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Scheme 2 Overall reaction mechanisms of silver-catalyzed C–H
functionalization.
On the basis of the observed experimental results, we presume
that two possible mechanistic pathways for the formation of the
3-methyleneoxindoles by C–H functionalization are outlined in
Scheme 2. The reaction is considered to be initiated by Ag
coordination with the carbonyl or diazo group and formation
of the intermediate 2A (Path A) or 3A (Path B). Then free
carbene is generated and quickly transferred into resonance
structures of allyl carbocation 2B or 3B by the release of N2.
Following, the allyl carbocation 2B or 3B undergoes cyclization
to form the intermediate 2C, after protonation to afford the
desired product 2a, releasing the metal center to reinitiate the
catalytic cycle. In any case, the rate-determining step of the whole
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By using AgOTf as a catalyst, we have developed a protocol for
the preparation of various oxindoles via aromatic C–H functiona-
lization. Substrate investigation shows that both electron-donating
and withdrawing groups worked smoothly to afford the desired
products in moderate to excellent yields. The NMR 13C data,
experimental results, and single crystal X-ray of product 2g proved
that solely 3-methyleneoxindole was obtained in this transformation.
The mechanistic study is ongoing in our laboratory.
We gratefully acknowledge the NSFC (NSFC21072079 and
NSFC20921120404) for financial support.
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This journal is The Royal Society of Chemistry 2011