Electrooxidation of alcohols in an N-oxyl-immobilized rigid network polymer particles/water disperse system

Article abstract of DOI:10.1016/j.tet.2006.03.020

The electrooxidation of alcohols in an aqueous disperse system with N-oxyl-immobilized poly(p-phenylene benzobisthiazole) network polymer particles (PBZT_NT-N-Oxyl) as a disperse phase was performed successfully in a simple beaker-type undivided cell under a constant current condition to afford the corresponding ketones, aldehydes, and/or carboxylic acid in moderate to good yields. Recycle use of both the PBZT_NT-N-Oxyl particles and the aqueous media could be achieved successfully by immobilization of additional N-oxyl moiety on the polymer particles in an appropriate interval. Notably, the shape and the particle size of PBZT_NT-N-Oxyl were not appreciably changed even after 60 times recycle use.

Full text of DOI:10.1016/j.tet.2006.03.020

Tetrahedron 62 (2006) 4769–4773
Electrooxidation of alcohols in an N-oxyl-immobilized rigid
network polymer particles/water disperse system
*
Jun Kubota, Toru Ido, Manabu Kuroboshi, Hideo Tanaka, Tetsuya Uchida and Kaoru Shimamura
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima-Naka 3-1-1, Okayama 700-8530, Japan
Received 17 February 2006; revised 6 March 2006; accepted 7 March 2006
Available online 27 March 2006
Abstract—The electrooxidation of alcohols in an aqueous disperse system with N-oxyl-immobilized poly(p-phenylene benzobisthiazole)
network polymer particles (PBZTNT-N-Oxyl) as a disperse phase was performed successfully in a simple beaker-type undivided cell under
a constant current condition to afford the corresponding ketones, aldehydes, and/or carboxylic acid in moderate to good yields. Recycle
use of both the PBZTNT-N-Oxyl particles and the aqueous media could be achieved successfully by immobilization of additional N-oxyl
moiety on the polymer particles in an appropriate interval. Notably, the shape and the particle size of PBZTNT-N-Oxyl were not appreciably
changed even after 60 times recycle use.
Ó 2006 Elsevier Ltd. All rights reserved.
1
. Introduction
as a disperse medium, in which most of alcohols would be ad-
sorbed on the disperse phase and oxidize on the water/solid
interface. Recycle use of the disperse phase and the aqueous
disperse media was also investigated to offer a formally
closed oxidation system. The disperse phases, e.g., poly-
ethylene and poly(ethylene-co-acrylic acid) particles, and
silica gel, however, suffered gradual degradation during the
course of the recycle use. After repeating the use of the N-
oxyl-immobilized polymer particles for several times, signif-
icant degradation into fine pieces brought about difficulty in
the separation of the solid particles and the aqueous disperse
media. In a practical sense, therefore, mechanically more
tough solid particles (disperse phase) are desirable. In our
continuing studies, we investigated the use of poly(p-phenyl-
Electrooxidation of alcohols mediated with N-oxyl com-
pounds has been intensively investigated as a prominent tool
for organic synthesis. The N-oxyl-mediated electrooxidation
has been mainly carried out in polar organic solvents, such
as acetonitrile or dichloromethane, containing rather high
concentration of supporting electrolytes under a regulated
1
potential condition in a divided cell. The procedure is not
necessarily satisfactory in terms of operational simplicity,
manufacturing cost, and environmental stress. Torii and
co-workers have developed an organic/aqueous two-phase
2
system, e.g., CH Cl /H O, in which the electrooxidation
2
2
2
of alcohols is performed successfully by use of a simple
beaker-type undivided cell under a constant current condi-
tion; hence, the operations are remarkably simple. However,
there still remain serious problems in terms of environmental
stress arising from the use of the harmful organic solvents,
e.g., CH Cl .
