Helix-sense controlled polymerization of a single phenyl isocyanide enantiomer leading to diastereomeric helical polyisocyanides with opposite helix-sense and cholesteric liquid crystals with opposite twist-sense

Article abstract of DOI:10.1021/ja0576536

We report the unprecedented helix-sense controlled polymerization of enantiomerically pure phenyl isocyanides bearing an l- or d-alanine pendant with a long alkyl chain. The polymerization with an achiral nickel catalyst diastereoselectively proceeds, res

Full text of DOI:10.1021/ja0576536

Published on Web 12/27/2005
Helix-Sense Controlled Polymerization of a Single Phenyl Isocyanide
Enantiomer Leading to Diastereomeric Helical Polyisocyanides with Opposite
Helix-Sense and Cholesteric Liquid Crystals with Opposite Twist-Sense
Takashi Kajitani,† Kento Okoshi,*,† Shin-ichiro Sakurai,† Jiro Kumaki,† and Eiji Yashima*,†,‡
Yashima Super-structured Helix Project, Exploratory Research for AdVanced Technology (ERATO), Japan Science
and Technology Agency (JST), 101 Creation Core Nagoya, 2266-22 Anagahora, Shimoshidami, Moriyama-ku,
Nagoya 463-0003, Japan, and Graduate School of Engineering, Nagoya UniVersity, Nagoya 464-8603, Japan
Received November 10, 2005; E-mail: kokoshi@yp-jst.jp; yashima@apchem.nagoya-u.ac.jp
Optically active polymers with a controlled helix-sense have
recently attracted considerable interest not only for mimicking
biological helices, but also for developing novel chiral materials.1
Fully synthetic helical polymers have been prepared either by the
polymerization of optically active monomers, such as isocyanates,1a
silanes,1b acetylenes,1c or by the helix-sense selective polymerization
of achiral methacrylates,1d isocyanides,2 and carbodiimides3 with
bulky substituents by chiral catalysts or initiators. The former helical
polymers are dynamic in nature, and the population of the
interconverting right- and left-handed helices can be switched under
certain conditions,1a-c while the helical conformations of the latter
Figure 1. Structures of L-1 and L-2 and schematic illustration of
polymers are stable (static) in solution and their helical senses are
diastereomeric helical polyisocyanides produced by the helix-sense con-
kinetically determined during the polymerization.1d,2-4 We now
trolled polymerization of L-1. The helix-sense can be controlled by the
solvent polarity and temperature during the polymerization, resulting in the
report a unique helix-sense controlled polymerization of an enan-
tiomerically pure isocyanide, which produces both static diaster-
eomeric right- and left-handed helical polyisocyanides whose helical
sense can be controlled by the polymerization solvent and temper-
ature (Figure 1). Moreover, the resulting helical polyisocyanides
with the opposite helix-sense form lyotropic, cholesteric liquid
crystals (LCs) with opposite twist-senses, respectively. Similar
amino acid- or peptide-bound helical polyisocyanides have been
extensively studied,2,5 but one of the helices could be produced
from the polymerization of an isocyanide enantiomer. Accordingly,
the opposite helical polyisocyanide requires the polymerization of
the opposite enantiomer.
formation of static diastereomeric helical polyisocyanides, while L-2 yields
only the left-handed helical poly-L-2 independent of the polymerization
conditions. The helix-senses of the diastereomeric poly-L-1s were determined
by their AFM measurements (see text).
We synthesized novel enantiomerically pure phenyl isocyanides
bearing L- or D-alanine residues with a long alkyl chain as the
pendants through an amide linkage (for L-1, see Figure 1), and these
were then polymerized with an achiral nickel catalyst (NiCl2) in
various solvents at different temperatures.6 Figure 2A shows the
CD spectra of the poly-L-1s measured in CHCl3 at 25 °C.
Interestingly, the first Cotton effect signs that reflect the helix-
sense of the polymer backbone, such as CCl4 (poly-L-1a) and
toluene (poly-L-1b) obtained by the polymerization at ambient
temperature in nonpolar solvents, were opposite to that polymerized
in polar THF (poly-L-1c). This suggests that the helix-senses of
poly-L-1a and poly-L-1c may be opposite to each other, regardless
of the fact that the polymers have the same L-alanine pendants.
