Syntheses of 1-bromo-8-methylnaphthalene and 1-bromo-5-methylnaphthalene

Article abstract of DOI:10.1021/acs.joc.5b00730

The Diels-Alder reaction between 2-methylfuran and 3-bromobenzyne (3), which was generated under mild conditions from 1,3-dibromobenzene and lithium diisopropylamide (LDA), gives a mixture of regioisomeric 1,4-dihydro-1,4-epoxynaphthalenes 4 and 5. A subsequent two-step deoxygenation affords the corresponding 1-bromo-8-methylnaphthalene (1) and 1-bromo-5-methylnaphthalene (2) in high yields.

Full text of DOI:10.1021/acs.joc.5b00730

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Note
Syntheses of 1-Bromo-8-methylnaphthalene
and 1-Bromo-5-methylnaphthalene
Evans O Onyango, Anne R Kelley, David C. Qian, and Gordon Wayne Gribble
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b00730 • Publication Date (Web): 29 Apr 2015
Downloaded from http://pubs.acs.org on May 3, 2015
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Syntheses of 1-Bromo-8-methylnaphthalene and 1-Bromo-5-methylnaphthalene
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Evans O. Onyango, Anne R. Kelley, David C. Qian, and Gordon W. Gribble*
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Department of Chemistry, Dartmouth College, Hanover, NH 03755-3564, USA
Br
Br
Me Br
Br
LDA, – 78 oC
Me
+
O
O
Br
O
Me
3
4
5
1. H2/Pd
2. HCl
1. HN=NH
2. HCl
Br
Me Br
1
2
Me
ABSTRACT
The Diels–Alder reaction between 2-methylfuran and 3-bromobenzyne (3), which was generated under
mild conditions from 1,3-dibromobenzene and lithium diisopropylamide (LDA), gives a mixture of
regioisomeric 1,4-dihydro-1,4-epoxynaphthalenes 4 and 5. A subsequent two-step deoxygenation affords
the corresponding 1-bromo-8-methylnaphthalene (1) and 1-bromo-5-methylnaphthalene (2) in high
yields.
In connection with a synthesis project we needed sufficient quantities of pure 1-bromo-8-
methylnaphthalene (1) and 1-bromo-5-methylnaphthalene (2). Because the literature procedures1-6 are
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either cumbersome or low yielding, and direct bromination of 1-methylnaphthalene leads mainly to 1-
bromo-4-methylnaphthalene or 1-(bromomethyl)naphthalene depending on the reaction conditions,7, 8 we
employed the method described herein to achieve a practical synthesis of 1 and 2. Thus, based on our
earlier studies of the Diels−Alder cycloaddition of 3-fluorobenzyne with 2-alkylfurans,9 we anticipated
that 3-bromobenzyne (3) would behave similarly in an approach to the desired isomeric naphthalenes 1
and 2.
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Br
Me Br
Me
1
2
As shown in Scheme 1, treatment of the commercially available 1,3-dibromobenzene with lithium
o
diisopropylamide (LDA) at −78 C generated the transient 3-bromobenzyne (3), which on warming to
ambient temperature in the presence of excess 2-methylfuran was trapped to give a 59% yield of a 3:2 (1H
NMR) mixture of separable 1,4-dihydro-1,4-epoxynaphthalenes 4 and 5. Flash column chromatography
followed by recrystallization from hexanes afforded 4 as white crystals and 5 as yellow oil. The
structures of these regioisomers were distinguished by NMR spectroscopy and by subsequent conversion
to the respective naphthalenes (vide infra). The observed regioselectivity slightly favoring syn-
cycloadduct 4 was consistent with our earlier studies.9 This method is reproducible and was utilized to
prepare multigram quantities of 4 and 5.
