H. Terai et al. / Tetrahedron Letters 47 (2006) 1705–1708
1707
important starting materials for polyaromatic electronic
devices.7 However, the conventional methods often
require strong acids.8 The present reaction provides an
alternative and convenient method because of simple
operation and neutral reaction conditions.
regenerate the active species 3 with the formation of 7,
which undergoes dehydration to afford product 8.
The formation of fused aromatic compounds can be
explained by the polycondensation as shown in Scheme
2. Compound 8 would be converted into intermediate 9
via coordination of 3, c-proton abstraction by counter
anion,10 and O-to-C migration of rhodium. Condensation
with ketones affords dienone 10, which undergoes simi-
lar reaction sequence or direct electrocyclic reaction to
afford aromatic compounds.
The reaction can be rationalized by assuming the mech-
anism as shown in Scheme 1. The catalytically active
species seems to be coordinatively unsaturated cationic
rhodium complex 3, which would be formed by dissoci-
ation of g6-arene ligand.5 After coordination to ketones,
the resulting complex 4 undergoes a-proton abstraction
from the coordinated ketone to afford rhodium enolate
5 with releasing HBF4. Catalytic inactivity of HBF4
has been confirmed by the control experiments. Enolate
5 reacts with the substrate ketone to give aldolate inter-
mediate 6.9 Proton abstraction from HBF4 proceeds to
Acknowledgements
This work was supported by a Grant-in-Aid for Scien-
tific Research, the Ministry of Education, Science,
Sports, and Culture, Japan, Sasagawa Scientific
Research Grant from the Japan Science Society, and
Kansai Research Foundation for Technology Promotion.
H. Terai also acknowledges the financial support from
the center of excellence program ‘Creation of Integrated
EcoChemistry’, Osaka University.
References and notes
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Scheme 1.
6. Typical procedure: In a 25 mL sealed tube, 1-indanone
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Scheme 2.