CF ₃-Ph reductive elimination from [(Xantphos)Pd(CF ₃)(Ph)]

CF ₃-Ph reductive elimination from [(Xantphos)Pd(Ph)(CF ₃)] (1) and [(i-Pr-Xantphos)Pd(Ph)(CF ₃)] (2) has been studied by experimental and computational methods. Complex 1 is cis in the solid state and predominantly cis in solution, undergoing degenerate cis-cis isomerization (ΔG ^* _exp = 13.4 kcal mol ^-1; ΔG ^* _calc = 12.8 kcal mol ^-1 in toluene) and slower cis-trans isomerization (ΔG _calc = +0.9 kcal mol ^-1; ΔG ^* _calc = 21.9 kcal mol ^-1). In contrast, 2 is only trans in both solution and the solid state with trans-2 computed to be 10.2 kcal mol ^-1 lower in energy than cis-2. Kinetic and computational studies of the previously communicated (J. Am. Chem. Soc. 2006, 128, 12644), remarkably facile CF ₃-Ph reductive elimination from 1 suggest that the process does not require P-Pd bond dissociation but rather occurs directly from cis-1. The experimentally determined activation parameters (ΔH ^* = 25.9 ± 2.6 kcal mol ^-1; ΔS ^* = 6.4 ± 7.8 e.u.) are in excellent agreement with the computed data (ΔH ^* _calc = 24.8 kcal mol ^-1; ΔG ^* _calc = 25.0 kcal mol ^-1). ΔG ^* _calc for CF ₃-Ph reductive elimination from cis-2 is only 24.0 kcal mol ^-1; however, the overall barrier relative to trans-2 is much higher (ΔG ^* _calc = 34.2 kcal mol ^-1) due to the need to include the energetic cost of trans-cis isomerization. This is consistent with the higher thermal stability of 2 that decomposes to PhCF ₃ only at 100 °C and even then only in a sluggish and less selective manner. The presence of excess Xantphos has a minor decelerating effect on the decomposition of 1. A steady slight decrease in k _obs in the presence of 1 and 2 equiv of Xantphos then plateaus at [Xantphos]:1 = 5, 10, and 20. Specific molecular interactions between 1 and Xantphos are not involved in this kinetic effect (NMR, T ₁ measurements). A deduced kinetic scheme accounting for the influence of extra Xantphos involves the formation of cis-[(η ¹- Xantphos) ₂Pd(Ph)(CF ₃)] that, by computation, is predicted to access reductive elimination of CF ₃-Ph with ΔG ^* _calc = 22.8 kcal mol ^-1.