Anion-Binding Catalysis by Electron-Deficient Pyridinium Cations

A new activation principle in organocatalysis is presented: halide binding through Coulombic interactions. This mode of catalysis was realized by using 3,5-di(carbomethoxy)pyridinium ions that carry an additional electron-withdrawing substituent on the nitrogen atom, for example, pentafluorobenzyl or cyanomethyl. For the N-pentafluorobenzyl derivative, Coulombic interaction with the pyridinium moiety is complemented in the solid state by anion- interactions with the perfluorophenyl ring. Bromide and chloride are bound by these cations in a 1:1 stoichiometry. Catalysis of the CC coupling between 1-chloroisochroman (and related electrophiles) with silyl ketene acetals occurs at -78 degrees C and at low catalyst loading (2 mol%).