Synthesis and asymmetric anionic polymerization of substituted 7-aryl-2,6-dimethyl-1,4-benzoquinone methides

Article abstract of DOI:10.1002/pola.27449

Substituted 7-aryl-2,6-dimethyl-1,4-benzoquinone methides which have an electron-donating methoxy substituent at the para-position (p-OMe, 2a) or an electronwithdrawing chloro one at the para- (p-Cl, 2b), meta- (m-Cl, 2c), and ortho-positions (o-Cl, 2d) of the benzene ring were synthesized, and their asymmetric anionic polymerizations using the complex of lithium 4-isopropylphenoxide with (-)- sparteine were carried out in toluene at 0°C. The polymers with negative specific rotation were obtained for all of four monomers, and the polymer obtained from 2a showed smaller specific rotation value than that of polymer having no substituent (p-H, 1) on the phenyl group and the polymers obtained from 2b-d showed larger ones. It was found that the kind of a substituent and its substitution position on the phenyl group affect significantly the optical activity of polymers. The largest specific rotation value of [α]₄₃₅= -153.2° was obtained in the polymerization of 2d with an ortho-chloro substituent.

Full text of DOI:10.1002/pola.27449

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Synthesis and Asymmetric Anionic Polymerization of Substituted
7-Aryl-2,6-dimethyl-1,4-benzoquinone Methides
Takahiro Uno,1 Hiroshi Ohta,1 Atsushi Yamane,1 Masataka Kubo,2 Takahito Itoh1
1Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577 Kurimamachiya-cho, Tsu,
Mie 514-8507, Japan
2Graduate School of Regional Innovation Studies, Mie University, 1577 Kurimamachiya-cho, Tsu, Mie 514-8507, Japan
Correspondence to: T. Itoh (E-mail: itoh@chem.mie-u.ac.jp)
Received 23 July 2014; accepted 13 October 2014; published online 7 November 2014
DOI: 10.1002/pola.27449
ABSTRACT: Substituted
7-aryl-2,6-dimethyl-1,4-benzoquinone
ent (p-H, 1) on the phenyl group and the polymers obtained
from 2b–d showed larger ones. It was found that the kind of a
substituent and its substitution position on the phenyl group
affect significantly the optical activity of polymers. The largest
specific rotation value of [a]4355 2153.2ꢀ was obtained in the
methides which have an electron-donating methoxy substitu-
ent at the para-position (p-OMe, 2a) or an electron-
withdrawing chloro one at the para- (p-Cl, 2b), meta- (m-Cl,
2c), and ortho-positions (o-Cl, 2d) of the benzene ring were
synthesized, and their asymmetric anionic polymerizations
using the complex of lithium 4-isopropylphenoxide with (2)-
sparteine were carried out in toluene at 0 ꢀC. The polymers
with negative specific rotation were obtained for all of four
monomers, and the polymer obtained from 2a showed smaller
specific rotation value than that of polymer having no substitu-
C
V
polymerization of 2d with an ortho-chloro substituent.
2014
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.
2015, 53, 437–444
KEYWORDS: anionic polymerization; chiral; substituent effect;
synthesis
none methide reduces its reactivity and makes it isolable as
a monomer crystal. For example, 7,7-dicyano-1,4-benzoqui-
none methide,4 7-(alkoxycarbonyl)-7-cyano-1,4-benzoquinone
methides,5 7,7-bis(alkoxycarbonyl)-1,4-benzoquinone methides,6
and 7-cyano-7-phenyl-1,4-benzoquinone methide7 have already
been synthesized and isolated. Previously, we investigated the
polymerization behaviors of these isolable quinone methides
and found that the radical and anionic polymerizations of many
quinone methides take place between the substituted exo-
methylene carbon atom and exocarbonyl oxygen with the for-
mation of a stable aromatic ring to afford polymers with
characteristic main-chain structures, such as poly(oxy-1,4-phen-
ylene-substituted methylene). Furthermore, we reported the
asymmetric anionic polymerization of a prochiral monomer, 7-
phenyl-2,6-dimethyl-1,4-benzoquinone methide (1), using vari-
ous chiral anionic initiators, and the synthesis of an optically
active polymer (poly(1)) with configurational chirality in the
main chain (Scheme 1).8
INTRODUCTION Optically active polymers have attracted
interest as chiral stationary phases for high performance liq-
uid chromatography, polymeric reagents, and asymmetric
catalysts because of their excellent chiral recognition abilities
toward a wide range of racemic compounds.1 Asymmetric
polymerization is one of promising methods to introduce the
chirality into the polymer chain and to synthesize optically
active polymers. There are a large number of reports about
the asymmetric polymerizations based on vinyl monomers,
diene monomers, cyclic olefin monomers, aldehyde mono-
mers, isocyanate monomers, and so on.2 Among the various
asymmetric polymerization, asymmetric synthesis polymer-
ization (IUPAC nomenclature: asymmetric chirogenic poly-
merization) of prochiral monomer is the most effective
method for the synthesis of the optically active polymer with
configurational chirality in the main chain which is analo-
gous to biopolymers, such as proteins and polysaccharides.
However, there are not so many successful results of asym-
metric synthesis polymerization reported so far.
The asymmetric anionic polymerization of 1 using (2)-spar-
teine ((2)-Sp) as chiral ligand gave the optically active poly-
mer with a negative specific rotation ([a]435 5 222.5ꢀ).
