Cycloallenes, 13. - Cyclohexa-1,2,4-triene from 1-bromocyclohexa-1,4- diene

Article abstract of DOI:10.1002/(sici)1099-0690(200005)2000:10<1871::aid-ejoc1871>3.0.co;2-3

1-Bromocyclohexa-1,4-diene (3) was prepared from trans-4,5- dibromocyclohexene by elimination of hydrogen bromide. Treatment of a solution of 3 in furan or 2,5-dimethylfuran with KOtBu afforded the tetrahydroepoxynaphthalenes 4 and 5, respectively. The st

Full text of DOI:10.1002/(sici)1099-0690(200005)2000:10<1871::aid-ejoc1871>3.0.co;2-3

FULL PAPER
Cycloallenes, 13[°]
Cyclohexa-1,2,4-triene from 1-Bromocyclohexa-1,4-diene
Manfred Christl*[a] and Stefan Groetsch[a]
Dedicated to Dr. Karl Peters on the occasion of his 60th birthday with warm thanks for a very prolific
collaboration over many years
Keywords: Cycloallenes / Cycloadditions / Eliminations / Strained molecules / Rearrangements
1-Bromocyclohexa-1,4-diene (3) was prepared from trans-
4,5-dibromocyclohexene by elimination of hydrogen bro-
mide. Treatment of a solution of 3 in furan or 2,5-dimethylfu-
ran with KOtBu afforded the tetrahydroepoxynaphthalenes 4
and 5, respectively. The structure of these products is evi-
by KOtBu, and only upon the addition of 18-crown-6 did a
reaction occur. This reaction did not, however, furnish the
known [2+2] cycloadduct 9 of styrene and 1, but, instead, a
small amount of 1,2-diphenylethane (8) was formed. In
agreement with this finding, the conjecture that 1 was depro-
dence for the title cycloallene (isobenzene 1) being the reac- tonated to give the phenyl anion was confirmed by the treat-
tive intermediate. The compounds 4 and 5 were dehydrogen- ment of a solution of 3 and benzophenone in THF with
ated by DDQ to the known dihydroepoxynaphthalenes 6 and
7, respectively. These conversions unambiguously confirm
the structures of 4 and 5. In pure styrene, 3 was not attacked
KOtBu, which resulted in the formation of triphenylme-
thanol.
Introduction
A quantum chemical study using the AM1 method[18]
found 1b to be less stable than 1a by 2 kcal molϪ1, which
is only a slight deviation from the above result. However,
the heat of formation of 1a was computed to be consider-
ably different to the experimental value, i.e. only 72
kcal molϪ1 above that of benzene.[18] Later, a high level ab
initio calculation failed to get significantly closer to the ex-
perimental value, although it also favoured the allenic struc-
ture 1a.[19]
Here, we present a third access to the reactive interme-
diate 1, namely a β-elimination route, which allows us to
address the question of whether 1 is amenable to depro-
tonation with formation of the phenyl anion. In the use of
the DoeringϪMooreϪSkattebøl pathway to 1, this depro-
tonation is not apparent although methyllithium, a very
strong base, is the reagent.[3,7]
In spite of their extremely short lifetime, numerous six-
membered cyclic allenes including cyclohexa-1,2,4-triene (1)
can be trapped by addition and cycloaddition
reactions.[1Ϫ10] For a number of years, derivatives of 1 have
been considered to be formed initially as DielsϪAlder ad-
ducts in reactions of vinylacetylenes with acetyl-
enes.[1,11,12,13] Furthermore, two rearrangement reactions
are believed to proceed via derivatives of 1.[14,15] Conclusive
evidence for the parent isobenzene 1 was provided by the
isolation of interception products after exo-6-bromo-endo-
6-fluoro- or 6,6-dibromobicyclo[3.1.0]hex-2-ene had been
treated with methyllithium in the presence of an activated
olefin.[3,7] Another route to 1 is the electrocyclisation of
hexa-1,3-diene-5-yne.[16,17] A kinetic investigation of this re-
action in the presence of molecular oxygen gave the heat of
formation of 1 as 105 kcal molϪ1 [16]
As this value is 85
.
kcal molϪ1 greater than that of benzene, a comparison with
the value estimated from group equivalents indicates that 1
is more likely to be the diradical 1b than the allene 1a
(Scheme 1).
