Polybrominated methoxy- and hydroxynaphthalenes

Article abstract of DOI:10.3906/kim-1506-28

Regio- and stereoselective synthesis are described for convenient preparation of hydroxy- and methoxynaphthalenes starting from naphthalene (1). cis,cis,trans-2,3,5,8-Tetrabromo-4-methoxy-1,2,3,4-tetrahydronaphthalen-1-ol (6), cis,cis,trans-2,3,5,8-tetrabromo-1,4-dimethoxy-1,2,3,4-tetrahydronaphthalene (7), and cis,cis,cis-2,3,5,8-tetrabromo-1,4-dimethoxy-1,2,3,4-tetrahydronaphthalene (8) were obtained with silver-induced substitution of trans,cis,trans-1,2,3,4, 5,8-hexabromo-1,2,3,4-tetrahydronaphthalene (3). Base-promoted aromatization of dimethoxides 7 and 8 afforded 3,5,8-tribromo-1-methoxynaphthalene (9) and 2,5,8-tribromo-1-methoxynaphthalene (10). The reaction of 6 with sodium methoxide formed compounds 10 and 3,5,8-tribromonaphthalen-1-ol (16). Bromination of 9 and 16 with Br² in dichloromethane at room temperature produced 2,3,5,8-tetrabromo-1-methoxynaphthalene (14) and 2,3,4,5,8-pentabromonaphthalen-1-ol (18), respectively, while compound 10 did not react in the same conditions. Pyridine-induced elimination of hexabromide 3 afforded 1,4,6-tribromnaphthalene (21) in 99% yield and thermolysis of the hexabromide 3 gave mainly 1,4,6,7-tetrabromonaphthalene (22). Tetrabromide 22 was transformed to 1,4,6,7-tetramethoxynaphthalene (23) by copper-assisted nucleophilic substitution reaction.

Full text of DOI:10.3906/kim-1506-28

Turk J Chem
Turkish Journal of Chemistry
(2016) 40: 332 – 346
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http://journals.tubitak.gov.tr/chem/
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c
⃝ TUBITAK
doi:10.3906/kim-1506-28
Research Article
Polybrominated methoxy- and hydroxynaphthalenes
Kıymet BERKIL AKAR1,∗, Osman C¸ AKMAK2, Tuncay TUNC¸ 3
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1Department of Bioengineering, Faculty of Engineering and Natural Science, Gaziosmanpa¸sa University,
Tokat, Turkey
2
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Department of Chemistry, Faculty of Arts and Science, Yıldız Technical University, Istanbul, Turkey
3Department of Science Education, Faculty of Education, Aksaray University, Aksaray, Turkey
Received: 14.06.2015
•
Accepted/Published Online: 26.10.2015
•
Final Version: 02.03.2016
Abstract: Regio- and stereoselective synthesis are described for convenient preparation of hydroxy- and methoxynaph-
thalenes starting from naphthalene (1). cis,cis,trans-2,3,5,8-Tetrabromo-4-methoxy-1,2,3,4-tetrahydronaphthalen-1-ol
(6), cis,cis,trans-2,3,5,8-tetrabromo-1,4-dimethoxy-1,2,3,4-tetrahydronaphthalene (7), and cis,cis,cis-2,3,5,8-tetrabromo-
1,4-dimethoxy-1,2,3,4-tetrahydronaphthalene (8) were obtained with silver-induced substitution of trans,cis,trans-1,2,3,4,
5,8-hexabromo-1,2,3,4-tetrahydronaphthalene (3). Base-promoted aromatization of dimethoxides 7 and 8 afforded
3,5,8-tribromo-1-methoxynaphthalene (9) and 2,5,8-tribromo-1-methoxynaphthalene (10). The reaction of 6 with
sodium methoxide formed compounds 10 and 3,5,8-tribromonaphthalen-1-ol (16). Bromination of 9 and 16 with
Br2 in dichloromethane at room temperature produced 2,3,5,8-tetrabromo-1-methoxynaphthalene (14) and 2,3,4,5,8-
pentabromonaphthalen-1-ol (18), respectively, while compound 10 did not react in the same conditions. Pyridine-induced
elimination of hexabromide 3 afforded 1,4,6-tribromnaphthalene (21) in 99% yield and thermolysis of the hexabromide 3
gave mainly 1,4,6,7-tetrabromonaphthalene (22). Tetrabromide 22 was transformed to 1,4,6,7-tetramethoxynaphthalene
(23) by copper-assisted nucleophilic substitution reaction.
Key words: Methoxynaphthalene, silver-induced substitution, base-promoted elimination, hydroxynaphthalene, bromon-
aphthalene
1. Introduction
Substituted naphthalenes have gained significant industrial importance, especially in the field of pharmaceutical,
optical, and electronic materials. They are useful precursors for various natural products,1,2 e.g., their bromi-
nated analogues are admirable predecessors for the preparation of substituted naphthalenes such as phenols,
amines, ethers, thioethers, epoxides, and organometallics.
Despite the considerable synthetic and biological interest in naphthalene derivatives, very few general
synthetic routes are available, starting from naphthalene’s core.3 Therefore, many researchers are trying
to explore the new synthetic methods for the preparation of polysubstituted naphthalenes, and the bromo
naphthalenes may be the most feasible intermediates.
This paper describes mainly the preparation of new members of polysubstituted –OCH3 and –OH
naphthalenes from hexabrominated naphthalene 3 (prepared from 1 in two steps) by silver-induced reaction
followed by a base-promoted substitution, which were further functionalized via simple bromination.
∗Correspondence: kiymet.berkilakar@gop.edu.tr
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We found that hexabromide 3 is very useful intermediate for the polyfunctionalization of naphthalene and
regioselective novel brominated methoxy and hydroxy derivatives of naphthalene, whose further transformation
generates synthetically important novel naphthalene derivatives.
