Asymmetric synthesis of chiral polymers by means of repetitive addition of allylsilane to aldehyde

Article abstract of DOI:10.1055/s-2002-28502

Monomers having both allyltrimethylsilyl and formyl groups were prepared. Chiral (acyloxy)borane initiated the repetitive polyaddition of the monomers to give optically active polymers having main chain configurational chirality. Optical purity of the chiral polymer was evaluated by the analysis of the degradation product.

Full text of DOI:10.1055/s-2002-28502

SPECIAL TOPIC
941
Asymmetric Synthesis of Chiral Polymers by Means of Repetitive Addition of
Allylsilane to Aldehyde
Asymmetric
S
h
ynthesis of
C
i
hiral
P
n
olymers by A
i
dditio
c
ns of
A
llyls
h
ilane to
A
lde
i
hyde Itsuno,* Toshihiro Kumagai
Department of Materials Science, Toyohashi University of Technology, Toyohashi 441-8580, Japan
Fax +81(532)446813; E-mail: itsuno@tutms.tut.ac.jp
Received 3 March 2002
trimethylsilanes to aldehydes.5 Carreira reported excellent
examples of the asymmetric allylsilylation using a TiF4–
BINOL system.6 BINAP–AgF was used as a chiral cata-
lyst for the addition of allyltrimethoxysilane to aldehyde.7
Abstract: Monomers having both allyltrimethylsilyl and formyl
groups were prepared. Chiral (acyloxy)borane initiated the repeti-
tive polyaddition of the monomers to give optically active polymers
having main chain configurational chirality. Optical purity of the
chiral polymer was evaluated by the analysis of the degradation Chiral Lewis base-promoted addition of allylic trichlo-
rosilanes to aldehydes was also developed.8 Since the re-
product.
action applied to polymer synthesis should proceed
quantitatively without any side reaction, chiral Lewis acid
seems suitable for the catalysis of the asymmetric allyla-
tion polymerization. Monomers having both allyltrimeth-
ylsilyl and formyl groups may be easily prepared. In this
paper, we describe the successful example of this novel
type of asymmetric polymerization.
Key words: allylations, asymmetric catalysis, polymers, asymmet-
ric polymerization, optically active polymers
Generation of well-controlled configurational chirality in
the polymer main chain is highly desired for the molecular
design of the next generation of polymeric materials.1–3
Since the enantioselective addition of allylsilane to alde-
hyde is one of the most important carbon-carbon bond
forming reactions, repetition of such reaction between
monomers would be a promising synthetic method of new
optically active polymers that have stereogenic centers in
the repeat unit.4 Molecules having both the allylsilane
moiety and formyl group may be a potential monomer
structure of this new polymerization reaction. Repetitive
intermolecular addition of the allylsilane moiety to the
formyl group would give a polymer having a unique
main-chain structure with a homoallylic alcohol. Recent-
ly, various asymmetric catalysts have been developed for
the allylation reaction. Yamamoto reported that chiral
(acyloxy)boranes (CAB) derived from tartaric acid were
efficient catalyst for the enantioselective addition of allyl-
In the first place, we have examined enantioselective ad-
dition of -substituted allyltrimethylsilane (2) to benzal-
dehyde as a model reaction of asymmetric allylation
polymerization (Scheme 1). Since chiral oxazaborolidi-
nones were known to be powerful catalyst for enantiose-
lective aldol type reactions,9,10 we have tested their
catalytic activity in the allylation reaction. We found that
oxazaborolidinone 4 derived from L-valine showed cata-
lytic activity in the model reaction. However, complete
conversion was not achieved even after 24 h (Table 1, en-
try 1). We then used another chiral oxazaborolidinone 5,
derived from L-tryptophan, which was originally devel-
oped by Yamamoto as a chiral catalyst for the asymmetric
aldol reaction.10 This catalyst was also found to be active
in the allylation and resulted in quantitative conversion at
R
OH
R
chiral catalyst
PhCHO
+
Me3Si
Ph
1
3a: R=Me
3b: R=Ph
2a: R=Me
2b: R=Ph
OiPr
O
CO2H O
H
N
O
O
O
O
O
O
B
OiPr
N
B
Ts
O
Ph
CF3
4
6
N
B
Ts
F3C
CF3
5
F3C
Scheme 1
Synthesis 2002, No. 7, 21 05 2002. Article Identifier:
1437-210X,E;2002,0,07,0941,0944,ftx,en;C00702SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

