High temperature bromination. Part 22: Bromination of 1a,2,7,7a-tetrahydro-1H-cyclopropa[b]naphthalene

Article abstract of DOI:10.1016/j.tet.2007.05.124

The high-temperature bromination of 1a,2,7,7a-tetrahydro-1H-cyclopropa[b]naphthalene and its carboethoxy derivative was studied. Reaction of the title compound with 1 mol of bromine in refluxing carbon tetrachloride resulted in the formation of ring-opening products. In the case of the carboethoxy derivative, bromination took place both regio- and stereospecifically at the benzylic positions, the cyclopropane ring did not undergo bond cleavage. A mechanism for the formation of the products and their dehydrobromination reactions is discussed.

Full text of DOI:10.1016/j.tet.2007.05.124

Tetrahedron 63 (2007) 8151–8156
High temperature bromination. Part 22: Bromination of
1a,2,7,7a-tetrahydro-1H-cyclopropa[b]naphthalene
a
b
c,
b,
*
*
€
€
Demet Demirci-Gultekin, Duygu D. Gunbaxs, Yavuz Taxskesenligil and Metin Balci
aDepartment of Chemistry, Faculty of Science, Atatu€rk University, Erzurum 25240, Turkey
bDepartment of Chemistry, Middle East Technical University, Ankara 06531, Turkey
cDepartment of Chemistry, Faculty of Education, Atatu€rk University, Erzurum 25240, Turkey
Received 9 March 2007; revised 17 May 2007; accepted 31 May 2007
Available online 6 June 2007
Abstract—The high-temperature bromination of 1a,2,7,7a-tetrahydro-1H-cyclopropa[b]naphthalene and its carboethoxy derivative was
studied. Reaction of the title compound with 1 mol of bromine in refluxing carbon tetrachloride resulted in the formation of ring-opening
products. In the case of the carboethoxy derivative, bromination took place both regio- and stereospecifically at the benzylic positions, the
cyclopropane ring did not undergo bond cleavage. A mechanism for the formation of the products and their dehydrobromination reactions
is discussed.
Ó 2007 Elsevier Ltd. All rights reserved.
Br Br
1. Introduction
Br2
Br2
Although saturated hydrocarbons are readily available and
extremely cheap starting materials, they cannot be used in
synthetic chemistry without prior activation. The activation
of alkanes can be done via halogenation using an appropriate
method, which leads to the direct synthesis of haloalkanes.
However, the control of regioselectivity is very difficult.
We recently reported the bromination of hydrocarbons
such as norbornadiene,1 benzonorbornadiene,2 and homo-
benzonorbornadiene3 at high temperatures (80–150 ꢀC) in
nonpolar solvents (CCl4, decalin, etc.) and noticed that
mostly non-rearranged brominated products were formed;
these bromination reactions proceed via a free radical mech-
anism. Applying this high-temperature bromination to a sat-
urated hydrocarbon, decalin 14a as well as an unsaturated
one, octalin 3, results in bromination with remarkable regio-
and stereospecificity (Scheme 1). Bromination of 1 and 3 at
150 ꢀC gives the tetrabromide 2 as the major product.4a
150 °C
150 °C
3
1
Br
Br
2
Br
Br
Br
Br
Br2
+
50 °C, CHCl3
4
5
6
Scheme 1.
dibromocyclohexane 6. However, we showed that it is possi-
ble to interrupt the opening of the cyclopropane ring using
our high-temperature bromination method. An example of
this being the octalin derivatives 7 and 9, which have cyclo-
propane rings along with a double bond were treated with
bromine, in refluxing CCl4.4a The reaction produced the cor-
responding tetrabromides 8 and 10 with remarkable regio-
and stereoselectivity where the opening of the cyclopropane
ring is precluded (Scheme 2). On the other hand, simple bro-
mination of 9 at room temperature resulted in the addition of
bromine to the double bond.4
In addition, it is well established that treatment of a cyclopro-
pane ring with bromine produces the corresponding 1,3-di-
bromides.5 For example, Lambert et al.6 have reported that
the reaction of bicyclo[3.1.0]hexane 4 with bromine results
in the formation of (cis- and trans)-1,3-dibromocyclohexane
5 along with a small amount of (cis- and trans)-1,2-
Inspired by these results we decided to investigate the fate of
a cyclopropane ring that has active benzylic positions origi-
nating from the fused benzene ring, in high-temperature bro-
mination reactions. For this purpose benzonorcarane 12 was
chosen as a model compound. The results of this study
should give an important insight into the mechanism and
chemoselectivity of the reaction.
