C(sp²)-H Borylation of Heterocycles by Well-Defined Bis(silylene)pyridine Cobalt(III) Precatalysts: Pincer Modification, C(sp²)-H Activation, and Catalytically Relevant Intermediates

Well-defined bis(silylene)pyridine cobalt(III) precatalysts for C(sp2)-H borylation have been synthesized and applied to the investigation of the mechanism of the catalytic borylation of furans and 2,6-lutidine. Specifically, [(ArSiNSi)CoH3]·NaHBEt3 {ArSiNSi = 2,6-[EtNSi(NtBu)2CAr]2C5H3N, where Ar = C6H5 (1-H3·NaHBEt3) or 4-MeC6H4 (2-H3·NaHBEt3)} and trans-[(ArSiNSi)Co(H)2BPin] {Ar = C6H5 [1-(H)2BPin] or 4-MeC6H4 [2-(H)2BPin], and Pin = pinacolato} were prepared and employed as single-component precatalysts for the C(sp2)-H borylation of 2-methylfuran, benzofuran, and 2,6-lutidine. The cobalt(III) precursors, 2-H3·NaHBEt3 and 2-(H)2BPin, also promoted C(sp2)-H activation of benzofuran, yielding [(ArSiNSi)CoH(Bf)2] {Ar = 4-MeC6H4 [2-H(Bf)2], and Bf = 2-benzofuranyl}. Monitoring the catalytic borylation of 2-methylfuran and 2,6-lutidine by 1H NMR spectroscopy established the trans-dihydride cobalt(III) boryl as the catalyst resting state at low substrate conversions. At higher conversions, two distinct pincer modification pathways were identified, depending on the substrate and the boron source.