Non-hydrogen-bonding-based, solvent-dependent helix inversion between pure P-helix and pure M-helix in poly(quinoxaline-2,3-diyl)s bearing chiral side chains

Article abstract of DOI:10.1039/c001564d

Poly(quinoxaline-2,3-diyl)s bearing chiral (R)-2-butoxy side chains adopt pure right- or left-handed screw senses in CHCl₃ and 1,1,2-trichloroethane, respectively.

Full text of DOI:10.1039/c001564d

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COMMUNICATION
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Non-hydrogen-bonding-based, solvent-dependent helix inversion between
pure P-helix and pure M-helix in poly(quinoxaline-2,3-diyl)s bearing
chiral side chainsw
Tetsuya Yamada,a Yuuya Nagataa and Michinori Suginome*ab
Received 26th January 2010, Accepted 11th May 2010
First published as an Advance Article on the web 1st June 2010
DOI: 10.1039/c001564d
Poly(quinoxaline-2,3-diyl)s bearing chiral (R)-2-butoxy side
chains adopt pure right- or left-handed screw senses in CHCl3
and 1,1,2-trichloroethane, respectively.
A series of random copolymers of achiral 1 and (S)- or (R)-2,
which has (S)- or (R)-2-butoxymethyl groups as side chains,
respectively, was prepared with an achiral organonickel initiator
with various ratios of 1 and 2 (Scheme 1, Table 1).10 In addition
to those copolymers consisting of 40 monomer units in total,
homopolymers, poly-2, with higher polymerization degrees
(n = 100 and 200) were prepared in a similar way. All the
polymers obtained in these experiments are highly soluble in
common organic solvents such as hexane, toluene, THF, ether
and CHCl3. The averaged molecular weights as well as PDI
(Mw/Mn) were almost constant for all the 40 mers consisting of
various ratios of 1 and 2, suggesting that the reactivities of 1 and
2 are almost equal under the polymerization conditions.
Efficient control of the screw sense of helical macromolecules has
gained much attention during the search for their novel macro-
molecular functions.1 Particular attention has been paid to helical
polymers with no unracemizable stereogenic centers or axes in
the polymer main chain, because they are virtually inter-
convertible between the two mirror image helical senses. Their
nonracemic helical structures are typically induced by chiral
groups attached as side chains or as terminal groups. Recent
interest has focused on the reversible control of the helical sense
of such polymers by external, symmetric stimuli such as heat,2
light,3 solvent,4,5 and metal ions.6 It should be noted that most
reported macromolecular right–left (P–M) transitions of the
helical sense do not concern the degree of screw-sense induction,
although it must be the most important measure for the appli-
cability and controllability of chiral macromolecular systems.
Poly(quinoxaline-2,3-diyl)s are a unique class of helical
polymers, being prepared via living polymerization of
o-diisocyanobenzenes.7 We have succeeded in the screw-sense
selective synthesis of the polymers by introducing a chiral
terminal group, which is derived from the chiral initiators.8
Our recent report has demonstrated that poly(quinoxaline-2,3-
diyl)s bearing a metal-binding phosphorus group in the middle of
the polymer backbone induce enantioselectivity up to 87% ee in
the palladium-catalyzed asymmetric hydrosilylation of styrenes.9
In this Communication, we report the synthesis of a series of
poly(quinoxaline-2,3-diyl)s bearing chiral side chains devoid of a
chiral terminal group. We observed a nonlinear relationship
between the calculated s.e. (screw-sense excess) and the number
of chiral units in the polymer backbone, which can be
rationalized by a linear relationship in terms of free energy. We
Scheme 1 Random living copolymerization of achiral monomer 1
and enantiopure chiral monomer (R)- or (S)-2.
Table 1 Properties of homopolymers (poly-1 and poly-(R)-2) and
random copolymers poly-1/(R)-2a
Polymer
1/Nib 2/Nic Mn/103d Mw/Mn gabs/10À3e
found
a unique solvent-dependent helical-sense induction,
Poly-1
Poly-1/2 (95/5)
40
38
0
2
4
8
12
20
28
40
100
200
6.8
7.5
7.7
7.7
8.2
7.9
7.9
7.9
28
1.08
1.07
1.07
1.08
1.08
1.08
1.09
1.07
1.06
1.04
—
leading to formation of pure right-handed helix in chloroform
and of pure left-handed helix in 1,1,2-trichloroethane.