2. Results and discussion
Our recent report showed that photobromination of 9,10-dibromoanthracene (4) results in the formation of
one stereoisomer, hexabromide 5 as the sole product,4 while its polar bromination gave entirely different
stereoisomers.5 On the other hand, the sequential regioselective photobromination of naphthalene gave corre-
sponding hexabromide 3 with the same stereochemistry as that of anthracene hexabromide 5 (Scheme 1).4
°
Scheme 1. Polar and photobromination of naphthalene (1) and 9,10-dibromoanthracene (4).
In this paper, we demonstrate that hexabromide 3, which is produced by photobromination of 1,4-
dibromonaphthalene (2)6 is a key compound for tetralin and naphthalene derivatives.
The treatment of methanolic solution of hexabromide 3 reacted with 2 equiv of AgSO4 in ambient
conditions for 4 days in a dark and inert atmosphere gave a mixture of brominated derivates. Careful 1 H NMR
examination of the crude mixture revealed that exclusively derivative 6 was observed in 51% isolated yield.
Furthermore, compound 6 was isolated by the crystallization of the crude mixture, while the other derivatives,
compounds 7–10, were separated by preparative thin layer chromatography of the residue (Scheme 2).
There was no evidence of formation of hydroxy isomer 11 in this case (Scheme 3). However, we suggest
a reaction mechanism in which the stereoselective formation of compound 6 occurs by the neighboring group
participation as depicted in Scheme 4. Benzylic carbocation A might be stabilized by the neighboring bromide
to generate an intermediate B, which blocks the upper face of the ring and allows the methoxy group to approach
from the rear face to yield stereochemically controlled species C. The second benzylic carbocation F blocked
the lower face due to bromides, which may control the stereochemistry of the incoming sulfate group. The
nucleophilic attack of CH3 OH at the sulfur atom of sulfate ester G might proceed through breakage of the S–O
bond versus the C–O bond to form the corresponding alcohol 6.
The IR spectrum of 6 displayed the absorbance signal of the hydroxy group at 3455 cm−1
. The
appearance as a doublet at 2.86 ppm (J1,OH = 4 Hz) and a singlet at 3.97 ppm corresponded to hydroxy
and methoxy protons, respectively. The aromatic region consisted of two doublets at 7.49 ppm and 7.55 ppm
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°
°
Scheme 2. Silver induced-substitution of hexabromide 3 with Ag2 SO4 and AgClO4 .
6, 22.4093 kJ/mol
11, 25.6854 kJ/mol
Scheme 3. Two possible isomeric structures, 6 and 11, for hydroxymethoxide and their energy values and configurations
were established by a molecular mechanics program of CS Chem Office.3
(J6,7 = 8.4 Hz), while the aliphatic part showed the existence of four protons, H1 , H2 , H3 ,and H4 , at 5.48, 4.62,
5.06, and 4.72 ppm, respectively, and their coupling constants were also in agreement with their position and
configuration at the ring. Ultimately, the structure and configuration were determined by X-ray crystallographic
analysis. A single crystal of 6 was obtained as fawn crystals by slow evaporation from MeOH enantiomers that
crystallizes in the orthorhombic enantiomorphous space group P 21 21 21 with Z = 12 (Z′ = 3). There are
three molecules in the asymmetric unit; two of these have (RSRR) and the other has (SRSS) stereocenters
(Figure).
Only one possible isomeric structure is available for unsymmetrical dimethoxide 7 in this reaction, which
is further confirmed by 1 H NMR analysis. The splitting pattern of dimethoxy 7 consisted of six signals, two
singlets and four multiplets. The singlets belonged to two methoxy protons while the multiplets at 7.52, 5.00,
4.76 and 4.74 ppm corresponded to H6−7 , H4 , H1 and H2−3 , respectively. The 13 C NMR spectrum also
supports the asymmetry in the molecule with the expected eleven peaks.
One aromatic singlet, one aliphatic AA′ BB′ system, and one methoxy singlet in the 1 H NMR spectrum
of 8 are in agreement with the proposed structure. Moreover, the presence of six signals in the 13 C NMR
spectrum supports the symmetry. Two symmetrical stereoisomers, 12 and 8, can be expected to form (Scheme
5). However, only sterically hindered and high energy dimethoxide 8 was isolated, which has four substituents
on the same face of the ring system (Scheme 5).
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Scheme 4. The suggested reaction mechanism for the formation of 6, 7, and 8.
8, 38.5971 kJ/mol
12, 31.1361 kJ/mol
Scheme 5. Symmetrical dimethoxides 8 and 12.
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Figure. ORTEP diagram of compound 6. Thermal ellipsoids are drawn at the 30% probability level. Stereogenic centers
are as follows: C9(R), C10(S), C11(R), C12(R); C25(S), C26(R), C27(S), C28(S); C39(R), C40(S), C41(R), C42(R).
The formation of dimethoxides 7 and 8 also confirms that the reaction proceeds via carbocation interme-
diate D. Both sides of the intermediate D are available for incoming methoxy groups. The proposed mechanism
also explains the absence of the sterically less hindered and lower energy isomer 12 (Scheme 5).
It is important to note that when the reaction was performed using AgClO4 instead of Ag2 SO4 , it
resulted in the formation of dimethoxy compounds 7 and 8 (Scheme 2), which were isolated in 66% and 21%
yields, respectively. The difference between the two procedures can be attributed to the weak nucleophilicity of
perchlorate ion as compared to sulfate ion.
After selective synthesis, successful isolation, and characterization, compounds 7 and 8 were subjected
to sodium methoxide induced elimination at room temperature to afford aromatized products 1-methoxy-3-
Scheme 6. Aromatization of dimethoxides 7 and 8, followed by electrophilic bromination.
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bromide (9) and 1-methoxy-2-bromide (10), which were further separated by column chromatography (Scheme
6). The 1 H NMR spectra of 9 and 10 exhibited the expected aromatic coupling pattern (consisting of five
signal groups), an AB system (J3,4 = 8 Hz) between H3 and H4 in compound 10, and the meta coupling of
H2 and H4 in compound 9 (J2,4 = 2 Hz).
