posted on 2007-03-13, 00:00authored byG. Theodoor de Jong, F. Matthias Bickelhaupt
We have theoretically studied the oxidative addition of all halomethanes CH3X (with
X = F, Cl, Br, I, At) to Pd and PdCl-, using both nonrelativistic and zeroth-order-regular-approximation-relativistic density functional theory at BLYP/QZ4P. Our study covers the gas
phase as well as the condensed phase (water), where solvent effects are described with the
conductor-like screening model. The activation of the C*−X bond may proceed via two
stereochemically different pathways: (i) direct oxidative insertion (OxIn) which goes with retention
of the configuration at C* and (ii) an alternative SN2 pathway which goes with inversion of the
configuration at C*. In the gas phase, for Pd, the OxIn pathway has the lowest reaction barrier
for all CH3X's. Anion assistance, that is, going from Pd to PdCl-, changes the preference for all
CH3X's from OxIn to the SN2 pathway. Gas-phase reaction barriers for both pathways to C−X
activation generally decrease as X descends in group 17. Two striking solvent effects are (i)
the shift in reactivity of Pd + CH3X from OxIn to SN2 in the case of the smaller halogens, F and
Cl, and (ii) the shift in reactivity of PdCl- + CH3X in the opposite direction, that is, from SN2 to
OxIn, in the case of the heavier halogens, I and At. We use the activation strain model to arrive
at a qualitative understanding of how the competition between OxIn and SN2 pathways is
determined by the halogen atom in the activated C−X bond, by anion assistance, and by
solvation.