N-Alkyl-4-boronopyridinium Salts for Dehydrative Condensations

Significance

The polystyrene resin-supported N-alkyl-4-boronopyridinium salts were used as highly effective amide and ester condensation catalysts (1st and 2nd cited reports, respectively), which showed high reusability and stability under thermal conditions. Thus, for example, the reaction of 4-phenylbutanoic acid (1) with one equivalent of benzylamine (2) in the presence of N-polystyrene-bound 4-boronopyridinium bis(trifluoromethanesulfonyl) amide (3) gives the corresponding amide 4 with complete conversion (>99%). Resin 3 could be reused ten times without any significant loss of catalytic activity. The dehydrative esterification of α-hydroxy carboxylic acids with i-BuOH (as a solvent) was also successfully promoted by 3′.