Therefore, for further functionalization, electrophilic bromination of compounds 9 and 10 was investigated
in the dark, at room temperature, with molecular bromine. The reaction of 10 with bromine did not produce the
4-bromo product 13 under any conditions probably due to the γ -gauche effect. However, bromination of 9 with
one equivalent of molecular bromine smoothly proceeded and 2,3,5,8-tetrabromo-1-methoxynaphthalene (14)
was obtained in a quantitative yield (Scheme 6). Compound 9 was treated with excess (four equiv) bromine,
but the reaction did not provide the 4-bromo product either.
One aromatic singlet, one aromatic AB system and one methoxy singlet in the 1 H NMR spectrum of 14
are in agreement with the proposed structure.
In order to introduce aromatization in compound 6, the same experiment was repeated as depicted in
Scheme 7. Surprisingly, no aromatized product 19 was observed in this case. Instead, 1-methoxy-2-bromide
(10) and 1-hydroxy-3-bromide (16) were isolated in yields of 46% and 13%, respectively (Scheme 7). The
product outcome of this reaction shows that the reaction proceeds through a selective intermediate 17 as shown
in Scheme 7.
Scheme 7.
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The structure of product 16 was elucidated by NMR and IR spectra. The stretching vibration band
at 3401 cm−1 in the IR spectrum suggests the presence of a hydroxyl group. The hydroxyl proton appeared
as a singlet at δ 8.22 and δ 153.6 in the 1 H and 13 C NMR spectra, which is typical aromatic carbon atom
resonance bonded to hydroxyl. In addition, H2 and H4 appeared as doublets at δ 7.29 and δ 8.10 and their
meta coupling constant J2,4 = 2 Hz confirms that Br is flanked between these two protons. The upfield shifting
of H4 is due to the electron donating mesomeric effect of the –OH group, while downfield shifting of H2 is
attributed to the van der Waals interaction of (C5 –Br)–H4 and (C3 –Br)–H4 .
For further functionalization, electrophilic bromination of compounds 16 was also investigated in the
dark, at room temperature, with excess bromine to give 2,3,4,5,8-pentabromonaphthalen-1-ol (18) in 98%
yield (Scheme 7). The 1 H NMR spectrum of pentabromide 18 consists of one AB system for H5 and H6
and one hydroxy singlet. Nine peaks (ten peaks are expected, only nine were observed due to overlap at
the aromatic region) in the 13 C NMR spectrum are also in accord with the proposed structure of 2,3,4,5,8-
pentabromonaphthalen-1-ol (18).
Treatment of hexabromide 3 with pyridine afforded tribromide 21 in high yield as the sole product, while
thermolysis of hexabromide 3 gave tetrabromide 22, besides tribromide 21 and 1,4-dibromide 2 in a ratio of
79:16:5 (1 H NMR) (Scheme 8). After repeated crystallization, tetrabromide 22 was obtained in 69% yield.
Da¸stan et al. also obtained the mixture of tribromide 21 and tetrabromide 22 (isolated yields: 37% and 55%,
respectively) by base-promoted elimination of other stereoisomeric hexabromides or hexabromide mixtures.7
°
Scheme 8. Base-induced elimination and thermolysis of hexabromide 3.
The 1 H NMR spectrum of tribromide 21 exhibited a singlet of H2 and H3 , a meta coupling doublet
(J5,7 = 2 Hz) for H5 , and an AB system for H7 and H8 . In the 13 C NMR spectrum ten lines in the aromatic
region support the suggested structure. The 1 H and 13 C NMR spectra of tetrabromide 22 indicate symmetry
in its structure. Proton NMR spectra exhibited two singlets at δ 8.50 and 7.62. Five lines in the 13 C NMR
signals are consistent with the assigned structure.
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It is clear that tetrabromide is a good precursor for the corresponding methoxy derivatives of naphthalene.
Therefore, tetrabromide 22 was treated with sodium methoxide in DMF in the presence of copper iodide.
Copper-assisted nucleophilic substitution afforded corresponding tetramethoxide 23 as the sole product (Scheme
8). The structure of 23 was proved by its 1 H NMR spectrum. The mass spectral analysis of compound 23
confirmed the molecular ion at m/z 248 and 13 C NMR was also in agreement with the structure.
In our previous study,8 we explored the synthesis, methanolysis reactions, and aromatization of hexabro-
mide 24 (Scheme 9). In that reaction, aromatization of dimethoxy tetralin 25 with sodium methoxide gave
tribromide 9 and dibromide 27 (Scheme 9). The same reaction for stereoisomeric dimethoxy tetralins 7 and 8
afforded tribromonaphthalenes 9 and 10.
°
°
Scheme 9. Synthesis of tribromo methoxynaphthalenes from 1,4-dibromonaphthalene (2).
On the other hand, reaction of 3 with Ag2 SO4 gave mainly hydroxyl compound 6, whereas the same
reaction using the reagent AgClO4 produced dimethoxy compounds (7 and 8).
Next, the silver-promoted methanolysis of stereoisomeric hexabromide 3 followed by aromatization was
discussed. The synthesis of diastereostereoisomer 7 and 8 by subsequent aromatization of 7 and 8 gave two
naphthalene derivatives, 9 and 10.
We also observed that the reactivity and selectivity of the tetralin hexabromide 3 strongly depends on
both the reaction conditions and their stereochemistry as shown in the case of silver perchloride or silver sulfate
(Scheme 2). Methanolysis of hexabromide 3 with silver sulfate produced an unexpected product, 6, whose
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°
Scheme 10. Synthesis of tetra- and penta-substituted naphthalene derivatives.
structure was established by X ray and a mechanism was suggested for the formation of hydroxyl compound
6. We also observed that the hydroxy and methoxy compounds exhibit different reactivities towards the base.
For example, while the methoxy compounds 7 and 8 (pure or mixture) give the same products (mixture of
9 and 10), the hydroxyl compound 6 produces 10 and 16. Further, the bromination of compound 9 and 16
was also studied. Compound 9 gives 14 while 10 did not brominate under the same reaction conditions. We
also observed different reactivity of hydroxyl 16 and methoxy 9 towards bromination. It is interesting that
bromination of hydroxyl 16 afforded pentabromide 18 (dibromination), whereas methoxy 9 reacted with only
one equiv of bromine (monobromination) in the same reaction conditions.
