Noncatalytic, solvent-free thermal formation of cyclic trimers using 1,6-bis(acyloxymethyl)hexa-2,4-diyne derivatives

Article abstract of DOI:10.1016/j.tetlet.2004.01.134

The thermal reactivity of diacetylenes in the liquid phase was studied extensively to elucidate the cooperative mechanism of polymerization.

Full text of DOI:10.1016/j.tetlet.2004.01.134

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2671–2675
Noncatalytic, solvent-free thermal formation of cyclic trimers using
,6-bis(acyloxymethyl)hexa-2,4-diyne derivatives
1
a
Shin-ichi Taniguchi, Toshihiro Yokoi, Akira Izuoka, Michio M. Matsushita and
a
b
a
a,
*
Tadashi Sugawara
a
Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku,
Tokyo 153-8902, Japan
b
Department of Materials and Biological Sciences, Faculty of Science, Ibaraki University, 2-1-1 Bunkyo,
Mito, Ibaraki 310-8512, Japan
Received 22 October 2003; revised 22 January 2004; accepted 23 January 2004
Abstract—The thermal reactivity of diacetylenes in the liquid phase was studied extensively to elucidate the cooperative mechanism
of polymerization.
Ó 2004 Elsevier Ltd. All rights reserved.
Heat, UV light irradiation, or c-irradiation can induce
polymerization of diacetylenes in the solid state, espe-
cially when they are arranged in an appropriate orien-
R
R
R
R
R
R
R
R
R
R
1
tation for 1,4-addition. Polymerization can even occur
in the liquid crystalline phase if the diacetylenes are
sufficiently reactive in the temperature range over which
R
R
R
R
R
R
R
2
the mesophase exists.
R
5
a
5b
5c
Polymerization of diacetylenes in neat liquid without
catalysts would be advantageous from the environ-
mental standpoint. However, because the liquid-phase
polymerization of diacetylenes requires extreme condi-
R = CH2OCOCH3
Symmetrically disubstituted diacetylenes 1, 2 and 3, 4
were prepared by Eglinton coupling of the monosub-
stituted acetylene derivatives (Scheme 1). The differen-
tial scanning calorimetry thermogram (scanning rate
3
tions, it has scarcely been investigated. In this study, we
carried out the liquid-phase polymerization of diacetyl-
enes with long alkyl chains (1, 2) and acyloxymethyl
groups (3, 4) at both ends of a 1,3-butadiynyl moiety
and found that bis(acyloxymethyl)-1,3-butadiynes 3 and
4
2
°C/min) of triaconta-14,16-diyne (1) exhibited only an
endothermic peak at 54.5–55.0 °C, corresponding to the
melting process, whereas the thermogram of bis(acetyl-
oxymethyl) derivative 3 showed a broad exothermic
peak above 120 °C (DH ¼ 67:0 kJ/mol), accompanied by
an endothermic peak (melting process) at 30.0–30.5 °C
4
polymerized rapidly in the melt state at 120 °C.
Moreover, we isolated small amounts of cyclic trimers
a (1,2,4-triethynylbenzene derivative) and 5b (1,2,3-
5
triethynylbenzene derivative), and the corresponding
heptyl analogues, respectively. This result provided clues
about the cooperative mechanism of the cyclic trimer-
ization.
(
Fig. 1). The exothermic peak can be ascribed to the heat
of oligomerization of diacetylene 3.
The liquid-phase polymerization of diacetylenes 1, 2 and
, 4 was carried out in a sealed Pyrex tube under a
3
nitrogen atmosphere at 110 °C for 24 h. Although
diacetylene monomer 1, 2 did not polymerize under
these conditions, diacetylene 3 polymerized smoothly.
The conversion yield of 3 was estimated to be 76% by gel
Keywords: Liquid-phase polymerization; Diacetylene; Triethynylbenz-
ene; Dewar benzene derivative.
5
*
Corresponding author. Tel.: +81-3-5454-6742; fax: +81-3-5454-6997;
e-mail: suga@pentacle.c.u-tokyo.ac.jp
0
040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.01.134

2672
S. Taniguchi et al. / Tetrahedron Letters 45 (2004) 2671–2675
(
a)
H
H
CmH2m+1
CmH2m+1
CmH2m+1
1
2
( m =11)
( m =13)
(
b)
CH2OH
HOCH2
O
CH2OH
(
c)
O
CnH2n+1
CnH2n+1
O
O
3
4
( n =1)
( n =7)
Scheme 1. Reagents and reactions: (a) Cu(OAc)
56%. (c) C O, Et
2
ÆH
2
O/Pyridine, MeOH, 1 (m ¼ 11); 41%, 2 (m ¼ 13); 46%. (b) Cu(OAc)
2 2
ÆH O/Pyridine, MeOH,
n
H
2nþ1COCl/Et
2
3
N, 3 (n ¼ 1); 60%, 4 (n ¼ 7); 56%.