Because the right- and left-handed helices of poly-L-1s are
diastereomers, therefore their CD spectra, particularly in the pendant
aromatic regions (235-280 nm), differ from one another.7
Although the first Cotton effect intensity at around 360 nm
(∆ꢀfirst) of the as-prepared poly-L-1c was very weak (-1.3), it slowly
but dramatically increased with time at high temperature and
reached ∆ꢀfirst ) -8.5 after the polymer annealed in toluene at
100 °C for 12 days (Figure S1).8,9 Under the same conditions, the
poly-L-1a showed a decrease in the CD intensity of ca. 31% (Figure
S1). These results indicate that the helical conformation of the as-
Figure 2. (A) CD spectra of poly-L-1a (a, blue line), poly-L-1b (b, red
solid line), and poly-L-1c (c, green line) polymerized in CCl4, toluene, and
THF, respectively, at ambient temperature; poly-L-1d (d, red dotted line)
polymerized in toluene at 100 °C; and poly-L-1e (e, black line) after poly-
L-1d annealed in toluene at 100 °C for 6 days. The absorption spectrum of
poly-L-1a (f) is also shown. The CD and absorption spectra were measured
in CHCl3 at 25 °C (0.2 mg/mL). (B-D) Polarized optical micrographs of
cholesteric LC phases of poly-L-1a (B), a 1:1 mixture of poly-L-1a and
poly-L-1e (C), and poly-L-1e (D) in CHCl3 (15 wt %).
prepared poly-L-1c with a slight excess of one helical sense is not
stable and changed into a thermodynamically more stable, excess
single-handed helix upon heating, while the as-prepared poly-L-1a
may adopt a rather stable, kinetically controlled helical conformation
whose helix inversion barrier seems to be sufficiently high to
maintain the predominant helical sense. If this is the case, the
polymerization of L-1 in nonpolar solvents at high temperature will
yield helical polymers with handedness identical to that of the poly-
L-1c. In fact, poly-L-1 obtained by the polymerization of L-1 in
toluene at 100 °C (poly-L-1d) exhibited the opposite negative ∆ꢀfirst
value (-4.6) to that of poly-L-1b, and the CD intensity further
increased in the negative direction after the polymer annealed in
toluene at 100 °C for 6 days (poly-L-1e, ∆ꢀfirst ) -11.0; Figure
† ERATO, JST.
‡ Nagoya University.
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10.1021/ja0576536 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
such as LCs and chiral selectors.1 Further studies of the mechanism
of the helix-sense controlled polymerization and the three-
dimensional solid-state structures of the diastereomeric helical
polyisocyanides by X-ray diffraction12 will be the object of a future
investigation.
Supporting Information Available: Full experimental details. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 3. AFM phase images of self-assembled poly-L-1e (A) and poly-
L-1a (B) on HOPG. Scale ) 10 × 20 nm. Schematic representations of the
left-handed helical poly-L-1e (left) and right-handed helical poly-L-1a (right).
2D helix-bundles with periodic oblique pendant arrangements (blue and
pink lines, respectively) are also shown.
(1) (a) Green, M. M.; Park, J.-W.; Sato, T.; Teramoto, A.; Lifson, S.; Selinger,
R. L. B.; Selinger, J. V. Angew. Chem., Int. Ed. 1999, 38, 3138-3154.
(b) Fujiki, M. Macromol. Rapid Commun. 2001, 22, 539-563. (c)
Yashima, E.; Maeda, K.; Nishimura, T. Chem.sEur. J. 2004, 10, 42-
51. (d) Nakano, T.; Okamoto, Y. Chem. ReV. 2001, 101, 4013-4038.
