Organic Letters
Letter
undergoes ligand exchange providing intermediate B. Reductive
elimination furnishes the aryl vinyl sulfide and regenerates Pd .
ACKNOWLEDGMENTS
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The authors acknowledge the Charles Casey Undergraduate
Research Fellowship for funding support. Bryn Mawr College
and the Isabel H. Benham Fund for Faculty Research are thanked
for funding support. The authors acknowledge the National
Science Foundation for a Major Research Instrumentation
Award (CHE-0958996), which funded the acquisition of the
NMR spectrometer used in this work.
For the competitive C−S activation, we provide merely a
plausible catalytic cycle, as significant further work remains to
more accurately elucidate the mechanism. Qualitatively, we know
that in the synthesis of the diaryl sulfides, pressure builds in the
reaction vial as the reaction progresses. Additionally, we know
that 2 equiv of aryl bromide are needed to fully consume the
phenyl vinyl sulfide, and the des-halogenated aryl bromide is
observed via GC of the crude reaction mixture. We are unsure at
this juncture whether the reduction of the aryl bromide is
essential to the turnover of the catalyst or if it is simply a second
unproductive path for the system, and though the extent to which
the des-halogenated anisole was formed was not determined, a
full 2.0 equiv of the aryl bromide were needed to completely
consume the vinyl sulfide in this reaction. Work continues in this
area.
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Thus, for the competitive C−S activation pathway, oxidative
addition to the aryl vinyl sulfide to furnish putative intermediate
C could quickly undergo β-hydride elimination. Loss of acetylene
gas leads to intermediate D before reductive elimination
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regenerates the Pd catalyst and forms the thiophenol.
Thiophenols are excellent substrates for palladium-catalyzed
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cross-coupling and would almost certainly out-compete the
II
ligand exchange onto the Pd Intermediate A and form
8
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intermediate B′ (R = aryl) before furnishing the diaryl sulfide
upon reductive elimination. A preliminary control reaction has
been run, subjecting phenyl vinyl sulfide to tert-butoxide in
CPME at 90 °C. After 24 h, we were able to reisolate the phenyl
vinyl sulfide in >98%, indicating that the palladium catalyst seems
to be essential for the decomposition/initial activation of the aryl
vinyl sulfides.
In conclusion, we have reported a new catalytic approach to
synthesize aryl vinyl sulfides. Importantly, this approach takes
advantage of widely available aryl bromides instead of relying
upon less common thiophenols. The current reaction scope
indicates that this reaction is amenable to a wide range of
electronically diverse aryl bromides, and future plans for in-depth
mechanistic studies to more fully understand the mechanism are
underway. Finally, application of this methodology to include
oxathiolanes with C4 and/or C5 substitution will furnish more
highly substituted alkenyl sulfides and is currently underway.
(
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ASSOCIATED CONTENT
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Supporting Information
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(14) Bruno, N. C.; Tudge, M. T.; Buchwald, S. L. Chem. Sci. 2013, 4,
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16.
Detailed experimental procedures and characterization
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15) Zhang, J.; Bellomo, A.; Trongsiriwat, N.; Jia, T.; Carroll, P. J.;
Dreher, S. D.; Tudge, M. T.; Yin, H.; Robinson, J. R.; Schelter, E. J.;
Walsh, P. J. J. Am. Chem. Soc. 2014, 136, 6276.
16) Other base-crown ether combinations were tried with poor
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data for all compounds; H and C NMR spectra for all
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results, including 15-crown-5 and 12-crown-4 with NaOtBu, and 12-
crown-4 with LiOtBu.
AUTHOR INFORMATION
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17) Mao, J.; Jia, T.; Frensch, G.; Walsh, P. J. Org. Lett. 2014, 16, 5304.
18) (a) Munjanja, L.; Brennessel, W. W.; Jones, W. D. Organometallics
2015, 34, 1716. (b) Munjanja, L.; Brennessel, W. W.; Jones, W. D.
Organometallics 2015, 34, 4574.
ORCID
(19) For a recent review, see: Eichman, C. C.; Stambuli, J. P. Molecules
Notes
2011, 16, 590.
The authors declare no competing financial interest.
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Org. Lett. XXXX, XXX, XXX−XXX