Direct and Autocatalytic Reductive Elimination from Gold Complexes ([(Ph3P)Au(Ar)(CF3)(X)], X = F, Cl, Br, I): The Key Role of Halide Ligands

Kinetic and thermodynamic preferences for the reductive elimination of C-aryl-CF3, C-aryl-X, C-aryl-P, and CF3-X bonds and competitive phosphine dissociation from a series of Au-III complexes [(Ph3P)Au(Ar)(CF3)(X)] (1(X); Ar=4-Me-C6H4; X=F, Cl, Br, I) are studied computationally. Kinetically, the most favorable pathways were found to consist of an initial phosphine dissociation from complex 1(X), which furnished the respective three-coordinate Au-III complexes [Au(Ar)(CF3)(X)] (2(X)). The computed enthalpy barriers for various reductive elimination reactions from complex 2(X) by a direct (or uncatalyzed) mechanism showed that C-aryl-CF3 bond formation was the most favorable fate for any X group. When the direct elimination was compared with an autocatalytic mechanism that proceeded through the formation of a mixed-valent binuclear Au-III-Au-I intermediate, the preference for the formation of a C-aryl-CF3 bond is dependent on the nature of the bridging halide atom and follows the order F>Cl>Br>I. Concomitantly, the selectivity for the formation of C-aryl-X bonds for various X atoms follows the opposite trend. The preference for the direct and autocatalytic processes is controlled entirely by the nature of the halide ligand. The predicted mechanisms and product selectivity trends for various halides show excellent agreement with recent experimental observation. The selectivity of various reductive elimination pathways was rationalized by using molecular orbital theory and distortion-interaction model analyses. Attractive interactions between the Au-I complex and complex 2X were found to reduce the activation barrier for C-aryl-X elimination and critically control the selectivity of the product formation.