Carbanions basic principles

The basic principle of all diazotizations of aromatic amines with a hydroxy- or a sulfonamido group in the 4-position relative to the amino group involves a deprotonation of the OH or NH group, respectively, after diazotization of the amino group. There is also a case of a deprotonation of a CH group in the 4-position of an aniline derivative, namely in the diazotization of 4-aminophenylmalononitrile (2.41) which, by the sequence of steps shown in Scheme 2-23, yields 3-diazo-6-dicyanomethylene-1,4-cyclohexadienone (2.42), as found by Hartzler (1964). This product can also be represented by a zwitterionic carbanion-diazonium mesomeric structure. 

Substituent effects have a significant influence only on the carbanion intermediate. Since the reaction is basically an E2 elimination, substituent effects will result in a failure of the re activity-selectivity principle. 

For Michael additions of CH acidic compounds (e.g. diethyl malonate with a,3-unsaturated ketones) the following recommendations are given "When possible, relatively weak basic catalysts such as piperidine... should be selected. If stronger bases are required, it is normally appropriate to use only 0.1 to 0.3 equivalent of the base." The analogy of these conditions to those specified by our rule A is obvious (concentration control). On the other hand, preformed carbanions (organometallies) are usually employed when the addend is more basic than the enolate produced by attack at the unsaturated carbonyl compound. Though the nature of the metal ion plays a crucial rule in many "carbanionic" addition reactions, a first understanding of the principles involved can be... 

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