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

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