Synthesis and helical structures of poly(ω-alkynamide)s having chiral side chains: Effect of solvent on their screw-sense inversion

Article abstract of DOI:10.1002/chem.201402628

New ω-alkynamides, (S)-HC≡CCH₂CONHCH₂CH-(CH₃)CH₂CH₃ (1) and (S)-HC≡CCH₂CH₂CONHCH-(CH₃)CH₂CH₂CH₂CH₂CH₃ (2) were synthesized and polymerized with a rhodium catalyst in CHCl3 to obtain cis-stereoregular poly(ω-alkynamide)s (poly(1) and poly(2)). Polarimetric, CD, and IR spectroscopic studies revealed that in solution the polymers adopted predominantly one-handed helical structures stabilized by intramolecular hydrogen bonds between the pendent amide groups. This behavior was similar to that of the corresponding poly(N-alkynylamide) counterparts (poly(3) and poly(4)) reported previously, whereas the helical senses were opposite to each other. The helical structures of the poly(ω-alkynamide)s were stable upon heating similar to those of the poly(N-alkynylamide)s, but the solvent response was completely different. An increase in MeOH content in CHCl₃/MeOH resulted in inversion of the predominant screw-sense for poly(1) and poly(2). Conversely, poly(3) was transformed into a random coil, and poly(4) maintained the predominant screw-sense irrespective of MeOH content. The solvent dependence of predominant screw-sense for poly(1) and poly(2) was reasonably explained by molecular orbital studies using the conductor-like screening model (COSMO).

Full text of DOI:10.1002/chem.201402628

DOI: 10.1002/chem.201402628
Full Paper
&
Helical Structures
Synthesis and Helical Structures of Poly(w-alkynamide)s Having
Chiral Side Chains: Effect of Solvent on Their Screw-Sense
Inversion
[
a]
[a]
[b]
[c]
[d]
Yuji Suzuki, Yu Miyagi, Masashi Shiotsuki, Yoshihito Inai, Toshio Masuda, and
[e]
Fumio Sanda*
Abstract: New w-alkynamides, (S)-HCꢀCCH CONHCH CH-
tures of the poly(w-alkynamide)s were stable upon heating
similar to those of the poly(N-alkynylamide)s, but the solvent
response was completely different. An increase in MeOH
2
2
(
CH )CH CH
(1)
and
(S)-HCꢀCCH CH CONHCH-
3
2
3
2
2
(
CH )CH CH CH CH CH (2) were synthesized and polymer-
3
2
2
2
2
3
ized with a rhodium catalyst in CHCl to obtain cis-stereo-
content in CHCl /MeOH resulted in inversion of the predomi-
3
3
regular poly(w-alkynamide)s (poly(1) and poly(2)). Polarimet-
ric, CD, and IR spectroscopic studies revealed that in solution
the polymers adopted predominantly one-handed helical
structures stabilized by intramolecular hydrogen bonds be-
tween the pendent amide groups. This behavior was similar
to that of the corresponding poly(N-alkynylamide) counter-
parts (poly(3) and poly(4)) reported previously, whereas the
helical senses were opposite to each other. The helical struc-
nant screw-sense for poly(1) and poly(2). Conversely, poly(3)
was transformed into a random coil, and poly(4) maintained
the predominant screw-sense irrespective of MeOH content.
The solvent dependence of predominant screw-sense for
poly(1) and poly(2) was reasonably explained by molecular
orbital studies using the conductor-like screening model
(COSMO).
[
1]
Introduction
a controlled screw-sense. Synthetic helical polymers are cate-
gorized into two groups according to the rigidity of the main
chains. The first type has rigid main chains induced by bulky
side chains, and the helical structures are stable, for example,
Naturally occurring biopolymers such as proteins and DNA
commonly have helical conformations, which are essential for
their sophisticated and fundamental functions. Since the dis-
covery of helical structures in these biopolymers, researchers
have endeavored to develop artificial helical polymers with
[2]
polymethacrylates having bulky ester groups, poly(trichloro-
[3]
[4]
acetaldehyde), and polyisocyanides, some of which are ap-
[5]
plicable to stationary phases for chiral HPLC. The second type
[6]
has semi-flexible main chains, including polyisocyanates,
[7]
[8]
polysilanes, and polyacetylenes. Due to the small energetic
barriers for helix reversal, these dynamic helical polymers un-
dergo temperature-dependent changes in their helical pitch
and/or sense.
[
a] Y. Suzuki, Y. Miyagi
Department of Polymer Chemistry, Graduate School of Engineering
Kyoto University, Katsura Campus
Nishikyo-ku, Kyoto 615-8510 (Japan)
[
b] Prof. Dr. M. Shiotsuki
Biopolymers stabilize the helical conformations through the
formation of intramolecular hydrogen bonds. Peptides and
proteins commonly form the right-handed a-helix, in which
every amide NꢁH group forms a hydrogen bond to the C=O of
the amino acid four residues earlier in the sequence (i+4!i hy-
drogen bonding). DNA forms a double helix, whose comple-
mentary helical chains are connected by adenine–thymine and
cytosine–guanine hydrogen bonds. Numerous examples have
been reported regarding peptides homologues (b- and g-pep-
tides and their analogues), synthetic helical oligomers, poly-
mers, and supramolecular polymers that utilize hydrogen
Department of Chemistry and Energy Engineering
Faculty of Engineering, Tokyo City University, 1-28-1 Tamadutsumi
Setagaya-ku, Tokyo 158-8557 (Japan)
[
c] Prof. Dr. Y. Inai
Department of Frontier Materials, Shikumi College
Graduate School of Engineering, Nagoya Institute of Technology
Gokiso-cho, Showa-ku, Nagoya 466-8555 (Japan)
[
d] Prof. Dr. T. Masuda
Research Center for Environmentally Friendly Materials Engineering
Muroran Institute of Technology, 27-1 Mizumoto-cho
Muroran 050-8585 (Japan)
[
e] Prof. Dr. F. Sanda
Department of Chemistry and Materials Engineering
Faculty of Chemistry, Materials and Bioengineering, Kansai University
[9]
bonds to stabilize the secondary structure as well. Polyisocya-
nates bearing oligopeptides in the side chains take a predomi-
nantly one-handed helical structure, which is stabilized by hy-
3
-3-35 Yamate-cho, Suita, Osaka 564-8680 (Japan)
E-mail: sanda@kansai-u.ac.jp
[10]
drogen bonds formed between the side chains.
A poly-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402628.
(phenylacetylene) derivative having hydroxy groups is another
Chem. Eur. J. 2014, 20, 15131 – 15143
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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example of a synthetic polymer that stabilizes the helical struc-
[
11]
ture by means of hydrogen bonding.
We have reported that poly(N-alkynylamide)s such as
[
12]
[13]
poly(N-propargylamide)s
and
poly(N-butynylamide)s,
{
poly[HCꢀC(CH ) NHCOR], m=1 and 2, respectively}, adopt
2
m
helical structures stabilized by intramolecular hydrogen bonds
between the pendent amide groups. In the case of most
poly(N-propargylamide)s, the amide NꢁH group of a monomer
unit forms a hydrogen bond with the C=O group of the amide
two units earlier (i+2!i hydrogen bonding; Figure 1, top),
and the polymers exhibit a unimodal CD signal around
[
12a]
4
00 nm.
