Long-range chiral induction in chemical systems with helical organization. Promesogenic monomers in the formation of poly(isocyanide)s and in the organization of liquid crystals

Article abstract of DOI:10.1021/ja980474m

The preparation of optically active poly(isocyanide)s derived from chiral promesogenic monomers is reported. Remarkably, the stereogenic carbon atom in the monomer is able to pass its chiral 'information' to the growing polymer backbone which is at least

Full text of DOI:10.1021/ja980474m

9126
J. Am. Chem. Soc. 1998, 120, 9126-9134
Long-Range Chiral Induction in Chemical Systems with Helical
Organization. Promesogenic Monomers in the Formation of
Poly(isocyanide)s and in the Organization of Liquid Crystals
David B. Amabilino,† Elena Ramos,† Jose´-Luis Serrano,*,‡ Teresa Sierra,‡ and
Jaume Veciana*,†
Contribution from the Institut de Cie`ncia de Materials de Barcelona, Consejo Superior de InVestigaciones
Cient´ıficas (CSIC), Campus UniVersitaˆri, 08193-Bellaterra, Spain, and Instituto de Ciencia de Materiales
de Arago´n, Consejo Superior de InVestigaciones Cient´ıficas (CSIC), UniVersidad de Zaragoza,
50009-Zaragoza, Spain
ReceiVed February 11, 1998
Abstract: The preparation of optically active poly(isocyanide)s derived from chiral promesogenic monomers
is reported. Remarkably, the stereogenic carbon atom in the monomer is able to pass its chiral “information”
to the growing polymer backbone which is at least 14 atoms (approximately 16 Å) remote from it. The sense
of helical induction in these conformationally rigid polymers is compared to the helical sense of the cholesteric
phases, as well as to the helical senses of chiral smectic C phases, induced by the monomers in nematic and
smectic C phases, respectively. Both relate to the odd-even rules for chiral sense changes in liquid crystalline
phases. The role of noncovalent interactions in the polymerization has been proven by performing the reactions
at various concentrations and in different solventssthe chiral induction from monomer to polymer is greatest
in most concentrated reactions and in solvents with a balance between high dipolarity-polarizability and low
hydrogen bond accepting and cavitational terms, as determined by an LSER analysis.
Introduction
We were drawn to the latter family of macromolecules since
the stereoselectivity of the reactions leading to their formation
as well as their chiroptical properties have been studied in some
detail.12 Diastereoselective polymerization reactions of isocya-
nides have generally been performed on monomers in which
the stereogenic center is the carbon atom connected tosor in
close proximity tosthe nitrogen atom of the isocyanide group.
Helical chirality1 is a central feature of chemical systems
which are involved in a wide variety of biologically2 and
technologically3 important processes. This source of optical
activity may be a result of conformational preferences around
coValent bonds or of intermolecular relationships goVerned by
noncoValent interactions (Figure 1). These phenomena are
exemplified by atropoisomeric molecules, including polymers4
and chiral liquid crystals,5 respectively, and both these media
are of well-established interest in the study of organic stereo-
chemistry.
(7) (a) Okamoto, Y.; Susuki, K.; Otha, K.; Hatada, K.; Yuko, H. J. Am.
Chem. Soc. 1979, 101, 4763-4765. (b) Nakano, T.; Okamoto, Y.; Hatada,
K. J. Am. Chem. Soc. 1992, 114, 1318-1329.
(8) (a) Bur, A. J.; Fetters, L. J. Chem. ReV. 1976, 76, 727-746. (b) Green,
M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science
1995, 268, 1860-1866 and references therein. (c) Okamoto, N.; Mukaida,
F.; Gu, H.; Nakamura, Y.; Sato, T.; Teramoto, A.; Green, M. M.; Andreola,
C.; Peterson, N. C.; Lifson, S. Macromolecules 1996, 29, 2878-2884. (d)
Mu¨ller, M.; Zentel, R. Macromolecules 1996, 29, 1609-1617.
(9) (a) Boumann, M. M.; Meijer, E. W. AdV. Mater. 1995, 7, 385-387.
(b) Hu, Q.-S.; Vitharana, D.; Jain, V.; Wagaman, M. W.; Zhang, L.; Lee,
T. R.; Pu, L. Macromolecules 1996, 29, 1082-1084.
Helical or similar atropoisomeric conformations4 are present
in several synthetic polymers,6-10 including poly(isocyanide)s.11
† Institut de Cie`ncia de Materials de Barcelona.
‡ Instituto de Ciencia de Materiales de Arago´n.
(1) (a) Meurer, K. P.; Vo¨gtle, F. Top. Curr. Chem. 1985, 127, 1-76.
(b) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic
Compounds; Wiley: New York, 1994. See also: (c) Robbie, K.; Brett, M.
J.; Lakhtakia, A. Nature 1996, 384, 616.
