Cyano group carbanion stabilization

This reaction follows the Elcb mechanism (see Chapter 9). The cyano group helps stabilize the carbanion that is formed in the first step, so that the proton can be more easily removed. 

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 ... 

Depending on the substituents, either elimination reactions or solvolysis leads to the product 36 or 37, respectively. The formation of addition product 37 can be explained by heterolytic cleavage under formation of a Co(III) complex and a carbanion, which is then protonated by the solvent. Owing to the carbanion-stabilizing ability of the cyano group this pathway is pronounced in the reaction with acrylonitrile. The product 39 is formed... 

A route based on a benzyne intermediate can afford the isoindole ring via a category lb process. iV-Cyanomethyl-Af-methyl-0-chlorobenzylamine cyclizes to 1-methylisoindole under the influence of potassium amide in liquid ammonia (equation 36) (77T581). The cyano group plays two important roles. First, it provides stabilization of the carbanion... 

The overall result of a conjugate addition is the addition of a proton and a nucleophile to the CC double bond. However, this reaction differs greatly from the additions discussed in Chapter 11, in which the electrophile adds first. Here, the nucleophile adds in the first step. This reaction does not occur unless there is a group attached to the double bond that can help stabilize, by resonance, the carbanion intermediate. In many cases this is the carbonyl group of an aldehyde or a ketone. However, other groups, such as the carbonyl group of an ester or a cyano group, also enable this reaction to occur. 

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. 

The synthesis of the requisite P-nitro sulfones is accomplished by reacting a carbanion stabilized with a cyano and arylsulfonyl group with a 1-bromonitroalkane, as shown in equation (29). Carbanions stabilized by one arylsulfonyl group or by an arylsulfonyl and nitro group do not react. 

As expected, introduction of a second amino group on C in enamines with acceptors on lowers the barriers to C=C rotation. This can be ascribed to a better stabilization of the transition state, in which the carbocationic part assumes the character of an amidinium ion. Some typical barriers for l,l-bis(dimethylamino)ethylenes with acceptor groups in position 2 (24) are shown in Table 6. It is observed that, with a pair of good acceptors, the barriers become too low to be accessible with the NMR technique. With weaker acceptors, the order of the barriers follows the expected capacity of the acceptor groups to stabilize the carbanionic part of the transition state. Kessler has measured the C=C barriers in a number of p-substituted l,l-bis(dimethylamino)-2-cyano-2-phenylethenes (24a) and found that the logarithms of the estimated rate constants at 25 °C correlated well with the a values for the para substituents. Similar correlations with opposite signs for p and lower sensitivity were found for the C—N and C—aryl rotations. 

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