Solvent-dependent switch of helical main-chain chirality in sergeants-and-soldiers-type poly(quinoxaline-2,3-diyl)s: Effect of the position and structures of the sergeant chiral units on the screw-sense induction

Article abstract of DOI:10.1021/ja403391m

Poly(quinoxaline-2,3-diyl) copolymers bearing various sergeant chiral units with common soldier achiral units have been synthesized to investigate the efficiency of screw-sense induction and its dependence on the nature of the solvents. Optically active 2-alkoxymethyl side chains located at the 6- and 7-positions of the quinoxaline ring induced a single-handed helical conformation more efficiently than 3-methylpentyl or 2-methylbutoxy chiral side chains. Among the 2-alkoxymethyl side chains, those bearing higher 2-alkoxy groups induced a single-handed screw sense more efficiently. For instance, a monomer unit bearing (R)-2-octyloxymethyl groups stabilized the P-helix by 1.01 kJ/mol, whereas the monomer bearing (S)-2-butoxymethyl groups stabilized the M-helix by 0.59 kJ/mol. The effect of the position of the sergeant units in the polymer main chain on the screw-sense induction was also investigated using copolymers in which the positions of the sergeant units were carefully controlled by their synthesis via living polymerization. Chiral units placed sparsely could induce single-handed helical structure efficiently. Chiral units bearing 2-alkoxymethyl, 3-methylpentyl, and 2-methylbutoxy groups showed solvent-dependent helix inversion in CHCl₃ and 1,1,2-trichloroethane. No helix inversion was observed in those solvents with chiral units bearing 2-butoxy or (2-methylbutoxy)methyl side chains. The 40-mer of the (R)-2-octyloxymethyl units showed P-helical structures in THF, t-BuOMe, and c-C₅H₁₁OMe, toluene, pyridine, Et₃N, 1-BuOH, CHCl₃, CH₂Cl₂, 1,4-dichlorobutane, 1,1,-dichloroethane, and 1,1,1-trichloroethane, whereas M-helical structures were induced in 1-BuCN, 1-PrCN, 1,2-dichloroethane, 1,3-dichloropropane, and 2-BuOH.

Full text of DOI:10.1021/ja403391m

Article
pubs.acs.org/JACS
Solvent-Dependent Switch of Helical Main-Chain Chirality in
Sergeants-and-Soldiers-Type Poly(quinoxaline-2,3-diyl)s: Effect of
the Position and Structures of the “Sergeant” Chiral Units on the
Screw-Sense Induction
Yuuya Nagata,† Tetsuya Yamada,† Takumi Adachi,† Yuto Akai,† Takeshi Yamamoto,†
and Michinori Suginome*,†,‡
†Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501,
Japan
‡CREST, Japan Science and Technology Agency (JST), Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
S
* Supporting Information
ABSTRACT: Poly(quinoxaline-2,3-diyl) copolymers bearing
various “sergeant” chiral units with common “soldier” achiral
units have been synthesized to investigate the efficiency of
screw-sense induction and its dependence on the nature of the
solvents. Optically active 2-alkoxymethyl side chains located at
the 6- and 7-positions of the quinoxaline ring induced a single-
handed helical conformation more efficiently than 3-
methylpentyl or 2-methylbutoxy chiral side chains. Among
the 2-alkoxymethyl side chains, those bearing higher 2-alkoxy
groups induced a single-handed screw sense more efficiently. For instance, a monomer unit bearing (R)-2-octyloxymethyl groups
stabilized the P-helix by 1.01 kJ/mol, whereas the monomer bearing (S)-2-butoxymethyl groups stabilized the M-helix by 0.59 kJ/
mol. The effect of the position of the sergeant units in the polymer main chain on the screw-sense induction was also investigated
using copolymers in which the positions of the sergeant units were carefully controlled by their synthesis via living
polymerization. Chiral units placed sparsely could induce single-handed helical structure efficiently. Chiral units bearing 2-
alkoxymethyl, 3-methylpentyl, and 2-methylbutoxy groups showed solvent-dependent helix inversion in CHCl3 and 1,1,2-
trichloroethane. No helix inversion was observed in those solvents with chiral units bearing 2-butoxy or (2-methylbutoxy)methyl
side chains. The 40-mer of the (R)-2-octyloxymethyl units showed P-helical structures in THF, t-BuOMe, and c-C5H11OMe,
toluene, pyridine, Et3N, 1-BuOH, CHCl3, CH2Cl2, 1,4-dichlorobutane, 1,1,-dichloroethane, and 1,1,1-trichloroethane, whereas
M-helical structures were induced in 1-BuCN, 1-PrCN, 1,2-dichloroethane, 1,3-dichloropropane, and 2-BuOH.
structures by introduction of chiral side chains. Both effects
are based on the accumulation of small energy differences in the
helical macromolecules for the induction of a nonracemic
macromolecular helical conformation. For instance, these
macromolecular effects enabled induction of nonracemic helical
structure even by an enantioenriched α-monodeuterated
primary alkyl group attached to the nitrogen atoms of
polyisocyanates.5b
Remarkably, the dynamic helical polymers are able to
undergo a switch of the helical chirality by external stimuli,
such as temperature,9 light,10 metal ion,11 pH,12 and solvent.13
Because switchable helical structures would lead to the
development of new functional materials, much effort has
been directed to the exploration of macromolecular systems
that exhibit highly efficient induction of both helical senses
reversibly, depending on the external circumstances. However,
INTRODUCTION
■
Much attention has been paid to the synthesis, structures,
properties, and applications of helical macromolecules,1 whose
helical main chains are potentially interconvertible to the mirror
image helical structure. In addition to the helical macro-
molecules supported by chiral stereogenic centers in their main
chains, such as triphenylmethyl methacrylates2 and polysilanes,3
those devoid of stereogenic centers, such as polyacetylenes,4
polyisocyanates,5 and polyisocyanides,6 are known and have
attracted increasing attention. A major characteristic of such
helical macromolecules is the reversible, i.e., thermodynamic,
formation of right- and left-handed helical structures by
introduction of chiral groups at the side chains or termini7 of
the polymers or by interaction with guest chiral molecules.8
Introduction of chiral side chains to those dynamic helical
polymers has been the most reliable strategy to obtain
nonracemic helical structures. Since the 1980s, Green
established the majority-rule effect5g and the sergeants-and-
soldiers effect5h for the induction of nonracemic helical
Received: April 5, 2013
Published: June 17, 2013
© 2013 American Chemical Society
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it should be noted here that determination of the purity of the
single-handed screw sense has not been an easy task. Although
the nonracemic nature of the macromolecules is readily
indicated by circular dichroism (CD) spectrometry, the degree
of the purity of the helical sense cannot be estimated unless the
maximum CD intensity for the purely single-handed helical
structure is determined. It is therefore important to establish a
macromolecular system in which the main chains adopt “pure”
single-handed helical structure and undergo a “perfect” switch
of helical structure to the other helical sense by some external
stimulus.