(2) (a) Klug, A. Angew. Chem., Int. Ed. Engl. 1983, 22, 565-582. (b)
Urry, D. W. Angew. Chem., Int. Ed. Engl. 1993, 32, 819-841. (c) Tirrell,
J. G.; Fournier, M. J.; Mason, T. L.; Tirrell, D. A. Chem. Eng. News 1994,
Dec. 19, 40-51.
(3) (a) Kallard, T., Ed.; Liquid Crystal DeVices; Optosonic Press: New
York, 1973; Vol. 7, State of the Art Review. (b) Attard, G. S. Trends Polym.
Sci. 1993, 1, 79-86. (c) Zentel, R.; Brehmer, M. AdV. Mater. 1994, 6,
598-599. (d) Blackwood, K. M. Science 1996, 273, 909-912.
(4) (a) Tobolsky, A. V. J. Phys. Chem. 1964, 68, 2267-2271. (b) Wulff,
G. Angew. Chem., Int. Ed. Engl. 1989, 28, 21-37. (c) Okamoto, Y.; Nakano,
T. Chem. ReV. 1994, 94, 349-372. (d) Farina, M. Topics Stereochem. 1987,
17, 1-111.
(5) (a) Solladie´, G.; Zimmerman, R. G. Angew. Chem., Int. Ed. Engl.
1984, 23, 348-362. (b) Goodby, J. W. J. Mater. Chem. 1991, 1, 307-318.
(c) de Gennes, P.-G. Angew. Chem., Int. Ed. Engl. 1992, 31, 842-845.
(6) (a) Corley, L. S.; Vogl, O. Polym. Bull. 1980, 3, 211. (b) Ute, K.;
Hirose, K.; Kashimoto, H.; Hatada, K.; Vogl, O. J. Am. Chem. Soc. 1991,
113, 6305-6306.
(10) (a) Fujiki, M. J. Am. Chem. Soc. 1994, 116, 11976-11981. (b) Seitz,
M.; Plesnivy, T.; Schimossek, K.; Edelmann, M.; Ringsdorf, H.; Fischer,
H.; Uyama, H.; Kobayashi, S. Macromolecules 1996, 29, 6560-6574. (c)
Wagner, H.; Harms, K.; Koert, U.; Meder, S.; Boheim, G. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 2643-2646. (d) Dai, Y.; Katz, T. J. J. Org. Chem.
1997, 62, 1274-1285. (e) Janssen, H. M.; Peeters, E.; van Zundert, M. F.;
van Genderen, M. H. P.; Meijer, E. W. Angew. Chem., Int. Ed. Engl. 1997,
36, 122-125. (f) Obata, M.; Kakuchi, T.; Yokota, K. Macromolecules 1997,
30, 348-353. (g) Obata, K.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1997,
119, 11345-11346.
(11) Poly(isocyanide)s is the nomenclature recommended by IUPAC,
although these polymers have also been described as poly(iminomethylene)s
and more systematically as poly(carbonimidoyl)s. For reviews, along with
references therein, on these polymers, see: (a) Millich. F. J. Polym. Sci.,
Macromol. ReV. 1980, 15, 207-253. (b) Nolte, R. J. M. Chem. Soc. ReV.
1994, 23, 11-19. See also: (c) King, R. B.; Borodinsky, L. Macromolecules
1985, 18, 2117-2120. (d) Abdelkader, M.; Drenth, W.; Meijer, E. W. Chem.
Mater. 1991, 3, 598-602. (e) Hong, B.; Fox, M. A. Macromolecules 1994,
27, 5311-5317.
S0002-7863(98)00474-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/28/1998

Long-Range Chiral Induction
J. Am. Chem. Soc., Vol. 120, No. 36, 1998 9127
Scheme 1
Figure 1. Schematic representations of helices in the M and P forms
of atropoisomeric macromolecules (top), in the M organization of a
cholesteric mesophase (bottom left) and in the P organization of a chiral
smectic C mesophase (bottom right).
Which of the two diastereomeric conformations13,14 is adopted
preferentially is determined largely by steric interactions, in
polymerizations operating under predominantly kinetic control.15
In contrast, the helicity present in cholesteric and smectic liquid
crystals (Figure 1) is a result of the relative orientations between
separate molecules,5 governed by weak noncovalent inter-
molecular interactions, and dependent on the position of the
stereogenic center with regard to the rigid mesogenic core. The
odd-even rules are applicable to cholesteric16 (N*) as well as
to chiral smectic C17 (Sc*) liquid crystals, when the dipole
associated with the stereogenic center is considered.