In contrast, poly(N-butynylamide)s form intramolec-
ular hydrogen bonds in a different manner. The amide NꢁH
group of a monomer unit forms a hydrogen bond with the C=
O group of the amide three units earlier (i+3!i hydrogen
bonding; Figure 1, bottom), and the polymers show a bisignate
[
13]
CD signal around 300 nm. On the other hand, the secondary
structures of poly(w-alkynamide)s {poly[HCꢀC(CH ) CONHR],
2
m
m= ꢂ1} have not been examined so far, presumably because
the precursor w-alkynoic acids are commercially unavailable.
w-Alkynamides are the isomers of N-alkynylamides, but it is as-
sumed that their helix-formation tendencies are quite different
from each other, because the dipoles of their amide groups
Figure 1. Top: possible helical conformer of a poly(N-propargylamide), [ꢁ
[
14]
are opposite in direction. In fact, the direction and orienta-
tion of amide groups largely affect the formations of supra-
CH=C(CH
2
NHCOH)ꢁ]
n
, which incorporates two helically arranged, intramolec-
ular i+2!i hydrogen-bonding strands (green-dotted lines) formed between
the amide groups. Bottom: possible helical conformer of a poly(N-butynyla-
[
15]
[16]
[17]
molecular assemblies, organogel fibers, polymer crystals,
[
18]
mide), [ꢁCH=C(CH
2
CH
2
NHCOH)ꢁ]
n
, which contains three helically arranged,
and adsorption layers.
It is of interest to elucidate how
intramolecular i+3!i hydrogen-bonding strands. The main chains are col-
poly(w-alkynamide)s induce helical structures utilizing the
amide groups.
ored in yellow, and the methylene hydrogen atoms are omitted for clarity.
Thus far, it has been reported that several helical polymers
undergo solvent-driven screw-sense inversion. As for biological
backbones, polyproline adopts two helical forms, a right-
handed helix I with all-cis peptide bonds and a left-handed he-
act strongly with the polar amide groups, and this solvation
effect possibly causes the helix inversion depending on the rel-
ative solvent polarity. Most of these substituted polyacetylenes
undergo helix inversion upon temperature change as well. Al-
though solvent-driven helix inversion has been reported as de-
scribed above, theoretical study of the phenomena using the
molecular orbital (MO) method remains open as one of the
[
19]
lix II with all-trans peptide bonds.
The preference for the
screw-sense largely depends on the type of solvent. Helical
peptides based on unusual achiral residues also undergo sol-
vent-driven inversion of the helix. Oligopeptides containing de-
Z
Z
[20]
hydroamino acids, Boc-l-Ala-D Phe-Gly-D Phe-l-Ala-OMe
most challenging issues. Helix inversion of proline oligomers in
Z
Z
[21]
[19c]
and Boc-l-Val-D Phe-Gly-D Phe-l-Val-OMe
carbonyl, D Phe=Z-dehydrophenylalanine) show reversible
(Boc=t-butoxy-
solution was simulated through MO computation.
However,
Z
no such theoretical demonstration has been reported regard-
ing helical polyacetylenes to the best of our knowledge.
The present paper deals with the synthesis of new poly(w-al-
kynamide)s, poly(1) and poly(2), and comparison of the helical
nature with that of the corresponding poly(N-alkynylamide)s
(poly(3) and poly(4); Scheme 1) with opposite arrangements of
NꢁH and C=O in the amide group. We also elucidate the sol-
vent-driven helix inversion of the polymers, and discuss the ra-
tionale by comparing the simulated and observed CD spectra.
We explain the relationship between the dipole moment and
preferable helix sense of substituted polyacetylenes in polar
and nonpolar solvents. Namely, poly(1) and poly(2) prefer
forming more-polar right-handed helices in MeOH, whereas
the polymers prefer forming less-polar left-handed helices in
CHCl3.
screw-sense inversion of the 3 -helix by varying the composi-
10
tions of two mixed solvents. Oligopeptides possessing a single
l-residue (X) at the penultimate position, Boc-Aib-X-(Aib-
Z
D Phe) -Aib-OMe (Aib=a-aminoisobutyric acid), adopt a right-
2
handed 3 -helical conformation in CHCl , whereas a left-
10
3
[
22]
handed helix is favored in THF or MeOH. It seems that the
solvent-dependent screw-sense is brought about by a balance
between intramolecular and intermolecular hydrogen bonding.
Z
Optically inactive H-Gly-(D Phe-Aib) -OCH tends to adopt pref-
4
3
erentially a right-handed 3 -helical conformation in the pres-
10
[
23]
ence of Boc-l-Pro-OH
through the noncovalent chiral
[
24]
domino effect. The peptide inverts the induced helix sense
in CHCl /CH CN according to the mole ratios of the two
3
3
solvents.
Some optically active helical polyacetylenes substituted with
amide groups undergo preferential helical sense transforma-
[
25,26]
tion in response to solvent.
Polar solvent molecules inter-
Chem. Eur. J. 2014, 20, 15131 – 15143
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Full Paper
gions of the conjugated main
chain chromophore in CHCl3.
Hence, it is concluded that they
adopt helical structures with pre-
dominantly one-handed screw-
[30]
sense in that solvent. The CD
signals of poly(1)–poly(4) are
quite different from one another
in shape and wavelength, indi-
cating that the helical structures
of the polymers are also differ-
ent. It should be noted that all
the polymers have an (S)-stereo-
genic center at the same atomic
position counted from the poly-
acetylene backbone. Conse-
quently, the helical structures of
the polymers seem to be affect-
ed by the position and direction
of the amide group rather than
Scheme 1. Polymerization of w-alkynamides (1, 2) and N-alkynylamides (3, 4).
Results and Discussion
[
a]
Table 1. Polymerization of 1–4.
Polymerization
Monomer
Polymer
Rhodium catalysts efficiently polymerize monosubstituted ace-
[e]
[g]
[g]
[h]
D
Yield [%]
M
n
M
w
/M
n
[a]
[8]
tylenes to give the corresponding polyacetylenes with a cis-
[
b]
1
2
3
4
54
48
91
43
46300
7000
34000
9100
2.88
3.91
2.06
2.56
ꢁ1171
ꢁ86
[
27]
stereoregular main chain, which is indispensable for helical
[c]
[f]
structure formation. Thus, w-alkynamides 1 and 2 were poly-
+1929
+355
+
6
ꢁ
[d]
merized with [Rh(nbd)] [h -C H B (C H ) ] (nbd=bicyclo-
6
5
6
5 3
[2.2.1]hepta-2,5-diene (norbornadiene)) as a catalyst in CHCl3.
+
6
ꢁ
5
[a] Polymerized with [Rh(nbd)] [h -C H B (C H ) ] in CHCl at 308C for
6
6
5
3
3
Table 1 summarizes the results of the polymerization along
24 h. [M]
Rh]=50. [d] With [RhCl(nbd)]
part. [f] Hexane-insoluble part. [g] Estimated by GPC (CHCl
3
0
=1.0m, [M]
0
/[Rh]=100. [b] [M]
(Et N), [Et N]/[Rh]=2. [e] MeOH-insoluble
, polystyrene
at room temperature,
0 0 0
=0.5m. [c] [M] =0.1m, [M] /
[
2
3
3
with those for the poly(N-alkynylamide) counterparts 3 and
[
12g]
4
.
