Remarkable structure effects on chiroptical properties of polyisocyanides carrying proline pendants

Article abstract of DOI:10.1002/asia.201300297

Chiral polymers with simple chemical structures and high helical conformation stabilities are important for their applications as chiral supports and asymmetrical catalysts. We report herein the synthesis of a series of aliphatic polyisocyanides carrying proline pendants of different chiralities, and an investigation of the effects of the chemical structures of these pendants on the chiroptical properties of the polymers. The configuration of the chiral center at the 4-position of the proline pendants was changed from S to R to check its effect on the handedness of the helical conformation. To examine the effects of steric hindrance on the stabilities of the helical conformation for these aliphatic representatives, proline pendants with various substituents at both the carboxyl and amine terminals were designed. To further examine the steric effects of the proline pendants, aromatic counterparts were also prepared. In the latter case, the effects of hydrogen bonds between pendant units on the enhancement and stabilities of the helical conformation were investigated by switching from the ester to an amide linkage. The Cotton effects and signal intensities of both aliphatic and aromatic polyisocyanides from circular dichroism spectroscopy were compared based on the bulkiness of the pendant groups, solvent polarities, and solution temperatures. It was found that highly stable helical conformations of polyisocyanides could be imposed by small bulky monoproline pendants. Twisted sisters: Polyisocyanides with various proline-based pendant groups were synthesized and their chiroptical properties investigated. These chiral polymers show unprecedented stable helical conformations in different solvents at various temperatures, even in the absence of strong hydrogen-bonding interactions between the pendant groups (see picture). Copyright

Full text of DOI:10.1002/asia.201300297

FULL PAPER
FULL PAPER
Twisted sisters: Polyisocyanides with
various proline-based pendant groups
were synthesized and their chiroptical
properties investigated. These chiral
polymers show unprecedented stable
helical conformations in different sol-
vents at various temperatures, even in
the absence of strong hydrogen-bond-
ing interactions between the pendant
groups (see picture).
Chiral Polymers
Anqiu Xu, Guixia Hu, Yulong Hu,
Xiuqiang Zhang, Kun Liu,
Guichao Kuang,*
Afang Zhang*
&&&& — &&&&
Remarkable Structure Effects on
Chiroptical Properties of Polyisocya-
nides Carrying Proline Pendants
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Guichao Kuang, Afang Zhang et al.
DOI: 10.1002/asia.201300297
Remarkable Structure Effects on Chiroptical Properties of Polyisocyanides
Carrying Proline Pendants
Anqiu Xu, Guixia Hu, Yulong Hu, Xiuqiang Zhang, Kun Liu, Guichao Kuang,* and
[
a]
Afang Zhang*
Abstract: Chiral polymers with simple
chemical structures and high helical
conformation stabilities are important
for their applications as chiral supports
and asymmetrical catalysts. We report
herein the synthesis of a series of ali-
phatic polyisocyanides carrying proline
pendants of different chiralities, and an
investigation of the effects of the chem-
ical structures of these pendants on the
chiroptical properties of the polymers.
The configuration of the chiral center
at the 4-position of the proline pend-
ants was changed from S to R to check
its effect on the handedness of the heli-
cal conformation. To examine the ef-
fects of steric hindrance on the stabili-
ties of the helical conformation for
these aliphatic representatives, proline
pendants with various substituents at
both the carboxyl and amine terminals
were designed. To further examine the
steric effects of the proline pendants,
aromatic counterparts were also pre-
pared. In the latter case, the effects of
hydrogen bonds between pendant units
on the enhancement and stabilities of
the helical conformation were investi-
gated by switching from the ester to an
amide linkage. The Cotton effects and
signal intensities of both aliphatic and
aromatic polyisocyanides from circular
dichroism spectroscopy were compared
based on the bulkiness of the pendant
groups, solvent polarities, and solution
temperatures. It was found that highly
stable helical conformations of polyiso-
cyanides could be imposed by small
bulky monoproline pendants.
Keywords: chiroptical properties ·
helical structures · polyisocyanides ·
polymers · proline
[5]
[6]
Introduction
chiral recognition, chiral separations, and asymmetric
[7]
catalysis. For all of these applications, the amplified chirali-
ty and stability of helical conformation of these polymers
play crucial roles. Generally speaking, the amplification of
chirality and enhancement of helical conformation stability
can be rationalized by strong hydrogen bonding, hydropho-
Ordered secondary structures in biomacromolecules, such as
proteins and DNA, which have directed sophisticated func-
tions in living systems, stimulate advances across many sci-
[1]
entific disciplines. Inspired by elegant features in nature,
the development of various strategies to investigate poly-
mers with rationally designed synthetic architectures and or-
dered secondary structures has been attracting interest from
[3,8]
bic interactions, and/or steric hindrance.
Different from
helical polymers carrying linear pendants that show variable
conformation stabilities, depending on the bulkiness and hy-
[2]
[9]
chemists in recent decades. Helical polymers are among
the most interesting representatives of optically active poly-
mers and significant progress in this area has been made in
drogen-bonding interactions of the side groups, those car-
rying highly branched pendants normally display enhanced
[10]
conformation stabilities.
In extreme cases, dendronized
[3]
pioneering reports. These polymers can be realized by
achiral polymerization of optically active monomers or
through helix-sense-selective polymerization of achiral and
chiral monomers or chiral supramolecular polymeri-
polymers carrying chiral and large branched pendants show
[11]
thermally stable helical conformations. Although investi-
gations into helical polymers have resulted in a great boom
in recent decades, the development of efficient strategies to
amplify helical conformations and to further elucidate the
relationship between their chemical structures and helical
conformation stability remains challenging.
[3,4]
zation.
They have been developed to mimic biological
helices and employed to display versatile functions, such as
Polyisocyanides are an intriguing class of helical polymers,
which were first investigated by the groups of Millich, Nolte,
[
a] A. Xu, G. Hu, Y. Hu, X. Zhang, K. Liu, Dr. G. Kuang,
Prof. Dr. A. Zhang
[12]
and Drenth.
They contain an all-carbon backbone and
Lab of Polymer Synthesis, Department of Polymer Materials
Shanghai University, Nanchen Street 333
Shanghai 200444 (P.R. China)
Fax : (+86)21-66131720
E-mail: gckuang@shu.edu.cn
pendant groups at each carbon atom, which lead to limited
rotational freedom of the CÀC bonds, and thus, make these
polymers relatively rigid. They were revealed to form a 41
helical conformation when the carbon atoms were substitut-
ed with chiral pendants. Improved understanding of the heli-
cal structures of polyisocyanides came from a combination
of experimental and theoretical analyses. In general, bulky
azhang@shu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300297.
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Guichao Kuang, Afang Zhang et al.
chiral substituents and intramolecular hydrogen bonds be-
tween the pendant amide residues are two most important
factors to form and stabilize the helical conformation with
preferred handedness for polyisocyanides. Recently, poly-
merization temperature and solvent polarities were also
found to play a role in controlling the helical sense of poly-
of chiral and helical polymers. For example, l-proline was
utilized to synthesize helical poly(phenyl acetylene) by Ya-
[
19]
shima et al. whose chirality is responsive in polar solvents.
[20]
Both proline-based molecular brushes
and dendronized
reported by our group demonstrate that
[11a,c]
polymers
a highly stable helical conformation can be achieved with
proline moieties. Considering that the substituted pyrroli-
dine ring of proline may act as the simplest branched struc-
ture, we report herein the synthesis of a series of aliphatic
and aromatic polyisocyanides with proline moieties as pend-
ant groups (Figure 1). To examine the effects of steric hin-
[9c,13]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
iso
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
cyanides.
Polyisocyanides with chiral amino acid
pendant or bulky peptide moieties have gained great atten-
tion because they can assure versatile secondary structure
formation and, at the same time, may allow the develop-
ment of a variety of biocompatible and biodegradable chiral
[14]
materials.
Those with a single amino acid pendant can
also show helical structures with predominantly one-handed
screw sense, but these helical structures are tunable by tem-
perature and solvent polarities, even when a long linear
alkyl chain is attached to the terminus of the amino acid
moiety; thus, showing less stability to environmental stimu-
[
15]
li. Through the substitution of single amino acid moieties
with peptide pendants, for example, alanine dipeptide pend-
ants, the resulting polyisocyanopeptides were able to form
remarkably stable b helices, which were supposed to be sta-
bilized by strong hydrogen bonds between the pendant di-
[16]
peptides.
l-Proline is a nonessential, neutral, genetically coded
amino acid. It is the only protein-forming amino acid with
a secondary amino group. Pioneering work from the group
of List has shown that this specific structure makes l-proline
act as an efficient organocatalyst for asymmetric aldol and
[17]
Mannich reactions. Owing to conformational restrictions
imposed by the pyrrolidine ring, the proline moiety also
plays an important role within peptides and proteins by
acting as a turn inducer, although the amide linkage from
[18]
the secondary amine cannot form strong hydrogen bonds.