A combination of these two types of helicity within a single
family of chemical structures is an academically intriguing goal,
which might spawn macromolecules of technological interest.18
We therefore designed the monomer drawn in cartoon form at
the top of Scheme 1, in which the polymerizable isocyanide
group is separated from a stereogenic center by a promesogenic
core. Initially, we chose the phenyl benzoate group as the spacer
in monomer 1 (Scheme 1)sas this structural feature is frequently
encountered in thermotropic liquid crystals.19 Somewhat re-
markably, in certain instances the stereogenic center in the
monomer induces the preferential creation of one of the
diastereomeric forms of the polymer, at a distance of at least
16 Å.20 In this paper, we report the effects of changing the
position and environment of the stereogenic center attached to
the promesogenic core. The senses of the helices present in
the polymers at the macromolecular level are compared with
those induced by the promesogenic monomers at the mesoscopic
level in liquid crystalline mixtures forming the N* and Sc*
helical mesophases. This comparison leads to interesting
conclusions regarding the applicability of the odd-even rules
in the three types of media, which present distinct helical
organization.
(12) (a) van Beijnen, A. J. M.; Nolte, R. J. M.; Drenth, W.; Hezemans,
A. M. F. Tetrahedron 1976, 32, 2017-2019. (b) van Beijnen, A. J. M.;
Nolte, R. J. M.; Naaktegeboren, A. J.; Zwikker, J. W.; Drenth, W.;
Hezemans, A. M. F. Macromolecules 1983, 16, 1679-1689. (c) Kamer, P.
C. J.; Nolte, R. J. M.; Drenth, W. J. Am. Chem. Soc. 1988, 110, 6818-
6825.
(13) Some cautionary reports concerning the interpretation of chiroptical
properties of these polymers (not derived from aryl isocyanides) have been
published, which cast doubt on the 41 helical conformation. See: (a) Green,
M. M.; Gross, R. A.; Schilling, F. C.; Zero, K.; Crosby, C., III
Macromolecules 1988, 21, 1839-1846. (b) Pini, D.; Iuliano, A.; Salvadori,
P. Macromolecules 1992, 25, 6059-6062. For a view against the helical
structure based on the solution conformation of a trimer, see: (c) Spencer,
L.; Kim, M.; Euler, W. B.; Rosen, W. J. Am. Chem. Soc. 1997, 119, 8129-
8130.
(14) For calculations concerning the backbone conformations of aliphatic
oligo(isocyanide)s, see: Clericuzio, M.; Alagona, G.; Ghio, C.; Salvadori,
P. J. Am. Chem. Soc. 1997, 119, 1059-1071.
(15) Kamer, P. C. J.; Cleij, M. C.; Nolte, R. J. M.; Harada, T.; Hezemans,
A. M. F.; Drenth, W. J. Am. Chem. Soc. 1988, 110, 1581-1587.
(16) Gray, G. W.; McDonnell, D. G. Mol. Cryst., Liq. Cryst. 1977, 34,
211-217.
(17) (a) Goodby, J. W.; Chin, E.; Leslie, T. M.; Geary, J. M.; Patel, J.
S. J. Am. Chem. Soc. 1986, 108, 4729-4735. (b) Goodby, J. W.; Chin, E.
J. Am. Chem. Soc. 1986, 108, 4736-4742. (c) Goodby, J. W.; Blinc, R.;
Clark, N. S.; Lagerwall, S. T.; Osipov, M. A.; Pikin, S. A.; Sakurai, T.;
Yoshino, K.; Zeks, B. Ferroelectric Liquid CrystalssPrinciples Properties
and Applications; Ferroelectrics and Related Phenomena, Vol. 7; Gordon
and Breach Science Publishers: Philadelphia, PA, 1991.
(18) The functionalization of polymers with promesogenic groups is
widely used for the creation of side-chain liquid crystalline systems, although
in these cases the promesogenic group is generally separated from the
polymer backbone by a long and flexible spacer. See, for example: Walther,
M.; Finkelmann, H. Prog. Polym. Sci. 1996, 21, 951-979.
(19) For discussions concerning the role of the phenyl benzoate group
in liquid crystals, see: (a) Castellano, J. A.; McCaffrey, M. T.; Goldmacher,
J. E. In Liquid Crystals 3; Brown, G. H., Labes, M. M., Eds.; Gordon and
Breach: New York, 1972; Vol. II, pp 597-618. (b) Sakurai, Y.; Takenaka,
S.; Miyake, H.; Morita, H.; Ikemoto, T. J. Chem. Soc., Perkin Trans. 2
1989, 1199-1204.
(20) Ramos, E.; Bosch, J.; Serrano, J. L.; Sierra, T.; Veciana, J. J. Am.
Chem. Soc. 1996, 118, 4703-4704.

9128 J. Am. Chem. Soc., Vol. 120, No. 36, 1998
Amabilino et al.
Scheme 2
from the isocyanide moiety. None of the isocyanides, nor their
formamide precursors, presented thermotropic mesophases.
