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Asymmetric counteranion directed catalysis (ACDC) or chiral anion catalysis in enantioselective synthesis is the "induction of enantioselectivity in a 
The term ''asymmetric counteranion-directed catalysis'' was coined in 2006 by 
Chiral anions have been combined with 
Subsequently, Toste and coworkers advanced the concept of ''chiral anion phase transfer catalysis'', in which phase segregation (solid/organic or aqueous/organic) is used as a tactic to enforce the pairing of a cationic reagent or intermediate with a chiral counteranion. In the example below, the fluorinating reagent Selectfluor exhibits extremely limited solubility in nonpolar solvents and is therefore unreactive as an insoluble bulk solid. Exchange of its tetrafluoroborate anions with two equivalents of a chiral, lipophilic phosphate anion brings the reagent into solution as an ion-triplet which behaves as a source of chiral "F+":
reaction
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proceeding through a cationic
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
intermediate by means of ion pair
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 o ...
ing with a chiral
Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object.
An object or a system is ''chiral'' if it is distinguishable from i ...
, enantiomerically pure anion provided by the catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
". Although chiral Brønsted acid catalyzed reactions may well fall into this category of catalysis under the definition given here (e.g., protonation of an imine by a chiral phosphoric acid), the extent of proton transfer and the demarcation between hydrogen bonding and full proton transfer is often ambiguous. Hence, some authors may exclude ion pair formation by proton transfer as a type of chiral counteranion catalysis. The discussion below will focus on chiral ion pairs generated through means other than proton transfer. (''For a discussion about chiral Brønsted acid catalyzed reactions, see the article on hydrogen bond catalysis
Hydrogen-bond catalysis is a type of organocatalysis that relies on use of hydrogen bonding interactions to accelerate and control organic reactions. In biological systems, hydrogen bonding plays a key role in many enzymatic reactions, both in ori ...
.'')
The concept of utilizing the counteranion as the source of chirality in catalysis was first proposed by Arndtsen and coworkers in 2000, who applied chiral borate anions to copper-catalyzed olefin aziridination
Aziridine is an organic compound consisting of the three-membered heterocycle . It is a colorless, toxic, volatile liquid that is of significant practical interest. Aziridine was discovered in 1888 by the chemist Siegmund Gabriel. Its derivat ...
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 ...
. The enantioselectivities observed were low: <10% ee in cases where the chiral borate was the only source of chirality. However, the observation that enantioinduction occurred only in nonpolar solvents supports the proposed role of the borate as the chiral counteranion.
The term ''asymmetric counteranion-directed catalysis'' was coined in 2006 by List
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* List College, an undergraduate division of the Jewish Theological Seminary of America
* SC Germania List, German rugby unio ...
in the context of organocatalytic iminium reduction in the presence of a chiral phosphate anion:
Chiral anions have been combined with homogeneous catalyst
In chemistry, homogeneous catalysis is catalysis by a soluble catalyst in a solution. Homogeneous catalysis refers to reactions where the catalyst is in the same phase as the reactants, principally in solution. In contrast, heterogeneous catalysis ...
s, illustrated by chiral phosphate anions for intramolecular allene hydrofunctionalization reactions catalyzed by phosphinegold(I) complexes:
:
Subsequently, Toste and coworkers advanced the concept of ''chiral anion phase transfer catalysis'', in which phase segregation (solid/organic or aqueous/organic) is used as a tactic to enforce the pairing of a cationic reagent or intermediate with a chiral counteranion. In the example below, the fluorinating reagent Selectfluor exhibits extremely limited solubility in nonpolar solvents and is therefore unreactive as an insoluble bulk solid. Exchange of its tetrafluoroborate anions with two equivalents of a chiral, lipophilic phosphate anion brings the reagent into solution as an ion-triplet which behaves as a source of chiral "F+":
References
{{Reflist Catalysis