Thus, it is not surprising that proline is used in the synthesis
Abstract in Chinese:
Figure 1. Chemical structures of the proline-based polyisocyanides dis-
cussed herein. Boc=tert-butoxycarbonyl. Asterisks (*) donate chiral cen-
ters.
drance at the 1- or 2-position of the proline ring on the con-
formation and stabilities of the resulting polymers, proline
pendants with different structures were designed. The hand-
edness of the polymers was examined by changing the ste-
reogenic center at the 4-position of prolines. To further ex-
amine the steric effects of the proline pendants on the chi-
roptical properties of less hindered aromatic representatives,
poly(phenyl isocyanide)s were also prepared. By switching
the ester to amide linkage between the pendants and the
backbone, the effects of hydrogen bonds between the pend-
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ants on stabilizing the helical conformation of the resulting
aromatic polymers were investigated. The chirality of the
polyisocyanides can be amplified and stabilized by an indi-
vidual small proline pendant, which is a rigid five-membered
ring.
lines 14 and 15, respectively. Dehydration of the formamides
with triphosgene in dry CH Cl gave rise to the correspond-
2
2
ing isocyanide monomers 1 and 2 in high yields. A similar
methodology was applied for the synthesis of (4S)-3 (see
Scheme S1 in the Supporting Information). These new mon-
omers were characterized by NMR spectroscopy and high-
resolution mass spectrometry (HRMS). Polymerization was
readily conducted by using the achiral catalyst NiCl ·6H O
Results and Discussion
2
2
in CH Cl or THF at room temperature. Polymerizations
2
2
Synthesis of the Aliphatic Polyisocyanides
proceeded homogeneously and afforded the polyisocyanides
poly-1, poly-2, and poly-(4S)-3 with moderate to high molec-
ular weights. The polymerization results are summarized in
Table 1 (entries 1 to 7). Generally speaking, polyisocyanides
of higher molecular weights can be achieved with a higher
molar ratio of monomer to initiator (Table 1, entries 1–3),
but the large pendants in the monomers will contribute sig-
nificantly to steric hindrance, which clearly reduces the
molar masses of the resulting polymers (Table 1, entries 5
and 6). Contrary to polymers prepared in CH Cl , those ob-
The synthesis of aliphatic isocyanide monomers (4S)-1 and
(
4R)-1, which carry a single proline moiety, started from 4-
azidoprolines with either S or R configuration at the 4-posi-
tion. To examine the steric effects from the chiral substitu-
ents at the 2-position of proline on the chiroptical properties
of the resulting polymers, monomers (4S)-2 and (4R)-2,
which carry proline dipeptides, were prepared. To investi-
gate the steric effects of the substituent at the 1-position of
the proline moiety, acetylated isocyanide (4S)-3 was also
prepared for comparison with the Boc-protected com-
pounds. Detailed synthetic procedures for monomers 1 and
2
2
tained from a more polar solvent, THF, have much higher
molecular weights. All polymers have different solubility in
conventional solvents. Boc-protected polymers poly-(4S)-1,
poly-(4R)-1, poly-(4S)-2, and poly-(4R)-2 show good solubil-
ity in THF or CH Cl , but their solubility is much worse in
2
are outlined in Scheme 1. Saponification of 9 with LiOH
yielded acid 10. Amidation of acid 10 with proline methyl
ester afforded dipeptide 11 by using a typical peptide cou-
2
2
[21]
pling method in the presence of EDC and HOBt. Reduc-
methanol. Polymers with high molar masses are no longer
soluble in methanol. Acetylated polymer poly-(4S)-3 is solu-
ble in CH Cl or methanol, but sparingly so in THF. After
[22]
tion of azides 9 and 11 with triphenylphosphine gave the
corresponding 4-aminoproline derivatives 12 and 13, which
were treated with ethyl formate to give the N-formylpro-
2
2
[22]
deprotection with trifluoroacetic acid (TFA), the positive-
Scheme 1. Synthesis and polymerization of aliphatic isocyanides. Reagents and conditions: a) 9, LiOH·H
OMe·HCl, DIPEA, EDC, HOBt, CH Cl , THF/H O, 50 8C, 2 h (85–96%); d) 12 or 13, ethyl formate, sodium for-
, À158C, 5 h (72–77%); c) 9 or 11, PPh
mate, 508C, 5 h (87–90%); e) 14 or 15, TEA, triphosgene, CH Cl ·6H O (10 wt% in MeOH),
, À108C to RT, 5 h (81–84%); f) monomer 1 or 2, NiCl
CH Cl or THF, 258C (47–82%). DIPEA=diisopropylethylamine, EDC=1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, HOBt=1-hy-
2 2
O, H O/MeOH, 08C, 2 h (100%); b) 10, H-Pro-
2
2
3
2
2
2
2
2
2
2
droxy-1H-benzotriazole, THF=tetrahydrofuran, TEA=triethylamine.
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[
a]
Table 1. Polymerization of the isocyanide monomers with achiral catalyst NiCl
2
·6H
2
O.
couplets
De ).
1st
configuration at the 4-position
of the proline determines the
handedness of the helical con-
formations of the polyisocya-
nides. The S configuration at
the 4-position of proline leads
to the formation of polymers
with P helical structure, where-
as the R-configured chiral
center results in the formation
of polymers with the M helical
(positive
Therefore, the
for
[
b]
[23b,26,27]
Entry
Polymerization conditions
GPC results
À3
Monomer
[M]
0
/[I]
0
Time [h]
Solvent
Yield [%]
M
n
(ꢁ10
)
PDI
1
2
3
4
5
6
7
8
9
1
1
1
1
(4S)-1
(4S)-1
(4S)-1
(4R)-1
(4S)-2
(4R)-2
(4S)-3
(4S)-6
(4R)-6
(4S)-7
(4R)-7
(4S)-8
(4R)-8
50:1
100:1
200:1
100:1
100:1
100:1
200:1
100:1
100:1
80:1
24
24
48
24
72
72
24
72
48
48
48
72
48
CH
CH
THF
2
2
Cl
Cl
2
70
82
62
70
47
64
78
55
48
62
55
60
46
4.6
1.31
1.48
1.10
1.13
1.12
1.07
1.05
1.26
2.21
1.63
1.44
1.45
1.35
2
10.5
43.9
7.4
2.2
2.1
49.6
131.4
144.7
48.0
36.6
45.3
39.5
CH
CH
CH
THF
THF
THF
THF
THF
THF
THF
2
2
2
Cl
2
Cl
2
Cl
2
0
1
2
3
100:1
200:1
200:1
conformation.
[
a] General polymerization condition: nitrogen atmosphere at room temperature (258C). [M]
o
/[I]
o
represents
Second, to understand how
steric hindrance from the pro-
line pendants contributes to
control of the helical confor-
À1
the initial molar ratio of monomer to initiator. [M]
o
is in the range of 0.66–1.00 molL . [b] GPC measure-
ments were performed with N,N-dimethylformamide (DMF) as the eluent containing 0.10 wt% LiBr at 458C.
=number-average molecular weight. PDI=polydispersion index.
M
n
mation of the resulting poly-
ly charged polyisocyanides poly-(4S)-4 and poly-(4R)-5 are
soluble in water.
mers, poly-2 and poly-(4S)-3 were prepared (Scheme S1 in
the Supporting Information). Poly-2 carries a proline-based
dipeptide with either S- or R-configured chiral centers at the
4
-position of the first proline moiety, resulting in polymers
Chiroptical Properties of Aliphatic Polyisocyanides
poly-(4S)-2 and poly-(4R)-2, respectively. This allows us to
check the effect of the bulkiness arising from substitution at
the 2-position on the chiroptical properties of the polymers.
