Spectroscopic analysis of the products of the cycloaddition reaction of 1-Aryl-2-chlorocyclopropenes and cyclopentadiene

Article abstract of DOI:10.1002/jccs.201300216

The treatment of a series of 1-aryl-2,2-dihalocyclopropanes with t-BuOK at -10 °C produces the corresponding 1-aryl-2-halocyclopropenes, which react with cyclopentadiene to produce fairly good yield of [4+2]-cycloaddition products with more than 90% of the endo-isomer. The higher yield obtained from hexane medium then from methanol and ionic liquid demonstrates that the reaction is a nonpolar process. The treatment of a series of 1-aryl-2,2- dihalocyclopropanes with t-BuOK at -10 °C produces the corresponding 1-aryl-2-halocyclopropenes, which react with cyclopentadiene to produce fairly good yield of [4+2]-cycloaddition products with more than 90% of the endo-isomer. The higher yield obtained from hexane medium then from methanol and ionic liquid demonstrates that the reaction is a nonpolar process. Copyright

Full text of DOI:10.1002/jccs.201300216

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Spectroscopic Analysis of the Products of the Cycloaddition Reaction of
1-Aryl-2-chlorocyclopropenes and Cyclopentadiene
Mei-Fang Ding,a Chuan-Chen Lee,b Lian-Chun Lina and Shaw-Tao Lina,*
aDepartment of Applied Chemistry, Providence University, Sha-Lu, Taichung 433, Taiwan
bDepartment of Health and Nutrition Biotechnology, Asia University, Wu-Feng, Taichung 413, Taiwan
(Received: Apr. 26, 2013; Accepted: Aug. 13, 2013; Published Online: Oct. 31, 2013; DOI: 10.1002/jccs.201300216)
The treatment of a series of 1-aryl-2,2-dihalocyclopropanes with t-BuOK at -10 °C produces the corre-
sponding 1-aryl-2-halocyclopropenes, which react with cyclopentadiene to produce fairly good yield of
[4+2]-cycloaddition products with more than 90% of the endo-isomer. The higher yield obtained from
hexane medium then from methanol and ionic liquid demonstrates that the reaction is a nonpolar process.
Keywords: Cyclopropenes; Cycloaddition; endo-adduct; 2D NMR; MS.
INTRODUCTION
Cyclopropenes are of interest to both theoretical and
cyclopropene has been prepared from the reaction of 1-
phenyl-1,2,2-tribromocyclopropane and MeLi at -45 °C or
even lower, in order to prevent a ring opening reaction.4b-d
In this study, compounds 2, 3 were prepared from the reac-
tion of 1-aryl-2,2-dihalocyclopropanes with t-BuOK in
hexane at -10 °C for 4 h (Scheme I). The formation of 2 and
3 was monitored using GC-MS analysis. (Fig. 1) The cy-
clopropenes prepared from this reaction were used without
purification. The reaction of cyclopropene with freshly
distillated cyclopentadiene (1:1.5) was performed within a
sealed system at ambient temperature, because of thermal
instability of cyclopropenes and the lower boiling point of
cyclopentadiene (b.p. 42 °C).
experimental chemists, because they have the simplest
small ring with a double bond.1 This has 27.7 Kcal/mol of
olefinic strain energy and 52.2 Kcal/mol of strain energy1
with a short double bond of 1.296 Å2. Therefore it is ex-
pected that this will be a highly reactive molecule. Unsub-
stituted cyclopropene 1 is a potentially explosive gas and is
oligomerized rapidly via alkene reactions.3 In contrast to 1,
the substituted cyclopropenes, which are stabilized by the
substituent, are stable enough to be utilized as a dienophile.
The Diels-Alder reaction is a powerful tool in organic syn-
thesis and industrial chemistry as it permits the single-stage
construction of six-membered carbocycles that are con-
tained in important compounds used for drug delivery, bio-
chemical applications, material science and devices.4 The
cycloaddition of cyclopropenes has been studied to some
extent.5 The reactivity of dienes toward cyclopropene and
the steric selectivity strongly depend on the nature of the
diene and the substituent on the cyclopropene. In further
study the chemistry of the three-membered ring system,
this study performs the cycloaddition with cyclopenta-
diene. The effect of the substituent and solvent is studied.
The structure of the endo-product is confirmed using NMR
analysis.
