Organic Letters
Letter
(9) For NMR evidence of a similar equilibrium, see: Feigel, M.;
Kessler, H. Tetrahedron 1976, 32, 1575.
(10) Denton, R. M.; An, J.; Adeniran, B. Chem. Commun. 2010, 46,
general. The different stabilization effect of the tropylium ion
used in this work compared to that of the reported
cyclopropenium ion5 opens new avenues for the investigation
of different chemistry and new reactions. We envisage that
structural modification of the seven-membered ring of the
tropylium ion could allow optimization of activation for specific
reactions and substrates. Work on tuning this tropylium system
to activate substitution reactions with other types of
nucleophiles other than chloride as well as to activate other
types of chemical transformations is currently ongoing and will
be reported in due course.
3025.
(11) See the Supporting Information for more details.
(12) That the reaction occurred between 1,1-dichloroheptatriene 1
and the alcohol was confirmed by the treatment of alcohols in entries
1−3, Table 1 with oxalyl chloride or HCl. Oxalyl chloride gave
predominantly a mixture of oxalate esters whereas reactions with HCl
were very slow and low-yielding. See the Supporting Information for
more details.
(13) The tendency for an elimination reaction of tertiary alcohol to
occur was also reported by Parnes et al.; see ref 6b.
(14) Similar to the control experiment designed by Lambert et al. (ref
5a), the reaction in entry 11, Table 1 was also carried out in CDCl3
ASSOCIATED CONTENT
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1
S
* Supporting Information
and followed by H NMR to confirm the presence of the proposed
alkoxytropylium intermediate. See the Supporting Information for
more details.
Experimental procedures and product characterization data.
This material is available free of charge via the Internet at
(15) The color of the reaction mixture changed from yellow to dark-
brown upon addition of the bromide source to the solution of 1 in
dichloromethane. We suspected that ion exchange occurred to form
tropylium bromide/bromocycloheptatriene species; see ref 2. In these
reactions, chloro derivatives were also formed in negligible ratio to
bromo derivatives. See the Supporting Information for more details.
(16) Audi, G.; Bersillon, O.; Blachot, J.; Wapstra, A. H. Nucl. Phys. A
2003, 729, 3.
AUTHOR INFORMATION
Corresponding Author
■
Notes
(17) We thank one of the referees for pointing out that it is possible
that the more nucleophilic carboxylate group, generated from an acid−
base reaction with Et3N, would react faster with the tropylium ion,
hence increasing the reaction rate.
(18) Complete conversion of the acid to the acid chloride was
confirmed by 13C NMR before the addition of amine or alcohol,
except for entries 2, 5 (Table 2) where the reaction mixtures were
messy.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
T.V.N. thanks Curtin University for providing the Curtin ERC
Fellowship. Generous laboratory support and helpful dis-
cussions from Prof. Michael Sherburn (ANU) and Dr. Alan
Payne (Curtin) are gratefully acknowledged.
(19) A conjugated electron withdrawing group will render the
carboxylic acid or carboxylate group less nucleophilic or less likely to
participate in the formation of the tropylium carboxylate intermediate
5′; see ref 5b.
DEDICATION
■
This work is dedicated to Professor Dieter Enders on the
occasion of his retirement.
(20) The carboxylate −COO− group is more nucleophilic than the
alcohol hydroxyl −OH group. Thus, it is possible that it is easier to
form the tropylium carboxylate intermediate 5′ than the tropylium
alkoxide intermediate 5.
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