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Probing Hydrogen Atom Transfer at a Phosphorus(V) Oxide Bond Using a “Bulky Hydrogen Atom” Surrogate: Analogies to PCET

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posted on 2018-10-31, 20:44 authored by Jiaxiang Chu, Timothy G. Carroll, Guang Wu, Joshua Telser, Roman Dobrovetsky, Gabriel Ménard
Recent computational studies suggest that the phosphate support in the commercial vanadium phosphate oxide (VPO) catalyst may play a critical role in initiating butane C–H bond activation through a mechanism termed reduction-coupled oxo activation (ROA) similar to proton-coupled electron transfer (PCET); however, no experimental evidence exists to support this mechanism. Herein, we present molecular model compounds, (Ph2N)3VN–P­(O)­Ar2 (Ar = C6F5 (2a), Ph (2b)), which are reactive to both weak H atom donors and a Me3Si (a “bulky hydrogen atom” surrogate) donor, 1,4-bis­(trimethylsilyl)­pyrazine. While the former reaction led to product decomposition, the latter resulted in the isolation of the reduced, silylated complexes (Ph2N)3V–NP­(OSiMe3)­Ar2 (3a/b). Detailed analyses of possible reaction pathways, involving the isolation and full characterization of potential stepwise square-scheme intermediates, as well as the determination of minimum experimentally and computationally derived thermochemical values, are described. We find that stepwise electron transfer (ET) + silylium transfer (ST) or concerted EST mechanisms are most likely. This study provides the first experimental evidence supporting a ROA mechanism and may inform future studies in homogeneous or heterogeneous C–H activation chemistry, as well as open up a possible new avenue for main group/transition metal cooperative redox reactivity.

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