Lewis Acid-Assisted C(sp³)-C(sp³) Reductive Elimination at Gold

The phosphine-borane iPr₂P(o-C₆H₄)BFxyl₂ (Fxyl = 3,5-(F₃C)₂C₆H₃) 1-Fxyl was found to promote the reductive elimination of ethane from [AuMe₂(μ-Cl)]₂. Nuclear magnetic resonance monitoring revealed the intermediate formation of the (1-Fxyl)AuMe₂Cl complex. Density functional theory calculations identified a zwitterionic path as the lowest energy profile, with an overall activation barrier more than 10 kcal/mol lower than without borane assistance. The Lewis acid moiety first abstracts the chloride to generate a zwitterionic Au(III) complex, which then readily undergoes C(sp³)-C(sp³) coupling. The chloride is finally transferred back from boron to gold. The electronic features of this Lewis-assisted reductive elimination at gold have been deciphered by intrinsic bond orbital analyses. Sufficient Lewis acidity of boron is required for the ambiphilic ligand to trigger the C(sp³)-C(sp³) coupling, as shown by complementary studies with two other phosphine-boranes, and the addition of chlorides slows down the reductive elimination of ethane.