These studies showed that both hexabromide 3 and 1,4-dimethoxy compounds 7 and 8 are useful
intermediates for polyfunctionalization of naphthalenes. It is noteworthy that separation and crystallization of
methoxylated bromonaphthalenes are relatively easy compared to bromonaphthalenes, and this offers important
advantages for preparative purposes.
On the other hand, aromatization of hexabromide 3 opened up an efficient synthetic methodology for
both tribromide 21 and tetrabromide 22, which enables synthesis of the corresponding tri- and tetrasubstituted
naphthalene derivatives due to the bromine groups. Treatment of hexabromide 3 with pyridine afforded
tribromide 21 in high yields as the sole product, while thermolysis of hexabromide 3 mainly gave tetrabromide
22, which is easily separated from the mixture with a simple crystallization method.
Tetrabromide 22 is good precursor for corresponding tetrasubstituted naphthalenes as shown by the
synthesis of tetramethoxy naphthalene 23.
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3. Experimental
3.1. General
Thin layer chromatography was carried out on Merck silica F254 0.255 mm plates and spots were visualized
with UV fluorescence at 254 nm. Classic column chromatography was performed using Merck 60 (70–230 mesh)
silica gel. Melting points were determined on a Thomas-Hoover capillary melting point apparatus. Solvents
were concentrated at reduced pressure. IR spectra were recorded on a Jasco FT/IR 430 instrument. Mass
spectra were recorded on GC-MS PerkinElmer Clarus 500 under electron impact (EI) conditions. 1 H and 13 C
NMR spectra were recorded on a 400 (100) MHz Bruker spectrometer with CDCl3 as solvent. The single crystal
of the racemate 6 was used for data collection on a Bruker SMART BREEZE CCD diffractometer. Starting
material 1,4-dibromonaphthalene (2) was prepared according to our previous method starting from naphthalene
1 or 1-bromonaphthalene.6
3.2. Reaction of hexabromide 3 with silver sulfate
To a solution of hexabromide 3 (obtained from bromination of 1,4-dibromonaphthalene (2) (0.583 g, 0.963
mmol) in dry methanol (90 mL)) was added Ag2 SO4 (0.660 g, 2.12 mmol) under a nitrogen gas atmosphere in
the dark at room temperature. The resulting reaction mixture was stirred magnetically at room temperature
for 4 days. Reaction progress was monitored by TLC and 1 H NMR. After the removal of the silver bromide
by filtration, the reaction material was diluted with dichloromethane (100 mL) and washed with H2 O (3 × 50
mL). The organic layer was dried over Na2 SO4 and concentrated at reduced pressure. The residue (380 mg)
was analyzed by 1 H NMR. Product ratios were 68:16:7:4:5 for 6, 7, 8, 9, and 10, respectively. Recrystallization
from dichloromethane–hexane mixture in the refrigerator gave the hydroxymethoxide 6.
cis,cis,trans-2,3,5,8-Tetrabromo-4-methoxy-1,2,3,4-tetrahydronaphthalen-1-ol (6): Colorless
crystals, yield 51%, 244 mg, mp 155–157 ◦ C, Rf = 0.60 (1:9 ethyl acetate/hexane). 1 H NMR (400 MHz,
CDCl3) δ 7.55 (A part of AB system, J6,7 8.4 Hz, 1H, H6), 7.49 (B part of AB system, J6,7 8.4 Hz, 1H, H7),
5.48 (brt, 1H, H1), 5.06 (dd, J3,4 4 Hz, J2,3 2.8 Hz, 1H, H3), 4.72 (d, J3,4 4 Hz, 1H, H4), 4.62 (brt, 1H,
H2), 3.97 (s, 3H, OCH3), 2.86 (d,J1,OH 4 Hz, 1H); 13 C NMR (100 MHz, CDCl3) δ 136.9, 135.4, 135.2, 134.5,
125.6, 124.6, 78.7, 73.8, 63.7, 51.9, 48.9; MS (GCMS) m/z 494 (M+), 415 (M+ -Br), 397 (M+ -Br-H2 O-H),
379/381/383/385 (M+ -Br-OCH3 -4H), 362/364/366/368 (M+ -Br-H2 O-OCH3 -4H), 353, 331/333/335 (M+ -
2Br-3H), 314/316/318 (M+ -2Br-H2 O-2H), 300/302/304 (M+ -2Br-OCH3 -2H), 284/286/288 (M+ -2Br-H2 O-
OCH3 -2H), 271/273/275, 252/254 (M+ -3Br-2H), 236/238, 222/224 (M+ -3Br-OCH3 -H), 207, 193/195, 174
(M+ -4Br), 167, 156 (M+ -4Br-H2 O), 143 (M+ -4B-OCH3), 126, 115, 113, 102, 98, 87, 74, 63, 57, 50, 45, 39; IR
(νmax , cm−1): 3455, 3060, 2971, 2933, 2840, 1733, 1716, 1698, 1683, 1652, 1635, 1616, 1558, 1540, 1521, 1508,
1438, 1411, 1390, 1363, 1338, 1315, 1253, 1224, 1193, 1135, 1122, 1060, 1018, 952, 923, 842, 809, 771, 754, 642,
605, 578, 526, 418; Anal. Calcd for C11 H10 Br4 O2 (493.81): C, 26.75; H, 2.04%. Found: C, 26.78; H, 2.04%.
Preparative thin-layer chromatography of the residue on silica gel with ethyl acetate–hexane (1:9) gave
four products.