particular, it should be useful to find chiral side chains that
induce helically chiral backbones more efficiently in a chirality-
switchable manner. We herein report the synthesis of a series of
poly(quinoxaline-2,3-diyl)s bearing various chiral side chains to
evaluate their screw-sense induction in various organic solvents,
such as chloroform, 1,1,2-TCE, toluene, and THF. While the
detailed mechanism was still not clear, an efficient “sergeant”
chiral unit has been established for the solvent-dependent helix
inversion.
RESULTS AND DISCUSSION
■
We have developed aromatizing polymerization of 1,2-
diisocyanobenzenes promoted by transition-metal complexes.
The polymerization affords poly(quinoxaline-2,3-diyl)s, which
possess a helical main chain because of steric repulsion of the
two substituents at the 5- and 8-positions of the quinoxaline
rings. The rigidity of the helical main chain is largely dependent
on the bulkiness of the substituents at the 5,8-positions. For
example, poly(quinoxaline-2,3-diyl)s having small alkyl sub-
stituents, such as methyl groups, at the 5,8-positions behaved as
dynamic helical polymers, where the preference for the helical
sense is determined by the thermodynamic stabilities. We have
already reported that the single-handed screw sense of the
poly(quinoxaline-2,3-diyl)s could be efficiently induced by
chiral end groups.14
In our preliminary report, we discussed degrees of screw sense
induction of poly(quinoxaline-2,3-diyl)s bearing chiral side
chains prepared by random copolymerizations of achiral
monomer 1 and chiral monomer 2a. In the random polymer,
the chiral units are randomly dispersed throughout the polymer
main chain. We were interested in knowing if the position and
the sequence of the sergeant chiral monomer units change the
degree of screw-sense induction. Because this polymerization
proceeded in a living fashion, copolymers in which the
positions and sequence of chiral monomer units are known
can be readily obtained by controlling the addition of chiral and
achiral monomers. As shown in Table 1, copolymers 3a4−S1 to
3a4−S6, in which the positions of the sergeant monomer were
regulated, were synthesized by virtue of the living nature of the
polymerization, and their CD and UV−vis absorption spectra
were measured in CHCl3. Copolymers 3a4-S1, 3a4−S3, and
In our preliminary Communication,15 we reported that a
single-handed screw sense was induced also by introduction of
“sergeant” monomers bearing chiral side chains and inverted by
the effect of solvent (Figure 1). For instance, poly(quinoxaline-
Table 1. Synthesis and Properties of Random Copolymer 3a4
and Sequence-Regulated Copolymers 3a4−S1 to 3a4−S6
Figure 1. Solvent-dependent helix inversion of sergeants-and-soldiers-
type poly(quinoxaline-2,3-diyl)s bearing achiral and chiral side chains.
2,3-diyl)s bearing (R)-2-butoxymethyl side chains adopt a pure
right-handed helical structure in chloroform, while they adopt a
pure left-handed helix in 1,1,2-trichloroethane (1,1,2-TCE).
We also reported that the single-handed helical poly-
(quinoxaline-2,3-diyl) bearing chiral 2-butoxymethyl side chains
can serve as a highly effective scaffold to which a diary-
lphosphino pendant is attached covalently as the metal-binding
site.16 They served as highly effective chiral catalysts in which
the solvent-dependent switch of helical chirality enables highly
enantioselective production of both enantiomers from a single
chiral catalyst in various asymmetric reactions.17 To gain more
insight into the origin and mechanism of the helix inversion and
to find more applications, it is crucial to examine in detail the
effect of chiral side chains on the induction of helical sense. In
a
A polymer main chain is shown as an arrowed line, whose start and
end points mean initiating and terminating end groups, respectively.
The red circles schematically represent positions of chiral units in the
polymer chain. Molecular weights were determined by gel
permeation chromatography (GPC) with polystyrene standards. gabs
b
c
values (Kuhn’s dissymmetry factor) were measured at 366.0 nm.
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Table 2. Structures and Dissymmetry Factors gabs in CHCl3 and 1,1,2-TCE of 40-, 100-, and 200-mers 3ax−cx, 3d40, 3dx, and 3f40
d
gabs /10−3 (screw sense)
a
b
c
c
polymer
position
confign.