Me
Br
Br
Br
Me Br
O
O
LDA
+
O
rt
-78 oC
Br
59%
Me
4
5
3
Scheme 1
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Hydrogenation of the syn-cycloadduct 4 with hydrogen over Pd/C was not without incident
(Scheme 2). Somewhat surprisingly, our initial attempts achieved direct deoxygenation of 4 to the
desired 1-bromo-8-methylnaphthalene (1) instead of the expected epoxy naphthalene 6. However, we
also observed reductive debromination to give 1-methylnaphthalene (7). Analysis of a reaction aliquot by
TLC and 1H NMR showed that hydrogenation of the double bond occurs very rapidly to furnish 6, which
slowly dehydrates to 1 and then is further reductively debrominated to 7 upon prolonged hydrogenation.
By monitoring the reaction by TLC we could stop it after complete hydrogenation of the double bond in 4
to obtain a mixture consisting mainly of 6, a small amount of 1, and a trace of 7. Acid-catalyzed
dehydration of this mixture followed by recrystallization from hexanes gave 1 (65% yield for two steps)
as colorless crystals, consistent with the known 1-bromo-8-methylnaphthalene.1 Similarly, catalytic
hydrogenation of 5 to 8 and subsequent acid-catalyzed dehydration gave the known 1-bromo-5-
methylnaphthalene (2),10 a sequence that proceeded excellently without reductive debromination.
Because the reductive debromination involving 4 lowered the overall yield of 1, we chose a different
reduction protocol. Thus, treatment of a methanol solution of 4 with dipotassium azodicarboxylate
(PADA)11 and glacial acetic acid at room temperature, which generates diimide, afforded 6 in 99% yield
in less than 10 minutes. Dehydration in refluxing concentrated HCl gave 1-bromo-8-methylnaphthalene
(1) in 99% yield.
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Me Br
Me Br
Me Br
Me
Pd/C
+
+
O
O
MeOH
7
8
9
6
4
1
7
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reflux
65%
(2 steps)
conc.
HCl
Me Br
Me Br
Me Br
conc. HCl
KO2CN=NCO2K
O
O
MeOH, AcOH
99%
reflux
99%
4
6
1
Br
Br
Br
Pd/C
O
O
conc. HCl
MeOH
93%
reflux
75%
Me
Me
Me
8
2
5
Scheme 2
CONCLUSION
The simplicity and scalability of these twin reaction sequences provide a useful alternative to the
existing methods for the preparation of these two naphthalenes. Our results also showcase the generality
and utility of our earlier studies on the regioselective Diels−Alder reaction between 3-halobenzynes with
2-alkylfurans to afford substituted naphthalenes.
EXPERIMENTAL SECTION
The general supplemental methods are provided in the Supporting Information.
8-Bromo-1-methyl-1,4-dihydro-1,4-epoxynaphthalene (4) and 5-Bromo-1-methyl-1,4-dihydro-
1,4-epoxynaphthalene (5): To a solution of 1,3-dibromobenzene (22 g, 0.093 mol) in Et2O (120 mL) at
–78 °C was added dropwise LDA (2M in THF; 49 mL, 0.098 mol). The mixture was stirred for 1 hour at
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–78 °C, treated dropwise with 2-methylfuran (17.2 g, 0.209 mol), and allowed to warm to room
temperature overnight. The cooled reaction mixture was quenched with ice water and extracted twice with
Et2O. The organic layer was washed with brine, dried over anhydrous MgSO4, and concentrated in vacuo
to yield orange oil. The crude mixture was purified via silica gel chromatography (60/1 hexanes/ethyl
acetate) to afford 4 and 5 (13 g, 59%) as yellow-orange oils. The isomers were further separated by
crystallization from hexanes.