11
1
1
,3,5-triethynyl derivative 5c (15% yield), whereas the
,2,3-triethynyl derivative 5b was not obtained. This
12
result provides additional support for the assigned
structure of 5a–c. The formation of 5a and 5c by the
cobalt complex catalyzed trimerization can be explained
13
on the basis of previous reports.
The distribution of products 5a and 5b provides a clue
for elucidating the mechanism of the liquid-phase tri-
merization (Fig. 3). Since formation of 5b with the 1,2,3-
trisubstituted pattern can only be rationalized in terms
of the intermediacy of Dewar benzene (6b), formation of
Figure 1. DSC thermogram of 3.
5
opening of Dewar benzene derivative (6a,a ). Viehe
a is presumably also due to the thermally allowed ring
0
14
permeation chromatography (GPC, JAIGEL 1H 2H,
eluent: chloroform). The wide distribution of the
molecular weights of the oligomers obtained from 3 was
analyzed by GPC, and the fractions of the largest
retention volume (161 mL) and the second largest one
15
et al. reported that Dewar benzene derivatives can be
4
formed by [
p
s
þ
p s
2 ] addition of cyclobutadiene and
acetylene derivatives. Taking this result into consider-
ation, it is likely that cyclobutadiene (7a), which is
formed transiently in a stepwise manner, reacts with
monomer 3 immediately only when they are in close
proximity, affording Dewar benzene derivatives 6.
(
140 mL) were isolated. The first fraction contained tri-
meric isomers, and the second fraction (broad peak)
contained oligomers up to 12-mer as revealed by time-
6
of-flight (TOF) mass spectroscopy. The major and
minor isomers (4:1 ratio) of the isolated trimeric mixture
The formation of Dewar benzene 6a can be explained by
the addition of cyclobutadiene 7a with diacetylene
monomer 3, which approaches from the other side of the
ethynyl substituents of cyclobutadiene 7a (Fig. 3; mode
A). Judging from the symmetry of HOMO of 7a and
LUMO of 3 calculated by PM3 method (MOPAC 97)
(6% yield) were separated by liquid column chroma-
tography (eluent: 95:5 hexane–ethyl acetate). Elemental
1
13
analysis, EI mass spectrometry, and H NMR and
NMR spectroscopy suggested that the major isomer was
1
C
7
,2,4-triethynylbenzene derivative (5a). An authentic
sample of 5a was prepared independently by heating 3 in
the presence of Ni(CO) (PPh in refluxing benzene for
0 h. The product (42% yield), which was isolated and
2
3 2
)
8
1
purified by silica gel column chromatography, was
identical with the isolated major isomer 5a. Single
crystals of 5a were obtained by recrystallization from 1:1
(
graphic analysis unequivocally confirmed the structure
v/v) dichloromethane–hexane, and X-ray crystallo-
9
10
of 5a (Fig. 2). On the other hand, spectroscopic data
suggested that the minor trimeric isomer was 1,2,3-
triethynylbenzene derivative (5b). Since this derivative
has never been obtained from a transition metal cata-
lyzed reaction, product 5b is characteristic of the liquid-
phase oligomerization. Diacetylene 4 also exhibited
thermal reactivity, affording the corresponding products
(
data not shown). Incidentally, the transition metal
catalyzed oligomerization of 3 using CoCp(CO)2
Cp ¼ cyclopentadienyl) afforded 5a (21% yield) and
Figure 2. Molecular structure of 5a determined by X-ray diffracto-
metry.
(

S. Taniguchi et al. / Tetrahedron Letters 45 (2004) 2671–2675
2673
R
R
R
R
R
R
δ+
δ-
R
O
O
O
δ+
O
6a
R
δ-
R
R
R
O
R
δ-
δ+
O
R
R
R
O
3
-
δ
O
R
δ+
R
R
5a
R
R
R
R
1
,1-addition
6a'
R
thermally allowed products
R
R
R
R
R
R
R
R
R
R
3
7a
3
R
R
[
mode A ]
[ mode B ]
R R
R
R
R
R
R
5b
R = CH2OCOCH3
6b
thermally allowed product
8
a
Figure 3. Reaction mechanism for the formation of 1,2,4-, 1,2,3-trisubstituted ethynylbenzene. Preorganized diacetylene 3 affords cyclobutadiene 7a,
0
which adds to diacetylene 3 in mode A or B, to give Dewar benzene 6a, 6a , or 6b, respectively.