When monomers 1 and 2 were polymerized at 1m con-
calibration). [h] Measured by polarimetry in CHCl
3
centration, the reaction mixtures gelled to give solvent-insolu-
ꢁ1
c=0.056–0.110 gdL
.
[
28]
ble polymers. When the monomer concentrations were low-
ered to 0.5 and 0.1m, the monomers gave solvent-soluble
polymers. The polymers that had the same spacers between
the main chain and amide group possessed similar molecular
the position of the chiral center. Poly(1) and poly(3) having ꢁ
CH ꢁ between the main chain and the amide group exhibited
2
weights [poly(1) and poly(3): ꢁCH ꢁ; poly(2) and poly(4): ꢁ minus- and plus-signed unimodal Cotton effects around 340
2
(
CH ) ꢁ; the M values of the former two polymers (46300 and
and 390 nm, respectively. On the other hand, poly(2) and
poly(4) having ꢁCH CH ꢁ between the main chain and the
2
2
n
3
9
4000) were higher than those of the latter two (7000 and
2
2
[
29]
1
100). All these polymers showed a H NMR spectroscopic
amide group exhibited first-minus/second-plus and first-plus/
second-minus split-type Cotton effects around 280 nm,
respectively.
signal assignable to the olefinic proton of the cis-polyacetylene
main chain around d=6 ppm, and the integration ratio of the
signals indicated that the cis content was quantitative in every
As described in the Introduction, poly(3) [poly(N-propargyla-
mide)] and poly(4) [poly(N-butynylamide)] form intramolecular
i+2!i and i+3!i hydrogen bonds (Figure 1), respectively.
The CD spectroscopic patterns shown in Figure 2 suggest that
poly(1) adopts a helical conformation with i+2!i hydrogen
bonding in a manner similar to poly(3), and poly(2) adopts
a helical conformation with i+3!i hydrogen bonding in
a manner similar to poly(4). Judging from the opposite signs
of the optical rotations (Table 1) and CD signals (Figure 2), it is
considered that the predominant helical senses of poly(1) and
poly(2) are opposite to those of poly(3) and poly(4),
respectively.
case. The polymers were soluble in CHCl , and exhibited large
3
optical rotations (j[a] j=86–19298) compared to those of the
D
[
12g]
monomers (j[a] j=3.9–6.78),
strongly suggesting the for-
D
mation of helices with predominantly one-handed screw-
sense.
Secondary structure
We then investigated the secondary structures of the polymers
by CD and UV/Vis spectroscopies. As shown in Figure 2,
poly(1) and poly(2) exhibited Cotton effects at absorption re-
Chem. Eur. J. 2014, 20, 15131 – 15143
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[
a]
Table 2. Solution-state IR absorption data for 1–4 and poly(1)–poly(4).
ꢁ1
ꢁ1
Compound
Amide I [cm
]
Amide II [cm ]
1
1670
1631
1662
1634
1667
1634
1665
1631
1530
1549
1519
1542
1512
1541
1513
1548
poly(1)
2
poly(2)
3
poly(3)
4
poly(4)
(ꢁ39)
(ꢁ28)
(ꢁ33)
(ꢁ34)
(+19)
(+23)
(+29)
(+35)
[
a] Measured in CHCl
3
, c=1–50 mm. The wavenumber difference between
the monomer and the corresponding polymer is given in parentheses.
[12c]
single bonds on the main chain are around 1408.
This
loosely twisted helical structure (large pitch/diameter ratio) is
stabilized by intramolecular hydrogen bonds between the
amide groups at the ith and (i+2) th units. On the other hand,
poly(N-butynylamide)s, analogues of poly(2), become the most
stable when the dihedral angles are around 708; this tight heli-
cal structure (small pitch/diameter ratio) is stabilized by intra-
molecular hydrogen bonds between the i th and (i+3) th
[12 g]
units.
In the present study, we attempted the molecular
mechanics calculation of poly(w-alkynamide)s to gain knowl-
edge about the difference of the secondary structure from that
of the corresponding poly(N-alkynylamide)s.
We first constructed poly(N-methyl-3-butynamide) and
poly(N-methyl-4-pentynamide) (18-mers, A and B in Figure 3,
respectively), terminated with hydrogen atoms, as the models
[32]
for poly(1) and poly(2), respectively. The dihedral angles at
the single bonds in the main chain were then varied by the in-
crement of 5 or 108 in a range from 50 to 908, at which the
amide groups at the i th and (i+3) th units form hydrogen
bonds. The process was repeated from 100 to 1508 in a similar
fashion, wherein the amide groups at the i th and (i+2) th
units form hydrogen bonds. The geometries were optimized,
except for the dihedral angles, which were constrained at the
Figure 2. CD and UV/Vis spectra of poly(1)–poly(4) measured in CHCl
08C, c=0.15–0.74 mm.
3
at
2
Confirmation of hydrogen bonding
As described above, the nature of the intramolecular hydrogen
bonding seems to affect the secondary structure of the poly-
mers. We measured the solution-state IR spectra of the mono-
[33]
main chain using the MMFF94 force field.
As plotted in
Figure 3, poly(N-methyl-3-butynamide) (A) and poly(N-methyl-
4-pentynamide) (B) become the most stable when the dihedral
angles are fixed at 125 and 708, respectively. The former ini-
tiates intramolecular hydrogen bonds between the amide
groups at i th and (i+2) th units, whereas in the latter the hy-
drogen bonds occur at i th and (i+3) th units. Judging from
these results and previous reports, the arrangement of NꢁH
and C=O (ꢁNHCOꢁ or ꢁCONHꢁ) does not affect the suitable
combination (i and i+2 vs. i and i+3) of the two amide units
for forming hydrogen bonds. The Cotton effect of poly(1) was
observed at 340 nm, which is 50 nm shorter than the wave-
length for poly(3), as shown in Figure 2. It is considered that
the difference is caused by the more tightly twisted main
chain (smaller dihedral angle at the single bonds) of poly(1)
mers and corresponding polymers in CHCl at c=1–50 mm to
3
check for the presence of hydrogen bonds. As listed in Table 2,
the amide I absorption (C=O stretching) peaks of monomers
ꢁ
1
1
–4 were observed at 1662–1670 cm , whereas those of
ꢁ
1
poly(1)–poly(4) were observed at 28–39 cm lower positions,
irrespective of the concentrations. In addition, amide II absorp-
tion (NꢁH bending) peaks of 1–4 were observed at 1512–
ꢁ
1
1
1
530 cm , whereas those of poly(1)–poly(4) were observed at
ꢁ
1
[31]
9–35 cm higher positions.
These results clearly indicate
that the polymers form stabilizing intramolecular hydrogen
bonds between the amide groups in CHCl3.
