Asymmetric ion-pairing catalysis in
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
is a type of
asymmetric catalysis 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 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 and
Bronsted acid catalysis. It is one of several strategies in
enantioselective synthesis 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 based
phase transfer catalyst in the synthesis of
indacrinone. Important chiral cation scaffolds are
quaternary ammonium cation
In organic chemistry, quaternary ammonium cations, also known as quats, are positively-charged polyatomic ions of the structure , where R is an alkyl group, an aryl group or organyl group. Unlike the ammonium ion () and the primary, secondary, ...
s and
quaternary phosphonium cations. The first reported example of asymmetric cation-binding catalysis originates from
Cram and Sogah who in 1981 used a chiral
crown ether 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 aziridination and
cyclopropanation. Another regularly encountered scaffold is a chiral phosphate anion together with
iminium,
carbocationic or
oxocarbenium counterions. Chiral anion
phase-transfer catalysis 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
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Catalysis