Moreover, to understand the stereocontrol process in asym-
metric anionic polymerization of 1, we carried out the asym-
metric anionic polymerization of 1 at the various initiator
1,4-Benzoquinone methide, a member of the quinoid family,
possesses high reactivity and reacts spontaneously to afford
a dimer bonded between exocyclic carbons or oligomers at
room temperature.3 However, the introduction of substitu-
ents on the exocyclic carbon or quinoid skeleton of the qui-
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other commercially available materials were used without
further purification.
Synthesis of Monomers (2a–d)
3,5-Dimethyl-4-hydroxy-substituted-benzophenone (3)
General Preparation Method. A mixture of 2,6-xylenol (6.1 g,
50 mmol) and substituted benzoyl chloride (50 mmol) was
heated at 140 ꢀC for 4 h with stirring. To the mixture was
added 8.5 g (64 mmol) of aluminum chloride milled in a
mortꢀar with a pestle, and then the mixture was stirred at
SCHEME 1 Asymmetric anionic polymerization of 1.
concentrations and in various solvents, and found that the
aggregation state of the propagating chain end significantly
might affect the specific rotation of poly(1). However, the
specific rotation value of obtained poly(1) was still not so
high. Lin et al.9 reported that the rates of anionic polymer-
ization of several 7-aryl-2,6-dimethylquinone methides were
affected by the para-substituent of the 7-phenyl group. In
this work, we carried out the asymmetric anionic polymer-
ization of substituted 7-aryl-2,6-dimethyl-1,4-benzoquinone
methides which have an electron-donating methoxy substitu-
ent at para-position (2a) on the phenyl group or an
electron-withdrawing chloro one at the para-(2b), meta-(2c),
ortho-positions (2d) of the benzene ring (Chart 1) and inves-
tigated the effect of substituent on optical activity of the
resulting polymers (poly(2a–d)).
ꢀ
190 C for 45 min. After the mixture cooled to 0 C, aqueous
hydrochloric acid (6 N) was added to quench the reaction,
and the resulting mixture was extracted with CH2Cl2 (100 mL
3 3). The extracts were combined, washed twice with brine,
and dried over anhydrous magnesium sulfate. After the sol-
vent was evaporated, a residual solid was purified by recrys-
tallization or by silica-gel column chromatography, followed
by recrystallization.
3,5-Dimethyl-4-hydroxy-40-methoxybenzophenone (3a). From
4-methoxybenzoly chloride, 3a was obtained as yellow nee-
dles after purification by recrystallization from chloroform/
hexane 5 1/5 (33% yield). M.p.: 129 ꢀC. IR (KBr, cm21):
3260 (mOAH), 1565 (mC@O), 1484 (mC@C), 1244, 1010 (mCAO).
1H NMR (CDCl3, d, ppm): 7.78 (d, J 5 8.6 Hz, 2H), 7.47 (s,
2H), 6.96 (d, J 5 8.6 Hz, 2H), 5.21 (s, 1H), 3.89 (s, 3H), 2.29
(s, 6H). 13C NMR (CDCl3, d, ppm): 195.4 (C@O), 162.7 (Ar),
156.5 (Ar), 132.2 (Ar), 131.2 (Ar), 130.8 (Ar), 129.8 (Ar),
123.1 (Ar), 113.3 (Ar), 55.4 (OCH3), 16.0 (CH3).
EXPERIMENTAL
Measurements
All melting points were obtained with a Yanaco MP-S3 micro
melting-point apparatus. Infrared (IR) spectra were recorded
on a JASCO IR-700 spectrometer. 1H and 13C NMR spectra
were measured with a JEOL JNM-EX270 (270 MHz for 1H)
spectrometer in chloroform-d with tetramethylsilane as an
3,5-Dimethyl-4-hydroxy-40-chlorobenzophenone (3b). From 4-
chlorobenzoly chloride, 3b was obtained as white needles
after purification by recrystallization from EtOAc/hexane 5
1/5 (87% yield). M.p.: 130 C. IR (KBr, cm21): 3360 (mOAH),
ꢀ
internal standard. The specific rotation at 435 nm ([a]435
)
2950 (mCAH), 1632 (mC@O), 1601, 1487 (mC@C), 1090, 760
(mCACl). 1H NMR (CDCl3, d, ppm): 7.70 (d, J 5 8.6 Hz, 2H),
7.48 (s, 2H), 7.45 (d, J 5 8.3 Hz, 2H), 5.55 (s, 1H), 2.29 (s,
6H). 13C NMR (CDCl3, d, ppm): 195.0 (C@O), 156.9 (Ar),
138.1 (Ar), 136.7 (Ar), 131.4 (Ar), 131.1 (Ar), 129.1 (Ar),
128.4 (Ar), 123.1 (Ar), 15.9 (CH3).
was obtained with a JASCO P-1030 polarimeter. The number-
average (Mn), weight-average (Mw) molecular weights, and
polydispersity (Mw/Mn) of the polymers were determined by
size exclusion chromatography (SEC) on a Jasco PU-1580
chromatograph equipped with a Jasco RI-930 refractive-
index (RI) detector and two Tosoh TSKgel Multipore-HXL-M
columns with tetrahydrofuran (THF) as an eluent at a flow
rate of 1.0 mL/min and with polystyrene standards for cali-
bration at room temperature. The relationship of the molecu-
lar weight and the specific rotation of the polymer was
examined with SEC on a Shodex System-21 equipped with a
Shodex RI-71S refractive index (RI) detector and a JASCO
OR-990 polarimetric (PM) detector with two columns, Sho-
dex KF-803 and ꢀKF-806L, connected in series (eluent: THF,
temperature: 40 C).