Keywords: Bromination; Dehydrobromination; Cyclopropanes; Rearrange-
ments.
* Corresponding authors. Tel.: +90 442 231 4013; fax: +90 442 236 0955
(Y.T.); tel.: +90 312 210 5140; fax: +90 312 210 3200 (M.B.); e-mail
addresses: ytaskes@atauni.edu.tr; mbalci@metu.edu.tr
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.05.124

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D. Demirci-Gu€ltekin et al. / Tetrahedron 63 (2007) 8151–8156
Shea and Skell11 studied the photobromination of alkyl-
R
R
R
R
Br
cyclopropanes and showed that the first bromine radical
attacks the least substituted carbon atom, with the second
one going to the most substituted carbon atom. According
to this mechanism, the monobromide 13 may arise from
the unsymmetrical addition of bromine to 12 to give 20,
which can undergo a hydrogen bromide elimination to
form 13. The formation of 16 can be reasonably explained
by addition of bromine to the double bond in 13. Inde-
pendently, pure 13 was treated with bromine under the
same reaction conditions and only the tribromide 16 was
obtained.
Br2
Br
Br
Br
CCl4, reflux
8
7
R
R
Br
Br2
Br
Br
Br
CCl4, reflux
R
R
9
10
R = COOCH3
Scheme 2.
We assume that the addition of bromine to the double bond
in 13 is a trans-addition. Two bromine atoms attached to the
C-1 and C-2 carbon atoms in 16 can be either in axial–axial
or equatorial–equatorial conformations. Wiberg calculated
the energies of diaxial, axial–equatorial, and diequatorial
1,2-dibromocyclohexanes using the hybrid density func-
tional methods B3LYP and B3P86 as well as MP2 and
QCISD and the 6-311G* and 6-311+G(2df,p) basis sets. In
all cases the axial–axial conformer was found to have the
lowest energy by about 1.11–1.54 kcal/mol.12 One explana-
tion for the differences in energy between the conformers is
that they result from electrostatic interactions between the
C–Br dipoles. We have calculated the heat of formation
energies for the two conformers of 16 using AM1 geometry
optimization and found, as predicted, that the axial–axial
conformer has the lowest heat of formation energy by about
1.3 kcal/mol. In the axial–axial conformation, the dihedral
angle between the protons H1 and H2 is about 78ꢀ. The ob-
served broad singlet resonance for H-1 proton is in agree-
ment with this suggested trans-configuration as well as
with the aa-conformation. The fact that the H-2 also reso-
nates as a broad singlet, can be explained only with the
trans-configuration of the bromomethylene group. In that
case, the dihedral angle between the protons H-2 and H-3
is calculated to be 65ꢀ, which also is in agreement with the
observed vicinal coupling constant J23ꢁ1 Hz in the 1H
NMR spectrum.
2. Results and discussions
The starting material, cyclopropa[b]naphthalene 12,7 was
synthesized by addition of dichlorocarbene to the readily
available 1,4-dihydronaphthalene using a phase transfer ca-
talysis method followed by reductive dechlorination of the
carbene-adduct 11.8 For the high-temperature bromination
of 12, bromine was directly distilled into a hot solution of
12 in refluxing carbon tetrachloride. The reaction provided
a mixture consisting of five products. After repeated column
chromatography, we isolated 13–17 in yields of 37, 17, 16,
11, and 7%, respectively (Scheme 3).
