Aminopyridines as acylation catalysts for tertiary alcohols

The acylation of unreactive alcohols with acid anhydrides is greatly facilitated by the addition of a catalytic (0.02–0.1 equivalent) amount of a 4-dialkylaminopyridine. The reaction is faster in nonpolar than in polar solvents and acetyl chloride is not as effective as acetic anhydride. Several pyridine, pyridazine, and quinoline derivatives have been examined as potential acylation catalysts. Of the systems examined, only a few of the 4-substituted pyridines were found to be acylation catalysts, the most effective being 4-pyrrolidinopyridine 4 and 1,1,3,3-tetramethyl-4-(4-pyridyl)guanidine 8. The reaction of t-butanol with an isocyanate is also accelerated by the presence of 4 but not as much as in the case of acylations. The cause of the pronounced effect of these pyridine species in catalyzing acylation reactions seems to be a combination of the increased donor ability of the 4-substituent and the stabilizing effect that this substituent has on an acyl pyridinium intermediate.