(
considered to occur in the thermally allowed [ 2 ]
manner. If the reacting monomer 3 is located close to the
substituent side of cyclobutadiene 7a (Fig. 3; mode B),
the addition reaction gives rise to Dewar benzene 6b also
Fig. 4), the electro-cyclic reaction between 7a and 3 is
4
Although it is difficult to elucidate the mechanism of the
liquid-phase polymerization of diacetylenes 3 in detail at
the present stage, local alignment of 3 enables to form
p
s
þ
p s
16
1,1-(1,4)-addition to give diradical 8a. The diradical is
0
likely to afford Dewar benzene (6a,a ,b) through the
through the thermally allowed [
p
4
s
þ
p
2
s
] process. On the
transition state [7aꢀ ꢀ ꢀ3], which can be accessed only
when monomer 3 is prelocated closely to the transiently
formed cyclobutadiene. If the transition state [7aꢀ ꢀ ꢀ3]
for affording Dewar benzene cannot be formed, diradi-
cal 8a will decompose to two monomers 3, because of
the extremely short lifetime of cyclobutadiene 7a at this
other hand, 1,3,5-derivative 5c, which must be generated
from Dewar benzene 6c, is considered to be formed by
the addition of 1,3-disubstituted cyclobutadiene deriva-
tive 7b with diacetylene 3. This product 5c, however, was
not detected at all because of the poor thermodynamic
stability of biradical intermediate 8b (Fig. 5).
17
temperature (Fig. 3).
In contrast, monomer 9, which has an ethylene unit as a
spacer, does not exhibit any reactivity in the liquid
phase. Presumably, the conformational freedom of the
ethylene unit is too large to restrict monomers to the
plausible transition state for the formation of Dewar
benzenes. Incidentally, hybrid monomer 10 did not tri-
R
R
R
R
R
R
18
merize in the liquid phase. The lack of oligomerization
of diacetylenes with alkyl chains (1,2) also supports the
7
a
3
5
foregoing interpretation.
R
R
O
R
R
CH3
O
O
O
CH3
9
1
R
R
O
HOMO of 7a
LUMO of 3
O
CH3
C12H25
0
Figure 4. HOMO of cyclobutadiene 7a and LUMO of diacetylene 3.

2674
S. Taniguchi et al. / Tetrahedron Letters 45 (2004) 2671–2675
R
R
R
R
R
R
R
R
R
3
R
5
c
R
R
R
R
R
1, 3-addition
R
6c
R
R
3
3
8b
7b
6
a
5a
R = CH2OCOCH3
Figure 5. Reaction mechanism for the formation of 1,3,5-trisubstituted ethynylbenzene. This reaction, however, does not take place due to the
instability of biradical 8b.
In summary, we found that triethynylbenzenes 5 with a
specific substitution pattern were obtained simply by
heating an appropriate monomer without the use of
solvents or catalysts. This result suggests that acetylene
monomers are loosely preorganized for the 1,1- (1,4)-
addition to occur even in the liquid phase, presumably
due to the electro-static interaction between polar ester
groups. Furthermore, the isolation of triethynylbenz-
enes 5a and 5b provides a clue for characterizing the
structures of higher oligomers because triethynylbenz-
enes possibly react to form phenylene-type oligomers
3. (a) Kociolek, M. G.; Johnson, R. P. Tetrahedron Lett.
999, 40, 4141–4144; (b) Breitkopf, V.; Hopf, H.; Klarner,
F. G.; Witulski, B.; Zimny, B. Liebigs Ann. Chem. 1995, 1,
1
613–617; (c) Bertholet, M. Liebigs Ann. Chem. 1867, 141,
173.
4
5
. Ojima, J.; Katakami, T.; Nakaminami, G.; Nakagawa, M.
Bull. Chem. Soc. Jpn. 1976, 49, 292–296.
. Either dialkyl-substituted diacetylene 1 or 2 did not exhibit
any reactivity at all although these diacetylenes with long
alkyl chains are likely to align side-by-side due to the
dispersion force between alkyl groups. Although alkyl
chains are close to each other, it does not necessarily mean
that the reactive site of each 1 or 2 is located in proximity
intermolecularly.
