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 coordinating solvents are present. DFT computations 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.