Polyguanidines as chiral orienting media for organic compounds

Article abstract of DOI:10.1002/chem.201000940

(Figure Presented) Handy polymers: The lyotropic liquidcrystalline phases of helically chiral polyguanidines are proposed as new chiral orienting media for organic compounds. The enantiomers of isopinocampheol (IPC) are oriented in an enantiodifferentiati

Full text of DOI:10.1002/chem.201000940

DOI: 10.1002/chem.201000940
Polyguanidines as Chiral Orienting Media for Organic Compounds
Lena Arnold, Andreas Marx, Christina M. Thiele, and Michael Reggelin*[a]
The determination of the three-dimensional structure of
organic compounds by NMR spectroscopy usually involves
phous and enantiomers give rise to separate sets of signals.
There is already one very promising first report by Lesot
et al., who were able to determine the absolute configura-
tion of a chiral epoxide based on the comparison of quadru-
polar splittings observed for this compound with those for
compounds of known absolute configuration.[9]
In addition to enantiodiscrimination and compatibility
with organic solvents, the alignment medium should induce
weak orientation, such that dipolar couplings in the Hz
range are observed (“weak alignment”). This is best fulfilled
either by polymer gels[8a–h,10] or by lyotropic liquid crystalline
(LC) systems.[11]
Several aqueous chiral lyotropic LC systems have already
been used as enantiodiscriminating orienting media.[12] The
only known class of chiral orienting media for organic sol-
vents based on lyotropic liquid crystalline (LC) phases are
the LC phases of the homopolypeptides poly-g-benzyl-l/d-
3
the measurement of J coupling constants,[1] NOE values[2]
and to a lesser extent cross-correlated relaxation[3] to obtain
information about dihedral angles, distances and projection
angles, respectively. If the determination of the relative or
even the absolute configuration of stereogenic elements is
the goal of the structural investigation the elucidation of
conformation and configuration superimpose each other en-
tailing the necessity to determine both structural aspects si-
multaneously.[4] Especially in cases of conformational flexi-
bility, these conventional NMR restraints often do not lead
to unambiguous configurational assignments.
It has recently been shown that residual dipolar couplings
(RDCs) can yield information complementary to 3J cou-
plings and NOE parameters[5,6] and allow the determination
of relative configurations even in the presence of (a limited
degree of) motion.[7]
glutamate
(PBLG/PBDG),
poly-g-ethyl-l-glutamate
RDCs belong to the class of anisotropic NMR parameters
and therefore the compound in question needs to be orient-
ed with respect to the magnetic field in order to be able to
observe them. Recently, considerable progress has been
made in the area of orienting media for compounds, which
are not soluble in water.[8] In terms of future applications of
RDCs to determine absolute configurations, it is essential
that the compound in question is not only aligned with re-
spect to the magnetic field, it is furthermore necessary to
induce enantiodifferentiating alignment. This in turn de-
mands the orienting medium to be chiral, non-racemic such
that the interactions with the analyte become diastereomor-
(PELG/PEDG),
and
poly-e-carboxybenzoyl-l/d-lysine
(PCBLL/PCBDL).[13,14] These polymeric lyotropic LC sys-
tems have the additional advantage that, due to their high
molecular weight (MW), their magnetization relaxes fast and
therefore virtually no signal of the orienting medium itself is
detected.
The degree of orientation in these media sometimes hap-
pens to be too large, complicating the extraction of RDCs.
Some of us have shown recently that a significant improve-
ment of spectral quality and reliability of couplings can be
achieved, if high molecular weight (MW) polyglutamates
were used.[8i] With increasing MW the critical concentration
for the phase transition of the LC phase drops, which per-
mits measurements at lower polymer concentrations. This in
turn has beneficial effects on the line widths and reduces
the intensity of residual polymer signals. This dependence,
however, occurs only as long as the length of the polymer
does not exceed its persistence length.[15]
[a] Dipl.-Ing. L. Arnold, Dr. A. Marx, Dr. C. M. Thiele,
Prof. Dr. M. Reggelin
Technische Universitꢀt Darmstadt
It has been shown by the group of Courtieu[14c,d] that the
enantiodiscrimination in these chiral phases is due to a dif-
ferent orientation of the two enantiomers in the liquid crys-
talline phase. Recently, we investigated how different the
orientations for both enantiomers of isopinocampheol (IPC)
Clemens Schçpf Institut fꢁr Organische Chemie und Biochemie
Petersenstrasse 22, 64287 Darmstadt (Germany)
Fax : (+49)6151-165531
E-mail: re@punk.oc.chemie.tu-darmstadt.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000940.