9
ene benzobisthiazole) network polymer (PBZTNT, Fig. 1) as
the disperse phase. This network polymer is constituted with
S
N
2
2
N
S
N
S
S
COOH
N
S
HOOC
S
N
In the last decade, various kinds of solid-supported reagents
and catalysts have been developed for the oxidation of
alcohols. For example, N-oxyl-immobilized polymer parti-
cles and silica gel were prepared and employed as solid-
supported catalysts. In this concern, we developed the
electrooxidation of alcohols in a disperse system with N-
oxyl-immobilized polymer particles, e.g., polyethylene and
poly(ethylene-co-acrylic acid), or silica gel as a disperse
phase and an aqueous 20 wt % NaBr/satd NaHCO solution
3
S
N
N
HOOC
S
N
branch
point
S
N
N
S
S
N
terminal
point
3
S
S
N
N
S
4
5
N
S
N
N
S
S
N
HOOC
S
N
N
COOH
N
6
S
S
N
N
S
S
S
N
N
7
8
S
N
N
N
S
S
N
S
N
S
HOOC
S
COOH
S
N
S
N
S
N
N
S
Keywords: Electrooxidation; Alcohols; Polymer-supported N-oxyl; Dis-
perse system; Aqueous solution.
Corresponding author. Tel.: +81 86 251 8072; fax: +81 86 251 8079;
e-mail: tanaka95@cc.okayama-u.ac.jp
COOH
Figure 1. Structure of PBZTNT
*
.
0
040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.03.020

4770
J. Kubota et al. / Tetrahedron 62 (2006) 4769–4773
#
#
100
50
0
1
2
3
4
5
20
30 31 32
Run
48
52
57 58a 59
Alcohol 1a
Ketone 2a
# : Treatment with 4-amino-TEMPO.
a
:
4.0 F/mol of electricity was passed.
Figure 2. Recycle use of the PBZTNT-N-Oxyl.
both straight chain segments and the branch point. The
straight chain segments consist of benzobisthiazole and phe-
nyl rings and is called PBZT. The PBZT is a rigid-rod poly-
mer and the PBZT fiber is well known for its exceptional
mechanical properties as well as its high thermal and chemi-
cal stabilities. The PBZTNT also shows high mechanical,
thermal, and chemical stability. In addition, many voids exist
in the network and there are carboxyl groups at the branch
and/or terminal point of the polymer chain, which can be
used for immobilization of the N-oxyl moieties on the poly-
mer particles. Herein, we describe that the PBZTNT particles
were strong enough to survive without appreciable change in
their shape and size even after more than 60 times recycle use
for the electrooxidation of alcohols (Fig. 2).
0.5 mmol) in an aqueous satd NaHCO containing 20 wt %
3
2
NaBr was electrolyzed at 20 mA/cm under vigorous stirring
ꢀ
at 0 C. After passage of 2.5 F/mol (1.67 h) of electricity,
the PBZTNT-N-Oxyl was separated by filtration and washed
with EtOAc. The filtrates were extracted three times with
EtOAc and the extracts and the washings were combined.
Evaporation of the solvent afforded the corresponding ketone
2a in 94% yield (Table 1, entry 1). The presence of both NaBr
1
0
and NaHCO is indispensable, since in the absence of each of
3
them, the yields of 2a decreased to 16–43% (entries 2 and 3).
Neither NaCl nor NaI was effective since only 14% yield
or no appreciable amount of the ketone 2a was obtained by
similar electrolysis with NaCl or NaI (entries 4 and 5).
Next, the required amount of PBZTNT-N-Oxyl was examined
(
Table 2). When the electrooxidation of 1a (0.5 mmol) was
2
. Results and discussion
carried out with 400–200 mg of PBZTNT-N-Oxyl, the corre-
sponding ketone 2a was obtained in good yield (entries 1–3).
With less than 100 mg of the PBZTNT-N-Oxyl, the yield of 2a
significantly decreased to 55–35%, suggesting that more
than 200 mg of PBZTNT-N-Oxyl would be required for the
adsorption and the oxidation of 0.5 mmol of alcohol 1a on
the polymer surface.
The N-oxyl-immobilized poly(p-phenylene benzobisthia-
zole) network polymer particles (PBZTNT-N-Oxyl) were
prepared by treatment of the PBZTNT particles with 4-amino-
2,2,6,6-tetramethylpiperidine-N-oxyl (4-amino-TEMPO) in
acetonitrile in the presence of dicyclohexylcarbodiimide
ꢀ
(
DCC) at 50 C for 2 d (Scheme 1). The polymer particles
were separated by filtration, washed successively with aceto-
nitrile, water, ethanol, and ether, and dried under reduced
pressure to afford the PBZTNT-N-Oxyl. The weight of poly-
mer particles increased from 400 to 450 mg, suggesting
that ca. 0.6 mmol/g of the N-oxyl moiety was immobilized
The aqueous solution recovered after extractive workup pro-
cess could also be used repeatedly. As shown in Figure 3, no
significant change in the current efficiency and in the
a
Table 1. Effects of halide salts and NaHCO
OH
3
on the PBZTNT
.
