A new C-F bond-cleavage route for the synthesis of octafluoro[2.2]paracyclophane

Full text of DOI:10.1021/jo015720i

7
216
J . Org. Chem. 2001, 66, 7216-7218
A New C-F Bon d -Clea va ge Rou te for th e
Syn th esis of Octa flu or o[2.2]p a r a cyclop h a n e
Ch a r t 1
Hideki Amii, Yasushi Hatamoto, Motoharu Seo, and
Kenji Uneyama*
Department of Applied Chemistry, Faculty of Engineering,
Okayama University 3-1-1 Tsushimanaka,
Okayama 700-8530, J apan
Ch a r t 2
uneyamak@cc.okayama-u.ac.jp
Received April 30, 2001
In tr od u ction
Fluoropolymers have received intense attention with
implications for material science. Some of them possess
potentially important and interesting properties as an
electronic device. For example, poly(R,R,R′,R′-tetrafluoro-
p-xylylene) (2), known in the industry as Parylene VIP
AF4, is endowed with fascinating material properties
(trifluoromethyl)benzene (4) would provide a highly
efficient access to cyclophane 1.8 Herein, we report a
conceptually new route for the synthesis of AF4 involving
successive C-F bond cleavage processes of commercially
available 4 as a starting material (eq 1).
1
such as low dielectricity (dielectric constant: 2.36 ( 0.5),
2
high thermal and oxidative stability (450 °C in air), low
moisture absorption, and chemical inertness due to the
nature of C-F bonds (Chart 1). Thus, the Parylene AF4
polymer 2 satisfies many of the exacting requirements
for high-temperature applications and shows consider-
able promise as an on-chip dielectric interlayer medium
and as a coating of an electrical component near an
automobile engine.
Octafluoro[2.2]paracyclophane (AF4, 1), which is an
excellent precursor of the high-purity parylene polymer
3
2
via vapor deposition (CVD) process, is a key compound
of significant commercial interest. Despite its desirability
in the market, there have been few methodologies to
synthesize the cyclophane 1 (the precursors are sum-
marized in Chart 2); (a) pyrolytic/reductive processes of
Resu lts a n d Discu ssion
Th e F ir st C-F Bon d Clea va ge P r ocess for AF 4.
Mg(0)-promoted defluorinative silylation of 1,4-bis(tri-
fluoromethyl)benzene (4) affording R-trimethylsilyl-
R,R,R′,R′,R′-pentafluoroxylene (5) has been examined. In
general, the cleavage of a C-F bond is not easy due to
a -c,4 (b) reductive debromination of 3b using low-
,5
3
6a
valent titanium, or (c) using a stoichiometric amount
of Bu
/CsF. 6b
3
Sn-SiMe
3
-
1
9
the large bond energy (ca. 552 kJ mol ). However, the
bond breaking does occur rather easily when a CF group
Very recently, Dolbier et al. reported inexpensive and
highly scalable preparation of AF4 by (d) a reaction of
p-bis(chlorodifluoromethyl)benzene (3c) with Zn under
3
is attached to the π-system because electron acceptance
into a benzene ring and subsequent extrusion of a
fluoride ion may make large contributions to the driving
force for the reaction.10 Clavel et al. reported the selective
non-high-dilution conditions,6c and they also demon-
strated the functionalization of the benzene ring of AF4.7
However, to our knowledge, there has been no com-
mercially available precursor to AF4 (1). By taking
advantage of the use of readily available trifluoromethy-
lated compounds, selective defluorination of 1,4-bis-
synthesis of PhCF
of PhCF in the presence of chlorotrimethylsilane. For
the synthesis of PhCF SiMe from PhCF , the chemical
2 3
SiMe via an electrochemical reduction
1
1
3
2
3
3
metal reduction methods did not work well due
(
1) Moore, J . A.; Lang, C.-I. Vapor Deposition Polymerization as a
Route to Fluorinated Polymers. In Fluoropolymers 1; Hougham, G.,
Cassidy, P. E., J ohns, K., Davidson, T., Eds.; Plenum Press: New York,
999; p 273.
(8) Carbon-fluorine bond cleavage of 1,4-bis(trifluoromethyl)ben-
zene catalyzed by 1,4-bis(bromodifluoromethyl)benzene and zinc metal
in the synthesis of poly(tetrafluoro-p-xylene), see: Wu, P. K.; Yang,
G.-R.; You, L.; Mathur, D.; Cocoziello, A.; Lang,; C.-I.; Moore, J . A.;
Lu, T.-M. J . Electron. Mater. 1997, 26, 949. However, in this reaction,
special apparatus was required and the metal surface is corroded by
molecular fluorine.
