Acid-catalyzed conversion of 7-ethynyl- and 7-vinyleyclohepta-1,3, 5-trienes to substituted benzene derivatives

Article abstract of DOI:10.1002/ejoc.200300245

The acid-catalyzed rearrangement in THF/TFA (5:1, v/v) of cyclohepta-1,3,5-trienes containing an α,β-unsaturated substituent at C-7 was investigated. 7-Ethynylcyclohepta-1,3,5-triene (1a) and its 1,4-di-tert-butyl (1b), 1,5-di-tert-butyl (1c), and 2,5-di-tert-butyl (1d) derivatives underwent isomerization to phenylallenes 2, 3, 7, and 3, respectively. 2,5-Di-tert-butyl-7-vinylcyclohepta-1,3,5-triene (17) gave a mixture (66:34) of (E)- and (Z)-1,4-di-tert-butyl-2-(1-propenyl)benzene (20). A mechanism involving the protonation of the norcaradiene tautomer (NCD), which is in equilibrium with cycloheptatriene (CHT) 1 or 17, followed by the cleavage of a three-membered ring to give an arenium ion, is proposed. In contrast, 2,5-di-tert-butyl-7-cyanocyclohepta-1,3,5-triene (12) and bis(cyclohepta-2,4,6-trien-1-yl)ethyne (21) did not give the expected products, probably due to the unfavorable energetics of the ring-opening step. A thermal 1,5-hydrogen shift predominates in these cases. Kinetic studies indicated that the rearrangement of 1a is significantly accelerated by the presence of tert-butyl groups on the seven-membered ring. The order of reactivity, 1a 〈 1b 〈 1c 〈 1d, is in agreement with the population of NCD forms at equilibrium, as estimated by the calculated CHT-NCD energy-difference and the ¹³C NMR chemical shifts of 1a-d, indicating the importance of the equilibrium concentration of the norcaradiene form as a rate-controlling factor. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejoc.200300245

FULL PAPER
Acid-Catalyzed Conversion of 7-Ethynyl- and 7-Vinylcyclohepta-1,3,5-trienes to
Substituted Benzene Derivatives
[a]
[a]
[a]
[b]
Shinya Minegishi, Jun Kamada, Ken’ichi Takeuchi, Koichi Komatsu, and
Toshikazu Kitagawa*[
b]
Keywords: Rearrangement / Carbocations / Valence isomerization / Protonation / Allenes
The acid-catalyzed rearrangement in THF/TFA (5:1, v/v) of
cyclohepta-1,3,5-trienes containing an α,β-unsaturated sub-
give the expected products, probably due to the unfavorable
energetics of the ring-opening step. A thermal 1,5-hydrogen
stituent at C-7 was investigated. 7-Ethynylcyclohepta-1,3,5- shift predominates in these cases. Kinetic studies indicated
triene (1a) and its 1,4-di-tert-butyl (1b), 1,5-di-tert-butyl (1c),
and 2,5-di-tert-butyl (1d) derivatives underwent isomeriz-
ation to phenylallenes 2, 3, 7, and 3, respectively. 2,5-Di-tert-
that the rearrangement of 1a is significantly accelerated by
the presence of tert-butyl groups on the seven-membered
ring. The order of reactivity, 1a Ͻ 1b Ͻ 1c Ͻ 1d, is in agree-
ment with the population of NCD forms at equilibrium, as
estimated by the calculated CHT−NCD energy-difference
butyl-7-vinylcyclohepta-1,3,5-triene (17) gave
a mixture
(
66:34) of (E)- and (Z)-1,4-di-tert-butyl-2-(1-propenyl)ben-
zene (20). A mechanism involving the protonation of the nor- and the 13C NMR chemical shifts of 1a−d, indicating the im-
caradiene tautomer (NCD), which is in equilibrium with
cycloheptatriene (CHT) 1 or 17, followed by the cleavage of
a three-membered ring to give an arenium ion, is proposed.
In contrast, 2,5-di-tert-butyl-7-cyanocyclohepta-1,3,5-triene
portance of the equilibrium concentration of the norcaradi-
ene form as a rate-controlling factor.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
(
12) and bis(cyclohepta-2,4,6-trien-1-yl)ethyne (21) did not
Germany, 2003)
Introduction
mal 1,5-hydrogen shift, a reaction which is normally ob-
served for cyclohepta-1,3,5-trienes at temperatures higher
Although several investigations regarding the reactions of than 100 °C.[4]
-ethynylcyclohepta-1,3,5-triene (1a) have been reported,
including, for example, complex formation with transition catalyzed both by base and acid, but reports of isomeriz-
The acetyleneϪallene rearrangement[5Ϫ9] is known to be
7
[
1,2]
[10]
metals
and conversion into the corresponding tropylium ation in acidic media
have been rather few. A possible
[
2]
ion, no reports concerning the possibility of the re- explanation for this is that due to the reversible nature of
arrangement of this compound under acidic conditions the rearrangement under acidic conditions, the product is
have appeared. In a preliminary communication,[3] we re- frequently a mixture of the desired allene and isomeric
ported the clean conversion of 1a into phenylallene 2 in the acetylenes. In contrast, the rearrangement of 1a to 2 is
presence of acid (Scheme 1). The reaction was found to be irreversible, because of the formation of an aromatic ring.[3]
sufficiently rapid that it occurs without any significant ther-
The clean rearrangement of 1a suggests the possibility of
analogous acid-catalyzed conversions of other cyclohepta-
1,3,5-trienes bearing directly attached multiple bonds into
benzene derivatives. In this paper we report the reaction
of cyclohepta-1,3,5-trienes containing an ethynyl, vinyl, or
cyano group at C-7.
