Asymmetric allylation polymerization of bis(allylsilane) and dialdehyde containing arylsilane structure

Article abstract of DOI:10.1021/ma012023j

Arylsilane monomers, soluble at low temperature and used for the asymmetric allylation polymerization were prepared. Optically active polymers containing aryl carbon-silicon linkages in the main chain were obtained. Results showed that the chiral polymers

Full text of DOI:10.1021/ma012023j

Macromolecules 2002, 35, 5323-5325
5323
phenylboric acid14 were prepared according to the literature
procedure. Monomers 1a ,16 4,16 and 510 were prepared accord-
ing to the procedures reported in the previous papers.
Mea su r em en ts. Melting points were determined on a
Yanaco micro melting apparatus and were uncorrected. Optical
rotations were measured on a J ASCO DIP-140 digital pola-
rimeter using a 10 cm thermostated microcell. Both 1H (300
MHz) and 13C (75 MHz) spectra were recorded on Varian
Mercury 300 spectrometer using tetramethylsilane as an
internal standard. IR spectra were recorded with a J EOL J IR-
7000 FT-IR spectrometer and were reported in reciprocal
centimeters (cm-1). Elemental analyses were performed at the
microanalysis center of Kyoto University. HPLC analyses were
performed with a J ASCO HPLC system composed of a 3-Line
Degasser DG-980-50, a HPLC pump PV-980, and a Column
oven CO-965, equipped with a chiral column (Chiralpac AD,
Daicel) using hexane/propan-2-ol as an eluent (30 °C, flow
rate: 0.5 mL/min). A UV detector J ASCO UV-975 was used
for the peak detection. Size exclusion chromatography (SEC)
for the characterization of molecular weight and its distribu-
tion was conducted at 40 °C with a J ASCO PU-980 as a pump,
a J ASCO UVDEC-100-III as a UV detector, and Shodex
column A-802 and A-803 as columns. The eluent was THF and
flow rate was 1.0 mL/min. A molecular weight calibration
curve was obtained by using a series of polystyrene standards
(Tosoh Co., J apan).
Asym m etr ic Allyla tion P olym er iza tion of
Bis(a llylsila n e) a n d Dia ld eh yd e Con ta in in g
Ar ylsila n e Str u ctu r e
Tosh ih ir o Ku m a ga i a n d Sh in ich i Itsu n o*
Department of Materials Science, Toyohashi University of
Technology, Toyohashi, 441-8580 J apan
Received November 19, 2001
Revised Manuscript Received March 12, 2002
In tr od u ction
There is considerable interest in the development of
polymerizations to produce optically active polymers; a
variety of approaches have been developed.1-4 Some of
the chiral polymers have found an application to poly-
meric reagents and catalysts.5,6 Recently, considerable
attention has focused on asymmetric syntheses of chiral
polymers with main-chain configurational chirality.7
Although various chiral polymers have been synthesized
and reported to be optically active, determination of the
optical purity of the polymers is usually difficult, and
in many cases no information on the stereochemical
purity is given. We previously8 found that asymmetric
addition of allylsilanes to aldehyde (Sakurai-Hosomi
allylation)9 is a suitable reaction for preparing optically
active polymers with main-chain chirality.10,11 All of the
polymers obtained by the asymmetric allylation polym-
erization were optically active. Model reactions revealed
that the asymmetric allylation polymerization proceeded
in a stereoselective manner. However, the model reac-
tions show only the initial step of the asymmetric
polymerization. This may not be sufficient to fully
understand the asymmetric induction and optical purity
of the polymers. The most direct procedure to evaluate
the optical purity of chiral polymers is to degrade the
polymer to chiral repeating units. The stereoisomer ratio
of the chiral degraded products can be determined by
methods such as chiral GC or HPLC analysis. This
procedure, however, requires a very efficient degrada-
tion reaction. For this purpose, we have designed
arylsilane monomers (1-4, Scheme 1) that are stable
to the Lewis acid catalyst used in the asymmetric
polymerization and to the usual workup conditions.