3,5-Dimethyl-4-hydroxy-30-chlorobenzophenone (3c). From 3-
chlorobenzoly chloride, 3c was obtained as white needles
after purification by silica-gel column chromatography using
chloroform followed by recrystallization from dichlorome-
thane (63% yield). M.p.: 166 ꢀC. IR (KBr, cm21): 3440
(mOAH), 2952 (mCAH), 1638 (mC@O), 1485, 1416 (mC@C), 1028,
Materials
Toluene was purified in the usual manner and distilled over
sodium metal. n-Butyllithium (n-BuLi) (Kanto Chemicals,
1.65 M solution in hexane) was used as received. 4-
Isopropylphenol (Tokyo Chemical Industry Co., LTD. (TCI))
was recrystallized from hexane. (2)-Sp (TCI) was dried over
calcium hydride and distilled under reduced pressure. All
CHART 1 Chemical structures of monomers (1 and 2a–d).
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756 (mCACl). 1H NMR (CDCl3, d, ppm): 7.72 (s, 1H), 7.60 (d,
J 5 7.6 Hz, 1H), 7.53 (d, J 5 7.9 Hz, 1H), 7.49 (s, 2H), 7.41 (t,
J 5 7.8 Hz, 1H), 5.36 (s, 1H), 2.30 (s, 6H). 13C NMR (CDCl3, d,
ppm): 194.6 (C@O), 156.9 (Ar), 140.1 (Ar), 134.4 (Ar), 131.7
(Ar), 131.5 (Ar), 129.5 (Ar), 129.4 (Ar), 129.0 (Ar), 127.7
(Ar), 123.1 (Ar), 15.9 (CH3).
gel column chromatography using dichloromethane and recry-
stallization from hexane (50% yield). M.p.: 74 ꢀC. IR (KBr,
cm21): 3474 (mOAH), 2924 (mCAH), 1485 (mC@C), 1079, 725
1
(mCACl). H NMR (CDCl3, d, ppm): 7.15–7.20 (m, 3H), 7.05 (d,
J 5 7.3 Hz, 1H), 6.78 (s, 2H), 4.51 (s, 1H), 3.81 (s, 2H), 2.21 (s,
6H). 13C NMR (CDCl3, d, ppm): 151.2 (Ar), 144.3 (Ar), 134.6
(Ar), 132.3 (Ar), 130.1 (Ar), 129.5 (Ar), 129.3 (Ar), 127.4 (Ar),
126.6 (Ar), 123.6 (Ar), 41.2 (CH2), 16.3 (CH3).
3,5-Dimethyl-4-hydroxy-20-chlorobenzophenone (3d). From 2-
chlorobenzoly chloride, 3d was obtained as brown needles
after purification by recrystallization from EtOAc/hexane 5
2,6-Dimethyl-4-(20-chlorobenzyl)phenol (4d). From 3d, 4d
was obtained as white needles after purification by silica-gel
column chromatography using chloroform and recrystalliza-
tion from hexane (58% yield). M.p.: 80 ꢀC. IR (KBr, cm21):
1/5 (88% yield). M.p.: 177 C. IR (KBr, cm21): 3302 (mOAH),
ꢀ
1
2950 (mCAH), 1635 (mC@O), 1578 (mC@C), 1031, 747 (mCACl). H
NMR (CDCl3, d, ppm): 7.48 (s, 2H), 7.33-7.44 (m, 4H), 5.29
(s, 1H), 2.26 (s, 6H). 13C NMR (CDCl3, d, ppm): 194.6 (C@O),
157.8 (Ar), 139.5 (Ar), 131.7 (Ar), 131.3 (Ar), 130.9 (Ar),
130.2 (Ar), 129.2 (Ar), 129.1 (Ar), 126.8 (Ar), 123.5 (Ar),
16.1 (CH3).
1
3376 (mOAH), 2916 (mCAH), 1606 (mC@C), 1031, 744 (mCACl). H
NMR (CDCl3, d, ppm): 7.36 (d, J 5 8.3 Hz, 1H), 7.11–7.24 (m,
2H), 7.16 (d, J 5 8.6 Hz, 1H), 6.80 (s, 2H), 4.47 (s, 1H), 3.96
(s, 2H), 2.20 (s, 6H). 13C NMR (CDCl3, d, ppm): 150.6 (Ar),
139.3 (Ar), 134.0 (Ar), 131.0 (Ar), 130.8 (Ar), 129.4 (Ar),
129.1 (Ar), 127.4 (Ar), 126.7 (Ar), 123.0 (Ar), 38.2 (CH2),
15.9 (CH3).
2,6-Dimethyl-4-(substituted benzyl)phenol (4)
General Preparation Method. Zinc powder (19.6 g, 300
mmol) was added to a solution of mercuric chloride (0.5 g,
2.0 mmol) in 27 mL of water, and the mixture was stirred at
room temperature for 45 min. The supernatant water was
decanted off, and the precipitate was washed twice with
water. Then, 27 mL of water and 20 mL of concentrated
hydrochloric acid were added. To the suspension was added
a solution of 3 (24 mmol) in 45 mL of ethanol, and the mix-
ture was refluxed for 8 h. After cooling to room temperature,
the reaction mixture was extracted twice with diethyl ether.
The extracts were combined, washed with brine, and dried
over magnesium sulfate. After the diethyl ether evaporated,
the residual pale yellow oil was purified by silica-gel column
chromatography or distillation, followed by recrystallization.
