Unexpected catalysis: aprotic pyridinium ions as active and recyclable Bronsted acid catalysts in protic media

Simple pyridinium salt derivatives have been (rather unexpectedly) shown to promote highly efficient acetalization reactions of both aldehydes and ketones at ambient temperature. The optimum catalyst is aprotic, yet it can promote the formation of benzaldehyde dimethyl acetal at 0.1 mol % loading more efficiently than a protic Bronsted acid catalyst with a pK(a) of 2.2. The process is of wide scope with respect to both the nucleophilic and electrophilic components, and the ionic catalyst can be readily recovered by precipitation and reused without loss of activity.