Results and Discussion
Scheme 1
Acid-Catalyzed Rearrangement of 7-Ethynylcyclohepta-
Department of Energy and Hydrocarbon Chemistry, Graduate 1,3,5-trienes
[
a]
School of Engineering, Kyoto University,
Kyoto 606-8501, Japan
Heating 7-ethynylcyclohepta-1,3,5-triene (1a) and its tert-
butyl-substituted derivatives 1bϪd at 60 °C in a mixture
of THF and trifluoroacetic acid (TFA) (5:1, v/v) gave the
substituted benzene derivatives in nearly quantitative yields
[
b]
Institute for Chemical Research, Kyoto University,
Uji, Kyoto 611-0011, Japan
Fax: (internat.) ϩ 82-774/38-3178
E-mail: kitagawa@scl.kyoto-u.ac.jp
Eur. J. Org. Chem. 2003, 3497Ϫ3504
DOI: 10.1002/ejoc.200300245
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3497

S. Minegishi, J. Kamada, K. Takeuchi, K. Komatsu, T. Kitagawa
FULL PAPER
(Table 1). The rearrangement of 1a and 1d gave phenylall-
enes 2 and 3, respectively, as the sole products. A plausible
pathway, shown in Scheme 2, includes an initial cyclohepta-
triene (CHT)Ϫnorcaradiene (NCD) equilibrium.[11Ϫ13] The
three-membered ring of the protonated form of the NCD
tautomer 4 can be cleaved to give an arenium ion intermedi-
ate 5, which on deprotonation gives phenylallene 2 or 3.
Table 1. TFA-catalyzed rearrangement of 7-ethynylcyclohepta-
[
a]
1,3,5-trienes 1aϪd
Scheme 3
Substituents on sp C atoms Time [h][b] Product
2
contrast, the pentadienyl cation moiety of 10 contains only
one tert-butyl group at C-2, where the distribution of the
positive charge should be small.
In an attempt to trap the intermediate cationic species,
the rearrangements of 1a and 1d were carried out at 60 °C
1
1
1
1
a
b
c
none
960
24
24
3
2
1,4-(tBu)
1,5-(tBu)
2,5-(tBu)
2
2
2
3 ϩ 6 (68:32)
7 ϩ 8 (93.4:6.6)
3
d
[
a]
[b]
[14]
In THF/TFA (5:1, v/v), at 60 °C.
The reaction times corre- in methanol, containing 0.50  HCl.
Both compounds
rearranged much faster in this system than in THF/TFA,
and the same products as those obtained in THF/TFA (2
and 3, respectively) were formed quantitatively in 3 h. The
failure to trap the cationic intermediates by methanol or
chloride ion can be attributed to the rapid rearrangement of
the vinyl cation 4 to arenium ion 5, followed by immediate
deprotonation to form an aromatic ring. Such a rearrange-
ment would be thermodynamically favorable due to the re-
lease of the strain of the cyclopropane ring in 4 and the
resonance stabilization of 5. DFT calculations [B3LYP/6-
spond to 4Ϫ6 half lives of the consumption of the starting com-
pound.
3
11ϩG(2d,p)//B3LYP/6-31G(d)] indicated that the most
stable form of 4 (R ϭ H) has an exo configuration, and 5
R ϭ H) favors a conformation with the allenyl group anti-
(
orientated with respect to the six-membered ring. The latter
has been shown to be more stable than the former by 3.69
Ϫ1
kcal·mol (Figure 1).
An alternative pathway for the formation of dealkylation
products 6 and 8 is the acid-promoted elimination of a tert-
butyl cation from 3 and/or 7 (Scheme 5), which may form
the mono-tert-butyl derivatives 8 and 11. The migration of
the other tert-butyl group in these products can yield 6.
However, these possibilities were ruled out by control
experiments, in which heating 3 and 7 under the same con-
ditions for 24 h resulted in the quantitative recovery of the
starting materials. No dealkylation product (6, 8, or 11) was
detected, which is consistent with the formation of these
products by the mechanism shown in Scheme 4.
Scheme 2
Similarly, the rearrangement of 1b and 1c gave (di-tert-
butylphenyl)allenes 3 and 7, respectively (Scheme 3), which
are the products expected by similar mechanisms
(Scheme 4). In each case a phenylallene with a single tert-
butyl group (6 or 8) was also produced as a minor product.
The formation of 6 and 8 can be explained by the cleavage
of the CϪC bond of the three-membered ring by route b,
followed by elimination of a tert-butyl cation. The large dif-
ference between the yields of the two products (93.4:6.6) for
the reaction of 1c can be attributed to the large difference
in energy between the arenium ion intermediates 9 and 10.
Acid-Catalyzed Rearrangement of 7-Cyano- and 7-
Vinylcyclohepta-1,3,5-trienes
7-Cyanocyclohepta-1,3,5-trienes may be regarded as ni-
The 2,4-pentadien-1-yl cation moiety of 9 is effectively stab- trogen analogs of 7-ethynylcyclohepta-1,3,5-trienes, and are
ilized by the two tert-butyl groups at C-1 and C-5, where expected to undergo a similar rearrangement under acidic
the positive charge is expected to be largely distributed. In conditions to give keteneimine derivatives such as 15
3498
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Eur. J. Org. Chem. 2003, 3497Ϫ3504

Conversion of 7-Ethynyl- and 7-Vinylcyclohepta-1,3,5-trienes to Substituted Benzene Derivatives
FULL PAPER
Scheme 4
(Scheme 6). This transformation may be more favorable
than the rearrangement of 7-ethynyl derivatives, because an
electron-withdrawing substituent at C-7 is known to effec-
tively shift the CHTϪNCD equilibrium toward the NCD
[
15Ϫ17]
side.