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8-Bromo-1-methyl-1,4-dihydro-1,4-epoxynaphthalene (4): colorless crystalline solid; mp 41–42
°C; Rf = 0.28 (15:1 hexanes/ethyl acetate); 1H NMR (500 MHz, CDCl3) δ 7.11 (d, J = 6.95 Hz, 1H), 7.07
(dd, J = 8.2, 0.55 Hz, 1H), 7.02 (dd, J = 5.4, 1.7 Hz, 1H), 6.84 – 6.79 (m, 2H), 5.60 (d, J = 1.85 Hz, 1H),
13
2.08 (s, 3H) ppm; C NMR (150 MHz, CDCl3) δ 153.7, 149.5, 145.5, 144.7, 130.1, 127.1, 118.8, 114.6,
91.8, 81.7, 17.5 ppm; IR (NaCl): ν 1577, 1448, 1125, 1076, 892, 841, 771, 741, 713 cm-1; HRMS (EI)
calcd for C11H9BrO [M]+: 235.9837, found 235.9831.
5-Bromo-1-methyl-1,4-dihydro-1,4-epoxynaphthalene (5): yellow oil; Rf = 0.33 (15:1
1
hexanes/ethyl acetate); H NMR (500 MHz, CDCl3) δ 7.09 – 7.08 (m, 2H), 7.06 (dd, J = 8.05, 0.6 Hz,
1H), 6.87 (dd, J = 8.2, 6.95 Hz, 1H), 6.81 (d, J = 5.45 Hz, 1H), 5.74 (d, J = 1.85 Hz, 1H), 1.92 (s, 3H)
13
ppm; C NMR (150 MHz, CDCl3) δ 154.0, 151.1, 145.9, 143.7, 128.1, 127.0, 117.5, 114.2, 90.4, 82.1,
15.3 ppm; IR (NaCl): ν 1585, 1449, 1382, 1302, 1069, 972, 857, 766, 714, 646 cm-1; HRMS (EI) calcd
for C11H9BrO [M]+: 235.9837, found 235.9836.
1-Bromo-8-methylnaphthalene (1): To a solution of 4 (150 mg, 0.633 mmol) in CH2Cl2/MeOH
(1:1; 6 mL) was added 10% Pd-C (66.2 mg, 0.0633 mmol). The mixture was stirred vigorously at room
temperature under H2 until TLC showed the absence of starting material. The mixture was filtered
through Celite and concentrated in vacuo to yield orange oil. Proton NMR revealed the crude as a
mixture of 1, 6, and 7. This crude material was refluxed with concentrated hydrochloric acid (3 mL)
overnight. The cooled reaction mixture was quenched with ice water and extracted twice with CH2Cl2.
The organic layer was washed with brine, dried over anhydrous MgSO4, and concentrated in vacuo to
give crude material. Recrystallization from hexanes gave 1 as a colorless crystalline solid (91 mg, 65%).
1
mp 77–78 °C [Lit. 2 mp 77–78 °C]; H NMR (600 MHz, CDCl3) δ 7.83 (dd, J = 7.4, 1.1 Hz, 1H), 7.77
(dd, J = 8.1, 0.78 Hz, 1H), 7.71 (dd, J = 7.0, 2.3 Hz, 1H), 7.37 – 7.33 (m, 2H), 7.22 – 7.20 (m, 1H), 3.13
(s, 3H) ppm; which is in agreement with that reported.1
8-Bromo-1-methyl-1,2,3,4-tetrahydro-1,4-epoxynaphthalene (6): To a solution of 4 (100 mg,
0.422 mmol) in MeOH (5 mL) was added potassium azodicarboxylate (248 mg, 1.28 mmol). The mixture
was stirred while a solution of glacial acetic acid (0.2 mL) in MeOH (3 mL) was added dropwise. The
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crude mixture was treated with water and extracted twice with EtOAc. The organic layer was washed
with brine, dried over anhydrous MgSO4, and concentrated in vacuo to afford 8-Bromo-1-methyl-1,2,3,4-
tetrahydro-1,4-epoxynaphthalene (6) as a pale yellow solid in quantitative yield, which was used in the
next step without further purification; mp 68–70 °C; 1H NMR (500 MHz, CDCl3) δ 7.28 (dd, J = 8.05, 0.7