(9) For recent reviews on metal-mediated C-F bond activation,
see: (a) Burdeniuc, J .; J edlicka, B.; Crabtree, R. H. Chem. Ber./ Recueil
1997, 130, 145-154. (b) Richmond, T. G. In Activating of Unreactive
Bonds and Organic Synthesis. Topics in Organometallic Chemistry;
Murai, S., Ed.; Springer-Verlag: Berlin, 1999; Vol. 3, p 243.
(10) (a) Saboureau, C.; Troupel, M.; Sibille S.; P e´ richon, J . J . Chem.
Soc., Chem. Commun. 1989, 1138-1139. (b) Andrieux, C. P.; Combel-
las, C.; Kanoufi, F.; Sav e´ ant, J .-M.; A. Thi e´ bault, J . Am. Chem. Soc.
1997, 119, 9527-9540.
1
(
2) Williams, K. R. J . Thermal Anal. 1997, 49, 589-594.
(3) Wenk, H. H.; Sander, W.; Leonov, A.; de Meijere, A. Eur. J . Org.
Chem. 1999, 3287, 7-3290.
4) Chow, S. W.; Pilato, L. A.; Wheelwright, W. L. J . Org. Chem.
970, 35, 20-22.
5) Grechkina, E. V.; Sochilin, V. A.; Pebalk, A. V.; Kardash, I. E.
Russ. J . Org. Chem. 1993, 30, 1663-1665.
6) (a) Dolbier, W. R., J r.; Asghar, M. A.; Pan, H.-Q.; Celewicz, L. J .
(
1
(
(
Org. Chem. 1993, 58, 1827-1830. (b) Dolbier, W. R., J r.; Rong, X. X.;
Xu, Y.; Beach, W. F. J . Org. Chem. 1997, 62, 1827-1830. (c) Dolbier,
W. R., J r.; Duan, J .-X.; Roche, A. J . Org. Lett. 2000, 2, 1867-1869.
(
7) (a) Roche, A. J .; Dolbier, W. R., J r. J . Org. Chem. 1999, 64, 9137-
9
143. (b) Idem., Ibid. J . Org. Chem. 2000, 65, 5282-5290.
1
0.1021/jo015720i CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/20/2001

Notes
Ta ble 1. Mg-P r om oted Deflu or in a tive Silyla tion of 4
J . Org. Chem., Vol. 66, No. 21, 2001 7217
Sch em e 1. F lu or id e An ion -Ca ta lyzed
1
,6-Elim in a tion of 5
product (% yield)a
entry
Mg (equiv)
conditions
5
7
1
2
3
1.1
2.0
2.0
rt, 3 h
rt, 0.5 h
rt, 1 h
35
64 (48)
58
7
5
15
b
a
NMR yield, which was calculated by 19F NMR integration of
Ta ble 2. CsF -Ca ta lyzed 1,6-Elim in a tion of 5a
product 5 relative to the 2,2,2-trifluoroethanol internal standard.
b
Isolated yield (in parentheses).
to the lack of chemoselectivity resulting in contamination
with the persilylated products. Independently, we devel-
oped the electroreductive methods for silyl difluoroenol
ethers and difluoroenamines and R-silyldifluoroacetates
using trifluoromethyl ketones and imines and trifluoro-
product
(% yields)b
acetates as a starting material.12 Recently, we found Mg-
(0) metal proves useful for the C-F bond-breaking
entry
solvent
conditions
additive
1
9
process of trifluoromethyl ketones to provide a highly
efficient access to a variety of 2,2-difluoroenol silyl
ethers.13 We have thus tried to apply our Mg(0) method
as a convenient reduction system to the cleavage of
benzylic C-F bond of 4.
1
2
3
4
5
toluene
o-xylene
mesitylene 160 °C, 24 h
anisole
anisole
110 °C, 24 h
140 °C, 24 h
trace
21
33
20-45
61 (53)
42
c
c
c
c
160 °C, 24 h
160 °C, 24 h Pd2(dba)3‚
CHCl3/TBCa
e
1
,4-Bis(trifluoromethyl)benzene (4) was treated with
a
All reactions were carried out with 0.05 M concentration of 5.
magnesium (1.1 equiv) and chlorotrimethylsilane (4.0
equiv) in DMF at room temperature for 3 h. Defluorina-
tive silylation took place to afford not only the desired
R,R-difluorobenzylsilane 5 in 35% yield but the further
defluorination product 7 in 7% yield (Table 1, entry 1).
b
19
NMR yield, which was calculated by F NMR integration of
product 1 relative to the 2,2,2-trifluoroethanol internal standard.
c
d
Not determined. Pd2(dba)3‚CHCl3 (0.00125 equiv) and TBC (5.0
10-5 in anisole) were used. e Isolated yield (in parentheses).