However, the attempted acid-catalyzed re-
arrangement of 12 in THF/TFA (5:1, v/v) at 100 °C for 50 h
resulting in the complete conversion of starting material to
a multi-component mixture, in which no aromatic com-
1
pound was detected. The H NMR spectrum of the reac-
tion mixture showed signals at δ ϭ 2.23 (d, J ϭ 8.0 Hz) and
δ ϭ 5.49 (t, J ϭ 8.0 Hz) ppm with a 2:1 intensity ratio.
These signals can be assigned to 7-H and 6-H of 16, respec-
tively. Thus, it appears that a thermal 1,5-hydrogen shift to
form 16 has predominated over any acid-catalyzed re-
arrangement. Protonation of the norcaradiene tautomer of
Figure 1. Optimized structures and relative energies of the inter-
mediate cations 4 (R ϭ H) and 5 (R ϭ H) obtained by the B3LYP/
6-311ϩG(2d,p)//B3LYP/6-31G(d) level of theory
12 forms a resonance-stabilized cation 13a ǟǞ13b, which
makes ring opening to arenium ion 14 energetically unfa-
vorable. DFT calculations [B3LYP/6-311ϩG(2d,p)//B3LYP/
6
3
-31G(d)] indicated that 14 is higher in energy than 13 by
Ϫ1
.00 kcal·mol
.
On the other hand, 7-vinylcyclohepta-1,3,5-triene 17 cle-
anly rearranged in THF/TFA (5:1, v/v) at 60 °C to give (1-
propenyl)benzenes (E)-20 and (Z)-20, which were separable
Scheme 5
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S. Minegishi, J. Kamada, K. Takeuchi, K. Komatsu, T. Kitagawa
FULL PAPER
Scheme 6
Acid-Catalyzed Rearrangement of Bis(cyclohepta-2,4,6-
trien-1-yl)ethyne
The clean rearrangements of 7-ethynyl- and 7-vinylcyclo-
hepta-1,3,5-trienes led us to expect that bis(cyclohepta-
2,4,6-trien-1-yl)ethyne (21) would undergo a similar acid-
catalyzed rearrangement to initially form an allene 22. The
other cycloheptatrienyl group of this compound would
further rearrange to eventually give diphenylbutadiene (24)
(
Scheme 8). Heating of 21 at 100 °C for 50 h in THF/TFA
(5:1, v/v), however, gave only a complex mixture (Ͼ 7 com-
ponents) of unidentified products. The NMR analysis of
the mixture indicated the formation of aromatic products,
[
18]
but 24 was not detected.
A possible explanation for this
Scheme 7
result is that the rearrangement of the second cycloheptatri-
enyl group (i.e. from 22 to 24) is difficult, since the opening
of the three-membered ring of cation 23 is energetically un-
favorable due to delocalization of the positive charge over
the 3-phenylallyl group. The initial product 22, if produced,
appears to be consumed by thermal 1,5-hydrogen shifts to
form isomers of 22 that are inert to acid-catalyzed re-
arrangement.
by HPLC (SiO ), in an isomer ratio of 66:34 (Scheme 7).
2
Heating isolated (E)-20 and (Z)-20 under the same con-
ditions for 24 h showed no evidence of (E)/(Z) isomeriz-
ation. This indicates that the observed (E)-20/(Z)-20 ratio is
the result of the kinetically controlled opening of the three-
membered ring of 18 to isomers of 19.
Scheme 8
3500
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Eur. J. Org. Chem. 2003, 3497Ϫ3504

Conversion of 7-Ethynyl- and 7-Vinylcyclohepta-1,3,5-trienes to Substituted Benzene Derivatives
FULL PAPER
Rate Studies
The rate of the acid-catalyzed rearrangement of 7-ethyn-
ylcyclohepta-1,3,5-trienes 1aϪd is largely dependent on the
number and position of the tert-butyl substituents. For the
kinetic study, the reaction was conducted in [D ]THF/TFA
8
(
5:1, v/v) by using an NMR sample tube. The progress of
1
the reaction was monitored by integration of the H NMR
peaks. The decay of the starting compounds followed the
first-order kinetics. The obtained rate constants are listed
in Table 2.
Table 2. Rate constants (k
1
) for acid-catalyzed rearrangement, 13C
chemical shifts (δ), and calculated energy of the conformers/iso-
mers of 1aϪd
[
a]
δ(C-6)[b] [ppm] Relative energy[c] [kcal·molϪ1
k
[
1
]
10Ϫ6 sϪ1
]
1
1
1
1
a
b
c
0.879
28.6
46.8
322
122.7
117.7
102.5
91.5
2.75 (ax-CHT)
Scheme 9. Valence isomers and conformers of 7-ethynylcyclohepta-
1,3,5-triene 1 and its norcaradiene form
0
6
4
.00 (eq-CHT)
.62 (endo-NCD)
.65 (exo-NCD)
gap (kcal·molϪ1) decreased in the order 1a (4.65) Ͼ 1b
(4.13) Ͼ 1c (3.79) Ͼ 1d (1.17).
0.00 (ax-CHT)
0
4
5
.81 (eq-CHT)
.13 (endo-NCD)
.13 (exo-NCD)
The increase in the fraction of the norcaradiene form can
13
be also estimated by the C NMR chemical shifts. The
chemical shifts for C-1 and C-6 serve as indicators for the
position of the CHTϪNCD equilibrium, because these car-
bons change their orbital hybridization from sp2 to sp3
upon transformation from CHT to NCD. Due to rapid tau-
tomerization at room temperature, these carbon atoms
show a time-averaged signal between the aliphatic and the
aromatic regions. Assuming that the influence of the group
attached at C-5 (H or tBu) is negligible, the observed C-6
chemical shifts δ(C-6) reflect the position of equilibrium.