7-(40-Methoxyphenyl)-2,6-dimethyl-1,4-benzoquinone
methide (2a)
Monomer precursor 4a (0.75 g, 3.1 mmol) and silver oxide
(1.43 g, 6.2 mmol) were dried under reduced pressure, and
then filled by nitrogen. After addition of dry toluene
(13 mL), the suspension was stirred at room temperature
for 30 min. The reaction mixture was filtered under nitrogen
atmosphere to remove silver oxide. The yellow solution of
2a directly used for asymmetric anionic polymerization after
determined the concentration of 2a by 1H NMR measure-
ment. 1H NMR (CDCl3, d, ppm): 7.56 (s, 1H), 7.46 (d, J 5 8.9
Hz, 2H), 7.11 (s, 1H), 7.04 (s, 1H), 6.99 (d, J 5 8.9 Hz, 2H),
3.87 (s, 3H), 2.07 (s, 6H).
2,6-Dimethyl-4-(40-methoxybenzyl)phenol (4a). From 3a, 4a
was obtained as white needles after purification by silica-gel
column chromatography using chloroform and recrystalliza-
tion from hexane (36% yield). M.p.: 68 ꢀC. IR (KBr, cm21):
3346 (mOAH), 2880 (mCAH), 1484 (mC@C), 1228, 1028 (mCAO).
1H NMR (CDCl3, d, ppm): 7.09 (d, J 5 8.3 Hz, 2H), 6.82 (d,
J 5 8.3 Hz, 2H), 6.78 (s, 2H), 4.47 (s, 1H), 3.78 (s, 2H), 3.78
(s, 3H), 2.20 (s, 6H). 13C NMR (CDCl3, d, ppm): 157.7 (Ar),
150.3 (Ar), 133.9 (Ar), 133.1 (Ar), 129.6 (Ar), 128.8 (Ar),
123.0 (Ar), 113.7 (Ar), 55.1 (OCH3), 40.1 (CH2), 15.8 (CH3).
7-(Chlorophenyl)-2,6-dimethyl-1,4-benzoquinone methide
(2b–d)
General Preparation Method. Silver oxide (4.1 mmol) was
added to a solution of monomer precursor 3b–d (2.0 mmol)
in 25 mL of diethyl ether, and the mixture was stirred at
room temperature for 2 h. The reaction mixture was filtered
to remove silver oxide, and then diethyl ether was evapo-
rated. The residual solid was recrystallized from hexane or a
mixture of dichloromethane and hexane.
7-(40-Chlorophenyl)-2,6-dimethyl-1,4-benzoquinone
methide
2,6-Dimethyl-4-(40-chlorobenzyl)phenol (4b). From 3b, 4b
was obtained as white needles after purified by distillation
under reduced pressure and recrystallization from hexane
(43% yield). B.p.: 147 ꢀC/4.0 mmHg. M.p.: 57 ꢀC. IR (KBr,
cm21): 3406 (mOAH), 2924, 2854 (mCAH), 1625, 1498 (mC@C),
1093, 728 (mCACl). 1H NMR (CDCl3, d, ppm): 7.23 (d, J 5 8.6
Hz, 2H), 7.10 (d, J 5 8.6 Hz, 2H), 6.76 (s, 2H), 4.48 (s, 1H),
3.80 (s, 2H), 2.20 (s, 6H). 13C NMR (CDCl3, d, ppm): 150.5
(Ar), 140.3 (Ar), 132.1 (Ar), 131.6 (Ar), 130.1 (Ar), 128.9
(Ar), 128.4 (Ar), 123.1 (Ar), 40.3 (CH2), 15.8 (CH3).
(2b). 2b was obtained as yellow needles in 93% yield by
recrystallization from hexane. M.p.: 94 ꢀC. IR (KBr, cm21):
2966, 2920 (mCAH), 1616 (mC@O), 1560 (mC@C), 1091, 800
(mCACl). 1H NMR (CDCl3, d, ppm): 7.45 (s, 1H), 7.41 (m, 4H),
7.09 (s, 1H), 7.04 (s, 1H), 2.06 (s, 6H). 13C NMR (CDCl3, d,
ppm): 187.0 (C@O), 140.6 (CH), 138.4 (CH), 137.7 (Ar),
135.8 (>C@), 135.2 (CACl), 133.9 (@CACH3), 131.9
(@CACH3), 131.4 (Ar), 130.6 (@CAAr), 128.9 (Ar), 16.7
(CH3), 16.0 (CH3). Anal. calcd. for C15H13ClO: C 73.62, H
5.35, Cl 14.49, O 6.54; found: C 72.83, H 5.46.
2,6-Dimethyl-4-(30-chlorobenzyl)phenol (4c). From 3c, 4c
7-(30-Chlorophenyl)-2,6-dimethyl-1,4-benzoquinone
methide
was obtained as white needles after purification by silica-
(2c). 2c was obtained as yellow needles in 54% yield by
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under reduced pressure, filled with nitrogen, and then added
dry toluene). The resulting solution was cooled to 0 ꢀC. The
polymerization was initiated by adding the initiator solution,
which was prepared by mixing lithium 4-isopropylphenoxide
(iPrPhOLi) (1.0 equiv.) and a (2)-Sp (1.1 equiv.) in dry tolu-
ene at room temperature just before use, and the reaction
ꢀ
mixture was stirred at 0 C for a given time. The polymeriza-
tion was terminated by adding an excess amount of acetic
acid. The resulting solution was poured into a large excess
amount of methanol, and the deposited polymer was col-
lected by centrifugation, and dried in vacuo.