Results and Discussion
By treatment of 1-bromocyclohexene with potassium
tert-butoxide (KOtBu), Wittig and Fritze[20] generated
cyclohexa-1,2-diene and thus clearly demonstrated for the
first time the existence of a six-membered cyclic allene.
Later, such a β-elimination of hydrogen bromide was used
to access other six-membered cyclic allenes,[1,5,6,10] espe-
cially 1-oxacyclohexa-2,3-diene[5,6] and 3δ2-chromene (2,3-
didehydro-2H-1-benzopyran).[10] In the latter case, the
DoeringϪMooreϪSkattebøl approach is unsuccessful and,
thus, the β-elimination pathway provides the only route to
the reactive intermediate.[10]
Scheme 1
[°]
Part 12: Ref.[10]
Institut für Organische Chemie, Universität Würzburg,
[a]
Am Hubland, D-97074 Würzburg, Germany
Fax: (internat.) ϩ 49-(0)931/888-4606
E-mail: christl@chemie.uni-wuerzburg.de
Following the previous strategy,[20] we chose 1-bromocy-
clohexa-1,4-diene (3) as the precursor for 1 and prepared 3
Eur. J. Org. Chem. 2000, 1871Ϫ1874
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/0510Ϫ1871 $ 17.50ϩ.50/0
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M. Christl, S. Groetsch
FULL PAPER
ide a strong enough base for the β-elimination of HBr from
3, styrene, as a pure hydrocarbon, cannot do this. Therefore,
we added 18-crown-6 as a fourth component in order to
complex the potassium cation. The immediate darkening of
the mixture indicated the start of the reaction. However, the
expected [2ϩ2] cycloadduct 9[7] of 1 to styrene, previously
obtained by employing the DoeringϪMooreϪSkattebøl
route to 1, was not observed. Instead, we isolated 1,2-
diphenylethane (8), albeit in only 10% yield (Scheme 4).
Scheme 2
by heating of a mixture of trans-4,5-dibromocyclohexene (2)
and N,N-diethylaniline at 200Ϫ220 °C (Scheme 2). In addi-
tion to 3 (11Ϫ27% yield), benzene and cyclohexa-1,4-diene
were also formed. Whereas the latter is the result of a re-
ductive elimination,[21] benzene is formed from a two-fold
anti elimination and 3 from a syn elimination. This outcome
clearly demonstrates that, in this case, the syn elimination
is not as strongly disfavoured relative to the anti elimination
as textbooks try to make the reader believe.[22] Our result is
essentially analogous to the formation of 1-bromocyclohex-
ene, cyclohexa-1,3-diene, cyclohexene and benzene on heat-
ing of trans-1,2-dibromocyclohexane with quinoline.[20b]
Scheme 4
It is assumed that, under these reaction conditions, 3 is
converted into 1, from which two pathways to 8 are conceiv-
able. One of them involves the deprotonation of 1 by tert-
butoxide with formation of the phenyl anion, which could
add to the β-carbon atom of styrene as in the anionic poly-
merisation of styrene.[23] Thus generated, the corresponding
anion of 8 would be protonated by tert-butyl alcohol to give
8. Alternatively, the isobenzene 1 could undergo the known
[2ϩ2] cycloaddition with styrene with production of 9,[7]
which would have to be isomerised to 8 by the action of the
base, i.e. by deprotonation of the methylene group of the
cyclohexa-1,4-diene subunit, opening of the four-membered
ring, resulting in the anion of 8 mentioned above, and its
protonation. We have examined this possibility by a control
experiment. Indeed, the treatment of 9 with KOtBu in the
presence of 18-crown-6 gave rise to 8 in 76% yield. This
high yield militates against the formation of 9 as the major
pathway in the reaction of 3 with KOtBu in styrene, since
the yield of 8 should be much larger than 10% in this case.
