Apparent 6-endo-trig Carbofluorination of Alkenes Enabled by Palladium-Based Dyotropic Rearrangement

Alkene difunctionalization featuring an intramolecular carbometallation step has been developed into a powerful transformation in organic synthesis. The 5-exo-trig carbometallation is in general highly favored over the alternative 6-endo-trig cyclization. We report herein a formal 6-endo-trig carbofluorination protocol by combining the conventional 5-exo-trig carbopalladation with a chemoselective 1,2-aryl/Pd dyotropic rearrangement. Treatment of 2-(2-alkylallyloxy)phenyl boronic acid derivatives in the presence of a catalytic amount of Pd(dba)(2), 4,4 '-di-tert-butyl-2,2 '-bipyridine and Selectfluor affords chromanes with concurrent generation of a tertiary alkyl fluoride function. A sigma-alkyl-Pd-II species resulting from the 5-exo-trig carbopalladation process is isolated and characterized by X-ray crystallographic analysis. Its oxidation to Pd-IV complex triggers the chemoselective dyotropic rearrangement leading to the ring expansion product.