Poly-(4S)-3 carries a single acetylated proline moiety with
an S-configured chiral center at the 4-position and allows an
examination of how the bulkiness at the 1-position of the
proline moiety affects the chiroptical properties of the poly-
mer. The CD spectra of these polymers are shown in Fig-
ure 2b. Disregarding the S- or R-configured chiral centers at
the 4-position, poly-2 undergoes a small bathochromic shift
relative to poly-1, which indicates a tighter helical confor-
mation. Surprisingly, increasing the steric hindrance by in-
creasing the pendant size did not lead to a clear enhance-
ment of the Cotton effects. On the contrary, the intensities
of the first Cotton effects of poly-2 slightly decreased rela-
tive to those of poly-1, most probably as a result of the
lower molar masses of the polymers. Once the Boc protect-
ing group (for poly-1) was changed to an acetyl one (for
poly-(4S)-3), the bulkiness of pendants along the polymer
backbone reduced owing to a decrease in the size of the
acetyl group when compared with that of the Boc group.
From the CD spectrum of poly-(4S)-3, the first Cotton
effect shows a similar shape to those of poly-1, but with
slightly reduced intensities. The effect of the bulkiness of
the Boc group on the chiroptical properties was also investi-
gated by deprotection of poly-(4S)-1c and poly-(4R)-2 with
TFA to yield the corresponding deprotected, charged poly-
mers poly-(4S)-4 and poly-(4R)-5, respectively. As shown by
the CD spectra in Figure 2b, both deprotected polymers had
significantly decreased Cotton effects at around 294 nm.
Similar results were obtained for CD spectra in THF (data
not shown). The weakened Cotton effect indicates that de-
protected polymers hold less defined helical structures,
Because these polyisocyanides were prepared from optically
pure isocyanide monomers, a single-handed helix conforma-
tion for the resulting polymers was expected, and their chi-
roptical properties were thus characterized with circular di-
chroism (CD) spectroscopy. The CD spectra of the polyiso-
cyanides show Cotton effects in the range from 240 to
3
80 nm (Figure 2), which are ascribed to n–p* transitions of
[23]
the CÀN chromophores.
For comparison, CD spectra
from corresponding monomers were recorded (Figure S1 in
the Supporting Information), and no clear Cotton effects
were observed. Therefore, the Cotton effects suggest that
these polymers all contain an excess of one helical sense. As
expected, polymers with higher molar masses show stronger
Cotton effects (compare the curves of poly-(4S)-1a (entry 1
in Table 1), poly-(4S)-1b (entry 2 in Table 1) and poly-(4S)-
[28]
1
c (entry 3 in Table 1) in Figure 2a). This molecular weight
dependence of the Cotton effect is consistent with previous
[13b]
results.
First, the effect of the absolute configuration at the 4-po-
sition of the proline moiety on the chiroptical properties of
poly-1 are discussed. Poly-(4S)-1 and poly-(4R)-1 have S- or
R-configured chiral centers at the 4-position of the proline
moiety, respectively. For the former, the CD spectrum in
Figure 2a displays a negative sign at the first Cotton effect
with an intensity value (De ) of about À1.0, whereas, for
1
st
the latter, the CD spectrum (Figure 2a) shows the opposite
sign for the first Cotton effect with an intensity value (De )
1
st
of about +1.0. The opposite Cotton effect sign is indicative
of inversion of the helical conformation of the polymer
[24]
[25]
backbones. According to Tinoco
and DeVoe,
polyiso-
cyanides adopt right-handed helices if the couplets are Z
shaped (negative for De ), whereas a left-handed helical
1
st
conformation would be expected in the case of S-shaped
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polymers examined are stable within the measured tempera-
ture range. The Cotton effects of these polyisocyanides are
also similar to each other in different solvents. These results
suggest that these chiral polymers possess helical conforma-
tions that are stable to temperature and solvent polarity.
This high stability could be attributed to sufficient steric hin-
drance from the proline pendants, which makes the main
chain rigid enough to prevent conformation switching, even
though there is an absence of strong intramolecular hydro-
gen bonding.
Synthesis and Secondary Structures of Aromatic
Poly(phenyl isocyanide)s
From the results described above, it is clear that aliphatic
polyisocyanides carrying a monoproline moiety can adopt
stable helical conformations, and the handedness is mainly
dependent on the configuration at the 4-position of the pro-
line moiety. Motivated by these findings, we set out to ex-
amine the structural effects on the chiroptical properties of
poly(phenyl isocyanide)s with these proline-based pendants.
It is clear in poly(phenyl isocyanide)s that the chiral pend-
ants are a benzene unit away from the isocyanide backbone,
and thus, should show less steric hindrance effects for the
same pendants when compared to the situation in aliphatic
polyisocyanides. Therefore, poly(phenyl isocyanide)s with
three types of proline-based pendants were prepared to
check whether structural effects on the chiroptical proper-
ties of the aliphatic polyisocyanides can be inherited. For
this purpose, compounds (4S)-6 and (4R)-6, with proline
pendants joined through an ester linkage, and (4S)-7 and
(
4R)-7, with proline pendants joined through an amide link-
age, were designed and synthesized. The chiroptical proper-
ties of the corresponding polymers were investigated and
compared. To examine the effects of steric hindrance at the
1
-position of proline moiety on the optical properties of the
corresponding polymers, acetylated monomers (4S)-8 and
4R)-8 were also designed. The synthetic procedures for
(
these monomers are outlined in Scheme 2, and their poly-
merization results are summarized in Table 1 (entries 8–13).
With THF as the polymerization solvent, polymers of high
molecular weights were obtained.
First, the secondary structures of poly-(4S)-6 were charac-
terized by CD spectroscopy, in which the proline pendant
was connected to the main chain through an ester linkage.
The CD spectra in THF at various temperatures are shown
in Figure 3a. Interestingly, the positive first Cotton effect at
around l=358 nm (De1st = +2.84) reveals that the poly-
Figure 2. CD and UV spectra of a) poly-1 measured in THF at 258C;
b) poly-2 and poly-(4S)-3 in CH Cl , poly-(4S)-4 and poly-(4R)-5 in water
at 258C; and c) CD spectra of poly-1 in THF and poly-(4S)-3 in methanol
at various temperatures. c=0.05 wt%.
2
2
(
phenyl isocyanide) also adopts a preferred single-handed
helix conformation, even with less steric hindrance and in
the absence of strong hydrogen bonding. CD spectra at ele-
vated temperatures indicate that this conformation has mod-
erate thermal stability. The CD spectra of poly-(4S)-6 in sol-
vents with different polarities were also recorded (Fig-
ure 3b). The strongest first Cotton effect in THF was ob-
served; this showed a clear dependence of the chiroptical
properties on solvent polarity. To examine whether the con-
which might be caused by reduced steric hindrance together
with repulsions between the positive charges.
Third, the thermal stability of the helical conformation of
these chiral polymers was examined at various temperatures
[29]
(
20–508C); the CD spectra of poly-1 and poly-(4S)-3 are
shown in Figure 2c and negligible CD signal changes are ob-
served, even at 508C. Thus, the helical conformations of all
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Scheme 2. Synthesis and polymerization of phenyl isocyanides. Reagents and conditions: a) 16, ethyl formate, 508C, reflux, 24 h (93%); b) for X=O: 17,
Boc-Hyp-OMe, DEAD, PPh , THF, 0–58C, 12 h (57–62%); for X=NH: 12, 17, DIPEA, PyBOP, HOBt, CH Cl
, À158C, 5 h (85%); c) 18, 19, or 21,
TEA, triphosgene, CH Cl ·6H O (1 wt% in MeOH), THF, 258C, 48–72 h (50–80%); e) 17,
, À108C to RT, 5 h (79–86%); d) monomer 6, 7, or 8, NiCl
pentafluorophenol, DMF, EDC, overnight (70%); f) 20, 24, DMF, TEA, RT, 12 h (65–87%). DEAD=diethyl azodicarboxylate, PyBOP=(benzotriazol-
-yloxy)tripyrrolidinophosphonium hexafluorophosphate, Boc-Hyp-OMe=N-Boc-trans (or cis)-4-hydroxy-l-proline methyl ester.
3
2
2
2
2
2
2
1
figuration at the 4-position of the proline pendant influences
the handedness of the preferred helical conformation, poly-
nide backbone contributes a significant barrier to pendant
[30]
group rotation.
The enhanced Cotton effects should be
(
4R)-6 was prepared. Surprisingly, the sign of the first
Cotton effect of this polymer was the same as that of poly-
4S)-6 (Figure 3c). This is in contrast to the situation for ali-
partially achieved by strong hydrogen bonding within the
pendant groups. A clear contribution from strong hydrogen
bonding within these two polymers is also observed CD
spectra recorded at various temperatures in protic solvent
methanol (Figure 3d). Within the measured temperature
range, the intensities of the first Cotton effects show almost
no deviations, suggesting the high thermal stability of their
helical conformations. Further work was conducted to assess
the effects of steric hindrance. By substituting the Boc pro-
tecting group with an acetylated one, poly-(4S)-8 and poly-
(4R)-8 with less hindered pendants were synthesized. Based
on the CD spectra (Figure 3e), both polymers inherit from
their Boc-protected counterparts a preference to adopt
a single-handed helical conformation with inverted signs.
