3176
J . Org. Chem. 1997, 62, 3176-3182
F or m a tion of Br om oca r ben iu m Br om id e Ion P a ir s in th e
Electr op h ilic Br om in a tion of High ly Rea ctive Olefin s in
Ch lor in a ted Ap r otic Solven ts†
Giuseppe Bellucci, Cinzia Chiappe,* and Giacomo Lo Moro
Dipartimento di Chimica Bioorganica, via Bonanno 33, 56126 Pisa, Italy
Received November 1, 1996X
The kinetics and the products of bromination of several substituted stilbenes with tetrabutylam-
monium tribromide (TBAT) have been investigated in aprotic solvents at different temperatures.
Stilbenes bearing electron-withdrawing or moderately electron-donating substituents gave ste-
reospecifically the anti addition products. The reactions followed a second-order rate law, and an
inverse kinetic isotope effect (KIE), kH/ kD ) 0.85(0.05), was found for the bromination of cis-stilbene.
The reactions of cis- and trans-4,4-dimethoxystilbenes yielded mixtures of meso and d,l dibromides
-
both in chloroform and 1,2-dichloroethane. The rate constants (kBr ) measured for the latter olefins
3
deviated considerably from the Hammett correlations, and added bromide had a significant effect
on the rates. The reactions of these activated stilbenes with molecular Br2, carried out at low Br2
concentration, followed a mixed second/third-order rate law. The kinetic and product distribution
data for the reaction, with TBAT, of stilbenes bearing electron-withdrawing or moderately electron-
donating substituents are interpreted on the basis of the known mechanism involving a product-
and rate-determining nucleophilic attack by bromide on the olefin-Br2 π-complex. The data related
to the bromination of the more activated methoxystilbenes are rationalized considering that, for
these olefins, even in aprotic solvents, the ionization of the initially formed 1:1 π-complex to a
bromocarbenium bromide ion pair can compete both with the formation of a bromonium-tribromide
ion pair and with the nucleophilic attack by Br-. For this second-order process (first order in Br2),
the kinetic constants and the activation parameters have been measured in chloroform and 1,2-
dichloroethane and the activation parameters have been compared with those related to the third-
order Br2 addition and to the reaction with TBAT.
In tr od u ction
of the addition of bromide ion in protic and aprotic
solvents has also been investigated.2,6
Sixty years after the postulation of bromonium ions
as the intermediate of the electrophilic bromination of
olefins by Roberts and Kimball,1 important features
concerning the formation of transient species are still
under discussion and the object of extensive investiga-
tions.2 Not only has the involvement of olefin-Br2
π-complexes prior to the rate-determining step leading
to the ionic intermediate been established3 but also the
nature of the interaction has recently been revealed.4
Furthermore, evidence for the reversibilty of the ioniza-
tion step has been presented, and the factors affecting
this process have been extensively discussed.5 The effect
At least three alternative nonradical mechanistic
pathways, all of which involve olefin-Br2 π-complexes,
were identified for the bromination of olefins (Scheme 1).
In particular the first pathway (path a) has been pro-
posed for the reaction with molecular bromine in aprotic
solvents of modest polarity and consists in the ionization
of a 1:2 π-complex to give a bromonium-bromocarbenium
tribromide ion pair intermediate, which then collapses
to dibromide and molecular Br2.3,5b,6,7 This pathway,
involving a second-order dependence of the rate on Br2,
has been clearly demonstrated for brominations in sev-
eral chlorinated hydrocarbon solvents.
†Dedicated to the memory of Professor Giuseppe Bellucci (d. March
3, 1996).
A similar ionic mechanism, where a solvent-assisted
bromine-bromine bond breaking occurs in a 1:1 π-com-
plex and the reaction is first-order in Br2 (path b), has
been established for the bromination at low Br2 concen-
trations in hydroxylic solvents8,9 which provide specific
electrophilic solvation by hydrogen bonding to the leaving
bromide ion.10 At higher Br2 concentration, a third-order
process of type a, has been reported also in acetic acid.9b
X Abstract published in Advance ACS Abstracts, April 15, 1997.
(1) Roberts, I.; Kimball, G. E. J . Am. Chem. Soc. 1937, 59, 947.
(2) (a) Schmid, G. H. The Chemistry of Double-Bonded Functional
groups; Patai, S., Ed.; Wiley: New York, 1989; Suppl. A, Vol. 2, part
1, p 699. (b) Ruasse, M. F. Adv. Phys. Org. Chem. 1993, 28, 207.
(3) Bellucci, G.; Bianchini, R.; Ambrosetti, R. J . Am. Chem. Soc.
1985, 107, 2464.
(4) Bellucci, G.; Bianchini, R.; Chiappe, C.; Lenoir, D.; Herges, R.
J . Am. Chem. Soc. 1995, 117, 12001.
(5) (a) Brown, R. S.; Gedye, R.; Slebocka-Tilk, H.; Buschek, J . M.;
Kopecky, K. R. J . Am. Chem. Soc. 1984, 106, 4515. (b) Bellucci, G.;
Chiappe, C.; Marioni, F. J . Am. Chem. Soc. 1987, 109, 515. (c) Bellucci,
G.; Bianchini, R.; Chiappe, C.; Marioni, F.; Spagna, R. J . Am. Chem.
Soc. 1988, 110, 546. (d) Bellucci, G.; Chiappe, C.; Marioni, F.;
Marchetti, F. J . Phys. Org. Chem. 1991, 4, 387. (e) Bellucci, G.;
Bianchini, R.; Chiappe, C.; Brown, R. S.; Slebocka-Tilk, H. J . Am.
Chem. Soc. 1991, 113, 8012. (f) Bellucci, G.; Bianchini, R.; Chiappe,
C.; Ambrosetti, R.; Catalano, D.; Bennet, A. J .; Slebocka-Tilk, H.;
Aartsand, G. H. M.; Brown, R. S. J . Org. Chem. 1993, 58, 340. (g)
Zheng, C. Y.; Slebocka-Tilk, H.; Nagorski, R. W.; Alvarado, L.; Brown,
R. S. J . Org. Chem. 1993, 58, 2122. (h) Bellucci, G.; Bianchini, R.;
Chiappe, C.; Gadgil, V. R.; Marchand, A. P. J . Org. Chem. 1993, 58,
3575. (i) Bellucci, G.; Bianchini, R.; Chiappe, C.; Lenoir, D.; Attar, A.
J . Am. Chem. Soc. 1995, 117, 6243.
In the presence of added bromide salts, which in low
polarity nonprotic solvents bind Br2 as the highly stable
(6) Bellucci, G.; Bianchini, R.; Ambrosetti, R.; Ingrosso, G. J . Org.
Chem. 1985, 50, 3313.
(7) Modro, A.; Schmid, G. H.; Yates, K. J . Org. Chem. 1977, 42, 3673.
(8) Dubois, J .-E.; Mouvier, G. Tetrahedron Lett. 1963, 1325. Dubois,
J .-E.; Mouvier, G. Bull. Soc. Chim. 1968, 1426.
(9) Roston, J . H.; Yates, K. J . J . Am. Chem. Soc. 1969, 91, 1469.
Yates, K.; McDonald, R. S.; Shapiro, S. A. J . Org. Chem. 1973, 38,
2460.
(10) Garnier, F.; Donnay, R. H.; Dubois, J .-E. Chem. Commun. 1971,
829. Modro, A.; Schmid, G. H.; Yates, K. J . Org. Chem. 1979, 44, 4221.
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