After isolation of the optically active polymers, the aryl
carbon-silicon bonds in the main chain of the optically
active polymer can be easily cleaved by fluoride ion.12,13
This paper reports the synthesis of the new arylsilane
monomers and their polymerization using chiral (acy-
loxy)borane (CAB) as a catalyst.14 The resulting opti-
cally active polymers were degraded to the chiral
homoallyl alcohol repeating units, which were analyzed
by HPLC using a chiral stationary phase.
Di(4-for m ylp h en yl)d ip h en ylsila n e (1c). 4-Bromobenzal-
dehyde dimethyl acetal (7.40 g, 32 mmol) was dissolved in THF
(120 mL) under nitrogen. n-BuLi/hexane solution (1.6 M, 32
mmol, 20 mL) was added slowly at -78 °C over 30 min. After
stirring at -78 °C for 1 h, dichlorodiphenylsilane (1.52 mL,
12.5 mmol) was added to the above suspension. The reaction
mixture was stirred at -78 °C for 1 h, allowed to warm to
room temperature, and stirred for 12 h. The reaction mixture
was quenched with 2 N HCl and extracted with ether. The
organic phase was washed with brine and dried (MgSO4).
Evaporation of the solvent under reduced pressure gave the
crude product of acetal/aldehyde mixture. Acetic acid (10 mL)
and H2O (3 mL) were added to the mixture and stirred for 3
h at room temperature. The reaction mixture was poured into
saturated aqueous NaHCO3 and extracted with ether. The
combined extracts were washed with brine, dried (MgSO4),
filtered, and concentrated. The crude product was purified by
column chromatography (hexanes/EtOAc 4:1) to give dialde-
hyde 1c in 58% yield (2.85 g, 6.3 mmol) as a white solid; mp
1
98-100 °C. H NMR (CDCl3): δ 10.01 (s, 2H, CHO), 7.89 (d,
J ) 8.0 Hz, 4H, OHC-Ph-H), 7.74 (d, J ) 8.0 Hz, 4H, OHC-
Ph-H), 7.56-7.40 (m, 10H, Ph-H). 13C NMR (CDCl3): δ 192.7
(CdO), 142.2, 137.5, 137.2, 136.6, 132.4, 130.6, 129.1, and
128.6 (Carom). IR (KBr): 3042, 2829, and 2737 (C-H), 1701 (Cd
O), 1592 (CdC), 1208, and 815 cm-1 (Si-C). Anal. Calcd for
C26H20O2Si (392.5): C, 79.56; H, 5.14. Found: C, 79.52; H, 5.29.
Di(4-for m ylp h en yl)d ieth ylsila n e (1b). Yield 62%. Color-
1
less oil. H NMR (CDCl3): δ 10.03 (s, 2H, CHO), 7.86 (d, J )
8.0 Hz, 4H, Ph-H), 7.67 (d, J ) 8.0 Hz, 4H, Ph-H), 1.16 (q, J
) 7.0 Hz 6H, SiCH2CH3), 1.02 (t, J ) 7.0 4H, SiCH2CH3). 13C
NMR (CDCl3): δ 192.8 (CdO), 144.2, 137.1, 135.6, and 129.0
(Carom), 7.5 (CH3), 3.7 (CH2). IR (NaCl): 2956 and 2825 (C-
H),1700 (CdO), 1595 (CdC), 1210 and 813 cm-1 (Si-C). Anal.
Calcd for C18H20O2Si (296.4): C, 72.93; H, 6.80. Found: C,
72.85; H, 6.88.