Surprisingly, the intensities of the first Cotton effects (l=
364 nm, De1st = +8.50) are doubled compared with those of
the Boc-protected polymers; this suggests the existence of
a subtle balance between steric effects and optical proper-
ties. The positive De1st values consistently gradually de-
(
phatic polyisocyanides, for which polymers adopt a helical
conformation with inversed handedness upon changing the
configuration at the 4-position from S to R. One possible
reason for this contradiction could be due to free rotation of
the ester linkages in poly-(4S)-6 and poly-(4R)-6, which
dampen the configuration effect of the 4-position of the pro-
[30]
line moiety.
To examine how hydrogen bonding plays a rule in con-
trolling the helical conformation in such aromatic polymers,
poly-(4S)-7 and poly-(4R)-7 were synthesized. The CD spec-
tra recorded in THF are shown in Figure 3c. Interestingly,
the first Cotton effect of poly-(4S)-7 is much stronger (l=
3
62 nm, De1st = +4.87) than those from counterparts with
ester linkages. The signs of the first Cotton effects are in-
verted for poly-(4S)-7 and poly-(4R)-7, which suggests that
the amide linkage between the proline pendant and isocya-
Chem. Asian J. 2013, 00, 0 – 0
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Guichao Kuang, Afang Zhang et al.
Figure 3. CD and UV spectra of a) poly-(4S)-6 in THF at different temperatures; b) poly-(4S)-6 in different polarity solvents at 258C; c) poly-(4R)-6,
poly-(4S)-7, and poly-(4S)-7 in THF at 258C; d) poly-(4S)-6 and poly-(4R)-6 in methanol at 258C; and e) poly-(4S)-8 and poly-(4R)-8 in methanol at dif-
ferent temperatures. c=0.05 wt%.
creased from +8.50 to +6.84 when the solvent changed
ties. We speculate that these contradictions mainly come
from strong hydrogen bonding between the pendant groups
along the polymer backbone, which should have been en-
hanced with reduced steric hindrance at the 1-position of
the proline moiety. Enhanced interactions leave the main
chain more intact to prevent the conformation from switch-
ing, and thus, affording chiral polymers with stable helical
conformations.
from less polar CH Cl to polar MeOH. Furthermore, these
2
2
helical conformations are stable over the temperature range
of 20–508C, as shown in Figure 3e. These results are in con-
tradiction to our initial assumption. By comparing these re-
sults with those for the aliphatic polyisocyanides, aromatic
polyisocyanides carrying the same pendant groups should
have less steric hindrance, leading to the formation of less
ordered helical conformations with impaired thermal stabili-
[31]
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Guichao Kuang, Afang Zhang et al.
Conclusion
A thermally controlled quartz cell with a path length of 1 mm was used.
CD data are given as mean molar ellipticities based on repeating units.
We have prepared a series of novel aliphatic and aromatic
polyisocyanides with various mono- and diproline deriva-
tives as pendant groups. The effects of the bulkiness of the
pendant, the configuration of the chiral center, solvent po-
larity, and solution temperature on the helical conformation
of these polyisocyanides were evaluated by CD spectroscopy
and demonstrated that the chiral polymers with stable heli-
cal conformations could be realized by small, rigid five-
membered proline pendants, even in the absence of strong
hydrogen bonds. The handedness of the helices could be
mediated by the configuration at the 4-position of the pro-
line moieties. Steric effects from the five-membered ring in
the chiral substituents were proposed to be the most impor-
tant factor to induce and stabilize the helical conformation
for these polyisocyanides. Aromatic representatives with
proline pendants attached through ester or amide linkages
were prepared to confirm this conjecture. Hydrogen bonds
from the amide linkages among the pendants are important
in the less bulky cases to guarantee the rigidity of polymer
backbone, and thus, enhance and stabilize the helical confor-
mation. This work provides a novel perspective for control-
ling the helical conformation of polyisocyanides. Based on
their simplified chemical structures and enhanced stable hel-
ical conformation, these chiral polyisocyanides with proline
pendants may find promising applications in chiral materials
and asymmetrical catalysis.
General Procedure for Formylation (A)
Sodium formate (18.37 mmol) and the amino compound (16.14 mmol)
were dissolved in ethyl formate (50 mL). The mixture was heated to
reflux for 3 h. After the precipitate was filtered out and washed with
CH
was purified by column chromatography with MeOH/CH
to afford the formamide compounds as colorless oils or solids.
2
Cl
2
, the combined filtrate was concentrated in vacuo and the residue
2
Cl (1/20, v/v)
2
General Procedure for the Dehydration of Formamide with Triphosgene
B)
(
Triphosgene (2.19 mmol) in CH
ture of TEA (10.87 mmol) and the formamide (3.67 mmol) in dry CH
2
Cl
2
(5 mL) was added dropwise to a mix-
Cl
2
2
(
80 mL) over 30 min at À108C. The mixture was allowed to warm to 08C
3
before the addition of a saturated aqueous solution of NaHCO (20 mL)
and then the solution was stirred vigorously for 10 min. After washing
successively with brine, the organic phase was collected and dried over
MgSO . Purification by column chromatography with ethyl acetate/
4
hexane (1/5, v/v) afforded the monomer as a yellow oil, which was stored
under 08C before use.
General Procedure for Amide Coupling (C)
The deprotected compound (36.23 mmol) and acid (30.05 mmol) were
dissolved in dry CH
was cooled to À158C, TEA (89.93 mmol), HOBt (36.02 mmol), and EDC
35.99 mmol) or PyBOP (35.99 mmol) were added successively under N
The mixture was kept at that temperature for 2 h, then for 14 h at room
temperature. It was washed successively with NaHCO and brine, and all
of the aqueous phases were extracted with CH Cl three times. The com-
bined organic phases were dried over MgSO . After filtration, the solvent
2 2
Cl (80 mL) at room temperature. After the solution
(
2
.
3
2
2
4
was evaporated in vacuo. Purification by column chromatography with
ethyl acetate/hexane (1:1 then 2:1, v/v) yielded the title compounds as
colorless solids.
General Procedure for the Mitsunobu Reaction (D)
Experimental Section
DEAD (40% in toluene, 11.02 mmol) was added dropwise to a mixture
3
of hydroxyproline (8.19 mmol), 17 (9.08 mmol), and PPh (11.82 mmol)
Materials
in dry THF (80 mL) at 08C under a nitrogen atmosphere. The mixture
was stirred at 08C for 30 min and then at room temperature for 12 h
before the solvent was evaporated in vacuo. Purification by column chro-
[
22]
[22]
[22]
[22]
[20]
Compounds (4S)-9,
(4R)-9,
4R)-12 were prepared according to previous reports. All other chemi-
cals were purchased with purities higher than 98% and used without fur-
ther purification unless otherwise specified. NiCl ·6H O has a purity of
over 99.9999%. CH Cl was dried over CaH . THF was predried over
(4S)-10,
(4R)-10,
(4S)-12,
and
[
20]
(
matography with CH
as white solids.
2 2
Cl /MeOH (20/1, v/v) afforded the title compounds
2
2
2
2
2
General Procedure for Polymerization (E)
sodium and then heated at reflux over lithium aluminum hydride before
use. Analytical TLC was performed by using TLC plates precoated with
silica gel 60 G/UV254, 0.25 mm. Separation on the plates was visualized
by treatment with a solution of ninhydrin in ethanol (0.3 wt%) and sub-
sequent heating at around 2008C. Silica gel 60M (300–400 mesh) was
used as the stationary phase for column chromatography. All samples
were dried thoroughly under vacuum prior to analytical measurements to
remove strongly adhering solvent molecules.
Polymerization of the monomer was carried out in organic solvent
(
CH
A typical procedure was as follows: NiCl
wt%) was added to a solution of monomer in organic solvent. The
brown solution was stirred for 24–72 h at room temperature and then pu-
2
Cl
2
or THF) with NiCl
2
·6H
2
O as the catalyst at room temperature.
2
·6H O in MeOH (10 wt% or
2
1
rified by column chromatography with CH
polymer as a yellow solid.
