Designing Fluorinated Cinchona Alkaloids for Enantioselective Catalysis: Controlling Internal Rotation by a Fluorine‐Ammonium Ion gauche Effect (φNCCF)

TLDR: A fluoride-ammonium ion gauche effect constitutes a partial solution to the long-standing problem of governing internal rotations in cinchonium-based catalysts relying solely on a fluorine conformational effect.

Abstract: The C9 position of cinchona alkaloids functions as a molecular hinge, with internal rotations around the C8 C9 (tau(1)) and C9 C4' (tau(2)) bonds giving rise to four low energy conformers (1; anti-closed, anti-open, syn-closed, and syn-open). By substituting the C9 carbinol centre by a configurationally defined fluorine substituent, a fluorine-ammonium ion gauche effect (sigma(C-H) -> sigma(C-F)*; F-delta-...N+) encodes for two out of the four possible conformers (2). This constitutes a partial solution to the long-standing problem of governing internal rotations in cinchonium-based catalysts relying solely on a fluorine conformational effect.