(2) (a) Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk,
N. A. J. M. Chem. Rev. 2001, 101, 4039-4070. (b) Suginome, M.; Ito,
Y. AdV. Polym. Sci. 2004, 17, 77-136.
2A) (the specific rotation ([R]25D) also increased from -410 to
-814° (Table S1)).10
(3) Tian, G.; Lu, Y.; Novak, B. M. J. Am. Chem. Soc. 2004, 126, 4082-
We assume that these unusual helix-sense controlled polymeriza-
tions of L-1 depend on the solvent polarity and temperature and
that the change in the CD intensity upon heating may be governed
by the “on-off” fashion of the intermolecular hydrogen bonds
between the pendant amide residues of the growing chain end and
L-1 during the propagation reaction, which may force the poly-L-1
into either a right- or left-handed helix. In polar solvents and even
in nonpolar solvents at high temperature, such hydrogen bonding
will be hampered, resulting in the thermodynamically favorable
helical conformation (negative ∆ꢀfirst value).11 This speculation was
supported by the fact that poly-L-2 (Figure 1), in which the amide
linkage was replaced by the ester one, exhibited negative ∆ꢀfirst
values independent of the polymerization conditions (Figure S3),
and the CD intensity hardly changed after the polymer solution
annealed at 100 °C for 12 days (Figure S1).
Additional strong evidence of diastereomeric helical structures
of the poly-L-1s showing the opposite ∆ꢀfirst sign was obtained from
the polarized optical microscopy studies. Poly-L-1a (∆ꢀfirst ) +8.1)
and poly-L-1e (∆ꢀfirst ) -11.0) formed a lyotropic, cholesteric LC
in concentrated CHCl3 solutions due to their main-chain stiffness,
thus showing a fingerprint texture (Figure 2B and 2D, respectively);
the spacings of the fringes corresponding to the half pitch of the
cholesteric helical structure were 6.4 and 5.9 µm, respectively. A
1:1 mixture of the cholesteric solutions produced a significant
expansion of the spacing (18.9 µm; Figure 2C). This transformation
clearly demonstrated that the poly-L-1s showing the opposite ∆ꢀfirst
sign indeed have helical senses opposite from each other.
4083.
(4) There is a class of static helical polymethacrylates and polycarbodiimides
bearing optically active pendants whose helical conformations are ir-
reversibly inverted after polymerization by temperature due to a change
in the polymer chains from kinetically controlled to thermodynamically
controlled helical conformations. See ref 3 and Okamoto, Y.; Yashima,
E.; Hatada, K. J. Polym. Sci., Part C: Polym. Lett. 1987, 25, 297-301.
(5) Cornelissen, J. J. L. M.; Donners, J. J. J. M.; de Gelder, R.; Graswinckel,
W. S.; Metselaar, G. A.; Rowan, A. E.; Sommerdijk, N. A. J. M.; Nolte,
R. J. M. Science 2001, 293, 676-680.
(6) The molecular weight (Mn) and its distribution (Mw/Mn) in polymerized
CCl4 (poly-L-1a) and tetrahydrofuran (THF) (poly-L-1c) were 8.4 × 104
and 1.4, and 28 × 104 and 2.2, respectively (Table S1).8
(7) Similar diastereomeric helical polyisocyanides were prepared by the
copolymerization between achiral and chiral isocyanides or two different
chiral isocyanides based on the selective inhibition of the growth of a
one screw-sense. (a) Kamer, P. C. J.; Cleiji, M. C.; Nolte, R. J. M.; Harada,
T.; Hezemans, A. M. F.; Drenth, W. J. Am. Chem. Soc. 1988, 110, 1581-
1587. (b) Amabilino, D. B.; Ramos, E.; Serrano, J.-L.; Veciana, J. AdV.
Mater. 1998, 10, 1001-1005. For other helical polyisocyanides, see ref
2 and (c) Nolte, R. J. M. Chem. Soc. ReV. 1994, 11-19. (d) Amabilino,
D. B.; Ramos, E.; Serrano, J.-L.; Sierra, T.; Veciana, J. J. Am. Chem.
Soc. 1998, 120, 9126-9134.
(8) For more details, see Supporting Information.
(9) A similar increase in the CD intensity upon heating was recently reported
for the poly(aryl isocyanide)s with chiral pendants. Takei, F.; Onitsuka,
K.; Takahashi, S. Macromolecules 2005, 38, 1513-1516.