Mn /103
Mw/Mn
CHCl3
1,1,2-TCE
3a2
3a4
3
3
3
3
3
3
3
R
R
R
R
R
R
R
7.5
7.7
7.7
8.2
7.9
7.9
7.9
1.07
1.07
1.08
1.08
1.08
1.08
1.07
+0.56 (P)
+1.07 (P)
+1.76 (P)
+2.13 (P)
+2.38 (P)
+2.34 (P)
+2.36 (P)
−0.39 (M)
−0.67 (M)
−1.49 (M)
−1.81 (M)
−2.49 (M)
−2.78 (M)
−2.81 (M)
3a8
3a12
3a20
3a28
3a40
3b40
3b100
3b200
3
3
3
S
S
S
12.0
35.1
55.7
1.25
1.31
1.93
−0.53 (M)
−1.00 (M)
−1.23 (M)
+1.15 (P)
e
−
e
−
3c2
3c4
3
3
3
3
3
3
3
R
R
R
R
R
R
R
4.5
4.3
5.0
5.2
4.8
5.5
4.7
1.22
1.25
1.20
1.18
1.27
1.19
1.43
+0.50 (P)
+1.21 (P)
+1.86 (P)
+2.07 (P)
+2.14 (P)
+2.24 (P)
+2.27 (P)
−0.37 (M)
−1.08 (M)
−1.87 (M)
−2.28 (M)
−2.53 (M)
−2.80 (M)
−2.90 (M)
3c8
3c12
3c16
3c28
3c40
3d40
4
S
5.2
1.24
−0.99 (M)
−0.91 (M)
3e40
3e100
3e200
3
3
3
S
S
S
11.1
36.5
1.08
1.14
1.09
−1.60 (M)
−2.35 (M)
−2.38 (M)
+1.22 (P)
e
−
e
117.6
−
3f40
2
S
b
9.6
1.20
+1.39 (P)
+1.25 (P)
a
c
Position of the chiral center in the side chains. Absolute configuration of the chiral side chain. Molecular weights were determined by GPC with
d
polystyrene standards. gabs values were measured at the following wavelengths; 3ax: 366.0 nm, 3bx: 366.0 nm, 3cx: 366.0 nm, 3d40: 365.0 nm, 3ex:
e
370.5 nm, and 3f40: 370.5 nm. These samples could not be measured because of their low solubility.
3a4−S4, which have chiral units at polymer termini, showed
relatively small gabs values, i.e., Kuhn’s dissymmetry factor. This
suggested that chiral helix induction by chiral units placed at
polymer termini was not efficient. Comparison of 3a4−S2 with
3a4−S6 suggested that chiral units introduced sparsely could
induce single-handed helices more efficiently. Although
copolymer 3a4−S5 had chiral units introduced sparsely, its
gabs value was slightly smaller than that of 3a4−S6. This can be
attributed to the presence of the chiral unit at the terminus.
Note that random copolymer 3a4 exhibited a similar gabs value
to 3a4−S6. Therefore, in the following studies, all poly-
(quinoxaline-2,3-diyl)s were synthesized by random copoly-
merization, taking advantage of the easy preparation.
cooperative length,18 which was defined as the number of units
that take a single helical sense without a helical reversal, because
the degree of helix induction increases with the increase in DP,
if the DP is within the cooperative length. Polymers 3a4 and
3a4−20 to 3a4−200 having four chiral units with varied degrees
of polymerization were synthesized, and their gabs values were
determined. Polymer 3a4−20 showed a smaller gabs than
polymer 3a4, because of the high probability of having the chiral
unit at the termini of the polymer main chain. Polymers 3a4−
80 to 3a4−200 having larger DP showed identical gabs values to
polymer 3a4. We therefore concluded that the cooperative
length of poly(quinoxaline-2,3-diyl)s is larger than 200 repeat
units, which means that the helix reversal energy is larger than
12.9 kJ/mol according to the reported theory.5c
We then examined the effect of the degree of polymerization
(DP) on the single-handed screw-sense induction, while
keeping the same number of chiral units in a single polymer
main chain. This examination was carried out to estimate the
The most striking aspect of the helical structures of
poly(quinoxaline-2,3-diyl)s is its highly solvent-dependent
nature. Homopolymer 3a40 bearing (R)-2-butoxymethyl side
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chains adopts perfect right-handed helical structure in CHCl3,
whereas the same 3a40 adopts perfect left-handed structure in
1,1,2-TCE. To evaluate the solvent-dependent induction of
single-handed helical conformations in chloroform and 1,1,2-
TCE on the structures of the side chains, polymers 3b40−f40
were prepared by aromatizing polymerizations of the
corresponding 1,2-diisocyanobenzenes in the presence of an
organonickel complex. The polymerization was evaluated by
the measurements of molecular weights and polydispersity
index (PDI) using polystyrene standards. Although the values
based on polystyrene serve as a good measure of polymer-
ization outcome, the values can be deviated from the exact
values because poly(quinoxaline-2,3-diyl)s should be stiffer
than polystyrene. Therefore the DP values for following
theoretical calculations were estimated from the loading ratios
of the monomers to the initiator.
se = ([P] − [M])/([P] + [M])
Here, se is represented using Eh, N, R, and T as
(3)
se = {exp(−EhN/RT) − 1}/{exp(−EhN/RT) + 1}
= tanh(−EhN/2RT)
(4)
where hyperbolic tangent function (tanh) is involved. The
observed dissymmetry factor gabs is proportional to the screw-
sense excess, namely
se = gabs/gmax
(5)
where gmax is the g value for the purely single-handed
poly(quinoxaline-2,3-diyl)s. Equation 4 can be expressed
using gabs and gmax as
gabs = tanh(−EhN/2RT) × gmax
(6)
No significant difference in UV−vis absorption spectra of
polymers 3a40−f40 was observed in CHCl3 and 1,1,2-TCE.
Absorption peaks at wavelengths below 400 nm were generally
observed because of π−π* transitions of the quinoxaline ring.
CD spectra were measured to determine the degree of screw-
sense induction of polymers 3a40−f40 in CHCl3 and 1,1,2-TCE.
As reported before, the P-helical poly(quinoxaline-2,3-diyl)
showed a positive CD signal around 370 nm and a negative CD
signal at 290 nm, while the M-helical poly(quinoxaline-2,3-diyl)
gave its mirror image. Polymer 3a40 showed a solvent-
dependent helix inversion between CHCl3 and 1,1,2-TCE.
Polymer 3b40 showed a helix inversion, indicating that oxygen
atoms in the side chain were not essential for the helix
inversion. Polymer 3c40 also exhibited the helix inversion.
