Reductive Elimination Leading to C-C Bond Formation in Gold(III) Complexes: A Mechanistic and Computational Study

Abstract

The factors affecting the rates of reductive C-C cross-coupling reactions in gold(III) aryls were studied by using complexes that allow easy access to a series of electronically modified aryl ligands, as well as to gold methyl and vinyl complexes, by using the pincer compounds [(C^N^C)AuR] (R=C₆ F₅ , CH=CMe₂ , Me and p-C₆ H₄ X, where X=OMe, F, H, tBu, Cl, CF₃ , or NO₂ ) as starting materials (C^N^C=2,6-(4'-tBuC₆ H₃ )₂ pyridine dianion). Protodeauration followed by addition of one equivalent SMe₂ leads to the quantitative generation of the thioether complexes [(C^N-CH)AuR(SMe₂ )]⁺ . Upon addition of a second SMe₂ pyridine is displaced, which triggers the reductive aryl-R elimination. The rates for these cross-couplings increase in the sequence k(vinyl)>k(aryl)≫k(C₆ F₅ )>k(Me). Vinyl-aryl coupling is particularly fast, 1.15×10^-3 L mol^-1 s^-1 at 221 K, whereas both C₆ F₅ and Me couplings encountered higher barriers for the C-C bond forming step. The use of P(p-tol)₃ in place of SMe₂ greatly accelerates the C-C couplings. Computational modelling shows that in the C^N-bonded compounds displacement of N by a donor L is required before the aryl ligands can adopt a conformation suitable for C-C bond formation, so that elimination takes place from a four-coordinate intermediate. The C-C bond formation is the rate-limiting step. In the non-chelating case, reductive C(sp² )-C(sp² ) elimination from three-coordinate ions [(Ar¹ )(Ar² )AuL]⁺ is almost barrier-free, particularly if L=phosphine.

Keywords: C−C coupling; density functional calculations; gold; reaction mechanisms; reductive eliminations.