Chiral amplification in the transcription of supramolecular helicity into a polymer backbone

Article abstract of DOI:10.1002/anie.200462347

(Figure Presented) A chiral supramolecular "sergeant" as the structure-directing agent affects the backbone stereochemistry during polymerization of the achiral "soldier" monomers. The polymer obtained shows preferred helical folding despite its mixed mic

Full text of DOI:10.1002/anie.200462347

Angewandte
Chemie
Helical Structures
Chiral Amplification in the Transcription of
Supramolecular Helicity into a Polymer
Backbone**
Andrew J. Wilson, Mitsutoshi Masuda,
Rint P. Sijbesma,* and E. W. Meijer*
Dedicated to Professor Roeland Nolte
on the occasion of his 60th birthday
Herein, we describe the expression of chirality into a polymer
backbone during the photopolymerization of achiral mono-
mers. The monomers are brought into a chiral self-organized
supramolecular structure by using an enantiomerically pure
structure-directing agent. The polymer obtained, despite
incomplete tacticity, holds enough chiral information to fold
itself into an almost homochiral, helical structure. In reporting
this system we outline the first example of a chiral supra-
[
1]
molecular “sergeant” that affects backbone stereochemistry
during a polymerization process by controlling the intrinsic
helicity of self-assembled achiral monomers. It implies that
[
2–5]
self-organization
can play a critical role in amplifying the
enantiomeric purity of the monomeric constituents of poly-
[
6]
mers.
Cooperative expression and amplification of chirality
present in the monomeric components of biopolymers is
central to biological function, particularly with respect to
folding and hierarchical self-assembly of large functional
nanoscale ensembles. For synthetic chiral polymers, asym-
metric synthesis of the polymer backbone with chiral mono-
[
7,8]
mers, catalysts, initiators, and solvents is well established.
However, in investigating why biopolymers are composed of
enantiomerically pure monomers, it is useful to consider
[
9,10]
mechanisms
through which small unbalances in enantio-
[
11]
[12]
meric excess are amplified. In synthetic polymers with-
out stereocenters, the latent helical conformation of polyiso-
[
13]
[14,15]
[16]
cyanates,
polyisocyanides,
and polysilanes
can be
expressed by using small quantities of chiral monomer, chiral
[
++]
[+]
[
*] Dr. A. J. Wilson, Dr. M. Masuda, Dr. R. P. Sijbesma,
Prof. Dr. E. W. Meijer
Laboratory of Macromolecular and Organic Chemistry
Eindhoven University of Technology
PO Box 513, 5600 MB, Eindhoven (The Netherlands)
Fax: (+31)402-474-706
E-mail: r.p.sijbesma@tue.nl
e.w.meijer@tue.nl
+
[
] Present address: National Institute of Advanced Industrial Science
and Technology (AIST)
Tsukuba (Japan)
+
+
[
] Present address:
Department of Chemistry, University of Leeds (UK)
[
**] We acknowledge Dr. Jef Vekemans for stimulating discussions and
the Council for Chemical Sciences of the Netherlands Organization
for Scientific Research for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 2275 –2279
DOI: 10.1002/anie.200462347
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2275

