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Pivotal Role of Electronics in Preferred Alkene over Alkyne Ni-Carboryne Insertions and Absolute Regioselectivities

Abstract

The in-situ formation mechanisms of active Ni-carboryne species (COM1) and subsequent alkene/alkyne Ni-C bond insertion priorities, as well as relevant cycloaddition regioselectivities and kinetics, were investigated using the IDSCRF-B3LYP density functional theory (DFT) method, and all atoms equitably treated at the DGDZVP level. Results reveal the o-carborane species to be energetically hedged into a four-step path (barrier heights 5.3, 19.7, 18.4 and 0.3 kcal•mol-1, respectively) prior to being transferred into the active Ni-carboryne species (COM1) with the assistance of nBuLi and NiCl2(PPh3)2 at room temperature. In direct agreement with empirical trends, alkene insertion into Ni-C bonds on COM1 is exclusively favoured over the competing alkyne insertion. Electronic structure analyses of the corresponding transition structures resolved the preference of alkene to alkyne is due to different bonding characteristics during this insertion process, namely, back donation for alkene but donation for alkyne insertion, as evidenced by the molecular graphics and NBO charge distributions. Subsequent alkyne-additions (i.e. post alkene-insertion) arise as the rate-determining step (RDS) for each of the five differing reactions (a-e) explored. The solution free-energy barriers of these RDSs (30.5~38.5 kcal•mol-1) were in quantitative agreement with their corresponding experimental yields, evidencing the reliability of the DFT results to reproduce chemical phenomena and energetic trends in the real Ni-catalyzed carboryne-alkene/alkyne cycloadditions.

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Publication details

The article was accepted on 04 Apr 2018 and first published on 05 Apr 2018


Article type: Paper
DOI: 10.1039/C8DT01154K
Citation: Dalton Trans., 2018, Accepted Manuscript
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    Pivotal Role of Electronics in Preferred Alkene over Alkyne Ni-Carboryne Insertions and Absolute Regioselectivities

    W. Mu, R. Cheng, D. Fang and G. A. Chass, Dalton Trans., 2018, Accepted Manuscript , DOI: 10.1039/C8DT01154K

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