1
9
The best result [5 (64% yield) + 7 (5% yield) by
NMR] was obtained by the use of 2.0 equiv of Mg with
0 min of stirring (48% isolated yield after distillation,
95% purity) (Table 1, entry 2), while a longer reaction
time (1 h) promoted the formation of undesired product
[5 (58%) + 7 (15%)] (Table 1, entry 3). Raising the
F
eliminaion of o-(trimethylsilylalkyl)benzyltrimethylam-
1
4
monium halides for use in Diels-Alder reactions. A key
feature of this methodology is the mildness of the reac-
tion conditions to generate o-quinodimethanes, which are
attributed to the use of the trigger fluoride ion by virtue
of the large bond energy between silicon and fluorine. In
most cases, a stoichiometric amount of fluoride is re-
quired for these reactions due to the decreased ability of
the leaving groups to attack the silicon atom of the
precursors. By contrast, when the conjugative elimination
of 5 occurs and the leaving group is a fluoride ion that
can react again with 5, the process should be catalytic
with respect to fluoride. We have explored a catalytic
version of fluoride anion induced 1,6-elimination of 5 to
generate tetrafluoro-p-xylylene (6) (Scheme 1).
3
>
7
reaction temperature (>80 °C) and using large amounts
of Mg (>8 equiv) resulted in very sluggish reactions.
Although we could not prevent trace contamination with
7
because of close reduction potentials between the
1
1b
starting material 4 and the product 5,
the reaction
described here presents a simple and versatile example
of a practical synthetic method involving C-F bond
cleavage without any requirement of special techniques
and reagents.
Th e Secon d C-F Bon d Clea va ge P r ocess for AF 4.
Conjugative elimination is one of the most versatile and
efficient methods for the generation of o- and p-xylylene
intermediates. Ito et al. reported the generation of
o-xylylene intermediates by fluoride anion induced 1,4-
A toluene solution of 5 containing a catalytic amount
(0.05 equiv) of CsF was stirred at 110 °C only to give the
protodesilylation product (HCF C H CF , 9) in 42% yield
2 6 4 3
(Table 2, entry 1). After several attempts, we succeeded
in the generation of tetrafluoro-p-xylylene 6 to provide
cyclophane 1 by raising the reaction temperature. The
desired 1,6-elimination occurred in refluxing mesitylene
or anisole (160 °C) affording AF4 (1) in 20-45% yield
(
11) (a) Cavel, P.; Legar-lambert, M. P.; Biran, C.; Serein-Spirau,
F.; Bordeau, M.; Roques, N.; Marzouk, H. Synthesis 1999, 829-834.
b) Cavel, P.; Lessene, G.; Bordeau, M.; Roques, N.; Tr e´ vin, S.; de
Montauzon, D. J . Fluorine Chem. 2001, 107, 301-310.
12) (a) Uneyama, K.; Maeda, K.; Kato T.; Katagiri, T. Tetrahedron
(
(Table 2, entries 3 and 4). A higher reaction temperature
(
is considered to be essential to the elimination of the poor
leaving group such as a fluoride because the bond energy
Lett. 1998, 39, 3741-3744. (b) Uneyama K.; Kato, T. Tetrahedron Lett.
1
1
998, 39, 587-590. (c) Uneyama, K.; Mizutani, G. Chem. Commun.
999, 613-614. (d) Uneyama, K.; Mizutani, G.; Maeda, K.; Kato, T. J .
Org. Chem. 1999, 64, 6717-6723.
(14) (a) Ito, Y.; Nakatsuka, M.; Saegusa, T. J . Am. Chem. Soc. 1980,
102, 863-865. (b) Ito, Y.; Miyata, S.; Nakatsuka, M.; Saegusa, T. J .
Org. Chem. 1981, 46, 1043-1044. (c) Ito, Y.; Nakatsuka, M.; Saegusa,
T. J . Am. Chem. Soc., 1982, 104, 7609-7622. (d) Ito, Y.; Amino, Y.;
Nakatsuka, M.; Saegusa, T. J . Am. Chem. Soc. 1983, 105, 1586-1590.
(13) (a) Amii, H.; Kobayashi, T.; Hatamoto, Y.; Uneyama, K. Chem.
Commun. 1999, 1323-1324. (b) Mae, M.; Amii, H.; Uneyama, K.