Cl
Na, Ether
Cl
t-BuOH, 93%
11
12
1. Br2 (1 eq)
CCl4, reflux
2. SiO2
Br
Br
5
4
1
6
3
2
7
8
+
+
CH2Br
CH2Br
9
Br
14 (17%)
15 (12%)
13 (37%)
+
Br
1
OR
The hydrolysis product 17a was formed during column chro-
matography. The alcohol 17a was converted to its acetate
17b for further characterization. The H NMR spectrum of
the alcohol displayed a doublet (J¼9.2 Hz) at 5.01 ppm for
the alkoxy proton H1 and a doublet of doublets (J¼11.2
and 9.2 Hz) at 4.23 ppm for the proton H2. These large cou-
plings in the cyclohexane ring can be observed only in the
axial–axial orientation (trans-configuration) of the relevant
protons. This finding strongly supports the trans–trans con-
figuration of the substituents.
Br
2
Br
CH2Br
+
1
3
CH2Br
4
16 (11%)
17a R = H (7%)
17b R = COCH3
AcCl, CH2Cl2 (81%)
Scheme 3.
Compounds 13 and 159 have been previously reported by
Friedrich and Holmstead10 in the free radical NBS a-bromi-
nation of 12. They suggested that a-bromination products 18
and 19 are not stable under the reaction conditions and un-
dergo rearrangement to the thermodynamically more stable
bromides 13 and 15 via an ion-pair type mechanism upon
standing or heating.
The formation of the seven-membered ring 14 can arise ei-
ther from the 1,3-addition of bromine to the internal double
bond of the initially formed monobromide 18 or from the
addition of bromine to the cyclopropane in 12 followed by
bromination of one of the benzylic carbons.
Br
Br
For further structural characterization of the isolated com-
pounds, 13 and 16, they were subjected to a base-promoted
elimination reaction. The monobromide 13 gave the 2-meth-
ylnaphthalene (22) as the sole product in 65% yield (Scheme
4). The elimination of hydrogen bromide from 13 gives 21,
Br
Br
Br
18
19
20

D. Demirci-Gu€ltekin et al. / Tetrahedron 63 (2007) 8151–8156
8153
1
COOEt
which undergoes double bond isomerization to form 22. Due
COOEt
2
3
Br2, CCl4
reflux
to the structural similarity between the compounds 13 and
16 we assume that the elimination reaction of 16 follows
a similar pathway and produces 15 and 235c,13 in a ratio of
16:84.
Br
Br
26
27 (62%)
Scheme 6.
t-BuOK
CH2
t-BuOK/THF
The dibromide 27 was also observed in the free radical NBS
a-bromination of 26 but information about the stereochem-
istry of the bromine atoms in this case was not given.17 The
proton and carbon NMR spectra show a plane-symmetry in
the molecule. A nine-line 13C NMR spectrum is in agree-
ment with structure 27. The configuration of bromine atoms
was determined by a NOE experiment. Irradiation at the
resonance frequency of the benzylic protons (d 5.66) causes
enhancement at the resonance signals of the endo-cyclopro-
pane proton resonating at d 1.46 as well as of the other cyclo-
propane protons resonating at d 2.75. This NOE experiment
clearly indicates that the bromines are in a cis-configuration
and are located in an anti-position with respect to the cyclo-
propane ring in 27.
60-70 °C
CH2Br
CH3
CH3
13
21
22 (65%)
Br
Br
Br
t-BuOK/THF
+
60-70 °C, 60%
CH2Br
CH2Br
16
Scheme 4.
15
16:84
23
The structure of the tribromide 14 was determined by NMR
and extensive double resonance experiments. 13C NMR data
were consistent with the proposed structure showing five
aliphatic and six aromatic carbons. Chemical proof for the
structure was obtained by treatment of 14 with sodium and
tert-butyl alcohol to give benzo[a]cycloheptene 2514
(Scheme 5). Furthermore, the tribromide 14 was subjected
to dehydrobromination with potassium tert-butoxide to
give two products. The structure of the major product
(44%) was shown by spectroscopic methods to be 24. The
dehydrobromination product 24 was accompanied by a small
amount (7%) of the ring-constricted product 23.
