Cu(I)-mediated cycloaddition reaction of zirconacyclopentadienes with fumaronitrile and application for synthesis of monocyano-substituted pentacenes

Article abstract of DOI:10.1016/j.tetlet.2007.07.075

Cu(I)-mediated reactions of zirconacyclopentadienes with fumaronitrile afforded the corresponding dicyanocyclohexadiene and benzonitrile derivatives in good yields. These products were selectively prepared by controlling the reaction temperature. Furtherm

Full text of DOI:10.1016/j.tetlet.2007.07.075

Tetrahedron Letters 48 (2007) 6726–6730
Cu(I)-mediated cycloaddition reaction
of zirconacyclopentadienes with fumaronitrile and
application for synthesis of monocyano-substituted pentacenes
Tamotsu Takahashi,a,* Yanzhong Li,a Jinghan Hu,a Fanzhi Kong,a
Kiyohiko Nakajima,b Lishan Zhoua and Ken-ichiro Kannoa
aCatalysis Research Center, Hokkaido University, and SORST, Japan Science and Technology Agency (JST),
Sapporo 001-0021, Japan
bDepartment of Chemistry, Aichi University of Education, Igaya, Kariya 448-8542, Japan
Received 24 May 2007; revised 12 July 2007; accepted 17 July 2007
Available online 21 July 2007
Abstract—Cu(I)-mediated reactions of zirconacyclopentadienes with fumaronitrile afforded the corresponding dicyanocyclohexadi-
ene and benzonitrile derivatives in good yields. These products were selectively prepared by controlling the reaction temperature.
Furthermore, the reaction was applicable for monocyano-substituted pentacene derivatives.
Ó 2007 Published by Elsevier Ltd.
Transition-metal mediated cycloaddition reactions of
unsaturated molecules such as alkynes, alkenes, and so
on have been widely applied for construction of carbo-
cyclic and heterocyclic skeletons.1
R
R
R
R
R
R
MeO2C
R
R
CO2Me
CO2Me
CO2Me
CuCl, THF
ZrCp2
We have developed a series of cycloaddition reactions of
zirconacycles with various unsaturated compounds to
afford the corresponding cycloadducts in good to excel-
lent yields.2–4 In general the reaction produces a formal
[4+2] cycloadduct. When a zirconacyclopentadiene
reacts with an alkyne in the presence of copper(I) or
nickel(II) salt, the corresponding benzene derivatives
were produced.2 The reactions of zirconacyclopentadi-
enes with dimethyl fumarate or maleate, the correspond-
ing 1,3-cyclohexadiene derivative was obtained having
trans configuration of the two ester groups as shown
in Scheme 1.2d,e
Scheme 1.
is dependent on the reaction temperature. Furthermore,
the reaction was applied for the synthesis of monocy-
ano-substituted pentacenes.5
A
typical experimental procedure is as follows
(Scheme 2): To a solution of zirconacyclopentadiene
1a prepared from Cp2ZrCl2 (1.1 mmol), n-BuLi
(2.2 mmol), and 3-hexyne (2.0 mmol) in THF,6 fumaro-
nitrile (2.2 mmol), and CuCl (2.2 mmol) were added and
the mixture was stirred at room temperature for 3 h.
After quenching with 3 M aqueous HCl and usual work-
up afforded a mixture of the corresponding dicyano-
cyclohexadiene 2a and benzonitrile 3a in 39% and 40%
yields, respectively. The trans configuration of the
two cyano groups in cyclohexadiene 2a was confirmed
by X-ray crystallographical analysis. The formation
of benzonitrile 3 is significantly different from the reac-
tion with dimethyl fumarate or maleate as shown in
Scheme 1.
During the further investigation of the zirconium-medi-
ated cycloaddition reactions, we found that the reaction
of zirconacyclopentadienes 1 with fumaronitrile affor-
ded two types of formal [4+2] cycloadducts, dicyano-
cyclohexadiene 2 and benzonitrile 3 as shown in Scheme
2. It is interesting to note that the formation of 2 and 3
*
Corresponding author. Tel./fax: +81 11 706 9149; e-mail: tamotsu@
cat.hokudai.ac.jp
0040-4039/$ - see front matter Ó 2007 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2007.07.075

