Palladium-Catalyzed Homocoupling of Highly Fluorinated Arylboronates: Studies of the Influence of Strongly vs. Weakly Coordinating Solvents on the Reductive Elimination Process.

C-C reductive elimination from [PdL2(C6F5)2] to form polyfluorinated biaryls has been a challenge for over 50 years. Thus, palladium-catalyzed homocoupling of arylboronates (ArF-Bpin) containing two ortho-fluorine substituents is very difficult as the reaction typically stops at the [PdL2(ArF)2] stage after two transmetalation steps. The transmetalated complexes cis-[Pd(MeCN)2(C6F5)2] (3a), cis-[Pd(MeCN)2(2,4,6-C6F3H2)2] (3b), and cis-[Pd(MeCN)2(2,6-C6F2H3)2] (3e) have been isolated from the reaction of ArF-Bpin with Pd(OAc)2 in acetonitrile solvent, with no homocoupling observed. However, catalytic homocoupling proceeds smoothly in a "weakly-coordinating" arene solvent as long as no ancillary ligands or coordination solvents are present. DFT computations performed reveal that the active catalyst formed by arene solvent coordination leads to an overall reduced barrier for the reductive elimination step compared to the formation of stable [PdL2(ArF)2] complexes in the presence of a donor ligand or solvent L.