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 equals aryl, X equals halide)

The oxidative addition of RI to Pd(O) 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 F-19 NMR kinetic study of the isomerization of 2 in THF at 322.6 K reveals a first-order law r(iso) = k(iso)[2], with k(iso) = f + g[2](0) + (h + i[2](0))/[PPh3] + j)(f = (1.66 +/- 0.03) x 10(-4) s(-1), g = (2.5 +/- 0.2) x 10(-3) mol(-1) L s(-1), h = (1.3 +/- 0.7) x 10(-8) mol L-1 s(-1), i = (4 +/- 2) x 10(-7) s(-1), and j = (1.4 +/- 0.7) x 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(C6Cl2F3)I(PPh3)(mu-I)-[Pd]}. The later rearranges via terminal-for-bridging iodide exchange to trans-{Pd(C6Cl2F3)I(PPh3)(mu-I)]-[Pd]}, which is finally cleaved by PPh3 yielding complex 3. The other two concurrent pathways are assigned to the isomerization via two consecutive Berry pseudorotations in the pentacoordinated species derived from 2 by coordination of THF (coefficient f) or I-[Pd] (coefficient g). The apparent activation entropy associated with k(iso) is negative (Delta S double dagger = -21 +/- 3 J K-?(-1) mol(-1)), in agreement with the proposed bimolecular mechanisms.