Polymers 3d40 and 3e40 were synthesized using (S)-2-
methylbutanol as an inexpensive chiral source, which is easily
available as a byproduct of an alcoholic fermentation. Although
polymer 3d40 bearing the (S)-2-methylbutoxymethyl group did
not show helix inversion, polymer 3e40 bearing the (S)-2-
methylbutoxy group showed helix inversion. Polymer 3f40,
which possesses the (S)-2-butoxy group, did not show helix
inversion. Here, chirality-switchable polymers 3a40−c40 and
3e40 commonly have the chiral center at the third position in
the chiral side chains on the quinoxaline ring. It is suggested
that the position of the chiral center in the side chain may be
important to produce the solvent-dependent helix inversion.
The results obtained thus far indicate that polymers that have
an S or R chiral center at the third atom in the side chain form
P- or M-helices, respectively, and undergo the solvent-
dependent helix inversion.
Nonlinear least-squares fitting of gabs versus N was performed
by using the Solver Function in Microsoft Office Excel 2007.
Sums of the squares of the deviation were minimized by varying
two parameters, gmax and Eh.
To determine the gained energy difference per monomer
unit, Eh, a series of poly(quinoxaline-2,3-diyl)s having varied
numbers of chiral monomers N was synthesized (Table 2). The
parameters Eh and gmax were successfully converged and the
final values are summarized in Table 3. The gained energy
Table 3. Calculated Eh and gmax Values of Polymers 3a−c and
3e
CHCl3
Eh (kJ mol−1
1,1,2-TCE
Eh (kJ mol−1 gmax /10−3
polymer
)
gmax /10−3
)
3a
3c
3b
3e
−0.59
−0.70
−0.05
−0.10
+2.37
+2.23
+1.26
+2.40
−0.32
−0.46
−2.87
−2.81
a
a
−
−
a
a
−
−
a
Collected data were not sufficient for the nonlinear least-squares
fitting.
differences per monomer unit of polymer 3a were determined
as 0.59 kJ mol−1 in CHCl3 and 0.32 kJ mol−1 in 1,1,2-TCE. On
the other hand, a monomer unit of polymer 3c gained more
energy (0.70 kJ mol−1 in CHCl3 and 0.46 kJ mol−1 in 1,1,2-
TCE). This increment can be explained by enhanced
differences of the bulkiness of the substituents on the chiral
center (polymer 3a: methyl and ethyl groups, polymer 3c:
methyl and propyl groups). Because polymers 3b and 3e
showed smaller Eh, benzyl ether moieties contained in 3a and
3c seemed to be essential for effective chiral helix inductions.
On the basis of the calculated gmax, the relationships between
se and N are plotted in Figure 2. The observed se values (shown
by circles and rhombuses) are in good agreement with the
fitted curves in every polymer. The relationships between ΔG
and N are shown in Figure 3. The values of ΔG (shown by
circles and rhombuses) based on the observed gabs are in good
agreement with the fitted lines. These good agreements of the
observed and calculated plots indicate that the least-squares
fitting was performed correctly and efficiently. Figures 2 and 3
also show that helix induction was achieved very efficiently in
polymers 3a and 3c, while many chiral units are required to
maintain a single-handed helical main chain in polymers 3b and
3e.
According to the previous report by Lifson and Green,5c we
determined the gained energy difference between P and M-
helices per a chiral unit. On the basis of the supposition that the
helix persistence length is sufficiently long, the total energy
difference between P and M helical polymers is expressed as
ΔG = −RTln([P]/[M])
(1)
where R, T, [P], and [M] are the gas constant (8.314 J K−1
mol−1), operating temperature (293.15 K), and molar
concentration of P and M helical polymers ([P] > [M]). In
the case where the polymer has no conformations containing
helix reversal, ΔG is proportional to the number of chiral units
N and Eh, i.e., the gained energy difference between P and M-
helices per a chiral unit.
ΔG = EhN
(2)
The screw-sense excess (se) is defined as
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Table 4. Structures and Dissymmetry Factors gabs of 100-
mers (S)-4ax, 4cx, 4gx, 4hx, and 4ix in CHCl3, 1,1,2-TCE,
THF, and Toluene
Figure 2. Relationships between number of chiral units N and se of
polymers 3a−c and 3e in CHCl3 or 1,1,2-TCE. The circles and the
rhombuses were calculated from observed gabs and gmax according to eq
5. The lines were generated from Eh and N according to eq 4.
gabs /10−3
polymer
Mn /103 Mw/Mn CHCl3 1,1,2-TCE
THF
toluene
(S)-4a5
(S)-4a10
(S)-4a15
(S)-4a20
(S)-4a100
4c5
25.3
23.7
23.0
21.8
27.9
27.7
27.5
27.6
28.0
29.5
28.9
30.3
29.8
30.2
30.8
30.2
28.9
30.9
27.4
27.7
27.4
29.0
28.5
28.3
28.4
20.5
20.5
20.7
19.7
18.6
19.9
21.6
1.08
1.08
1.09
1.10
1.07
1.10
1.10
1.10
1.08
1.08
1.11
1.08
1.08
1.08
1.07
1.07
1.08
1.07
1.07
1.08
1.07
1.08
1.08
1.08
1.11
1.07
1.09
1.10
1.11
1.10
1.11
1.08
−1.45
−2.04
−2.25
−2.25
−2.41
+1.64
+2.11
+2.27
+2.30
+2.42
+1.04
+1.69
+1.99
+2.21
+2.27
+2.32
+2.39
+2.41
+1.13
+1.78
+2.03
+2.21
+2.25
+2.25
+2.39
−0.63
−1.20
−1.55
−1.91
−2.16
−2.24
−2.63
+1.15
+2.01
+2.51
+2.67
−0.83
−1.44
−1.79
−1.87
−2.24
+0.96
+1.51
+1.72
+1.92
+2.22
−0.56
−1.04
−1.40
−1.58
−2.23
+0.81
+1.38
+1.64
+1.83
+2.16
a
−
−1.57
−2.41
−2.68
−2.78
4c10
4c15
4c20
4c100
4g2.5
4g5
a
−
b
b
b
−
−
−
−1.68
+0.93
+0.76
b
b
b
4g7.5
4g10
−
−
−
−2.60
−2.91
−2.96
+1.53
+1.83
+1.92
+1.39
+1.69
+1.90
4g15
4g20
b
b
b
4g50
−
−
−
a
Figure 3. Relationships between number of chiral units N and the total
energy difference between P- and M-helical polymers of polymers 3a−
c and 3e in CHCl3 or 1,1,2-TCE. The circles and the rhombuses were
calculated from observed gabs and gmax according to eq 6. The lines
were generated from Eh and N according to eq 2.