PBZTNT-N-Oxyl
20 wt% NaX-aq. sat. NaHCO3
Pt)-(Pt), Undivided Cell
O
4
-amino-TEMPO
N
S
S
DCC
R
R
CO2H
(
1
a
2a
N
n
CH3CN, 50 °C, 2 d
2
0 mA, 2.5 F/mol, ice bath
(
0.50 mmol)
PBZTNT
R = 4-ClC H
6
4
O
b
b
N
S
S
H
Entry
NaX
NaHCO
3
Yield 2a (%)
Recovered 1a (%)
C
N
N O
N
n
1
2
3
4
5
NaBr
None
NaBr
NaCl
NaI
Satd
Satd
None
Satd
Satd
94
16
43
14
—
4
75
51
74
94
PBZTNT-N-Oxyl
Scheme 1.
a
Electrooxidation of 1a (0.50 mmol) using PBZTNT-N-Oxyl (450 mg) in
aqueous solution was carried out under a constant current (20 mA,
Electrolysis was carried out in a simple beaker-type undi-
vided cell fitted with two platinum electrodes (1ꢁ1 cm
2
2
.5 F/mol, 1.67 h) in an undivided cell under ice bath.
b
each). A typical procedure is as follows: a mixture of
PBZTNT-N-Oxyl (450 mg), and alcohol 1a (R¼4-Cl–C H ;
Yields were determined by GC analysis using acetophenone as an internal
standard.
6
4

J. Kubota et al. / Tetrahedron 62 (2006) 4769–4773
4771
a
Table 2. Effects of the amount of PBZTNT-N-Oxyl
OH
PBZTNT-N-Oxyl
Table 3. Electrooxidation of alcohols in the PBZTNT-N-Oxyl dispersed water
system
a
O
b
2
0 wt% NaBr-aq. sat. NaHCO3
(Pt)-(Pt), Undivided Cell
Entry
Substrate
OH
F/mol
2.5
Products (Yield, %)
R
R
1a
2a
(
0.50 mmol) 20 mA, 2.5 F/mol, ice bath
O
6
R = 4-ClC H4
1
4
-Me-C H
4-Me-C H
6 4
6
4
1
b
2b (84)
Entry
PBZTNT-N-Oxyl
N-Oxyl
(mmol)
Yield 2a
(%)
Recovered 1a
(%)
b
b
(mg)
OH
O
2
3
2.5
4.0
2c (73)
1
c
1
2
3
4
5
450
400
200
100
50
0.27
0.24
0.12
0.06
0.03
94
94
94
55
35
4
4
4
41
63
Ph
Ph
OH
O
4-t-Bu-C H
4-t-Bu-C H
6 4
6
4
1
d
2d (68)
a
Electrooxidation of 1a (0.50 mmol) using PBZTNT-N-Oxyl in aqueous
solution was carried out under a constant current (20 mA, 2.5 F/mol,
OH
O
1
.67 h) in an undivided cell under ice bath.
4
5
4.0
2.5
b
Ph
Ph
Yields were determined by GC analysis using acetophenone as an internal
standard.
1
e
2e (90)
O
4-Cl-C6H4
OH
100
4-Cl-C6H4 cH
1f
2f (30)
d
8
0
0
0
0
0
6
1f
2f (86)
O
Ph
Ph
OH
OH
e,f
7
2.5
6
4
2
1g
Ph
H
2g (61)
O
f
8
OH
4.5
Ph
OH
OH 2h (73)
1h
d
9
OH
OH
1
2
3
4
5
O
4
.5
Run
:
5a
1
i
O
2i (80)
Figure 3. Recycle use of both the PBZTNT-N-Oxyl and the aqueous media.
a
Electrooxidation of 1a (0.50 mmol) using PBZTNT-N-Oxyl (450 mg) in
aqueous solution was carried out under a constant current (20 mA) in
an undivided cell under ice bath.
conversion yields was observed when both the aqueous solu-
tion and PBZT -N-Oxyl were recovered and reused for
the electrooxidation of 1a. The recycling system, formally
offers a totally closed system as illustrated in Scheme 2.
b
c
d
e
f
NT
Isolated yields.