RESULTS AND DISCUSSION
Monomer Synthesis
Substituted 7-aryl-2,6-dimethyl-1,4-benzoquinone methide
monomers (2a–d) were synthesized according to the route
as shown in Scheme 2.
Esterification reactions of 2,6-xylenolꢀwith methoxy- or chloro-
substituted benzoyl chloride at 140 C followed by Fries rear-
rangement in the presence of aluminum chloride at 190 ꢀC
gave 3,5-dimethyl-4-hydroxy-substituted benzophenones (3a–
d) in 33–88% yields as yellow needles for 3a, white needles
for 3b and 3c, and brown needles for 3d, respectively.
SCHEME 2 Synthesis route of substituted 7-aryl-2,6-dimethyl-
1,4-benzoquinone methides (2a–d).
2,6-Dimethyl-4-(substituted benzyl)phenols (4a–d) were synthe-
sized in 36–58% yields as white needles by Clemensen reduc-
tion reaction of 3a–d using zinc–mercury amalgam. Oxidations
of 4a–d with silver oxide in diethyl ether at room temperature,
followed by recrystallization from a mixture of dichloromethane
and hexane for 2c or hexane for 2b and 2d gave 2b in 93%,
2c in 75%, and 2d in 92% yields, respectively, as yellow nee-
dles. All monomers except for 2a were identified by 1H, 13C
NMR, IR spectroscopies, and elemental analysis. Unfortunately,
2a is labile, and could not be isolated as pure monomer crystal
because 2a reacted spontaneously to form a dimer coupled
between exocyclic carbons as shown in Scheme 3.
recrystallization from a mixture of dichloromethane and hex-
ane. M.p.: 143 C. IR (KBr, cm21): 2952 (mCAH), 1611 (mC@O),
ꢀ
1552 (mC@C), 1024, 770 (mCACl). 1H NMR (CDCl3, d, ppm):
7.37–7.42 (m, 4H), 7.34 (s, 1H), 7.06 (s, 1H), 7.03 (s, 1H),
2.07 (s, 6H). 13C NMR (CDCl3, d, ppm): 187.2 (C@O), 140.1
(CH), 138.4 (CH), 138.1 (>C@), 137.3 (Ar), 136.2 (CACl),
134.7 (@CACH3), 132.6 (@CACH3), 130.6 (Ar), 129.9 (Ar),
129.0 (Ar), 128.4 (@CAAr), 16.8 (CH3), 16.1 (CH3). Anal.
calcd. for C15H13ClO: C 73.62, H 5.35, Cl 14.49, O 6.54; found:
C 73.53, H 5.51.
Asymmetric Anionic Polymerization
7-(20-Chlorophenyl)-2,6-dimethyl-1,4-benzoquinone
methide
Firstly, we investigated the asymmetric anionic polymeriza-
tion of 2a (p-MeO). Although 2a (p-MeO) was not obtained
as pure monomer crystal, it was found from 1H NMR mea-
surement that 2a (p-MeO) could exist as a pure monomer
state in solution, confirmed by the oxidation reaction of 4a
(p-MeO) in chloroform-d. Therefore, we carried out the
asymmetric anionic polymerization of 2a (p-MeO) without
isolation as monomer crystal by using 2a (p-MeO) solution
(2d). 2d was obtained as yellow needles in 92% yield by
recrystallization from hexane. M.p.: 117 ꢀC. IR (KBr, cm21):
2920 (mCAH), 1617 (mC@O), 1553 (mC@C), 1051, 759 (mCACl).
1H NMR (CDCl3, d, ppm): 7.48 (d, J 5 9.2 Hz, 1H), 7.39-7.42
(m, 2H), 7.36 (d, J 5 9.6 Hz, 1H), 7.30 (s, 1H), 7.29 (s, 1H),
7.13 (s, 1H), 2.08 (s, 3H), 2.03 (s, 3H). 13C NMR (CDCl3, d,
ppm): 187.3 (C@O), 138.7 (CH), 138.2 (CH), 138.0 (Ar),
136.2 (>C@), 134.8 (CACl), 133.8 (@CACH3), 132.6
(@CACH3), 132.2 (Ar), 131.1 (Ar), 130.3 (@CAAr), 129.9
(Ar), 126.7 (Ar), 16.7 (CH3), 16.1 (CH3). Anal. calcd. for
C15H13ClO: C 73.62, H 5.35, Cl 14.49, O 6.54; found: C
73.47, H 5.34.
Asymmetric Anionic Polymerization
Asymmetric anionic polymerization was carried out in a
glass ampoule equipped with a three-way stopcock. A given
amount of dry toluene solution of monomer 2a was placed
in the ampoule by a syringe (In the case of monomer 2b–d,
a given amount of 2b–d was placed in the ampoule, dried
SCHEME 3 Dimer coupled between exocyclic carbons
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TABLE 1 Asymmetric Anionic Polymerization of 2a (p-OMe)
withdrawing chloro substituent at the para-position on the
7-phenyl group. The specific rotation values of obtained pol-
y(2b)s were dependent upon the [M]/[I] ratios, and
increased with an increase in the [M]/[I] ratio, reached a
maximum value ([a]435 5 233.0ꢀ) at the [M]/[I] ratio of 10
(Table 2, Entry 2), and then decreased, though the molecular
weights are almost same. Similar behavior was observed in
the asymmetric anionic polymerization of 1 (p-H)8 and 7-
cyano-7-alkoxycarbonyl-1,4-benzoquinone methides.10
i
with PrPhOLi/(2)-Sp Initiator at Various [M]/[I] Ratiosa
Entry [M]/[I] Time (h) Yield (%)b Mn
Mw/Mn
[a]435
c
c
d
1
2
3
4
5
5
48
36
17
34
18
4
2,800 2.22
4,200 2.55
3,900 2.35
3,900 2.58
4,300 2.87
21.5ꢀ
20.4ꢀ
20.6ꢀ
21.1ꢀ
10
20
50
100
72
72
120
168
e
–
In these previous works,8,10 we proposed that the aggrega-
tion state of the propagating chain ends, which composed of
the propagating anion, lithium cation, and (2)-Sp, signifi-
cantly affected the specific rotation of obtained polymers.