The chemical shift data in Table 2 show that the population
of the norcaradiene form increases in the order 1a Ͻ 1b
Ͻ 1c Ͻ 1d, in agreement with the DFT calculations. This
0.00 (ax-CHT)
0
3
4
.41 (eq-CHT)
.79 (endo-NCD)
.57 (exo-NCD)
d
3.47 (ax-CHT)
0
3
1
.00 (eq-CHT)
.44 (endo-NCD)
.17 (exo-NCD)
[
a]
[b]
, 23 °C. [c]
In CDCl
3
In [D
8
]THF/TFA (5:1, v/v), at 60 °C.
Obtained at the B3LYP/6-311ϩG(2d,p)//B3LYP/6-31G(d) level.
Thermal CHTϪNCD equilibrium is a key step in the tendency parallels the order of the rate constant, indicating
mechanism of this rearrangement, as shown in Schemes 2 that the CHTϪNCD composition at equilibrium is closely
and 4. Although unsubstituted cyclohepta-1,3,5-triene is far related to the reactivity toward the acid-catalyzed re-
more stable than norcaradiene, the energy level of the CHT arrangement.
form is known to be increased by the introduction of bulky
substituents to the olefinic carbon atoms.[
19Ϫ22]
For ex-
ample,
2,5-di-tert-butyl-7-ethynylcyclohepta-1,3,5-triene Conclusion
(1d) is known to exist as an equilibrium mixture of CHT
[22]
and NCD in a ratio 79.5:20.5.
7-Ethynyl- and 7-vinyl-substituted cyclohepta-1,3,5-
The energies calculated for the four conformers/isomers trienes were found to undergo clean rearrangement to form
of 1aϪd (Scheme 9) indicated that the most stable forms phenylallene and (1-propenyl)benzene derivatives respec-
are eq-CHT for 1a and 1d and ax-CHT for 1b and 1c tively, under acidic conditions. The rearrangement of 7-
(Table 2). The increased energy of the eq-CHT conformers ethynylcyclohepta-1,3,5-triene (1a) is very slow (half life of
of 1b and 1c relative to the axial forms may be due to steric 9 d at 60 °C in THF/TFA, 5:1, v/v), but is still sufficiently
repulsion between the 7-ethynyl group and the 1-tert-butyl rapid so as to occur without any significant thermal 1,5-
group. The most easily accessible NCD isomers are exo for hydrogen shift. This slow rate can be attributed to the very
1a and 1d and endo for 1b and 1c for a similar reason. low concentration of the norcaradiene form at CHTϪNCD
The difference in energy between CHT and NCD is 4.65 equilibrium, but a significant acceleration was observed
Ϫ1
kcal·mol for 1a. This value is only slightly lower than the when tert-butyl groups were introduced, which shifted the
reported CHTϪNCD free energy difference of 5.6 equilibrium to the NCD side. Ring-opening of the three-
Ϫ1
kcal·mol
obtained for unsubstituted cyclohepta-1,3,5- membered ring of the norcaradiene structure should be en-
[23]
triene by the HF/6-31G(d) level. The CHTϪNCD energy ergetically favorable for a clean acid-catalyzed rearrange-
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S. Minegishi, J. Kamada, K. Takeuchi, K. Komatsu, T. Kitagawa
ment to be realized. This requirement somewhat limits the (ϵCH), 83.4 (ϪCϵCH), 117.0 (C-6), 118.7 (C-2), 123.4 (C-3),
FULL PAPER
1
25.6 (C-5), 141.9 (C-1 or C-4), 150.1 (C-1 or C-4) ppm. IR (KBr):
scope of the present transformation, as illustrated by the
cases of cyano- and allenyl-substituted cyclohepta-1,3,5-
trienes, where side-reactions such as thermal 1,5-hydrogen
shifts override the acid-catalyzed rearrangement.
Ϫ1
ν˜ ϭ 3278 (ϵCϪH) cm . C17
H24 (228.37): calcd. C 89.41, H 10.59;
found C 89.16, H 10.42.
1,5-Di-tert-butyl-7-ethynylcyclohepta-1,3,5-triene (1c): Colorless
1
crystals, m.p. 43.2Ϫ44.5 °C. H NMR (270 MHz, CDCl
3
): δ ϭ 1.12
s, 9 H, tBu), 1.16 (br. s, 9 H, tBu), 1.68 (br. s, 1 H, ϵCH), 3.83
br. s, 1 H, 7-H), 4.95 (br. d, J ϭ 8.2 Hz, 1 H, 6-H), 5.96 (d, J ϭ
.4 Hz, 1 H, 2-H), 6.53 (dd, J ϭ 11.0, 6.4 Hz, 1 H, 3-H), 6.65 (dd,
(
(
6
Experimental Section
1
3
J ϭ 11.0, 1.3 Hz, 1 H, 4-H) ppm. C NMR (68 MHz, CDCl
δ ϭ 25.1 (br., C-7), 29.2 (br., CH ), 29.9 (CH ), 34.8 [br., C(CH
5.9 [C(CH ], 65.2 (br., ϵCH), 83.4 (br., ϪCϵCH), 102.5 (br.,
3
):
General: 1H and 13C NMR spectra were obtained with JEOL-
3 3
)
],
1
13
1
13
3
3
EX400 ( H, 400 MHz; C, 100 MHz), AL300 ( H, 300 MHz; C,
7
3
3 3
)
1
13
5 MHz), and GSX270 ( H, 270 MHz;
C, 68 MHz) spec-
C-6), 118.3 (C-2), 127.3 (br., C-3), 129.0 (br., C-4), 134.2 (br., C-1
or C-5), 146.3 (br., C-1 or C-5) ppm. IR (KBr): ν˜ ϭ 3279 (ϵCϪH)
cm . C17
H 10.80.