2 2
Cl as the eluent to afford the
Synthesis of (4S)-11
Instrumentation and Measurements
1
According to general procedure C, from Pro-OMe·HCl (6.00 g,
6.23 mmol), (4S)-10 (7.70 g, 30.05 mmol), TEA (9.10 g, 89.93 mmol),
H NMR spectra were recorded on a Bruker AV500 (500 MHz) spec-
3
3
trometer at room temperature with CDCl as the solvent (unless noted
HOBt (4.87 g, 36.02 mmol), and EDC (6.90 g, 35.99 mmol), (4S)-11 was
otherwise). All chemical shifts were reported in ppm relative to tetrame-
thylsilane. Gel permeation chromatography (GPC) measurements were
carried out on a Waters GPC e2695 instrument with a three-column set
(
(
1
afforded as a colorless solid (7.98 g, 72%). H NMR: d=1.40 and 1.46
(
2ꢁs, 9H; H-Boc), 1.93–2.27 (m, 5H; CH
2
), 2.58–2.64 (m, 1H; CH
2
),
3
.37–3.43 (m, 1H; CH ), 3.55–3.91 (m, 6H; CH
2
2
, CH ), 4.02–4.11 (m, 1H;
3
Styragel HR3+HR4+HR5) equipped with a refractive index detector
À1
CH), 4.41–4.64 ppm (m, 2H; CH).
Waters 2414) and DMF (containing 1 mgmL LiBr) as the eluent at
4
58C. The calibration was performed with poly(methyl methacrylate)
Synthesis of (4R)-11
standards in the range of M =2580–981000 (Polymer Standards Service
p
USA, Inc.). High-resolution MALDI-TOF-MS analyses were performed
on IonSpec Ultra instruments. CD measurements were performed on
a JASCOJ-815 spectropolarimeter (continuous scanning mode; scanning
According to general procedure C, from Pro-OMe·HCl (6.00 g,
36.23 mmol), (4R)-10 (7.70 g, 30.05 mmol), TEA (9.10 g, 89.93 mmol),
2 2
HOBt (4.87 g, 36.02 mmol), and EDC (6.90 g, 36.10 mmol) in dry CH Cl
À1
1
speed: 50 nmmin ; data pitch: 1 nm; response: 1 s; band width: 2.0 nm).
(80 mL), (4R)-11 was yielded as a colorless solid (8.47 g, 77%). H NMR:
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Guichao Kuang, Afang Zhang et al.
d=1.41 and 1.46 (2ꢁs, 9H; H-Boc), 1.93–2.35 (m, 6H; CH
m, 7H; CH , CH ), 4.24–4.33 (m, 1H; CH), 4.50–4.67 ppm (m, 2H;
CH).
2
), 3.46–3.91
Synthesis of (4R)-1
According to general procedure B, from (4R)-14 (1.00 g, 3.67 mmol), tri-
(
2
3
phosgene (0.65 g, 2.19 mmol), and TEA (1.10 g, 10.87 mmol) in CH Cl
(60 mL) under an N atmosphere, (4R)-1 was afforded as thick yellowish
needles (0.77 g, 83%). H NMR: d=1.44 and 1.49 (2ꢁs, 9H; H-Boc),
.25–2.35 (m, 1H; CH ), 2.47–2.62 (m, 1H; CH ), 3.66–3.84 (m, 5H; CH
CH ), 4.20–4.29 (m, 1H; CH), 4.43–4.56 ppm (m, 1H; CH); HRMS
ESI): m/z calcd for C12
2
2
Synthesis of (4S)-13
2
1
3
A solution of (4S)-11 (1.00 g, 2.72 mmol) and PPh (1.07 g, 4.08 mmol) in
THF (50 mL) and water (5 mL) was stirred at 508C for 4 h. The solvent
was evaporated and the residue was purified by column chromatography
2
2
2
3
,
2
+
(
19 2 4
H N O [M+H] : 255.1345; found: 277.1155.
with MeOH/CH
2
Cl
2
(1/20, v/v) to afford (4S)-13 as a colorless oil (0.79 g,
1
8
5%). H NMR: d=1.40 and 1.46 (2ꢁs, 9H; H-Boc), 1.88–2.27 (m, 5H;
Synthesis of (4S)-2
According to general procedure B, from (4S)-15 (0.60 g, 1.62 mmol),
CH
2
), 2.43–2.52 (m, 1H; CH
2 2
), 3.41–3.47 (m, 1H; CH ), 3.53–3.85 (m,
7
H; CH
2
, CH ), 4.41–4.64 ppm (m, 2H; CH).
3
TEA (0.50 g, 4.94 mmol), and triphosgene (0.29 g, 0.98 mmol) in CH Cl
2
2
Synthesis of (4R)-13
(40 mL) under an N atmosphere, (4S)-2 was afforded as thick yellowish
needles (0.46 g, 81%). H NMR: d=1.40 and 1.46 (2ꢁs, 9H; H-Boc),
2
1
3
A solution of (4R)-11 (1.00 g, 2.72 mmol) and PPh (1.07 g, 4.08 mmol) in
THF (50 mL) and water (5 mL) was stirred at 508C for 4 h. The solvent
was evaporated and the residue was purified by silica chromatography
1
.95–2.32 (m, 5H; CH
CH , CH ), 4.02–4.13 (m, 2H; CH
ESI): m/z calcd for C17 [M+H] : 352.1872; found: 352.1874.
2
), 2.74–2.83 (m, 1H; CH
2
), 3.54–3.83 (m, 6H; CH,
2
3
2
), 4.38–4.66 ppm (m, 2H; CH); HRMS
+
(
26 3 5
H N O
with MeOH/CH
2
Cl
2
(1/20, v/v) to afford (4R)-13 as a colorless oil (0.89 g,
1
9
6%). H NMR: d=1.40 and 1.45 (2ꢁs, 9H; H-Boc), 1.92–2.25 (m, 6H;
Synthesis of (4R)-2
According to general procedure B, from (4R)-15 (0.50 g, 1.35 mmol), tri-
2 2 2 3
CH ), 3.08–3.28 (m, 1H; CH ), 3.56–3.87 (m, 7H; CH, CH , CH ), 4.52–
4
.64 ppm (m, 2H; CH).
Synthesis of (4S)-14
According to general procedure A, from compound (4S)-12 (1.50 g,
phosgene (0.24 g, 0.81 mmol), TEA (0.55 g, 5.44 mmol), and CH Cl
2
2
(45 mL) under an N atmosphere, (4R)-2 was afforded as thick yellowish
2
1
needles (0.39 g, 82%). H NMR: d=1.42 and 1.47 (2ꢁs, 9H; H-Boc),
1
.93–2.56 (m, 6H; CH
CH ), 3.77–3.93 (m, 2H; CH
m, 2H; CH); HRMS (ESI): m/z calcd for
52.1872; found: 352.1860.
2
), 3.58–3.72 (m, 2H; CH
2
), 3.72–3.77 (m, 3H;
6
.14 mmol) and sodium formate (1.58 g, 18.37 mmol) in ethyl formate
50 mL), (4S)-14 was afforded as colorless solid (1.50 g, 90%).
H NMR: d=1.44 and 1.48 (2ꢁs, 9H; H-Boc), 1.92–2.01 (m, 1H; CH ),
), 3.79 and 3.81 (2ꢁs,
), 4.37 (d, J=9.0 Hz, 1H; CH), 4.78 (s, 1H; CH), 6.75–6.99 (m,
H; NH), 8.11 ppm (s, 1H; CHO).
3
2
), 4.28–4.41 (m, 1H; CH), 4.53–4.74 ppm
(
a
+
(
3
C
17 26 3 5
H N O [M+H] :
1
2
2
3
1
.43–2.57 (m, 1H; CH
H; CH
2 2
), 3.49–3.67 (m, 2H; CH
3
Synthesis of 17
A solution of 4-aminobenzoic acid (3.00 g, 21.88 mmol) in ethyl formate
Synthesis of (4R)-14
(
50 mL) was heated to reflux for 24 h. Then the precipitate was filtered
and washed thoroughly with CH Cl to afford product 17 as a white solid
]DMSO): d=7.69 and 7.71 (2ꢁs, 2H; Ph-
H), 7.90 and 7.92 (2ꢁs, 2H; Ph-H), 8.31–8.36 (m, 1H; CHO), 10.53 (s,
H; NH), 12.81 ppm (br, 1H; COOH).
