High temperature bromination. 7. Bromination of norbornadiene

Full text of DOI:10.1021/jo960225l

J . Org. Chem. 1996, 61, 8297-8300
8297
High Tem p er a tu r e Br om in a tion . 7.1
Br om in a tion of Nor bor n a d ien e
Sch em e 1
†
‡
†
Ahmet Tutar, Yavuz Ta s¸ kesenligil, Osman C¸ akmak,
§
,‡
Riza Abbaso gˇ lu, and Metin Balci*
Department of Chemistry, Faculty of Art and Sciences,
Atat u¨ rk University, 25240 Erzurum, Turkey, Department of
Chemistry, Faculty of Art and Sciences, Gaziosmanpa s¸ a
University,Tokat, Turkey, and Department of Chemistry,
Faculty of Art and Sciences, Karadeniz Technical University
Trabzon, Turkey
Received February 2, 1996
Sch em e 2
Constitution and configuration of the products formed
by electrophilic addition to norbornene and norborna-
2
dienes are interesting. The reaction of these systems
have been used as a mechanistic probe to elucidate the
mechanism of different reactions. The electrophiles add
to norbornene preferentially (or exclusively) by exo-
attack, while in norbornadiene there is possibility of both
3
exo and endo-attack. Halogenation of norbornadiene has
been studied less intensively. Winstein4 has studied
bromination of norbornadiene and has pointed out the
dangerous properties of the products.
Addition of bromine to norbornadiene at 0 to -20 °C
in chlorinated solvents results in the products of Wag-
ner-Meerwein rearrangement and homoallylic conjuga-
in carbon tetrachloride was added a hot solution of
bromine in carbon tetrachloride in one portion. The color
of bromine disappeared immediately. After distillation
of the reaction mixture followed by fractional crystalliza-
tion and silica gel chromatography, we isolated five
products, three of them with nonrearranged skeleton
(Scheme 2).
4
tion; no products of trans-addition have been detected.
The formation of halogenonium ion cannot compete with
that of nonclassical ions. The formation of 2-4 can be
rationalized in terms of initially formation of exo-attack
(Scheme 1).
In the course of studying the bromination reactions of
Dibromo nortricyclanes 3 and 4 were formed in 54%
the unsaturated bicyclic systems we noticed that the
reaction temperature has a dramatic influence on product
distribution. Bromination at room and lower tempera-
tures gives rearranged products via Wagner-Meerwein
rearrangement with accompanying alkyl and aryl (in the
case of benzannelated systems) migration. However, the
bromination of these hydrocarbons at higher tempera-
tures (80-150 °C) resulted in the formation of non-
rearranged products.5 High temperature bromination
prevents skeletal rearrangement. In connection with our
continuing work in the temperature bromination reac-
1
total yield in a ratio of 3:7. The H NMR and especially
13
the C NMR spectra are in good agreement with struc-
tures 3 and 4. Besides these products we isolated three
nonrearranged tetrabromo compounds 5-7 with norbor-
nane structure. Carefully examination of the reaction
mixture did not reveal the formation of any trace of
Wagner-Meerwein rearrangement product 2.
The determination of the exact structures 5-7 would
allow us to propose the mechanism by addition of
electrophiles to norbornadiene. The structural assign-
ment to the tetrabromo compound 6 was achieved by
5
tions we have been interested in the bromination reac-
1
means of proton and carbon NMR data. The H NMR
tion of norbornadiene at high temperature in order to see
the effect of the temperature on skeletal rearrangement
and in the synthesis of disubstituted norbornadiene
derivatives which opens up entry to further substituted
norbornadiene derivatives.