The homopolymers 5 were prepared (Scheme 2) in dry CH2-
Cl2, using NiCl2‚6H2O as catalyst, at a concentration of freshly
prepared isocyanide monomer of approximately 200 mM. Air
was allowed to be present during the reactions, as it has been
shown to be involved during the catalysis in polymerizations
of this type.29,30 The polymerizations proceeded in good to
excellent yields, ranging from 68% up to 96%, the majority
being in excess of 80%. The conversion of the monomers into
1
the polymers was confirmed by IR, H, and 13C NMR spec-
troscopies, gel permeation chromatography (GPC), differential
scanning calorimetry (DSC), thermogravimetric analysis (TGA),
polarimetry, UV-vis spectroscopy, and circular dichroism (CD)
spectrometry.
The IR spectra of the solids showed no signal corresponding
to the isocyanide moiety, but were dominated by signals from
the promesogenic units. Weak broad bands from the imine
groups constituting the polymer backbone were observed at
approximately 1655 cm-1. The 1H NMR of the polymers show
extremely broad resonances in the aromatic region from
approximately 8.0 to 5.5 ppm corresponding to the phenyl
benzoate moiety, their high-field positions indicating the
proximity of these groups in the polymer. The hydrogen atom-
(s) attached to the aliphatic carbon atoms in close proximity to
the phenyl ring is also broad, but the resonances of the aliphatic
chain become increasingly well-resolved with increasing dis-
tance from the polymer backbone. The 13C NMR spectra reveal
similar characteristics to the 1H NMR spectra, in that the
resonances arising from the carbon atoms close to the polymer
backbone are very broad, while those from the carbon atoms in
the aliphatic side chains of the macromolecules are quite well-
resolved, reflecting the higher mobility of the atoms at the end
of the side chain.
In common with other polymers generated using the NiCl2‚
6H2O catalyst,12 the samples had polydispersities (GPC)31
between 1.1 and 1.7, with a mean value of 1.4, in accord with
the nonliving nature of the polymerization.29 Differential
scanning calorimetry (DSC) and thermogravimetric analysis
(TGA) of the polymers showed no significant phase transitions
in the range of 40-200 °C, but they decomposed (weight loss
from sample witnessed by TGA) above approximately 275 °C.
None of the polymers presented thermotropic mesophases,
presumably because of their rigid nature. All of the homopoly-
mers were very soluble in organic solvents such as toluene,
methylene chloride, chloroform, THF, and DMF, while they
show low solubility in hexane, acetonitrile, and short-chain
alcohols, and remain insoluble in water.
Results and Discussion
Synthesis and Characterization of the Materials. The
synthesis of isocyanide monomers 1 and the corresponding poly-
(isocyanide)s 5, which differ in the nature of the tails a-g
attached to the promesogenic cores, described in this work is
depicted in Scheme 2.
Treatment of 4-aminobenzoic acid with formic acid and acetic
anhydride in ethyl acetate gave 4-formamidobenzoic acid (2).
The phenol 3a-(R)C1 was prepared from the racemic 1-(4-
acetylphenyl)ethanol using a lipase-catalyzed acetylation step21
to achieve the resolution of the enantiomers. The phenols 3b,c
were prepared by coupling appropriate commercial alkyl alco-
hols with 4-(benzyloxy)phenol using the Mitsunobu protocol,22
followed by removal of the protecting group using Pd(OH)2/C
and cyclohexene in ethanol. Williamson ether synthesis using
4-(benzyloxy)phenol and the appropriate alkyl tosylates,23-25
followed by debenzylation, provided the phenols 3d-g. The
coupling26 of 2 with the phenols 3 using dicyclohexyl carbo-
diimide (DCC) in the presence of 4-(dimethylamino)pyridine
(DMAP) afforded the intermediate formamides 4, whose
dehydration was achieved27 employing diphosgene. The con-
version of the formamides to the unstable isocyanides 1 was
evidenced by symmetrization in NMR spectra28 as well as by
the characteristic IR band (at approximately 2120 cm-1) arising
(21) Naemura, K.; Fukuda, R.; Konishi, M.; Hirose, K.; Tobe, Y. J.
Chem. Soc., Perkin Trans. 1 1994, 1253-1256.
Circular Dichroism Studies
(22) (a) Mitsunobu, O. Synthesis 1981, 1-28. (b) Hughes, D. L. In
Organic Reactions; Paquette, L. A., et al., Eds.; Wiley: New York, 1992;
Vol. 42, pp 335-656.
A. Circular Dichroism Spectra of the Polymers. The
chiroptical data of the polymers 5 reveal some interesting trends
(23) Sierra, T.; Ros, M. B.; Omenat, A.; Serrano, J. L. Chem. Mater.
1993, 5, 938-942. This paper describes the preparation of the 2,6-
naphthalene analogue.
(29) (a) Deming, T. J.; Novak, B. M. Macromolecules 1993, 26, 7092-
7094. (b) Deming, T. J.; Novak, B. M. J. Am. Chem. Soc. 1993, 115, 9101-
9111.
(30) Euler, W. B.; Huang, J.-T.; Kim, M.; Spencer, L.; Rosen, W. Chem.
