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

• Published in: Chemistry : a European journal 2018-06, Vol.24 (35), p.8893-8903
• Main Authors: Rocchigiani, Luca, Fernandez‐Cestau, Julio, Budzelaar, Peter H. M., Bochmann, Manfred
• Format: Article
• Language: English
• Subjects: Anions; Aromatic compounds; Chelation; Chemical bonds; Chemical reactions; Chemistry; Computation; Computer applications; Conformation; Couplings; C−C coupling; density functional calculations; Gold; Ligands; Nitrogen dioxide; Phosphine; Pyridines; reaction mechanisms; reductive eliminations
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201801277

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=C6F5, CH=CMe2, Me and p‐C6H4X, where X=OMe, F, H, tBu, Cl, CF3, or NO2) as starting materials (C^N^C=2,6‐(4′‐tBuC6H3)2pyridine 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(C6F5)>k(Me). Vinyl−aryl coupling is particularly fast, 1.15×10−3 L mol−1 s−1 at 221 K, whereas both C6F5 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. Kick it out: The protolytic Au−C bond cleavage of [(C^N^C)AuR] pincer complexes, followed by addition of two or more equivalents of SMe2 leads to a reductive aryl−R elimination, with reactions rates decreasing in the sequence k(vinyl)>k(aryl)≫k(C6F5)>k(Me) (see figure). Vinyl−aryl coupling is very fast even at −52 °C.