2
2
According to general procedure A, from (4R)-12 (1.50 g, 6.14 mmol) and
sodium formate (1.58 g, 18.37 mmol) in ethyl formate (50 mL), (4R)-14
was afforded as a colorless oil (1.51 g, 90%). H NMR: d=1.43 and 1.47
1
(
3.35 g, 93%). H NMR ([D
6
1
1
(
3
6
2ꢁs, 9H; H-Boc), 2.21–2.38 (m, 2H; CH
.70–3.93 (m, 4H; CH , CH
2 3
2
), 3.31–3.47 (m, 1H; CH
), 4.24–4.45 (m, 1H; CH), 4.61 (s, 1H; CH),
.02–6.40 (m, 1H; NH), 8.10–8.18 ppm (m, 1H; HCO).
2
),
Synthesis of (4S)-18
According to general procedure D, from DEAD (40% in toluene, 4.80 g,
Synthesis of (4S)-15
1
9
1.02 mmol), (4R)-hydroxyproline (2.01 g, 8.19 mmol), 17 (1.50 g,
.08 mmol), and PPh (3.10 g, 11.82 mmol), (4S)-18 was afforded as
3
According to general procedure A, from compound (4S)-13 (0.80 g,
1
a white solid (1.84 g, 57%). H NMR: d=1.46 and 1.50 (2ꢁs, 9H; H-
Boc), 2.37–2.63 (m, 2H; CH ), 3.66–3.87 (m, 5H; CH , CH ), 4.46–4.64
m, 1H; CH), 5.54 (s, 1H; CH), 7.11–7.16 (m, 1H; NH), 7.61–8.18 (m,
2
.34 mmol) and ethyl formate (40 mL), (4S)-15 was afforded as a colorless
1
2
3
2
solid (0.77 g, 89%). H NMR: d=1.41 and 1.45 (2ꢁs, 9H; H-Boc), 1.96–
.31 (m, 5H; CH ), 2.40–2.52 (m, 1H; CH), 3.52–3.94 (m, 7H; CH
CH ), 4.49–4.63 (m, 2H; CH), 4.78–4.85 (m, 1H; CH), 7.70 (dd, J=9.3,
.2 Hz, 1H; NH), 8.02–8.08 ppm (m, 1H; HCO).
(
2
2
2
,
4
H; Ph-H), 8.44–8.92 ppm (m, 1H; CHO).
3
9
Synthesis of (4R)-18
According to general procedure D, from DEAD (40% in toluene, 4.80 g,
Synthesis of (4R)-15
1
9
1.02 mmol), (4S)-hydroxyproline (2.00 g, 8.19 mmol), 17 (1.50 g,
.08 mmol), and PPh (3.10 g, 11.82 mmol), (4R)-18 was afforded as
According to general procedure A, from (4R)-13 (0.80 g, 2.34 mmol) and
3
sodium formate dihydrate (0.73 g, 7.02 mmol) in ethyl formate (40 mL),
1
a white solid (2.00 g, 62%). This compound was roughly purified by
column chromatography and used directly in subsequent syntheses.
(
4R)-15 was afforded as a colorless solid (0.75 g, 87%). H NMR: d=
1
7
8
.40 and 1.45 (2ꢁs, 9H; H-Boc), 1.92–2.44 (m, 6H; CH
H; CH , CH ), 4.45–4.68 (m, 3H; CH , CH), 5.88–6.28 (m, 1H; NH),
.05–8.18 ppm (m, 1H; HCO).
2
), 3.17–3.58 (m,
2
3
2
Synthesis of (4S)-19
According to general procedure C, from compound (4S)-12 (1.00 g,
Synthesis of (4S)-1
4
.09 mmol), HOBt (0.68 g, 5.03 mmol), DIPEA (1.93 g, 14.93 mmol),
PyBOP (2.60 g, 5.00 mmol), and 17 (0.82 g, 4.97 mmol) in dry DMF
5 mL) and CH Cl (50 mL), the residue was purified by silica chroma-
tography with ethyl acetate/hexane (from 1/1 to 2/1, v/v) to afford (4S)-
According to general procedure B, from (4S)-14 (1.00 g, 3.67 mmol),
TEA (1.10 g, 10.87 mmol), and triphosgene (0.65 g, 2.19 mmol), (4S)-
(
2
2
1
1
1
1
was afforded as thick yellowish needles (0.78 g, 84%). H NMR: d=
1
1
9 as a white foam (1.36 g, 85%). H NMR: d=1.46 and 1.48 (2ꢁs, 9H;
H-Boc), 2.03–2.14 (m, 1H; CH ), 2.48–2.61 (m, 1H; CH ), 3.60–3.86 (m,
H; CH , CH ), 4.33–4.48 (m, 1H; CH), 4.87–4.95 (m, 1H; CH), 7.13–
.44 and 1.50 (2ꢁs, 9H; H-Boc), 2.34–2.43 (m, 1H; CH
H; CH ), 3.64–3.76 (m, 1H; CH ), 3.80 (s, 3H; CH ), 3.83–3.96 (m, 1H;
), 4.11–4.19 (m, 1H; CH), 4.33–4.40 (m, 1/2H; CH), 4.43–4.50 ppm
2
), 2.56–2.69 (m,
2
2
2
2
3
5
2
3
CH
2
+
7.20 (m, 1H; NH), 7.48–7.91 (m, 5H; NH, Ph-H), 8.42–8.86 ppm (m, 1H;
CHO).
(
19 2 4
m, 1/2H; CH); HRMS (ESI): m/z calcd for C12H N O [M+H] :
2
55.1345; found: 255.1343.
Synthesis of (4R)-19
According to general procedure C, from (4R)-12 (1.00 g, 4.09 mmol),
HOBt (0.68 g, 5.03 mmol), DIPEA (1.93 g, 14.93 mmol), PyBOP (2.60 g,
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Guichao Kuang, Afang Zhang et al.
5
.00 mmol), and 17 (0.82 g, 4.97 mmol) in dry DMF (5 mL) and CH
2
Cl
2
Synthesis of (4R)-21
solution of (4R)-24 (see the Supporting Information, 0.56 g,
(
50 mL), the residue was roughly purified by silica chromatography with
A
ethyl acetate/hexane (from 1/1 to 2/1, v/v) and used directly for subse-
quent syntheses.
3
.01 mmol) and DIPEA (1.30 g, 10.06 mmol) in dry CH Cl (40 mL) was
added dropwise to a solution of 20 (1.20 g, 3.62 mmol) in dry DMF
10 mL) at room temperature. After the reaction mixture had been
stirred overnight, it was washed successively with a saturated aqueous so-
lution of NH Cl and brine. The organic phase was dried with MgSO . Pu-
rification by column chromatography with CH Cl /MeOH (20/1, v/v) as
the eluent afforded (4R)-21 as colorless crystals (0.87 g, 87%). H NMR
[D ]DMSO): d=2.02–2.16 (m, 4H; CH , CH ), 2.26–2.39 (m, 1H; CH ),
.38–3.49 (m, 2H; CH ), 3.64 (s, 3H; CH ), 4.42–4.61 (m, 2H; CH), 7.62–
2
2
(
Synthesis of (4S)-6
According to general procedure B, from (4S)-18 (0.60 g, 1.53 mmol), tri-
4
4
phosgene (0.39 g, 1.31 mmol), and TEA (1.00 g, 9.88 mmol) in CH
60 mL) under an N atmosphere, (4S)-6 was afforded as thick yellowish
needles (0.47 g, 82%). H NMR: d=1.46 and 1.50 (2ꢁs, 9H; H-Boc),
2 2
Cl
2
2
(
2
1
1
(
6
2
3
2
2
2
1
8
.42–2.64 (m, 2H; CH
2 3
), 3.69 and 3.70 (2ꢁs, 3H; CH ), 3.71–3.86 (m,
3
7
1
2
3
H; CH ), 4.49 (dd, J=1.4, 8.5 Hz, 1/2H; CH), 4.62 (dd, J=1.2, 9.3 Hz,
2
.68 (m, 2H; Ph-H), 7.76–7.83 (m, 2H; Ph-H), 8.41–8.53 (m, 1H; CHO),
0.18 ppm (s, 1H; NH).
/2H; CH), 5.52–5.62 (m, 1H; CH), 7.46 (d, J=8.3 Hz, 2H; Ph-H),
.04 ppm (d, J=8.1 Hz, 2H; Ph-H).