1
3
and C NMR spectra of 6 were highly symmetrical
according to the symmetry in the molecule. Especially,
a three-line 13C NMR spectrum is in good agreement
either for all exo- or all endo-configuration of the tetra-
bromides. The configuration of 6 has been confirmed by
Norbornadiene was submitted to high temperature
bromination. To a refluxing solution of norbornadiene
1
differential H NMR nuclear overhauser enhancement
(
NOE) studies. Irradiation of bridge methylene protons
at δ 2.33 induces an enhancement only of the bridgehead
protons not the methine protons where bromine atoms
are attached, which clearly indicates the all exo-orienta-
tion of bromine atoms (Scheme 3).
†
Gaziosmanpa s¸ a University.
‡
§
Atat u¨ rk University.
Karadeniz Technical Universtiy.
1) For Part VI see: Da s¸ tan, A.; Balci, M.; H o¨ kelek, T.; U¨ lk u¨ , D.;
(
B u¨ y u¨ kg u¨ ng o¨ r, O. Tetrahedron 1994, 50, 10555-10578.
(
2) (a) Traylor, T. G. Acc. Chem. Res. 1969, 2, 152. (b) Fahey, R. C.
Top. Stereochem. 1969, 3, 253.
3) (a) Alvernhe, G.; Anker, D.; Laurent, A.; Haufe, G.; Beguin, C.
The chemical shift of the protons on the bridging
carbon are highly sensitive against the configuration of
substituent attached to C
, C , C , and C carbon atoms.
2 3 5 6
Any steric repulsion between methylene protons and
substituents related to the van der Waals effect causes
a paramagnetic contribution to the shielding constants
of methylene protons which results in a shift to lower
field.6 The methylene protons of norbornane resonates
at δ 1.21. Exo-orientation of two bromine atoms as in
(
Tetrahedron 1988, 44, 3551-3563. (b) Gregorcic, A.; Zupan, M.
Tetrahedron 1977, 33, 3241-3246.
(
4) (a) Winstein, S. Shatavsky, M. Chem. Ind. 1956, 1210. (b)
Winstein, S. J . Am. Chem. Soc. 1961, 83, 1516-1517.
(
5) (a) Da s¸ tan, A., Demir, U¨ ., Balci, M. J . Org. Chem. 1994, 59,
6
4
1
534-6538. (b) Balci, M.; C¸ akmak, O.; H o¨ kelek, T. Tetrahedron 1992,
8, 3163-3182. (c) Balci, M.; C¸ akmak, O.; H o¨ kelek, T. J . Org. Chem.
992, 57, 6640-6643. (d) Balci, M.; C¸ akmak, O.; Harmandar, M.
Tetrahedron Lett. 1985, 26, 5469.
S0022-3263(96)00225-3 CCC: $12.00 © 1996 American Chemical Society

8
298 J . Org. Chem., Vol. 61, No. 23, 1996
Notes
Sch em e 3
Sch em e 4
the case of 7 causes a remarkable chemical shift differ-
ence of methylene protons (δ 1.69 and δ 2.56). However,
the chemical shift difference is getting smaller in the case
of exo,endo, exo,exo-orientation of tetrabromine atoms in
Ta ble 1. Str a in En er gies of Tetr a br om o Nor bor n a n e
Der iva tives fr om th e MM2 F or ce F ield Ca lcu la tion s
(
en er gies in k ca l/m ol)
van der stretch- electro-
5
. On the basis of these chemical shifts we can also
molecule total bond angle torsional Waals
bend
static
conclude the correct configuration of 5. In saturated
cyclic systems, the steric effect causes shielding of carbon
resonances when two substituted carbon atoms are
γ-gauche relative to each other. Halogens raise the
γ-effect up to -7 ppm.7 The high-field resonance of the
methylene carbon in 6 compared to norbornane indicates
clearly the all exo-orientation of bromine atoms. The
trends of 13C chemical shifts of the bridge carbons toward
high field also nicely support the proposed structures.