Commun. 1997, 257-258.
(31) The polymers all had molecular masses in excess of 30 kDa and up
to 60 kDa, with a mean of 45 kDa, when determined by GPC against
polystyrene standards using THF as eluant. When determined by vapor phase
osmometry, the molecular masses were considerably lower, the mean value
being of the order of 20 kDa. This discrepancy probably arises from the
rigid-rod nature of the polymers, which are known to exhibit exageratedly
large molecular masses by GPC when compared against random-coil
polymers. The lower limit of molecular mass corresponds to approximately
70 monomer units (if the backbone conformation were a 41 helix this
molecular mass would correspond to approximately 18 turns).
(24) Decobert, G.; Dubois, J.-C. New J. Chem. 1986, 10, 777-782.
(25) Decobert, G.; Dubois, J.-C. Mol. Cryst., Liq. Cryst. 1984, 114
(1-3), 237-247.
(26) See, along with references therein: (a) Williams, A.; Ibrahim, I. T.
Chem. ReV. 1981, 81, 589-636. (b) Balcom, B. J.; Petersen, N. O. J. Org.
Chem. 1989, 54, 1922-1927.
(27) Skorna, G.; Ugi, I. Angew. Chem., Int. Ed. Engl. 1977, 16, 259-
260.
(28) The NMR of the formanilide derivatives described here are
complicated by the presence of unequal populations of the cis and trans
isomers about the amide bond, a phenomenon noted previously: Bourn,
A. J. R.; Gillies, D. G.; Randall, E. W. Tetrahedron 1964, 20, 1811-1818.
This asymmetry vanishes from the NMR spectrum upon conversion of the
formamide to the isocyanide.

Long-Range Chiral Induction
J. Am. Chem. Soc., Vol. 120, No. 36, 1998 9129
Table 1. Chiroptical Data for Monomers 4 and the Corresponding Polymers 5,32 the Configuration of the Chiral Tail (with the all-trans
Conformation) Showing the Position of the Methyl Group with Respect to the Promesogenic Core, the Helical Senses of the Polymer
Backbones, and the Helical Senses of the Mesophases Induced by the Monomers 4 in Nematic and Smectic C Liquid Crystals
a ins. indicates that the monomers were insoluble in this phase. Specific optical rotations in units of deg cm2.g-1 were measured in a 1 cm cell
at concentrations of 6-10 mg mL-1 in CHCl3 at ambient temperature. CD spectra (∆ꢀ in L mol-1 cm-1) were recorded in THF at a controlled
temperature of 25 °C. No signal could be detected for this polymer 5g-(S)C5.
as can be seen in Table 1, where they are compared with the
the broader absorption centered on 363 nm arises from the imino
chromophore’s n-π* transition, whose sign and relation with
P or M helicity has been established in related poly(iso-
cyanide)s,12,15,34 and presents clear evidence that the polymer
backbone has an optically active conformation. The more
intense and narrow band centered on 252 nm is consistent with
a 1Lb absorption arising from the aromatic chromophore attached
to the imino group. The spectra bear a more than passing
resemblanceswith appropriate displacement to longer wave-
length because of the presence of aromatic chromophoressto
calculated CD spectra for helical conformations of aliphatic
poly(isocyanide)s.14
Polymers 5c-(R)C2 and 5c-(S)C2 exhibit mirror-image spectra
with differential molar absorptivities of greatest intensity,
paralleling the associated molar optical rotations. The largest
∆ꢀ values observed for the Cotton effects centered on 252 and
363 nm are -8.80 and +5.86 L mol-1 cm-1, respectively, for
5c-(R)C2 (Figure 2). In addition, there is a weaker Cotton effect
centered on 282 nm, arising from the n-π* absorption of the
carbonyl group adjacent to the stereogenic carbon atom. Indeed,
the monomer precursor 4c-(R)C2 exhibits a Cotton effect of
weaker intensity in its CD spectrum near this wavelength (Figure
2). It is interesting to note that the incorporation of the ester
group close to the stereogenic center increases the preference
for one helical form of the polymer since the Cotton effects at
252 and 363 nm associated with the polymers 5b-(R)C2 or
5b-(S)C2 are considerably weaker.35 In liquid crystals the ester
corresponding precursor formamides 4.32 First, there are some
parallels between the orders of the specific optical rotations
25
546
[R] and the degrees of the differential molar absorptivities
(∆ꢀ363) from the CD spectra. The molar optical rotation is
greatest for the polymer with tail c, the value for the polymers
being an order of magnitude greater than the corresponding
monomers. Polymers with tail b also exhibit large molar optical
rotations; however, as the chiral center in the promesogenic unit
is positioned further away from the relatively rigid aromatic
core to which the alkyl chain is connected (i.e., chiral tails f-(S)-
C4 and g-(S)C5), the optical rotations of the resulting polymers
approach those of the monomers. The racemic modifications
of 5b-(RS)C2 and 5c-(RS)C2 exhibited no optical rotation.