4g100
4h2.5
4h5
−
+2.16
+2.10
b
b
−0.95
−1.74
−2.20
−2.47
−2.69
−2.80
−
−
+0.90
+0.83
b
b
4h7.5
4h10
4h15
4h20
−
−
+1.45
+1.72
+1.89
+2.06
+1.40
+1.67
+1.87
+1.94
We then synthesized additional homopolymers and copoly-
mers having various benzyl−ether-type chiral side chains from
diisocyanobenzenes (S)-2a, 2c, 2g, 2h, and 2i, as shown in
Table 4. Note that (S)-4ax, in which x indicates the number of
chiral units, is derived from monomer (S)-2a, which is the
enantiomer of monomer 2a used thus far. Using (S)-4ax, we
could confirm induction of the opposite helix, compared with
the polymer obtained from 2a. Note that all of the
polymerizations proceeded in a living fashion, giving the
corresponding polymers with narrow PDI despite the presence
of higher side chains. In Table 4, the observed values of gabs in
various solvents, including THF and toluene along with CHCl3
and 1,1,2-TCE, are shown.
a
4h100
4i2.5
−
b
b
b
−
−
−
4i5
+1.22
+0.15
−0.07
b
b
b
4i7.5
−
−
−
4i10
+1.90
+2.26
+2.41
+0.23
+0.30
+0.33
−2.14
−0.16
−0.36
−0.57
−1.99
4i15
4i20
a
4i100
−
a
These samples could not be measured because of their low solubility.
Not measured.
b
The parameters Eh and gmax of 100-mers (S)-4a, 4c, 4g−i in
several solvents, which were determined by nonlinear least-
squares fitting using eq 6 with the gabs values in Table 4, are
summarized in Table 5. The calculated parameters Eh and gmax
of 100-mers (S)-4a and 4c showed good agreement with the
values calculated for 40-mers. While the Eh values were slightly
larger, this could be explained by the influence of the chiral
groups located at the termini. The Eh values increased with
increase in the difference in bulkiness of the substituents on the
chiral centers. Polymer 4h showed the largest Eh (1.01 kJ
mol−1), which was 1.7 times larger than the original structure
(polymers 3a and (S)-4a). On the other hand, polymer 4i
bearing 3-octyloxymethyl groups showed a smaller Eh
compared with polymer 4h bearing 2-octyloxymethyl groups.
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structure. Helix induction by the chiral units derived from 2h
was found to be more efficient than the chiral units derived
from 2a not only in CHCl3 but also in other solvents, such as
1,1,2-TCE, THF, and toluene.
Next, we carried out CD and UV measurements of polymers
3a40 and 3h40 (40-mers) in 1,1,2-TCE/CHCl3 with varied
ratios (Figure 5). The observed g value was plotted against the
Table 5. Calculated Eh and gmax Values of Polymers (S)-4a,
4c, 4g−i
CHCl3
1,1,2-TCE
polymer
Eh (kJ mol−1
)
gmax/10−3
Eh (kJ mol−1
)
gmax/10−3
(S)-4a
4c
−0.59
−0.82
−0.88
−1.01
−0.50
−2.37
+2.33
+2.35
+2.29
−2.44
−0.42
−0.62
−0.62
−0.70
−0.49
+2.87
−2.81
−3.02
−2.79
+2.46
4g
4h
4i
THF
toluene
polymer
Eh (kJ mol−1
)
gmax/10−3
Eh (kJ mol−1
)
gmax/10−3
(S)-4a
4c
−0.37
−0.42
−0.44
−0.44
−2.18
+2.13
+2.10
+2.02
−0.24
−0.36
−0.39
−0.44
−2.21
+2.10
+2.06
+1.95
4g
4h
a
a
a
a
4i
−
−
−
−
a
The nonlinear least-squares fitting did not converge.
This can be explained by the smaller differences in bulkiness of
the substituents on the chiral center (polymer 4h: methyl and
hexyl groups, polymer 4i: ethyl and pentyl groups). It should be
noted that parameters Eh and gmax of polymer 4i in THF or
toluene were not in accordance with eq 6. In particular, while
polymer 4i in THF gave positive gabs values when the number
of chiral units N was small (N = 2.5−20), the gabs value became
negative when N was large (N = 100) (Table 4). Although the
reason for this phenomenon is not clear at this moment, we
assume that the chirality induction by the chiral unit is
susceptible to the influence of neighboring monomer units in
the case where the chiral group becomes bulkier.
Figure 5. Relationships between percentage by volume of 1,1,2-TCE
in 1,1,2-TCE/CHCl3 mixed solvent and dissymmetry factor g at 366.0
nm of polymers 3a40 and 3h40.
The relationships between the number of chiral units N and
the se in CHCl3 or 1,1,2-TCE calculated with the parameters Eh
and gmax are plotted in Figure 4. Polymer 4h shows steeper
ratio of the solvents. The polymers both adopted P-helical
structures in pure CHCl3. Polymer 3a40 started to invert the
helical sense at the 1,1,2-TCE/CHCl3 ratio of ca. 40/60 and
became racemic at a ratio around 60/40. Polymer 3h40
exhibited sharper inversion of its helical sense as indicated by
the steeper curve, which starts to descend at a 55:45 ratio. By
virtue of the larger Eh value, 3h40 can complete the inversion
within the narrower range of the 1,1,2-TCE/CHCl3 ratio.
