Organic Letters
Letter
a
can also be tolerated, and indolin-3-ones 6e−i were obtained
in 73−84% yields.
Scheme 3. Substrate Scope of Coumaran-3-one Synthesis
Then, we turned our attention to the reaction of
disubstituted alkynes. Substrates 7a−c were synthesized, but
unfortunately, the desired 2-substituted coumaran-3-ones and
indolin-3-ones 8 were not obtained. Instead, fast consumption
of starting materials was observed in all cases, leading to the
formation of 2-substituted benzofurans and indoles 8a′−c′ in
good yields (Scheme 5).
Scheme 5. Reaction of Disubstituted Alkynes under
Standard Conditions
There is rational doubt about the mechanism of these
transformations in terms of whether α-oxo mercury carbene F
has formed during the catalytic process because the formation
of α-oxo gold carbenes has frequently been found after alkynes
are treated with N-oxides and gold(I) catalysts.14 However,
when phenylacetylene was treated with our standard
conditions in the presence of trans-stilbene or cis-stilbene, no
desired cyclopropane products 9 were detected (Scheme 6, eq
a
All reactions were performed with 1 (0.5 mmol), Hg(OTf)2 (0.025
mmol), and 2-Cl-pyridine N-oxide (0.6 mmol) in CH2Cl2 (2.5 mL) at
23 °C for 1 h. Isolated yields of 2 are listed. Ten millimoles of 1a was
employed.
b
Next, we focused on the synthesis of indolin-3-ones under
our standard conditions. Due to the fact that 2-ethynylaniline
failed to afford the desired indolin-3-one, we decided to
employ tosyl-substituted 2-ethynylaniline 5a, as it can be more
easily deprotonated under the reaction conditions, increasing
its nucleophilicity. The reaction of substrate 5a proceeded well
to give N-Ts-indolin-3-one 6a in 85% yield, with a slightly
higher Hg(OTf)2 loading (10 mol %) and a longer reaction
time (3 h). Then various N-Ts-2-ethynylanilines were
examined under these conditions, and the results are listed
in Scheme 4. Similar to the synthesis of coumaran-3-ones, the
synthesis of indolin-3-ones featured good functional group
tolerance. Substrates 5b−d bearing Me and OMe groups
afforded the corresponding indolin-3-one products 6b−d,
respectively, in excellent yields. Halogen atoms and CO2Me
Scheme 6. Attempts to Trap Potential α-Oxo Mercury
Carbene
a
Scheme 4. Substrate Scope of Indolin-3-one Synthesis
1). The intramolecular trapping of the potential mercury
carbene was performed by exposing 10 to our standard
conditions, but the expected cyclopropane product 11 could
not be found (Scheme 6, eq 2). These results led us to propose
a mechanism for our reactions without the formation of
mercury carbene. With 1a as an example, the alkynyl moiety
can be activated by Hg(OTf)2 and the addition of 2-Cl-
pyridine N-oxide takes place to give intermediate G. Intra-
molecular proton transfer leads to the protodemercurization of
intermediate G to generate enolonium species H. Then an
intramolecular SN2′ reaction occurs, which affords coumaran-
3-one 2a (Scheme 7).
In conclusion, we have demonstrated the first mercury-
catalyzed enolate umpolung reaction that allows the efficient
construction of various coumaran-3-ones and indolin-3-ones.
This method features high yields and excellent functional
group tolerance. Obviously, the formation of enolonium
intermediates by mercury catalysis has the potential to be a
a
All reactions were performed with 5 (0.5 mmol), Hg(OTf)2 (0.05
mmol), and 2-Cl-pyridine N-oxide (0.6 mmol) in CH2Cl2 (2.5 mL) at
23 °C for 3 h. Isolated yields of 6 are listed.
C
Org. Lett. XXXX, XXX, XXX−XXX