In conclusion, the high-temperature bromination of 12
resulted in the formation of ring-opening products, and the
regio- and stereoselectivity was not observed. In the case
of 26, the high-temperature bromination proceeded regio-
and stereospecifically at the benzylic positions. The carbonyl
group bound to the cyclopropane ring in 26 prevents the
opening of the ring.
3. Experimental
3.1. General
Br
Br
Br
Br
t-BuOK/THF
+
60-70 °C
CH3
Br
€
14
Melting points were determined on a Buchi model 530 appa-
24 (44%)
23 (7%)
ratus and are uncorrected. Infrared spectra were recorded on
a Mattson model 1000 FTIR and Shimadzu model 8300
Na/t-BuOH
ether
1
FTIR spectrophotometer. H and 13C NMR spectra were
recorded on 200 (50)-MHz spectrometers. Mass spectra
(EI) were recorded at 70 eVas m/z. Column chromatography
was performed on silica gel 60 (70–230 mesh, Merck). TLC
was carried out on Merck 0.2 mm silica gel 60 F254 analyti-
cal aluminum plates. Elemental analysis was determined on
a Leco CHNS-932 instrument.
25 (85%)
Scheme 5.
The high-temperature bromination of 12 did not show any
regio- and stereoselectivity as observed for decalin and its
derivatives. This is probably due to the lack of a carbonyl
group bound to the cyclopropane ring, which suppress the
cyclopropane bond cleavage leading to the ring-opening
3.2. 1,1-Dichloro-1a,2,7,7a-tetrahydro-1H-cyclo-
propa[b]naphthalene (11)
The dichlorocyclopropane compound was prepared in 70%
yield as described in the literature.8b
products. Hoffmann and Gunther15 explained this phenome-
€
non on the basis of HOMO and LUMO interactions between
the cyclopropane ring and its substituents. The dominant in-
teraction is assumed to be between the cyclopropane 3E’
Walsh-type orbital and the p* orbital on the substituent,
which causes a shortening of the distal bond between (C-2
and C-3) and stabilization of this bond. In order to see the
effect of a carbonyl group on the product distribution we
synthesized the exo-ester 264b,16 and subjected it to the
high-temperature bromination reaction. The reaction of 26
with bromine, in refluxing carbon tetrachloride, yielded
the corresponding dibromide 27 as the sole product in 62%
yield (Scheme 6).
3.3. 1a,2,7,7a-Tetrahydro-1H-cyclopropa[b]naphtha-
lene (12)
Metallic sodium (10.9 g, 474 mmol) was cut into small
pieces and combined with 80 mL of anhydrous ether. This
was magnetically stirred under nitrogen atmosphere while
a mixture of the dichlorocarbene-adduct 11 (10.0 g,
46.9 mmol), tert-butanol (34.3 g, 464 mmol), and ether
(50 mL) was added dropwise over a period of 2 h. The reac-
tion mixture was stirred for additional 24 h at room temper-
ature and methanol (20 mL) and then water (40 mL) was

8154
D. Demirci-Gu€ltekin et al. / Tetrahedron 63 (2007) 8151–8156
added dropwise. The mixturewas diluted with water (50 mL)
and the aqueous solution was extracted with ether
(3ꢂ100 mL), washed with water (2ꢂ50 mL), and dried
over CaCl2. After removal of the solvent, 6.29 g (93%)
of 12 was obtained as a crystalline solid, mp 31–32 ꢀC
(lit.7 31–32 ꢀC). Comparison of the spectral data of this
compound with those reported in the literature7 was in full
agreement.
(76). Anal. Calcd for C11H11Br3: C, 34.50; H, 2.90. Found:
C, 34.85; H, 2.74.