Scheme I
At the end of reaction following the work-up, a mix-
ture of endo- and exo-isomer was obtained using flash
chromatographic separation and the isomeric ratios were
estimated according to the areas of the peaks in the GC-MS
analysis. In general, the molecular ions of the adducts ap-
pear with moderated to low abundance of the molecular ion
along with [M-Cl]+ as the base peaks. The intensity of the
molecular ion for the endo-isomer is lower than that for its
exo-counterpart (Fig. 2), because the pi-electron of double
bond participates in the cleavage of C-Cl bond. The simpler
RESULTS AND DISCUSSION
Although cyclopropene can be stabilized by its sub-
stituent, arylcyclopropenes 2 often decomposes to form ei-
ther an allene or acetylene, in the presence of a strong base
or at an elevated reaction temperature.6 1-Bromo-2-phenyl-
* Corresponding author. E-mail: sdlin@pu.edu.tw
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Ding et al.
to interact with the double bond of the adduct.7 This favors
for the formation of the endo-isomer. In this series, the ab-
sence of a substituent on the C3 of cyclopropene means that
the SOI cannot explain the formation of the endo-isomer.
The congestion between the aryl ring and the double bond
in the exo-isomer resulted in favorable for the formation of
endo-isomer. This result is consistent with the conclusion
of Apeloig “the parent cyclopropene and 1,2-disubstituted
cyclopropenes are expected to yield endo-adducts exclu-
sively or predominantly”.8
Scheme II
spectrum that results from the initial cleavage of C-Cl and
following fragmentation as a main process for the endo-
isomer is evidence of the endo-form confirmation from this
series reaction. The amount of exo-isomer is too small to be
isolated for further characterization.
The 1H chemical shifts of the tricyclic system are as-
signed according to the correlation in the COSY and
NOESY spectra. The chemical shifts of Ha (~1.9 ppm) and
Hb (~2.5 ppm) differ by 0.6 ppm, because a double bond
shields the Ha. The COSY spectra also demonstrates a cor-
relation between Ha and Hd. (Fig. 3) This unusual 5J long
range coupling (J 2.4 Hz) indicates the presence of the W
form in a rigid structure. The correlations between Hb, Hd
and Hg with the aromatic protons and between Hc, and He,
Hf in the NOESY spectrum confirm the assigned 1H chemi-
cal shifts. (Fig. 4)
Fig. 2. GC-MS analysis of the resultant from the cy-
cloaddition of 1-chloro-2-(3-methylphenyl)cy-
clopropene and cyclopentadiene; (A) gas chro-
matograph of the resultant after flash chromato-
graphic purification; (B) mass spectrum of 2-
chloro-4-(3-methylphenyl)-exo-tricyclo-
[3.2.102,4]oct-6-ene (5f) (RT: 14.27 min); (C)
mass spectrum of 2-chloro-4-(3-methylphen-
yl)-endo-tricyclo[3.2.102,4]oct-6-ene (4f) (RT:
15.21 min).
A secondary orbital interaction (SOI) takes place
when the dienophile contains functional group that is able
Fig. 1. GC-MS analysis of the resultant from the de-
hydrochloride of 1,1-dichloro-2-(3-methyl-
phenyl)cyclopropane. (A) GC chromatography
from the resultant after 2 h; (B) gas chromatog-
raphy from the resultant after 4 h; (C) mass
spectrum of 1,1-dichloro-2-(3-methylphenyl)-
cyclopropane; (D) 1-chloro-2-(3-methylphen-
yl)cyclopropene.
Fig. 3. The portion of COSY spectrum of 2-chloro-4-
(3-bromophenyl)-endo-tricyclo[3.2.102,4]oct-
6-ene (4d) in CDCl3 solution.
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CHEMICAL SOCIETY
Products from Cyclopropenes and Cyclopentadiene
Table 1. Results for the Diels-Alder addition of 1-aryl-2-halo-
cyclopropenes and cyclopentadienea
Entry Substituent Isolated yield (%)
Endo-
Exo-
1
2
3
4
5
6
7
8
H
61.8
45.0b
36.3c
29.8b,c
22.7d
17.1b,d
58.6e
63.5
56.2
66.7
58.4
35.7
92.0%
---
---
---
---
8.0%
---
---
---
---
---
92.7
---
7.3
3-Cl
4-Cl
3-Br
97.4%
95.4%
93.0%
95.9%
94.2%
92.4%
88.9%
2.6%
4.6%
7.0%
4.1%
5.8%
8.6%
11.1%
9
10
11
12
13
14
4-Br
3-CH3
4-CH3
4-OCH3
39.5
29.2
Fig. 4. The NOESY spectrum of 2-chloro-4-(3-bromo-
phenyl)-endo-tricyclo[3.2.102,4]oct-6-ene (4d)
in CDCl3 solution.
a. The endo-: exo-isomer ratios are determined based on the GC-
MS analysis; b. open system; c. methanol is the solvent; d.
-
[NMP+][HSO4 ] is the solvent; e. 1-bromo-2-phenylcyclopropene
is used.