Conformational analysis
[
12c]
(
f=1258) compared with poly(3) (f=1408),
leading to the
[34]
We have previously analyzed the conformation of poly(N-prop-
argylamide)s, analogues of poly(1), by molecular mechanics
and molecular orbital calculations to prove that they become
the most energetically stable when the dihedral angles of the
shorter conjugation of the polyacetylene backbone.
Chem. Eur. J. 2014, 20, 15131 – 15143
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Vis absorption at 390 nm, corresponding to the regularly twist-
ed conjugated polyacetylene backbone, whereas the absorp-
tion at 320 nm of the predominantly unregulated structure in-
[12a]
creased.
On the contrary, poly(1) maintained the helical
polyacetylene-based UV/Vis absorption at 340 nm upon MeOH
addition. These results indicate that, upon MeOH addition,
poly(3) transformed from a helix into a random coil, whereas
poly(1) remained as a helix, but with less predominant screw-
[35]
sense. It is concluded that the populations of left- and right-
handed helices of poly(1) become equal when the MeOH con-
tent is 60%. Recently, unique examples of the reversible chi-
roptical switching phenomenon of poly(carbodiimide)s having
polyarene groups were reported, in which it was suggested
that changes in the helical pitches and directions of the transi-
tion dipole moments of polyarenes cause the phenomenon
[36]
rather than helix reversal. On the contrary, the chiroptical
switching of the present poly(w-alkynamide)s is definitely
caused by inversion of the helical sense, because the CD sig-
nals at 300–400 nm cannot arise mainly from the side chains,
but rather from the conjugated polyacetylene backbones.
Further, poly(2) and poly(4) exhibited behavior quite differ-
ent from that of poly(1) and poly(3) upon MeOH addition. The
CD intensity for poly(4) barely decreased, whereas the bisig-
nate CD signal of poly(2) varied from first-minus/second-plus
to first-plus/second-minus upon increasing MeOH content.
Both polymers maintained the helix-based UV/Vis absorption
peak irrespective of MeOH content. Poly(4) kept the helical
conformation with predominantly one-handed screw-sense,
whereas poly(2) reversed the screw-sense upon MeOH addi-
tion, keeping the total helix content the same as that of the in-
itial stage. The arrangement of NꢁH and C=O dramatically af-
fected the responses of the secondary structures of the poly-
mers to MeOH.
Figure 3. Relationships between the energy and dihedral angle f at the
single bond in the main chain of poly(N-methyl-3-butynamide) (18-mer, A)
and poly(N-methyl-4-pentynamide) (18-mer, B) calculated by MMFF94. The
conformers with f of 50–908 form i+3!i hydrogen bonding (HB), and
those with f of 100–1508 form i+2!i hydrogen bonding between the
amide groups.
Stability of the helical structures
Helical conformations of poly(1) in solution
The helical structures of poly(N-propargylamide)s and poly(N-
butynylamide)s are affected by external stimuli such as heating
We attempted to gain a theoretical understanding of the
effect of the solvent on helicity. The left- and right-handed
helices of poly(1) were energy-minimized by PM6 in
[
12,13]
and addition of polar solvents.
To examine the effects of
amide position and arrangement on the thermal stability of
the helical structures of the present polymers, we measured
[37,38]
MOPAC2009
in vacuum, CHCl , and MeOH. In the “conduc-
3
[37,39]
the CD and UV/Vis spectra in CHCl at various temperatures
tor-like screening model” (COSMO)
calculation, “RSOLV” for
3
(
Figure 4) and in CHCl /MeOH with various compositions at
the effective radius of solvent molecule was commonly set to
default, whereas the dielectric constant (“EPS”) was specified
3
2
08C (Figure 5). As depicted in Figure 4, the CD intensity for all
[37,39,40]
the polymers gradually decreased as the temperature was in-
creased, but they still exhibited an intense Cotton effect even
at 558C. By comparing the thermal responses between poly(1)
and poly(3), and also between poly(2) and poly(4) (Figure 4), it
is concluded that the arrangement of NꢁH and C=O in the
amide groups of the polymers does not affect the thermal sta-
bility of the helical structure so much. In contrast, the arrange-
ment led to the significantly different response to MeOH addi-
for each solvent.
focus on the influence of solvent polarity on helical stability.
Thus the present simulation should
[41]
Figure 6 and Table 3 summarize the structures obtained in
each medium and their energies, respectively. In these six heli-
ces, there are two types of the main-chain dihedral angles for
each monomer unit: f for C=CꢁC=C and t for CꢁC=CꢁC. In
the left- and right-handed helices, the values of f and t in the
[42]
three media are similar to each other. Thus the left- and
right-handed helical structures adopted in vacuum are essen-
tion to CHCl solutions of the polymers as shown in Figure 5.
3
For the predominantly one-handed helical structure, the
Cotton effect intensities of both poly(1) and poly(3) decreased
as the concentration of MeOH increased; the Cotton effect
almost disappeared when the MeOH content reached 60 and
tially maintained in CHCl and MeOH. Table 3 indicates that
3
poly(1) prefers a left-handed helix in vacuum, whereas the
population of a right-handed helix increases in the order of
vacuum<CHCl <MeOH. It is likely that the reason for this is
3
20%, respectively. For poly(3), there was a decrease in the UV/
the larger dipole moment of a right-handed helix, which is
Chem. Eur. J. 2014, 20, 15131 – 15143
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Therefore, the conformational
analysis and CD simulation ex-
plain the experimental tendency
that a left-handed helix is ener-
getically favorable in less-polar
solvents (e.g., CHCl ), but that
3
more-polar media permit a right-
handed helix. Such solvent-in-
duced helical propensity may at
least partially originate from the
dipole moment of the entire
molecule, as listed in Table 3. In
every medium, a right-handed
helix possesses
a more-polar
structure than the corresponding
left-handed helix. This implies
that the polar helical structure is
more preferentially stabilized by
solvation with polar molecules.
Helical conformations of
[43]
poly(2) in solution
The effect of solvent on poly(2)
was also theoretically simulated.
[41]
Figure 8 displays helical struc-
tures of poly(2) (12-mer) con-
verged in vacuum, CHCl , and
3
MeOH. In the left-handed helices
(
Figure 8a–c),
f
angles in
vacuum seem to be somewhat
different from those in CHCl
3
[44]
and MeOH.
However, on the
whole, the left- and right-
handed helical structures in
3
Figure 4. CD and UV/Vis spectra of poly(1)–poly(4) measured in CHCl at various temperatures (c=0.15–0.74 mm).
more stable in a more-polar medium. Here, each dipole
moment is placed on a line essentially parallel to the respec-
Table 3. Energies and dipole moments of left- and right-handed helical
[a]
poly(1) (18-mer) in vacuum, CHCl
3
, and MeOH.
[41]
tive helical axis (see the Supporting Information, Figure S2 ).
Figure 7 illustrates the CD spectra and UV/Vis absorption
profiles simulated for the helical structures of poly(1) (18-mer)
Medium
vacuum
Left-handed helix
Right-handed helix
energy
kJmol
dipole
moment [D]
energy
[kJmol
dipole
moment [D]
ꢁ1
ꢁ1
[
]
]
shown in Figure 6. Each oscillator strength (f ) and rotatory
vel
ꢁ4912.07
65.3
75.9
77.8
ꢁ4860.47
ꢁ5121.47
ꢁ5224.40
78.1
93.6
strength (R ) in velocity form is sumperimposed. The three
vel
(
ꢁ51.60)
left-handed helices (Figure 6a–c) yielded markedly negative CD
signals for a strong absorption band around 350–360 nm. On
the other hand, the right-handed helices (Figure 6d–f) showed
the opposite-signed CD signals at shorter wavelengths (ca.