3.4.4. 1,2-Dibromo-3-(bromomethyl)-1,2,3,4-tetrahydro-
naphthalene (16). Pale yellow powder (the fifth fraction),
mp 72–74 ꢀC from ether/hexane. 1H NMR (200 MHz,
CDCl3) d 7.16 (dd, J¼7.7 and 1.4 Hz, 1H, ArH), 7.07 (m,
2H, ArH), 6.93 (br d, J¼7.3 Hz, 1H, ArH), 5.57 (d,
J¼2.2 Hz, 1H, H1), 4.87 (br s, 1H, H2), 3.33 (dd, A-part of
AB-system, J¼10.2, 5.9 Hz, 1H, –CH2Br), 3.28 (dd, B-part
of AB-system, J¼10.2, 8.3 Hz, 1H, –CH2Br), 2.87 (dd, A-
part of AB-system, J¼16.1, 5.1 Hz, 1H, H4), 2.79 (m, 1H,
H3), 2.66 (dd, B-part of AB-system, J¼16.1, 11.3 Hz, 1H,
H4); 13C NMR (50 MHz, CDCl3) d 133.5 (s), 132.6 (s),
131. 1 (d), 129.1 (d), 129.0 (d), 127.1 (d), 55.3 (d), 51.3 (d),
36.01 (d), 35.1 (t), 30.6 (t); nmax (KBr) 2845, 1490, 1425,
1336, 1234, 1143, 1097, 1031 cmꢃ1; MS (EI, m/z, % relative
intensity) 386/384/382/380 (M+, 0.5/1/0.8/0.4), 305/303/301
(M+ꢃBr, 11/23/13), 223/221 (M+ꢃ2Br, 30/27), 143.1 (57),
141.1 (M+ꢃ3Br, 100), 129.2 (97). Anal. Calcd for
C11H11Br3: C, 34.50; H, 2.90. Found: C, 34.38; H, 2.79.
3.4. Bromination of 1a,2,7,7a-tetrahydro-1H-cyclo-
propa[b]naphthalene (12) at 77 ꢀC
Two grams (13.8 mmol) of 12 was dissolved in 30 mL of
carbon tetrachloride in a 100 mL two necked flask equipped
with reflux condenser and an inlet glass tube touching the
bottom of the reaction flask. The inlet glass tube was con-
nected to a 2 mL round-bottom flask, which contains
2.25 g (14.06 mmol) of bromine. Bromine vapors obtained
by heating of the flask to 100 ꢀC, were transferred directly
to the refluxing solution (2–3 min) while stirring magneti-
cally. The resulting reaction mixture was refluxed for
5 min. After being cooled to room temperature the solvent
was evaporated. The oily residue was chromatographed on
silica gel (110 g) eluting with hexane. The first fraction
was the unreacted starting material 12 (1.0 g). From the
other fractions four compounds were isolated in the follow-
ing order: 13 (0.58 g, 37%), 15 (0.18 g, 12%), 14 (0.44 g,
17%), 16 (0.30 g, 11%). Then the column was eluted with
ether/hexane (1:9) and 0.154 g (7%) of 17a was isolated as
the last fraction.
3.4.5. 2-Bromo-3-(bromomethyl)-1,2,3,4-tetrahydro-
naphthalene-1-ol (17a). Colorless needles (the sixth frac-
tion), mp 103–104 ꢀC from ether/hexane. 1H NMR
(200 MHz, CDCl3) d 7.55 (br d, J¼6.8 Hz, 1H, ArH), 7.26
(m, 2H, ArH), 7.12 (br d, J¼6.7 Hz, 1H, ArH), 4.95 (dd,
J¼9.1, 3.7 Hz, 1H, H1), 4.18 (dd, J¼11.1, 9.1 Hz, 1H, H2),
3.79 (dd, A-part of AB-system, J¼10.4, 5.3 Hz, 1H,
–CH2Br), 3.70 (dd, B-part of AB-system, J¼10.4, 2.5 Hz,
1H, –CH2Br), 3.02 (d, J¼7.4 Hz, 2H, H4), 2.80 (d,
J¼4.0 Hz, 1H, –OH), 2.44 (m, 1H, H3); 13C NMR
(50 MHz, CDCl3) d 135.4 (s), 133.4 (s) 128.2 (d), 128.1 (d),
127.0 (d), 126.9 (d), 75.2 (d, C-1), 63.5 (d, C-2), 40.6 (t,
–CH2Br), 39.1 (d, C-3), 33.9 (t, C-4); nmax (KBr) 3280, 2835,
1488, 1433, 1326, 1230, 1174, 1053, 1022, 904 cmꢃ1; MS
(EI, m/z, % relative intensity) 322/320/318 (M+, 9/18/11),
239/241 (M+ꢃBr, 95/100), 159 (M+ꢃ2Br, 21). Anal. Calcd
for C11H12Br2O: C, 41.28; H, 3.78. Found: C, 41.01; H, 3.95.