In order to test whether the deprotonation of 1 can pro-
ceed under the reaction conditions, we tried to intercept
the suspected phenyl anion with an electrophile other than
styrene. For this purpose, we added KOtBu to a solution of
3 and benzophenone in THF and, indeed, obtained triphen-
ylmethanol (Scheme 5). The low yield (5%) may have its
origin in the possible protonation of the phenyl anion by
tert-butyl alcohol with formation of benzene, which was not
looked for. Thus, the route by which 8 is formed from 3
remains unclear.
Scheme 3
Treatment of 3, dissolved in pure furan, with KOtBu af-
forded a 30% yield of the tetrahydroepoxynaphthalene 4
(Scheme 3), which had been prepared previously in low
yield from 6,6-dibromobicyclo[3.1.0]hex-2-ene. Apparently,
in the DoeringϪMooreϪSkattebøl reaction and the β-elim-
ination of hydrogen bromide from 3, the same intermediate
is generated, namely 1. Analogously, use of 2,5-dimethylfu-
ran instead of furan resulted in the formation of 5 (13%).
The configuration of 4 as endo-substituted 7-oxanorbonene
derivative is revealed by the magnitude of the coupling con-
1
stant J4,4a ഠ 4 Hz in the H NMR spectrum.[7] Because of
a torsional angle of ca. 90 °, a value close to 0 Hz has to
be expected for the exo-isomer. Based on the similarity of
the NMR spectroscopic data of 4 and 5, the endo-config-
uration of 5 seems highly probable. In support of the consti-
tutions of 4 and 5, these compounds were converted into
the known dihydroepoxynaphthalenes 6 and 7, respectively,
on treatment with DDQ (Scheme 3).
Our attempt to trap the isobenzene 1 generated from 3
by styrene had an unexpected result. At first it turned out
that 3, dissolved in pure styrene, did not react with KOtBu.
Whereas furan and its 2,5-dimethyl derivative obviously
solvate the potassium cation sufficiently to make tert-butox- Scheme 5
1872
Eur. J. Org. Chem. 2000, 1871Ϫ1874

Cycloallenes, 13
FULL PAPER
pure 3 (150Ϫ357 mg, 11Ϫ27% in several experiments) was obtained
by distillation, b.p. 50 °C/10 mbar m. p. 23Ϫ25 °C. Ϫ IR (film):
ν˜ ϭ 3034 cmϪ1, 2880, 2822, 1678, 1428, 1350, 964, 929, 661, 633.
Conclusion
In the presence of furan, the treatment of 6,6-dibromo-
bicyclo[3.1.0]hex-2-ene with methyllithium[7] and the β-
elimination of hydrogen bromide from 1-bromocyclohexa-
1,4-diene (3) described here furnish the same product. This
is good evidence for the same intermediate in both reac-
tions, i.e. the isobenzene 1 is unassociated with fragments
of the precursors. In addition, the reaction conditions offer
a test as to whether 1 can be transformed to the phenyl
anion by deprotonation. The finding that performing the
reaction in the presence of benzophenone gives rise to tri-
phenylmethanol provides an unequivocally positive answer.
For the preparation of cycloadducts of 1 in quantity, the
use of 3 is inferior to the one-pot procedure starting from
cyclopentadiene,[7] in particular because of the low yield in
the synthesis of 3. Moreover, the possibility of the cycload-
ducts being isomerised under the influence of KOtBu/18-
crown-6 limits the scope of the method, as shown by the
styrene adduct 9 of 1.