For example, the propagating species may exist as a mono-
meric (non-aggregating) state at a low concentration, and
the dimeric and oligomeric aggregates, as reported for butyl-
lithium,11 might be formed at medium and high concentra-
tions, respectively (Scheme 4). If only the dimeric aggregate
contributes to the high stereoselectivity in the propagation
reactions, the maximum specific rotations will be obtained at
a medium initiator concentration, regardless of the monomer
concentration. Actually, the specific rotation of poly(2b)s
were also dependent on the concentrations of monomer and
initiator, and the values became smaller with a decrease in
the concentrations of the monomer and initiator at a con-
stant [M]/[I] ratio of 10 (Table 2, Entries 2 and 6–8). There-
fore, this behavior observed in the asymmetric anionic
polymerization of 2b (p-Cl) is considered to proceed with
same stereocontrol mechanism depending upon the mono-
mer and initiator concentrations in polymerization system.
a
b
c
[M] 5 0.476 M in toluene, temp. 5 0 ꢀC.
Methanol-insoluble part.
Determined by SEC in THF (polystyrene standard).
Measured in CHCl3 (c 5 1.0).
Not determined.
d
e
in dry toluene, which was prepared by filtration of oxidation
reaction mixture of monomer precursor with silver oxide
under nitrogen atmosphere. Table 1 shows the results of the
asymmetric anionic polymerizations of 2a (p-MeO) using the
complex of lithium 4-isopropylphenoxide with (2)-sparteine
(iPrPhOLi/(2)-Sp) as a chiral anionic initiator at various
monomer/initiator feed ratios ([M]/[I] ratios). The polymer-
ization conditions were chosen on the basis of the effective
conditions for the stereocontrol on the polymerization of 1
(p-H) in the previous work.8
Polymerization system became gradually heterogeneous, due
to low solubility of the resulting polymer (poly(2a)) toward
toluene. Poly(2a)s were obtained in relatively low yields,
which have the number-average molecular weights in the
range 2800–4300 and the polydispersity in the range 2.22–
2.87. The molecular weights of obtained polymers were not
in agreement with the values expected from their [M]/[I]
ratios, probably due to slow polymerization rate and the
presence of an unreacted monomer after the termination of
polymerization. And also, broad molecular weight distribu-
tions are presumably ascribed to instability of 2a (p-MeO)
and low solubility of the poly(2a). The specific rotations of
obtained poly(2a)s were very small negative values regard-
less of the [M]/[I] ratios. These results indicate that both of
polymerization control and stereocontrol are difficult in the
asymmetric anionic polymerization of 2a (p-MeO).
Table 3 shows the results of the asymmetric anionic poly-
i
merizations of 2c (m-Cl) using PrPhOLi/(2)-Sp as a chiral
anionic initiator with various [M]/[I] ratios in toluene at 0
ꢀC. The polymerization was carried out at a constant mono-
mer concentration of 0.216 mol/L because of relatively low
solubility of 2c (m-Cl) toward toluene. The polymerization
proceeded in heterogeneous state for all experiments (Table
3, Entries 1–4), and afforded optically active polymers
TABLE 2 Asymmetric Anionic Polymerization of 2b (p-Cl) with
iPrPhOLi/(2)-Sp Initiator at Various [M]/[I] Ratiosa
Entry [M]
[M]/[I] Time (h) Yield (%)b Mn
Mw/Mn [a]435
c
c
d
Next, we investigated the asymmetric anionic polymeriza-
tions of the p-, m-, o-chloro substituted monomers 2b–d.
Table 2 shows the results of the asymmetric anionic poly-
1
2
3
4
5
6
7
8
0.328 5
24
24
24
48
96
36
48
72
26
31
30
38
27
38
43
12
4,300 1.20 220.4ꢀ
4,700 1.20 233.0ꢀ
5,200 1.25 227.4ꢀ
4,600 1.42 23.8ꢀ
4,000 1.55 11.5ꢀ
3,300 1.40 225.2ꢀ
3,300 1.36 217.4ꢀ
3,900 1.39 211.6ꢀ
0.328 10
0.328 20
0.328 50
0.328 100
0.221 10
0.164 10
0.109 10
i
merizations of 2b (p-Cl) using PrPhOLi/(2)-Sp as a chiral
anionic initiator with various [M]/[I] ratios in monomer con-
ꢀ
centrations of 0.109–0.328 mol/L in toluene at 0 C. In con-
trast to the polymerization of 2a (p-MeO), all polymerization
reactions of 2b (p-Cl) proceeded homogenously, and afforded
optically active polymers (poly(2b)) with the number-
average molecular weights in the range 4000–5200 and the
polydispersity in the range 1.20–1.55. The polymerization
rate of 2b (p-Cl) was faster than that of 2a (p-MeO). This
likely comes from a decrease in the electron density on the
exocyclic carbon atom of 2b (p-Cl) induced by an electron-
a
b
c
[M] 5 0.328–0.109 M in toluene, temp. 5 0 ꢀC.
