Angewandte
Chemie
DOI: 10.1002/anie.200903368
Vinyl Cations
Facile Generation of a Strained Cyclic Vinyl Cation by Thermal
Solvolysis of Cyclopent-1-enyl-l3-bromanes**
Kazunori Miyamoto, Motoo Shiro, and Masahito Ochiai*
Ever since the first direct generation of vinyl cations by
=
solvolysis of arylvinyl bromides (ArC(Br) CH2) reported in
1964 by Grob (father of vinyl cations) and Cseh,[1] their
chemistry has quickly evolved.[2] The ease of formation of
cyclic vinyl cations is dependent upon ring size, because vinyl
cations prefer sp-hybridized linear arrangements at the
positive carbon atom, which can be more easily accommo-
dated by increasingly larger rings.[3] The bent sp2-hybridized
having a hyperleaving group (phenyl-l3-iodanyl),[10] which
structure 2 of the parent vinyl cation with a C C H bond
shows a leaving group ability about 106 times greater than that
of the triflate superleaving group, is stable upon heating at
508C in methanol for two weeks, whereas the cyclohexenyl
analogue 8 readily undergoes heterolysis to give a cyclic vinyl
cation under mild reaction conditions.[11] These results
indicate that even these super- and hyperleaving groups will
not be good enough leaving groups to produce strained
cyclopent-1-enyl cation by thermal solvolysis. The attempt to
generate the cyclopent-1-enyl cation by diazotization of N-
silylated cyclopent-1-enylamines was also found to be fruit-
less.[12] Herein, we report the first solvolytic generation of the
five-membered cyclic vinyl cation with a singlet ground state
from cyclopent-1-enyl-l3-bromanes 6, where the reaction
proceeds at a reasonable rate even at room temperature. The
vastly enhanced nucleofugality of the aryl-l3-bromanyl
groups is a driving force for this solvolytic reaction.[13]
= À
angle of 1208 is 40 kcalmolÀ1 higher in energy than the linear
sp-hybridized form 1.[4] Ab initio calculations did not locate a
stationary-state structure for the smallest cyclic vinyl cation,
cycloprop-1-enyl cation 3 (n = 3), which opens to the prop-
argyl structure 4,[4] whereas cyclobut-1-enyl cation 3 (n = 4) is
readily generated by solvolysis owing to its highly stabilized
nature associated with the bridged nonclassical structure.[5] In
contrast to vinyl cations 3 (n ꢀ 6) that have larger rings,
strongly bent cyclopent-1-enyl cation 3 (n = 5, V= 1418)[6] is
thought to be too strained to be generated during
SN1 solvolysis.[2] Hence, generation of a cyclopent-1-enyl
cation by simple solvolysis is among one of the challenges
facing modern organic chemistry and still remains to be
established experimentally.[7,8]
Over a period of 35 years, much effort has been directed
toward generating the last cyclic vinyl cation 3 (n = 5) by
solvolysis. Attempted solvolysis of cyclopent-1-enyl sulfonate
5 with a superleaving group, triflate,[2] does not exhibit
unimolecular dissociation[3] and the vinyl triflate 5 was
recovered unchanged even after heating in trifluoroethanol
at 1008C for 10 days.[9] Cyclopent-1-enyl(phenyl)-l3-iodane 7
Cyclic vinyl-l3-bromane 6a was prepared by ligand
[14]
exchange of p-CF3C6H4BrF2
on the bromine(III) atom
with potassium cyclopentenyltrifluoroborate (5 equiv) in
MeCN at low temperature (À45–08C) in 61% yield.[15] The
bromane 6a can be kept at À208C for months without any
change, but gradually decomposes under ambient conditions.
Complexation of 6a with [18]crown-6 increases the thermal
stability of some labile alkynyl-l3-iodanes and bromanes
through hypervalent IIII···O and BrIII···O interactions.[16,17]
Thus, slow evaporation of a n-hexane/ethyl acetate/dichloro-
methane solution of a 1:2 mixture of 6a and [18]crown-6 at
48C afforded colorless crystals of
a 1:1 complex of
6a·[18]crown-6. This complex is thermally stable and no
decomposition was detected when it was left standing under
ambient conditions for two weeks. In solution (CDCl3, 218C,
air), the half-life (t1/2 = 2.5 days) of 6a was extended to
six days when complexed to [18]crown-6.
The structure of 6a was firmly established by single-
crystal X-ray analysis of the crown ether complex, which
showed the presence of two independent but closely related
molecules: one of which is depicted in Figure 1. The hyper-
valent bromine(III) atom has contacts with three adjacent
oxygen atoms (O2, O3, and O4) of [18]crown-6, which
probably increases the stability of 6a.
[*] Prof. Dr. K. Miyamoto, Prof. Dr. M. Ochiai
Graduate School of Pharmaceutical Sciences
University of Tokushima
1-78 Shomachi, Tokushima 770-8505 (Japan)
Fax: (+81)88-633-9504
E-mail: mochiai@ph.tokushima-u.ac.jp
M. Shiro
Rigaku Corporation
3-9-12 Matsubara, Akishima, Tokyo 196-8666 (Japan)
[**] This work was supported by a Grant-in-Aid for Scientific Research
(B) funded by the JSPS.
Solvolysis of 6a in aqueous solvents and alcohols at 508C
predominantly produced cyclopentanone (9) and p-
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2009, 48, 8931 –8934
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8931