Main-chain ionic chiral polymers: Synthesis of optically active quaternary ammonium sulfonate polymers and their application in asymmetric catalysis

Full text of DOI:10.1021/ja909972d

Published on Web 02/15/2010
Main-Chain Ionic Chiral Polymers: Synthesis of Optically Active Quaternary
Ammonium Sulfonate Polymers and Their Application in Asymmetric
Catalysis
Shinichi Itsuno,* Devproshad K. Paul, M. A. Salam, and Naoki Haraguchi
Department of Materials Science, Toyohashi UniVersity of Technology, Toyohashi 441-8580, Japan
Received December 14, 2009; E-mail: itsuno@tutms.tut.ac.jp
Recently, polymer-immobilized asymmetric catalysts have at-
tracted considerable attention. Although substantial research per-
taining to the use of side-chain-functionalized polymers has been
carried out, only a limited number of investigations to elucidate
the use of main-chain-functionalized polymers have been performed.
Some examples of main-chain-functionalized polymers include
readily to afford the corresponding quaternary ammonium sulfonate
in quantitative conversion. For example, reaction of 1 and 2
occurred immediately to afford chiral sulfonates 3, which could be
easily extracted using an organic solvent.
Scheme 1
1
2
3
polymeric chiral salen ligand, poly(amino acid), poly(tartrate),
4
5
polybinaphthols, and helical polymers. These main-chain chiral
polymers have been successfully used as chiral catalysts in various
kinds of asymmetric reactions. The rigid and sterically regular
polymer catalysts may have a better defined microenvironment at
the catalytic sites and have allowed systematic modification of their
catalytic properties.
In asymmetric synthesis, chiral organocatalysts have received
considerable attention in recent years, as asymmetric reactions with
6
chiral organocatalysts satisfy green chemistry requirements. One
class of important chiral organocatalysts is optically active qua-
ternary ammonium salts, which can be used in various kinds of
asymmetric transformations. Polymer-immobilized versions of chiral
7
organocatalysts have also been reported. These chiral organocata-
lysts are all side-chain-modified polymer-immobilized ammonium
salts, and the chiral quaternary ammonium salt is attached randomly
as a pendant group of the polymer support.
We have found that quaternary ammonium sulfonates, such as
3
, can be prepared easily and are extremely stable in both aqueous
8
,9
and organic media. We have recently reported polymer-im-
mobilized chiral ammonium sulfonate obtained by radical polym-
erization of the chiral vinyl monomer 3b. The obtained chiral
polymer was successfully used as an asymmetric catalyst for an
alkylation reaction. This approach involved the attachment of the
chiral catalyst moiety to an achiral and sterically irregular polymer
backbone.
10
We then applied the quaternary ammonium sulfonate formation
reaction to the polymer synthesis. First, we prepared chiral
bis(quaternary ammonium salt) 6 containing the cinchonidine unit.
Reaction of 2 equiv of cinchonidine derivative 4 with dihalide 5
afforded 6, as shown in Scheme 1. The other monomer for this
polymerization is a sodium disulfonate. We chose disodium 2,6-
naphthalenedisulfonate (7) as the disulfonate monomer.
Disulfonate 7 is soluble only in water. Bis(quaternary ammonium
halide) 6 is soluble in organic solvents, such as methanol, N,N-
dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). Our
first polymerization attempt involved the reaction of an aqueous
solution of 7 with a DMSO solution of 6. In this homogeneous
system, surprisingly, no reaction occurred after 3 h. The product
solution contained a mixture of 7 and 6. With the addition of
chloroform to the reaction mixture, 7 completely precipitated out.
Even when solid disulfonate 7 was added to the DMSO solution
of 6, no reaction occurred. Interestingly, when an aqueous solution
of 7 was treated with powdered 6 without any organic solvent, we
found a white precipitate of 8. The precipitated product was
Because of the exceptional stability of quaternary ammonium
sulfonates and their excellent catalytic activity, we used the
quaternary ammonium sulfonate formation reaction for chiral
polymer synthesis and employed the resulting chiral polymers as
novel asymmetric catalysts. The reaction between chiral bis(qua-
ternary ammonium salt) 6 and disulfonate 7 results in the formation
of 8, a main-chain chiral polymer of quaternary ammonium
sulfonate that contains an ionic bond in the main chain. To the
best of our knowledge, this is the first instance of the synthesis of
chiral polymers that involve main-chain quaternary ammonium
sulfonate bonding. In this study, we focused on the synthesis of
the chiral quaternary ammonium sulfonate polymers and their use
in the asymmetric benzylation of N-diphenylmethylidene glycine
tert-butyl ester (9).
Synthesis of quaternary ammonium sulfonate is usually very
simple and quantitative (Scheme 1). The ion-exchange reaction
between quaternary ammonium halide and sodium sulfonate occurs
2
864 9 J. AM. CHEM. SOC. 2010, 132, 2864–2865
10.1021/ja909972d  2010 American Chemical Society