Methanol-insoluble part.
Determined by SEC in THF (polystyrene standard).
Measured in CHCl3 (c 5 1.0).
d
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SCHEME 4 Aggregation state of the propagating chain end composed of the propagating anion, lithium cation, and (2)-Sp.
(poly(2c)) with the number-average molecular weights in
the range 2200–3700 and the polydispersity in the range
1.40–2.00. The specific rotation values of obtained poly(2c)s
were dependent on the [M]/[I] ratio as well as the polymer-
ization of 2b (p-Cl), and increased with an increase in the
[M]/[I] ratio, reached a maximum value ([a]435 5 229.3ꢀ) at
the [M]/[I] ratio of 20 (Table 3, Entry 3), and then
decreased. However, the effect of the [M]/[I] ratio on specific
rotation value of poly(2c)s was not clear, because the poly-
merization of 2c proceeded heterogeneously.
(m-Cl), and increased with an increase in the [M]/[I] ratios,
reached a maximum value ([a]435 5 2153.2ꢀ) at the [M]/[I]
ratio of 10 (Table 4, Entry 2), and then decreased. The trend
of change in the specific rotation with the [M]/[I] ratios, that
is, with the concentrations of initiator, was similar to those
observed in the polymerizations of 2b (p-Cl) and 2c (m-Cl). It
is considered that the changes of aggregation state depending
on the concentrations of initiator, as shown in Scheme 4, con-
trols the optical rotation of obtained polymers. However, all of
the poly(2d)s had significantly larger specific rotation values
than both poly(2b)s and poly(2c)s in all [M]/[I] ratios. The
chloro substituent at the ortho-position might contribute to the
formation of a more favorable aggregate state for the stereose-
lectivity compared with chloro substituent at the para- and
meta-positions.
Table 4 shows the results of the asymmetric anionic polymer-
i
izations of 2d (o-Cl) using PrPhOLi/(2)-Sp as a chiral anionic
initiator with various [M]/[I] ratios in a constant monomer
concentration of 0.315 mol/L in toluene at 0 ꢀC. All polymer-
ization reactions of 2d (o-Cl) proceeded homogenously, and
afforded optically active polymers (poly(2d)) with the number-
average molecular weights in the range 1900–4600 and the
polydispersity in the range 1.34–1.80. The specific rotation val-
ues of obtained poly(2d)s were greatly dependent on the [M]/
[I] ratios as well as the polymerizations of 2b (p-Cl) and 2c
Table 5 summarizes the results of the asymmetric anionic
polymerization of the substituted 7-aryl-2,6-dimethyl-1,4-
benzoquinone methide monomers 2a–d at the optimum
[M]/[I] ratios which gave the polymer with the highest
TABLE 4 Asymmetric Anionic Polymerization of 2d (o-Cl) with
iPrPhOLi/(2)-Sp Initiator at Various [M]/[I] Ratiosa
TABLE 3 Asymmetric Anionic Polymerization of 2c (m-Cl) with
iPrPhOLi/(2)-Sp Initiator at Various [M]/[I] Ratiosa
Entry [M]/[I] Time (h) Yield (%)b Mn
Mw/Mn [a]435
c
c
d
Entry [M]/[I] Time (h) Yield (%)b Mn
Mw/Mn
[a]435
c
c
d
1
2
3
4
5
5
48
22
50
73
89
94
1,900 1.80
1,900 1.34
3,300 1.62
3,600 1.79
4,600 1.88
263.2ꢀ
1
2
3
4
5
48
7
3,700 1.50
2,700 1.40
2,200 1.80
2,400 2.00
210.9ꢀ
29.4ꢀ
229.3ꢀ
223.3ꢀ
10
20
50
100
48
2153.2ꢀ
2144.7ꢀ
275.2ꢀ
261.7ꢀ
10
20
50
48
9
72
72
53
42
168
168
168
a
b
c
a
b
c
[M] 5 0.216 M in toluene, temp. 5 0 ꢀC.
Hexane-insoluble part.
[M] 5 0.315 M in toluene, temp. 5 0 ꢀC.
Hexane-insoluble part.
Determined by SEC in THF (polystyrene standard).
Determined by SEC in THF (polystyrene standard).
d
d
Measured in CHCl3 (c 5 1.0).
Measured in CHCl3 (c 5 1.0).
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TABLE 5 Asymmetric Anionic Polymerization of Monomers (1
with the chloro substituents at the para- and meta-positions.
The steric effect of the chloro substituent is considered to con-
tribute more effectively to the high enantioface-selectivity for
incoming monomer because the chloro substituent at the
ortho-position exists more close to the reactive methylene car-
bon in comparison with those at the para- and meta-positions.
i
and 2a–d) with PrPhOLi/(2)-Sp Initiator at the Optimum [M]/[I]
Ratio for Each Monomer
Monomer
(R)
Time Yield
c
c
d
[M]/[I] (h)
(%)b
Mn
Mw/Mn
[a]435
1a (H)
20
5
72
48
24
72
48
59
36
31
53
50
4,300
2,800
4,700
2,200
1,900
1.31
2.22
1.20
1.80
1.34
222.5ꢀ
21.5ꢀ
233.0ꢀ
229.3ꢀ
2153.2ꢀ
Figure 1 shows the SEC curves of polymers (poly(1),
poly(2a), poly(2b), and poly(2d)) in Table 5 monitored with
RI (bottom chromatogram) and PM detectors (top chromato-
gram). The scale was normalized by the intensities of RI
chromatograph.
