Publication | Open Access
C(sp<sup>2</sup>)–H Borylation of Heterocycles by Well-Defined Bis(silylene)pyridine Cobalt(III) Precatalysts: Pincer Modification, C(sp<sup>2</sup>)–H Activation, and Catalytically Relevant Intermediates
36
Citations
65
References
2020
Year
Well-defined bis(silylene)pyridine cobalt(III) precatalysts for C(sp<sup>2</sup>)-H borylation have been synthesized and applied to the investigation of the mechanism of the catalytic borylation of furans and pyridines. Specifically, [( <sup><i>Ar</i></sup> SiNSi)CoH<sub>3</sub>]·NaHBEt<sub>3</sub> ( <sup><i>Ar</i></sup> SiNSi = 2,6-[EtNSi(N<sup>t</sup>Bu)<sub>2</sub>CAr]<sub>2</sub>C<sub>5</sub>H<sub>3</sub>N, <i>Ar</i> = C<sub>6</sub>H<sub>5</sub> (<b>1-H</b> <sub><b>3</b></sub> <b>·NaHBEt</b> <sub><b>3</b></sub> ), 4-MeC<sub>6</sub>H<sub>4</sub> (<b>2-H</b> <sub><b>3</b></sub> <b>·NaHBEt</b> <sub><b>3</b></sub> )) and <i>trans</i>-[( <sup><i>Ar</i></sup> SiNSi)Co(H)<sub>2</sub>BPin] (<i>Ar</i> = C<sub>6</sub>H<sub>5</sub> (<b>1-(H)</b> <sub><b>2</b></sub> <b>BPin</b>), 4-MeC<sub>6</sub>H<sub>4</sub> (<b>2-(H)</b> <sub><b>2</b></sub> <b>BPin</b>), Pin = pinacolato) were prepared and employed as single component precatalysts for the C(sp<sup>2</sup>)-H borylation of 2-methylfuran, benzofuran and 2,6-lutidine. The cobalt(III) precursors, <b>2-H</b> <sub><b>3</b></sub> <b>·NaHBEt</b> <sub><b>3</b></sub> and <b>2-(H)</b> <sub><b>2</b></sub> <b>BPin</b> also promoted C(sp<sup>2</sup>)-H activation of benzofuran, yielding [(<sup>Ar</sup>SiNSi)CoH(Bf)<sub>2</sub>] (Ar = 4-MeC<sub>6</sub>H<sub>4</sub>, <b>2-H(Bf)</b> <sub><b>2</b></sub> , Bf = 2-benzofuranyl). Monitoring the catalytic borylation of 2-methylfuran and 2,6-lutidine by <sup>1</sup>H NMR spectroscopy established the <i>trans</i>-dihydride cobalt(III) boryl as the catalyst resting state at low substrate conversion. At higher conversion two distinct pincer modification pathways were identified, depending on the substrate and the boron source.
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