NHC•Alane Adducts as Hydride Sources in the Hydrodefluorination of Fluoroaromatics and Fluoroolefins

We present herein the utilization of NHC-stabilized alane adducts of the type (NHC)center dot AIH(3) [NHC = Me(2)Im (1), Me(2)Im(Me) (2), iPr(2)Im (3), iPr(2)Im(Me) (4), Dipp(2)Im (5)] and (NHC)center dot AliBu(2)H [NHC = iPr(2)Im (6), Dipp(2)Im (7)] as novel hydride transfer reagents in the hydrodefluorination (HDF) of different fluoroaromatics and hexafluoropropene. Depending on the alane adduct used, HDF of pentafluoropyridine to 2,3,5,6-tetrafluoropyridine in yields of 15-99% was observed. The adducts 1, 2, and 5 achieved a quantitative conversion into 2,3,5,6-tetrafluoropyridine at room temperature immediately after mixing the reactants. Studies on the HDF of fluorobenzenes with the (NHC)center dot AIH(3) adducts 1, 3, and 5 and (Dipp(2)Im)center dot AliBu(2)H (7) showed the decisive influence of the reaction temperature on the H/F exchange and that 135 degrees C in xylene afforded the best product distribution. Although the HDF of hexafluorobenzene yielded 1,2,4,5-tetrafluorobenzene in moderate yields with traces of 1,2,3,4-tetrafluorobenzene and 1,2,4-trifluorobenzene, pentafluorobenzene was converted quantitatively into 1,2,4,5-tetrafluorobenzene, with (Dipp(2)Im)center dot AliBu(2)H (7) showing the highest activity and reaching complete conversion after 12 h at 135 degrees C in xylene. The HDF of hexafluoropropene with (Me(2)Im)center dot AIH(3) (1) occurred even at low temperatures and preferably at the CF2 group with the formation of 1,2,3,3,3-pentafluoropropene (with 0.4 equiv. of 1) or 2,3,3,3-tetra-fluoropropene (with 0.9 equiv. of 1) as the main product. .