CF3–Ph Reductive Elimination from [(Xantphos)Pd(CF3)(Ph)]

CF3–Ph reductive elimination from [(Xantphos)­Pd­(Ph)­(CF3)] (1) and [(i-Pr-Xantphos)­Pd­(Ph)­(CF3)] (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 (ΔGcalc = +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 CF3–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 CF3–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 PhCF3 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 kobs 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, T1 measurements). A deduced kinetic scheme accounting for the influence of extra Xantphos involves the formation of cis-[(η1-Xantphos)2Pd­(Ph)­(CF3)] that, by computation, is predicted to access reductive elimination of CF3–Ph with ΔG≠calc = 22.8 kcal mol–1.