Synthesis of a polymerizable benzocyclobutene that undergoes ring-opening isomerization at reduced temperature

Article abstract of DOI:10.1055/s-0033-1339925

1-Ethoxyvinylbenzocyclobutene is a substituted benzocyclobutene that undergoes radical polymerization to produce polymers that can be crosslinked at 100-150 °C. The 4- and 5-vinyl isomers are synthesized in a 1:4 ratio via a halogenated benzyne intermediate produced from anthranilic acid, followed by cycloaddition with ethyl vinyl ether and replacement of the halogen atom with a vinyl group. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0033-1339925

148▌
LETTER
lSetyternthesis of a Polymerizable Benzocyclobutene that Undergoes Ring-Opening
Isomerization at Reduced Temperature
Synthesis of a Polymerizable 1-Ethoxybenzocyclobutene
Coleen Pugh,* James S. Baker, William K. Storms
The University of Akron, Department of Polymer Science, Maurice Morton Institute of Polymer Science and Polymer Engineering, Akron,
OH 44325-3909, USA
Fax +1(330)9725290; E-mail: cpugh@uakron.edu
Received: 04.09.2013; Accepted: 12.09.2013
and crosslinked at more reasonable temperatures than
Abstract: 1-Ethoxyvinylbenzocyclobutene is a substituted benzo-
those used to crosslink benzocyclobutene.
cyclobutene that undergoes radical polymerization to produce poly-
mers that can be crosslinked at 100–150 °C. The 4- and 5-vinyl
isomers are synthesized in a 1:4 ratio via a halogenated benzyne in-
termediate produced from anthranilic acid, followed by cycloaddi-
tion with ethyl vinyl ether and replacement of the halogen atom with
a vinyl group.
Δ
Key words: benzocyclobutene, cycloaddition, ring-opening isom-
erization, Kumada coupling, benzyne
•
•
+
Bicyclo[4.2.0]octa-1,3,5-triene, or benzocyclobutene
BCB),1 has been widely used in polymer science based
,2
(
on the ring-opening ability of the strained cyclobutene
3
ring. It is stable at room temperature, but undergoes an
electrocyclic rearrangement at elevated temperature
oligomers
5%
7
(
>200 °C) to the highly reactive o-quinodimethane. The
o-quinodimethane undergoes Diels–Alder reactions, and
in the absence of an external dienophile, generates
2
5%
1
,2,5,6-dibenzocyclooctane (~25%) and higher oligomers
(
~75%) (Scheme 1).4,5 Since the thermally unstable spi- Scheme 1 Electrocyclic ring-opening of benzocyclobutene and its
4
rodimer does not generate any 1,2,5,6-dibenzocyclooc- subsequent [4+2] cycloaddition with another o-quinodimethane
tane upon heating, it apparently forms by a different
5
mechanism than the oligomers. This thermal activation
and the lack of condensation byproducts makes benzocy-
The ring-opening isomerization temperature of benzocy-
clobutenes can be lowered by introducing both electron-
donating (raises the ground state energy) and electron-
withdrawing (lowers the transition state energy) substitu-
6
clobutene chemistry especially attractive for crosslinking
and for polymer-forming reactions by a step (condensa-
3
tion-like) mechanism. Benzocyclobutene-containing vi-
nyl monomers have also been polymerized by chain
mechanisms.7
Although the high temperature of the ring-opening isom- ether) isomerize at ≥60 °C. (The temperature range over
erization reaction is useful for applications such as the which the exothermic isomerization transitions occur is
crosslinking of engineering polymers, which are synthe- usually broad.)
5
ents onto the cyclobutene ring. For example, 1-methoxy-
11
benzocyclobutene isomerizes at ≥110 °C, and the
–10
cyclobutene rings of poly(1-benzocyclobutenyl vinyl
1
2
sized at elevated temperatures, it also limits the use of
benzocyclobutene in other applications, such as those in-
volving temperature-sensitive materials. For example, it
would be difficult to exploit the benzocyclobutene cross-
linking reaction to create nanostructured flexible films by
field-assisted roll-to-roll processing if the glass transition
temperature of the substrate polymer film and/or the avail-
able thermal field are significantly less than 200 °C. We
are particularly interested in developing nanostructured
flexible films using block copolymers that can be aligned
Block copolymers with controlled chain lengths are pre-
pared by chain polymerizations, such as living anionic,
cationic or radical polymerizations of vinyl monomers.
Therefore, the preparation of well-defined block copoly-
mers that can be aligned and crosslinked by benzocyclo-
butene chemistry at reduced temperatures requires the
synthesis of a benzocyclobutene functionalized with both
a polymerizable vinyl group, and a substituent that lowers
the temperature of ring-opening isomerization, such as an
ethyl ether moiety.