CHCl
3
ꢁ5106.48
(+14.99)
ꢁ5186.52
[
b]
MeOH
100.2
(
+37.88)
[
37–39]
300 nm). Comparison between the simulated (Figure 7) and ex-
[a] The geometries were optimized by MOPAC2009.
The energy dif-
ꢁ
1
ference between the left- and right-handed helices [kJmol ] in each
medium is given in parentheses. [b] The MOPAC output
perimental (Figure 5) CD spectra can confirm the CD assign-
[
37–39]
says that its
ment of the helical screw-sense; that is, poly(1) preferentially
structure is “not a stationary point”, but is “essentially stationary” for
energy.
adopts a left-handed helix in CHCl . Furthermore, the decrease
3
of the CD signals with increasing MeOH content and/or the ap-
pearance of weakly positive signals suggest that other confor-
mational species yielding the opposite CD signs (mainly right-
handed helices) are significantly populated in more polar
media.
vacuum are essentially maintained in solution. A right-handed
helix is more stable than the corresponding left-handed helix
in each medium, as seen in Table 4, which summarizes the en-
Chem. Eur. J. 2014, 20, 15131 – 15143
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well with the experimental CD
pattern in CHCl . Thus, it is obvi-
3
ous that poly(2) adopts preferen-
tially a left-handed helix in that
solvent. In contrast, the three
right-handed helices (Figure 9d–
f) indicate positive CD signals for
the major absorption band
around 270–280 nm. It is reason-
able that the inversion of a helix
sense gives rise to opposite chi-
roptical signs. The experimental
CD spectra induce opposite
(
positive) CD signals at longer
wavelengths with increasing
[45–47]
MeOH content.
Therefore,
the energy and spectral simula-
tions strongly suggest that the
left-handed helix favored in
CHCl3 is converted to a right-
handed helix through addition
[48]
of MeOH.
The CD spectra of poly(Et-2)
(
12-mer and 18-mer), in which
all n-pentyl side chains of poly(2)
were replaced with ethyl groups,
were also simulated in vacuum,
[49]
CHCl , and MeOH. It was con-
3
firmed that the energies and CD
spectra of poly(Et-2) show trends
similar to those of poly(2). The
DH
values of poly(Et-2) in-
LꢁR
crease in the order of vacuum<
CHCl <MeOH, indicating that
3
a right-handed helix is more sta-
Figure 5. CD and UV/Vis spectra of poly(1)–poly(4) measured in CHCl
c=0.15–0.74 mm).
3
/MeOH with various compositions at 208C
[50]
bilized with solvent polarity.
(
ergies and dipole moments. However, the relative stability of
a right-handed helix depends on the medium. That is, whereas
the energy difference in left- and right-handed helices
Table 4. Energies and dipole moments of left- and right-handed helical
[
a]
poly(2) (12-mer) in vacuum, CHCl
3
, and MeOH.
Medium
vacuum
Left-handed helix
Right-handed helix
(
DH =DH ꢁDH ) is relatively small in vacuum (+
LꢁR
L
R
energy
[kJmol
dipole
moment [D]
energy
[kJmol
dipole
moment [D]
ꢁ
1
ꢁ1
ꢁ1
2
.8 kJmol ), it increases significantly in CHCl3 (ca.
]
]
ꢁ
1
ꢁ1
+
13 kJmol ) and even more in MeOH (ca. +16 kJmol ). As
ꢁ4131.24
(+2.80)
29.9
37.0
39.9
ꢁ4134.04
ꢁ4351.95
ꢁ4438.61
33.1
41.4
45.8
a result, the preference for a right-handed helix tends to in-
crease with solvent polarity. This prediction also implies that
an increase in solvent polarity for poly(2) promotes the inter-
conversion from a left-handed helix to a right-handed helix.
Similarly to the case of poly(1), each dipole moment places on
a line essentially parallel to the respective helical axis (see the
CHCl
3
ꢁ4338.56
(
+13.39)
MeOH
ꢁ4422.23
+16.38)
(
[
37–39]
[
a] The geometries were optimized by MOPAC2009.
The energy dif-
ꢁ1
ference between the left- and right-handed helices [kJmol ] in each
medium is given in parentheses.
[
41]
Supporting Information, Figure S4 ).
Figure 9 shows the theoretical CD spectra and absorption
profiles for these six helices of poly(2) (12-mer). The three left-
handed helices (Figure 9a–c) show an intense absorption band
with large fvel values around 270–280 nm. The corresponding
CD spectra give largely negative signals. This CD profile agrees
Chem. Eur. J. 2014, 20, 15131 – 15143
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[
37–39]
Figure 6. Left-handed helices (a–c) and right-handed helices (d–f) of poly(1) (18-mer), the geometries of which were optimized by MOPAC2009
vacuum (a, d), CHCl (b, e), and MeOH (c, f).
in
3
Herein, specific water molecule(s) are hydrogen-bonded to ter-
minal free-amide groups and internal hydrogen-bonding
amide groups in the peptide helix.
We have applied such a specific solvation model to our pres-
ent system in MeOH. In poly(1) and poly(2), MeOH molecules
were placed near all amide CO and terminal free-NH groups:
The number of MeOH molecules added is 20 for poly(1) (18-
mer) and 15 for poly(2) (12-mer). The complex of polymer and
MeOH molecules was energy-minimized in vacuum and in
COSMO-based
MeOH
media
by
using
PM6/
[38,39,52,53]
MOPAC2012.
The geometry-optimized structures of
[
41]
poly(1) and poly(2) are displayed in Figures 10 and 11, re-
spectively. In all cases, helical structures are essentially main-
tained, and MeOH molecules are located near amide CO and
free NH groups, as a result, being helically arranged along
each polymer chain. Thus, the helical structures supported by
internal hydrogen bonds should not be disturbed by specific
solvation by hydrogen-bonding MeOH molecules.
Figure 7. Simulated CD spectra and UV/Vis absorption profiles of poly(1)
18-mer) in left-handed helices (a–c) and right-handed helices (d–f) that
were energy-minimized in vacuum (a, d), CHCl (b, e), and MeOH (c, f). The
(
3
corresponding structures are given in Figure 6.
The energies and dipole moments of these polymer-MeOH
complexes are listed in Table 5. Poly(1)-MeOH complex in
vacuum prefers a left-handed helix, unlike experimental obser-
vation in MeOH. However, the complex slightly stabilizes
a right-handed helix in MeOH (COSMO) media. On the other
hand, complex of poly(2) with MeOH molecules in vacuum
prefers a right-handed helix as experimentally observed in
MeOH. The energetic advantage of a right-handed helicity is
promoted in the MeOH (COSMO) media. Poly(2)-MeOH com-
plex in both vacuum and MeOH media yields a somewhat
Effect of individual MeOH molecules
In the COSMO method, specific solvent interaction at molecu-
[
39,48]
lar level is not involved.