The CD spectra33 of the polymers and monomers are more
informative than the specific optical rotations, since this
technique allows observation of Cotton effects associated with
the component chromophores in the macromolecules and
molecules, respectively. The most optically active polymers
reported here have two bands centered on 252 and 363 nm which
have opposite signs and different widths. In the monomers,
the former band has a very low intensity and the latter is
nonexistant. It is generally accepted that the Cotton effect in
(32) The molar optical rotations and differential molar absorptivities for
the polymers are based on the molecular mass of a monomer unit (identical
molecular mass to the corresponding isocyanide precursor), hence they are
directly comparable with those of the precursor formamides, since the
isocyanides are relatively unstable. This comparison is made and considered
valid, given the specific optical rotations for 1b-(R)C2 and 4b-(R)C2 are
the same.
(33) For discussions on CD, see: (a) Analytical Applications of Circular
Dichroism; Purdie, N.; Brittain H. G., Eds.; Techniques and Instrumentation
in Analytical Chemistry; Elsevier: Amsterdam, The Netherlands, 1994; Vol.
14. (b) Circular Dichroism, Principles and Applications; Nakanishi, K.,
Berova, N., Woody, R. W., Eds.; VCH: Weinheim, 1994. (c) Rodger, A.;
Norde´n, B. Circular Dichroism and Linear Dichroism; Oxford University
Press: Oxford, 1997.
(34) The CD of related chiral poly(quinoxaline-2,3-diyl)s display similar
bands, see: (a) Ito, Y.; Ihara, E.; Murakami, M. Angew. Chem., Int. Ed.
Engl. 1992, 31, 1509-1510. (b) Ito, Y.; Kojima, Y.; Murakami, M.
Tetrahedron Lett. 1993, 34, 8279-8282. (c) Ito, Y.; Ohara, T.; Shima, R.;
Suginome, M. J. Am. Chem. Soc. 1996, 118, 9188-9189.
(35) It is unlikely that the chiral group attached to one extreme of the
promesogenic unit could influence the electronic transition associated with
the Cotton effect of the iminomethylene chromophore of the polymer
backbone.

9130 J. Am. Chem. Soc., Vol. 120, No. 36, 1998
Amabilino et al.
the intensity or form of the CD spectra of the poly(isocyanide)s
reported here has been observed when they have been recorded
in chloroform, THF, or 2:1 ethanol:THF. This result also
contrasts with the solvent-dependent CD spectra observed for
poly(thiophene)s38 or poly(isocyanate)s,8b,c both of which are
conformationally mobile, and suggests that our poly(isocyanides)
are conformationally rigid.39 Indeed, when polymer 5b-(S)C2
was refluxed in toluene for 24 h, no change in its CD spectrum
could be detected confirming a prodigious conformational
stiffness. This result also indicates (along with the polymeriza-
tions performed at various concentrations, vide infra) that the
polymers have structures whose conformation is determined
largely during their formation and which are not free to reorient
dramatically once they are formed, in contrast with other more
flexible helical polymers.8b,c,38,39
We considered that the inductions observed over unusually
large distances in these polymerizations might be a result of
the promesogenic nature of the core linking the stereogenic
center to the polymerizable group. Therefore, since none of
the monomers (or the polymers) exhibit liquid crystalline phases,
we investigated their helix-inducing properties in mesophases.
B. Liquid Crystal Induced Circular Dichroism (LCICD)
Spectra of the Promesogenic Monomers. Among the meso-
phases exhibiting helical organization, the two most deeply
studied are the cholesteric (N*) and chiral smectic C (Sc*). We
thus selected nematic and smectic C “hosts” exhibiting their
mesophases at room temperature in order to study the ability
of our promesogenic monomer “guests” to induce N*40 and Sc*41
phases by virtue of stereoselective noncovalent interactions with
the host. To ensure a good transfer of chiral “information” from
the guest to the host, we sought host mesogens with relatively
close structural features to those of the promesogenic monomers,
such as a twist between aromatic rings in the core. As room-
temperature nematic hosts, both p-methoxybenzylidine-p′-(n-
butyl)aniline (MBBA), which in common with the phenyl
benzoate-derived monomers42 possesses a single alkyl chain and
a twist between its two component aromatic rings,37 and the
eutectic mixture (2:1 by weight) of 4-(pentylphenyl)-4-methoxy
Figure 2. Comparison of the circular dichroism spectra of the
monomeric precursor formamide 4c-(R)C2 (dashed line) and the poly-
(isocyanide) 5c-(R)C2 derived from it (solid line) both recorded in THF.
group linked directly to the stereogenic center in columnar liquid
crystals leads to an enhanced rigidity in the tail relative to purely
alkyl derivatives, such as those derived from 2-octanol.36 In
the solution state, this less mobile moiety may rigidify the
asymmetric part of the monomer, leading to greater chiral
induction in the polymers with tails c derived from butyl lactate.
