Stereoselective Radical Acylfluoroalkylation of Bicyclobutanes via N-Heterocyclic Carbene Catalysis.

Abstract

Cyclobutanes are prominent structural components in natural products and drug molecules. With the advent of strain-release-driven synthesis, ring-opening reactions of bicyclo[1.1.0]butanes (BCBs) provide an attractive pathway to construct these three-dimensional structures. However, the stereoselective difunctionalization of the central C-C σ-bonds remains challenging. Reported herein is a covalent-based organocatalytic strategy that exploits radical NHC catalysis to achieve diastereoselective acylfluoroalkylation of BCBs under mild conditions. The Breslow enolate acts as a single electron donor and provides an NHC-bound ketyl radical with appropriate steric hindrance, which effectively distinguishes between the two faces of transient cyclobutyl radicals. This operationally simple method tolerates various fluoroalkyl reagents and common functional groups, providing a straightforward access to polysubstituted cyclobutanes (75 examples, up to >19:1 d.r.). The combined experimental and theoretical investigations of this organocatalytic system confirm the formation of the NHC-derived radical and provide an understanding of how stereoselective radical-radical coupling occurs.