Mechanistic Studies of the Rhodium NHC Catalyzed Hydrodefluorination of Polyfluorotoluenes

The six-membered-ring NHC complexes Rh(6-NHC)(PPh3)2H (6-NHC = 6-iPr (1), 6-Et (2), 6-Me (3)) have been employed in the catalytic hydrodefluorination (HDF) of C6F5CF3 and 2-C6F4HCF3. Stoichiometric studies showed that 1 reacted with C6F5CF3 at room temperature to afford cis- and trans-phosphine isomers of Rh(6-iPr)(PPh3)2F (4), which re-form 1 upon heating with Et3SiH. Although up to three consecutive HDF steps prove possible with C6F5CF3, the ultimate effectiveness of the catalysts is limited by their propensity to undergo C–H activation of partially fluorinated toluenes to give, for example, Rh(6-iPr)(PPh3)2(C6F4CF3) (7), which was isolated and structurally characterized.