The results gathered in Table 1 indicate that the CD
absorptions at 363 nm (sign and order of the differential molar
absorptivity) associated with the polymer backbonesand hence
the sense and predominance of a given atropoisomersare
affected dramatically by the position of the stereogenic center.
Indeed, the differential molar absorptivities diminish rapidly and
alternate in sign as the chiral center in the promesogenic unit is
positioned away from the isocyanide group. The reduced
induction can be rationalized by considering the greatly
increased rotational freedom of the stereogenic center relative
to the promesogenic core with increasing numbers of intervening
methylene groups. The sense of the conformation induced in
the polymer backbone follows odd-even alternance as the
stereogenic center is positioned sequentially along the alkyl tail
if the same configuration is maintained. Thus, 5b-(S)C2 and
5c-(S)C2 have predominantly P, while 5d-(S)C3 and 5e-(S)C3
have predominantly M, and 5f-(S)C4 is inclined toward P helical
conformations. The only apparent exception to this general
behavior is the polymer with the stereogenic center in position
1 (5a-(R)C1). This apparent discrepancy results from the
misleading nomenclature R for this center when compared with
the other compounds. The Cahn-Ingold-Prelog rules describe
the relative positions of ligands relative to the stereogenic carbon
rather than to the promesogenic core which is our reference
point.
(37) For discussions concerning the twist between the component
aromatic rings in benzylidineaniline derivatives, see: (a) Brocklehurst, P.
Tetrahedron 1962, 18, 299-304. (b) Smith, W. F. Tetrahedron 1963, 19,
445-454. (c) Bu¨rgi, H. B.; Dunitz, J. D. HelV. Chim. Acta 1970, 53, 1747-
1764. (d) van der Veen, J.; Grobben, A. H. In Liquid Crystals 3; Brown,
G. H., Labes, M. M., Eds.; Gordon & Breach: New York, 1972; Vol. II,
pp 589-595. (e) Bar, I.; Bernstein, J. Tetrahedron 1987, 43, 1299-1305.
(f) Clegg, W.; Elsegood, M. R. J.; Heath, S. L.; Houlton, A.; Shipman, M.
A. Acta Crystallogr. C. 1996, 52, 2548-2552. (g) Navon, O.; Bernstein, J.
Struct. Chem. 1997, 8, 3-11. For an example of an X-ray crystal structure
of a mesogen incorporating both phenyl benzoate and benzylideneaniline
moieties, see: (h) Baumeister, U.; Hartung, H.; Gdaniec, M. Acta Crys-
tallogr. C. 1987, 43, 1117-1119.
(38) Bidan, G.; Guillerez, S.; Sorokin, V. AdV. Mater. 1996, 8, 157-
160.
(39) This view contrasts with that expressed concerning much lower
molecular mass poly(phenylisocyanide) prepared in MeOH. See: Huang,
J.-T.; Sun, J.; Euler, W. B.; Rosen, W. J. Polym. Sci. A. Polym. Chem.
1997, 35, 439-446.
(40) (a) Stegemeyer, H.; Mainush, K. J. Chem. Phys. Lett. 1970, 6, 5-6.
(b) Stegemeyer, H.; Mainush, K. J. Naturwissenschaften 1971, 58, 599-
602. (c) Saeva, F. D.; Sharpe, P. E.; Olin, G. R. J. Am. Chem. Soc. 1973,
95, 7656. (d) Gottarelli, G.; Samori, B.; Marzocchi, S. Tetrahedron Lett.
1975, 1981-1984.
(41) (a) Li, J.; Takezoe, H.; Fukuda, A.; Watanabe, J. Liq. Cryst. 1995,
18, 239-250. (b) Yamada, K.; Takanishi, Y.; Ishikawa, K.; Takezoe, H.;
Fukuda, A.; Osipov, M. A. Phys. ReV. E 1997, 56, R43-R46.
(42) For discussions concerning the twist between the component
aromatic rings in benzoate derivatives, see: (a) Schweizer, W. B.; Dunitz,
J. D. HelV. Chim. Acta 1982, 65, 1547-1554. (b) Bicerano, J.; Clark, H.
A. Macromolecules 1988, 21, 585-597. (c) Bicerano, J.; Clark, H. A.
Macromolecules 1988, 21, 597-603. (d) Coulter, P.; Windle, A. H.
Macromolecules 1989, 22, 1129-1136. (e) Emsley, J. W.; Furby, M. I. C.;
de Luca, G. Liquid Crystals 1996, 21, 877-883.
