Direct coupling of unactivated alkynes and C(sp³)–H bonds catalyzed by a Pt(ii, iv)-centered catalyst: a computational study
2015
Li, Zhi-Feng | Li, Hui-Xue | Yang, Xiao-Ping | Liu, Xin-Wen | Zuo, Guo-Fang | Zhao, Cunyuan
Direct coupling of unactivated alkynes and C(sp³)–H bonds catalyzed by a Pt(ii, iv)-centered catalyst X (X = PtCl₂, PtBr₂, PtI₂ and PtI₄) (J. Am. Chem. Soc. 2009, 131, 16525) have been theoretically investigated with density functional theory (DFT). A comprehensive mechanistic DFT study of these reactions was carried out to better understand the experimental outcomes, and divergent and substrate-/catalyst-dependent mechanisms for the formation of ether derivatives were uncovered based on the computational results. Free energy diagrams for three types of mechanisms were computed, (a) in Mechanism I, the transition state implies a directed 1,5-hydrogen shift (pathways a1–a4), (b) Mechanism II leads to the formation of a Pt(ii, iv) vinyl carbenoid (pathway b), and (c) Mechanism III involves an O-coordinated Pt and includes 5,6-hydrogen migration (pathway c). Results suggest that the catalytic mechanism with PtI₄ is different to PtCl₂, PtBr₂ and PtI₂ catalysts. When PtCl₂, PtBr₂ and PtI₂ were used the insertion reaction pathway a2 is favored while PtI₄ adopted the pathway a1. Comparing the energy profiles, the pathway a1 with PtI₄ is the most favored. Through 1,5-hydrogen transfer, the concerted insertion pathway a1 with carbocationic intermediate is favored while the vinyl carbenoid mechanism is implausible.
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