942
S. Itsuno, T. Kumagai
SPECIAL TOPIC
Table 1 Enantioselective Addition of -Substituted Allylsilane to
Benzaldehyde as a Model Reaction of Asymmetric Polymerization
–78 °C with an enantioselectivity of 77% (entry 2). When
chiral (acyloxy)borane (CAB) (6) was used, the same re-
action occurred smoothly at –78 °C in propionitrile to
yield the product homoallyl alcohol in quantitative con-
version with higher enantioselectivity. Based on these re-
sults, CAB 6 is a choice of catalyst for the asymmetric
allylation polymerization.
Entry Allylsi- Catalyst Temp Time Yielda Eeb
Config
lane
(°C)
–78
–78
–20
–78
–78
(h)
24
1
(%)
79
93
99
99
99
(%)
34
77
75
89
79
1
2a
4
5
6
6
6
R
R
R
R
R
2
2a
We then prepared monomers for the asymmetric allyla-
tion polymerization. Since allyltrimethylsilane is stable
enough to coexist with an aldehyde in the absence of cat-
alyst, compounds having both the allylsilane moiety and
formyl group may be isolable. We have designed a novel
monomer structure consisting of these functionalities.
Monomer 10 was prepared from aldehydic acid 7 by the
procedure described in Scheme 2. The allylsilane moiety
was easily generated by Peterson elimination reaction of
9. Care must be taken at the deprotection step of the acetal
9. Strong acidic condition afforded 10 contaminated with
desilylated compound. We have found that pyridinium p-
toluenesulfonate (PPTS) gave the best result in this trans-
formation without any undesirable reaction. Another
monomer containing the phenyl–Si linkage was prepared
3
2a
1
4c
5
2a
2
2b
3
a Isolated yield.
b Determined by HPLC using a chiral column (Chiralpac AD).
c Reported in the literature.5
as shown in Scheme 3. The allyltrimethylsilane moiety
was again constructed by Peterson elimination reaction.
Methoxysilyl-containing acetal 12 was coupled with lithi-
ated -phenylallylsilane to give the monomer precursor
16, which was converted to monomer 17 by treatment
with PPTS.
O
OMe
Me3SiCH2MgCl-CeCl3
CH(OMe)3, H2SO4
H
OMe
THF
MeOH, reflux, 12 h
HOOC
MeOOC
7
8
OMe
OMe
O
PPTS
SiMe3
H
THF-H2O, rt
Me3Si
HO
9
10
SiMe3
Scheme 2
Me
Br
OMe
Si
BuLi, Me2Si(OMe)2
THF
Me
MeO
MeO
OMe
11
OMe
12
Me3Si
OH
SiMe3
COOMe
PPTS
Me3SiCH2MgCl-CeCl3
hexane
SiMe3
THF
Br
Br
Br
13
15
14
Me Me
Si
Me Me
Si
BuLi, 12
PPTS
THF-H2O
SiMe3
THF
MeO
OHC
SiMe3
OMe
17
16
Scheme 3
Synthesis 2002, No. 7, 941–944 ISSN 0039-7881 © Thieme Stuttgart · New York