13
4
-Vinylbenzocyclobutene (VBCB) was first synthesized
SYNLETT 2014, 25, 0148–0152
Advanced online publication: 21.11.2013
by a Wittig reaction starting from 4-chloromethylbenzo-
cyclobutene, and subsequently from 4-bromobenzocy-
clobutene with conversion into the corresponding
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
8
DOI: 10.1055/s-0033-1339925; Art ID: ST-2013-S0856-L
Georg Thieme Verlag Stuttgart · New York
©

LETTER
Synthesis of a Polymerizable 1-Ethoxybenzocyclobutene
149
7
benzaldehyde. It has also been synthesized by palladi- sions were low. The Grignard halogen exchange was also
um(0)-catalyzed Heck coupling of 4-bromobenzocyclo- not sufficiently selective, and we obtained several side
butene with ethylene,14 by nickel(0)-catalyzed Kumada products that were difficult to separate from the desired
coupling of 4-bromobenzocyclobutene with vinyl bro- product. The Grignard route is not useful because the
9
mide, and by palladium(0)-catalyzed coupling of 4-bro- elimination step requires a higher temperature (≥65 °C)
1
5
mobenzocyclobutene
with
tri(n-butyl)vinyltin.
than the boiling point of ethyl vinyl ether (bp 33 °C).
Although 4-vinylbenzocyclobutene has been synthesized
by several routes, there are no literature reports of the syn-
thesis of 1-substituted 4-vinylbenzocyclobutenes.
We were able to synthesize 1-ethoxybenzocyclobutene by
generating the benzyne in the presence of ethyl vinyl ether
via either metallation–elimination starting from 2-bromo-
Fuji holds a patent that uses 1-ethoxy-4-vinylbenzocy- fluorobenzene, or by first converting anthranilic acid into
1
6
clobutene; however, its synthesis was not reported. An benzenediazonium-2-carboxylate using isoamyl nitrite in
ether-substituted vinylbenzocyclobutene with an isomeri- the presence of a catalytic amount of trifluoroacetic acid
21
zation temperature at approximately 100 °C would be ide- in tetrahydrofuran, followed by elimination of carbon
al for creating nanostructured flexible films, because dioxide and nitrogen; the Grignard route was not used be-
many living chain polymerizations are performed at tem- cause the elimination step required a higher temperature
peratures lower than 100 °C, and 100–150 °C is accessi- (≥65 °C) than the boiling point of ethyl vinyl ether.
ble in field-assisted roll-to-roll processing equipment.
Unfortunately, although benzocyclobutene is readily bro-
Our goal was therefore to introduce an ethoxy group onto minated using bromine in acetic acid, with only a minor
2
2
the cyclobutene ring to decrease its isomerization temper- amount of dealkylated (ring-opened) side product, the
ature, and a halogen (Br or I) onto the aromatic ring to pro- same conditions oxidize 1-ethoxybenzocyclobutene into
vide a site for attaching the polymerizable vinyl group. benzocyclobutenone. For example, reaction of 1-ethoxy-
The most direct method for synthesizing 1-substituted benzocyclobutene with 1.2 equivalents of bromine pro-
1
7
benzocyclobutenes is by [2+2] cycloaddition of benzyne
duced
approximately
equimolar
amounts
of
with an alkene, especially electron-rich alkenes such as benzocyclobutenone and unreacted 1-ethoxybenzocy-
1
8,19
vinyl ethers and vinyl acetate.
Due to the many routes clobutene. +Other halogenation agents, such as
– 23–25 + –
for converting 4-bromobenzocyclobutene into vinylben- PhCH NMe , ICl /ZnCl ,
PhCH NMe , Br3
2
3
2
2
2
3
zocyclobutene,7
,9,14,15
we considered the three possible /CaCO3 and NH I /H O produced only a minor
24
+ –
4 2 2
starting materials shown in Scheme 2 for synthesizing 4- amount of side product with the remaining material unre-
bromo-1-ethoxybenzocyclobutene via a benzyne interme- acted. Therefore, the aromatic ring must be halogenated
diate. Since iodine exchanges faster than bromine in lithi- prior to functionalizing the cyclobutene ring. Note that al-
2
0
26
um–halogen exchange using n-butyllithium, 4-bromo-2- though 2-(trimethylsilyl)aryl triflates generate arynes
fluoroiodobenzene would be the most direct reagent for under very mild conditions, replacement of 4-bromo-2-
synthesizing 4-bromo-1-ethoxybenzocyclobutene if met- fluoroiodobenzene and 2-bromofluorobenzene, in
al–iodine exchange with either n-butyllithium or possibly Scheme 2, with a 2-silylaryl triflate will lead to the same
the Grignard reagent occurs selectively, thereby generat- problems for vinylation, either in the synthesis of the ap-
ing the benzyne by elimination of lithium fluoride (LiF) or propriately functionalized 2-silylaryl triflate, or in its use.