trometers. Peak assignments are based on DEPT, COSY, and HϪC
COSY measurements. High-resolution mass spectra were obtained
with JEOL JMS-HX110 and JMS700 mass spectrometers. Gel per-
meation chromatography (GPC) was performed using a Japan
Analytical Industry JAIGEL 1Hϩ2H polystyrene columns with
Ϫ1
H24 (228.37): calcd. C 89.41, H 10.59; found C 89.12,
2,5-Di-tert-butyl-7-ethynylcyclohepta-1,3,5-triene (1d): Pale-yellow
oil. H NMR (270 MHz, CDCl ): δ ϭ 1.11 (s, 18 H, tBu), 1.91 (br.
were synthesized by pre- s, 1 H, 7-H), 2.11 (d, J ϭ 2.6 Hz, 1 H, ϵCH), 4.37 (br. s, 2 H, 1,6-
[24]
1
chloroform as the eluent. Tropylium tetrafluoroborate
and 1,4-
3
di-tert-butyltropylium perchlorate[
25]
1
3
viously reported methods. THF was purified by distillation from
benzophenone ketyl prior to use. DFT calculations were performed
using the Gaussian 98 program.[ Geometry optimizations were
verified by frequency calculations.
H), 6.44 (br. s, 2 H, 3,4-H) ppm. C NMR (68 MHz, CDCl , 60
°C): δ ϭ 23.6 (br., C-7), 29.6 (CH ), 34.7 [C(CH ) ], 67.8 (ϵCH),
86.9 (ϪCϵCH), 91.5 (br., C-1,6), 125.7 (C-3,4), 144.5 (C-2,5) ppm.
IR (neat): ν˜ ϭ 3314 (ϵCϪH), 2110 (CϵC) cm . C H (228.37):
calcd. C 89.41, H 10.59; found C 89.51, H 10.60. The H NMR
spectroscopic data for 1d are in agreement with previously re-
ported data.
3
3
3 3
26]
Ϫ1
1
7
24
1
7-Ethynylcyclohepta-1,3,5-triene (1a): Acetylene gas (passed
through H SO ) was introduced into a graduated test tube contain-
2
4
[22]
ing 10 mL of THF at Ϫ66 °C until the volume of the solution
increased by 6.4 mL. The collected acetylene [approximated as 4.5 g
2,5-Di-tert-butyl-7-cyanocyclohepta-1,3,5-triene (12): A solution of
1,4-di-tert-butyltropylium perchlorate (2.05 g, 6.8 mmol) in
(
0.17 mol) assuming a density of 0.7 g mLϪ1 [27]] in THF was trans-
ferred through a cannula to a four-necked flask containing 50 mL CH
of THF at Ϫ66 °C. A 1.38 mol·L solution of BuLi in hexane
3
CN (70 mL) was added over a 5 min period to sodium cyanide
(1.74 g, 36 mmol) suspended in CH CN (200 mL) at Ϫ25 °C. The
mixture was stirred at Ϫ25 °C for 10 min and then for a further
tetrafluoroborate (3.04 g, 0.0171 mol) was added, and the mixture 10 min at room temperature. The solution was decanted and the
was stirred at 0 °C for 90 min. After the solvent had been evapo-
solvents evaporated. The precipitate was dissolved in Et O, and the
rated, the residual brown oil was purified by GPC to afford 7- solution was washed with 10% NaCl and dried (MgSO ) to give a
yellow oil (1.64 g). The oil was purified by MPLC (SiO , hexane/
Ϫ1
3
(12.0 mL, 0.0166 mol) was added dropwise over 20 min. Tropylium
2
4
ethynylcyclohepta-1,3,5-triene (1a, 1.35 g, 68%) and bis(cyclo-
hepta-2,4,6-trien-1-yl)ethyne (21, 99 mg, 5.6%).
2
1
Et
300 MHz, CDCl
.44 (br. s, 2 H, 1,6-H), 6.49 (br. s, 2 H, 3,4-H) ppm. C NMR
75 MHz, CDCl , 60 °C): δ ϭ 20.6 (br., C-7), 29.3 (CH ), 35.0
C(CH ], 86.0 (br., C-1,6), 120.9 (CN), 126.2 (br., C-3,4), 145.4
2
O, 9:1) to afford 12 (0.23 g, 15%) as a colorless oil. H NMR
(
4
(
[
3
): δ ϭ 1.12 (s, 18 H, tBu), 2.20 (br. s, 1 H, 7-H),
1
1
2
6
a: Pale-yellow oil. H NMR (400 MHz, CDCl
3
): δ ϭ 2.15 (d, J ϭ
13
.4 Hz, 1 H, ϵCH), 2.49 (br. s, 1 H, 7-H), 5.32 (m, 2 H, 1,6-H),
3
3
.15 (m, 2 H, 2,5-H), 6.62 (m, 2 H, 3,4-H) ppm. 13C NMR
3 3
)
(68 MHz, CDCl
3
): δ ϭ 31.3 (C-7), 68.4 (ϵCH), 85.5 (ϪCϵCH),
Ϫ1
(
(
C-2,5) ppm. IR (neat): ν˜ ϭ 2240, 2250 (CϵN) cm . C16
229.36): calcd. C 83.79, H 10.11; found C 83.71, H 10.20. The H
23
H N
1
1
(
22.7 (C-1,6), 124.8 (C-2,5), 130.9 (C-3,4) ppm. IR (neat): ν˜ ϭ 3303
Ϫ1
1
ϵCϪH), 2126 (CϵC) cm . The H NMR spectroscopic data are
NMR spectroscopic data are in agreement with previously re-
ported data.
[28]
in agreement with previously reported data.