Ϫ
1H NMR (CDCl3): δ ϭ 2.77 ("ttd", line distances 8.7, 3.5,
1.8 Hz, 3-H2), 3.04 ("tdt", line distances 8.7, 3.2, 1.8 Hz, 6-H2), 5.59
(dtt, J4,5 ϭ 10.1, J5,6 ϭ 3.2, J3,5 ϭ 1.8 Hz, 5-H), 5.70 (dttd, J3,4
3.2, J4,6 ϭ 2.0, J2,4 ϭ 1.3 Hz, 4-H), 6.05 (tq, J2,3 ϭ 3.7, J2,6
ϭ
ϭ
1.6 Hz, 2-H); the assignment is based on a NOESY experiment. Ϫ
13C NMR (CDCl3): δ ϭ 28.9 (C-3), 35.4 (C-6), 119.6 (C-1), 123.0
(C-4), 123.9 (C-5), 126.1 (C-2); the assignment is based on a C,H-
COSY spectrum. Ϫ C6H7Br (159.0): calcd. C 45.31, H 4.44; found
C 45.63, H 4.70.
(1α,4α,4aα)-1,4,4a,7-Tetrahydro-1,4-epoxynaphthalene (4): Under
nitrogen, potassium tert-butoxide (767 mg, 6.84 mmol) was added
in small portions to a stirred solution of 3 (543 mg, 3.41 mmol) in
freshly distilled furan (15 mL) over a period of 10 min at room
temperature. The mixture was stirred vigorously for 1 h and then
treated with water (5 mL). After separation of the layers, the aque-
ous phase was extracted with diethyl ether (3 ϫ 10 mL). The com-
bined organic layers were dried with MgSO4 and concentrated in
vacuo. The residue was purified by flash chromatography (SiO2,
light petroleum for the first 200 mL of eluate, light petroleum/di-
ethyl ether, 20:1 for the next 400 mL and 15:1 finally) to give 4
(150 mg, 30%) as a pale yellow liquid, which was shown by 1H
NMR spectroscopy to contain a few per cent of the dehydrogena-
Experimental Section
1
tion product 6. Ϫ H, 13C NMR: ref.[7] Ϫ MS (70 eV): m/z (%) ϭ
Instrumentation: See ref.[24]
146 (16) [Mϩ], 145 (95), 131 (36), 128 (32), 127 (36), 117 (61), 116
(30), 115 (100), 91 (71), 78 (20), 77 (23), 68 (44), 65 (20), 51 (25),
39 (65). Ϫ HRMS (C10H9O [Mϩ Ϫ H]): calcd. 145.0653; found
145.0653. Ϫ C10H10O (146.2): calcd. C 82.16, H 6.90; found C
81.59, H 6.78.
trans-4,5-Dibromocyclohexene (2): This compound was obtained
according to a literature procedure.[25] Thus, cyclohexa-1,4-diene
(15.21 g, 189.8 mmol, containing 1.29 g benzene, as obtained by the
Birch reduction of benzene[26]) was dissolved in 50 mL of anhyd-
rous chloroform. The stirred solution was cooled to Ϫ78 °C and
bromine (32.9 g, 206 mmol) was added slowly so that the temper-
ature did not exceed Ϫ75 °C. The solvent was then evaporated at
20 °C in vacuo and the remaining colourless solid was subjected to
distillation. The product 2 (28.04 g, 62%; ref.[25] 90%) was obtained
at 112 °C/8Ϫ10 mbar and solidified rapidly in the receiver, m.p. 37
°C (ref.[27] b.p. 119 °C/20 Torr; ref.[25] m.p. 34Ϫ37 °C). The solid
residue (15.7 g, 21%) was shown to be a 93:7 mixture of 1r,2t,4c,5t-
and 1r,2t,4t,5c-tetrabromocyclohexane[27] by NMR spectroscopy.
2: 1H NMR (CDCl3): δ ϭ 2.60, 3.20 (2 ϫ dm, Jgem ഠ 19 Hz, 3-
H2), 4.52 (m, 4-H), 5.66 (m, 1-H). Ϫ 13C NMR (CDCl3): δ ϭ 30.9
(t, C-3), 48.4 (d, C-4), 122.0 (d, C-1).
