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 isomeri zation, which are involved in the Stille reaction and other Pd-catalyz ed syntheses, have been studied. C6Cl2F3I (1, C6Cl2F3 = 3,5-dichlorotr ifluorophenyl) adds to Pd(PPh3)(4) in THF at room temperature giving c is-[Pd(C6Cl2F3)I(PPh3)(2)] (2), which could be isolated before isomeri zation to the more stable trans-[Pd(C6Cl2F3)I(PPh3)(2)] (3). A F-19 NM R 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 assign ed to the associative replacements of PPh3 coordinated to 2 by an iodi de ligand of I-[Pd] (I-[Pd] = 2 or 3), both THF-assisted (coefficient h) or direct (coefficient i), leading to a monoiodide-bridged intermed iate cis-{Pd(C6Cl2F3)I(PPh3)(mu-I)-[Pd]}. The later rearranges via ter minal-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 con secutive 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 w ith the proposed bimolecular mechanisms.