Synthesis of (4S)-8
Synthesis of (4R)-6
According to general procedure B, from (4S)-21 (0.42 g, 1.26 mmol),
According to general procedure B, from (4R)-18 (0.60 g, 1.53 mmol),
TEA (0.78 g, 7.71 mmol), and triphosgene (0.31 g, 1.04 mmol) in CH Cl
2
2
TEA (1.00 g, 9.88 mmol), and triphosgene (0.35 g, 1.18 mmol) in CH
2
Cl
2
(50 mL) under an N atmosphere, (4S)-8 was afforded as thick yellowish
2
1
under an N atmosphere, (4R)-6 was afforded as thick yellowish needles
2
needles (0.33 g, 83%). H NMR: d=2.03–2.11 (m, 4H; CH , CH ), 2.51–
3
2
1
(
(
4
0.48 g, 84%). H NMR: d=1.45 and 1.48 (2ꢁs, 9H; H-Boc), 2.30–2.40
m, 1H; CH ), 2.49–2.60 (m, 1H; CH ), 3.66–3.90 (m, 5H; CH , CH ),
.40–4.57 (m, 1H; CH), 5.52–5.57 (m, 1H; CH), 7.48 (d, J=8.2 Hz, 2H;
2 3 2
2.59 (m, 1H; CH ), 3.70–3.87 (m, 5H; CH , CH ), 4.50–4.59 (m, 1H;
CH), 4.98–5.04 (m, 1H; CH), 7.46–7.51 (m, 2H; Ph-H), 7.89–7.94 (m,
2H; Ph-H), 8.03–8.16 ppm (m, 1H; NH).
2
2
2
3
Ph-H), 8.07 ppm (d, J=8.3 Hz, 2H; Ph-H).
Synthesis of (4R)-8
Synthesis of (4S)-7
According to general procedure B, from (4R)-21 (0.40 g, 1.20 mmol),
According to general procedure B, from (4S)-19 (0.60 g, 1.53 mmol),
TEA (1.00 g, 9.88 mmol), and triphosgene (0.39 g, 1.31 mmol) in CH Cl
under an N atmosphere, (4S)-7 was afforded thick yellowish needles
TEA (0.75 g, 7.41 mmol), and triphosgene (0.27 g, 0.91 mmol) in CH
2 2
Cl
(50 mL) under an N atmosphere, (4R)-8 was afforded as thick yellowish
2
2
2
needles (0.30 g, 79%). This monomer was used directly for polymeri-
zation.
2
1
(
(
4
0.45 g, 79%). H NMR: d=1.46 and 1.48 (2ꢁs, 9H; H-Boc), 2.02–2.12
m, 1H; CH ), 2.49–2.61 (m, 1H; CH ), 3.60–3.86 (m, 5H; CH , CH ),
.33–4.48 (m, 1H; CH), 4.87–4.95 (m, 1H; CH), 7.48 (d, J=8.3 Hz, 2H;
Ph-H), 7.77–8.04 ppm (m, 3H; Ph-H, NH); HRMS (ESI): m/z calcd for
2
2
2
3
Synthesis of Poly-(4S)-1
According to general procedure E, NiCl
MeOH, 3.91ꢁ10 mmol) was added to (4S)-1 (0.10 g, 0.39 mmol) in
2
·6H
2
O (9.30 mg, 10 wt% in
+
À3
19 24 3 5
C H N O [M+H] : 374.1716; found: 374.1710.
CH
perature for 24 h. Purification by column chromatography with MeOH/
CH Cl (from 1/20 to 1/10,v/v) as the eluent afforded poly-(4S)-1 as
a brown solid (82 mg, 82%). H NMR: d=1.05–2.09 (br, 11H; H-Boc,
CH ), 3.33–4.04 (br, 5H; CH , CH ), 4.04–5.50 ppm (br, 2H; CH, CH).
2 2 2
Cl (0.6 mL) or THF (0.4 mL) under an N atmosphere at room tem-
Synthesis of (4R)-7
According to general procedure B, from (4R)-19 (0.60 g, 1.53 mmol),
TEA (1.00 g, 9.88 mmol), and triphosgene (0.39 g, 1.31 mmol) in CH
2
2
1
2 2
Cl
under an N atmosphere, (4R)-7 was afforded as thick yellowish needles
2
2
3
2
1
(
(
4
0.49 g, 86%). H NMR: d=1.43 and 1.47 (2ꢁs, 9H; H-Boc), 2.26–2.49
m, 2H; CH ), 3.35–3.96 (m, 5H; CH , CH ), 4.29–4.52 (m, 1H; CH),
.66–4.81 (m, 1H; CH), 6.40–6.85 (m, 1H; CH), 7.46 (d, J=8.3 Hz, 2H;
Ph-H), 7.80–7.91 ppm (m, 2H; Ph-H); HRMS (ESI): m/z calcd for
Synthesis of Poly-(4R)-1
2
2
3
According to general procedure E, from monomer (4R)-1 (0.10 g,
0.39 mmol) and NiCl ·6H O (9.30 mg, 10 wt% in MeOH, 3.91ꢁ
2
2
+
À3
C
19
H
24
N
3
O
5
[M+H] : 374.1716; found: 374.1720.
10 mmol) in CH Cl (0.6 mL), poly-(4R)-1 was afforded as a brown
2 2
1
solid (70 mg, 70%). H NMR: d=1.15–1.65 (br, 9H; H-Boc), 1.75–2.70
br, 2H; CH ), 2.81–4.06 (br, 5H; CH , CH ), 4.07–4.85 ppm (br, 2H;
CH).
Synthesis of 20
(
2
3
2
Acid 17 (2.00 g, 12.11 mmol) and pentafluorophenol (2.60 g, 14.13 mmol)
were dissolved in DMF (100 mL) and stirred for 10 min before EDC
Synthesis of Poly-(4S)-2
(
2.78 g, 14.50 mmol) was then added. After stirring for 18 h and evapora-
tion of the solvents in vacuo, the reaction mixture was dissolved in ethyl
ethanoate and washed with a saturated aqueous solution of NH Cl. After
the organic phase had been dried over MgSO , evaporation of the sol-
vents in vacuum yielded 20 as a white solid (2.81 g, 70%). H NMR
[D ]DMSO): d=7.84 and 7.86 (2ꢁs, 2H; Ph-H), 8.15 and 8.16 (2ꢁs,
H; Ph-H), 8.38–8.43 (m, 1H; CHO), 10.77 ppm (s, 1H; NH).
According to general procedure E, from (4S)-2 (100 mg, 0.28 mmol) and
À3
NiCl
2
·6H
2
O (6.70 mg, 2.82ꢁ10 mmol, 10 wt% in MeOH) in CH
2
Cl
2
4
(0.6 mL), poly-(4S)-2 was afforded as a brown solid (47 mg, 47%).
4
1
1
H NMR: d=0.95–1.65 (br, 9H; H-Boc), 1.65–2.89 (br, 6H; CH
4.17 (br, 7H; CH , CH ), 4.07–5.57 ppm (br, 3H; CH).
2
), 2.89–
(
6
3
2
2
Synthesis of Poly-(4R)-2
Synthesis of (4S)-21
According to general procedure E, from (4R)-2 (100 mg, 0.28 mmol) and
À3
NiCl
2
·6H
2
O (6.70 mg, 2.82ꢁ10 mmol, 10 wt% in MeOH) in CH
2
Cl
2
A solution of (4S)-24 (see the Supporting Information, 0.47 g, 2.52 mmol)
and DIPEA (1.30 g, 10.06 mmol) in dry CH Cl (40 mL) was added drop-
2 2
wise to a solution of 20 (1.00 g, 3.02 mmol) in dry DMF (10 mL) at room
temperature. After the reaction mixture had been stirred overnight, it
(
0.6 mL), poly-(4R)-2 was afforded as a brown solid (64 mg, 64%).
1
H NMR: d=1.07–1.67 (br, 9H; H-Boc), 1.67–2.53 (br, 5H; CH
2
), 2.53–
2
3
.96 (br, 1H; CH
H; CH).
), 3.05–4.17 (br, 7H; CH , CH ), 4.17–5.13 ppm (br,
2 3 2
was washed successively with a saturated aqueous solution of NH
brine. The organic phase was dried with MgSO . Purification by column
chromatography with CH Cl /MeOH (20/1, v/v) as the eluent afforded
4S)-21 as colorless crystals (0.55 g, 65%). H NMR: d=2.07–2.16 (m,
4
Cl and
4
Synthesis of Poly-(4S)-6
2
2
1
(
According to general procedure E, from (4S)-6 (100 mg, 0.26 mmol) and
À3
4
4
7
H; CH
3
, CH
2
), 2.51–2.60 (m, 1H; CH
2
), 3.76–3.88 (m, 5H; CH
3
, CH
2
),
NiCl
2
·6H
2
O
(64 mg, 2.69ꢁ10 mmol, 1 wt% in MeOH) in THF
.54–4.59 (m, 1H; CH), 4.98–5.05 (m, 1H; CH), 7.12–7.21 (m, 1H; NH),
.62–8.01 (m, 5H; NH, Ph-H), 8.41–8.87 ppm (m, 1H; CHO).