A question should be raised: How
does specific solvation such as the hydrogen-bonding interac-
tion affect the present COSMO-based results? Recently, solva-
tion of a helical peptide with specific water molecules has
[51]
been elegantly simulated in vacuum and solvent media.
Chem. Eur. J. 2014, 20, 15131 – 15143
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Figure 8. Left-handed helices (a–c) and right-handed helices (d–f) of poly(2) (12-mer) energy-minimized in vacuum (a, d), CHCl
3
(b, e), and MeOH (c, f).
Figure 11. Left-handed helices (a, b) and right-handed helices (c, d) of
poly(2) (12-mer) containing 15MeOH molecules. These geometries were op-
[
38,39,52, 53]
timized by MOPAC2012
in vacuum (a, c) and MeOH (b, d).
Figure 9. Simulated CD spectra and UV/Vis absorption profiles of poly(2)
12-mer) in left-handed helices (a–c) and right-handed helices (d–f) that
were energy-minimized in vacuum (a, d), CHCl (b, e), and MeOH (c, f). The
corresponding structures are given in Figure 8.
(
smaller dipole moment for more stable right-handed helix. In
3
[39,52]
contrast, the COSMO area
of the right-handed helix-MeOH
complex is increased by about 1.2-fold, compared with the cor-
responding left-handed one. Thus, the preference for a right-
handed helix in polar MeOH might arise not only from the
total dipole moment of the system, but also from the surface
[39,52,54]
area.
The experimental preference for the helicity of poly(1) and
poly(2) seems not to be well-reproduced only by specific solva-
tion of MeOH molecules in vacuum. Simulation for specific sol-
vation of such a polymeric system should be considerably
complicated, because it depends largely on various conditions
to be assumed about the number and position of MeOH mole-
cules inserted. Rather, the preference for a right-handed helici-
ty of poly(1) and poly(2) in MeOH has been demonstrated by
the COSMO approximation, whether specific MeOH solvation is
[39,52]
considered or not. Therefore, the COSMO method
should
be simple and effective for solvent effect on the helicity of the
present polymers.
Conclusion
Figure 10. Left-handed helices (a, b) and right-handed helices (c, d) of
poly(1) (18-mer) containing 20MeOH molecules. These geometries were op-
In the present study, we have demonstrated the synthesis and
[
38,39,52, 53]
timized by MOPAC2012
in vacuum (a, c) and MeOH (b, d).
polymerization of new w-alkynamides (1 and 2) having chiral
Chem. Eur. J. 2014, 20, 15131 – 15143
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yacetylenes based on analysis by the COSMO method. We be-
lieve that the present study will serve as a basis for further
considerations of solvent effects on the secondary structures
of synthetic polymers by the molecular orbital method.
Table 5. Energies and dipole moments of poly(1) (18-mer) and poly(2)
[
a]
(12-mer) containing individual MeOH molecules in vacuum and MeOH.
Polymer-MeOH/
medium
Energy
[kJmol
Dipole
moment [D] [kJmol
Energy
Dipole
moment [D]
ꢁ1
ꢁ1
]
]
poly(1)-20MeOH
molecules
vacuum
Left-handed helix
Right-handed helix
Experimental Section
ꢁ9509.47
ꢁ15.83)
ꢁ9876.87
+1.99)
55.8
73.2
ꢁ9493.64
72.8
(
Measurements
MeOH
ꢁ9878.86 114.9
(
Melting points (m.p.) were measured with a Yanako micromelting
point apparatus. Specific rotations ([a] ) were measured with
D
poly(2)-15MeOH
molecules
vacuum
Left-handed helix
Right-handed helix
a JASCO DIP-1000 digital polarimeter. IR spectra were obtained
1
with a JASCO FTIR-4100 spectrophotometer. NMR ( H: 400 MHz;
ꢁ7648.85
+18.92)
ꢁ7909.36
+33.48)
24.1
40.0
ꢁ7667.77
ꢁ7942.84
23.6
35.0
13
C: 100 MHz) spectra were recorded on a JEOL EX-400 spectrome-
(
ter. Elemental analyses were conducted at the Microanalytical
Center of Kyoto University. Number-average molecular weights
MeOH
(
(
M ) and molecular weight distributions (M /M ) of polymers were
n
w
n
[
38, 39,52,53]
[a] The geometries were optimized by MOPAC2012.
The values of
estimated by SEC (Shodex columns K803, K804, K805) eluted with
energy and the dipole moment are given for the corresponding complex
of polymer and MeOH molecules. The energy difference between the
left- and right-handed helices [kJmol ] in each medium is given in pa-
rentheses.
CHCl calibrated by polystyrene standards. CD and UV/Vis spectra
3
were recorded on a JASCO J-820 spectropolarimeter.
ꢁ1
Materials
3
-Butyn-1-ol (Wako), 4-pentyn-1-ol (Aldrich), (S)-(ꢁ)-2-methylbutyla-
substituents to obtain cis-stereoregular poly(w-alkynamide)s
mine (Aldrich), (S)-(+)-2-heptylamine (Aldrich), and 4-(4,6-dime-
(
poly(1) and poly(2)) with moderate molecular weights. In
thoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (Tokuyama)
[57]
CHCl solution, the polymers adopted helical structures with
were used as received. Pyridinium dichromate (PDC)
and
3
+
6
ꢁ
[58]
[
Rh(nbd)] [h -C H B (C H ) ] were prepared according to the lit-
predominantly one-handed screw-sense stabilized by intramo-
lecular hydrogen bonds between the pendent amide groups.
Compared with the corresponding poly(N-alkynylamide)s
6
5
6
5 3
erature. CHCl used for polymerization was distilled prior to use.
3
(poly(3) and poly(4)), inversion of the arrangement of NH and
Synthesis of monomers 1–4
CO in the amide groups led to inversion of the predominant
helical sense. Conformational analysis revealed that the combi-
nation of hydrogen-bonded amide pairs (i th and (i+2) th or
i th and (i+3) th units) was the same as that of the corre-
sponding poly(N-alkynylamide)s. It was confirmed that the pre-
dominant screw-sense was not simply ruled by the stereo-
chemistry of the chiral center of the side chain; that is, reversal
of the direction of the amide group (ꢁCONHꢁ or ꢁNHCOꢁ) re-
sulted in the opposite helical sense (poly(1) vs. poly(3), and
poly(2) vs. poly(4)). The helical structures of the poly(w-alkyna-
mide)s did not change significantly upon heating, as is the
case with the poly(N-alkynylamide)s, whereas the response of
[12d]
[13b]
Monomers 3
and 4
were synthesized according to the litera-
ture. Monomer 1 was synthesized as follows: A solution of 3-
butyn-1-ol (1.0 g, 14.3 mmol) in acetone (14 mL) was added drop-
wise over a 1 h period to a solution of concentrated H SO
(19.2 mL, 106 mmol) and CrO
72 mL) at 08C. After stirring at 08C for 3.5 h, the reaction mixture
was extracted with Et O (100 mL) six times. The organic layers
were combined and washed with 2m HCl, dried over anhydrous
MgSO , and concentrated on a rotary evaporator to afford 3-buty-
2
4
(2.78 g, 27.8 mmol) in distilled water
3
(
2
4
noic acid in 48% yield. (S)-(ꢁ)-2-Methylbutylamine (583 mg,
6
.7 mmol) and 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpho-
linium chloride (2.18 g, 6.7 mmol) were added to a THF solution of
the 3-butynoic acid (563 mg, 6.7 mmol) obtained above at room
temperature. The resulting solution was stirred at room tempera-
ture for 24 h. The white precipitate that formed in the reaction was
filtered, and the filtrate was concentrated on a rotary evaporator
to afford a residual mass. Et O (ca. 100 mL) was added to the resi-
due, and the resulting solution was washed with 2m HCl, saturated
aqueous NaHCO , and saturated aqueous NaCl, was dried over an-
the helical structures upon addition of MeOH to CHCl was dra-
3
matically different for the two new polymers.