C O M M U N I C A T I O N S
thoroughly washed with dichloromethane and water to remove
unreacted 6 and 7, respectively. The obtained solid 8 was insoluble
both in water and some organic solvents, such as toluene,
chloroform, and ethanol, but was soluble in DMSO and DMF. The
H NMR spectrum of product 8 contained both the cinchonidine
and naphthalene moieties. The inherent viscosity ([η] ) 0.1-0.2)
showed that product 8 is a polymeric material.
Even when the ionic polymer 8 was treated with a strong aqueous
alkaline solution during the reaction, no destruction of its salt
structure was detected. The polymeric catalyst 8 was easily
separated from the reaction mixture. When 8ba was used as a chiral
organocatalyst, the reaction occurred smoothly to afford 11 in 92%
ee (entry 12). In the presence of 8bb, the same reaction occurred
with a high yield and 91% ee. The enantioselectivity obtained from
polymeric catalyst 8bb was obviously higher than that obtained
from model catalyst 6bb (entry 15 vs 7). At -20 °C, a higher
enantioselectivity of 94% ee was obtained using the same polymer
1
Table 1. Asymmetric Benzylation of N-Diphenylmethylidene
a
Glycine tert-Butyl Ester
(
entry 16). Since the polymeric chiral catalyst was not soluble in
the organic solvent used for the reaction, the polymer could be
easily separated from the reaction mixture. The recovered polymer
could be reused for the same reaction (entries 16-18). Although
the reaction mechanism with the polymeric organocatalyst is not
clear at this moment, the transition state should involve the
polymeric ion complex to give the product.
In summary, we have prepared a novel type of main-chain chiral
polymer that comprises a quaternary ammonium sulfonate repeating
unit. This chiral polymer was successfully used as a catalyst for
asymmetric alkylation of a glycine derivative. The use of chiral
ionic polymers as organocatalysts may be extended to other types
of asymmetric transformations.
11
b
entry
catalyst
temp. (°C)
time (h)
yield (%)
ee (%)
config.
c
1
1
3a
0
0
rt
0.5
0.5
15
2
5
12
4
78
88
72
91
94
91
92
92
88
75
75
86
85
92
90
93
91
90
83
90
69
68
57
90
95
80
80
86
86
86
60
92
94
82
91
94
93
93
87
93
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
c
2
c
3
6aa
6ba
6ba
6ab
6bb
6bb
6ac
6ac
8aa
8ba
8ba
8ab
8bb
d
4
0
d
5
-20
6
0
0
d
7
d
8
-20
6
6
e
9
0
e
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
-20
rt
0
-20
0
0
-20
-20
-20
0
12
12
15
20
15
20
20
20
20
48
48
Acknowledgment. This work was financially supported by a
Grant-in-Aid from the Ministry of Education, Culture, Sports,
Science and Technology (Japan).
c
Supporting Information Available: Details pertaining to the
experimental procedure, NMR data, and viscosity data. This material
is available free of charge via the Internet at http://pubs.acs.org.
8bbf
8bb
8bb
g
8ac
8ac
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JA909972D
J. AM. CHEM. SOC. 9 VOL. 132, NO. 9, 2010 2865