19
under the reaction conditions.
6
. TOF mass spectra were obtained with a Bruker Daltonics
Autoflex spectrometer, using a-cyano-4-hydroxycinnamic
acid as a matrix.
Acknowledgements
1
7
. Spectral data for 5a: H NMR (270 MHz, CDCl
2
6H), 4.87 (s, 2H), 4.92 (s, 2H), 4.93 (s, 2H), 5.30 (s, 2H),
3
/TMS) d
.02 (s, 3H), 2.03 (s, 3H), 2.06 (s, 3H), 2.10 (s, 3H), 2.11 (s,
We thank Dr. T. Kudo (Bruker Daltonics) for skillful
TOF measurements. This work was supported by a
grant-in-aid from The 21st Century COE (Center of
Excellence) program (Research Center for Integrated
Science) of the Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
1
3
5.31 (s, 2H), 5.33 (s, 2H); C NMR (126 MHz, CDCl
3
) d
), 52.34, 52.46, 52.50, 61.64,
O–), 80.59, 81.41, 81.85, 93.33, 94.14, 94.63
2
0.5, 20.6, 20.65, 20.70 (CH
3.26 (–CH
3
6
2
(
(
CBC), 125.72, 127.43, 127.62, 137.79, 138.21, 140.10
Ar), 170.09, 170.14, 170.37, 170.62 (C@O); EI mass
þ
spectrum m=z (rel intensity) 582.2 (M , 20), 420.2 (100).
Anal. Calcd for C30
30
H O12: C, 61.85; H, 5.19. Found: C,
6
1.75; H, 5.19.
8
9
. Chalk, A. J.; Jerussi, R. A. Tetrahedron Lett. 1972, 13, 61–
62.
. Crystal structure data for 5a: colorless needles (crystallized
from dichloromethane–hexane); crystal system mono-
References and notes
1
. (a) Baughman, R. H. J. Appl. Phys. 1972, 43, 4362–4370;
b) Baughman, R. H. J. Polym. Sci., Polym. Chem. Ed.
(
1
1
clinic; space group P2
1
=n; Z ¼ 4; cell parameters
ꢀ
ꢀ ꢀ
974, 12, 1511–1535; (c) Wegner, G. Makromol. Chem.
972, 154, 35–48; (d) Tykwinski, R. R.; Gubler, U.;
a ¼ 8:237ð1Þ A, b ¼ 18:847ð2Þ A, c ¼ 19:131ð3Þ A, b ¼
ꢀ
3
102:168ð2Þ°; V ¼ 2903:2ð7Þ A ; radiation (Mo Ka) k ¼
ꢀ
0:71073 A; 379 variables for 2597 reflections; final
Martin, R. E.; Diederich, F.; Bosshard, C.; G u€ nter, P.
J. Phys. Chem. B. 1998, 102, 4451–4465; (e) Nakanishi, H.;
Katagi, H. Supramol. Sci. 1998, 5, 289–295; (f) Garito,
A. F.; Teng, C. C.; Wong, K. Y.; Zammaniꢀ Khamiri, O.
Mol. Cryst. Liq. Cryst. 1984, 106, 219–258; (g) Iwamura,
H. Adv. Phys. Org. Chem. 1984, 26, 179–253; (h)
Hermann, J. P.; Ducuing, J. J. Appl. Phys. 1974, 45,
2
wR
2
ðF Þ ¼ 0:114; R
1
ðF Þ ¼ 0:057; atomic coordinates have
been deposited at the Cambridge Crystallographic Data
Centre as supplementary publication number
CCDC221909. Copies of the data can be obtained, free
of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [fax: +44(0)-1223-336033 or
5
100–5102.
e-mail: deposit@ccdc.cam.ac.uk].