Tetrahedron Lett. 2000, 41, 7893-7897. (c) Amii, H.; Kobayashi, T.;
Uneyama, K. Synthesis 2000, 2001-2003.

7
218 J . Org. Chem., Vol. 66, No. 21, 2001
Notes
of a C-F bond is much larger than that of other C-X
bonds (X: good leaving groups).
r-Tr im eth ylsilyl-r,r,r′,r′,r′-p en ta flu or oxylen e (5). To a
mixture of Mg (1.73 g, 72 mmol) and chlorotrimethylsilane (15.6
g, 144 mmol) in distilled DMF (80 mL) was added 1,4-bis-
Interestingly, the yield of AF4 dramatically increased
to 61% (53% isolated yield) when the reaction was
conducted in the presence of a very small amount of Pd-
(
trifluoromethyl)benzene (4) (7.70 g, 36 mmol) dropwise, and
then the mixture was stirred for an additional 20-30 min under
an argon atmosphere at room temperature. H O was added to
2
(
0) catalyst15 with good reproducibility (Table 2, entry 5).
quench the reaction, the reaction mixture was extracted with
hexane, and the combined organic extracts were dried (MgSO4),
filtered, and concentrated. Kugelrohr distillation afforded 6 as
Pd(0) catalyst is considered to accelerate the intramo-
lecular coupling reaction of the diradical intermediate.6
In conclusion, a new sequence was developed in which
two C-F bonds were cleaved, the first through Mg(0)-
promoted defluorinative silylation and the second by
subsequent conjugative 1,6-elimination induced by the
catalytic use of CsF. The present example demonstrates
a promising application of selective C-F bond cleavage
processes in material science.
1
a colorless oil (4.64 g, 48%): bp 100 °C (30 mmHg); H NMR
(
7
200 MHz, CDCl
.68 (d, J ) 8.6 Hz, 2 H); 19F NMR (188 MHz, CDCl
an internal standard): δ 48.7 (s, 2F), 99.0 (s, 3F). Anal. Calcd
for C11 Si: C, 49.24; H, 4.88. Found: C, 49.18; H, 5.14.
,1,2,2,9,9,10,10-Octa flu or o[2.2]p a r a cyclop h a n e (AF 4, 1).
3
): δ 0.15 (s, 9H), 7.46 (d, J ) 8.6 Hz, 2 H),
3
, C as
6 6
F
13 5
H F
1
A mixture of 5 (3.21 g, 12 mmol) and CsF (91.2 mg, 0.6 mmol)
and Pd (dba) ‚CHCl (24.7 mg, 24 µmol) in anisole (80 mL;
2
3
3
contaminated with 1.8 mg of 4-tert-butylcatechol) was heated
at reflux for 24 h. The cooled reaction mixture was filtered
through a pad of Celite. Anisole was removed by distillation.
Kugelrohr distillation afforded 1 as a colorless solid (1.12 g,
Exp er im en ta l Section
1H and 19F NMR spectra were recorded at 200 and 188 MHz,
respectively, using CDCl
3
as a solvent. The chemical shifts are
6
1
1
53%): bp 150 °C (3 mmHg); mp 261 °C (lit. mp 262 °C); H
reported in δ (ppm) values relative to TMS (δ 0 ppm for H NMR)
1
9
NMR (200 MHz, CDCl ) δ 7.16 (s, 8 H); 19F NMR (188 MHz,
and C
6
F
6
(d 0 ppm for F NMR). Coupling constants are reported
3
in hertz (Hz). Commercially available Mg turnings (Nacalai
tesque, Inc.) were used without further activation of the mag-
nesium surface, such as iodine, dibromoethane, or ultrasound
treatment. All air- and/or water-sensitive reactions were carried
out under argon atmosphere with dry, freshly distilled solvents
using standard syringe-cannula/septa techniques. DMF, toluene,
xylene, mesitylene, and anisole were distilled from CaH . All
2
other reagents and solvents were employed without further
purification.
CDCl
for C16
3
C
H
6
8
F
F
6
8
as an internal standard) δ 43.5 (s, 8 F). Anal. Calcd
: C, 54.56; H 2.29. Found: C, 54.86; H, 2.44.
Ack n ow led gm en t. This work has been supported
by Ministry of Education, Science, Sports, Culture, and
Technology of J apan (No. 12450356). We also thank the
1
SC-NMR Laboratory of Okayama University for H and
1
9
F NMR analyses.
(
15) Dolbier, W. R., J r.; Duan, J .-X.; Roche, A. J ., US Pat. No. 5,-
8
41,005, Nov 24, 1998.
J O015720I