Chemistry - An Asian Journal
10.1002/asia.201600761
COMMUNICATION
6
). The transformation starts with a C-H bond activation of 2a by
along with 3ak. Notably, the exchanged products were the major
product in both cross reactions.
Cu
2
O catalyst to give a copper-isocyanide complex A with
O. A [3 + 2] cycloaddition between 1a
concurrent formation of H
2
In summary, we have developed a new method for the
synthesis of di-substituted or tri-substituted pyrroles by copper-
catalyzed cyclization of allenoates with activated isocyanides.
This new process features a skeletal rearrangement in which the
aryl sulfonyl moiety of the isocyanide translocates to the γ-
and intermediate A takes place to give intermediate B. The latter
undergoes protonolysis leading to the formation of intermediate
C and the regeneration Cu O. A Cu O-assisted elimination of
2 2
the 4-methylbenzenesulfinate group (Ts ) produces an
-
-
electrophilic species D. Recombination of D with Ts affords the
carbon of the starting allenoate. A plausible mechanism
pyrrole copper complex E. Protonolysis of compound E leads to
the formation of product 3aa and the regeneration of the copper
catalyst.
involving an elimination and re-addition of a Ts group was
proposed. Application of this catalytic system to construct other
types of nitrogen-containing heterocycles is underway in our
laboratory.
Acknowledgements
The authors sincerely thank the financial support from
National Science Foundation of China (Grants 21202118,
21572158, 21202119), China International Science and
technology cooperation projects (2013DFA31160). We thank
Prof. Hongchao Guo (China Agricultural University) for providing
the allenoates (synthesized in the National Key Technologies
R&D Program of China, 2012BAK25B03, CAU) for substrate
screening. We are thankful to the Research Centre of Modern
Analytical Technology, Tianjin University of Science and
Technology for high resolution mass spectrum analysis.
Keywords: Pyrrole Synthesis • Allenoates • Isocyanides •
Copper-catalyzed • Cyclization
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Scheme 6. Proposed Mechanism for Copper-catalyzed Cyclization between
Allenoate 1a and Isocyanide 2a.
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While the isolation of 3cj′ in the reaction of 1c and 2j (Scheme
) is consistent with the proposed mechanism and supports the
5
formation of a nucleophilic sulfinate intermediate, two cross
reaction experiments were carried out to establish the
elimination and re-recombination process of sulfinate group.
When 1.2 equiv. of allenoate 1a and 1.0 equiv. of isocyanide 2a
were used as substrates, in the presence of sodium
benzenesulfinate (4a) (1.0 equiv.), both the ‘desired product’ 3aa
and crossover product 3ak were obtained (Scheme 7). When a
similar reaction between 1a and isocyanide 2k was performed in
the presence of 1.0 equiv. of sodium 4-methylbenzenesulfinate
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[
7
(
4b) (1.0 equiv.), the exchanged product 3aa was obtained
[
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