[29]
Di-tert-butyl-7-ethynylcyclohepta-1,3,5-trienes 1b؊d: A mixture of
isomers of di-tert-butyl-7-ethynylcyclohepta-1,3,5-trienes was ob-
tained by the addition of 1,4-di-tert-butyltropylium perchlorate
2,5-Di-tert-butyl-7-vinylcyclohepta-1,3,5-triene (17): A solution of
vinylmagnesium bromide in THF (40 mL) was prepared from vinyl
bromide (3.4 g, 32 mmol) and magnesium (0.75 g, 31 mmol). 1,4-
Di-tert-butyltropylium perchlorate (4.5 g, 15 mmol) was added to
this solution at 0 °C, and the temperature was raised to room tem-
perature. Aqueous workup gave a yellow oil (3.7 g), a portion
(
(
3.09 g, 10.2 mmol) to a THF solution (110 mL) of ethynyllithium
12.1 mmol) by a procedure similar to that employed for the syn-
2
thesis of 1a. Separation by MPLC (SiO , hexane) gave a fraction
containing pure 1d (0.46 g, 20%) and one (1.85 g) containing 1b (0.91 g) of which was purified by HPLC (Dupont Zorbax SIL, hex-
1
and 1c. A portion (0.88 g) of the latter fraction was further sepa-
ane) to afford 17 (0.22 g, 26%) as a colorless oil. H NMR
rated by HPLC (Waters µPorasil, hexane) to give 1b (0.30 g, 27%) (270 MHz, CDCl ): δ ϭ 1.10 (s, 18 H, tBu), 1.53 (m, 1 H, 7-H),
3
and 1c (0.51 g, 46%).
3.94 (br. d, J ϭ 4.3 Hz, 2 H, 1,6-H), 5.02 (d, J ϭ 10.2 Hz, 1 H, ϭ
CH
CH
2
), 5.09 (d, J ϭ 17.2 Hz, 1 H, ϭCH
2
), 5.93 (m, 1 H, ϪCHϭ
), 6.34 (s, 2 H, 3,4-H) ppm. C NMR (68 MHz, CDCl ): δ ϭ
), 34.1 (br., C-7), 34.8 [C(CH ], 85.4 (br., C-1,6), 113.2
), 123.8 (br., C-3,4), 141.0 (ϪCHϭCH ), 144.3 (C-2,5) ppm.
1
,4-Di-tert-butyl-7-ethynylcyclohepta-1,3,5-triene (1b): Colorless
1
3
1
2
3
crystals, m.p. 84.5Ϫ86.5 °C. H NMR (270 MHz, CDCl
3
): δ ϭ 1.18
2
(
9.5 (CH
ϭCH
HRMS (EIϩ): calcd. for C17
3
3 3
)
(s, 9 H, tBu), 1.20 (s, 9 H, tBu), 1.68 (br. s, 1 H, ϵCH), 3.97 (br.
2
2
s, 1 H, 7-H), 5.43 (t, J ϭ 9.3 Hz, 1 H, 6-H), 6.07 (d, J ϭ 6.7 Hz, 1
H, 2-H), 6.30 (d, J ϭ 9.3 Hz, 1 H, 5-H), 6.54 (dd, J ϭ 6.7, 1.2 Hz,
1
7
ϩ
H26 230.2035 [M ]; found 230.2042.
H, 3-H) ppm. 13C NMR (68 MHz, CDCl
, 60 °C): δ ϭ 29.0 (C-
], 36.2 [C(CH
Bis(cyclohepta-2,4,6-trien-1-yl)ethyne (21):[28] A 1.40 mol·LϪ1 solu-
3 3
) ], 66.4 tion of BuLi in hexane (2.1 mL, 2.9 mmol) was added to 1a (0.34 g,
3
), 29.7 (CH
3
), 30.2 (CH
3
), 35.6 [C(CH
3
)
3
3502
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2003, 3497Ϫ3504

Conversion of 7-Ethynyl- and 7-Vinylcyclohepta-1,3,5-trienes to Substituted Benzene Derivatives
FULL PAPER
2
.9 mmol) in dry THF (50 mL) at Ϫ68 °C over a 10 min period. 1,3-Di-tert-butyl-2-(1,2-propadienyl)benzene (7): Colorless oil. 1
H
Tropylium tetrafluoroborate (0.52 g, 2.9 mmol) was added at Ϫ68
NMR (270 MHz, CDCl
3
): δ ϭ 1.45 (s, 18 H, tBu), 4.69 (d, J ϭ
°
C, and the reaction mixture was stirred at room temperature for 6.9 Hz, 2 H, ϭCH
2
), 6.53 (t, J ϭ 6.9 Hz, 1 H, ϪCHϭ), 7.16 (t,
1
3
2
h. Water (1 mL) was added, and the THF was evaporated under
vacuum. Et O and 10% NaCl were added to the residue and the
layers separated. The Et O layer was dried (MgSO ) and the sol-
vents were evaporated to give 21 (0.58 g, 97%) as a yellow oil. H
J ϭ 7.9 Hz, 1 H, 5-H), 7.35 (d, J ϭ 7.9 Hz, 2 H, 4,6-H) ppm.
C
2
NMR (68 MHz, CDCl
CH ), 94.7 (ϪCHϭ), 124.4 (C-4,6), 126.7 (C-5), 132.8 (C-2), 149.5
(C-1,3), 208.4 (ϭCϭ) ppm. IR (neat): ν˜ ϭ 1959 (CϭCϭC) cm
3 3 3 3
): δ ϭ 31.8 (CH ), 36.8 [C(CH ) ], 74.8 (ϭ
2
4
2
1
Ϫ1
.
ϩ
NMR (270 MHz, CDCl
3
): δ ϭ 2.53 (t, J ϭ 4.6 Hz, 2 H, 7-H), 5.37 HRMS (FABϩ): calcd. for C17
H
24 228.1878 [M ]; found 228.1884.
(
m, 4 H, 1,6-H), 6.17 (m, 4 H, 2,5-H), 6.66 (m, 4 H, 3,4-H) ppm.