(10) The observed remarkable changes in the CD intensity may be accompanied
by configurational isomerization around the CdN double bonds (syn-
anti isomerization) of the polymer backbone. However, the 1H and 13C
NMR of the poly-L-1s were too broad to explore the isomerization before
and after annealing. Ishikawa, M.; Maeda, K.; Mitsutsuji, Y.; Yashima,
E. J. Am. Chem. Soc. 2004, 126, 732-733. When D-1 was instead
polymerized under the same various conditions as for L-1, poly-D-1s with
macromolecular helicities opposite to those of the poly-L-1s were obtained,
which further underwent similar irreversible changes in their ∆ꢀfirst values
at high temperatures (Table S1 and Figure S2).
Finally, the helical structures of the diastereomeric helical
polyisocyanides were investigated by atomic force microscopy
(AFM). Figure 3 shows typical high-resolution AFM images of
poly-L-1e (A) and poly-L-1a (B) spin cast from dilute toluene
solutions (0.2 mg/mL) on highly oriented pyrolytic graphite
(HOPG), followed by benzene vapor exposure at ca. 20 °C for 12
h.12 The polymers self-assembled into well-defined (A) and slightly
irregular (B) 2D helix bundles. The bundle structures of poly-L-1e
are clearly resolved into individual polymer chains packed parallel
to each other, in which the left-handed helices with a helical pitch
of 1.3 nm are predominant (Figure 3A). However, the poly-L-1a
chains most likely consist of right-handed helical segments with a
helical pitch of 1.25 nm, together with minor, but distinctly left-
handed, helical segments.13 This remarkable pseudo-mirror-image
relationship suggests that poly-L-1a and poly-L-1e have predomi-
nantly right- and left-handed helical structures, respectively.14
Consequently, the macromolecular helicity and mesoscopic,
supramolecular cholesteric twist can be controlled by the molecular
chirality of the pendant of a single enantiomeric phenyl isocyanide
through polymerization under either kinetic or thermodynamic
control, and their helical senses can be determined by direct AFM
observations. The present results not only demonstrate this new
phenomenon but will also provide new chiral materials in areas
(11) The formation of intramolecular hydrogen bonds was clearly proved by
the IR spectra of the poly-L-1’s regardless of the polymerization conditions,
indicating that both helices are stabilized more or less by hydrogen bonds
after the polymerization (Table S2). For the hydrogen-bond assisted helical
polymers, see ref 5 and (a) Nomura, R.; Tabei, J.; Masuda, T. J. Am.
Chem. Soc. 2001, 123, 8430-8431. (b) Li, B. S.; Cheuk, K. K. L.; Ling,
L.; Chen, J.; Xiao, X.; Bai, C.; Tang, B. Z. Macromolecules 2003, 36,
77-85. (c) Okoshi, K.; Sakajiri, K.; Kumaki, J.; Yashima, E. Macromol-
ecules 2005, 38, 4061-4064.
(12) This method is very useful for constructing highly ordered two-dimensional
(2D) helix-bundles with a controlled helicity for helical poly(phenylacety-
lene)s bearing the same alanine-based pendants on HOPG, and their helical
structures were visualized by AFM. Sakurai, S.-i.; Okoshi, K.; Kumaki,
J.; Yashima, E. Angew. Chem., Int. Ed. 2006. In press.
(13) These AFM results imply that the helix-sense excesses of the diastereo-
meric helical polyisocyanides may be determined from the statistical
analysis of a series of such AFM images, and work along this line is now
in progress. For the AFM images of larger areas, see Figure S4. Similar
AFM observations of poly-D-1a (∆ꢀ1st ) -8.0) suggest that the polymer
has a predominantly left-handed helical structure (Figure S5).
(14) This assignment of the helical sense in the poly(phenyl isocyanide)s agrees
with that determined by the exciton-coupled CD method: (a) Takei, F.;
Hayashi, H.; Onitsuka, K.; Kobayashi, N.; Takahashi, S. Angew. Chem.,
Int. Ed. 2001, 40, 4092-4094. For other references of helical-sense
assignments of polyisocyanides, see: (b) van Beijnen, A. J. M.; Nolte,
R. J. M.; Drenth, W.; Hezemans, A. M. F. Tetrahedron 1976, 32, 2017-
2019. (c) Cornelissen, J. J. L. M.; Sommerdijk, N. A. J. M.; Nolte, R. J.
M. Macromol. Chem. Phys. 2002, 203, 1625-1630.
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