Nitrile anion
Nitrile anions are nitriles lacking a proton at the position α to the nitrile group. They undergo nucleophilic addition and substitution reactions with various electrophiles.
Although nitrile anions are functionally similar to enolates, the extra multiple bond in nitrile anions provides them with a unique ketene-like geometry. Additionally, deprotonated cyanohydrins can act as masked acyl anions, giving products impossible to access with enolates alone. The mechanisms of nitrile addition and substitution are well understood; however, strongly basic conditions are usually required, limiting the reaction's synthetic usefulness.
Nitrile anions are most often generated through the action of an appropriate base. However, the pKas of nitriles span a wide range—at least 20 pKa units. Thus, the proper choice of base is usually substrate dependent. Acetonitriles containing an extra stabilizing electron-withdrawing group (such as an aromatic ring) can usually be deprotonated using hydroxide or alkoxide bases. Unstabilized nitriles, on the other hand, usually require either alkali metal amide bases (such as NaNH2) or metal alkyls (such as butyllithium) for effective deprotonation. In the latter case, competitive addition of the alkyl group to the nitrile takes place.
IR spectroscopy studies have demonstrated the existence of at least two tautomeric forms of the nitrile anion (see above).
Polyanions of nitriles can also be generated by multiple deprotonations, and these species produce polyalkylated products in the presence of alkyl electrophiles.
Alternative methods to produce nitrile anions include conjugate addition to α,β-unsaturated nitriles, reduction, and transmetallation.
The mechanisms of reactions involving nitrile anions depend primarily on the nature of the electrophile involved. Simple alkylations take place by SN2 displacement and are subject to the usual stereoelectronic requirements of the process. Phase-transfer catalysis has been employed in alkylations of arylacetonitriles. Nitrile anions can also be involved in Michael-type additions to activated double bonds and vinylation reactions with a limited number of polarized, unhindered acetylene derivatives.
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