Insight into the Modes of Activation of Pyridinium and Bipyridinium Salts in Non‐Covalent Organocatalysis

Abstract A series of pyridinium and bipyridinium salts were prepared and their catalytic properties were evaluated in the aza‐Diels‐Alder reaction between imines and Danishefsky diene. Depending on the substituents of the pyridinium/bipyridinium rings and on the nature of the counterion, two mechanisms of activation were demonstrated. In case of non‐substituted rings, the substrate is activated through charge transfer involving the aryl ring on the C‐side of the imine. When halogen atoms were introduced on the catalysts, the activation mode switched to halogen bond involving the imine nitrogen lone pair. Moreover, alternative activation modes based on hydrogen bonding and radical cation were ruled out. This work allowed us to develop two families of catalysts whose potential was demonstrated in the cycloaddition of various imines with Danishefsky diene. The first family is composed of the simple methyl pyridinium triflate and dioctyl bipyridinium triflate. The former is active only with imines bearing a p ‐methoxyphenyl group on the C‐side and the latter was found to be efficient with imines bearing different substituents on both the N‐ and C‐sides of the imines. The second family is based on halogenated pyridinium salts which proved active with almost all considered imines. magnified image