0.56
1.07
1.76
2.13
2.38
2.34
2.36
2.32
2.46
Poly-1/2 (90/10) 36
Poly-1/2 (80/20) 32
Poly-1/2 (70/30) 28
Poly-1/2 (50/50) 20
Poly-1/2 (30/70) 12
Poly-2
Poly-2 (100)
Poly-2 (200)
a Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan.
0
0
0
E-mail: suginome@sbchem.kyoto-u.ac.jp; Fax: +81-75-383-2722;
Tel: +81-75-383-2723
58
b JST, CREST (Creation of Next-Generation Nanosystems through
Process Integration), Kyoto University Katsura, Nishikyo-ku,
Kyoto 615-8510, Japan
a
Data for poly-1/(S)-2 are shown in the supplementary information.
c
Molar ratio of 1 and the initiator. Molar ratio of 2 and the
b
d
e
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for new compounds. See DOI:
10.1039/c001564d
initiator. Polystyrene standard. Dissymmetry factor (De/e) per
monomer unit at 366 nm.
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Fig. 1 Plots of screw-sense excesses (curve) and energy difference
(straight line) in chloroform against mean number of chiral (R)- or (S)-
2 units in poly-1, poly1/2, and poly-2.
Fig. 3 Plots of screw-sense excesses (curve) and energy difference
(straight line) in 1,1,2-trichloroethane against mean number of chiral
(R)- or (S)-2 units in poly-1, poly1/2, and poly-2.
As a measure of the degree of single-handed screw sense, CD
spectra were measured in CHCl3. All polymers containing (R)-2
showed typical spectra for the P-helical poly(quinoxaline-2,3-
diyl)s with varying intensities, which are represented by
dissymmetry factors (De/e) per monomer unit, i.e., gabs
(Table 1).11 The nonlinear relationship between gabs and mean
numbers of the chiral units in 40 mers has been converged into a
hyperbolic tangent curve, when 100% pure P-helix is assumed to
have gabs of 2.39 Â 10À3 (the dotted curve in Fig. 1).12,13 This
assumption leads to the establishment of a linear relationship
between the energy difference between P and M helices and the
mean numbers of chiral units (Fig. 1, the straight line). From the
linear relationship, we conclude that poly(quinoxaline-2,3-diyl)s
are in an equilibrium between P- and M-helices, of which the
energy difference between the two screw senses is proportional to
the number of chiral units in the polymer chain. Moreover, it also
leads us to the conclusion that each polymer chain has no
jointing of P- and M-helices. It is assumed that each chiral
monomer unit gains 0.59 kJ molÀ1 for the P-helix in CHCl3.
We observed an intriguing dependence of the CD intensities of
poly-(R)-2 (n = 40) on solvent (Fig. 2). In particular, use of 1,1,2-
trichloroethane (1,1,2-TCE) resulted in a complete switch of its
screw sense to M (the red curve in Fig. 2). The CD spectra
continuously changed with the change of the 1,1,2-TCE/CHCl3
ratio. In a 60 : 40 mixture of 1,1,2-TCE and CHCl3, no Cotton
effect was observed, indicating that the left- and right-handed
helical macromolecules coexist almost in a 1 : 1 ratio. The helix
inversion could be repeated several times with no drop of s.e. by
changing the solvent through evaporation in vacuo.14 The plot of
DG against the content of chiral monomer units also shows their
linear relationship, from which the gained energy difference per
(R)- or (S)-2 unit in 1,1,2-TCE was determined to be 0.35 kJ molÀ1
for the M-helix (Fig. 3). Although the magnitude of this value was
considerably smaller than that in CHCl3 (0.59 kJ molÀ1),
accumulation of 40 monomer units was sufficient to form an
Fig. 4 Estimated screw-sense excesses of poly-(R)-2 in various solvents.
The screw-sense excesses are estimated by comparison of the g values
obtained in each solvent with the maximum g value (2.39 Â 10À3) in CHCl3.
almost perfect M-helix. CD measurements for poly-(R)-2 (40 mer)
in various solvents revealed that the P-helical conformation was
strongly preferred in CHCl3, CH2Cl2, THF, and 1-butanol, while
moderate P-helix induction was observed in 1,1,1-TCE, 1-chloro-
butane, and 1-bromobutane (Fig. 4). In valeronitrile (BuCN), 1,2-
dichloroethane (1,2-DCE), and 1,3-dichloropropane (1,3-DCP),
moderate M-helix induction was observed.