T. Takahashi et al. / Tetrahedron Letters 48 (2007) 6726–6730
6727
R
R
R
CN
CN
-30 °C
2
3
NC
(2 equiv)
R
R
R
CN
R
R
(2 equiv)
2
n
-BuLi
R
R
Cp2ZrCl2
Cp2ZrBu2
ZrCp2
rt, 3 h
CuCl (2 equiv),
THF
THF, -78 °C
R
R
CN
R
50 °C
1
R
Scheme 2.
Table 1. Reaction of zirconacyclopentadienes with fumaronitrile in the
Table 2. Temperature-dependence of the reaction of zirconacyclo-
presence of CuCl at room temperature
pentadienes with fumaronitrile
Entry
Zirconacyclopentadiene
Time
(h)
Yielda
(%)
Entry
Zirconacyclopentadiene
Temp
(°C)
Yielda (%)
2
3
2
3
1
2
3
4
5
1a (R = Et)
1a
50
À30
50
À30
—
(75)
—
(70)
(43)
74(64)
—
78(64)
—
R
R
R
R
1
2
3
4
1a (R = Et)
1b (R = Pr)
1b
3
3
24
3
39
29
19
—
40
33
41
44
R
R
R
R
1b (R = Pr)
1b
ZrCp2
Pr
ZrCp2
R
1c (R = Ph)
1c (R = Ph) À30
—
6
7
50
À30
—
(87)
82(66)
—
ZrCp2
1d
5
6
1d (R = Pr)
1e (R = Ph)
3
3
22
—
51
50
ZrCp2
Pr
a GC yields. Isolated yields were given in parentheses.
R
a Isolated yields.
cyclohexadienes 2 were produced at À30 °C in compara-
Table 1 summarizes the result of the reactions of a series
of zirconacyclopentadienes with fumaronitrile at room
temperature. The product ratio of cyclohexadiene 2 to
benzonitrile 3 was dependent on the reaction time and
substituents on the zirconacycles employed. When the
reaction of zirconacyclopentadiene 1a or 1b with fumaro-
nitrile was carried out at room temperature for 3 h, the
product ratio of 2–3 was almost 1:1 (entries 1 and 2). On
the other hand, when the reaction time was prolonged
for 24 h, the amount of benzonitrile 3b increased with
decrease of cyclohexadiene 2b (entry 3). This suggested
that conversion of cyclohexadiene 2 into 3 occurred dur-
ing the reaction.
ble yields (entries 2, 4, 5, and 7).
Interestingly, the cycloaddition of zirconacycle 1 with
fumaronitrile occurred only at the C@C bond to afford
the carbocycles 2 and 3. No pyridine derivatives via the
cycloaddition at the C„N bond were observed. This is
striking contrast to the reaction of cobaltacyclopentadi-
ene with acrylonitrile to afford a mixture of the corre-
sponding cyclohexadiene and pyridine derivatives.7
Thus, the carbocycle-selective nature is a unique charac-
teristic of the present Zr-mediated reaction.
Table 3 shows the results of the reaction of zirconacyclo-
pentadiene 1 with maleonitrile. Compared with fumaro-
nitrile, the reactivity of maleonitrile was lower, and at
À30 °C the reaction did not take place (entry 1). At
0 °C, the reaction with zirconacycle 1b (R = Pr) pro-
ceeded to afford the corresponding cis-dicyanocyclohex-
adiene 4b along with a small amount of trans-isomer 2b
(entry 2). At room temperature, the ratio of cis-isomer 4b
was decreased on increasing the ratio of trans-isomer 2b
and benzonitrile 3b (entry 3). This suggests that isomer-
ization of cis-isomer 4b into trans-isomer 2b occurred
during the reaction. The similar phenomena were ob-
served in the reaction of zirconacycle 1a (R = Et, entries
4 and 5). The difference of the product ratios between the
reaction of 1a (R = Et) and 1b (R = Pr) could be attrib-
uted to the different steric effects of their substituents.
In contrast to the alkyl-substituted zirconacycles, tetra-
phenyl-substituted one 1c afforded the corresponding
benzonitrile 3c as a single product (entry 4). The same
tendency was observed in the reactions with bicyclic
zirconacyclopentadienes, that is, the mixture of cyclo-
hexadiene 2d and benzonitrile 3d was obtained from
the reaction with alkyl-substituted zirconacycle 1d,
whereas only the corresponding benzonitrile derivative
3e was obtained from phenyl-substituted 1e (entry 6).
It is worth noting that by controlling the reaction tem-
perature highly selective formation of each product
was achieved. Table 2 summarized the results of temper-
ature-dependence of the cycloaddition reactions. When
the reactions were carried out at 50 °C, the correspond-
ing benzonitrile derivatives 3 were obtained as single
products (entries 1, 3, and 6). On the other hand, only
The reaction was applied for the synthesis of mono-
cyano-substituted pentacenes as shown in Scheme 3. In