Finally, we measured the dissymmetry factors g of polymer
3h40 in various solvents to explore solvents for helix inversion
(Figure 6; the abbreviations of solvents are summarized in the
Figure 4. Relationships between number of chiral units N and se of
polymers 4a, 4c, 4g, and 4h in CHCl3 or 1,1,2-TCE. The circles and
the rhombuses were calculated from observed gabs and gmax according
to eq 5. The lines were generated from Eh and N according to eq 4.
curves than the other polymers, suggesting that the chiral unit
in polymer 4h induced an asymmetric helix in the main chain
more efficiently than the other polymers. In general, according
to the relationship between the Gibbs free energy and the
equilibrium constant, helix induction to the level of 99.0% se
requires 12.9 kJ mol−1 at 20 °C. Therefore, polymer 4h requires
only 13 chiral units derived from chiral monomer 2h to gain
13.1 kJ mol−1 for the formation of a single-handed helical
Figure 6. Dissymmetry factors g at 366.0 nm of polymer 3h40 in
various solvents. DCE: dichloroethane, DCP: dichloropropane, DCB:
dichlorobutane, TCE: trichloroethane, MTBE: methyl tert-butyl ether,
and CPME: cyclopentyl methyl ether.
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caption of Figure 6). Polymer 3h40 exhibited positive
dissymmetry factors because of the P-helix in ethers (THF,
MTBE, and CPME), aromatic solvents (toluene and pyridine),
amine (NEt3), alcohol (1-BuOH), and halogenated hydro-
carbons such as CH2Cl2, 1,4-DCB, CHCl3, 1,1,-DCE, and 1,1,1-
TCE. We found several solvents other than 1,1,2-TCE for the
formation of an M-helix with polymer 3h40. 1-BuCN and 1-
PrCN induced an M-helix to the polymer 3h40, although 3h40
was not soluble in other nitrile solvents, such as acetonitrile
(MeCN) or adiponitrile (NC−(CH2)4−CN). 2-BuOH also
induced an M-helix in contrast to 1-BuOH. It is notable that
1,2-DCE and 1,3-DCP induced an M-helix, while dichloro-
methane and 1,4-DCB showed P-helix induction.
Preparative GPC was performed on JAI LC-908 equipped with
JAIGEL-1H and -2H columns in a series (CHCl3). UV spectra were
recorded on a JASCO V-500 spectrometer equipped with a JASCO
ETC-505T temperature/stirring controller at 20 °C. CD spectra were
recorded on a JASCO J-750 spectrometer equipped with a JASCO
PTC-423L temperature/stirring controller at 20 °C. Optical rotations
were recorded on a JASCO DIP-1000 digital polarimeter at room
temperature, using the sodium D line. Flash chromatography was
performed using a Biotage Isolera One flash purification system with
silica gel flash cartridges. Ethylmagnesium bromide (EtMgBr) was
freshly prepared just before use. THF and toluene were dried and
deoxygenized using an alumina/catalyst column system (Glass 3
Contour Co.). 4,7-dibromo-5,6-bis(bromomethyl)benzo[c][1,2,5]-
thiadiazole 5,19 2,3-Bis(bromomethyl)-1,4-dimethylbenzene 6,20 acetic
formic anhydride (AFA),21 3,6-dimethyl-4,5-dinitrobenzene-1,2-diol
9,22 cyanomethylenetributylphosphorane (CMBP),23 (S)-1-bromo-2-
methylbutane,24 monomers 1,19 2a,15 (S)-2a,15 2e,22b and o-TolNiCl-
(PMe3),25 were prepared according to the reported procedure. Other
chemical reagents were purchased from the commercial sources and
were used without further purification.
CONCLUSIONS
■
In summary, we have investigated the induction of a single-
handed helical sense to the main chain of poly(quinoxaline-2,3-
diyl)s by several “sergeant” chiral monomer units bearing
different chiral side chains. The efficiency of screw-sense
induction has been evaluated by the helix stabilization energy
per monomer unit (Eh = ΔG/N), which was determined by CD
measurements of copolymers consisting of the chiral and
achiral units in a varied ratios. We compared (R)-2-
butoxymethyl, (R)-3-methylpentyl, and (R)-2-methylbutoxy
chiral side chains at 6- and 7-positions on the quinoxaline
rings, all of which consist of six nonhydrogen elements and
have stereogenic carbon centers at the third atom. The three
chiral units all showed P-helix induction in CHCl3. Among the
three, the most efficient helix-sense induction was made by the
(R)-2-butoxymethyl side chain (Eh = 0.59 kJ mol−1). In 1,1,2-
TCE, the same chiral unit induced a left-handed helical sense
(Eh = 0.32 kJ mol−1). This solvent-dependent switch of helical
chirality from P to M in CHCl3 and 1,1,2-TCE was found to be
general for the three “sergeant” units bearing stereogenic
centers at the third carbon atoms. Chiral units bearing
stereogenic centers at the second or fourth carbon atoms in
the side chain showed no helix inversion between CHCl3 and
1,1,2-TCE. A series of 2-alkoxymethyl chiral side chains was
compared in terms of the efficiency of screw sense induction,
indicating that higher (R)-2-alkoxymethyl side chains exhibited
larger Eh both for the P-helix in CHCl3 and the M-helix in 1,1,2-
TCE. (R)-2-Octyloxy side chains showed Eh of 1.01 kJ mol−1 in
CHCl3 for the P-helix and of 0.70 kJ mol−1 in 1,1,2-TCE for the
M-helix. Studies for exploring applications of the single-handed
helical poly(quinoxaline-2,3-diyl)s as a new class of chiral
materials are currently being undertaken in this laboratory
along with the elucidation of the origin of the solvent-
dependent helix inversion.