1
2
. (a) Okuno, T.; Izuoka, A.; Ito, T.; Kubo, S.; Sugawara, T.;
Sato, N.; Sugawara, Y. J. Chem. Soc., Perkin Trans. 2
10. Spectral data for 5b: H NMR (270 MHz, CDCl
2.04 (s, 3H), 2.06 (s, 6H), 2.13 (s, 3H), 2.14 (s, 6H), 4.96 (s,
3
/TMS) d
1
3
1998, 889–895; (b) Tsibouklis, J. Adv. Mater. 1995, 7, 407–
4H), 4.99 (s, 2H), 5.23 (s, 2H), 5.36 (s, 4H); C NMR
(126 MHz, CDCl ) d 20.69, 20.70, 20.76, 20.77 (CH ),
52.50, 52.62, 59.60, 61.82 (–CH O–), 81.57, 82.76, 92.69,
408; (c) Okuno, T.; Izuoka, A.; Kume, K.; Sato, N.;
3
3
Sugawara, T. Mol. Cryst. Liq. Cryst. 1993, 225, 393–398;
d) Okuno, T.; Fukada, M.; Izuoka, A.; Sato, N.;
Sugawara, T. Mol. Cryst. Liq. Cryst. 1993, 217, 59–64;
e) Izuoka, A.; Okuno, T.; Ito, I.; Sugawara, T.; Sato, N.;
Kamei, S.; Tohyama, K. Mol. Cryst. Liq. Cryst. 1993, 226,
01–205.
2
(
93.49 (CBC), 127.83, 129.19, 136.04, 137.94 (Ar), 170.13,
170.18, 170.37, 170.38 (C@O); EI mass spectrum m=z (rel
intensity) 582.2 (M , 21), 420.2 (100).
þ
(
11. Wakatsuki, Y.; Yamazaki, H. Bull. Chem. Soc. Jpn. 1985,
58, 2715–2716.
2

S. Taniguchi et al. / Tetrahedron Letters 45 (2004) 2671–2675
2675
1
1
1
2. Spectral data for 5c: H NMR (270 MHz, CDCl
.10 (s, 9H), 2.13 (s, 9H), 4.90 (s, 6H), 5.38 (s, 6H);
NMR (126 MHz, CDCl ) d 20.56, 20.61 (CH ), 52.25,
3
/TMS) d
17. Noro, M.; Masuda, T.; Ichimura, A. S.; Koga, N.;
Iwamura, H. J. Am. Chem. Soc. 1994, 116, 6179–6190.
18. Hybrid-type derivative 9 was synthesized as follows. To a
solution of 0.3 mmol of 1-tetradecyne in 10 mL of dry
diethyl ether was added dropwise 0.19 mL of a 1.58 M
solution of n-butyllithium in hexane at )78 °C. After 1 h,
1
3
2
C
3
3
6
2
3.19 (–CH O–), 80.11, 93.89 (CBC), 125.93, 139.75 (Ar),
1
sity) 582.2 (M , 21), 420.2 (100).
70.04, 170.55 (C@O); EI mass spectrum m=z (rel inten-
þ
3. (a) Hardesty, J. H.; Koerner, J. B.; Albright, T. A.; Lee, G.
J. Am. Chem. Soc. 1999, 121, 6055–6067; (b) Vollhardt, K.
P. C. Angew. Chem., Int. Ed. Engl. 1984, 23, 539–556; (c)
Schore, N. E. Chem. Rev. 1988, 88, 1081–1119.
2
I powder was added, and the mixture was allowed to
react for 3 h. The reaction mixture was hydrolyzed with ice
water and extracted with hexane. The organic phase was
washed with water and dried over MgSO . Thereafter,
4
Glaser coupling was performed according to the proce-
dure reported in the literature. Ben-Efraim, D. A. Tetra-
hedron 1973, 29, 4111–4125.
0
1
4. Dewar benzene 6a is an optical antipode of 6a, which is
formed through the approach from the right side (mode A).
5. (a) Viehe, H. G.; Merenyi, R.; Oth, J. F. M.; Senders, J.
R.; Valange, P. Angew. Chem., Int. Ed. Engl. 1964, 3, 755–
1
19. Snow, A. W. Nature 1981, 292, 40–41, and references cited
therein. The proposed structure of the polymers in this
earlier work may be re-examined on the basis of the
present results. Preliminary experiment revealed that the
relative yield of cyclic trimers 5a,b became maximum after
12 h, decreasing gradually thereafter. This tendency sug-
gests that the cyclic trimers reacted with a monomer, or
trimers, affording phenylene-type oligomers, at least, to
some extent.
7
56; (b) Viehe, H. G.; Merenyi, R.; Oth, J. F. M.; Valange,
P. Angew. Chem., Int. Ed. Engl. 1964, 3, 746–751; (c)
Viehe, H. G. Angew. Chem., Int. Ed. Engl. 1965, 4, 746; (d)
Maier, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 309–332.
6. Possibility of formation of cyclobutadiene 7a through 1,4-
addition cannot be excluded. In this case, however, the
ring closure of the geminate biradical 8a to cyclobutadiene
1
7
a requires the conformational change of the biradical 8a.