3.4.1. 2-(Bromomethyl)-1,2-dihydronaphthalene (13).10
Pale yellow liquid (the second fraction), 1H NMR
(200 MHz, CDCl3) d 7.21–7.04 (m, 4H, ArH), 6.55 (dd,
J¼9.7, 1.3 Hz, 1H, H4), 5.93 (dd, J¼9.7, 4.0 Hz, 1H, H3),
3.44 (dd, J¼9.9, 5.5 Hz, 1H, H1), 3.33 (dd, J¼9.9, 7.4 Hz,
1H, H1), 2.95 (d, J¼7.2 Hz, 2H, –CH2Br), 2.87 (m, 1H,
H2); 13C NMR (50 MHz, CDCl3) d 135.4, 135.1, 131.2,
131.1, 130.2, 129.6, 128.8, 128.2, 38.5, 38.1, 34.2.
3.4.2. 2-(Bromomethyl)naphthalene (15).9,10 Colorless
plates (the third fraction), mp 52–53 ꢀC (lit.9 51–53 ꢀC)
from ethanol. 1H NMR data for 15 were identical with those
reported in the literature. 13C NMR (50 MHz, CDCl3)
d 137.1, 135.2, 135.1, 130.7, 129.9, 129.8, 129.7, 128.7,
128.5, 128.4, 35.6.
3.5. Reaction of 1,2-dibromo-3-(bromomethyl)-1,2,3,4-
tetrahydronaphthalene 16 with potassium tert-butoxide
To a stirred solution of tribromide 16 (100 mg, 0.26 mmol)
in 5 mL of dry THF, potassium tert-butoxide (70 mg,
0.62 mmol) was added. The reaction mixture was heated at
60–70 ꢀC for 3 h and then cooled to room temperature.
The mixture was diluted with water (20 mL) and the aqueous
solution was extracted with ether (3ꢂ30 mL), washed with
water (20 mL), and dried over CaCl2. Evaporation of the sol-
vent gave an oil whose 1H NMR spectrum indicated a mix-
3.4.3. 5,6,8-Tribromo-6,7,8,9-tetrahydro-5H-benzo[a]cy-
cloheptene (14). Colorless plates (the fourth fraction), mp
105–106 ꢀC from ether/hexane. 1H NMR (200 MHz,
CDCl3) d 7.38–7.15 (m, 4H, ArH), 5.43 (d, J¼5.2 Hz, 1H,
H5), 4.70 (m, 1H, H6), 4.50 (tt, J¼12.1, 2.1 Hz, 1H, H8),
3.96 (dd, A-part of AB-system, J¼14.3, 12.1 Hz, 1H, H9),
3.54 (ddd, A-part of AB-system, J¼14.9, 10.8, 3.0 Hz, 1H,
H7), 3.42 (dt, B-part of AB-system, J¼14.3, 2.0 Hz, 1H,
H8), 2.90 (dm, B-part of AB-system, J¼14.9 Hz, 1H, H7);
13C NMR (APT, 50 MHz, CDCl3) d 139.8, 138.3, 134.0
(ꢃ), 133.4 (ꢃ), 131.9 (ꢃ), 129.4 (ꢃ), 58.1 (ꢃ), 54.1 (ꢃ),
48.5, 47.6 (ꢃ), 47.3; nmax (KBr) 3018, 2337, 1492, 1433,
1367, 1253, 1205, 1170, 1118, 1083, 931 cmꢃ1; MS (EI,
m/z, % relative intensity) 386/384/382/380 (M+, 0.6/2.2/
2.3/0.6), 305/303/301 (M+ꢃBr, 12/22/11), 223.1/221.1
(M+ꢃ2Br, 50/47), 142.2 (72), 141.1 (M+ꢃ3Br, 100), 128.2
1
ture of 23 and 15 in a ratio of 84:16 (according to the H
NMR spectrum), respectively. The crude product was puri-
fied by preparative TLC (silica gel, hexane) to give 33 mg
(57%) of 23 and 1.7 mg (3%) of 15.
