Meisenheimer complexes reactions with acid

Previous attempts (8,10,12,13,14) to develop the J- acidity function scale from equilibrium measurements have been unsuccessful. Most frequently studied addition reactions of hydroxide ions are those involving nitroaromatic compounds (formation of Meisenheimer complexes). Measurements of equilibria with hydroxide ions involving nitro compounds were found complicated by consecutive reactions (8, 9,10), by uptake of a second hydroxide ion (13), or by complicated changes in absorption spectra (13,14). 

The last but one in situ procedure for activating carboxylic acids is shown in Figure 6.16. There, the oc-chlorinated A-methylpyridinium iodide A reacts with the carboxylic acid by an SN reaction at a pyridine carbon. This leads to the pyridinium salt C, presumably via the Meisenheimer complex B and its deprotonation product D as intermediates. The activated carboxylic acid C is not only an aryl ester but one in which the aryl group is positively charged. This charge keeps the single-bonded O atom of this species completely from providing any resonance stabilization by its +M effect to the C=0 double bond (cf. discussion of Table 6.1). 

The order of reactivity of picryl halides is the reverse of that observed in the reactions of the corresponding alkyl halides.1 Alkyl fluorides react only slowly with nucleophiles whereas picryl fluoride reacts extremely rapidly with hydroxide ions to produce picric acid. Addition of sodium hydroxide to a picryl halide in aqueous dioxan produces a transient red intermediate. The present study has been concerned with the measurement of the rates of formation and decay of the intermediates derived from picryl fluoride, chloride and bromide using stopped-flow spectrophotometry. The spectra of the intermediates are similar to those attributed to Meisenheimer complexes. Picryl fluoride reacts to give this intermediate in a few milliseconds. The rate of appearance of the intermediate was therefore only measurable at very low base concentrations (less than 6 x 10 M hydroxide). Under these conditions the rate was found to... 

As a Carbon Nncleophile in Fluoride Ion-Catalyzed Reactions. The reactions with aldehydes, ketones (eq 23), and o , -unsaturated esters (eq 24) can also be catalyzed by fluoride ion, usually introduced as tetra-n-butylammonium fluoride (TBAF), or other silicophihc ions such as alkoxide. These reactions produce silyl ether intermediates, which are usually hydrolyzed before workup. The stereochemistry of attack on chiral ketones can sometimes be different for the Lewis acid- and fluoride ion-catalyzed reactions. In addition some electrophiles only react in the fluoride-catalyzed reactions, as with the addition to trinitrobenzene giving an allyl Meisenheimer complex . ... 

---