Cyanoform anion

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 study of gas-phase reactions of benzyl and methoxide anions with alkyl formate and other esters has revealed some differences in behaviour of these anions of comparable basicity.184 The delocalized benzyl anion and localized methoxide ion engage in exclusive transacylation and proton transfer, respectively, on reaction with alkyl formates. However, proton transfer is sufficiently exothermic to dominate when benzyl anion reacts with methyl acetate. Both anions react with methyl benzoate, methyl trifluoroacetate, and methyl cyanoformate by competing transacylation and S 2 reactions. 

In the course of our studies devoted to the scaling up of ethyl cyanoformate preparation, we noticed that the side reaction is strongly related to the nucleophilicity of the cyanide anion which depends on the structure of the counter cation of the catalyst. Quite good results can be achieved by a proper choice of the catalyst as depicted in scheme 47. 

The mechanism of the Krapcho dealkoxycarbonylation is dependent on the structure of the substrate ester and the type of anion used. In the case of a,a-disubstituted diesters (especially the methyl esters), the anion from the salt (cyanide ion in the scheme) attacks the alkyl group of the ester in an Sn2 fashion and the decarboxylation results in the formation of a carbanionic intermediate that is quenched by the water. In the case of a-monosubstituted diesters the cyanide attacks the carbonyl group to form a tetrahedral intermediate, which breaks down to give the same carbanionic intermediate and a cyanoformate, which is hydrolyzed to give carbon dioxide and an alcohol. 

Following additional trial experiments which revealed the need to activate 200 by C-acylation [104], the anion of this lactone was condensed with methyl cyanoformate [118] to produce 210 (Scheme XXIV). In line with plans to achieve convergency by means of Stille coupling [119], it was necessary to activate 210 as... 

By using cyanoformate esters as electrophiles, Johnson and coworkers were able to employ catalytic amounts of the cyanide initiator in reactions with acyl silanes. This process led to carbon-carbon bond formation via C-acylation. For example, acyl silane 61 and ethylcyanoformate combined under these conditions to provide the tertiary alcohol silyl ether 62 in nearly quantitative yield. Johnson s group has also developed an asymmetric version of this process, utilizing the Jacobsen (salen)aluminum system as a chiral carrier for the cyanide anion. ... 

Johnson s group developed a catalytic asymmetric cyanation/1,2-Brook rearrangement/C-acylation of acylsilanes with cyanoformates (Scheme 19.14). In the presence of (i ,/ )-(salen)Al 19, the corresponding cyanohydrin trimethylsilyl ethers of a-keto esters were obtained in moderate to good enantioselectivities (61-82% enantiomeric excess). Access to chiral (silyloxy)nitrile anions is facilitated by metal cyanide-promoted Brook rearrangement reaction of acylsilanes. 

Higher Alkyl Cyanoformates. A range of other alkyl cyanoformates has been successfully utilized for the acylation of enolate anions, including ethyl, allyl, benzyl, and p-methoxybenzyl, but not r-butyl cyanoformate, which appears to be insufficiently reactive. Enantiomerically enriched cyanoformates derived from (+)-menthol, (-)-bomeol, and the Oppolzer alcohol were reported to furnish good chemical yields, but the level of enantioselectivity was disappointingly low (eq 18). ... 

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