SPECIAL TOPIC
Asymmetric Synthesis of Chiral Polymers by Additions of Allylsilane to Aldehyde
943
We have examined the allylation polymerization of metric polymerization of this monomer to give the
monomer 10. When TiCl4 was added to the monomer so- optically active polymer 19 in high yield. Si–phenyl link-
lution of propionitrile at 0 °C, the monomer was con- ages in the polymer main chain were readily cleaved by a
sumed rapidly and the polymeric product was precipitated treatment with fluoride anion.12 Tetrabutylammonium flu-
scheme 4). This polymer was unfortunately insoluble in oride (TBAF) gave the best result in the degradation of the
usual organic solvents (Table 2, entry 1). Among various chiral polymer to give 3b in high yield (Scheme 5). Chiral
achiral Lewis acids, we have found that Sc(OTf)3 gave HPLC analysis of the degraded product showed 78% ee
quantitative conversion in the model reaction.11 Polymer- with R configuration (Table 3, entry 1). Degradation of
ization also smoothly occurred with this catalyst to give a the chiral polymer obtained from D-6 resulted in 3b with
soluble polymer in high yield (entry 2). We then used opposite configuration (entry 2). These results revealed
chiral Lewis acid 6 for the asymmetric allylation polymer- that the asymmetric allylation polymerization occurred
ization of 10. Although some precipitation occurred when stereoselectively with a similar selectivity to that expected
the polymerization was performed at 23 °C, the optically from the model reaction.
active polymer 18 was obtained (entry 3). Lowering the
temperature prevented the formation of insoluble poly-
mers and raised the optical rotation value of the polymer
(entries 4, 5). When D-6, derived from D-tartaric acid was
used as a catalyst, the same polymer having an opposite
Me Me
Si
CAB
17
rotation value was obtained (entry 6). This is the first ex-
ample of asymmetric self-polyaddition using asymmetric
allylation reaction.
OH
n
19
TBAF
3b
OH
CAB
Scheme 5
10
Table 3 Asymmetric Allylation Polymerization of Monomer 17 in
Propionitrile at –78 °C
n
18
a
a
b
Entry CAB Yield Mw
Mw/Mn [ ]405 Ee
(%)c
Config
Scheme 4
(%)
1
2
L-6d
D-6e
89
91
11000 2.5
11000 2.6
–596
+611
78
75
R
S
Table 2 Asymmetric Allylation Polymerization of 10 in Propioni-
trile
a
a
b
Entry Lewis acid Temp Time Yield Mw
Mw/Mn [ ]405
a Estimated by size exclusion chromatography (SEC) with polysty-
rene standards.
(°C) (h)
(%)
b Molar optical rotation was measured as THF solution (c 1.0, THF).
c Determined by HPLC using a chiral column (Chiralpac AD).
1
2
3
4
5
6
TiCl4
Sc(OTf)3
L-6c
0
23
2
6
5
2
8
8
99
94
66
88
99
91
–
–
–
d
L-6 derived from L-tartaric acid was used.
D-6 derived from D-tartaric acid was used.
8700 3.0
7200 3.1
7300 2.8
16000 2.6
15000 2.2
–
e
23
–298
+525
–667
+627
L-6
–20
–78
–78
In conclusion, we have chosen CAB as a catalyst for the
asymmetric allylation polymerization of monomers hav-
ing both allyltrimethylsilyl and formyl groups. Asymmet-
ric self-polyaddition of the monomer successfully took
place in propionitrile to yield optically active polymer
having main-chain configurational chirality. The optical
purity of the chiral polymers was also determined by a
degradation method, which showed good agreement with
those in the corresponding model reaction.
L-6
D-6d
a Estimated by size exclusion chromatography (SEC) with polystyrene
standards.
b Molar optical rotation was measured as THF solution (c 1.0, THF).
c
L-6 derived from L-tartaric acid was used.
D-6 derived from D-tartaric acid was used.
d
Optical purity of the stereogenic centeres created in the
chiral polymer may be estimated from the results of the
model asymmetric reaction demonstrated in Scheme 1.
However, it is difficult to determine the optical purity of
the obtained chiral polymer itself by direct analytical
method. We have thus designed a new monomer 17 con-
taining an Si–phenyl linkage. CAB 6 initiated the asym-
Asymmetric Allylation Polymerization
An anhyd propionitrile (1 mL) solution of chiral (acyloxy)borane 6
prepared from (2R,3R)-2-O-(2,6-diisopropoxybenzoyl) tartrate (74
mg, 0.2 mmol) and 3,5-bis(trifluoromethyl)phenylboronic acid (51
mg, 0.2 mmol) was added to a solution of monomer 17 (352 mg, 1.0
mmol) in propionitrile (2 mL) at –78 °C. The mixture was stirred at
the same temperature for 12 h and quenched with 2N aq HCl (1
mL). The whole mixture was poured into MeOH–H2O (2:1, 50 mL),
Synthesis 2002, No. 7, 941–944 ISSN 0039-7881 © Thieme Stuttgart · New York