2a (p-OMe)
2b (p-Cl)
2c (m-Cl)
2d (o-Cl)
10
20
10
a
Data in ref. 8.
Poly(2a) with an electron-donating methoxy substituent
showed hardly intense peak on PM chromatograph. On the
other hand, the PM detector demonstrated negative peaks
for poly(1) without substituent and poly(2b) and poly(2d)
with an electron-withdrawing chloro substituent, and also
peak patterns of PM chromatograms are quite similar to cor-
responding RI chromatograms, which show unimodal SEC
curves. These results indicate that the optical rotations of
poly(1), poly(2b), and poly(2d) do not depend upon their
molecular weights, and the configurations of all asymmetric
carbons in the polymer chain are controlled in almost same
degree. In other words, the addition reactions of a propagat-
ing anion to the monomer must take place with the same
stereoselectivity in every propagating step. Moreover, the
negative peak of PM chromatograph for poly(2d) is signifi-
cantly larger than those for poly(1) and poly(2b). These
results suggests that an introduction of the substituent into
the 7-phenyl group changes the aggregation state composed
of the several propagating anions in the polymerization sys-
tem and the more suitable aggregation for the stereocontrol
on asymmetric anionic polymerization of 7-phenyl-2,6-
dimethyl-1,4-benzoquinone methide monomer is formed by
b
Methanol-insoluble part.
c
Determined by SEC in THF (polystyrene standard).
Measured in CHCl3 (c 5 1.0).
d
specific rotation value for each monomer, together with the
previous result of 1 (p-H) for comparison. Substitution of the
hydrogen at para-position on the 7-phenyl group with an
electron-donating methoxy substituent led to decrease in the
specific rotation of the resulting polymer (poly(2a), [a]435 5
21.5ꢀ) in comparison with the polymer (poly(1), [a]4355
222.5ꢀ) from 1 (p-H). On the other hand, substitution with
an electron-withdrawing chloro substituent at the para-,
meta-, and ortho-positions provided the polymers (poly(2b),
[a]435 5 233.0ꢀ; poly(2c), [a]435 5 229.3ꢀ; and poly(2d),
[a]435 5 2153.2ꢀ) with larger specific rotation values than
the poly(1). Of course, since the asymmetric carbons in the
main chain of these polymers (poly(1) and poly(2a–2d)) are
composed of different group such as phenyl, methoxyphenyl,
and chlorophenyl group, it was expected that these polymers
show different specific rotation even if these polymers have
same stereoregularity, that is, enantiomeric excess (ee) of the
main chain. However, the specific rotations of (R)21-phenyl-
ethanol and its derivatives with para-methoxy, para-chloro,
and ortho-chloro group were reported to be [a]D 5 238.4ꢀ at
73%ee, 229.3ꢀ at 65%ee, 239.5ꢀ at 75%ee and 249.3ꢀ at
94%ee, respectively.12 From those values, the specific rota-
tions at 100%ee were calculated to be [a]D 5 252.6ꢀ for
(R)21-phenylethanol, [a]D 5 245.0ꢀ for (R)-1-(para-methoxy-
phenyl)ethanol, [a]D 5 252.7ꢀ for (R)-1-(para-chlorophenyl)
ethanol, and [a]D 5 252.4ꢀ for (R)-1-(ortho-chlorophenyl)
ethanol, and the difference of specific rotation by the introduc-
tion of methoxy or chloro substituents were within a range of
1.2 times magnitude. This suggests that large difference of
specific rotations between the polymers with different substit-
uent summarized in Table 5 is caused by ee’s of the main
chain. In other words, these results indicate that introduction
of an electron-donating substituent on the 7-aryl group have a
disadvantage for the stereocontrol of the polymers. On the
other hand, the substitution of the 7-aryl group with an
electron-accepting chloro substituent yielded the polymers
with different specific rotation values depending on the substi-
tution positions. The polymer with the chloro substituent at
the ortho-position of the benzene ring shows the significantly
larger specific rotation value of 2153.2ꢀ than the polymers
FIGURE 1 SEC curves of poly(1) (black dashed lines), poly(2a)
(blue dotted lines), poly(2b) (green solid lines), and poly(2d) (red
bold lines) summarized in Table 5 monitored with RI (bottom
chromatogram) and PM (top chromatogram) detectors. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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introduction of electron-withdrawing chloro substituent on
ortho-position, whereas the aggregation with poor stereocon-
trol is formed by the introduction of electron-donating
methoxy substituent on para-position.
CONCLUSIONS
Asymmetric anionic polymerization of prochiral quinone
methide monomers, substituted 7-aryl-2,6-dimethyl-1,4-
i
benzoquinone methides (2a–d), were examined using Pr-
PhOLi/(2)-Sp as a chiral anionic initiator. Poly(2a) with an
electron-donating methoxy group showed very small negative
specific rotation, while the specific rotations of poly(2b–d)
with an electron-withdrawing chloro group were larger nega-
tive values than that of poly(1). It was found that the introduc-
tion of the electron-withdrawing substituent is effective for
the stereocontrol on the asymmetric anionic polymerization of
7-phenyl-2,6-dimethyl-1,4-benzoquinone methide, and also
the introduction of the chloro substituent at the ortho-position
on the 7-phenyl group is more effective for the stereocontrol.
Further studies on the inductive and mesomeric effects of sub-
stituent for asymmetric anionic polymerization of 7-aryl-2,6-
dimethyl-1,4-benzoquinone methides with various substitu-
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