The computational simulations showed that the CD spectro-
scopic changes upon MeOH addition originate from helix re-
versal. Thus far, several examples of solvent-driven helix-coil
and/or screw-sense inversions of synthetic helical polymers
2
3
hydrous MgSO , and concentrated. The residue was purified by
4
[
1b]
have been reported. Although the driving forces have been
quantitatively analyzed in a few cases, including the solvent-
driven helix–coil-folding transition based on p-stacking of m-
silica gel column chromatography eluted with hexane/ethyl ace-
tate=1:1 (v/v) to yield monomer 1 as a colorless liquid (202 mg,
1.32 mmol, 20% yield). [a]
ꢁ
1
= +4.28 (c=0.10 gdL
in CHCl ).
3
D
1
[
55]
H NMR (CDCl ): d=0.87–0.96 (CH CH , CH*CH , m, 6H), 1.14–1.21
phenylene ethynylene oligomers and the change of screw-
sense inversion temperature of helical polysilanes based on
3
2
3
3
(
(
C*HCHHCH , m, 1H), 1.37–1.44 (C*HCHHCH , m, 1H), 1.56–1.62
3 3
C*H, m, 1H), 2.38 (HCꢀ, s, 1H), 3.07–3.15 (NHCHH, m, 1H), 3.18–
[
56]
the molecular topology of the solvent, the principles of sol-
vent effects have not been clearly explained, or have been
qualitatively assumed in most cases. As far as we know, the
present study is the first successful research that explains the
solvent-driven screw-sense inversion of substituted helical pol-
3
.26 (NHCHH, m, 1H), 3.21 (CH C=O, s, 2H), 6.56 ppm (NH, s, 1H);
2
13
C NMR (CDCl ): d 11.20, 17.00, 26.89, 27.36, 34.71, 45.29, 74.12,
3
7
7.64, 165.9 ppm; IR (in CHCl ): 3426, 3307, 3017, 2966, 1670, 1530,
3
1217, 774, 741 cm ; elemental analysis calcd (%) for C H NO: C
70.55; H 9.87; N 9.14; found: C 70.48; H 9.75, N 9.17.
ꢁ1
9
15
Chem. Eur. J. 2014, 20, 15131 – 15143
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Monomer 2 was synthesized as follows: Pyridinium dichromate
PDC; 40 g, 106 mmol) was added to a DMF solution (80 mL) of 4-
a solvent were simulated using the SCF (ZINDO/S) method in Gaus-
[40,46,59]
(
sian 03.
Two hundred low-energy transition states, including
pentyn-2-ol (2.33 g, 27.7 mmol), and the reaction mixture was
stirred at room temperature for 24 h. The mixture was poured into
each fvel and Rvel in velocity form, were computed under the condi-
tion of a CI number of 200ꢁ200. According to references [12g],
[13b], and [24e], the CD spectra and UV/Vis absorption profiles
were produced by using a wavelength-based Gaussian function
with a half of 1/e-bandwidth (10 nm). In theoretical spectra, [q], fvel,
and Rvel were expressed with respect to monomer units.
water (500 mL) and extracted with Et O. The organic layer was
2
washed with 2m HCl, dried over anhydrous MgSO , and concen-
4
trated to afford 4-pentynoic acid in 41% yield. Monomer 2 was
synthesized by the condensation of the obtained 4-pentynoic acid
and (S)-(+)-2-heptylamine in a similar way to monomer 1. White
By using a procedure similar to that for poly(1), the structure of
ꢁ
1
powder. M.p. 52.0–53.08C; [a] = +3.98 (c=0.10 gdL
in CHCl3);
D
poly(2) (12-mer) was energy-minimized in vacuum, CHCl , and
3
1
H NMR (CDCl ): d=0.88 (CH CH , t, J=6.8 Hz, 3H), 1.13 (CH C*H,
3
2
3
3
MeOH, and then the corresponding CD spectra were also simulat-
ed. The simulation of poly(2) (18-mer) could not be performed due
to our computational limitations. Instead, all n-pentyl side chains in
poly(2) were replaced by ethyl groups to create poly(Et-2) with a re-
duced total atom number. Left- and right-handed helices of poly-
(Et-2) in an 18-mer or 12-mer were energy-minimized in vacuum.
d, J=6.4 Hz, 3H), 1.29 (CH CH CH CH , br, 6H), 1.39–1.43 (C*HCH ,
2
2
2
3
2
m, 2H), 1.99 (HCꢀ, s, 1H), 2.37 (ꢀCCH , t, J=6.8 Hz, 2H), 2.53
2
(
(
CH C=O, t, J=6.8 Hz, 2H), 3.95–4.03 (NHCH, m, 1H), 5.43 ppm
2
1
3
NH, s, 1H); C NMR (CDCl ): d=13.99, 15.03, 21.00, 22.54, 25.62,
3
3
1.66, 35.61, 36.90, 45.37, 69.26, 83.08, 170.1 ppm; IR (in CHCl ):
3
ꢁ
1
n˜ =3750, 3307, 2931, 1662, 1519, 1221, 727, 669 cm ; elemental
analysis calcd (%) for C H NO: C 73.80, H 10.84, N 7.17; found: C
Subsequently, a single-point computation in CHCl and MeOH was
3
12
21
carried out for each helical structure converged in vacuum. A CI
number of 212ꢁ212 for the 18-mer or 142ꢁ142 for the 12-mer
was used in the CD simulation of poly(Et-2).
7
3.74, H 10.94, N 7.17.
Polymerization
A solution of a monomer in distilled CHCl was added to a solution
Specific MeOH solvation of poly(1) (18-mer) and poly(2) (12-mer)
was simulated as follows (for a similar calculation procedure in
a peptide helix-water system, see ref. [51]). In each of the helical
structures in vacuum (Figure 6a and d, and Figure 8a and d),
MeOH molecules were placed near all amide CO groups and hy-
drogen bond-free amide NH groups at one terminus. The distance
of hydroxy H (MeOH)ꢁO (amide) and of O (MeOH)ꢁH (amide) was
3
+
6
ꢁ
of [Rh(nbd)] [h -C H B (C H ) ] in distilled CHCl under dry nitro-
6
5
6
5 3
3
gen, and the resulting solution (1: [M] =0.5m, [M] /[Rh]=100, 2:
0
0
[
M] =0.1m, [M] /[Rh]=50) was kept at 308C for 24 h. The reaction
0 0
mixture was poured into a large amount of MeOH or hexane to
precipitate the formed polymer, which was separated by filtration
and dried under reduced pressure.
set to about 2.5 ꢂ, which is also used for criteria of protein hydro-
[60]
gen bonds.