C NMR (68 MHz, CDCl ): δ ϭ 31.7 (C-7), 81.8 (CϵC), 123.9
3
1
13
1-tert-Butyl-3-(1,2-propadienyl)benzene (8): Colorless oil. H NMR
(
270 MHz, CDCl
H, ϭCH ), 6.17 (t, J ϭ 6.6 Hz, 1 H, ϪCHϭ), 7.09Ϫ7.37 (m, 4 H,
arom. H) ppm. C NMR (68 MHz, CDCl
[C(CH ], 78.7 (ϭCH ), 94.3 (ϪCHϭ), 123.7, 123.9, 124.0, 128.3
(arom. CH), 133.4, 151.5, 209.8 (arom. C) ppm. IR (neat): ν˜ ϭ
3
): δ ϭ 1.32 (s, 9 H, tBu), 5.14 (d, J ϭ 6.6 Hz, 2
(C-1,6), 124.6 (C-2,5), 131.0 (C-3,4) ppm.
2
1
3
Acid-Catalyzed Rearrangement in THF/TFA. General Procedure: A
weighed amount of cycloheptatriene derivative was dissolved in dry
THF (5 mL) under argon. TFA (1 mL) was added, and the solution
was heated at 60 °C until TLC analysis indicated that all the start-
ing compound had been consumed. The reaction mixture was di- [M ]; found 172.1253.
luted with Et O and washed with 5% NaHCO and with 10%
NaCl. The organic layer was dried (MgSO ) and the solvents were
3 3
): δ ϭ 31.3 (CH ), 34.7
3
)
3
2
Ϫ1
1943 (CϭCϭC) cm . HRMS (FABϩ): calcd. for C13H16 172.1252
ϩ
2
3
D. Rearrangement of 1d in THF/TFA: Heating of compound 1d
44.0 mg) in THF/TFA (5:1, v/v) at 60 °C for 3 h gave a yellow oil,
which was shown to be essentially pure 3 by H NMR spectroscopy.
Purification by HPLC (DuPont Zorbax SIL, hexane) gave 3
4
(
evaporated. The residue was subject to separation and structure
analysis.
1
1
13
A. Rearrangement of 1a in THF/TFA: Heating of compound 1a
(41.6 mg, 95%), which showed H and C NMR spectra identical
(39.1 mg, 0.384 mmol) in THF/TFA (5:1, v/v) at 60 °C for 40 d
to those of 3 obtained by the rearrangement of 1b.
gave a yellow oil, the purification of which by HPLC (DuPont Zor-
bax SIL, hexane) gave 2 (35.8 mg, 92%) as a colorless oil. H NMR
1
E. Rearrangement of 17 in THF/TFA: Heating of compound 17
(
90.0 mg) in THF/TFA (5:1, v/v) at 60 °C for 29 h gave a yellow
(
270 MHz, [D
CH ), 6.12 (t, J ϭ 6.8 Hz, 1 H, ϪCHϭ), 7.05Ϫ7.33 (m, 5 H, arom.
H) ppm. 1 C NMR (68 MHz, [D
CH ), 94.3 (ϪCHϭ), 127.3 (C-2,6 or C-3,5), 127.5 (C-4), 129.2 (C-
,6 or C-3,5), 134.9 (C-1), 210.5 (ϭCϭ) ppm. These spectroscopic
8
]THF/TFA, 5:1): δ ϭ 5.06 (d, J ϭ 6.8 Hz, 2 H, ϭ
oil, which was shown to be a mixture of (E)-20 and (Z)-20 (66:34
2
1
3
molar ratio) by H NMR spectroscopy. The mixture was separated
8
]THF/TFA, 5:1): δ ϭ 78.5 (ϭ
by HPLC (DuPont Zorbax SIL, hexane) to afford (E)-20 (41.8 mg,
2
46%) and (Z)-20 (21.3 mg, 24%).
2
[30,31]
data are in agreement with previously reported data.
(
E)-1,4-Di-tert-butyl-2-(1-propenyl)benzene [(E)-20]: Colorless oil.
1
H NMR (270 MHz, CDCl
tBu), 1.90 (dd, J ϭ 6.6, 2.0 Hz, 3 H, ϭCHCH
5.2, 6.6 Hz, 1 H, ϭCHCH ), 7.01 (dd, J ϭ 15.2, 2.0 Hz, 1 H,
3
): δ ϭ 1.32 (s, 9 H, tBu), 1.40 (s, 9 H,
B. Rearrangement of 1b in THF/TFA: Heating compound 1b
34.5 mg) in THF/TFA (5:1, v/v) at 60 °C for 24 h gave a yellow
3
), 5.85 (dq, J ϭ
(
1
3
oil, which was shown to be a mixture of 3 and 6 (68:32 molar ratio)
1
ϪCHϭCHCH ), 7.18 (dd, J ϭ 8.3, 2.4 Hz, 1 H, 5-H), 7.28 (d, J ϭ
by H NMR spectroscopy. The mixture was separated by GPC to
3
8
.3 Hz, 1 H, 6-H), 7.32 (d, J ϭ 2.4 Hz, 1 H, 3-H) ppm. 1 C NMR
3
afford 3 (18.7 mg, 54%) and 6 (6.0 mg, 18%) in pure forms.
(
68 MHz, CDCl
3
): δ ϭ 18.8 (ϭCHCH
], 34.2 [C(CH ], 35.3 [C(CH
), 126.5 (C-3), 133.7 (ϪCHϭCHCH
143.7, 148.4 (arom. C) ppm. C17 26 (230.39): calcd. C 88.63, H
ϪCHϭ), 7.17 (dd, J ϭ 8.6, 2.3 Hz, 1 H, 5-H), 7.31 (d, J ϭ 8.6 Hz, 11.37; found C 88.46, H 11.64.
