Organic Letters
Letter
Scheme 4. Example of the Assumed Catalytic Cycle
Experimental procedures and spectral data for all new
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AUTHOR INFORMATION
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Corresponding Author
ORCID
Notes
the C−Br bond and the vicinal C−C bond. Compound 12 is
protonated biphenyl and, as such, a strong Brønsted acid that
will drive the catalytic cycle by protonation of substrate 4a.
Either the bromine atom in 3 or one of the bis-allylic
hydrogen atoms of the cyclohexa-1,4-diene could, in principle,
act as the nucleofuge in the reaction with the vinyl cation.
However, we knew from our previous study7 that hydride
abstraction leads to Wagner−Meerwein rearrangement16 of the
surrogate, and we did not observe any of that with surrogate 3.
To probe the possibility of this reaction pathway, we treated 3
with the triyl cation that would chemoselectively abstract the
hydride (Scheme 5). Indeed, rearranged 14 did form in 49%
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This research was supported by the China Scholarship Council
(predoctoral fellowship to W.C., 2016−2020) and the
Deutsche Forschungsgemeinschaft (Oe 249/18-1). M.O. is
indebted to the Einstein Foundation Berlin for an endowed
professorship. We thank Dr. Alice Lefranc for her initial
attempts to prepare HBr surrogates based on benzene as well
as Dr. Johannes C. L. Walker and Dr. Huaquan Fang for
fruitful discussions and Dr. Elisabeth Irran for the X-ray
analysis (all TU Berlin). We also thank Dr. Takeshi Komiyama
of Chuo University for his experimental contributions.
Scheme 5. Rearrangement of the HBr Surrogate Induced by
Hydride Abstraction
REFERENCES
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(1) (a) For transition-metal-catalyzed transfer processes, see:
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yield after 5 days; the reaction likely involves the intermediacy
of Wheland complex 13.16 This suggests that hydride
abstraction is not competing with the bromonium-ion
formation in the transfer hydrobromination.
To summarize, we have reported here a metal-free
hydrobromination of terminal, internal, and especially
electron-deficient alkynes. The reaction is less general than
the related transfer hydroiodination7 as it requires even higher
reaction temperatures (140 and 160 °C instead of 80−120
°C). However, the new method avoids handling of gaseous
hydrogen bromide or its aqueous solution, thereby allowing for
the hydrobromination of a useful subset of C−C triple bonds.
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ASSOCIATED CONTENT
* Supporting Information
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(8) For transition-metal-catalyzed hydrobromination, see: (a) Gao,
F.; Hoveyda, A. H. J. Am. Chem. Soc. 2010, 132, 10961−10963.
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The Supporting Information is available free of charge on the
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