In order to test whether tetrabromine compounds 5-7
are primary or secondary products which can be formed
by reaction of nortricyclanes 3 and 4 with excess bromine
at high temperature, we submitted nortricyclanes 3 and
8
5
7
6
10
36.71 1.45 12.99 11.74
40.49 1.62 13.48 12.63
44.21 1.66 16.48 12.08
44.56 1.80 13.93 13.55
52.12 1.65 24.49 10.35
5.18
6.53
7.38
7.84
7.68
-0.270
-0.155
-0.08
-0.042
-0.022
5.63
6.39
6.70
7.49
8.17
of the nonrearranged products. Rearranged product 2
and nortricyclanes 3 and 4 were formed in a yield of 85%.
Careful 200 MHz H NMR measurements did not reveal
1
the formation of any trace of 5-7 with norbornane
structure. Furthermore, when free radical inhibitors are
added to bromination reaction at -10 °C, we obtained
exactly the same product distribution as by the reaction
in the absence of radical scavengers. On the basis of
these observations, we can conclude that the reaction
mechanism of bromination at 0-20 °C is ionic. Moreover,
these observations support strongly the assumption that
there is a competition between radical and ionic reactions
at high temperatures. Tetrabromides 5-7 with non-
rearranged norbornane structures are formed exclusively
by a radical mechanism. If we compare the results
obtained from the bromination reaction of norbornadiene
at different temperatures we notice that molecular re-
arrangement is getting suppressed by going to higher
temperatures.
4
to high temperature bromine reaction. We have
determined that these products are stable in refluxing
CCl and in the presence of excess bromine.
4
All products isolated after high temperature bromina-
tion show no Wagner-Meerwein type rearranged prod-
ucts. It is evident from the bromine configuration in 5-7
that the initial attack by the bromine has occurred from
the sterically less favored exo-side of the π-system. In
the case of 7 there is also endo-attack to one of the double
bonds. Giese and Kay have already observed an endo-
attack in 5% yield during the radical addition of bromo-
trichloromethane to norbornadiene.8
Studies concerning the mechanism of syn-addition
show that the syn-adduct can arise either from direct syn-
collapse of an ion pair or from rotation followed by anti-
collapse.9 Because of the rigid skeleton in 1, a bond
rotation is out of the question. In this case, we assume
that the high temperature bromination is occurring by a
free radical mechanism. Radical intermediates are much
less likely to rearrange. The fact that we do not observe
any trace of Wagner-Meerwein rearranged products
indicates that the trans addition of bromine in 5 can also
arise from the radical intermediates.
Generally seven nonrearranged isomers can be ex-
pected from the reaction of bromine with norbornadiene
1
0
(Scheme 4). The MM2 force field calculations on some
possible isomers indicate that the endo,exo,endo,exo
isomer 8 has the most stable structure (36.7 kcal/mol
total strain energy) whereas the endo,endo,endo,endo
isomer 10 is of much higher energy as a consequence of
cis orientation of bromine atoms which causes a strong
dipole-dipole and van der Waals interactions.
Strong steric repulsion between the vicinal brom atoms
in the case of endo,endo (or exo,exo) orientation can also
be seen nicely between Br9 and Br10 (Br8 and Br11)
atoms in 10. The MM2 force field calculations show that
the distances between Br8-Br9 (3.467 Å) and Br8-Br11-
Conducting the bromination reaction at reflux tem-
perature of CCl
4
in the presence of free radical inhibitors
like 2,4,6-tri-tert-butylphenol suppressed the formation
(3.476 Å) in 10 are not much different from each other.
(
6) G u¨ nther, H. NMR Spectroscopy; J ohn Wiley: New York, 1980;
The important feature of this isomer 10 is a marked
interaction between all four bromine atoms. The calcu-
lated strain energy of a value of 52 kcal/mol predicts the
existence of this strong interaction and prevents the
formation of this isomer. It is not surprising that this
isomer was not detected under the reaction products. On
the other hand, we were also not able to detect any trace
p 88.
