Communications
that the observed intermediate is not symmetric, in agreement
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with an intermediate with a tribromide structure analogous to
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Because of the high stability of the tribromide adduct one
could expect that this species is also an intermediate in
bromination reactions of alkenes. Even in very polar solvents
the two conformations of the tribromide adducts 15 and 16
are significantly more stable than the bridged bromonium ion,
both as a separated ion pair 18a as well as the contact ion pair
18b (see Scheme 4). However, the activation barrier for the
[11] S. Uemura, H. Okazaki, M. Okano, J. Chem. Soc. Perkin Trans. 1
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[14] J. Odenthal, Dissertation, Universitꢂt Heidelberg, 1975.
[15] U. Hꢃpfner, Dissertation, Universitꢂt Heidelberg, 1979.
[16] C. Ciappe, unpublished results.
[17] G. Wittig, H.-L. Dorsch, Liebigs Ann. Chem. 1968, 711, 46 – 54.
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926.
Scheme 4. Nonionic mechanisms for the bromination of acetylene.
The energies of the stationary points (B3LYP/6-31G*+ZPE in
kcal molꢀ1) are relative to the most stable p complex 13. The SP
solvent energies in dichloromethane (B3LYP/6-31G* in kcalmolꢀ1) are
also relative to the p complex 13.
[21] M. L. Munzarova, R. Hoffmann, J. Am. Chem. Soc. 2001, 123,
4787 – 4795.
[22] There is no minimum for a contact ion pair composed of the
bromirenium cation and a tribromide anion at the levels of
theory that we applied (B3LYP/6-31G*, B3LYP/6-31(2d,f),
MP2/6-31G*). The stationary point of lowest order that was
found for such a species is the C2v-symmetrical structure 12b
(saddle point). Structure 12b is suitable as a model to estimate
the lower level of the relative energy because the charges come
very close to each other and hence minimize the dipole moment.
[23] S. Miertus, J. Tomasi, Chem. Phys. 1982, 65, 239 – 245.
[24] S. Miertus, E. Scrocco, J. Tomasi, Chem. Phys. 1981, 55, 117 – 129.
[25] M. Cossi, M. Persico, J. Tomasi, J. Am. Chem. Soc. 1994, 116,
5373 – 5378.
formation of 15 from the p complex 14, at least in the parent
system acetylene, is too high to compete with the classical
mechanism via the bromonium ion. In substituted systems
like acenaphthylene, which adds bromine preferentially cis,
however, an intermediate analogous to 15 is likely.[7,8]
Received: August 13, 2004
Revised: November 3, 2004
Published online: January 21, 2005
[26] We are grateful to Prof. Dr. R. Boese, Universitꢂt Essen, for his
attempts to determine the X-ray structure of the intermediate in
the bromination of 4.
Keywords: alkynes · bromination · density functional
calculations · electrophilic addition · reaction mechanisms
.
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