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 (Delta G(exp)(not equal) = 13.4 kcal mol(-1); Delta G(calc)(not equal)= 12.8 kcal mol(-1) in toluene) and slower cis trans isomerization (Delta G(calc) = +0.9 kcal mol(-1); Delta G(calc)(not equal) = 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 (Delta H-not equal = 25.9 +/- 2.6 kcal mol(-1); Delta S-not equal = 6.4 +/- 7.8 e.u.) are in excellent agreement with the computed data (Delta H-calc(not equal) = 24.8 kcal mol(-1); Delta G(calc)(not equal), = 25.0 kcal mol(-1)). Delta G(calc)(not equal) 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 (Delta G(calc)(not equal) = 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 degrees 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-[(eta(1)-Xantphos)(2)Pd(Ph)(CF3)] that, by computation, is predicted to access reductive elimination of CF3-Ph with Delta G(calc)(not equal), = 22.8 kcal mol(-1).