4
-Chlorobenzoic acid (7%) was obtained.
Methyl ethyl ketone (1.5 mL) was used as a co-solvent.
-Phenylpropanoic acid (28%) was obtained.
Acetonitrile (1.5 mL) was used as a co-solvent.
3
It is of interest to note that PBZT -N-Oxyl could be easily
NT
recovered and used repeatedly. Thus, PBZT -N-Oxyl was
NT
recovered by filtration after the electrolysis and used for
the subsequent electrolysis. The results of the recycle use
of PBZT -N-Oxyl are shown in Figure 2. The yields of
N-Oxyl moiety, the efficiency of the N-oxyl-mediated oxida-
tion of 1a was almost completely recovered, affording 2a in
93% yield (run 32). After additional 25 times recycle use
(run 57), the yield of 2a decreased to 67%. It is of interest
to note that the yield of 2a increased to over 90% by only
passage of excess amount (3.0 F/mol) of electricity (run
58). Above all, it is likely that the immobilization process
of the N-oxyl moiety on the PBZTNT particles in an appro-
NT
the ketone 2a varied in the range of 79–90% through runs
–20. After 30 times recycle use (run 30), however, the yield
1
of 2a decreased to 55% and 40% of 1a was recovered. It is
likely that some of the N-oxyl moieties would be peeled
off from the PBZT -N-Oxyl during the repetition of the
NT
electrolysis. Indeed, when the recovered PBZT -N-Oxyl
NT
was treated again with 4-amino-TEMPO to immobilize the
priate interval enables the recycle use of the PBZTNT
N-Oxyl for almost semi-permanent times (run >60).
-
OH
PBZT -N-Oxyl
NT
R
R'
2
3
4
Polymer
Particles
Filtration
Rinse
Organic
Solvent
1
Electrolysis
4
5
O
Washing
Filtrates
Extraction
Concentration
(
Extracts)
R
R'
5
aq. NaBr/NaHCO
3
Scheme 2. A totally closed electrolysis system.

4772
J. Kubota et al. / Tetrahedron 62 (2006) 4769–4773
The present electrooxidation in the PBZT -N-Oxyl/Water
NT
were separated by filtration and washed successively with
acetonitrile, H O, MeOH, and Et O (20 mL each). The
solids were dried under reduced pressure to afford N-oxyl-
immobilized PBZTNT (451 mg, 0.6 mmol/g of the N-oxyl
moiety was immobilized): black solids.
disperse system could be successfully applied to various
alcohols 1b–i. The representative results are shown in
Table 3. The electrooxidation of benzylic as well as aliphatic
sec-alcohols 1b–e proceeded smoothly to afford the corre-
sponding ketones 2b–e in good to excellent yields (entries
2
2
1
–4). In contrast, the electrooxidation of p-chlorobenzyl
alcohol (1f) under the standard condition afforded only
0% yield of the corresponding aldehyde 2f together with
4.3. Electrooxidation of alcohols. A typical procedure
3
A mixture of N-oxyl-immobilized PBZTNT (450 mg) and
1-(4-chlorophenyl)ethanol 1a (78.6 mg, 0.50 mmol) in an
aqueous satd NaHCO containing 20 wt % NaBr (5.0 mL)
the corresponding carboxylic acid (7%) (entry 5). When this
electrooxidation was carried out in the presence of methyl
ethyl ketone as a co-solvent, the yield of 2f increased to
3
was placed in a beaker-type undivided cell. After stirring
for 15 min, two platinum electrodes (1ꢁ1 cm ) were im-
2
8
6% and no appreciable amount of the carboxylic acid was
obtained (entry 6). The electrooxidation of aliphatic prim-
alcohol 1g and diols 1h and 1i was also performed by the
use of methyl ethyl ketone or acetonitrile as a co-solvent to
afford the corresponding aldehyde 2g, carboxylic acid 2h
and lactone 2i (entries 7–9).
mersed into the reaction mixture, and a constant current
ꢀ
(20 mA, 1.67 h, 2.5 F/mol) was supplied at 0 C under vig-
orous stirring. After electrolysis, the PBZTNT particles were
separated by filtration and washed with EtOAc. The aqueous
layer was extracted three times with EtOAc. The extracts and
the washings were combined and dried over Na SO . Most
2
4
of the solvents were evaporated and the residue was chroma-
tographed on a silica gel column (hexane/EtOAc: 5/1) to
afford 4-chloroacetophenone (2a, 70.0 mg, 0.45 mmol,
3
. Conclusion
In conclusion, the electrooxidation of alcohols 1 was suc-
cessfully achieved in the disperse system with N-oxyl-
immobilized poly(p-phenylene benzobisthiazole) network
polymer (PBZT -N-Oxyl) as a disperse phase and an aque-
NT
ous 20 wt % NaBr/saturared NaHCO as a disperse media.