The first fraction: 3,5,8-Tribromo-1-methoxynaphthalene (9). Colorless crystals, yield 2%, 7 mg,
mp 125–127 ◦ C, Rf = 0.78 (1:9 ethyl acetate/hexane). 1 H NMR (400 MHz, CDCl3) δ 8.10 (d, J2,4 2 Hz,
1H, H4), 7.63 (A part of AB system, J6,7 8 Hz, 1H, H5), 7.58 (B part of AB system,J6,7 8 Hz, 1H, H8), 7.07
(d, J2,4 2 Hz, 1H, H3), 3.99 (3H, s, OCH3); 13 C NMR (100 MHz, CDCl3) δ 156.4, 135.5, 133.1, 131.5, 123.5,
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122.7, 122.2, 121.2, 116.7, 111.3, 55.9 (OCH3); MS (GCMS) m/z 392/394396/398 (M+ -2H), 377/379/381/383
(M+ -CH3 -2H), 362/364/366/368 (M+ -OCH3), 349/351/353/355, 315 (M+ -Br), 300, 284 (M+ -OCH3 -Br),
270/272/274, 234/236 (M+ -2Br), 221, 204, 206 (M+ -OCH3 -2Br), 191/193, 167, 155 (M+ -3Br), 141, 125
(M+ -OCH3 -3Br), 117, 112, 102, 98, 96, 86, 74, 62, 56, 50, 38; IR (νmax , cm−1): 3087, 3002, 2956, 2923, 2852,
1843, 1714, 1697, 1683, 1635, 1596, 1575, 1556, 1488, 1457, 1442, 1382, 1349, 1290, 1255, 1195, 1178, 1126,
1087, 995, 906, 836, 821, 808, 765, 665, 580; Anal. Calcd for C11 H7 Br3 O (394.88): C, 33.46; H, 1.79%. Found:
C, 33.52; H, 1.78%.
The second fraction: 2,5,8-Tribromo-1-methoxynaphthalene (10). Colorless crystals, yield 1%, 3
mg, mp 111–112 ◦ C, Rf = 0.85 (1:9 ethyl acetate/hexane). 1 H NMR (400 MHz, CDCl3) δ 8.02 (A part
of AB system, J6,7 9.2 Hz, 1H, H7), 7.77 (B part of AB system,J6,7 9.2 Hz, 1H, H6), 7.70 (A part of AB
system,J3,4 8 Hz, 1H, H4), 7.61 (B part of AB system,J3,4 8 Hz, 1H, H3), 3.94 (s, 3H, OCH3); 13 C NMR
(100 MHz, CDCl3)δ 153.0, 134.3, 134.0, 132.3, 130.8, 128.2, 125.5, 123.0, 117.6, 115.4, 62.4; MS (GCMS) m/z
392/394/396/398 (M+ -2H), 377/379/381/383 (M+ -CH3 -2H), 349/351/353/355, 316 (M+ -Br), 301, 284 (M+ -
OCH3 -Br), 270/272/274, 252, 236 (M+ -2Br), 219, 206 (M+ -OCH3 -2Br), 204, 191/193, 165, 155 (M+ -3Br),
141/143, 136, 125 (M+ -OCH3 -3Br), 119, 112, 98, 86, 77, 74, 69, 62, 55, 50, 38; IR (νmax , cm−1): 3068, 2998,
2931, 2846, 1866, 1749, 1716, 1698, 1683, 1652, 1575, 1548, 1452, 1392, 1319, 1280, 1232, 1180, 1068, 981, 906,
819, 802, 786, 769, 678, 628; Anal. Calcd for C11 H7 Br3 O (394.88): C, 33.46; H, 1.79%. Found: C, 33.55; H,
1.78%.
The third fraction: cis,cis,trans-2,3,5,8-tetrabromo-1,4-dimethoxy-1,2,3,4-tetrahydronaphtha-
lene (7). Colorless crystals, yield 7%, 32 mg, mp 155–157 ◦ C, Rf =0.60 (1:9 ethyl acetate/hexane). 1 H
NMR (400 MHz, CDCl3)δ 7.52 (m, 2H, H6 and H7), 5.00 (m, 1H, H4), 4.96 (m, 1H, H1), 4.74 (m, 2H, H2
and H3), 4.00 (s, 3H, OCH3), 3.63 (s, 3H, OCH3); 13 C NMR (100 MHz, CDCl3) δ 137.2, 135.1, 134.6, 133.6,
126.2, 125.4, 82.5, 78.8, 64.5, 58.9, 48.4; MS (GCMS) m/z 425/427/429/431 (M+ -Br-H), 393/395/397/399
(M+ -Br-OCH3 -4H), 362/364/366/368 (M+ -Br-2OCH3 -2H), 353, 347 (M+ -2Br-H), 333, 314/316/318 (M+ -
2Br-OCH3 -3H), 301, 289, 286 (M+ -2Br-2OCH3), 271/273/275, 263, 251, 236/238, 223, 206, 193/195, 169,
158, 143, 126, 113, 102, 99, 87, 75, 63, 50, 45, 39; IR (νmax , cm−1): 3066, 2962, 2940, 2927, 2898, 2829, 1612,
1434, 1365, 1317, 1265, 1193, 1128, 1068, 973, 931, 821, 763, 692, 638, 611, 582, 534, 501, 420; Anal. Calcd for
C12 H12 Br4 O2 (507.84): C, 28.38; H, 2.38%. Found: C, 28.29; H, 2.38%.