Communications
catalysts, and/or chiral solvents during
polymerization. Similarly, noncovalent
interaction of chiral molecules with the
backbone of an achiral polymer can bias
and express a preferred helical confor-
[
17]
mation, while subsequent exchange for
achiral guests results in a chiral memory
[
18]
effect. Wholly self-organizing systems
take advantage of cooperative noncova-
lent interactions to elicit so-called supra-
[
1,19]
molecular induction of chirality
and
[
20]
supramolecular memory of chirality.
However, the kinetic lability of self-
assembled architectures renders them
subject to environmental stress. Recently,
Feringa and co-workers used labile supra-
molecular chirality to enhance signifi-
cantly the asymmetric induction in a
Figure 1. Representation of regular conformations of polymerizable self-assembled columns
[
24]
of 1, based on the crystal structure of a benzene tricarboxamide. Octyl side chains have
been omitted for clarity. a) “Zigzag” conformation of sorbyl-containing side chains to give an
achiral polysorbyl backbone. b) Helical conformation of sorbyl-containing side chains to give a
chiral polysorbyl backbone.
[
21]
photochemical ring closure.
We propose herein the use of cova-
[
22]
lent fixation after self-assembly
as a
[
1,24–28]
means of obtaining kinetically robust chiral nanoscale archi-
tectures, as an extension of our previously described self-
assembly and polymerization of achiral discotic 1 in apolar
the axis of the column
with equal probabilities of left-
and right-handed helices in the absence of further sources of
chirality. This proposal for the structure of 1 is based on the
single-crystal structure of another substituted benzene-1,3,5-
[
23]
solvents (Scheme 1 and Figure 1).
By introducing an
[
24]
enantiomerically pure structure-directing agent, we found
one handedness of the helix only. After polymerization and
removal of the structure-directing agent, the polymer
obtained from achiral monomers is chiral and folds into a
preferred helical superstructure. Surprisingly, the folding
process is more directed by the supramolecular interactions
than by the tacticity of the polymer backbone.
Self-assembly of 1 in cyclohexane (10 –10 m) occurs
through cooperative hydrogen bonding, aromatic stacking,
and van der Waals interactions as proposed in Figure 1. The
proposed mode of assembly results in the cooperative
formation of a triple helical seam of hydrogen bonds down
tricarboxamide.
effect of the supramolecular sergeants-and-soldiers experi-
It is therefore important to explore the
[
1,25]
ment during polymerization because it results in the
formation of two new chiral centers per discotic 1. As has
been observed in several other systems,
[
1,25,27,28]
in the absence
of any chiral additives equal quantities of left- and right-
handed helices composed of discotic 1 are present in solution
(0.8 ꢀ 10 m, cyclohexane). However, in the presence of chiral
À5
À2
À3
discotic 2a a large negative circular dichroism (CD) effect is
observed (Figure 2). The CD effect is induced by a supra-
molecular sergeant-and-soldiers effect as the spectrum
reflects the absorption spectrum of monomer 1, while its
Scheme 1. Self-assembly and polymerization of discotic 1.
2
276
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Chemie
or 2a, nor products thereof are detected. Importantly, the
presence of varying amounts of 2a does not significantly
affect these parameters through radical C–H abstraction
mechanisms, that is, the sergeant does not act as a chain
stopper. (Table 1)
Table 1: Conversion, degree of polymerization (DP), and polydispersity
index (PDI) of photoinitiated polymerization of 1 in the presence of
varying amounts of sergeant 2a.
2
a [%]
Conversion [%]
DP
PDI
0
58
51
50
56
47
50
50
45
58
110
103
130
63
100
54
77
1.76
2.1
1.53
1.68
1.2
1.7
1.57
1.6
0
0
0
0
5
0
0
1
1
1
2
2
Induced chirality was studied by CD spectroscopy on 1a
after careful removal of unconverted monomer and sergeant
2
a by exhaustive Soxhlet extraction with diethyl ether.
C NMR spectroscopy of a methanolyzed sample demon-
1
3
strated the absence of any sergeant (incorporation at a level
below 0.3 molecules of 2a per polymer chain, see ESI). CD
spectra were recorded in C H /CHCl /CH OH (95:4.5:0.5), a
6
12
3
3
Figure 2. a) CD spectra of monomer 1 in the presence of different
amounts of sergeant 2a and b) anisotropy factor of monomer 1 as a
solvent combination in which the polymer has sufficient
solubility and in which monomer and sergeant 2a do not form
columnar stacks as outlined above. Remarkably, the polymers
still display optical activity in the CD spectrum (Figure 3)
despite the complete removal of chiral sergeant 2a from the
product. The CD spectra were concentration independent
À4
function of added sergeant 2a. [a] [1]=8.1ꢀ10 m, cyclohexane,
À4
0
.1 cm pathlength. [b] [1]=8.1ꢀ10 m, 95:4.5:0.5 C H /CHCl /
6
12
3
CH OH, 0.1 cm path length.
3
À3
À4
magnitude increases nonlinearly with the quantity of added
sergeant” 2a (leveling off at around 10%). The effect is
completely lost upon increasing the solvent polarity (C H /