H, 6-H), 7.50 (d, J ϭ 2.3 Hz, 1 H, 3-H) ppm. 13C NMR (68 MHz,
CDCl ): δ ϭ 31.2 (CH ), 31.4 (CH ), 34.2 [C(CH ], 35.0
], 77.5 (ϭCH ), 94.4 (ϪCHϭ), 123.9 (C-5), 125.8 (C-6),
26.9 (C-3), 131.2 (C-2), 143.6 (C-1 or C-4), 148.5 (C-1 or C-4),
3
), 31.30 [C(CH
], 123.6 (C-5), 125.3 (C-
), 137.6,
3 3
) ], 31.32
1
,4-Di-tert-butyl-2-(1,2-propadienyl)benzene (3): Colorless oil. 1
H
[C(CH
3
)
3
3
)
3
3 3
)
NMR (270 MHz, CDCl
tBu), 5.09 (d, J ϭ 6.8 Hz, 2 H, ϭCH
3
): δ ϭ 1.31 (s, 9 H, tBu), 1.43 (s, 9 H, 6), 126.1 (ϭCHCH
), 6.81 (t, J ϭ 6.8 Hz, 1 H,
3
3
2
H
1
(
Z)-1,4-Di-tert-butyl-2-(1-propenyl)benzene [(Z)-20]: Colorless oil.
3
3
3
3 3
)
1
H NMR (270 MHz, CDCl
tBu), 1.63 (dd, J ϭ 6.9, 1.8 Hz, 3 H, ϭCHCH
11.2, 6.9 Hz, 1 H, ϭCHCH ), 6.86 (dd, J ϭ 11.2, 1.8 Hz, 1 H,
3
): δ ϭ 1.30 (s, 9 H, tBu), 1.36 (s, 9 H,
[
C(CH
3
)
3
2
3
), 5.74 (dq, J ϭ
1
2
Ϫ1
09.5 (ϭCϭ) ppm. IR (neat): ν˜ ϭ 1942 (CϭCϭC) cm . HRMS
3
ϩ
ϪCHϭCHCH ), 7.09 (d, J ϭ 2.2 Hz, 1 H, 3-H), 7.20 (dd, J ϭ 8.2,
(FABϩ): calcd. for C17
H
24 228.1878 [M ]; found 228.1875.
3
2
.2 Hz, 1 H, 5-H), 7.33 (d, J ϭ 8.2 Hz, 1 H, 6-H) ppm. 1 C NMR
3
1
1
-tert-Butyl-4-(1,2-propadienyl)benzene (6): Colorless oil. H NMR
(68 MHz, CDCl
], 34.1 [C(CH
), 125.4 (C-6), 129.3 (C-3), 133.9 (ϪCHϭCHCH
3
3
): δ ϭ 14.3 (ϭCHCH
3
), 30.6 [C(CH
], 123.2 (C-5), 125.0 (ϭ
), 136.2,
3 3
) ], 31.3
(400 MHz, CDCl
3
): δ ϭ 1.31 (s, 9 H, tBu), 5.12 (d, J ϭ 6.8 Hz, 2 [C(CH
3
)
3
3
)
3
], 35.4 [C(CH
3 3
)
H, ϭCH
2
), 6.15 (t, J ϭ 6.8 Hz, 1 H, ϪCHϭ), 7.23 (d, J ϭ 8.6 Hz, CHCH
3
2
H, 2,6-H or 3,5-H), 7.33 (d, J ϭ 8.6 Hz, 2 H, 2,6-H or 3,5-H)
145.1, 147.8 (arom. C) ppm. HRMS (EIϩ): calcd. for C17H
26
230.2035[M ]; found 230.2038.
13
ϩ
ppm. C NMR (100 MHz, CDCl
C(CH ], 78.6 (ϭCH ), 93.6 (ϪCHϭ), 125.6 (C-2,6 or -C-3,5),
26.4 (C-2,6 or C-3,5), 130.9 (C-4), 150.0 (C-1), 209.8 (ϭCϭ) ppm.
3
): δ ϭ 31.3 (CH
3
), 34.5
[
1
3
)
3
2
Kinetic Measurements of the Acid-Catalyzed Rearrangements of
1a؊d: A solution of a starting compound (0.02Ϫ0.57 mmol) in a
Ϫ1
IR (neat): ν˜ ϭ 1942 (CϭCϭC) cm . The NMR spectroscopic data
are in agreement with previously reported data.[
32]
mixture of [D ]THF (0.7 mL) and TFA (0.14 mL) was placed in an
8
NMR sample tube. The tube was filled with argon, sealed, and
C. Rearrangement of 1c in THF/TFA: Heating of compound 1c
heated in a thermostatted oil bath at 60 °C. The reaction was moni-
1
(31.5 mg) in THF/TFA (5:1, v/v) at 60 °C for 24 h gave a yellow tored by the integrations of the following H NMR peaks. 1a, 1,4-
oil, which was shown to be a mixture of 7 and 8 (93.4:6.6 molar
H; 1b, 6-H; 1c, 2-H; 1d, 1,6-H; 2, ϭCH
2 2
; 3, ϭCH and ϪCHϭ;
ratio) by 1H NMR spectroscopy. The mixture was separated by 6, ϭCH
; 7, ϭCH ; 8, ϪCHϭ. The reaction followed first-order
2
2
GPC to afford 7 (28.0 mg, 89%) and 8 (2.0 mg, 6%).
www.eurjoc.org
kinetics over three half lives.
Eur. J. Org. Chem. 2003, 3497Ϫ3504
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3503

S. Minegishi, J. Kamada, K. Takeuchi, K. Komatsu, T. Kitagawa
FULL PAPER
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This work was supported by a Grant-in-Aid for Scientific Research
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17c]
E.
[31]
[32]
[
The absence of the (E,E), (E,Z), and (Z,Z) isomers of 1,4-di-
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1
phenyl-1,3-butadiene was confirmed by comparing the
H
NMR spectrum of the reaction mixture with those reported:
Received April 22, 2003
3504
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Eur. J. Org. Chem. 2003, 3497Ϫ3504