Negatively charge conjugates

Proton removal for the purpose of preparing negatively charged conjugate bases in conditions suitable for direct NMR observation may be effected on exposure of the free amine to strongly basic conditions such as potassium amide in liquid ammonia for ben-zazonine (5c) and potassium mirror in THF for aza[13]annulene (37a). 

Organomagnesium halide reagents also add to vinylsilanes, a reflection perhaps of some ability of Si to reduce negative charge conjugatively on an adjoining carbon ° ° ... 

Both negatively charged conjugate bases Itiivc the same negative atom (O), but the carboxylate ion is stabilized by inductive elfeets and rc.soniince. Because the carboxylate is more stable, the carboxylic acid is a. stronger acid than the alcohol. 

Nucleophilicity can also be compared among species having the same nucleophilic atom. A negatively charged conjugate base of a neutral species (conjugate acid) is more nucleophilic than its corresponding neutral species. For example, HO is a better nucleophile than HjO. 

Notice that a neutral acid forms a negatively charged conjugate base, whereas a positively charged acid forms a neutral conjugate base. (The difference in charge decreases by one because the acid loses IT. )... 

Figure4.2 An electrically neutral acid such as acetic acid will dissociate in water to form a negatively charged conjugate base. Any acid that is charged, such as ammonium ion, will dissociate to form a conjugate base with a charge one unit less than the acid.

An interesting case are the a,/i-unsaturated ketones, which form carbanions, in which the negative charge is delocalized in a 5-centre-6-electron system. Alkylation, however, only occurs at the central, most nucleophilic position. This regioselectivity has been utilized by Woodward (R.B. Woodward, 1957 B.F. Mundy, 1972) in the synthesis of 4-dialkylated steroids. This reaction has been carried out at high temperature in a protic solvent. Therefore it yields the product, which is formed from the most stable anion (thermodynamic control). In conjugated enones a proton adjacent to the carbonyl group, however, is removed much faster than a y-proton. If the same alkylation, therefore, is carried out in an aprotic solvent, which does not catalyze tautomerizations, and if the temperature is kept low, the steroid is mono- or dimethylated at C-2 in comparable yield (L. Nedelec, 1974). 

Neutral Lewis bases such as water alcohols and carboxylic acids are much weaker nucleophiles than their conjugate bases When comparing species that have the same nucleophilic atom a negatively charged nucleophile is more reactive than a neutral one... 

The conjugate base of a hydrocarbon is called a carbanion It is an anion in which the negative charge is borne by carbon Because it is derived from a very weak acid a car banion such as CH3 is an exceptionally strong base... 

The structure of the conjugate base is more like resonance structure B than A because the negative charge is on the more electronegative atom (O versus S). 

The negatively charged ring in the transition state and intermediate complex presumably exerts little or no inductive electron attraction on a substituent. So, as one might expect, the transition-state effect of an azine methoxy group can differ from its (conjugated)... 

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