The last fraction: cis,cis,cis-2,3,5,8-tetrabromo-1,4-dimethoxy-1,2,3,4-tetrahydronaphthalene
(8). Colorless crystals, yield 3%, 12 mg, mp 145–146 ◦ C, Rf =0.42 (1:9 ethyl acetate/hexane). 1 H NMR
(400 MHz, CDCl3)δ 7.49 (s, 2H, H6 and H7), 4.76 (A part of AA′ BB′ system, 2H, H1 and H4), 4.63
(B part of AA′ BB′ , 2H, H2 and H3), 3.77 (s, 3H, OCH3); 13 C NMR (100 MHz, CDCl3)δ 135.9, 135.0,
124.8, 78.7, 60.2, 50.8; MS (GCMS) m/z 446 (M+ -2OCH3), 425/427/429/431 (M+ -Br-H), 397 (M+ -Br-
OCH3), 363/365/367/369 (M+ -Br-2OCH3 -3H), 346 (M+ -2Br-2H), 331, 316 (M+ -2Br-OCH3 -H), 307, 303,
284/286/288 (M+ -2Br-2OCH3), 271/273/275, 263, 251, 236/238, 223, 207, 193/195, 182, 169, 158, 143, 126,
113, 99, 87, 75, 63, 56, 50, 45, 39; IR (νmax , cm−1): 3056, 2956, 2927, 2913, 2821, 1731, 1558, 1538, 1457,
1430, 1396, 1340, 1322, 1301, 1265, 1226, 1191, 1135, 1076, 1020, 968, 910, 809, 765, 744, 624, 543, 528, 474,
433, 410; Anal. Calcd for C12 H12 Br4 O2 (507.84): C, 28.38; H, 2.38%. Found: C, 28.45; H, 2.37%.
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3.3. Reaction of hexabromide 3 with 2 equiv silver perchlorate
To a stirred solution of hexabromide 3 (0.950 g, 1.57 mmol) in dry and freshly distilled methanol (140 mL) was
added silver perchloride (0.715 g, 3.45 mmol) under an argon gas atmosphere in the dark. The resulting reaction
mixture was stirred magnetically at room temperature for 6 days. Reaction progress was monitored by TLC
for the consumption of the starting material. After completion of the reaction, the precipitated silver bromide
was removed by filtration. The reaction mixture was diluted with methylene chloride (50 mL), washed with
water (3 × 50 mL), dried over Na2 SO4 , and concentrated at reduced pressure. Then the relative percentages
of the products were determined by 1 H NMR as 74:26 for dimethoxide 7 and dimethoxide 8, respectively.
The residue (727 mg) was chromatographed on silica gel (70 g), eluting with hexane and ethyl acetate–hexane
(5:95) to give cis,cis,trans-2,3,5,8-tetrabromo-1,4-dimethoxy-1,2,3,4-tetrahydronaphthalene (7) (524 mg, 66%)
and cis,cis,cis-2,3,5,8-tetrabromo-1,4-dimethoxy-1,2,3,4-tetrahydronaphthalene (8) (167 mg, 21%).
3.4. Aromatization of dimethoxides 7 and 8
To a solution of the dimethoxides 7 and 8 (1 H NMR rate: 67:33) (0.415 g, 0.817 mmol) in freshly distilled
THF (20 mL) was added dropwise a solution of sodium methoxide (0.132 g, 2.45 mmol) in dried THF (20 mL)
under argon atmosphere. The resulting reaction mixture was magnetically stirred for 12 h at 25 ◦ C. After the
reaction was complete (TLC), the reaction material was diluted with diethyl ether (50 mL) and washed with
water (3 × 40 mL). After drying with sodium sulfate and removing the solvent, 1 H NMR of the residue (0.273
g, total yield: 85%) showed two products in a ratio of 40:60 for 9 and 10, respectively. The residue (0.273 g)
was chromatographed over silica gel (50 g). Eluting with hexane gave 1-methoxy-3,5,8-tribromonaphtlanene (9)
(130 mg, 40%) and 1-methoxy-2,5,8-tribromonaphthalene (10) (87 mg, 27%).
3.5. Aromatization of hydroxymethoxide 6
A stirred solution of the hydroxymethoxide 6 (0170 g, 0.344 mmol) in dry and freshly distilled THF (20 mL)
was combined with a solution of sodium methoxide (0.056 g, 1.03 mmol) in freshly distilled THF (20 mL)
under argon atmosphere. The resulting reaction mixture was magnetically stirred for 12 h at 25 ◦ C. Ether (50
mL) was added to the reaction mixture and the resulting precipitate was washed with water (3 × 40 mL) and
dried over anhydride sodium sulfate. The solvent was evaporated. 1 H NMR of the residue (0.102 g) showed
two products in a ratio of 67:33 for 10 and 16, respectively. The residue (0.102 g) was chromatographed
on silica gel (15 g), eluting with hexane to give 1-methoxy-2,5,8-tribromonaphthalene (10) (62 mg, 46%) and
1-hydroxy-3,5,8-tribromonaphthalene (16).
1-Hydroxy-3,5,8-tribromonaphthalene (16). Colorless crystals, yield 13%, 17 mg, mp 106.5–107.5
◦ C, Rf =0.44 (1:9 ethyl acetate/hexane). 1 H NMR (400 MHz, CDCl3) δ 8.22 (s, 1H, OH), 8.10 (d, J2,4 2 Hz,
1H, H4), 7.61 (A part of AB system, J6,7 8 Hz, 1H, H7), 7.49 (B part of AB system, J6,7 8 Hz, 1H, H6), 7.29
(d, J2,4 1.6 Hz, 1H, H2); 13 C NMR (100 MHz, CDCl3) δ 153.6, 135.6, 131.6, 131.2, 123.1, 122.9, 122.8, 120.3,
117.8, 114.9; MS (GCMS) m/z 378/380/382/384 (M+), 351, 314, 299/301/303, 271/273/275, 235, 220/222,
193, 191, 167, 150, 143, 136, 117, 113, 96, 86, 77, 74, 63, 56, 50, 38; IR (νmax , cm−1): 3401, 3079, 2962, 2927,
2873, 1600, 1575, 1554, 1481, 1396, 1357, 1294, 1265, 1226, 1168, 1110, 1037, 923, 844, 831, 815, 759, 638, 582,
539; Anal. Calcd for C10 H5 Br3 O (380.86): C, 31.54; H, 1.32%. Found: C, 31.61; H, 1.31%.