(1α,4α,4aα)-1,4,4a,7-Tetrahydro-1,4-dimethyl-1,4-epoxynaphthalene
(5): According to the procedure described for the preparation of 4,
from 3 (300 mg, 1.88 mmol) in 2,5-dimethylfuran (15 mL), 5
(43 mg, 13%) was obtained as a colourless oil after flash chromato-
graphy (SiO2, light petroleum/diethyl ether 10:1). Ϫ 1H NMR
(CDCl3): δ ϭ 1.63, 1.70 (2 ϫ s, 2 ϫ Me), 2.62Ϫ2.70 (m, 4a-H, 7-
H2), 5.44 (m, 8-H), 5.76Ϫ5.81 (m, 5-, 6-H), 5.87, 6.16 (2 ϫ d, J2,3 ϭ
5.4 Hz, 2-, 3-H). Ϫ 13C NMR (CDCl3): δ ϭ 15.2, 17.7 (2 ϫ Me),
27.9 (C-7), 48.0 (C-4a), 87.0 (double intensity, C-1, C-4), 111.0 (C-
8), 126.7, 128.0 (C-5, C-6), 133.9 (C-3), 139.9 (C-2), 148.4 (C-8a).
Ϫ MS (70 eV): m/z (%) ϭ 174 (13) [Mϩ], 159 (26), 132 (12), 131
(100), 130 (17), 129 (28), 128 (17), 116 (19), 115 (32), 91 (52), 77
(11), 43 (49). Ϫ HRMS (C12H13O [Mϩ Ϫ H]): calcd. 173.0966;
found 173.0965.
1r,2t,4c,5t-Tetrabromocyclohexane: 1H NMR: δ ϭ 2.88 (br. s,
CH2), 4.51 (m, CH). Ϫ 13C NMR (CDCl3): δ ϭ 39.7 (CH2), 50.6
(CH).
1r,2t,4t,5c-Tetrabromocyclohexane: 1H NMR: δ ϭ 2.46, (m, Jgem ϭ
Ϫ14.0 Hz, Jvic ϭ ϩ12.0 Hz, 3-Hax), 3.14 (dm, Jvic ϭ ϩ4.6 Hz, 3-
Heq), 3.95 (m, J1,2 ϭ ϩ10.5 Hz, 1-H); the coupling constants were
determined by simulation. Ϫ 13C NMR (CDCl3): δ ϭ 46.8 (CH2),
51.6 (CH).
1,4-Dihydro-1,4-epoxynaphthalene (6): To a solution of 4 (10.0 mg,
0.0684 mmol) in benzene (0.7 mL) was added DDQ (18.6 mg,
0.0819 mmol) at room temperature. The mixture was shaken until
the DDQ had dissolved and then left at 25 °C for 24 h. The precip-
itate formed was removed by filtration and the filtrate was concen-
trated in vacuo. The residue was shown to be virtually pure 6 by the
1H and 13C NMR spectra on comparison with literature data.[28]
1-Bromocyclohexa-1,4-diene (3): A flask, equipped with a reflux
condenser and containing a mixture of 2 (2.00 g, 8.33 mmol) and
N,N-diethylaniline (2.49 g, 16.7 mmol) under nitrogen, was heated
1,4-Dihydro-1,4-dimethyl-1,4-epoxynaphthalene (7): According to
with an electric mantle to 200Ϫ220 °C within 10 min and kept at the procedure described for the reaction of 4, compound 5
that temperature for 5 min. Then, the heater was turned off and
the flask was allowed to cool within the mantle. The mixture was
treated with water and diethyl ether until two layers resulted, which
were then separated. The aqueous layer was extracted with diethyl
ether (3 ϫ 5 mL). The combined organic layers were washed with
2  hydrochloric acid (3 ϫ 5 mL) and water (5 mL), dried with
MgSO4, and concentrated in vacuo. From the residue, analytically
(22.0 mg, 0.126 mmol) was treated with DDQ. The precipitate
formed was removed by filtration through basic Al2O3 (activity IV)
with benzene as eluant. The filtrate was concentrated in vacuo and
the remaining colourless oil (20 mg, 92%) was shown to be virtually
pure 7 by NMR spectroscopy. The 1H NMR chemical shifts
(CDCl3) reported in ref.[29] deviate somewhat from our values: δ ϭ
1.90 (s, Me), 6.78 (s, 2-H ), 6.98, 7.12 ( 2 ϫ m, 5-, 6-H). Ϫ 13C
Eur. J. Org. Chem. 2000, 1871Ϫ1874
1873

M. Christl, S. Groetsch
FULL PAPER
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NMR (CDCl3): δ ϭ 15.2 (Me), 88.6 (C-1), 118.3 (C-5), 124.7 (C-
6), 146.8 (C-2), 152.7 (C-4a).