between 10 and 10 m (see ESI) which suggests that the
signal is a consequence of intramolecular organization within
molecularly dissolved polymers. If the variation in the
magnitude of the CD spectrum is plotted against the fraction
of sergeant 2a present during the polymerization, almost
complete induction of helicity is observed at around 10% of
added sergeant 2a (once again the use of sergeant 2b
produces an opposite response of equal intensity, see ESI).
The shape of the curve matches that observed during the self-
assembling sergeants-and-soldiers experiment and the aniso-
tropy value is of similar magnitude, suggesting that the helical
bias present before polymerization is almost completely
(>80%) retained in the polymer. The Cotton effect com-
pletely disappears when the methanol content is increased to
1.5%. Removal of all solvent and redissolution in the original
solvent mixture leads to complete recovery of the original
Cotton effect. The process can be repeated several times
without significant loss of CD signal. The process amounts to
an unfolding–refolding cycle of the polymer and indicates that
the helical bias induced by sergeant 2a in self-assembled
stacks of monomer 1 is locked in the polymer and that the
chiral information is encoded in the stereochemistry of the
sorbyl main chain. The complete sequence of assembly,
fixation, sergeant removal, and reversible unfolding is sum-
marized in Figure 4.
“
6
12
CHCl /CH OH 95:4.5:0.5). Gratifyingly, the use of a mirror
3
3
image “sergeant” 2b with opposite chirality results in an
induced CD signal of opposite and similar magnitude (see
ESI).
We next investigated whether the chirality induced by
sergeant 2a was retained upon polymerization of monomer 1
in the presence of varying amounts of sergeant 2a. As
[
23]
reported previously by us, 1,4-polymerization of the sorbyl
moiety in 1 provides sufficient distance to bridge stacked
aromatics. Photoinitiated polymerization (365 nm) of the self-
À2
assembled stacks (1 ꢀ 10 m solution in cyclohexane) of 1, in
the presence of 2,2-dimethoxyphenylacetophenone as initia-
tor, furnishes columnar polymers 1a. Full analysis of the
polymers requires careful removal of monomer 1 and
structure-directing agent 2a by Soxhlet extraction followed
[
29]
by methanolysis of the polymer formed to detach most of
the trimesic amide moieties from the polysorbyl backbone.
The highly soluble and non-aggregating polymethyl sorbate
1
13
1
b can be analyzed by H NMR, C NMR, and size-exclusion
chromatography (SEC). The latter analysis shows that the
photopolymerization results in polymers with DPs of 50–130
and polydispersities of ꢀ 1.7, while no signals of remaining 1
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Figure 3. CD spectroscopy of polymer 1a made in the presence and absence of sergeant 2a (0.1 cm path length, 95:4.5:0.5 C H /CHCl /
6
12
3
À4
CH OH). a) CD spectra of polymer 1a (9.5ꢀ10 m). b) Variation in the anisotropy factor g of the polymer 1a as a function of sergeant 2a present
3
during polymerization. c) Variation in the anisotropy factor g as a function of added methanol (polymer 1a with 9.4% sergeant 2a present during
the polymerization). d) Folding and unfolding behavior (as shown by g value) upon addition and removal of methanol
(
polymer 1b made with 15% sergeant 2a present during the polymerization).
Deeper understanding of the structural
details responsible for the chiral amplification
effect required a more detailed analysis of the
polymer backbone. This was achieved with
transesterified polymer 1b, which has higher
mobility relative to 1a resulting in higher-
1
3
1
1
1
quality C- and H-correlated spectra. H, H
COSY of 1b reveals the interconnection along
the backbone and is entirely consistent with a
1
,4-trans polymerization (confirmed with
C NMR spectroscopy) in which both erthyro
1
3
and threo stereocoupled centers are
[
30,31]
formed.
The addition of sergeant 2a or
2
b to a solution of the self-assembled stack
prior to polymerization has very little effect on
the NMR spectrum of the polymer. However,
the unfolding–refolding experiments show that
the helical bias is strongly retained in the
backbone of the polymer. This implies that
despite the fact that the polymer has a mixed
microstructure, at least part of the polysorbyl
backbone has been formed with asymmetric
preference. It is highly surprising that the
mixed microstructure results in a very high
stereoselectivity in the folding process. It can
Figure 4. Sequence of events leading to locking of supramolecular chirality into columnar
self-assemblies of 1.
2
278
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Chemie
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formed in a perfect helical assembly and that the backbone,
despite its mixed microstructure, fits perfectly into the
refolded helical polymer. Notably, order in polymer back-
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1
Keywords: helical structures · polymerization · polymers ·
self-assembly · supramolecular chemistry
.
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