Although, the cyclopropene has a high ring strain and
is expected to be reactive in the addition, in order to release
the strain energy, the yields are sensitive to the substituent
of the phenyl ring. The reactivity of cyclopropene is deacti-
vated by the presence of an electron-donating group (en-
tries 12~14).
tained using GC/MS (Thermo Focus GC coupled with Polaris Q)
at an ionization potential of 70 eV. Elemental analysis was per-
formed at the Instrumental Analytical Center at National Chung
Hsien University. Cyclopentadiene was freshly distillated from
dicyclopentadiene. 1-Aryl-2,2-dihalocyclopropanes were pre-
pared from the reaction of styrene and CHX3 (X = Br, Cl) in hex-
anes solution in the presence of t-BuOK.10 1-Aryl-2-halocyclo-
propenes 2, 3 were prepared by the dehydrohalogenation of
1-aryl-2,2-dihalocyclopropanes using t-BuOK at 0 °C.5 The ex-
tension of the conversion from cyclopropanes to cyclopropenes
monitored using GC/MS analysis.5e Compounds 2 and 3 were iso-
lated as a hexanes solution, which was used without purification.
Typical procedure for cycloaddition: A mixture of freshly
prepared 1-chloro-2-phenylcyclopropene (90 mg, 0.6 mmol) and
cyclopentadiene (60 mg, 0.9 mmol) and hexane (2 mL) was
sealed in a glass tube and stirred at ambient temperature for 12 h.
After removing the volatile fraction using a rotor evaporator the
residue was purified by flash chromatography on a silica gel col-
umn to give 80 mg (61.8% yield) of 2-chloro-4-phenyl-endo-
tricyclo[3.2.102,4]oct-6-ene 4a as a highly viscous yellow liquid.,
1H NMR d 1.75 (m, 2H), 1.94 (d, J = 3.6 Hz , 1H), 2.63 (d, J = 2.4
Hz , 1H), 3.13 (s, 2H), 6.11 (t, J = 4.0 Hz, 1H), 6.24 (t, J = 4.0 Hz ,
1H), 7.36 (m, 5H); 13C NMR d 32.1, 33.6, 52.4, 55.9, 61.4, 126.6,
128.4, 128.7, 135.4, 136.1, 139.5; MS (m/z, %) 216 (M+., 15), 181
(100), 115 (3); Anal. calcd. for C14H13Cl: C, 75.59; H, 6.05.
Found: C, 75.62; H, 6.03.
The formation of an adduct is also affected by the sol-
vent used.9 A highly polar solvent and an ionic liquid both
reduce the yields of the adduct (entries 3, 5). This demon-
strates that the interaction between the polar solvent
(MeOH) or a cation (ion liquid) and cyclopentadiene can
reduces the electron density of the diene and consequently
reduces its reactivity toward the dienophile.
CONCLUSION
Cycloaddition between 1-aryl-2-chlorocyclopropenes
and cyclopentadiene yields the endo-isomer as the main
product. The dienophile, 1-aryl-2-chlorocyclopropenes, is
electrophilic, so the electron-donating group on the phenyl
ring can deactivate the cyclization process. The rigid sys-
tem leads to a long range 5J coupling, which gives useful
information for the assignment of the proton chemical
shift.
EXPERIMENTAL
The 1H and 13C NMR spectra were recorded at 400 and 100
MHz, respectively, and at an ambient temperature, using a Bruker
Advance-400. The chemical shifts for the samples in CDCl3 solu-
tion are reported in d units relative to TMS. Mass spectra were ob-
1
2-Bromo-4-phenyl-endo-tricyclo[3.2.102,4]oct-6-ene, H
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NMR d 1.87 (m, 2H), 2.01 (d, J = 7.5 Hz , 1H), 2.73 (d, J = 7.5 Hz ,
1H), 3.15 (s, 1H), 3.33 (s, 1H), 6.16 (s, 1H), 6.25 (s, 1H), 7.45 (m,
5H); 13C NMR d 32.6, 33.2, 46.9, 52.1, 54.7, 126.4, 128.1, 128.4,
134.6, 135.7, 140.2; MS (m/z, %) 260 (M+., 2), 181 (100), 115
(33); Anal. calcd. for C14H13Br: C, 64.39; H, 5.02. Found: C,
64.37; H, 5.05.
Found: C, 78.05; H, 6.51.
2-Chloro-4-(4-methylphenyl)-endo-tricyclo[3.2.102,4]oct-
6-ene 4g, 1H NMR d 1.71 (dd, J = 2.4, 6.4 Hz, 1H), 1.74 (d, J = 6.4
Hz, 1H), 1.93 (d, J = 7.2 Hz , 1H), 2.62 (d, J = 7.2 Hz , 1H), 3.08
(s, 1H), 3.12 (s, 1H), 6.09 (t, J = 4.0 Hz, 1H), 6.23 (t, J = 4.0 Hz,
1H), 7.17 (d, J = 8.0 Hz , 2H), 7.29 (d, J = 8.0 Hz , 2H); 13C NMR d
21.1, 32.0, 33.6, 52.3, 55.7, 61.2, 128.5, 129.0, 135.4, 135.7,
135.9, 137.1; MS (m/z, %) 230 (M+., 17), 195 (100), 129 (9); Anal.
calcd. for C15H15Cl: C, 78.08; H, 6.55. Found: C, 78.02; H, 6.51.