(7) (a) Tori, K.; Ueyama, M.; Tsuji, T.; Matsamura, H.; Tanida, H.;
Iwamura, H.; Kushida, K.; Nishida, T.; Satoh, S. Tetrahedron Lett.
974, 327. (b) Davis, S. G.; Whitman, G. H. J . Chem. Soc., Perkin
Trans. 2 1975, 861. (c) Cristl, M.; Leininger, H.; Brunn, E. J . Org.
Chem. 1982, 47, 662. (d) Gheorghiu, M. D.; Olteanu, E. J . Org. Chem.
987, 52, 5158.
(
(
1
1
8) Giese, B.; Kay, J . Chem. Ber. 1979, 112, 298.
9) Heasley, G. H.; Bower, T. R.; Dougharty, K. W.; Easdon, J . C.;
Heasley, V. L.; Arnold, S.; Carter, T. L.; Yaeger, D. B.; Gipe, B. T.;
Shellhamer, D. F. J . Org. Chem. 1980, 45, 5150.
(10) Allinger, N. L. J . Am. Chem. Soc, 1977, 99, 8127.

Notes
J . Org. Chem., Vol. 61, No. 23, 1996 8299
4
1
Sch em e 5
exo,exo-3,5-Dibr om on or tr icycla n e (3): colorless liquid; H
NMR (200 MHz, CDCl ) δ 3.97 (s, 2H, H and H ), 2.35 (s, 1H,
), 2.15 (s, 2H, H ), 1.68 (m, 3H, H , H , and H
MHz, CDCl ) δ 53.17, 46.24, 31.24, 24.45, 15.61.
exo,en d o-3,5-Dibr om on or tr icycla n e (4): colorless liquid;
3
3
5
1
3
H
4
7
1
2
6
); C NMR (50
3
4
1
H NMR (200 MHz, CDCl
.30 (s, 1H, H ), 2.13 (δ, J ) 11.4 Hz, 1H, H7b), 1.80 (t, J ) 4.7
Hz, 1H), 1.66 (t, J ) 4.9 Hz, 1H), 1.57 (m, 2H); C NMR (50
MHz, CDCl ) δ 56.50, 55.94, 46.08, 32.27, 24.27, 21.48, 17.63.
The distillation residue was filtered on a short silica gel
column (10 g) eluted with petroleum ether-CHCl (4:1) to give
3 5 3
) δ 4.60 (s, 1H, H ), 3.96 (s, 1H, H ),
2
4
1
3
3
of 8 and 9 in the reaction mixture after a careful search
by NMR. Nonexistence of these thermodynamically most
stable compounds forces us to assume that the first
bromine attack to the double bond in norbornadiene takes
place from the exo-face and in a cis-fashion. During the
second addition, most stable compounds are formed
preferentially in the following order 5 > 6 > 7. Total
strain energies of 6 and 7 indicate that the isomer 6 has
slightly more strain than 7. The fact that 6 and 7 was
formed in a ratio of 9:1 indicates that exo-attack is
preferred.
After successful synthesis and characterization of these
nonrearranged products 5-7 which have the requisite
skeletal arrangement and functionality to permit the
easy introduction of two double bonds, we submitted
either pure isomer 5 or an isomeric mixture consisting
of 5-7 to a dehydrobromination reaction with 2 mol of
potassium tert-butoxide and isolated 11 and 12 in a ratio
of 54:46 (total yield 74%) which was separated by silica
gel column chromatography eluting with petroleum ether
3
5.70 g (40%) of crude product which was identified as tetra-
bromonorbornanes 5-7. A part of the crude tetrabromonor-
bornanes (1.80 g) was chromatographed on silica gel (200 g).