Exp er im en ta l Section
Di(2-for m ylp h en yl)d im eth ylsila n e (2). Yield 52%. Color-
Ma ter ia ls. Tetrahydrofuran (THF) was distilled from so-
dium benzophenone ketyl under nitrogen immediately before
use. Propionitrile was distilled from CaH2. Commercially
available n-BuLi (1.6 M in n-hexane, Mitsuwa Pure Chemical
Co., Ltd.) was used without purification. Tetrabutylammonium
fluoride (TBAF) (95+%) was from Aldrich (as a 1.0 M solution
in THF). Dichlorodimethylsilane (99+%), dichlorodiethylsilane
(99+%), and dichlorodiphenylsilane (99+%) (Shin-Etsu Chemi-
cal Co., Ltd.) were used without purification. (2R,3R)-2-O-(2,6-
Diisopropoxybenzoyl) tartrate15 and 3,5-bis(trifluoromethyl)-
1
less viscous oil. H NMR (CDCl3): δ 9.93 (s, 2H, CHO), 7.89-
7.54 (m, 8H, Ph-H), 0.67 (s, 6H, SiCH3). 13C NMR (CDCl3): δ
193.4 (CdO), 141.9, 140.7, 136.5, 133.6, 132.5, and 129.9
(Carom), 0.29 (CH3). IR (KBr): 3056, 2956, 2837, and 2742 (C-
H), 1695 (CdO), 1584 (CdC), 1252 (Si-CH3), 758 cm-1 (Si-
C). Anal. Calcd for C16H16O2Si (268.3): C, 71.60; H, 6.01.
Found: C, 71.54; H, 6.01.
Di(3-for m ylp h en yl)d im eth ylsila n e (3). Yield 52%. Color-
less viscous oil. 1H NMR (CDCl3): δ 10.00 (s, 2H, CHO), 8.02-
10.1021/ma012023j CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/16/2002

5324 Notes
Macromolecules, Vol. 35, No. 13, 2002
Sch em e 1. Mon om er s for th e Asym m etr ic Allyla tion P olym er iza tion
Sch em e 2. Mod el Rea ction s for th e Asym m etr ic Allyla tion P olym er iza tion 10
Sch em e 3. Asym m etr ic Allyla tion P olym er iza tion of Dia ld eh yd e a n d Bis(a llylsila n e)
7.51 (m, 8H, Ph-H), 0.65 (s, 6H, SiCH3). 13C NMR (CDCl3): δ
192.9 (CdO), 140.4, 139.1, 136.0, 135.6, 131.0, and 129.0
(Carom), -2.37 (CH3). IR (KBr): 3051, 2957, and 2817 (C-H),
1696 (CdO), 1584 (CdC), 1257 (Si-CH3), 814 cm-1 (Si-C).
Anal. Calcd for C16H16O2Si (268.3): C, 71.60; H, 6.01. Found:
C, 71.07; H, 5.98.
Asym m etr ic P olym er iza tion of 1c a n d 4. A dry propio-
nitrile (1 mL) solution of chiral (acyloxy)borane prepared from
(2R,3R)-2-O-(2,6-diisopropoxybenzoyl) tartrate (74 mg, 0.2
mmol) and 3,5-bis(trifluoromethyl)phenylboric acid (51 mg, 0.2
mmol) was added to a solution of bis(allylsilane) 4 (219 mg,
0.5 mmol) and dialdehyde 1c (196 mg, 0.5 mmol) in propioni-
trile (1 mL) at -78 °C. The mixture was stirred at -78 °C for
12 h and quenched with 2 N HCl (1 mL). The mixture was
poured into MeOH/H2O (2:1), filtered, and dried under vacuum
129.9, 128.2, 125.9, 125.6, and 125.2 (Carom), 116.5 (CdCH2),
72.3 (CH-OH), 46.1 (CH2), -2.1 (CH3).
Degr a d a tion of P olym er 6c. The chiral polymer 6c (100
mg) was dissolved in TBAF/THF solution (1.0 M, 3 mL) and
heated at 60 °C for 24 h. The reaction mixture was diluted
with ether (50 mL) and washed with 2 N HCl and brine, dried
over MgSO4, and concentrated. The crude product was purified
by flash column chromatography (hexanes/EtOAc 4:1) to give
homoallyl alcohol 8 (85%). The enantiomers ratio of 86.9:13.1
(73.8% ee) was determined by HPLC analysis using a chiral
stationary phase column (Daicel, Chiralpac AD; hexane/
propan-2-ol 30:1; 0.5 mL/min): (R)-8 tr ) 34.1 min; (S)-8 tr )
38.2 min.