As a result, 20 MeOH molecules were added to
poly(1) having two free NH groups, whereas 15MeOH molecules
to poly(2) having three free NH groups. These initial structures of
polymer-MeOH complexes were energy-minimized in vacuum and
Spectroscopic data of the polymers
1
Poly(1): H NMR (CDCl ): d=0.89 (CH CH , CH*CH , br, 6H), 1.16
3
2
3
3
[38,39,52,53]
in MeOH (COSMO) by the PM6 method in MOPAC2012.
(
C*HCHHCH , br, 1H), 1.48 (C*HCHHCH , br, 2H), 3.00 (NHCH , br,
3 3 2
2
1
7
H), 3.10 (CH C=O, br, 2H), 6.08 (HC=C, br, 1H), 8.02 ppm (NH, br,
2
H); IR (in CHCl ): n˜ =3905, 3855, 3020, 1631, 1549, 1217, 765,
3
ꢁ
1
39 cm .
Acknowledgements
1
Poly(2): H NMR (CDCl ): d=0.88 (CH CH , br, 3H), 1.12 (CH C*H, br,
3
2
3
3
3
H), 1.28 (CH CH CH CH , br, 6H), 1.49 (C*HCH , br, 2H), 2.36 (=
2 2 2 3 2
This research was partly supported by a Grant-in-Aid for Sci-
ence Research in a Priority Area “Super-Hierarchical Structures
CCH , br, 2H), 2.49 (CH C=O, br, 2H), 3.90 (NHCH, br, 1H), 5.92
2
2
(HC=C, br, 1H), 8.04 ppm (NH, br, 1H); IR (in CHCl ): n˜ =3905, 3752,
3
(
No. 446)” from the Ministry of Education, Culture, Sports, Sci-
ꢁ1
1
634, 1542, 1224, 789, 781 cm .
ence, and Technology, Japan. We are grateful to Prof. Ken-
neth B. Wagener and Dr. Kathryn R. Williams at the University
of Florida for their helpful suggestions and comments.
Conformational analysis of poly(1) and poly(2)
Energy minimization of an 18-mer molecule of poly(1) was carried
[37,38]
out by the PM6 method in MOPAC2009.
The initial conforma-
Keywords: chirality · helical structures · hydrogen bonding ·
polymers · rhodium
tions were generated by Wavefunction Inc., Spartan 06 for Win-
[33]
dows using the MMFF94 force field for a right-handed helix. The
backbone dihedral angle of C=C-C=C (f) was estimated to be
about +1258, whereas the f of a left-handed helix was set to
about ꢁ1258. All the bond lengths, bond angles, and dihedral
angles of the initial conformations were varied during the minimi-
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[48] Poly(2) backbone that readily undergoes helix inversion in solution im-
plies that the stabilities of the left- and right-handed helices are almost
close to each other. Thus, we infer that complete interpretation of the
inversion is somewhat difficult here. The structure simulation is based
on the heat of formation and COSMO method in semi-empirical MO
method (refs.[37–39]). That is, conformational stability is discussed in
terms of only enthalpy, but not free energy involving the entropy term.
In addition, solvent in COSMO is treated as “conductor-like” media
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[
[42] The f and t of poly(1) (18-mer) are listed in the Supporting Informa-
tion, Table S2 and Figure S1.
[
43] In the current theoretical CD assignment, minor CD and absorption
bands of around 315 nm (Figure 9) are not taken into account. In our
previous work, we proposed that the minor absorption band should be
the origin of a longer-wavelength component in a split-type CD pat-
[
13b]
tern.
However, this minor absorption band consisting of several
DHLꢁR of poly(2) shows
a tendency to increase in the order of
small fvel states does not appear to be clearly reproduced, whereas
vacuum<CHCl <MeOH. This tendency supports the experimental
3
there is a tail at around 350 nm in experimental absorption spectra
view that a right-handed helix is more preferred in polar solvent.
[49] Left- and right-handed helices of poly(Et-2) (12-mer and 18-mer)
energy-minimized in vacuum are shown in Figure S5 (the Supporting
(
Figure 5). Thus, the present simulation may overestimate the absorp-
tion intensity and the CD signal at longer wavelengths, and may hardly
reproduce a split-type CD pattern. It should be also noted that the ge-
ometry-optimized structures of poly(2) and poly(2-Et) shown here have
unfavorable and distorted side-chain orientations, which are severely
overlapped with each other. Such unfavorable orientations did not
occur in their initial conformers in vacuum. However, the structures ge-
ometry-optimized by the semi-empirical MO method gave lower
energy value than the corresponding initial ones in vacuum.
3
Information; ref. [41]). The energies of these four helices in CHCl and
MeOH were obtained by single-point computation and listed in Ta-
bles S4 and S5, and the simulated CD spectra for these helices are
given in Figure S6 (the Supporting Information).
[50] The solvent effect on poly(2) and poly(Et-2) can be observed in
Tables 4, and the Supporting Information, S4 and S5. Each DHLꢁR of
poly(Et-2) (12-mer) is larger than that of poly(2) in the corresponding
medium. A considerably larger DHLꢁR is also obtained in the case of
poly(Et-2) (18-mer), but the dependence of medium on DHLꢁR seems to
be somewhat weak. Thus, solvent-driven helix inversion of poly(2) is
possibly facilitated by the presence of the long alkyl side-chain groups
and of chain-terminal segments.
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poly(1)ꢁMeOH complex, 0.98 in vacuum and 1.03 in MeOH; for poly(2)
alone, 1.12 in vacuum and 1.13 in MeOH; for poly(2)ꢁMeOH complex,
1.24 in vacuum and 1.19 in MeOH. On the whole, poly(2) yields a larger
Area(r/l) than poly(1).
[
44] f and t of poly(2) (12-mer) are listed in the Supporting Information,
Table S3 and Figure S3.
[
45] The present CD simulation was carried out at the semi-empirical MO
level (ZINDO/S) (ref. [46]) but does not reproduce the experimental
split-type CD patterns. Electronic transition and chiroptical properties of
helical poly(alkynamide)s seem to be complicated, because their transi-
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due to varying the degree of conjugation in ꢁ(C=C)ꢁ. Actually, main-
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bles S2 and S3). Accurate CD reproduction for the structure-sensitive
transition states of poly(2) might not be fully covered at the semi-em-
pirical MO level. Instead, another elaborate method (e.g., TD-DFT level;
ref. [47]) might be recommended. On the other hand, TD-DFT-based CD
simulation for such a large polymeric system will exceed our computer
specifications. However, the assignment of CD spectra to helical screw-
sense will be made at the present simulation. The CD spectrum of
poly(2) in CHCl
3
(Figure 5) affords a split-type pattern. Herein, a negative
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Received: March 17, 2014
Published online on September 26, 2014
Chem. Eur. J. 2014, 20, 15131 – 15143
www.chemeurj.org
15143
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim