Simple and efficient synthesis of bromine-substituted 1,3-dienes and 1,3,5-cycloheptatriene by vacuum pyrolysis of gem-dibromocyclopropanes

Article abstract of DOI:10.1016/S0040-4039(01)00460-9

To establish whether the results obtained by the gas phase pyrolysis of 6,6-dihalobicyclo[3.1.0]hexanes by HeI photoelectron spectroscopy using a high power CW CO₂ laser as a directed heat source can be achieved on a preparatory scale using a modified apparatus, we carried out the gas phase pyrolysis of a few representative gem-dibromocyclopropanes such as 1,1-dibromo-2,2,3,3-tetramethylcyclopropane (1), 1,1-dibromo-2,2-dimethyl-cyclopropane (2), 1,1-dibromo-cis-2,3-dimethylcyclopropane (3), 1,1-dibromo-trans-2,3-dimethylcyclopropane (4), 6,6-dibromobicyclo[3.1.0]hexane (5) and 7,7-dibromobicyclo[4.1.0]heptane (6). Except 7,7-dibromobicyclo[4.1.0]heptane, that gave 1,3,5-cycloheptatriene in 72% yield at 525°C, 1, 2, 3, 4 and 5 readily lose HBr at 400-560°C in the gas phase to produce β-bromo-1,3-dienes in high chemical yields and purity. The dienes are potentially useful starting substrates for the Diels-Alder reactions.

Full text of DOI:10.1016/S0040-4039(01)00460-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3255–3258
Simple and efficient synthesis of bromine-substituted 1,3-dienes
and 1,3,5-cycloheptatriene by vacuum pyrolysis of
gem-dibromocyclopropanes
Nick H. Werstiuk* and Chandra D. Roy
Department of Chemistry, McMaster University, Hamilton, Ont., Canada L8S 4M1
Received 16 March 2001; accepted 19 March 2001
Abstract—To establish whether the results obtained by the gas phase pyrolysis of 6,6-dihalobicyclo[3.1.0]hexanes by HeI
photoelectron spectroscopy using a high power CW CO2 laser as a directed heat source can be achieved on a preparatory scale
using a modified apparatus, we carried out the gas phase pyrolysis of a few representative gem-dibromocyclopropanes such as
1,1-dibromo-2,2,3,3-tetramethylcyclopropane (1), 1,1-dibromo-2,2-dimethyl-cyclopropane (2), 1,1-dibromo-cis-2,3-dimethylcyclo-
propane (3), 1,1-dibromo-trans-2,3-dimethylcyclopropane (4), 6,6-dibromobicyclo[3.1.0]hexane (5) and 7,7-dibromobicyclo-
[4.1.0]heptane (6). Except 7,7-dibromobicyclo[4.1.0]heptane, that gave 1,3,5-cycloheptatriene in 72% yield at 525°C, 1, 2, 3, 4 and
5 readily lose HBr at 400–560°C in the gas phase to produce b-bromo-1,3-dienes in high chemical yields and purity. The dienes
are potentially useful starting substrates for the Diels–Alder reactions. © 2001 Published by Elsevier Science Ltd.
gem-Dihalocyclopropanes constitute a very important
class of compounds in organic synthesis due to their wide
spectrum of specific chemical reactivity.1 These valuable
transformations include: (i) generation of divalent car-
bon species, i.e. cyclopropylidenes by the a-elimination
of molecular halogen which ultimately isomerize by ring
opening to form allenes2 (in the absence of any carbene
trapping agents); (ii) loss of hydrogen halide by b-elim-
ination to produce cyclopropenes which can undergo
nucleophilic addition,3 tautomerization4 or ring open-
ing;5 (iii) 1,4-dehydrohalogenation to give halogen sub-
stituted 1,3-dienes6,7 if hydrogen atoms are present in an
appropriate exocyclic positions (Scheme 1). A facile ring
opening of gem-dihalocyclopropanes with or without
electrophilic and nucleophilic reagents to give b-bromo-
allylic compounds is a very useful synthetic method for
extending the carbon chain of olefins, which leads to
several molecules otherwise difficult to access. The so
called ‘cyclopropyl-allyl transformation’ always pro-
duces b-bromoallyl bromides and occasionally b-bromo-
1,3-dienes in good yield.8
A
versatile chemical
intermediate 1,3,5-cycloheptatriene has been prepared by
the thermal rearrangement of 7,7-dichloro- and 7,7-
dibromonorcaranes.9 It is evident from the literature that
the ring opening process depends on whether it is carried
out neat, in solution or in the gas phase. Therefore, this
transformation becomes interesting from both a syn-
thetic and mechanistic point of view.
H
H
1,2-elimination
1,1-elimination
X
X
H
X
X = halogen
1,4-elimination
X
Scheme 1.
Keywords: vacuum pyrolysis; gem-dibromocyclopropanes; b-bromo-1,3-dienes; 1,3,5-cycloheptatriene.
* Corresponding author. Fax: 905-522-2509; e-mail: werstiuk@mcmaster.ca
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(01)00460-9

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N. H. Werstiuk, C. D. Roy / Tetrahedron Letters 42 (2001) 3255–3258
To facilitate the preparation and detection of highly
reactive transients, we interfaced a tuneable, high-
power CW CO2 laser with a modified locally built
ultraviolet photoelectron spectrometer (UPS). The
focussed laser beam was used as a directed heat source
to produce a short (1–2 mm), hot contact zone at the
tip of a 3 mm OD diameter quartz tube constricted to
an ID of 1 mm. The temperature of the nozzle was
altered by tuning the laser to an appropriate line. Our
interest in preparing and detecting the highly reactive
distorted cyclical allene 1,2-cyclohexadiene led us to use
this CO2 laser/UPS apparatus—its first application to
research in this field—to study the gas phase (10−2 to
10−3 Torr) pyrolysis of 6,6-dihalobicyclo[3.1.0]hexanes.
We chose to study these compounds because cleavage
of the ClꢀC5 bond followed by extrusion of dihalo-
gen—the reaction could be concerted—is a possible
route to the 1,2-diene. This work led us to discover a
very selective generation of b-halo-1,3-dienes.10 This
finding motivated us to exploit the synthetic utility of
this technique by synthesising a few representative b-
bromo-1,3-dienes at preparatory scales with relatively
high chemical purity using an electrically heated fur-
nace. In this article, we wish to report the results of our
study on the gas phase pyrolysis of a representative
group of gem-dibromocyclopropanes and selective trap-
ping of pyrolysates based on their boiling points.
lent chemical yield and purity (Scheme 3). There were
no traces of any ‘cyclopropyl-allyl’ rearrangement
product, 2,3-dibromo-2,4-dimethylpent-3-ene or other
products. Upon pyrolysis at 500°C, 1,1-dibromo-2,2-
dimethylcyclo-propane (2) also produced 2-bromo-3-
methyl-1,3-butadiene (9) as
a
sole product.
Interestingly, it afforded 1,2-dibromo-3-methyl-2-
butene (10) in 60% chemical yield at 195–210°C under
neat condition.12 This observation clearly suggests that
dehydrobromination occurs in concert with ring open-
ing in the gas phase.
The thermolysis of 1,1-dibromo-cis-2,3-dimethylcyclo-
propane (3) at 510°C afforded a mixture (cis and trans)
of 1,3-dienes and 3-bromo-1,3-pentadienes (12 and 13)
as major products in the ratios of 4:1 along with
15–20% b-bromoallylic bromide and 2,3-dibromopent-
3-ene (14) (Scheme 4). The stereochemistry of the major
1
diene was established by H NMR spectroscopy using
the nuclear Overhauser enhancement (NOE) technique.
The irradiation at C4-H (l 6.05 ppm) enhanced the
signal intensities of the C2-H (l 6.32 ppm) and the
C4-CH3 (l 1.89 ppm), which suggests that the methyl
protons and olefinic protons are closer to C4-H in
space. When C2-H (l 6.32 ppm) was irradiated, only
resonance enhancement at C4-H (l 6.05 ppm) was
observed, not at C4-CH3 (l 1.89 ppm). These experi-
ments established the geometry of the major diene in
which the methyl group and the bromine are oriented
cis to each other. In contrast to Sandler’s observation,
when 1,1-dibromo-cis-2,3-dimethylcyclopropane (3)
was heated neat at 125–130°C for 72 h under a nitrogen
atmosphere only 2,3-dibromopent-3-ene (14) was
We selected six representative gem-dibromocyclo-
propanes. These gem-dibromocyclopropanes (1–6) were
prepared by reacting the dibromocarbene (generated in
situ from CHBr3 and tert-BuOK) with corresponding
olefins following a procedure reported in the literature
(Scheme 2).11
1
formed as a major product (80%), as seen by H NMR
spectroscopy, the remaining being unrearranged start-
ing material. The amount of 1,3-diene formed was
<2–3%. Sandler reported the formation of only tars at
150°C for 3 h when 3 was heated neat.13
1,1-Dibromo-2,2,3,3-tetramethylcyclopropane (1) under-
went very clean and quantitative pyrolysis at 500°C to
yield 3-bromo-2,4-dimethyl-1,3-pentadiene (7) in excel-
H3C
H3C
H3C
H3C
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
H3C
H3C
H3C
H3C
H3C
H3C
1
2
3
4
5
6
Scheme 2.
Scheme 3.
H3C
H3C
Br
H
H
CH3
H
H
Br
CH3
Br
H3C
Br
H3C
H3C
Br
H
H
Br
H3C
CH3
CH2Br
H3C
H
Br
H
H3C
11
H3C
8
10
7
9
Br
H
H
H
H
H
H
H3C
H
Br
Br
H
CH3
H
H
Br
Br
12
CH3
Br
H3C
17
13
14
15
16
Scheme 4.

N. H. Werstiuk, C. D. Roy / Tetrahedron Letters 42 (2001) 3255–3258
Table 1. Gas phase thermolysis of gem-dibromocyclopropanes15,16
3257
Entry
gem-Dibromocyclopropane
Pyrolysis temp./trap temp. (°C)
Product
Yield (%)
1
2
3
4
5
6
1
2
3
4
5
6
500/−40 to −50
500/−60
560/−65
500/−50 to −60
435/−40 to −60
525/−55 to −65
7
9
90
96
83
75
85
72
11 and 12
11 and 12
14 and 15
17
Similar results were obtained when 1,1-dibromo-trans-
2,3-dimethylcyclopropane (4) was pyrolysed under
identical conditions. When it was heated neat at 130–
140°C under a nitrogen blanket, it also afforded the
same 2,3-dibromopent-3-ene (14) in 60% yield as
obtained from the cis-cyclopropane precursor. The only
notable difference between cis and trans compounds
was that the 1,1-dibromo-trans-2,3-dimethyl-cyclo-
propane (4) underwent rearrangement much slower and
less clean in comparison with the 1,1-dibromo-cis-2,3-
dimethylcyclopropane (3). Again, the formation of only
tars was reported when the trans compound was heated
at 150–170°C for 3 h.
triene (17) in high chemical yield and purity from easily
prepared starting materials using a simple, modified
pyrolysis technique in the gas phase. This modified
technique also allows us to trap various pyrolysates
cleanly and selectively by just adjusting the temperature
of the traps by taking advantage of their boiling points.
This technique is quite different from the previously
mentioned calcium oxide packed electric furnace. These
dienes are potentially useful substrates for Diels–Alder
and Ene reactions.
Acknowledgements
It was of interest to examine several bicyclic systems
such as 6,6-dibromobicyclo[3.1.0]hexane (5) and 7,7-
dibromobicyclo[4.1.0]heptane (6), mainly because of the
fact that the reactivity of such fused systems depends
upon the ring strain with varied ring size. When 6,6-
dibromobicyclo-[3.1.0]hexane (5) was pyrolysed at
435°C, a mixture of 2-bromo-1,3-cyclohexadiene (15)
and 1-bromo-1,3-cyclohexadiene (16) were obtained
along with a small amount of 2,3-dibromocyclohexene.
6,6-Dibromobicyclo[3.1.0]hexane has been known to
rearrange to 2,3-dibromocyclohexene during distilla-
tion.
We thank the Natural Sciences and Engineering
Research Council of Canada for financial support.
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2. Skattebol, L. Tetrahedron Lett. 1965, 6, 2175.
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As expected, 7,7-dibromobicyclo[4.1.0]heptane (6) was
observed to be thermally much more stable in compari-
son with the 6,6-dibromobicyclo[3.1.0]hexane (5). At
443°C, 6 underwent only partial pyrolysis, as evident by
the presence of 65% starting material in the trap along
with 1,3,5-cycloheptatriene (17) (15%) and a small
amount of other isomeric bromocycloheptadienes by
1H NMR spectroscopy. When the thermolysis was car-
ried out at 525°C, 1,3,5-cycloheptatriene (17) was
obtained as a sole product in 72% chemical yield. The
1H NMR spectrum of the pyrolysate trapped did not
show any other products such as bromine substituted
cycloheptene, cycloheptadiene or toluene. There was an
indication of the formation of toluene as reported in
the literature in the case of 7,7-dichlorobicyclo-
[4.1.0]heptane.14 The results of the pyrolysis reactions
of gem-dibromocyclopropanes are summarized in Table
1.
4. Billups, W. E.; Shields, T. C.; Chow, W. Y.; Deno, N. C.
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In conclusion, we have successfully prepared several
synthetically difficult molecules such as 3-bromo-2,4-
dimethyl-1,3-pentadiene (7), 2-bromo-3-methyl-1,3-
butadiene (9), 3-bromo-1,3-pentadiene (12 and 13),
2-bromo-1,3-cyclohexadiene (15) and 1,3,5-cyclohepta-

3258
N. H. Werstiuk, C. D. Roy / Tetrahedron Letters 42 (2001) 3255–3258
1
Perkin Trans. 1 1990, 1881; (m) Wagner, R. A.; Weber,
J.; Brinker, U. H. Chem. Lett. 2000, 246.
pentadiene (13): H NMR (CDCl3): l 6.52–6.65 (dd, 1H,
C2-H), 6.15 (q, 1H, C4-H), 5.61 (d, J 16 Hz, 1H, C1-H,
trans to C2-H), 5.32 (d, J 10 Hz, 1H, C1-H, cis to C2-H),
1.82 (d, J 8 Hz, 3H, C4-CH3). 2,3-Dibromopent-3-ene
(14): 1H NMR (CDCl3): l 6.20 (q, 1H, C4-H), 4.90 (q,
1H, C2-H), 1.85 (d, J 8 Hz, 3H, C4-CH3), 1.80 (d, J 8 Hz,
3H, C2-CH3). 2-Bromo-1,3-cyclohexadiene (14): 1H
NMR (CDCl3): l 6.05 (m, 1H, C1-H), 5.70–6.00 (m, 2H,
C3-H and C4-H), 2.10–2.30 (m, 4H, C5-H and C6-H).
1,3,5-Cycloheptatriene (17): 1H NMR (CDCl3): l 6.60
(m, 2H), 6.20 (m, 2H), 5.30–5.50 (m, 2H), 2.25 (t, 2H).
16. Experimental procedure: The pyrolysis reactions were
conducted in a horizontally oriented electrically heated
furnace (11 cm in length) (packed with quartz helices)
equipped with valves to control the flow of the starting
material. The outlet of the furnace was connected to three
U-shaped traps in series. The last trap was immersed in
liquid nitrogen and ultimately connected to a high vac-
uum pump. The precursors were allowed to pass through
an electric furnace under high vacuum (3×10−2 Torr) and
the temperatures of various cooling traps were adjusted
on the basis of the boiling points of the pyrolysates. The
trapped pyrolysis products were analyzed and character-
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Asahara, T.; Ono, K.; Tanaka, K. Bull. Chem. Soc. Jpn.
1971, 44, 1130.
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72, 2537.
11. Sandler, S. R. Chem. Ind. 1968, 1481.
12. Sandler, S. R. J. Org. Chem. 1967, 32, 3876.
13. Sandler, S. R. J. Org. Chem. 1968, 33, 4537.
14. Winberg, H. E. J. Org. Chem. 1959, 24, 264.
15. Spectral data: 3-Bromo-2,4-dimethyl-1,3-pentadiene (7).
1H NMR (CDCl3): l 5.00 (m, 1H, C1-H), 4.90 (m, 1H,
C1-H), 1.85 (s, 6H, C4-CH3), 1.81 (s, 3H, C2-CH3). 3-
1
Bromo-3-methyl-1,3-butadiene (9): H NMR (CDCl3): l
5.80–5.15 (s, with weak allylic and homoallylic couplings,
3H, C1-H and C4-H), 1.93 (s, with weak allylic and
homoallylic couplings, 3H, C4-CH3); 13C NMR (CDCl3):
l 140.87 (C2), 133.19 (C3), 120.92 (C1), 118.61 (C4), 20.79
(C5). 1,2-Dibromo-3-methyl-2-butene (10): 1H NMR
(CDCl3): l 4.32 (s, 2H, C1-H), 1.84 (d, 6H, C3-CH3).
1
3-Bromo-1,3-pentadiene (12): H NMR (CDCl3): l 6.20–
6.40 (dd, 1H, C2-H), 6.05 (q, 1H, C4-H), 5.50 (d, J 16 Hz,
1H, C1-H, trans to C2-H), 5.14 (d, J 10 Hz, 1H, C1-H, cis
to C2-H), 1.89 (d, J 8 Hz, 3H, C4-CH3). 3-Bromo-1,3-
1
ized by H NMR spectroscopy. The chemical purities of
the products were also checked by gas chromatography.
.
.