Synthesis of 1,2-Diisocyanobenzene monomers. The syn-
thesis of 1,2-diisocyanobenzenes monomers 1, 2a, (S)-2a, 2c, 2g−i
bearing alkoxymethyl groups is outlined in Scheme 1. These
monomers were prepared from compound 5 according to our
previous report with minor modifications.14b,19 A representative
procedure is shown for the synthesis of 2h.
Scheme 1. Synthesis of monomers 1, 2a, (S)-2a, 2c, 2g−i
Synthesis of 2h-Br. To a solution of (R)-2-octanol (8.44g, 64.8
mmol) in THF (10 mL) was added EtMgBr (0.85 M in THF, 70 mL,
59.6 mmol) at 0 °C. After stirring at room temperature for 30 min,
hexamethylphosphoramide (HMPA, 27.0 mL, 155 mmol) and 4,7-
dibromo-5,6- bis(bromomethyl)benzo[c][1,2,5]thiadiazole 5 (12.4 g,
25.9 mmol) were added. The mixture was refluxed for 15 h. After
concentration under reduced pressure, the mixture was diluted with
water and extracted with Et2O. The organic phase was washed with
water and brine and dried over MgSO4. After evaporation of the
solvent, the residue was purified by silica gel column chromatography
(hexane:Et2O = 98:2 to 10:1) to give 2h-Br as pale-yellow oil (12.9 g,
86% yield). 1H NMR (CDCl3) δ 5.08 (2H, d, J = 10.4 Hz), 4.94 (2H,
d, J = 10.4 Hz), 3.69−3.56 (2H, m), 1.80−1.51 (4H, m), 1.50−1.15
(16H, m), 1.28 (6H, d, J = 6.0 Hz), 0.86 (6H, t, J = 6.8 Hz); 13C NMR
(CDCl3) δ 152.7, 140.1, 117.6, 76.2, 67.2, 36.7, 31.9, 29.3, 25.6, 22.6,
19.8, 14.1; IR (ATR, neat) 2957, 2926, 2854, 1464, 1373, 1337, 1271,
1248, 1229, 1119, 1059, 922, 879, 841, 825, 723 cm−1; HRMS (ESI+)
m/z calcd for C24H38Br2N2O2S + H+ (M + H+): 579.1073, found:
579.1057.
Synthesis of 2h-Me. To a mixture of 2h-Br (12.9 g, 22.3 mmol),
KF (8.54 g, 147 mmol) and (MeBO)3 (2.80 g, 22.3 mmol) were added
a solution of tris(dibenzylideneacetone)dipalladium(0) (256 mg, 0.455
mmol), P(tert-Bu)3 (180 mg, 0.891 mmol), and water (1.20 mL, 66.8
mmol) in THF (46 mL). The mixture was heated under reflux for 1 h.
The reaction mixture was cooled to room temperature and
concentrated under reduced pressure. To the mixture was diluted
with water and extracted with Et2O. The organic phase was washed
EXPERIMENTAL SECTION
■
All reactions were carried out under an atmosphere of nitrogen with
magnetic stirring. 1H and 13C NMR spectra were recorded on a Varian
Mercury-VX 400, 400-MR, or JEOL JNM-A500 spectrometer at
ambient temperature. 1H NMR data are reported as follows: chemical
shift in ppm downfield from tetramethylsilane (δ scale), multiplicity (s
= singlet, d = doublet, t = triplet, q = quartet, quint = quintet, sex =
sextet, m = multiplet, and br = broad), coupling constant (Hz), and
integration. 13C NMR chemical shifts are reported in ppm downfield
from tetramethylsilane (δ scale). All 13C NMR spectra were obtained
with complete proton decoupling. IR spectra were obtained using a
Shimadzu FTIR-8400 Fourier transform infrared (FT-IR) spectrom-
eter equipped with PIKE MIRacle attenuated total reflection (MIR-
ATR) attachment. The GPC analysis was carried out with TSKgel
G4000HHR or TSKgel GMHXL (CHCl3, polystyrene standards).
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with water and brine and dried over MgSO4. After evaporation of the
solvent, the residue was purified by silica gel column chromatography
(hexane:Et2O = 20:1) to give 2h-Me as pale-yellow oil (8.70 g, 87%
yield). 1H NMR (CDCl3) δ 4.81 (2H, d, J = 10.0 Hz), 4.65 (2H, d, J =
10.0 Hz), 3.68−3.54 (2H, m), 2.79 (6H, s), 1.96−1.54 (4H, m), 1.53−
1.37 (4H, m), 1.37−1.18 (18H, m), 0.88 (6H, t, J = 6.4 Hz); 13C
NMR (CDCl3) δ 155.4, 136.3, 128.6, 76.0, 64.4, 36.8, 31.9, 29.4, 25.8,
22.6, 19.8, 14.3, 14.1; IR (ATR, neat) 2957, 2926, 2854, 1456, 1373,
1337, 1119, 1074, 1051, 910, 878, 835, 723 cm−1; HRMS (ESI+) m/z
calcd for C26H44N2O2S + H+ (M + H+): 450.3230, found: 450.3218.