3.5.1. 1-Bromo-3-methylnaphthalene (23).5c,13 Pale yel-
1
low liquid, H NMR (200 MHz, CDCl3) d 8.20 (m, 1H,
ArH), 7.76–7.45 (m, 5H, ArH), 2.51 (s, 3H, –CH3); 13C
NMR (APT, 50 MHz, CDCl3) d 138.4, 137.1, 134.3 (ꢃ),
132.7, 130.0 (ꢃ), 129.3 (2 ꢂ,ꢃ), 129.1 (ꢃ), 128.7 (ꢃ),
125.0, 23.7 (ꢃ); nmax (film) 3051, 2920, 2856, 1599, 1558,
1497, 1365, 1259, 1207, 854, 748 cmꢃ1
.

D. Demirci-Gu€ltekin et al. / Tetrahedron 63 (2007) 8151–8156
8155
3.6. Reaction of 2-(bromomethyl)-1,2-dihydronaphtha-
lene 13 with potassium tert-butoxide
81%) as white needles, mp 101–102 ꢀC. 1H NMR
(200 MHz, CDCl3) d 7.32–7.12 (m, 4H, ArH), 6.34 (d,
J¼7.8 Hz, 1H, H1), 4.36 (dd, J¼9.7, 7.8 Hz, 1H, H2), 3.83
(dd, A-part of AB-system, J¼10.4, 5.3 Hz, 1H, –CH2Br),
3.72 (dd, B-part of AB-system, J¼10.4, 4.0 Hz, 1H,
–CH2Br), 3.21–2.99 (m, 2H, H4), 2.54 (m, 1H, H3), 2.22
(s, 3H, –CH3); 13C NMR (50 MHz, CDCl3) d 172.6, 136.1,
134.6, 130.6 (2ꢂ), 129.6, 129.0, 77.3, 56.5, 43.3, 40.2,
34.9, 23.1; nmax (KBr) 3004, 2899, 2849, 1747, 1613,
The reaction was carried out as described above by using
200 mg (0.83 mmol) of 13 and 150 mg (1.34 mmol) of po-
tassium tert-butoxide. 2-Methylnaphthalene (83 mg, 65%)
was obtained as a white solid.
3.7. Reduction of tribromide 14
1478, 1425, 1374, 1230, 1107, 1072, 1039, 915 cmꢃ1
.
Metallic sodium (116 mg, 5.04 mmol) was cut into small
pieces and combined with 10 mL of anhydrous ether. This
was magnetically stirred at reflux under nitrogen atmo-
sphere while a mixture of tribromide 14 (127 mg,
0.33 mmol), tert-butanol (375 mg, 5.07 mmol), and ether
(5 mL) was added dropwise over a period of 30 min. After
stirring at reflux temperature for 48 h, the heat was removed
and methanol (0.5 mL) and then water (1 mL) were added
dropwise. The mixture was diluted with water (20 mL)
and the aqueous solution was extracted with ether
(3ꢂ50 mL), washed with water (20 mL), and dried over
CaCl2. After removal of the solvent, the residue was chro-
matographed on silica gel with hexane yielding 41 mg
(85%) of 25.14
Anal. Calcd for C11H12Br2O: C, 41.28; H, 3.78. Found: C,
41.01; H, 3.95.
3.10. exo-Ethyl-1a,2,7,7a-tetrahydro-1H-cyclopropa[b]-
naphthalene-1-carboxylate (26)
The exo-ester 26 was prepared in 42% yield as described in
the literature.16 Comparison of the spectral data of this com-
pound with those reported in the literature4b was in full
agreement.