Resu lts a n d Discu ssion
to yield a white solid (305 mg, 89%); Mw ) 10 200, Mw/Mn
)
2.4, [Φ]405 -1182 (c 1.0, THF).17 1H NMR (CDCl3): δ 7.55-
7.36 (m, 16H, Ph-H), 5.47 (s, 2H, CdCH2), 5.20 (s, 2H,
CdCH2), 4.74 (m, 2H, CHOH), 2.90 (m, 4H, CH2dCCH2), 2.10
(b, 2H, OH), 0.57 (s, 6H, Ph-SiCH3). 13C NMR (CDCl3): δ
145.6 (CdCH2), 145.1, 141.2, 138.1, 136.9, 136.7, 134.7, 134.5,
The dialdehydes 1-3 were prepared by coupling a
lithiated benzaldehyde dimethylacetal with dichloro-
silane, followed by deprotection of the acetal moiety. All
of these monomers were readily prepared in high purity.

Macromolecules, Vol. 35, No. 13, 2002
Ta ble 1. Rep etitive Asym m etr ic Ad d ition of Bis(a llylsila n e) w ith Dia ld eh yd ea
Notes 5325
chiral polymer
degradation product
yield
(%)
10
8
R:S
b
b
c
entry
dialdehyde
bis(allylsilane)
Mw
Mw/Mn
[Φ]405
(R,R):(R,S):(S,S)
1d
2
3
4
5
1a
1b
1c
2
4
4
4
4
4
5
5
5
5
5
95
83
89
0
95
92
94
93
95
95
57 000
28 800
10 200
6.9
4.8
2.4
-990
-1050
-1182
86.1:13.9
81.6:18.4
86.9:13.1
3
35 000
27 400
34 200
10 800
33 000
9 600
5.6
5.6
7.8
3.6
8.0
3.2
-528
-1034
+1146
-693
-942
-908
85.0:15.0
86.8:13.2g
13.7:86.3g
79.5:20.5g
83.7:16.3g
86.2:13.8g
6
1a
1a
1a
1b
1c
75.5:22.6:1.9
2.1:23.2:74.7
63.7:31.6:4.7
72.1:23.1:4.8
74.3:23.7:2.0
7e
8f
9
10
a
b
Asymmetric polymerization was performed in propionitrile using L-CAB at -78 °C for 12 h, unless otherwise stated. Determined by
SEC calibration using polystyrene standards in THF solution. c Molar rotation value measure in THF.17 See ref 12. D-CAB was used.
d
e
fAt -20 °C. R:S ratio calculated from the stereoisomers distribution of the degradation product 10.
g
We have reported the enantioselective addition of
allylsilane to aldehyde in the presence of chiral (acyl-
oxy)borane (CAB) as a model reaction of asymmetric
allylation polymerization.10 For example, â-phenyl-
allylsilane (7) reacted with benzaldehyde in the pres-
ence of L-CAB at -78 °C to produce the corresponding
R-homoallyl alcohol in quantitative conversion with 79%
ee (Scheme 2).10 Thus, we have examined asymmetric
polymerization of bis(allylsilane)s and dialdehydes con-
taining arylsilane structure. The polymerization using
CAB occurred smoothly in propionitrile at -78 °C to
yield the optically active polymers in high yield (Scheme
3). Introduction of a silyl group into the monomers
improved their solubility at low temperature. The
asymmetric polymerization must be performed at lower
temperature to attain higher asymmetric induction. The
resulting optically active polymers were then treated
with terabutylammonium fluoride (TBAF) in THF to
give the corresponding chiral repeating units. A refer-
ence experiment of TBAF treatment on the enantioen-
riched homoallyl alcohol proved that this process caused
no racemization. Degradation of 6 led to 8, which was
then analyzed by chiral HPLC to determine the enan-
tioselectivity. The enantiomers ratios are listed in Table
1. In most cases, the asymmetric induction degrees were
over 70% ee (R/S > 85/15), which are similar to that of
the model reaction. Ortho-substituted dialdehyde 2
resulted in no detectable reaction under the same
reaction conditions due to its steric congestion (entry
4). Even when bis(allylsilane) 5 without Si-phenyl
linkage was used as a monomer, the use of silicon-
containing dialdehyde yielded chiral polymers that could
be degraded to the chiral repeating units of 10. From
the stereoisomers distribution of 10, the R/S ratios of
the stereogenic centers in the chiral polymer main chain
were calculated (entries 6-10). Stereoselectivities simi-
lar to those obtained from 4 and dialdehydes were
attained in the case of polymerization of 5 and 1. Phenyl
substituent in the monomer 1 resulted in somewhat
lower enantioselectivity (entries 2 and 9). Polymeriza-
tion at -20 °C gave lower selectivity than that at -78
°C as expected (entry 8).
