Cyano groups anion stabilization

A family of ionic liquids has developed around anions containing a central element coordinated by one or more cyano groups. The stability of the carbon-nitrogen triple... 

These acids (51) are organic molecules that contain a plurality of cyano groups and are readily ionized to hydrogen ions and resonance-stabilized anions. Typical cyanocarbon acids are cyanoform, methanetricarbonitrile (5) 1,1,3,3-tetracyanopropene [32019-26-4] l-propene-l,l,3,3-tetracarbonitrile (52) 1,1,2,3,3-pentacyanopropene [45078-17-9], l-propene-l,l,2,3,3-pentacarbonitrile (51) l,l,2,6,7,7-hexacyano-l,3,5-heptatriene [69239-39-0] (53) 2-dicyanomethylene-l,l,3,3-tetracyanopropane [32019-27-5] (51) and l,3-cyclopentadiene-l,2,3,4,5-pentacarbonitrile [69239-40-3] (54,55). Many of these acids rival mineral acids in strength (56) and are usually isolable only as salts with metal or ammonium ions. The remarkable strength of these acids results from resonance stabilization in the anions that is not possible in the protonated forms. 

A cyanide anion as a nucleophile adds to an aldehyde molecule 1, leading to the anionic species 3. The acidity of the aldehydic proton is increased by the adjacent cyano group therefore the tautomeric carbanion species 4 can be formed and then add to another aldehyde molecule. In subsequent steps the product molecule becomes stabilized through loss of the cyanide ion, thus yielding the benzoin 2 ... 

A carbanion can be a good leaving group in SNl-type solvolyses when the anion is highly stabilized by strong electron-withdrawing substituents. Mitsuhashi reported some examples of such reactions for substrates which eject an anion stabilized by cyano (and nitro) groups [81] (Mitsuhashi, 1986), [82] (Mitsuhashi and Hirota, 1990) and [83] (Hirota and Mitsuhashi,... 

An overview on cycloproparenyl anions has also been reported.3 According to theoretical calculation, cyclopropabenzenyl anion is by ca 145 kJmol-1 more stable than the parent cyclopropenyl anion. It has been shown that the stability of the cyclopropabenzenyl anion could be considerably enhanced by substitution of the aromatic ring with fluorine and cyano groups, and also by a linear extension of the aromatic backbone. 

Efforts directed towards the synthesis of stable, perhaps isolable, cyclopropenyl anions having three identical anion stabilizing groups, such as cyano or benzoyl, have been attempted without much success. Tricyanocyclopropene (22a) could not be prepared except as a transient intermediate which could be trapped with diphenylisobenzofuran The corresponding triketone (22b) was obtainable, but attempted generation of the corresponding anion was thwarted by conjugate (Michael) addition of the base to the double bond. The cyclopropenyl anion remains an illusive species ... 

Since the degree of stabilization of boron-stabilized carbanions is similar to that for anions stabilized by a formyl or a cyano group, the comparative lack of knowledge of boron-stabilized carbanions must be due either to kinetic factors associated with their production or to the availability and nature of their precursors. 

Other examples of cyclopropanation with stabilized 5-ylides, i.e. alkyl- or aryl-(dimeth-ylamino)oxosulfonium alkanides/ " are presented in Table 15. 5-Ylides stabilized by an anion-stabilizing group at the nucleophilic carbon atom have also been successfully utilized for the cyclopropanation of Michael acceptors. The stabilizing group was a ketone/ " ester/carboxylate anion/ phosphonate diester/ or a cyano function (Table 16). 

This polymerization goes so quickly because the anionic intermediate is highly resonance stabilized by the carbonyl and the cyano groups. A stable intermediate suggests a low activation energy which translates to a fast reaction. 

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