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3.6. Bromination of 1-methoxy-3,5,8-tribromonaphthalene (9) with 1.2 equiv molecular bromine
The solution of 1-methoxy-3,5,8-tribromonaphthalene 9 (150 mg, 0.39 mmol) in CH2 Cl2 (1.5 mL) was protected
from light and to the solution was added Br2 (24 mg, 0.15 mmol) in one portion. The mixture was allowed
to stand at room temperature. Reaction progress was monitored by TLC. After completing the reaction in 2
days, the solvent was removed in vacuo to afford pure 2,3,5,8-tetrabromo-1-methoxynaphthalene (14).
2,3,5,8-Tetrabromo-1-methoxynaphthalene (14). Colorless crystals, yield 95%, 170 mg, mp 144–
146 ◦ C, Rf = 0.33 (hexane). 1 H NMR (400 MHz, CDCl3) δ 8.50 (s, 1H, H4), 7.72 (A part of AB system, J6,7
8
Hz, 1H, H6), 7.63 (B part of AB system, J6,7 8 Hz, 1H, H7), 3.94 (s, 3H, OCH3); 13 C NMR (100 MHz, CDCl3) δ
154.6, 134.5, 134.1, 131.6, 128.5, 126.8, 125.4, 121.8, 121.2, 115.6, 62.5; MS (GCMS) m/z 472/474/476 (M+ -
H), 457/459/461 (M+ -H-CH3), 429/431/433, 393, 380 (M+ -CH3 -Br), 364, 348/350/352/354, 312/314/316,
282/284/286, 269/271/273, 235, 218, 205, 190, 182, 166, 154, 142, 135, 124, 111, 102, 98, 95, 85, 74, 69, 61, 55,
50, 37; IR (νmax , cm−1): 3073, 2940, 2850, 1862, 1683, 1587, 1563, 1535, 1465, 1436, 1371, 1321, 1270, 1220,
1195, 1168, 1106, 1085, 981, 912, 856, 835, 817, 690, 671, 549; Anal. Calcd for C11 H6 Br4 O (473.78): C, 27.89;
H, 1.28%. Found: C, 27.91; H, 1.28%.
3.7. Bromination of 3,5,8-tribromonaphthalen-1-ol (16)
The solution of 3,5,8-tribromonaphthalen-1-ol (16) (50 mg, 0.026 mmol) in CH2 Cl2 (2.5 mL) was protected
from light and to the solution was added Br2 (19 mg, 0.12 mmol) in one portion. The mixture was allowed to
stand at room temperature. After completing the reaction (TLC) in 8 h the crude product was passed through
a short silica gel column eluting with CH2 Cl2 to give 2,3,4,5,8-pentabromonaphthalen-1-ol (18).
2,3,4,5,8-Pentabromonaphthalen-1-ol (18). Yellow crystals, yield 99%, 14 mg, mp 148–150 ◦ C,
Rf =0.46 (1:9 ethy acetate/hexane). 1 H NMR (400 MHz, CDCl3) δ 7.71 (A part of AB system, J6,7 8.4 Hz,
1H, H7), 7.66 (B part of AB system, J6,7 8 Hz, 1H, H6), 6.13 (s, 1H, OH); 13 C NMR (100 MHz, CDCl3) δ
174.4, 141.7, 141.0, 137.7, 131.3, 129.2, 123.4, 123.1, 96.1; IR (νmax , cm−1): 3446, 3104, 3060, 2962, 2921,
2850, 1670, 1592, 1558, 1438, 1278, 1238, 1193, 1108, 1072, 916, 844, 833, 815, 707, 696, 655, 590, 522, 503, 418;
Anal. Calcd for C10 H3 Br5 O (538.65): C, 22.30; H, 0.56%. Found: C, 22.27; H, 0.56%.
3.8. Synthesis of 1,4,6-tribromonaphthalene (21)
To a solution of the hexabromide 3 (0.410 g, 0.677 mmol) in toluene (20 mL) was added freshly distilled pyridine
(1.96 g, 24.8 mmol, 2 mL). The mixture was stirred magnetically at room temperature for 4 days. The mixture
was filtered and pyridine and solvent were then removed in vacuo. Tribromonaphthalene 21 was purified in 99%
yield (246 mg) by passing a short silica gel column eluting with hexane.
1,4,6-Tribromnaphthalene (21). Colorless crystals, yield 99%, 246 mg, mp 84–85 ◦ C, Rf =0, 89
(hexane) (Lit.7 : 86–87 ◦ C). 1 H NMR (400 MHz, CDCl3) δ 8.45 (d, J5,7 2 Hz, 1H, H5), 8.13 (A part of AB
system, J7,8 8.8 Hz, 1H, H8), 7.72 (d of B part of AB system, J7,8 8.8 Hz, J57 2 Hz, 1H, H7), 7.66 (s, 2H, H2
and H3); 13 C NMR (100 MHz, CDCl3) δ 133.9, 131.6, 131.5, 131.1, 130.5, 130.0, 129.6, 123.0, 122.5, 121.2; MS
(GCMS) m/z 362/364/366/368 (M+), 283/285/287 (M+ -Br), 204/206 (M+ -2Br), 182, 142, 125 (M+ -3Br),
103, 99, 86, 74, 62, 50; IR (νmax , cm−1): 3081, 3066, 2923, 2852, 1918, 1857, 1741, 1637, 1604, 1573, 1481, 1423,
1405, 1330, 1290, 1245, 1176, 1143, 1072, 964, 871, 815, 619, 484, 406; Anal. Calcd for C10 H5 Br3 (364.86): C,
32.92; H, 1.38%. Found: C, 32.97; H, 1.39%.
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3.9. Synthesis of 1,4,6,7-tetrabromonaphthalene (22)
Hexabromide 3 (1.295 g, 2.13 mmol) was put into a round bottom flask and heated by a heating mantle at
130–145 ◦ C for 10 h. 1 H NMR investigation of the reaction mixture (871 mg) showed formation of tetrabromide
22, tribromide 21, and 1,4-dibromide 2 in a ratio of 79:16:5, respectively. The solid was purified by repeated
crystallization procedures from dichloromethane to give pure tetrabromide 22.
