On the Configuration Resulting from Oxidative Addition of RX to Pd(PPh3)4 and the Mechanism of the cis-to-trans Isomerization of [PdRX(PPh3)2] Complexes (R = Aryl, X = Halide)

The oxidative addition of RI to Pd(0) and further cis-to-trans isomerization, which are involved in the Stille reaction and other Pd-catalyzed syntheses, have been studied. C6Cl2F3I (1, C6Cl2F3 = 3,5-dichlorotrifluorophenyl) adds to Pd(PPh3)4 in THF at room temperature giving cis-[Pd(C6Cl2F3)I(PPh3)2] (2), which could be isolated before isomerization to the more stable trans-[Pd(C6Cl2F3)I(PPh3)2] (3). A 19F NMR kinetic study of the isomerization of 2 in THF at 322.6 K reveals a first-order law riso = kiso[2], with kiso = f + g[2]0 + (h + i[2]0)/([PPh3] + j) (f = (1.66 ± 0.03) × 10-4 s-1, g = (2.5 ± 0.2) × 10-3 mol-1 L s-1, h = (1.3 ± 0.7) × 10-8 mol L-1 s-1, i = (4 ± 2) × 10-7 s-1, and j = (1.4 ± 0.7) × 10-5 mol L-1). A four-pathway mechanism accounts for these results:  Two are assigned to the associative replacements of PPh3 coordinated to 2 by an iodide ligand of I−[Pd] (I−[Pd] = 2 or 3), both THF-assisted (coefficient h) or direct (coefficient i), leading to a monoiodide-bridged intermediate cis-{Pd(C...