944
S. Itsuno, T. Kumagai
SPECIAL TOPIC
filtered and dried under vacuum to yield 19 as a white solid (250
mg, 89%); Mw = 11000, Mw/Mn = 2.5, [ ]405 –596 (c 1.0, THF).
IR (film): 3419, 2952, 1597, 1251, 814 cm–1.
1H NMR (300 MHz, CDCl3): = 7.55–7.30 (Ph), 5.46 (br s, =CH2),
5.19 (br s, =CH2), 4.75–4.70 (m, CH–OH), 3.04-2.76 (m, CH2),
2.10 (br s, OH), 0.56 (s, Si–CH3).
13C NMR (75 MHz, CDCl3): = 145.2, 141.1, 138.2, 137.6, 129.0,
125.9, 125.6, 72.3, 46.0, –2.0.
(4) In our previous paper, we reported that bis(allylsilane) and
dialdehyde were polymerized in the presence of Lewis acid
catalyst, see: (a) Kumagai, T.; Itsuno, S. Macromolecules
2000, 33, 4995. (b) Kumagai, T.; Itsuno, S. Macromol.
Rapid Commun. 2001, 22, 741. (c) Kumagai, T.; Itsuno, S.
Macromolecules 2001, 34, 7624.
(5) Ishihara, K.; Mouri, M.; Gao, Q.; Maruyama, T.; Furuta, K.;
Yamamoto, H. J. Am. Chem. Soc. 1993, 115, 11490.
(6) (a) Bode, J. W.; Gauthier, D. R. Jr.; Carreira, E. M. Chem.
Commun. 2001, 2560. (b) Gauthier, D. R. Jr.; Carreira, E.
M. Angew. Chem. Int. Ed. Engl. 1996, 35, 2363.
(7) Yanagisawa, A.; Kageyama, H.; Nakatsuka, Y.; Asakawa,
K.; Matsumoto, Y.; Yamamoto, H. Angew. Chem. Int. Ed.
1999, 38, 3701.
(8) (a) Denmark, S. E.; Fu, J. J. Am. Chem. Soc. 2000, 122,
12021. (b) Denmark, S. E.; Fu, J. J. Am. Chem. Soc. 2001,
123, 9488. (c) Denmark, S. E.; Coe, D. M.; Pratt, N. E.;
Griedel, B. D. J. Org. Chem. 1994, 59, 6161.
(d) Chataigner, I.; Piarulli, U.; Gennari, C. Tetrahedron Lett.
1999, 40, 3633. (e) Nakajima, M.; Saito, M.; Shiro, M.;
Hashimoto, S. J. Am. Chem. Soc. 1998, 120, 6419. (f)Iseki,
K.; Mizuno, S.; Kuroki, Y.; Kobayashi, Y. Tetrahedron
1999, 55, 977. (g) Iseki, K.; Mizuno, S.; Kuroki, Y.;
Kobayashi, Y. Tetrahedron Lett. 1998, 39, 2767.
(9) Kiyooka, S. J. Synth. Org. Chem. Jpn. 1997, 55, 313.
(10) Ishihara, K.; Kondo, S.; Yamamoto, H. J. Org. Chem. 2000,
65, 9125.
Cleavage of Chiral Allylation Polymer 19
The chiral polymer 19 (100 mg) was dissolved in TBAF–THF solu-
tion (1.0 M, 3 mL) and heated at 60 °C for 24 h. The reaction mix-
ture was diluted with Et2O (50 mL) and washed with 2 N aq HCl and
brine, dried (MgSO4) and concentrated. The crude product was pu-
rified by flash column chromatography (packing; solvent hexane–
EtOAc, 4:1) to give 3b (87%). The enantioselectivity was deter-
mined by HPLC analysis using a chiral stationary phase column
[Daicel Chiralpak AD; hexane–propan-2-ol, 30:1 v/v, flow
rate = 0.5 mL/min, tR = 34.1 min (R), 38.2 min (S)].
Acknowledgement
This work was supported by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Science, Sports and Culture, Japan.
(11) Benzaldehyde reacted with 1 equiv of
References
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CH2Cl2, DMF, toluene, nitroethane gave no product. THF
was polymerized in the presence of Sc(OTf)3.
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Synthesis 2002, No. 7, 941–944 ISSN 0039-7881 © Thieme Stuttgart · New York