Rates and products of electrophilic bromination of
ring-substituted cis- and trans-stilbenes have
been
investigated in acetic acid, trifluoroethanol, ethanol, methanol, and
water−methanol mixtures. The mY
Br
relationships
(linear for nucleophilic solvents only, with m = 0.8), the
deviations of the two nonnucleophilic solvents from the
mY
Br plots (ΔAcOH and
ΔTFE positive, negative, or negligible), the kinetic
solvent isotope effects
(k
MeOH/k
MeOD
=
1.1−1.6), the chemoselectivity (predominant dibromide, DB, or
solvent-incorporated adducts, MA), and the high
dependence of the stereochemistry on the solvent and the substituents
(from stereoconvergency to stereospecificity)
are discussed and interpreted in terms of a mechanistic scheme,
analogous to the Jencks scheme for aliphatic
nucleophilic substitutions, in which preassociation, free-ion, and
ion-pair pathways compete. In particular, the
stereochemical outcome of these reactions is consistent with a marked
change in the nucleophilic partners of the
product-forming ionic intermediate arising from different ionization
routes. Return, i.e. change in the rate-limiting
step from ionization to product formation, is shown to be related to
substituent-dependent, but not solvent-dependent,
bromine bridging.
The bromine additions to methylenecyclopropane (1), bicyclopropylidene (2), and spirocyclopropanated methylenecyclopropanes and bicyclopropylidenes 3-6 in methanol at 25 degrees C proceed essentially with the same rate as those to the corresponding oligomethyl-substituted ethylenes. An increasing number of spiroannelated three-membered rings enhances the rate of bromination and stabilizes the intermediate cyclopropyl bromonium cations against ring opening in the course of bromine addition. Calculations at the B3LYP/6-311G(d,p) level show that unsymmetrical bromonium ions are the intermediates, and that they are stabilized by the spiroannelation with cyclopropane rings. The bromonium ion derived from 1 is less stable by 6.3 kcal mol-1 than that from isobutene. One or two spirocyclopropane rings as in 3 and 4 stabilize the corresponding bromonium ion by 9.6 and 16.4 kcal mol-1, respectively, while one or two alpha-cyclopropyl substituents as in ethenylcyclopropane (7) and 1,1-dicyclopropylethene (8) stabilize the corresponding bromonium ions by 13 and 29 kcal mol-1, respectively. The experimental bromination rates of all the studied alkenes correlate reasonably well (r2 = 0.93) with calculated relative energies of the corresponding bromonium ions. The correlation is even better within the series of methylenecyclopropanes 1, 3, and 4 (r2 = 0.974) and bicyclopropylidenes 2, 5, and 6 (r2 = 0.999). The experimental bromination rates also correlate fairly well with the first ionization energies of the corresponding alkenes 1-12 (with r2 = 0.963) and 13-19 (with r2 = 0.991). The calculated preferred nucleophilic attack of a water molecule at both the C1' and C1 atoms of representative bromonium ions conforms well to the experimentally observed product distribution.
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