Angewandte
Chemie
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[7] There is yet another possible mechanism in Cu-promoted
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Scheme 3. Cyclization of o-allenylaniline. a) [Rh(cod)2]OTf (10 mol%),
dppp (10 mol%), HFIP, reflux, 3 h, 74% yield. b) No catalyst, HFIP,
reflux, 3 h, 51% yield. c) No catalyst, 1,2-dichloroethane, reflux, 3 h,
53% yield.
[8] The aromatic Claisen rearrangements of propargyl ether deriv-
atives usually afford fused arenes containing a six-membered
ring: a) U. Koch-Pomeranz, H.-J. Hansen, H. Schmid, Helv.
Chim. Acta 1973, 56, 2981 – 3004; b) L. I. Olsson, A. Claesson,
Synthesis 1979, 743; c) see also: H. Ishii, T. Ishikawa, S. Takeda,
S. Ueki, M. Suzuki, T. Harayama, Chem. Pharm. Bull. 1992, 38,
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respectively, we believe that the present process would consist
of an initial Rh-catalyzed amino-Claisen rearrangement of N-
propargyl aniline[15] and the subsequent cyclization of o-
allenylaniline by path a in Scheme 1. Since heating 20 in HFIP
or 1,2-dichloroethane gave 13b in moderate yield, cyclization
of the o-allenylaniline intermediate is induced by both the Rh
catalyst and by heating.
In summary, we have reported the facile formation of
indoles by an aromatic amino-Claisen rearrangement of N-
propargyl aniline derivatives in the presence of a [Rh-
(cod)2]OTf/dppp catalyst in HFIP. We believe that these
findings raise new possibilities for the formation of 2,3-
disubstituted indoles under mild conditions. Investigation of
the synthetic applications and detailed mechanistic studies of
this reaction are underway.
Received: December 21, 2006
Revised: January 23, 2007
Published online: April 11, 2007
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[12] Although the formation mechanism of 4c and 18 remains
unclear at present, it may involve the hydrogenation of a 1,2-
dihydroquinoline derivative formed from 1c or 16 according to
Scheme 1. However, the corresponding quinoline compounds,
which are considered to be oxidized products, were not detected.
See also: T. P. Forrest, G. A. Dauphinee, S. A. Deraniyagala,
Can. J. Chem. 1985, 63, 412 – 417 and references [4a] and [16].
[13] A similar observation has been reported in the Brønsted acid-
catalyzed rearrangement of N-propargyl anilines. See referen-
ce [5c].
Keywords: alkynes · cyclization · heterocycles · rearrangement ·
rhodium
.
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[14] The authors acknowledge the referees for their suggestion to
examine the reaction of 1 f.
[15] Although it has been reported that the activation mechanism in a
transition-metal-promoted amino-Claisen rearrangement prob-
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Angew. Chem. Int. Ed. 2007, 46, 3931 –3933
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