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Role of N, N′‐Diboryl‐4, 4′‐bipyridinylidene in the Transition‐Metal‐Free Borylation of Aryl Halides and Direct C−H Arylation of Unactivated Benzene

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Sub-stoichiometric amount of N,N′-diboryl-4,4′-bipyridinylidene along with anionic base proved to be a radical initiator common for both the transition metal-free borylation of aryl halides and arene C−H functionalization with aryl iodide. The combination results in the bipyridine radical anion formation together with the single electron transfer to the aryl halide thereby generating the aryl radical species necessary for the metal-free catalytic cycle.


Abstract

N,N′-Diboryl-4,4′-bipyridinylidene ( B BiPy) has been utilized in sub-stoichiometric amounts along with anionic base as one electron donor species for both transition metal-free borylation of aryl halides and arene C−H functionalization with aryl iodides for biaryl syntheses. Reaction between BBiPy and potassium tert-butoxide or methoxide has led to the formation of 4,4′-bipyridine radical anion (BiPy⋅− ) with the release of an electron. The BiPy⋅− has been structurally characterized. The single electron released is transferred to aryl halides generating aryl radicals, which then react with diboron in the presence of methoxide to form aryl boronate and with unactivated benzene in the presence of tert-butoxide to form biaryls. Substrate scopes studies showed that aryl iodides and bromides undergo borylation (1B12B), while only aryl iodides undergo C−C bond formation with arene C−H (1C8C) through radical chain reaction. Mechanisms for C−B and C−C bond formation have been proposed based on the experimental findings.

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