3.11. Bromination of exo-ester 26 at 77 ꢀC
The reaction was carried out as described above by using
110 mg (0.51 mmol) of 26 and 170 mg (1.06 mmol) of bro-
mine. After removal of the solvent, the residue was filtered
through a short silica gel column and eluted with chloro-
form. Evaporation of the solvent afforded an oil, which
was crystallized from ether/hexane to give the dibromo ester
2717 (118 mg, 62%).
3.7.1. 6,7,8,9-Tetrahydro-5H-benzo[a]cycloheptene
(25).14 Pale yellow oil, 1H NMR (200 MHz, CDCl3) d 7.10
(s, 4H, ArH), 2.84 (m, 4H, benzylic protons), 1.88 (m,
2H), 1.71 (m, 4H); 13C NMR (APT, 50 MHz, CDCl3)
d 144.4, 130.2 (ꢃ), 127.2 (ꢃ), 38.1, 34.1, 29.7; nmax (film)
2922, 2854, 1454, 746 cmꢃ1
.
3.11.1. exo-Ethyl-2,7-dibromo-1a,2,7,7a-tetrahydro-1H-
cyclopropa[b]naphthalene-1-carboxylate (27).17 White
solid, mp 123–124 ꢀC, 1H NMR (200 MHz, CDCl3)
d 7.36–7.24 (m, 4H, ArH), 5.66 (t, J¼1.4 Hz, 2H, benzylic),
4.12 (q, J¼7.2 Hz, 2H, –OCH2), 2.75 (dt, J¼4.1, 1.4 Hz, 2H,
cyclopropane), 1.46 (t, J¼4.1 Hz, 1H, cyclopropane), 1.24
(t, J¼7.2 Hz, –CH3); 13C NMR (APT, 50 MHz, CDCl3)
d 172.1, 134.3, 132.3 (ꢃ), 131.9 (ꢃ), 63.1, 45.9 (ꢃ), 30.3
(ꢃ), 28.1 (ꢃ), 16.2 (ꢃ); nmax (KBr) 2970, 2926, 1690,
3.8. Reaction of tribromide 14 with potassium
tert-butoxide
The reaction was carried out as described for 16 by using
195 mg (0.51 mmol) of 14 and 125 mg (1.21 mmol) of
potassium tert-butoxide. After workup, the residue was
purified by TLC (silica gel, hexane) to give 8 mg (7%) of
23 and 50 mg (44%) of 24.
1441, 1361, 1176, 1140, 1016, 913 cmꢃ1
.
3.8.1. 9-Bromo-5H-benzo[a]cycloheptene (24). Pale yel-
low oil, H NMR (200 MHz, CDCl3) d (200 MHz, CDCl3)
1
7.86 (dd, J¼7.5, 1.8 Hz, 1H, ArH), 7.42–7.11 (m, 4H, H8
and ArH), 5.99–5.84 (m, 2H, H6 and H7), 3.09 (d,
J¼5.8 Hz, 2H, H5); 13C NMR (APT, 50 MHz, CDCl3)
d 139.7, 137.3, 133.2 (ꢃ), 131.5 (ꢃ), 130.8 (ꢃ), 130.3
(ꢃ), 128.3, 128.1 (ꢃ), 127.2 (ꢃ), 126.9 (ꢃ), 35.8; nmax
(film) 3028, 2924, 2850, 1585, 1439, 1169, 1043, 896,
760, 708 cmꢃ1; MS (EI, m/z, % relative intensity) 220/218
(M+, 2), 139 (M+ꢃBr, 100), 115 (85), 86 (14). Anal.
Calcd for C11H9Br: C, 59.76; H, 4.10. Found: C, 59.48; H,
4.21.
Acknowledgements
The authors are indebted to the Department of Chemistry
€
(Faculty of Science and Faculty of Education) at Ataturk
University and TUBITAK (MISAG-216) for financial sup-
port of this work and the State Planning Organization of
Turkey (DPT) for purchasing a 200 MHz NMR and liquid
nitrogen cryogenerator.
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