Reductive elimination leading to C-C bond formation in gold(III) complexes: A mechanistic and computational study

The factors affecting the rates of reductive C-C cross-coupling reactions in gold(III) aryls were studied using complexes that allow easy access to a series of electronically modified aryl ligands, as well as to gold methyl and vinyl complexes, using the pincer compounds (C^N^C)AuR (R = C₆F₅, CH=CMe₂, Me and p-C₆H₄X, where X = OMe, F, H, Bu^t, Cl, CF₃, or NO₂) as starting materials (C^N^C = 2,6-(4′-Bu^tC₆H₃)₂pyridine dianion). Protodeauration followed by addition of 1 equiv. 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 reductive aryl-R elimination. The rates for these cross-couplings increase in the sequence k(vinyl) > k(aryl) >> k(C6F5) > k(Me). Vinyl-aryl coupling is particularly fast, 1.15 × 10^–3 L mol^–1 s^–1 at 221 K, while both C6F5 and Me couplings encountered higher barriers for the C-C bond forming step. Using P(p-tol)₃ in place of SMe₂ greatly accelerates 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. C-C bond formation is rate limiting. In the non-chelating case, reductive C(sp²)-C(sp²) elimination from three-coordinate cations [(Ar¹)(Ar²)AuL]+ is almost barrierless, particularly if L = phosphine.