N-alkyl-4-boronopyridinium salts as thermally stable and reusable amide condensation catalysts.

[reaction: see text] N-Alkyl-4-boronopyridinium salts are highly effective and reusable catalysts for the dehydrative amide condensation reaction between equimolar mixtures of carboxylic acids and amines. N-Alkylboronopyridinium salts are thermally stabilized in the order N-alkyl-2-boronopyridinium salt << N-alkyl-3-boronopyridinium salt < N-alkyl-4-boronopyridinium salt. Homogeneous catalysts, such as 4-borono-N-methylpyridinium iodide, are more effective in the presence of ionic liquid and can be recovered by extraction with ionic liquid. In contrast, heterogeneous catalysts, such as polystyrene-bound 4-boronopyridinium salts, are effective even in the absence of ionic liquid and can be recovered by filtration.