3
9
0%): a colorless liquid; R ¼0.46 (hexane/EtOAc: 5/1);
f
1
H NMR (200 MHz, CDCl ) d 2.60 (s, 3H, CH ), 7.44
3 3
(
(
d, J¼8.6 Hz, 2H, Ar), 7.89 (d, J¼8.6 Hz, 2H, Ar); IR
ꢂ1
neat) 3006, 2970, 2904, 1687, 1590, 1572, 1176 cm
.
The disperse phase and the aqueous disperse media could
be recovered and used repeatedly for the electrooxidation
of alcohols, thereby offering a totally closed electrolysis
system (Scheme 2).
1
4.3.1. 4-Methylacetophenone (2b). A colorless liquid; H
NMR (200 MHz, CDCl ): d 2.42 (s, 3H, CH –Ar), 2.58 (s,
3
3
3
IR (neat) 3004, 2923, 1683, 1607, 1183 cm
H, CH ), 7.26 (m, 2H, Ar), 7.86 (d, J¼8.2 Hz, 2H, Ar);
3
ꢂ1
.
1
4. Experimental
4.3.2. Acetophenone (2c). A colorless liquid; H NMR
200 MHz, CDCl ): d 2.61 (s, 3H, CH ), 7.43–7.60 (m,
(
3
3
4.1. Preparation of poly(p-phenylene benzobisthiazole)
network (PBZTNT)
3H, Ar), 7.96 (d, J¼7.6 Hz, 2H, Ar); IR (neat) 3004, 2923,
ꢂ1
1686, 1599, 1266, 761 cm
.
To a 500-mL reaction flask, equipped with a mechanical
stirrer and an argon inlet/outlet adapters, were placed
4.3.3. 1-(4-tert-Butylphenyl)-1-propanone (2d). A color-
less liquid; H NMR (200 MHz, CDCl ): d 1.22 (t, J¼
1
3
2
2
,5-diamino-1,4-benzenedithiol$dihydrochloride (6.17 g,
5.2 mmol) and PPA (115%, 125 g). The mixture was stirred
7.2 Hz, 3H, CH ), 1.34 (s, 9H, (CH ) C), 2.98 (q, J¼
3
3 3
7.2 Hz, 2H, CH ), 7.47 (d, J¼8.6 Hz, 2H, Ar), 7.91 (d,
2
under reduced pressure (<3 mmHg) and gradually heated up
to 100 C. On completion of degassing of hydrogen chlo-
J¼8.6 Hz, 2H, Ar); IR (neat) 2967, 1687, 1607, 1228,
ꢀ
ride, a stoichiometric amount of terephthalic acid (2.08 g,
ꢂ1
1192, 801 cm
.
1
1
2.6 mmol) and trimesic acid (1.75 g, 8.39 mmol) were
4.3.4. 4-Phenyl-2-butanone (2e). A colorless liquid; H
NMR (200 MHz, CDCl ): d 2.14 (s, 3H, CH ), 2.71–2.79
(m, 2H, CH –Ar), 2.86–2.95 (m, 2H, CH CO), 7.16–7.31
ꢀ
added. The mixture was then heated to 140 C under an
argon atmosphere. The polymerization was carried out,
while the solution viscosity was measured by a torque meter,
and the gel point of the system was evaluated by the begin-
ning of the precipitous climb of the viscosity. By stopping
the reaction before the gelation, PBZT network particle
was prepared. After the polymerization, the product was
washed with sulfuric acid and water, and dried under
3
3
2
2
(m, 5H, Ar); IR (neat) 3027, 2923, 1717, 1497, 1454,
1162, 750 cm
ꢂ1
.