[4]
[5]
M. Christl, M. Braun, Chem. Ber. 1989, 122, 1939Ϫ1946.
Treatment of 3 with KOtBu in the Presence of 18-Crown-6 and Sty-
rene: At room temperature, KOtBu (695 mg, 6.19 mmol) was added
over a period of 30 min in small portions to a stirred solution of 3
(500 mg, 3.15 mmol) and 18-crown-6 (830 mg, 3.14 mmol) in sty-
rene (20 mL), kept under nitrogen. Stirring was continued for 2 h
and water (5 mL) was then added to the mixture. After separation
of the layers, the aqueous layer was extracted with diethyl ether (3
ϫ 10 mL); the combined organic layers were dried with MgSO4
and concentrated in vacuo. From the residue, 1,2-diphenylethane
(8, 55 mg, 10%) was obtained as a colourless oil by distillation at
100 °C (kugelrohr)/0.15 mbar. The NMR spectra are in agreement
with literature data.[30]
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[11]
Treatment of exo-7-Phenylbicyclo[4.2.0]octa-1,4-diene (9) with
KOtBu in the Presence of 18-Crown-6: KOtBu (616 mg, 5.49 mmol)
was added in small portions to a stirred solution of 9 (500 mg,
2.74 mmol) in anhydrous THF (20 mL), kept under nitrogen, at
room temperature. Since no reaction occurred within 1 h, 18-
crown-6 (728 mg, 2.75 mmol) was added. This caused the mixture
to turn from almost colourless to a deep brownish red instanta-
neously. Stirring was continued for 1 h, after which the solvent was
removed in vacuo. The residue was treated with light petroleum
(10 mL) and water (5 mL) and the layers were separated. The aque-
ous layer was extracted with light petroleum (3 ϫ 15 mL). The
combined organic layers were dried with MgSO4 and concentrated
in vacuo. From the residue, 1,2-diphenylethane (8, 380 mg, 76%)
was obtained by distillation at 100 °C (kugelrohr)/0.15 mbar as a
colourless oil, which solidified, m.p. 45Ϫ47 °C (ref.[30] 45Ϫ50 °C,
ref.[31] 52 °C). The NMR spectra were the same as those of the
product of the above experiment.
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[13]
[14]
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Reaction of 3 with KOtBu in the Presence of Benzophenone: To a
solution of 3 (685 mg, 4.30 mmol) and benzophenone (3.13 g,
17.2 mmol) in anhydrous THF (10 mL), kept under nitrogen, was
added KOtBu (2.24 g, 20.0 mmol). The mixture was stirred at room
temperature for 1 h and then treated with water (5 mL) and diethyl
ether, until two layers resulted. These were separated and the aque-
ous layer was extracted with diethyl ether (3 ϫ 10 mL). The com-
bined organic layers were dried with MgSO4 and concentrated in
vacuo. The residue was purified by flash chromatography (SiO2,
light petroleum/diethyl ether, 10:1 for the first 300 mL of eluate
and then 5:1) to give triphenylmethanol (55 mg, 5%) as colourless
[21]
[22]
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