Site-Selective Synthesis of 2,4,6-Collidinium Salts via Electrooxidative C–H Functionalization

Abstract

2,4,6-Trisubstituted pyridinium salts have emerged as versatile pseudohalides for SET-mediated radical cross couplings. However, the widely utilized 2,4,6-triphenylpyridinium Katrtizky salt is plagued by poor atom econ-omy and high cost of synthesis. Thus, there is a growing need for developing more practical scaffolds, along with novel strategies for pyridinium salt formation that will support both diverse heterocyclic motifs and sub-strates. A recent report demonstrated 2,4,6-collidinium salts as more atom-economical and cost-effective electron-acceptors, however their steric hinderance limits their synthesis via traditional nucleophilic substitu-tion. Herein, we report the synthesis of benzylic 2,4,6-collidinium salts via the electrooxidative C–H functional-ization of electron-rich arenes. The method proceeds under mild conditions, has broad functional group tol-erance, is applicable to the synthesis of both primary and secondary collidinium salts, and uses an inexpen-sive, recoverable collidine-based electrolyte system. The resulting salts can be isolated via trituration and di-rectly used in subsequent coupling reactions. This method represents the first synthesis of alkyl pyridinium salts via a net C¬¬–H functionalization, providing a complementary approach to traditional substitution and condensation reactions of pre-functionalized substrates.