Synthesis of 2h. To a solution of 2h-Me (8.70 g, 19.4 mmol) in
EtOH (110 mL) and toluene (110 mL) were added NaBH4 (7.34 g,
194 mmol) and CoCl2·6H2O (92.3 mg, 0.39 mmol) at room
temperature. The mixture was heated gradually to 50 °C, stirred for
1 h, and passed through a pad of Celite. The resultant solution was
evaporated in vacuo. Extraction with AcOEt followed by column
chromatography on silica gel (hexane:AcOEt = 2:1) afforded diamine
compound (7.25 g, 89% yield). To a CH2Cl2 (100 mL) solution of the
diamine compound (7.25 g, 17.2 mmol) was added AFA (6.07 g, 68.9
mmol) at 0 °C. The mixture was stirred for 1 h with gradual warming
up to room temperature. The mixture containing a diformamide
compound was subjected to evaporation of volatile materials in vacuo
and used for the next step without further purification. To a CH2Cl2
(40 mL) suspension of the diformamide compound (2.60 g, 5.45
mmol) and Et3N (7.60 mL, 54.5 mmol) cooled to 0 °C, POCl3 (1.53
mL, 2.51 g, 16.4 mmol) was added. After stirring for 45 min at 0 °C,
saturated NaHCO3 aq was added to the reaction mixture. Extraction
with CH2Cl2 followed by column chromatography on silica gel
(hexane:Et2O = 10:1) gave 2h as yellow oil (1.10 g, 2.50 mmol, 46%
yield). 1H NMR (C6D6) δ 4.32 (2H, d, J = 10.4 Hz), 4.10 (2H, d, J =
10.4 Hz), 3.38−3.20 (2H, m), 2.13 (6H, s), 1.60−1.45 (2H, m), 1.44−
1.17 (18H, m), 1.10 (6H, d, J = 6.4 Hz), 0.91 (6H, t, J = 7.0 Hz); 13C
NMR (C6D6) δ 175.6, 138.4, 133.9, 76.1, 64.2, 37.2, 32.3, 29.8, 26.0,
23.1, 19.7, 15.3, 14.3; IR (ATR, neat) 2957, 2928, 2856, 2114, 1456,
1373, 1337, 1227, 1198, 1136, 1119, 1084, 1049, 970, 922, 891, 723,
631, 623 cm−1; HRMS (ESI+) m/z calcd for C28H44N2O2 + H+ (M +
Scheme 3. Synthesis of Monomers 2e and 2f
nitrated under acidic conditions and then deprotected by treatment
with BBr3 to give compound 9. Monomer 2f was prepared from 9 by
Mitsunobu reaction mediated by a phosphorane reagent.23,27
A typical procedure for synthesis of sequence-regulated
copolymers: Synthesis of 3a4-S6. THF solutions of 1 (0.10 M)
and 2a (0.011 M) were prepared. A THF solution of o-TolNiCl-
(PMe3)2 (0.050 M, 50 mL, 2.5 mmol) was diluted with THF (3.9
mL). To the solution was added a solution of PMe3 in THF (1.0 M,
2.5 mL, 2.5 mmol). After stirring for 15 min, the solution of 1 (0.18
mL, 18 mmol) was added at room temperature. After stirring for 15
min, the solution of 2a (0.23 mL, 2.5 mmol) was added at room
temperature. The additions of the solutions of 1 and 2a were repeated
four times in the same manner. After 15 min from fourth addition of
2a, the solution of 1 (0.18 mL, 18 mmol) was added at room
temperature. After 15 min, a THF solution of o-TolMgBr (1.0 M, 50
mL, 50 mmol) was added to the reaction mixture at room
temperature. After stirring for 15 min at room temperature, water
was added. Extraction with CHCl3 followed by preparative GPC gave
H+): 442.3509, found: 442.3501; [α]29.7 41.2 (c 0.54, CHCl3).
D
Monomers 2b and 2d were synthesized from compound 6 as shown
in Scheme 2. We adopted this route for the synthesis of monomer 2b,
Scheme 2. Synthesis of Monomers 2b and 2d
1
3a4-S6 as orange solid (25.9 mg, 85%). H NMR (CDCl3) δ 5.36−
4.03 (40 × 4H, m), 3.46 ((4 × 2+36 × 4)H, br s), 2.57−1.92 (40 ×
6H, m), 1.85−1.12 ((4 × 10 + 36 × 4)H, m), 1.05−0.63 (40 × 6H,
m); GPC (CHCl3, g/mol): Mn = 8.5 × 103, Mw/Mn = 1.12.
Synthesis of 3h40. A THF solution of o-TolNiCl(PMe3)2 (10.0
mM, 250 μL, 2.5 μmol) was diluted with THF (3.0 mL). A THF
solution of 2h (44.1 mg, 100.0 mmol) was diluted with THF (2 mL).
The solution of the monomer 2h was added to the solution of o-
TolNiCl(PMe3)2. After stirring for 3 h at room temperature, NaBH4
(33.3 mg, 0.88 mmol) was added to the reaction mixture and stirred
for an additional 1 h. The mixture was diluted with water (10 mL) and
extracted with CH2Cl2 (15 mL). The organic extract was washed with
brine (10 mL), dried over MgSO4, and evaporated under vacuum. The
residue was subjected to preparative GPC to give 3h40 as a beige solid
(40.0 mg, 90 mmol, 90%). 1H NMR (CDCl3) δ 4.61 (40 × 2H, br s),
4.50 (40 × 2H, br s), 3.47 (40 × 2H, br s), 2.28 (40 × 6H, br s), 1.88−
1.04 (40 × 26H, m), 0.84 (40 × 6H, br s); GPC (CHCl3, g/mol): Mn
= 9.7 × 103, Mw/Mn = 1.11.
because the Grignard reagent was too reactive to modify compound 5
and caused undesired side reactions. Monomer 2d was synthesized via
compound 7 by using Williamson ether synthesis mediated by
anhydrous FeSO4.26
Monomers 2e and 2f were synthesized from compound 8 as shown
in Scheme 3. Williamson ether synthesis with compound 8 and (S)-1-
bromo-2-methylbutane successfully proceeded in the presence of
KOH in dimethylsulfoxide (DMSO). It should be noted that this
reaction was unsuccessful under other conditions using NaH/
dimethylformamide (DMF) or K2CO3/acetone. Hydroxy groups of
8 were protected by methyl groups. The dimethoxy compound was
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and spectral data for the new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
suginome@sbchem.kyoto-u.ac.jp
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the Japan Science and Technology
Corporation (CREST, “Development of High-performance
Nanostructures for Process Integration” Area) and in part by a
Grant-in-Aid for Scientific Research from Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
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