Elution with petroleum ether gave as the first fraction exo,
endo,exo,exo-2,3,5,6-tetrabromonorbornane 5 (1.20 g, total
yield: 26%): colorless crystals, mp 67-68 °C from chloroform/
1
n-hexane (3:1); H NMR (200 MHz, CDCl
3
) δ 4.83 (dd, J ) 6.9,
) 4.38 (tm, J ) 2.9 Hz, 1H, H ), 4.22 (dd, J ) 6.9,
), 3.86 (t, J ) 2.9 Hz, 1H, H ), 2.84 (m, 2H, H
), 2.45 (dm, J ) 11.8 Hz, 1H, H7b), 2.13 (dm, J ) 11.8 Hz,
2.1 Hz, 1H, H
5
3
2
.0 Hz, 1H, H
6
2
1
and H
4
1
3
1
5
H, H7a); C NMR (50 MHz, CDCl
3
) δ 57.64 (C
5
), 57.44 (C
3
),
5.55 (C ), 54.23 (C ), 52.10 (C ), 51.64 (C
6
2
1
4
), 31.49 (C
7
); IR (KBr,
-1
cm ) 2970, 2950, 1440, 1300, 1230, 920, 850; MS m/ z 408/410/
+
+
4
12/414/416 (M , 22), 327/329/331/333 (M - Br, 100), 249/251/
+
+
+
253 (M - 2Br, 30), 169/171(M - 3Br, 60), 91 (M - 4Br, 67),
6
2
5 (50). Anal. Calcd for C
0.52; H, 1.85.
7 8 4
H Br ; C, 20.42; H, 1.96. Found: C,
Continued elution with the same solvent afforded endo,
endo,exo,exo-2,3,5,6-tetrabromonorbornane (7) as the second
fraction (35 mg, total yield: 1%): colorless crystals, mp 184-
85 °C from chloroform/n-hexane (1:1); 1H NMR (200 MHz,
1
(
Scheme 5). Structural assignments to 11 and 12 were
CDCl
Hz, 2H, H
1.9 Hz, 1H, H7b), 1.69 (br d, 1H, H7a); 1 C NMR (50 MHz, CDCl
δ 54.93 (C and C ), 53.10 (C and C ), 52.43 (C and C ), 33.11
); IR (KBr, cm ) 2970, 1450, 1300, 1280, 1210, 1140, 910,
3
) δ 4.88 (d, J ) 2.0 Hz, 2H, H
5
and H
6
), 4.50 (t, J ) 2.4
achieved by means of proton and carbon NMR data. A
2
and H ), 2.85 (m, 2H, H and H
3
1
4
), 2.56 (dt, J ) 11.8,
3
four-line 1 C-NMR spectrum of 11 and a five-line C-
3
13
3
)
2
3
-
5
6
1
4
NMR spectrum of 12 strongly supports the symmetrical
structures. With the completion of the synthesis of 11
and 12, we opened up an entry to nongeminal disubsti-
tuted norbornadiene derivatives.
1
(C
7
+
9
00; MS m/ z 408/410/412/ 414/416 (M , 18), 327/329/331/333
+
+
+
(M - Br, 100), 249/251/253 (M - 2Br, 41), 169/171(M - 3Br,
+
69), 91 (M - 4Br, 64), 65 (57). Anal. Calcd for C
0.42; H, 1.96. Found: C, 20.31; H, 1.83.
Further elution with CH Cl -petroleum ether (3:1) yielded
7 8 4
H Br ; C,
2
2
2
Exp er im en ta l Section
exo, exo,exo,exo-2,3,5,6-tetrabromonorbornane (6) as the last
P r eca u tion : Norbornadiene bromides are dangerous com-
pounds. Prof. S. Winstein has reported4b that of three co-workers
who have used the norbornadiene dibromides, two later devel-
oped similar pulmonary disorders which contributed to their
subsequent deaths. Gen er a l. Commercial reagents were pur-
chased from standard chemical suppliers and purified to match
the reported physical and spectral data. Solvents were concen-
trated at reduced pressure. Melting points are uncorrected.