(0.2 mL), poly-(4S)-6 was afforded as brown solid (55 mg, 55%).
1
2
H NMR: d=0.94–1.77 (br, 9H; H-Boc), 1.79–2.91 (br, 2H; CH ), 3.00–
Chem. Asian J. 2013, 00, 0 – 0
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Guichao Kuang, Afang Zhang et al.
4
.10 (br, 4H; CH
3
, CH
2
), 4.18–4.72 (br, 1H; CH
2
), 4.72–6.28 (br, 2H;
[3] E. Yashima, K. Maeda, H. Iida, Y. Furusho, K. Nagai, Chem. Rev.
2009, 109, 6102–6211.
CH), 6.28–8.35 ppm (br, 2H; Ph-H).
[
4] a) T. Aoki, T. Kaneko, N. Maruyama, A. Sumi, M. Takahashi, T.
Sato, M. Teraguchi, J. Am. Chem. Soc. 2003, 125, 6346–6347;
b) T. F. A. De Greef, M. M. J. Smulders, M. Wolffs, A. P. H. J. Schen-
ning, R. P. Sijbesma, E. W. Meijer, Chem. Rev. 2009, 109, 5687–
Synthesis of Poly-(4R)-6
According to general procedure E, from (4R)-6 (100 mg, 0.26 mmol) and
À3
NiCl
2
·6H
2
O
(64 mg, 2.69ꢁ10 mmol, 1 wt% in MeOH) in THF
5
2
754; c) J. Cui, A. Liu, H. Cao, X. Wan, Q. Zhou, Chem. Asian J.
010, 5, 1139–1145; d) C. K. W. Jim, J. W. Y. Lam, C. W. T. Leung,
(
0.2 mL), poly-(4R)-6 was afforded as a brown solid (48 mg, 48%).
1
H NMR: d=1.03–1.70 (br, 9H; H-Boc), 1.86–2.91 (br, 2H; CH
.71 (br, 5H; CH , CH ), 4.71–6.30 (br, 2H; CH), 6.28–8.09 ppm (br, 2H;
Ph-H).
2
), 3.05–
A. Qin, F. Mahtab, B. Z. Tang, Macromolecules 2011, 44, 2427–
2437; e) Z. Zhu, J. Cui, J. Zhang, X. Wan, Polym. Chem. 2012, 3,
4
3
2
6
68–678.
5] a) K. Shinohara, T. Aoki, T. Kaneko, E. Oikawa, Polymer 2001, 42,
51–355; b) T. Miyabe, H. Iida, M. Banno, T. Yamaguchi, E. Yashi-
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Deng, W. Yang, J. Mater. Chem. 2010, 20, 781–789.
Synthesis of Poly-(4S)-7
[
3
According to general procedure E, from (4S)-7 (100 mg, 0.26 mmol) and
À3
NiCl
2
·6H
2
O
(77 mg, 3.24ꢁ10 mmol, 1 wt% in MeOH) in THF
(
0.2 mL), poly-(4S)-7 was afforded as a brown solid (62 mg, 62%).
1
[6] a) M. Nakagawa, Y. Ikeuchi, M. Yoshikawa, Polymer 2008, 49,
612–4619; b) M. Hatanaka, Y. Nishioka, M. Yoshikawa, Macromol.
H NMR: d=1.27–1.57 (br, 9H; H-Boc), 1.66–2.96 (br, 2H; CH
.10 (br, 5H; CH , CH ), 4.12–6.06 (br, 2H; CH), 6.08–9.27 ppm (br, 4H;
Ph-H, NH).
2
), 2.99–
4
4
3
2
Chem. Phys. 2011, 212, 1351–1359.
7] a) R. P. Megens, G. Roelfes, Chem. Eur. J. 2011, 17, 8514–8523;
b) H. Iida, S. Iwahana, T. Mizoguchi, E. Yashima, J. Am. Chem. Soc.
[
[
[
Synthesis of Poly-(4R)-7
2
012, 134, 15103–15113; c) D. Zhang, C. Ren, W. Yang, J. Deng,
According to general procedure E, from (4R)-7 (100 mg, 0.26 mmol) and
Macromol. Rapid Commun. 2012, 33, 652–657.
À3
NiCl
2
·6H
2
O
(64 mg, 2.69ꢁ10 mmol, 1 wt% in MeOH) in THF
8] a) A. R. A. Palmans, E. W. Meijer, Angew. Chem. 2007, 119, 9106–
9126; Angew. Chem. Int. Ed. 2007, 46, 8948–8968; b) P. N. Shah, J.
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9] a) H. Nakako, R. Nomura, T. Masuda, Macromolecules 2001, 34,
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J. Kumaki, E. Yashima, J. Am. Chem. Soc. 2006, 128, 708–709.
(
0.2 mL), poly-(4R)-7 was afforded as a brown solid (55 mg, 55%).
1
H NMR: d=0.99–1.78 (br, 9H; H-Boc), 1.92–2.99 (br, 2H; CH
.06 (br, 5H; CH , CH ), 5.06–7.66 (br, 2H; CH), 7.78–9.20 ppm (br, 1H;
Ph-H).
2
), 2.99–
5
3
2
Synthesis of Poly-(4S)-8
According to general procedure E, from (4S)-8 (100 mg, 0.31 mmol) and
À3
NiCl
2
·6H
2
O
(38 mg, 1.60ꢁ10 mmol, 1 wt% in MeOH) in THF
[
10] B. M. Rosen, C. J. Wilson, D. A. Wilson, M. Peterca, M. R. Imam, V.
Percec, Chem. Rev. 2009, 109, 6275–6540.
(
0.2 mL), poly-(4S)-8 was afforded as a brown solid (60 mg, 60%).
1
H NMR (CD
3 3 2
OD): d=1.69–3.07 (br, 5H; CH , CH ), 3.41–4.16 (br, 5H;
[
11] a) A. Zhang, F. Rodrꢂguez-Ropero, D. Zanuy, C. Alemꢃn, E. W.
Meijer, A. D. Schlꢄter, Chem. Eur. J. 2008, 14, 6924–6934; b) T. Ka-
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molecules 2009, 42, 560–567; c) K. Liu, X. Zhang, X. Tao, J. Yan, G.
Kuang, W. Li, A. Zhang, Polym. Chem. 2012, 3, 2708–2711.
CH , CH ), 4.62–6.20 (br, 3H; CH, NH), 6.26–7.29 ppm (br, 2H; Ph-H).
3
2
Synthesis of Poly-(4R)-8
According to general procedure E, from (4R)-8 (100 mg, 0.31 mmol) and
À3
NiCl
2
·6H
2
O
(38 mg, 1.60ꢁ10 mmol, 1 wt% in MeOH) in THF
[
[
[
12] a) F. Millich, Chem. Rev. 1972, 72, 101–113; b) R. J. M. Nolte,
(
0.2 mL), poly-(4R)-8 was afforded as a brown solid (46 mg, 46%).
A. J. M. van Beijnen, W. Drenth, J. Am. Chem. Soc. 1974, 96, 5932–
1
3 3 2
H NMR (CD OD): d=1.73–2.98 (br, 5H; CH , CH ), 3.39–4.24 (br, 5H;
5
933.
13] a) T. Kajitani, K. Okoshi, E. Yashima, Macromolecules 2008, 41,
601–1611; b) G. X. Hu, W. Li, Y. L. Hu, A. Q. Xu, J. T. Yan, L. X.
CH
3
, CH
2
), 4.43–5.92 (br, 3H; CH, NH), 6.62–7.76 ppm (br, 2H; Ph-H).
1
Liu, X. Q. Zhang, A. Zhang, Macromolecules 2013, 46, 1124–1136.
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8
430–8431; b) J. Tabei, M. Shiotsuki, F. Sanda, T. Masuda, Macro-
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Dr. Hongmei Deng from the Instrumental Analysis of Research Center
Shanghai University) is thanked for her assistance with NMR measure-
ments. This work is financially supported by National Natural Science
Foundation of China (nos. 21034004, 21104043, 21204047 and 20974020),
the Science and Technology Commission of Shanghai (no. 10520500300
and 11PJ1404100).
(
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ÝÝ These are not the final page numbers!

www.chemasianj.org
Guichao Kuang, Afang Zhang et al.
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[
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[
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Received: March 5, 2013
Published online: && &&, 0000
Chem. Asian J. 2013, 00, 0 – 0
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