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Chem. Eur. J. 2010, 16, 10342 – 10346

COMMUNICATION
in polyglutamates are. Interestingly, these orientations are
rather similar as can be seen from the eigenvector plots in
Figure 1b.[8j]
For a number of reasons we decided to work with poly-
guanidines (Scheme 1). First of all the synthesis of a broad
range of symmetrical or unsymmetrical carbodiimide mono-
Scheme 1. Synthesis and structure of polyguanidines.
mers via easy to access ureas or thioureas is straightforward.
Helically chiral, configurationally stable polymers are ob-
tained by living polymerization of carbodiimides with chiral
substituents.[23] Alternatively, single-handed polymers can be
synthesized by screw sense selective polymerization of achi-
ral carbodiimides with chiral titanium complexes.[24] In con-
trast to polyisocyanates and polyisocyanides, properties such
as solubility, polarity and even the configurational stability
can be adjusted independently (via R1) from structural fea-
tures controlling the helical sense (via R2). Polyguanidines
are intrinsically stiffer than polyisocyanates and form aniso-
tropic phases at rather low concentrations[22b] in a variety of
non-polar solvents.
Finally, polyguanidines with low helix inversion barriers
and chiral side chains can be thermally equilibrated to the
thermodynamically more stable helix conformation.[22c,25]
Note that the helix inversion barriers and therefore the dis-
tinction between stiff or dynamic helices in polyguanidines
depend on the side chains.
Figure 1. Graphical illustration of the eigenvectors of the Saupe matrix
for (+)- and (À)-IPC in a) (R)-PPEMG, b) and PBLG.
The polyguanidine investigated here, poly-(N-methyl-N’-
((R)-1-phenylethyl)guanidine ((R)-PPEMG, p1), is known
to exhibit cholesteric liquid crystalline phases in chloroform
at 12.5% w/w.[22b] The carbodiimide monomer 1 was ob-
tained by bromotriphenylphosphonium bromide mediated
dehydration[26] of the corresponding urea 4, which was ob-
tained from the enantiopure isocyanate (2) by reaction with
methylammonium chloride (3).[27] The enantiopure carbodii-
mide 1 was subjected to a living polymerisation using the ti-
We wondered whether helically chiral polymers other
than homopolypeptides would also allow for the orientation
of organic compounds and the discrimination of enantio-
mers. Thus, we were searching for chiral, non-racemic poly-
mers displaying high persistence lengths capable of forming
lyotropic LC phases. These kinds of polymers are expected
to be characterized by low critical concentrations, a low in-
duced degree of order for the compound in question (Da ꢀ
1ꢃ10À4) and enantiodifferentiating properties.
Since many years helically chiral synthetic polymers at-
tract the interest of polymer chemists.[16] Potential applica-
tions include enantiomer separations, chirality sensing and
asymmetric catalysis. Our interest in these materials began
with the idea to develop uniformly configured multiple-site
catalysts based on soluble polymers.[17] On the other hand,
as rigid-backbone polymers they should form lyotropic
liquid crystalline phases[18] which makes them interesting
candidates for the development of new chiral alignment
media. Indeed, one-handed helical polymers such as polyiso-
cyanates,[19] polyacetylenes,[20] polyisocyanides[21] and poly-
guanidines[22] do form lyotropic LC phases.
tanium complex [CpTi
(Scheme 2).
(NMe2)Cl2] (5)[23a,b,28] as initiator
ACHTUNGTRENNUNG
Scheme 2. Synthesis of N-methyl-N’-((R)-1-phenylethyl)carbodiimide (1)
and the corresponding polymer p1.
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M. Reggelin et al.
We conducted polymerisations at different monomer to
initiator ratios and determined the MW values of the result-
ing polymers via GPC (Table 1, for experimental details see
Supporting Information). As expected, with increasing mo-
nomer to initiator ratios the molecular weights of the poly-
phases all measurements were conducted at concentrations
slightly above the critical concentration (see Table S4 for
more information concerning the exact composition of the
samples). The two enantiomers of IPC were oriented in (R)-
PPEMG and RDCs were extracted from w1-coupled
HSQCs[31] (Figure 2). Due to
the rather strong orientation in-
duced (axial component of the
Table 1. Polymerisations of carbodiimide 1 with titanium complex 5.