Infrared spectra were obtained from films on NaCl plates for
liquids or from solution in 0.1 mm cells or KBr pellets for solids
fraction (80 mg, total yield: 9%): colorless crystals, mp 261-
1
262 °C from chloroform; H NMR (200 MHz, CDCl
3
) δ 4.17 (br
), 2.33 (br s,
, C , and C ),
7
); IR (KBr cm ) 2970, 1450, 1310,
s, 4H, H
2H, H
); 1 C NMR (50 MHz, CDCl
52.40 (C and C ), 29.29 (C
1270, 1150, 910, 850; MS m/ z 408/410/412/414/416 (M , 16), 327/
2
, H
3
, H5, and H
6
), 2.96 (br s, 2H, H
1
and H
4
3
7
3
) δ 58.94 (C
2
, C
3
5
6
-
1
1
4
+
+
+
329/331/333 (M - Br, 100), 249/251/253 (M - 2Br, 46), 169/
+
+
171 (M - 3Br, 58), 91 (M - 4Br, 56), 65 (51). Anal. Calcd for
Br ; C, 20.42; H, 1.96. Found: C, 20.15; H, 2.01.
Br om in a tion of Nor bor n a d ien e (1) in th e P r esen ce of
C
7
H
8
4
1
13
on a regular instrument. The H and C NMR spectra were
recorded on 200 (50)- and 60-MHz spectrometers. Mass spectra
2,4,6-Tr i-ter t-Bu tylp h en ol a t 77 °C. To a solution of norbor-
nadiene (100 mg, 1.08 mmol) and 2,4,6-tri-tert-butylphenol (600
mg, 2.29 mmol) in 3 mL of refluxing carbon tetrachloride was
added a hot solution of bromine (346 mg, 2.16 mmol) in 0.35
mL of carbon tetrachloride in one portion. The color of bromine
disappeared. After cooling to room temperature, the solvent was
evaporated. NMR analysis of the residue indicated the forma-
tion of Wagner-Meerwein rearrangement product 2 and nortri-
cyclane derivatives 3 and 4 in 85% yield.
Br om in a tion of Dibr om on or tr icycla n es 3 a n d 4 a t 77
°C. To a solution of a mixture of dibromonortricylanes (0.44 g,
1.76 mmol) in 5 mL of refluxing carbon tetrachloride was added
a hot solution of bromine (0.32 g, 2 mmol) in 1 mL of hot carbon
tetrachloride in one portion. The resulting reaction mixture was
refluxed for 0.5 h and cooled to room temperature, and the
solvent was evaporated. NMR analysis of the residue indicated
that the dibromonortricyclanes were not changed and are
stabilized under the reaction conditions.
(electron impact) were recorded at 70 eV as m/ z. Column
chromatography was performed on silica gel (60 mesh, Merck).
Br om in a tion of Nor bor n a d ien e (1) a t 77 °C. Norborna-
diene 1 (3.2 g , 34.78 mmol) was dissolved in 30 mL of CCl in
4
a 100 mL flask which was equipped with reflux condenser. The
solution was heated until carbon tetrachloride started to reflux
while stirring magnetically. To the refluxing solution was added
a hot solution (65-70 °C) of bromine (8.79 g, 55 mmol) in 10
mL of carbon tetrachloride in one portion. The color of bromine
disappeared immediately. The resulting reaction mixture was
heated for 5 min at reflux temperature. After being cooled to
room temperature, the solvent was evaporated. The oily residue
2 2
(12.34 g) was dissolved in 40 mL of CH Cl -hexane (1:1) and
allowed to stand for several days in a refrigerator. A 1.01 g (7%)
amount of crystalline exo,exo,exo,exo-2,3,5,6-tetrabromonor-
bornane (6) was isolated. After filtration of the tetrabromide 6,
the organic solvent was evaporated, and the oily residue (10.83
g) was fractionated under reduced pressure. The distillate (bp
Rea ction of Tetr a br om on or bor n a n es 5-7 w ith P ota s-
siu m ter t-Bu toxid e. To a stirred solution of a mixture of
tetrabromides 5-7 (1.64 g, 3.9 mmol) in dry and freshly distilled
THF (40 mL) was added 1.23 g (10.9 mmol) of potassium tert-
butoxide. The resulting reaction mixture was refluxed for 3 h
and then cooled to room temperature. The mixture was diluted
with water, and the aqueous solution was extracted with ether,
7
4
5-85 °C/10 mm) was identified as dibromonortricyclanes 3 and
(4.70 g, 54%) in a ratio of 31:69 (by NMR). A mixture of
dibromonortricyclanes (550 mg) was chromatographed on silica
gel (110 g) with n-hexane to give 3 (28 mg, 5%) and 4 (40 mg,
7
%).

