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Scheme 1 Postulated catalytic pathway for a Pd-catalyzed cascade
process.
asymmetric diarylacetylene (Table 4), two regioisomers were
obtained with ca. 1 : 1 ratio (Table 4, 3r, 3s–v, and 3x–z).
Interestingly, if the phenyl ring bearing two CF3 groups on the
meta position or the TMS group on the para position, an excellent
ratio of regioisomers could be achieved (Table 4, 3s and 3w, ratio
of regioisomers: 92 : 8 and 93 : 7, respectively).
Our postulated reaction pathway is summarized in
Scheme 1. In the initial step, a regioselective direct electro-
philic aromatic palladation at the 3-position of the coumarin
forms a palladium complex I. The subsequent insertion of two
diphenylacetylene molecules forms a dienylpalladium inter-
mediate III, which will then undergo intramolecular 6-exo-dig
insertion into coumarin leads to a polycyclic palladium
intermediate IV. Subsequent trapping by the OH group
from coumarin generated intermediate V. Intermediate V
undergoes a set of rearrangements to offer a cyclopentadiene
fused intermediate VII. The intermediate VII is transferred
to intermediate VIII via reductive elimination and Pd0 is
reoxidized by CuBr2 to regenerate the PdII catalyst for the
next catalytic cycle. Finally, the desired product 3a is obtained
via a phenyl ring migration. The configuration of the
compound was determined by X-ray crystal structure
analysis of a suitable single crystal (Scheme 1, product 3a)
(see ESIw).
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In summary, we have disclosed an unprecedented
palladium-catalyzed cascade reaction between coumarins and
alkynes. The coupling of coumarins with alkynes in the presence
of Pd(OAc)2 enabled us to trigger a cascade process to furnish the
highly substituted cyclopentadiene fused chromone framework in
moderate to high chemical yields (47–88%).
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We gratefully acknowledge the National University of
Singapore for financial support of this work (Academic Research
Grant: R143000408133, R143000408733 and R143000443112).
We also thank Prof. Tamio Hayashi (Kyoto University) and
Prof. Shin Seunghoon (Hanyang University) for their discussion
and valuable advice on the reaction mechanism.
c
5424 Chem. Commun., 2011, 47, 5422–5424
This journal is The Royal Society of Chemistry 2011