文摘
We have theoretically studied the oxidative addition of all halomethanes CH3X (withX = 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 gasphase as well as the condensed phase (water), where solvent effects are described with theconductor-like screening model. The activation of the C*-X bond may proceed via twostereochemically different pathways: (i) direct oxidative insertion (OxIn) which goes with retentionof the configuration at C* and (ii) an alternative SN2 pathway which goes with inversion of theconfiguration at C*. In the gas phase, for Pd, the OxIn pathway has the lowest reaction barrierfor all CH3X's. Anion assistance, that is, going from Pd to PdCl-, changes the preference for allCH3X's from OxIn to the SN2 pathway. Gas-phase reaction barriers for both pathways to C-Xactivation 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 andCl, and (ii) the shift in reactivity of PdCl- + CH3X in the opposite direction, that is, from SN2 toOxIn, in the case of the heavier halogens, I and At. We use the activation strain model to arriveat a qualitative understanding of how the competition between OxIn and SN2 pathways isdetermined by the halogen atom in the activated C-X bond, by anion assistance, and bysolvation.