Hz, 1H), 7.14 (d, J = 7.15 Hz, 1H), 7.00 (dd, J = 8.0, 7.2 Hz, 1H), 5.29 (d, J = 5.1 Hz, 1H), 2.24 – 2.18
(m, 1H), 2.00 (s, 3H), 1.79 – 1.74 (m, 1H), 1.58 – 1.53 (m, 1H), 1.43 – 1.38 (m, 1H) ppm; 13C NMR (150
MHz, CDCl3) δ 149.3, 145.3, 131.2, 128.5, 117.7, 113.9, 87.8, 78.1, 32.3, 29.4, 19.6 ppm; IR (NaCl): ν
1569, 1450, 1384, 1344, 1171, 1136, 1008, 912, 881, 834, 799, 754, 620 cm-1; HRMS (EI) calcd for
C11H11BrO [M]+: 237.9993, found 237.9999. The crude 6 was refluxed with concentrated hydrochloric
acid (5 mL) overnight. The cooled reaction mixture was quenched with ice water and extracted twice with
CH2Cl2. The organic layer was washed with brine, dried over anhydrous MgSO4, and concentrated in
vacuo to afford 1 as a pale brown solid in quantitative yield. The NMR agrees with that listed above.
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5-Bromo-1-methyl-1,2,3,4-tetrahydro-1,4-epoxynaphthalene (8): To a solution of 5 (150 mg,
0.633 mmol) in CH2Cl2/MeOH (1:1, 6 mL) was added 10% Pd-C (66.2 mg, 0.0633 mmol). The mixture
was stirred vigorously at room temperature under H2 until TLC showed the absence of starting material.
The mixture was filtered through Celite and concentrated in vacuo to yield brown oil (141 mg, 93%),
1
which was used in the next step without further purification. H NMR (500 MHz, CDCl3) δ 7.27 (d, J =
1.75 Hz, 1H), 7.25 (d, J = 1.75 Hz, 1H), 7.08 – 7.03 (m, 1H), 5.40 (d, J = 5.1 Hz, 1H), 2.25 – 2.19 (m,
13
2H), 1.83 (s, 3H), 1.56 – 1.42 (m, 2H) ppm. C NMR (150 MHz, CDCl3) δ 150.0, 146.6, 129.6, 128.6,
116.5, 113.8, 86.8, 78.9, 32.6, 28.3, 17.8 ppm. IR (NaCl): ν 1576, 1455, 1382, 1343, 863, 759, 620 cm-1.
HRMS (EI) calcd for C11H11BrO [M]+: 237.9993, found 238.0000.
1-Bromo-5-methylnaphthalene (2): The crude 8 was refluxed with concentrated hydrochloric acid
(3 mL) overnight. The cooled reaction mixture was quenched with ice water and extracted twice with
CH2Cl2. The organic layer was washed with brine, dried over anhydrous MgSO4, and concentrated in
vacuo to yield a mixture of brown and white solids. Recrystallization from hexanes gave 2 as a colorless
crystalline solid (98 mg, 75%). mp 58–59 °C [lit.4 mp 59–60 °C]; 1H NMR (500 MHz, CDCl3) δ 8.14 (d,
J = 8.6, 1H), 7.99 (d, J = 8.5, 1H), 7.79 (dd, J = 7.4, 1.0 Hz, 1H), 7.48 (dd, J = 8.5, 7.0 Hz, 1H), 7.38 –
7.34 (m, 2H), 2.71 (s, 3H) ppm; which is in agreement with that reported.10
ASSOCIATED CONTENT
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1H NMR and 13C NMR spectra for new compounds. This material is available free of charge at
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AUTHOR INFORMATION
Corresponding Author
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*gordon.w.gribble@dartmouth.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work was supported in part by the Donors of the Petroleum Research Fund, administered by the
American Chemical Society.
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