[c]
[d]
Polymer
[M]/[I][a]
Yield [%]
Mn [gmolÀ1
]
Mw [gmolÀ1
]
PDI[e]
(w/w)[f] [%]
ACHTUNGTRENNUNG
alignment tensor Da =2.7ꢃ10À3
for the phase containing (À)-
IPC after normalisation to the
quadrupolar splitting of (+)-
IPC (see below) and Da =2.3ꢃ
10À3 for (+)-IPC), lines are
rather broad. The total cou-
plings, scalar couplings and re-
p1-1
p1-2
p1-3
80:1
100:1
250:1
65
10002
15964
43931
17886
20978
104460
1.79
1.31
2.38
26.1 g]
n.d.[h]
18.7[i]
98[b]
82
[a] Monomer to initiator ratio. [b] Polymer contaminated by unknown impurities. [c] Number average molecu-
lar weight, calibrated against polystyrene standards. [d] Weight average molecular weight, calibrated against
polystyrene standards. [e] PDI (polydispersity index)=Mw/Mn. [f] Critical concentration for the liquid crystal-
line phase in CDCl3 in weight percent as determined by 2H NMR spectroscopy. [g] DnQ,min
ACTHNUGTRNE(UNG CDCl3)=540 Hz
(303 K). [h] Not determined. [i] DnQ,min (CDCl3)=1077 Hz (303 K).
mers increased (Table 1) and the liquid crystalline phase can
be observed at lower concentrations. The completeness of
the phase transition was judged from the disappearance of
the isotropic 2H NMR signal (singlet) of the solvent
(CDCl3) and its replacement by a doublet split by the mini-
mum quadrupolar coupling (DnQ,min) characterizing the ani-
sotropic phase. This molecular weight dependence of the
minimum lower concentration to reach the anisotropic state
is in accordance with calculations by DuPrꢄ[15] and also cor-
roborates our experimental findings with PBLG.[8i]
Contrasting our experiences with PBLG it is worth men-
tioning here that with the low molecular weight polymer p1-
1 a lower DnQ,min (compared with p1-3) was measured at a
higher polymer concentration.
As reported by Novak et al.,[23a] polyguanidines tend to
absorb on the chromatography columns used in gel permea-
tion chromatography (GPC), which leads to pronounced
peak broadening and to non-reliable PDIs. We managed to
reduce the absorption of the polymer significantly when
using diethanolamine (DEA) as an additive (see Supporting
Information), but the PDIs reported above are still affected
by that phenomenon and therefore molecular weight distri-
butions are expected to be smaller than those reported in
Table 1.
The resulting (R)-PPEMGs exhibit negative Cotton ef-
fects at l=262 nm in their CD spectra (see Supporting In-
formation) with molar ellipticities comparable to those re-
ported in the literature (e.g., q262 =À3872.35 mdegm2 molÀ1
for p1-3 in Table 1), which are assigned to P helices.[27]
We chose the polymer sample with highest molecular
weight (p1-3) for the investigations towards the adequacy of
polyguanidines as orienting media.
Figure 2. Sections (corresponding to the C5-H5 cross peak) of w1-coupled
HSQCs of a) (+)-IPC and b) (À)-IPC in the liquid crystalline phase of
(R)-PPEMG in CDCl3. The isotropic spectrum is shown in black for com-
parison. Note that the additional splitting in w1 observable in b) is due to
long-range C–H coupling.[29]
Table 2. Scalar (J), total (T) and dipolar (D) one bond couplings
(1TCÀH =1JCÀH +2·1DCÀH) of (+)- and (À)-IPC in (R)-PPEMG. Each split-
ting was evaluated three times. The error given is the difference between
the largest and smallest value extracted (maximum difference) in the
three measurements (divided by 2 for D).
(+)-IPC
(À)-IPC[a]
1
1
1
1
Spin pairs[b] JCH [Hz]
1TCH [Hz] DCH [Hz] TCH [Hz] DCH [Hz]
C1–H1
C2–H2
C3–H3
C4–H4s
C4–H4a
C5–H5
C8–H8
C9–H9
C10–H10
139.5Æ0.5 216Æ6
38Æ3
13Æ1
66Æ1
62Æ2
À8Æ1
1Æ1
157Æ4
150Æ8
268Æ2
206Æ10
100Æ4
248Æ10
172Æ2
94Æ2
9Æ2
12Æ4
125.8Æ0.5 152Æ2
139.6Æ0.5 271Æ2
126.0Æ0.5 250Æ4
126.0Æ0.5 110Æ2
143.7Æ0.5 146Æ2
123.0Æ0.5 169Æ2
64Æ1
40Æ5
À13Æ2
52Æ5
23Æ1
À20Æ2
À8Æ1
25Æ1
It exhibits a liquid crystalline phase at a critical concentra-
tion of 18.7% w/w in CDCl3 (monitored by observation of
the concentration dependence of the 2H spectra) with a
quadrupolar splitting of the solvent signal of 1077 Hz.[30] To
study the alignment properties of (R)-PPEMG and to be
able to compare it to PBLG, we chose isopinocampheol
(IPC) as the analyte. To maintain stable liquid crystalline
124.6Æ0.5
85Æ4
À15Æ1
124.4Æ0.5 108Æ2
87Æ2
À18Æ1
[a] As the two samples used where not identically concentrated the total
and dipolar couplings of (À)-IPC were normalized by the ratio of quad-
rupolar splittings of the two samples (1D(À)-IPC,norm =1D(À)-IPC,meas* 1258/
1382) in order to provide comparability. [b] The total and dipolar cou-
plings for C7–H7a/s are not observable.
