Phase pyridine/carboxylic acid systems

Solid-liquid phase systems with no added solvent produce esters in high yield [e.g. 2, 3] and are particularly Useful when using less reactive alkyl halides [e.g. 15], for the preparation of sterically hindered esters [16], or where other basic sites within the molecule are susceptible to alkylation, e.g. anthranilic acid is converted into the esters with minimal A-alkylation and pyridine carboxylic acids do no undergo quat-emization [17]. Excellent yields of the esters in very short reaction times (2-7 minutes) are also obtained when the two-phase system is subjected to microwave irradiation [18]. Direct reaction of the carboxylic acids with 1,2-dichloroethane under reflux yields the chloroethyl ester [19], although generally higher yields of the esters are obtained under microwave conditions [20]. 

The two-phase epoxidation of alkenes by hydrogen peroxide in water-dichloromethane system, catalysed by manganese(III)-porphyrin, is strongly accelerated by addition of catalytic amounts of a carboxylic acid and lipophilic imidazole or pyridine axial ligand365,366. Manganese(III)-porphyrin bound to colloidal anion-exchange particles is more active in the selective epoxidation of styrene by aqueous hypochlorite than the same catalyst in aqueous solution367. 

Derivatives of pyridine [248], derivatives of pyrimidine [249], pyrazine [250] and other substance classes contain lateral hydroxyl groups, which give rise to strong association effects. The formation of mesophases in binary systems of pyridine derivatives and carboxylic acids is a specific field of research [250a-253, 253 a, 253b]. In most cases complex formation leads to an increase in the length-to-breadth ratio, and different liquid crystalline phases can be induced. This is illustrated by the example shown in Figure 17 [250]. 

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