or more new elements of chirality are formed in a substrate molecul ...

and of some relevance to academic research. The first reported example of this type of catalysis is attributed to Dolling, Davis & Grabowski who in 1984 used a chiral

cinchonidine Cinchonidine is an alkaloid found in '' Cinchona officinalis'' and ''Gongronema latifolium''. It is used in asymmetric synthesis Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IU ...

based

phase transfer catalyst In chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the transition of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of heterogeneous catalysis. Ionic ...

in the synthesis of

indacrinone Indacrinone is a loop diuretic. It can be used in patients of gout with hypertension as an antihypertensive because it decreases reabsorption of uric acid, while other diuretics increase it. Chirality and biological activity Indacrinone is a ...

. Important chiral cation scaffolds are

quaternary ammonium cation In chemistry, quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure , R being an alkyl group or an aryl group. Unlike the ammonium ion () and the primary, secondary, or tertiary ammonium cat ...

s and

quaternary phosphonium cation In polyatomic cations with the chemical formula (where R is a hydrogen or an alkyl, aryl, or halide group). These cations have tetrahedral structures. The salts are generally colorless or take the color of the anions. Types of phosphonium c ...

s. The first reported example of asymmetric cation-binding catalysis originates from

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and Sogah who in 1981 used a chiral

crown ether In organic chemistry, crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups (). The most common crown ethers are cyclic oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., . I ...

in combination with a potassium base in an enantioselective Michael addition. The development of chiral anionic catalysists is a more recent research field. In 2000 Llewellyn, Adamson & Arndtsen used a chiral borate anion in

olefin In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

aziridination and

cyclopropanation In organic chemistry, cyclopropanation refers to any chemical process which generates cyclopropane () rings. It is an important process in modern chemistry as many useful compounds bear this motif; for example pyrethroids and a number of quinolon ...

. Another regularly encountered scaffold is a chiral phosphate anion together with

iminium In organic chemistry, an iminium cation is a polyatomic ion with the general structure . They are common in synthetic chemistry and biology. Structure Iminium cations adopt alkene-like geometries. The central C=N unit is nearly coplanar with a ...

, carbocationic or

oxocarbenium An oxocarbenium ion (or oxacarbenium ion) is a chemical species characterized by a central sp2-hybridized carbon, an oxygen substituent, and an overall positive charge that is delocalized between the central carbon and oxygen atoms. An oxocarbenium ...

counterions. Chiral anion

phase-transfer catalysis In chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the transition of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of heterogeneous catalysis. Ionic ...

was introduced by Toste in 2007. ''Chiral Anion-Mediated Asymmetric Ring Opening of meso-Aziridinium and Episulfonium Ions'' Gregory L. Hamilton, Toshio Kanai, and F. Dean Toste Journal of the American Chemical Society 2008 130 (45), 14984-14986

References

{{Reflist Catalysis