2-Chloro-4-(4-methoxyphenyl)-endo-tricyclo[3.2.102,4]-
oct-6-ene 4h, 1H NMR d 1.66 (dd, J = 2.0, 6.4 Hz, 1H), 1.74 (d, J =
6.4 Hz , 1H), 1.92 (d, J = 7.2 Hz , 1H), 2.60 (d, J = 7.2 Hz , 1H),
3.04 (s, 1H), 3.12 (s, 1H), 6.09 (t, J = 4.0 Hz, 1H), 6.22 (t, J = 4.0
2-Chloro-4-(3-chlorophenyl)-endo-tricyclo[3.2.102,4]oct-
6-ene 4b, 1H NMR d 1.72 (dd, J = 2.4, 6.4 Hz, 1H), 1.79 (d, J = 6.4
Hz , 1H), 1.95 (d, J = 7.2 Hz , 1H), 2.59 (d, J = 7.2 Hz , 1H), 3.10
(s, 1H), 3.14 (s, 1H), 6.10 (t, J = 4.0 Hz, 1H), 6.25 (t, J = 4.0 Hz,
1H), 7.28 (m, 3H), 7.38 (s, J = 8.4 Hz , 1H); 13C NMR d 32.3, 33.5,
52.2, 53.6, 55.9, 61.4, 126.8, 128.9, 129.6, 134.3, 135.5, 135.9,
141.8; MS (m/z, %) 250 (M+., 14), 215 (100), 180 (54); Anal.
calcd. for C14H12Cl2: C, 66.95; H, 4.82. Found: 66.92; H, 4.87.
2-Chloro-4-(4-chlorophenyl)-endo-tricyclo[3.2.102,4]oct-
6-ene 4c, 1H NMR d 1.69 (dd, J = 2.4, 6.4 Hz, 1H), 1.78 (d, J = 6.4
Hz , 1H), 1.93 (d, J = 7.2 Hz , 1H), 2.58 (d, J = 7.2 Hz , 1H), 3.07
(s, 1H), 3.13 (s, 1H), 6.10 (t, J = 4.0 Hz, 1H), 6.24 (t, J = 4.0 Hz,
1H), 7.32 (s, 4H); 13C NMR d 32.3, 33.2, 52.3, 53.7, 55.8, 61.4,
128.5, 130.0, 132.5, 135.4, 135.9, 138.2; MS (m/z, %) 250 (M+.,
19), 215 (100), 180 (44); Anal. calcd. for C14H12Cl2: C, 66.95; H,
4.82. Found: 66.89; H, 4.86.
2-Chloro-4-(3-bromophenyl)-endo-tricyclo[3.2.102,4]oct-
6-ene 4d, 1H NMR d 1.72 (dd, J = 6.4, 4.0 Hz, 1H), 1.79 (d, J = 6.4
Hz, 1H), 1.95 (d, J = 7.2 Hz, 1H), 2.58 (d, J = 7.2 Hz, 1H), 3.10 (s,
1H), 3.14 (s, 1H), 6.09 (t, J = 4.0 Hz, 1H), 6.25 (t, J = 4.0 Hz , 1H),
7.22 (t, J = 8.0 Hz , 1H), 7.33 (d, J = 8.0 Hz, 1H), 7.40 (d, J = 8.0
Hz, 1H), 7.53 (s, 1H); 13C NMR d 32.4, 33.5, 52.3, 53.5, 55.9,
61.4, 122.5, 127.3, 129.8, 129.9, 131.9, 135.5, 135.9, 142.1; MS
(m/z, %) 294 (M+., 9), 259 (25), 180 (100); Anal. calcd. for
C14H12BrCl: C, 56.88; H, 4.09. Found: 56.90; H, 4.05.
Hz , 1H), 6.90 (d, J = 8.4 Hz , 2H), 7.32 (d, J = 8.4 Hz , 2H); 13
C
NMR d 32.0, 33.5, 52.4, 54.0, 55.8, 61.3, 114.3, 129.4, 130.5,
135.3, 135.9, 156.8; MS (m/z, %) 246 (M+., 23), 211 (100), 145
(11); Anal. calcd. for C15H15ClO: C, 73.02; H, 6.13. Found: C,
72.98; H, 6.20.
ACKNOWLEDGEMENTS
The financial support provided by the National Sci-
ence Council of the Republic of China (NSC 90-2113-M-
126-007) and the technical support of the HSCC are both
gratefully acknowledged.
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