8
300 J . Org. Chem., Vol. 61, No. 23, 1996
Notes
washed with water, and dried over MgSO . After removal of
the solvent, the residue (a pale yellow oil) was filtered on a short
silica gel column (10 g) eluted with n-hexane to give 700 mg
4
Rea ction of exo,en d o,exo,exo-2,3,5,6-Tetr a br om on or bor -
n a n e (5) w ith P ota ssiu m ter t-Bu toxid e. The reaction was
carried out as described above by using 1.11 g (2.7 mmol) of pure
tetrabromide 5 and 0.75 g (6.75 mmol) of potassium tert-butoxide
in 30 mL of dry THF, and 500 mg (74%) of 2,5- and 2,6-
dibromonorbornadiene (11, 12) was obtained as an isomeric
mixture. The relative percentage of products 11 and 12 was
(70%) of crude 2,5- (11) and 2,6-dibromonorbornadiene (12) in a
ratio of 52:48 (by NMR). The crude product (700 mg) was
chromatographed on silica gel (110 g). Elution with petroleum
ether gave 65 mg (6%) of pure 2,5-dibromonorbornadiene (11)
as the first fraction. The other isomer, 2,6-dibromonorborna-
diene (12), was not isolated in purity (purity over 95%).
1
obtained from the integration of H NMR signals for the olefinic
hydrogens and found to be in a 54:46 ratio for 11 and 12,
1
2
,5-Dibr om on or bor n a d ien e (11): colorless liquid; H NMR
(
200 MHz, CDCl ) δ 6.80 (t, J ) 2.1 Hz, 2H, olefinic H), 3.51 (m,
3
respectively.
1
3
2
H, bridgehead H), 2.36 (m, 2H, bridge H); C NMR (50 MHz,
) δ 139.85 (dCH), 138.53 (dCBr), 74.61 (C and C ), 61.63
); IR (film, cm ) 2950, 1600, 1575, 1300, 1200, 980, 850. Anal.
Calcd for C Br : C, 33.63; H, 2.42. Found: C, 33.21; H, 2.32.
,6-Dibr om on or bor n a d ien e (12): colorless liquid; H NMR
200 MHz, CDCl ) δ 6.71 (d, J ) 2.9 Hz, 2H, olefinic H), 3.60
m, 1H, H ), 3.47 (m, 1H, H
), 2.35 (m, 2H, bridge H); 13C NMR
50 MHz, CDCl ) δ 141.65 (dCH), 136.72 (dCBr), 74.80 (C ),
7.71 (C ), 54.84 (C ).
The dibromonorbornadienes 11 and 12 were found to be
unstable at room temperature. However, at low temperature
under -30 °C) or basic conditions, it can be kept for several
days.
CDCl
(C
7
3
1
4
-1
Ack n ow led gm en t. The authors are indebted to the
Department of Chemistry (Atat u¨ rk University) and the
Department of Chemistry (Gaziosmanpa s¸ a University
Grant 1993/5) for financial support of this work and
State Planning Organization of Turkey (DPT) for pur-
chasing a 200 MHz NMR spectrometer.
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