1,4,6,7-Tetrabromonaphthalene (22). Colorless crystals, yield 69%, 652 mg, mp 169–170 ◦ C (lit.7 :
168–169 ◦ C), Rf =0.87 (hexane). 1 H NMR (400 MHz, CDCl3)δ 8.50 (s, 2H, H5 and H8), 7.62 (s, 2H, H2 and
H3); 13 C NMR (100 MHz, CDCl3)δ 132.4, 132.3, 131.3, 125.5, 121.0; MS (GCMS) m/z 440/442/444/446/448
(M+), 361/363/365/367 (M+ -Br), 282/284/286 (M+ -2Br), 222, 203/205 (M+ -3Br), 181, 154, 142, 124, 190
(M+ -4CH3), 175, 160, 147, 131, 124 (M+ -4Br), 101, 98, 85, 74, 62, 50, 37; IR (νmax , cm−1): 3073, 1863,
1739, 1643, 1562, 1457, 1403, 1294, 1247, 1191, 1170, 1110, 970, 875, 831, 819, 617; Anal. Calcd for C10 H4 Br4
(443.75): C, 27.07; H, 0.91%. Found: C, 27.08; H, 0.90%.
3.10. Synthesis of 1,4,6,7-tetramethoxynaphthalene 23
Freshly cut sodium (0.415 g, 18 mmol) was added to dry methanol (20 mL) under argon atmosphere. When
dissolution was completed, the warm solution was diluted with dry DMF (10 mL), which was followed by
the addition of copper(I) iodide (0.429 g, 2.25 mmol). After dissolution, 1,4,6,7-tetrabromonaphthalene (22)
(0.5 g, 1.13 mmol) in dry DMF (30 mL) was added. The reaction mixture was stirred magnetically under
argon gas atmosphere at reflux for 5 days. Reaction progress was monitored by TLC. After cooling to room
temperature, diethyl ether (70 mL) was added to the reaction, followed by washing with H2 O (4 × 50 mL).
Then the organic layer was separated, dried over Na2 SO4 , and concentrated at reduced pressure. 1,4,6,7-
Tetramethoxynaphthalene (23) was obtained in 66% yield (184 mg) after recrystallization of the crude product.
1,4,6,7-Tetramethoxynaphthalene (23). Colorless crystals, yield 66%, 184 mg, mp 117–119 ◦ C,
Rf =0.39 (1:9 ethylacetate/hexane). 1 H NMR (400 MHz, CDCl3) δ 7.53 (s, 2H, H5 and H8), 6.64 (s, 2H,
H2 and H3), 4.05 (s, 3H, OCH3), 3.98 (s, 3H, OCH3); 13 C NMR (100 MHz, CDCl3) δ 149.3, 148.6, 121.5,
101.9, 101.1, 55.9, 55.6; MS (GCMS) m/z 248 (M+), 233 (M+ -CH3), 218 (M+ -2CH3), 203 (M+ -3CH3), 190
(M+ -4CH3), 175, 160, 147, 131, 124, 119, 101, 91, 89, 76, 63, 50, 39; IR (νmax , cm−1): 3091, 2999, 2956, 2931,
2829, 2055, 1949, 1749, 1708, 1679, 1602, 1511, 1484, 1461, 1452, 1427, 1334, 1265, 1205, 1176, 1089, 1014, 970,
862, 811, 784, 723, 568; Anal. Calcd for C14 H16 O4 (248.27): C, 67.65; H, 6.50%. Found: C, 22.27; H, 6.53%.
3.11. X-ray analysis
The single crystal of the racemate 6 was used for data collection on a Bruker SMART BREEZE CCD diffrac-
˚
tometer. The graphite-monochromatized MoKα radiation (λ = 0.71073 A) and oscillation scans technique
with ∆ω = 5◦ for one image were used for data collection. The lattice parameters were determined by the
least-squares methods on the basis of all reflections with F2 > 2σ (F2). Integration of the intensities, cor-
rection for Lorentz and polarization effects, and cell refinement was performed using Bruker SAINT (Bruker
AXS Inc., 2012) software.9 The structure was solved by direct methods using SHELXS-97 10 and refined by a
full-matrix least-squares procedure also using SHELXL-97.9 H atoms were positioned geometrically and refined
using a riding model. The final difference Fourier maps showed no peaks of chemical significance. Crystal data
for 9: C15 H12 Br4 O2 ; crystal system, space group: orthorhombic, P 21 21 21 ; (no: 19); unit cell dimensions:
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◦
3
˚
˚
a = 10.9427(5), b = 22.4790(11), c = 20.1385(9) A, α = 90, β = 90, γ =90 ; volume: 4953.7(4) A ; Z = 12;
calculated density: 2.188 g cm−3 ; absorption coefficient: 9.749 mm−1 ; F (000): 3096; θ-range for data collec-
tion 1.4–28.5◦ ; refinement method: full-matrix least-square on F2 ; data/parameters: 6837/569; goodness-of-fit
on F2 : 0.920; final R indices [I > 2σ(I)]: R1 = 0.056, wR2 = 0.111 largest diff. peak and hole: 0.831 and
−3
˚
–0.782 e A
.
The crystallographic data (excluding structure factors) were deposited in CCDC-1038474 registration
number and are available free of charge upon request to CCDC, 12 Union Road, Cambridge, UK (fax: +44-
1223-336033, e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgments
The authors thank the Department of Chemistry at Gaziosmanpa¸sa University and Yıldız Technical University
for financial support. The authors acknowledge the Aksaray University, Science and Technology Application and
Research Center, Aksaray, Turkey, for the use of the Bruker SMART BREEZE CCD diffractometer (purchased
under grant No. 2010K120480 from the State of Planning Organization).
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