magnesium bromide fluoride (MgBrF), respectively, in
the presence of ethyl vinyl ether. However, the conver-
can be successfully synthesized by first regioselectively
As outlined in Scheme 3, 1-ethoxyvinylbenzocyclobutene
F
Br
F
I
Br
EtMgBr, THF
reflux
Br
EtMgBr, THF
reflux
OEt
NH3+
CO2–
N2+
Br
isoamyl nitrite
ClCH CH Cl
OEt
Br2, AcOH
25 °C, 2 d
2
2
OEt
CF3CO2H (cat.)
THF, 25 °C, 2.5 h
reflux, 5 h
CO2–
OEt
n-BuLi, hexanes
–
78 °C to 25 °C
OEt
F
Br
F
I
Br
n-BuLi, hexanes
78 °C to 25 °C
–
Br
Scheme 2 Potential synthetic routes to 4- and/or 5-bromo-1-ethoxybenzocyclobutenes via benzyne intermediates
©
Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 148–152

150
C. Pugh et al.
LETTER
iodinating anthranilic acid para to the amine group using The resulting 1-ethoxyvinylbenzocyclobutene can be po-
ammonium iodide, similar to brominations using ammo- lymerized by conventional or living radical polymeriza-
2
7
nium bromide, in combination with hydrogen peroxide. tions. For example, copolymerization of an 8:2 mixture of
2
8
The corresponding benzenediazonium-2-carboxylate is styrene and 1-ethoxyvinylbenzocyclobutene by atom
then generated using isoamyl nitrite, and converted into transfer radical polymerization (ATRP), using methyl 2-
the iodinated benzyne at reflux in 1,4-dioxane or 1,2-di- bromopropionate as the initiator and copper(I) bromide–
chloroethane in the presence of ethyl vinyl ether. Genera- tris[2-(dimethylamino)ethyl]amine (CuBr–Me TREN) as
6
tion of the benzyne intermediate is the yield-limiting step, the catalyst system at 60 °C, produced poly(styrene-co-1-
4
both because of the lower yields of halogenated benzene- ethoxyvinylbenzocyclobutene) (M = 5.50 × 10 g/mol
n
diazonium-2-carboxylates,29 and due to the low boiling and pdi = 1.41) in 43% purified yield. The first differential
point of ethyl vinyl ether. 1-Ethoxy-4-vinylbenzocyclob- scanning calorimetry (DSC) heating scan presented in
utene and 1-ethoxy-5-vinylbenzocyclobutene are pro- Figure 2 demonstrates that this copolymer exhibits a glass
duced in approximately a 1:4 ratio using either a two-step transition at ~100 °C, followed by a broad exotherm with
7
Wittig reaction, or by a direct Kumada coupling with vi- a maximum at 135 °C due to ring-opening isomerization
3
0
31
1
nyl bromide as the final step. Figure 1 presents the H and crosslinking. Due to crosslinking, this material exhib-
NMR spectrum of 1-ethoxy-5-vinylbenzocyclobutene its only a glass transition temperature at 116 °C on the
produced by the Kumada coupling route and isolated from second and third heating scans.
its product mixture with 1-ethoxy-4-vinylbenzocyclobu-
tene. The resonance at 5.04 ppm corresponds to the me-
thine proton, and those at 3.10 and 3.44 ppm correspond
to the methylene protons of the benzocyclobutene ring.
st
nd
Figure 2 Normalized DSC traces (10 °C/min; 1 heating: blue, 2
rd
heating: red, 3 heating: green) of poly(styrene-co-1-ethoxyvinylben-
zocyclobutene) (80:20 mol% feed ratio); M = 5.50 × 10 g/mol, pdi
4
n
=
1.41
In summary, we have established a synthetic route to 1-
ethoxyvinylbenzocyclobutene. 1-Ethoxyvinylbenzocyclo-
Figure 1 H NMR (500 MHz, CDCl ) spectrum of 1-ethoxy-5-vi- butene is a polymerizable benzocyclobutene that under-
nylbenzocyclobutene synthesized by, and isolated from, the Kumada goes ring-opening isomerization and crosslinking at 100–
1
3
coupling of 1-ethoxy-4-iodobenzocyclobutene and 1-ethoxy-5-iodo-
benzocyclobutene with vinyl bromide
150 °C, which is ~100 °C lower than that of the unsubsti-
tuted vinylbenzocyclobutene-containing polymers. The
vinyl group was introduced via a halogen functionality,
CO2–
NH3+
CO2–
NH3+
isoamyl nitrite
Cl3CCO2H
CO2–
N2+
I
+ –
I
I
NH4 I , H2O2
ClCH2CH2Cl
reflux, 2 h
AcOH
5 °C, 1.5 h
THF, 25 °C, 1.5 h
2
OEt
O
EtMgBr
THF, Et2O
MgBr
I
H
DMF
OEt
OEt
OEt
reflux, 0.5 h
5 °C, 2 h
2
5 °C, 24 h
2
CH2PPh3
NiCl (dppp)
2
THF, 25 °C, 15 h
Br 25 °C, 12 h
OEt
+
OEt
1
: 4
Scheme 3 Synthesis of 4- and 5-vinyl-1-ethoxybenzocyclobutene (1:4 ratio) by first halogenating anthranilic acid
Synlett 2014, 25, 148–152
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of a Polymerizable 1-Ethoxybenzocyclobutene
151
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(
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(
Acknowledgment
4
We acknowledge the National Science Foundation for support of
this research through DMR-1006195 and a Special Creativity
Award. We also gratefully acknowledge the Ohio Board of Regents
for a partial matching award.