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Chem. Eur. J. 2010, 16, 10342 – 10346

Chiral Orienting Media for Organic Compounds
COMMUNICATION
sidual dipolar couplings of (+) and (À)-IPC in (R)-PPEMG
are listed in Table 2.
non-racemic compounds. Future work will be directed
toward the development of polymers with enhanced stiffness
to reduce the critical concentration needed to reach the ani-
sotropic state by enhancing the steric bulk of the side
chains. Moreover, we try to separate the alignment proper-
ties from enantiodifferentiation by embedding of the helical-
ly chiral polymers into polymer gels. This may entail the op-
portunity to induce alignment by strain,[8a–h] thus avoiding
the necessity to prepare a LC phase, and enantiodifferentia-
tion by the embedded helical structures.
As can clearly be seen, the observed total and dipolar
couplings are significantly different for the two enantiomers
of IPC in the helically chiral polymeric lyotropic liquid crys-
tal. Thus enantiodiscrimination is taking place. The RDCs
reported in Table 2 were used for the calculation of the
order tensors of the two enantiomers applying the RDC
module of hotFCHT[32] in order to find out how large the
difference in orientation for the two enantiomers of IPC in
Table 3. Orientational properties of (À)-IPC and (+)-IPC in (R)-
Experimental Section
PPEMG.
[c]
DnQ [Hz][b]
Da [10À3
]
a [8][d]
b [8][d]
g [8][d]
See Supporting Information.
(À)-IPC[a]
33.8Æ0.46
(+)-IPC[f]
39.8Æ0.44
1382
1258
2.70[e]
2.30
78.3Æ0.74
84.5Æ0.37
175.1Æ7.8
118.8Æ1.8
[a] 42 mg (À)-IPC, 123.4 mg (R)-PPEMG, 405 mg CDCl3 (21.6% w/w
polymer. [b] Absolute value of the quadrupolar splitting of the solvent.
[c] Axial component of the orienting tensor. [d] Euler angles relating the
principal axis system and the initial molecular axis frame. [e]After nor-
malisation with the ratio of quadrupolar splittings (see footnote of
Table 2). [f] 42 mg (+)-IPC, 145.8 mg (R)-PPEMG, 494 mg CDCl3
(21.4% w/w polymer).
Acknowledgements
The authors would like to thank the German Research Council (DFG)
for the funding of the Research Unit FOR 934 (RE 1007/7-1, TH 1115/3-
1) and PSS (Polymer Standard Service, Mainz) for help with the GPC
analysis.
Keywords: alignment media
spectroscopy · polyguanidines
· liquid crystals · NMR
(R)-PPEMG is. The results of these calculations are com-
piled in Table 3 and the difference in orientations is graphi-
cally illustrated in Figure 1a. The eigenvectors of the Saupe
matrix for all four possible orientations of each (+) and
(À)-IPC are shown as intersections with a sphere of radius
unity for both media. The uncertainties of the eigenvectors
were determined by a Monte-Carlo based method.[33,34] As
can be taken from Figure 1a the Szz component is deter-
mined most precisely, reflected by its narrow distribution on
the surface of the sphere.
On the other hand, from the rather broad distribution of
the corresponding eigenvectors on the sphere, it is obvious
that the Sxx and Syy components[35] (especially for (À)-IPC)
are less well defined for (R)-PPEMG. Nevertheless, the dif-
ference in orientations for (+) and (À)-IPC is obviously sig-
nificant. Interestingly, this difference and therefore also the
enantiodiscriminating power of the orienting medium (R)-
PPEMG for IPC seems to be larger than the one of PBLG
(see Figure 1b), in which differences in orientations were
rather small.[8j] Whether this is also the case with other sol-
utes, is currently under investigation.
In summary, we have presented a member of a new class
of chiral, enantiodiscriminating orienting media for organic
compounds based on helically chiral polyguanidines. Al-
though RDCs obtained are rather large, which is an aspect
we want to improve in the future, the orientations of the
two enantiomers of IPC are clearly different and can be de-
termined reliably. For IPC the difference in orientation ob-
served in (R)-PPEMG appears to be larger than the one ob-
served in PBLG. This will be essential for the development
of a method to determine absolute configurations of chiral,
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Even if that was the case here, the difference in orientations would
still be significant.
Received: April 13, 2010
Published online: July 30, 2010
10346
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