Catalytic Carbon-Halogen Bond Activation: Trends in Reactivity, Selectivity, and Solvation

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• 作者：G. Theodoor de Jong ; F. Matthias Bickelhaupt
• 刊名：Journal of Chemical Theory and Computation
• 出版年：2007
• 出版时间：March 2007
• 年：2007
• 卷：3
• 期：2
• 页码：514 - 529
• 全文大小：225K
• 年卷期：v.3,no.2(March 2007)
• ISSN：1549-9626

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 S_N2 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 S_N2 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 S_N2 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 S_N2 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 S_N2 pathways isdetermined by the halogen atom in the activated C-X bond, by anion assistance, and bysolvation.