(
(
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performed in a Parr reactor.
(
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1
1
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6
630. (b) Wong, P. K. US Patent 4722974, 1988; Chem.
1-Ethoxyvinylbenzocyclobutene was prepared via a
Kumada coupling in 50–75% yield as in the following
example. A solution of iodinated 1-ethoxybenzocyclobutene
(3.7 g, 14 mmol; 1:4 ratio of the 4- and 5-iodo isomers) in
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2
2
8, 5947. (b) Chino, K.; Takata, T.; Endo, T. J. Polym. Sci.,
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at r.t. The solution was stirred at r.t. until H NMR analysis
of MeOH-quenched aliquots confirmed that the formation of
the Grignard reagent was complete (18 h). The solution was
then transferred to the glass sleeve of a Parr reactor, followed
1
Polym. Chem. Ed. 1999, 37, 1555. (c) Chino, K.; Endo, T.
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(
(
(
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2
6
715.
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extracted with Et O (3 × 25 mL). The combined organic
2
layers were washed with sat. aq NaHCO solution (2 × 30
3
mL) and H O (30 mL), and dried over MgSO . The product
2
4
32
was purified by column chromatography using silica gel as
(14) Kirchhoff, R. A.; Schrock, A. K.; Hahn, S. F. US Patent
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2
f
4
724260, 1988; Chem. Abstr. 1988, 108, 95095.
1.5 g (64%) of a colorless oil composed of a 1:4 ratio of the
4- and 5-vinyl-1-ethoxybenzocyclobutene isomers.
5-Vinyl-1-ethoxybenzocyclobutene
(
15) Blomberg, S.; Ostberg, S.; Harth, E.; Bosman, A. W.; van
Horn, B.; Hawker, C. J. J. Polym. Sci., Polym. Chem. Ed.
1
3
2
002, 40, 1309.
H NMR (300 MHz, CDCl ): δ = 1.29 (t, J = 7.0 Hz, 3 H,
3
2
3
(
16) Makino, N.; Oohashi, H. US Patent 6780567 B2, 2004;
Chem. Abstr. 2013, 1620468.
CH ), 3.10 (dd, J = 14.4 Hz, J = 1.6 Hz, 1 H, CHHAr), 3.44
3
(dd, J = 14.4 Hz, J = 4.4 Hz, 1 H, CHHAr), 3.66 (dq, J =
.1 Hz, J = 7.0 Hz, 1 H, OCHH), 3.73 (dq, J = 9.1 Hz, J =
2
3
2
3
2
3
9
©
Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 148–152

152
C. Pugh et al.
.0 Hz, 1 H, OCHH), 5.04 (dd, J = 4.4 Hz, J = 2.0 Hz, 1 H,
LETTER
3
3
7
Anal. Calcd for C H O: C, 82.72; H, 8.10. Found: C, 82.46;
1
2
14
3
2
CHOEt), 5.18 (dd, J = 10.9 Hz, J = 0.8 Hz, 1 H,
H, 8.11.
BX
AB
3
2
HCHB,trans=), 5.68 (dd, J = 17.6 Hz, J = 0.9 Hz, 1 H,
(32) Alternatively, an inhibitor such as pyrogallol can be added to
the crude product, which can then be purified by distillation
at 77 °C/1 mmHg.
AX
AB
3
3
HCHA,cis=), 6.70 (dd, J = 17.6 Hz, J = 10.9 Hz, 1 H,
AX
BX
3
CH =), 7.09 (d, J
= 8.4 Hz, 1 H, aromatic C3H), 7.24 (s,
X
ortho
3
aromatic C6H), 7.35 (d, J
= 8.5 Hz, 1 H, aromatic C4H).
ortho
Synlett 2014, 25, 148–152
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