10.1002/anie.201910051
Angewandte Chemie International Edition
COMMUNICATION
Based on these observations, we propose the mechanism
shown in Scheme 3. Irradiation of EDA complex 12 formed
between thionocarbonate 1, B2cat2 and Et3N results in
photoinduced electron transfer to generate aryl iodide radical
anion 13 and radical cation complex 14. Elimination of iodide from
13 provides aryl radical 2, which after 5-endo-trig cyclization and
subsequent fragmentation of 15, gives alkyl radical 4 and cyclic
thionocarbonate 3. Borylation of 4 likely proceeds via the same
pathway as previously described[5f,8,16] with homolytic substitution
at boron of B2cat2 (via 16 with a weak B–B one electron σ-bond)
facilitated by either DMF or Et3N. The resulting Lewis base
stabilized boryl radical 17 can then react with thionocarbonate 1,
either via iodine atom abstraction[5f] or SET,[8c] to regenerate aryl
radical 2 and form the I–Bcat Lewis base adduct 18.
Keywords: Boronic Esters • Borylation • Deoxygenation •
Photochemistry • Radical Reactions
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Scheme 3. Proposed mechanism.
In conclusion, we have developed a deoxygenative borylation
reaction which proceeds efficiently under mild, operationally
simple conditions, with only a slight excess of the diboron reagent.
The process is noteworthy in that it does not require a
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We thank H2020 ERC (670668) for financial support and Prof.
Weisheng Tian from Shanghai Institute of Organic Chemistry for
providing the steroidal natural product precursors for 6r–6u.
R.M.B. acknowledges the CRC 1176 funded by the German
Research Foundation (DFG) in the context of project A4 and the
Karlsruhe House of Young Scientists (KHYS) for funding. L.G.
thanks the DFG for a research fellowship (GU 1935/1-1). We
thank Alexi Sedikides and Dr. Alastair Lennox (UoB) for
performing cyclic voltammetry experiments.
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