1
4.3.5. 4-Chlorobenzaldehyde (2f). White solids; H NMR
(200 MHz, CDCl ): d 7.52 (d, J¼8.5 Hz, 2H, Ar), 7.82 (d,
3
J¼8.5 Hz, 2H, Ar), 9.98 (s, 1H, CHO); IR (KBr) 3019,
ꢀ
ꢂ1
reduced pressure at 100 C to afford PBZT network as
.
2985, 2862, 1693, 1590, 1576, 1209 cm
.
ꢂ1
dark-brown powder: IR (KBr) 1708, 1100, 960, 690 cm
4
.3.6. 3-Phenylpropionaldehyde (2g). A colorless liquid;
1
4.2. Preparation of N-oxyl-immobilized PBZTNT
a typical procedure
:
H NMR (200 MHz, CDCl ): d 2.74–2.82 (m, 2H, CH –
3 2
Ar), 2.90–3.02 (m, 2H, CH CO), 7.16–7.34 (m, 5H, Ar),
2
9
1455, 1180, 747 cm
.82 (s, 1H, CHO); IR (neat) 3029, 2928, 1725, 1604,
.
ꢂ1
A mixture of PBZTNT (400 mg), 4-amino-2,2,6,6-tetra-
methylpiperidine-N-oxyl (105 mg, 0.61 mmol) and DCC
(
5
119 mg, 0.58 mmol) in acetonitrile (10 mL) was heated at
ꢀ
4.3.7. 3-Phenylpropanoic acid-2-ol (2h). White solids;
H NMR (200 MHz, CDCl ): d 2.94–3.26 (m, 2H, CH ),
1
0 C for 2 d under an Ar atmosphere. The solid particles
3
2

J. Kubota et al. / Tetrahedron 62 (2006) 4769–4773
4773
4
3
.49–4.55 (m, 1H, CH), 7.23–7.38 (m, 5H, Ar); IR (KBr)
006, 2966, 1687, 1590, 1488, 1262, 1176 cm
1990, 57; (d) Inokuchi, T.; Matsumoto, S.; Torii, S. J. Org.
Chem. 1991, 56, 2416; (e) Inokuchi, T.; Liu, P.; Torii, S.
Chem. Lett. 1994, 1411; (f) Kuroboshi, M.; Yoshihisa, H.;
Cortona, M. N.; Kawakami, Y.; Gao, Z.; Tanaka, H. Tetrahe-
dron Lett. 2000, 41, 8131.
ꢂ1
.
4.3.8. cis-8-Oxabicyclo[4.3.0]nonan-7-one (2i). A colorless
liquid; H NMR (200 MHz, CDCl ): d 1.13–1.36 (m, 3H),
1
3
1
2
3
.52–1.70 (m, 3H), 1.76–1.84 (m, 1H), 2.06–2.15 (m, 1H),
.39–2.52 (m, 1H, CH), 2.59–2.69 (m, 1H, CH–C(O)–),
3. (a) Sourkouni-Argirusi, G.; Kirschning, A. Org. Lett. 2000, 2,
3781; (b) Minghu, W.; Guichun, Y.; Zuxing, C. React. Funct.
Polym. 2000, 44, 97; (c) Hinzen, B.; Ley, S. V. J. Chem. Soc.,
Perkin Trans. 1 1997, 1907; (d) M u¨ lbaier, M.; Giannis, A.
Angew. Chem., Int. Ed. 2001, 40, 4393; (e) Sorg, G.; Mengel,
A.; Jung, G.; Rademann, J. Angew. Chem., Int. Ed. 2001, 40,
.94 (d, J¼9 Hz, 1H, –C(O)OCH –), 4.18 (dd, J¼9 Hz,
2
J¼5 Hz, 1H, –C(O)OCH –); IR (neat) 2935, 1857, 1774,
2
ꢂ1
1
376, 1160, 1129 cm
.
4
2
395; (f) Ficht, S.; M u¨ lbaier, M.; Giannis, A. Tetrahedron
001, 57, 4863.
Acknowledgment
4
. (a) Miyazawa, T.; Endo, T. J. Polym. Sci. 1985, 23, 2487; (b)
Miyazawa, T.; Endo, T. J. Mol. Catal. 1988, 49, L31; (c) Osa,
T.; Akiba, U.; Segawa, I.; Bobbitt, J. M. Chem. Lett. 1988,
We thank the SC-NMR Laboratory of Okayama University
for H and C NMR analyses.
1
13
1
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