chiral repeating units. The R/S ratio was determined
by chiral HPLC analysis of the degraded product. This
method may be applicable to other types of asymmetric
synthesis polymerizations and to the evaluation of
asymmetric induction.
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research from the Ministry
of Education, Science, Sports and Culture of J apan.
Refer en ces a n d Notes
(1) Okamoto, Y.; Nakano, T. Chem. Rev. 1994, 94, 349-372.
(2) Coates, G. W. In Comprehensive Asymmetric Catalysis;
J acobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer:
Berlin, 1999; Vol. III, pp 1329-1349.
(3) Okamoto, Y.; Nakano, T. In Catalytic Asymmetric Synthesis
Second Edition; Ojima, I., Ed.; Wiley-VCH: New York, 2000;
pp 757-796.
(4) (a) Pu, L. Chem. Rev. 1998, 98, 2405-2494. (b) Wulff, G.;
Zweering, U. Chem. Eur. J . 1999, 5, 1898-1904.
(5) (a) Pu, L. Chem. Eur. J . 1999, 5, 2227-2232. (b) Onitsuka,
K.; Takahashi, S. Kobunshi 1999, 48, 936-941. (c) Itsuno,
S. Kobunshi 1997, 46, 90-94.
(6) Fan, Q.; Ren, C.; Yeung, C.; Hu, W.; Chan, A. S. C. J . Am.
Chem. Soc. 1999, 121, 7407-7408.
(7) (a) Nozaki, K. J . Synth. Org. Chem., J pn. 2001, 59, 496-
497. (b) Nozaki, K. Kobunshi Ronbunshu 2001, 58, 375-
381.
(8) Asymmetric Diels-Alder polymerization: (a) Kamahori, K.;
Tada, S.; Ito, K.; Itsuno, S. Macromolecules 1999, 32, 541-
547. Asymmetric aldol polymerization: (b) Komura, K.;
Itsuno, S. Chem. Lett. 2001, 730-731.
(9) (a) Hosomi, A.; Sakurai, H. Tetrahedron Lett. 1976, 1295-
1298. (b) Hosomi, A. Acc. Chem. Res. 1988, 21, 200-206.
(10) Kumagai, T.; Itsuno, S. Macromolecules 2001, 7624-7628.
(11) Kumagai, T.; Itsuno, S. Macromolecules 2000, 33, 4995-
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(12) Larson, G. L. In The Chemistry of Organosilicon Compounds
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1989; pp 763-808.
(13) Colvin, E. W. Silicon in Organic Synthesis; Butterworth:
London, 1981.
(14) CAB as catalyst for enantioselective allylation was originally
developed by Yamamoto. See: Ishihara, K.; Mouri, M.; Gao,
Q.; Maruyama, T.; Furuta, K.; Yamamoto, H. J . Am. Chem.
Soc. 1993, 115, 11490-11495.
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2509-2516.
In conclusion, arylsilane monomers were prepared,
which were soluble even at the low temperature used
for the asymmetric allylation polymerization. Optically
active polymers containing aryl carbon-silicon linkages
in the main chain were obtained. These chiral polymers
were easily degraded by treatment with TBAF to the
(17) Molar optical rotation [Φ] was calculated from the following
equation: [Φ] ) [R](M/100), where M is the molecular
weight of the repeating polymer unit.
MA012023J