ChemComm
Communication
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Scheme 2 Tentative mechanism of the reaction.
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The 3-methyl-1-phenylbut-3-en-1-one oxime acetate 1n was tolerated
in the reaction and afforded the 4-methyl-2-phenyl-1H-pyrrole 2n in
64% yield (Table 2, entry 1). However, a complex mixture was observed
when the cyclohex-2-en-1-yl(phenyl)methanone oxime acetate 1o or
2,2-dimethyl-1-phenylbut-3-en-1-one oxime acetate 1p was used as
the substrate (Table 2, entries 2 and 3). Additionally, an unexpected
4,4-dimethyl-5-phenyl-2-(prop-1-en-2-yl)-3,4-dihydro-2H-pyrrole 2q was
obtained in 80% yield when 2,2,5-trimethyl-1-phenylhex-4-en-1-one
oxime acetate 1q was employed as the substrate (Table 2, entry 4).
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studied to examine the reactivity. It was found that propionate,
pivalate, and benzoate show similar reactivity to acetate to give
2-phenyl-1H-pyrrole 2a in 80–84% yield (Table 2, entries 5–7). How-
ever, no reaction occurred when 1-phenylbut-3-en-1-one oxime tert-
butyl carbonate 1u was used as the substrate (Table 2, entry 8).
On the basis of the above results and previous studies, a
tentative mechanism for the cyclization reaction is proposed in
Scheme 2. Firstly, Cu(I) initiated N–O bond cleavage of the
ketoxime acetate generates an imino radical A and a Cu(II)
species.13 An anti-Baldwin intramolecular 5-endo-trig radical
cyclization of intermediate A produces an intermediate B.14
Then, single-electron oxidation of intermediate B by Cu(II)
species forms a 2-phenyl-3H-pyrrole intermediate C and the
active Cu(I) catalyst.7b,13 Finally, tautomerization of intermediate
C gives the desired 2-phenyl-1H-pyrrole 2.
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In summary, we have developed a novel and efficient Cu-catalyzed
intramolecular 5-endo-trig cyclization of ketoxime carboxylates for the
synthesis of 2-arylpyrroles. The reaction tolerates a range of functional
groups and is a good protocol for rapid synthesis of valuable
2-arylpyrroles in high yields under mild conditions. A further study
of the reaction scope and mechanism is underway in our laboratory.
This work was supported by generous grants from the National
Natural Science Foundation of China (NSFC21272183) and the Fund
of the Rising Stars of Shanxi Province (2012KJXX-26).
Notes and references
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Chem. Commun., 2014, 50, 7437--7439 | 7439