6728
T. Takahashi et al. / Tetrahedron Letters 48 (2007) 6726–6730
Table 3. Reaction of zirconacyclopentadienes with maleonitrile
A plausible mechanism for the reaction of a zirconacy-
clopentadiene with fumaronitrile is shown in Scheme
4. In the reaction of zirconacyclopentadiene 1 with
CuCl, transmetalation between zirconium and cop-
per(I)15,16 occurs to generate dimetalated butadiene
intermediate 8, which adds to fumaronitrile to produce
intermediate 9. Then, oxidative ring closing of 9 can
produce cyclohexadiene 2 along with elimination of
the low valent metal species. At low temperature, 2 is
stable. After treatment of the low valent metal species
with 3 M HCl, 2 is isolated as a single product. At
50 °C, 2 reacts with the low valent metal species, which
act as a base to remove HCN from 2. As a result, benzo-
nitrile 3 is formed as a single product.
R
R
R
R
R
CN
CN
R
R
CN
CN
R
R
CN
NC
CN
1
+
+
CuCl, THF
12 h
R
R
R
4
2
3
Entry
R
Temp
Yielda (%)
4
2
3
1
2
3
4
5
Pr
Pr
Pr
Et
Et
À30 °C
0 °C
rt
0 °C
rt
0
72
7
29
0
0
2
28
14
15
0
0
41
32
55
In order to make clear the elimination of HCN from the
once formed 2 in the solution, the following reactions
were carried out. First, the reaction mixture of zircona-
cyclopentadiene 1a (R = Et) with fumaronitrile and
CuCl was quenched by D2O. No deuterated products
were obtained. This indicates that metalated species of
2 or 3 were not formed as the products before hydrolysis
at low temperature. In addition, to the reaction mixture
of 1b with fumaronitrile, which gives 2b as a product,
dicyanocyclohexadiene 2a was added as shown in
Scheme 5. Then dicyanocyclohexadiene 2a was con-
verted into 3a. This result suggests that the reaction
mixture after formation of cyclohexadiene 2 is basic
enough to cause the elimination of HCN from 2. These
results strongly support our proposed mechanism,
which explains the temperature-dependent products.
a NMR yields.
1997, it was reported that non-substituted pentacene
had high carrier mobility comparable to amorphous
silicon.8 Since then non-substituted pentacene has
shown the highest carrier mobility among organic com-
pounds as an organic film. However, non-substituted
pentacene has a critical problem, which is insoluble in
organic solvents at ambient temperature. Therefore,
we reported the preparation method (homologation
method) of the substituted pentacenes as soluble pentac-
enes.5,9 The substituents can control the properties of
pentacenes. After our report, several groups reported
substituted pentacene derivatives.10–14
Here, we applied our novel method to the formation of
monocyano-substituted soluble pentacenes. In this reac-
tion, the same reaction conditions were used for
bis(propargyl)anthracenes 5a,b to afford the corre-
sponding monocyano-substituted dihydropentacenes
6a,b, in 45% and 70% yield, respectively. These dihydro-
pentacene derivatives were aromatized with DDQ to
produce the corresponding monocyanopentacenes 7a,b
as dark-blue solid in 48% and 50% yield, respectively.
In summary, cycloaddition reaction of zirconacyclo-
pentadiene with fumaronitrile was demonstrated.
Although a mixture of cyclohexadiene 2 and benzonit-
rile 3 was obtained from the reaction at room tempera-
ture, a simple control of the reaction temperature led to
selective preparation of each product. Furthermore, the
method was applied to synthesize monocyano-substi-
tuted pentacenes.
R
R
Bu
Bu
R
R
Bu
Bu
Bu
Bu
R
R
Bu
Bu
Bu
Bu
R
R
R
R
CN
1) Cp2ZrBu2
DDQ
R
R
CN
Bu
Bu
2)
toluene
100 °C, 1 h
NC
CN
CuCl, THF, 50 °C
5a (R = H)
5b (R = Et)
6a (R = H): 45%
6b (R = Et): 70%
7a (R = H): 48%
7b (R = Et): 50%
Scheme 3.
NC
R
CN
R
R
R
R
R
[M]
R
R
R
CN
CN
NC
Cu
[M]
R
R
CN
R
R
CN
CuCl
Cu
[M]
-30 °C
2 [M]
ZrCp2
50 °C
R
R
2
R
HCN
R
1
R
8
R
3
R
9
[M] = Cp2ZrCl or Cu
Scheme 4.

T. Takahashi et al. / Tetrahedron Letters 48 (2007) 6726–6730
6729
Et
Et
Et
CN
CN
Et
Et
Pr
Pr
Et
Et
CN
Pr
Pr
CN
Pr
+
Pr
ZrCp2
Pr
Pr
Pr
CN
CN
Et
Pr
Pr
NC
2b
2a (0.5 mmol)
CN
50 °C, 12 h
3a (38%)
3b (40%)
CuCl, THF
-30 °C, 3 h
Pr
+
+
2a (38%)
+ 2b (24%)
low valent metal species
1b (0.5 mmol)
Scheme 5.
Tour, J. M.; Miller, J. A.; Cederbaum, F. E.; Swanson, D.
R.; Takahashi, T. J. Am. Chem. Soc. 1989, 111, 3336–
3346.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2007.
07.075.
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