B(C ₆F ₅) ₃-Catalyzed C(Sp ³)-F Bond Activation With/Without the Cooperation of HFIP: Synthesis of Spirobiindanes, Monofluoroalkenes, and Indane Derivatives

Abstract

The activation of C(sp³)-F bond in saturated fluorocarbons, especially for aliphatic gem-difluoroalkanes, remains challenging and less reported. Herein, we describe the selective activation of inert C(sp³)-F bonds catalyzed by B(C₆F₅)₃. In hexafluoro-2-propanol (HFIP), chemically robust aliphatic gem-difluorides are converted in high yields to the corresponding substituted 2,2',3,3'-tetrahydro-1,1'-spirobiindenes via a B(C₆F₅)₃-catalyzed intramolecular cascade Friedel-Crafts cyclization, whereby a silicon-based trapping reagent is not required. However, in the absence of a hydrogen-bonding donor solvent such as HFIP, the aliphatic gem-difluorides preferentially engage in a defluorination/elimination process that provides monofluorinated alkenes in good yields. In addition, a series of substituted 1-alkyl-2,3-dihydro-1H-indenes was obtained in high yield from the B(C₆F₅)₃-catalyzed defluorinative cyclization of aliphatic secondary monofluorides in HFIP. The protocol could inspire the development of a new class of main-group Lewis acids-catalyzed C(sp³)-F bond activation in general unactivated fluorocarbons.

Keywords: fluorine, C-F bond activation, Cyclization, catalysis, spiro compounds, fluorro-olefine