Another interesting feature is the dependence of the CD
intensity on polymerization degree (Fig. 4). Thus, poly-(R)-2
(40 mer) showed ca. 50% s.e. in 1-chlorobutane, suggesting each
monomer unit favors the P-helix over the M-helix by just
0.06 kJ molÀ1. The corresponding 100 mer (the light blue bar)
and 200 mer (the dark blue bar) resulted in the formation of P-helix
with 75% and >95% s.e, respectively. This significant increase in
the screw-sense excess clearly relies on the long helix persistent
length of poly(quinoxaline-2,3-diyl)s. Although a solvent-dependent
switch of screw sense has been reported for several chiral polymer
systems,4 most of them rely on hydrogen bonding at the polymer
side chains containing amide groups. As far as we are aware, there
is only one precedent for non-hydrogen-bonding-based switch of
helical chirality by achiral solvents, in which cis-polyacetylene
bearing chiral 2-methylbutoxy groups was measured in CHCl3
and hexane.4d In that system, however, no s.e. was determined and
helix induction in CHCl3 was much less than that in hexane.
Although the origin of the change of helical chirality in poly-2
is not clear at this moment, the phenomenon is not limited to the
polyquinoxalines bearing 2-butoxymethyl side chains. The corres-
ponding polyquinoxaline poly-3 bearing 3-methylpentyl side
chains, in which the oxygen atom of the 2-butoxymethyl group
is replaced with a methylene group, showed a similar solvent
dependent switch of helical chirality. Thus, poly-(S)-3, whose side
chains have virtually the same chirality as those in poly-(S)-2,
took (M)- or (P)-helical structure in CHCl3 and 1,1,2-TCE,
respectively (Fig. 5). Although the degree of helix sense induction
in each solvent has not been determined yet, this result suggests
Fig. 2 CD spectra of poly-(R)-2 in 1,1,2-trichloroethane/CHCl3 with
varied ratios.
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In summary, we have established that poly(quinoxaline-2,3-
diyl)s bearing chiral (R)-2-butoxy side chains adopt pure
right- and left-handed screw senses in CHCl3 and 1,1,2-trichloro-
ethane, respectively. The degrees of helix-sense induction were
determined by analyses of free energy differences of copolymers
consisting of achiral and chiral monomer units with varying
ratios. We assume that the helix switch may depend on solvent-
induced conformational changes of the chiral side chains.
Although each chiral group may gain a very small energy for
one helix, its accumulation in the helical polymer backbones can
cause a large macromolecular conformational change.12 Studies
for establishing the origin of the conformational change are
currently being undertaken in this laboratory.
Fig. 5 CD spectra of poly-(S)-3 in 1,1,2-trichloroethane and CHCl3.
that the solvent-dependent helix inversion does not rely crucially
on the presence of an oxygen atom in the side chain.
This work is supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Notes and references
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It seems to be interesting to discuss the cause of the solvent
dependent helical sense inversion. Fig. 6 shows four representative
conformers A–D of helical poly(quinoxaline-2,3-diyl) poly-(R)-2
bearing chiral (R)-2-butoxy group, of which only 5 quinoxaline
units are shown. The (R)-2-butoxy groups except for those on the
central quinoxaline ring are omitted for clarity. The anti orienta-
tion of the two (R)-2-butoxy groups should be preferred for the
steric reason. Steric repulsion between the (R)-2-butoxy group
and the quinoxaline ring in conformers B and C may make these
conformations unfavorable, resulting in the preferential forma-
tion of conformers A and D. We assume that, for any reason, the
conformer A is favorable in chloroform, and conformation D is
favored in 1,1,2-TCE, although the cause of the solvent-
dependent conformational change of the chiral groups should
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chiral alkyl group, which lacks an oxygen atom.
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13 See the Supplementary Information.
Fig. 6 Possible conformations of poly-(R)-2. Only five quinoxaline
units, of which chiral side chains except for those on the central
quinoxaline ring are omitted for clarity, are shown.
14 When solid poly-(R)-2 was obtained by evaporation of CHCl3 and
dissolved in 1,1,2-TCE at rt, gradual P-to-M helical inversion was
observed in the CD spectra. It takes a few hours to complete.
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This journal is The Royal Society of Chemistry 2010
4916 | Chem. Commun., 2010, 46, 4914–4916