The dominant factor in determining the helical sense of the
polymers appears to be steric, since when the conformation of
the tails relative to the phenyl benzoate is fixed in an all-trans
conformation, a given configuration of the stereogenic center
(with methyl group above or below the plane of the paper in
Table 1) always leads to the same handed backbone. In the
particular case of 5a-(R)C1, the enantiomer conformationally
related with the (S) configuration in an odd position, in
comparison with tails d-(S)C3, e-(S)C3, and g-(S)C5), is the
(R) enantiomer.
It should be noted that while aromatic imines are known to
have absorption spectra which change both in intensity and
position of maxima,37 no significant solvent dependence of either
(36) Barbera´, J.; Iglesias, R.; Serrano, J. L.; Sierra, T.; de la Fuente, M.
R.; Palacios, B.; Pe´rez-Jubindo, M. A.; Va´zquez, J. T. J. Am. Chem. Soc.
1998, 120, 2908-2918.

Long-Range Chiral Induction
J. Am. Chem. Soc., Vol. 120, No. 36, 1998 9131
benzoate and 4-(pentylphenyl)-4-(hexyloxy) benzoate43 (phase
ZLI-1052) were employed. The senses of the cholesteric phases
induced in the nematic phases formed by addition of the
monomers were identical in both host mesogens. As Sc host, a
phenylpyrimidine mesogen,44 known as phase MX8056, which
shows a broad phase around room temperature was employed.
First, two enantiomerically pure stereoisomers as well as the
racemate of isocyanide monomer 1b-C2 were studied individu-
ally for their ability to induce chiral nematic phases. The
racemate 1b-(RS)C2 caused no bulk induction of twisted nematic
phases, and the CD of the resulting solution showed no optical
response. Conversely, when either 1b-(S)C2 or 1b-(R)C2 were
incorporated into the nematic phases, they induced cholesteric
phases with large chiroptical effects of opposite sign centered
on approximately 380 nm in the case of MBBA and 300 nm in
ZLI-1052. The (S) stereoisomer induces a P helical sense in
the nematic phase while the (R) enantiomer induces an M helical
sense.45 Concerned about the stability of the isocyanide
compounds in the nematic phases, we established that the
corresponding formamides 4b-(S)C2 and 4b-(R)C2 induce the
same helicities in the nematic phases as the derived isocyanides.
Subsequently, the formamides 4 were all studied for their
ability to induce N* and Sc* phases in the host mesogens. The
results are collected in Table 1. The N* phases induced by the
monomers 4a,b,d-gsin which a methyl group (and a chlorine
atom in 4e-(S)C3) associated with the stereogenic center is
increasingly remote from the promesogenic core of the guest
along an alkyl chain for the series of S enantiomerssshow
helicities of alternating sense, in accordance with the odd-even
rules16 developed for the prediction of the helicities present in
mesophases.17 Contrastingly, the monomers 4c-(S)C2 and 4c-
(R)C2 incorporating the butyl lactate chain induce cholesteric
phases of opposite helicity to those of the corresponding
enantiomers of 4b in which the 2-octyloxy chain has the same
absolute configuration, despite the fact that the chains have
approximately the same conformation with respect to the phenyl
ring.46 This behavior is thought to be a result of the differing
dipole moments resulting from the two groups (the ester group
in 4c has a dipole moment directed mainly through the long
axis of the promesogenic monomer. This type of longitudinal
dipole can influence dramatically the calamitic nematic behav-
ior,47 overruling the steric factors as the principle cause for the
observance of the alternance rules.
Figure 3. The circular dichroism spectra (recorded in THF) of the
poly(isocyanide)s 5c-(R)C2 (positive Cotton effect at 363 nm) and
5b-(S)C2 (negative Cotton effect at 363 nm) prepared at the concentra-
tions indicated.
group (4d), because of the opposite inductive effect of the groups
and consequent opposed transversal dipole moments of the
molecules as Goodby et al. proposed in similar chlorine-
containing derivatives.48 Again, dipolar characteristics of the
chiral tail are superimposed on pure steric factors in determining
the helicity induced in the mesophase.
It is worth emphasizing that, contrary to the situation in the
polymers, monomer 4a-(R)C1 obeys the odd-even rules in the
induction of both N* and Sc* phases, although its longitudinal
dipole is different to the rest of the promesogenic monomers
where an oxygen atom is the link between the phenyl ring and
the tail. This result, together with the behavior of c-C2 in N*
and e-C3 in Sc* helical phases, supports the hypothesis that the
most important factor in the induction of a given helical sense
in the polymers is a steric one. The operation of steric effects
over such a large distance implied that other noncovalent
interactions may be playing a role in the polymerizations and
led us to study the influence of certain external factors on this
reaction.
Influence of External Factors in Polymerizations of
Promesogenic Monomers
A. Variable Concentration. To investigate the possible role
that noncovalent interactions play in the transfer of the chiral
message from the remote stereogenic center to the polymer
backbone, we designed a series of experiments such that the
initial concentrations of the monomer and catalyst (at fixed
stoichiometry) were varied under otherwise identical conditions.
Monomers 1b-(S)C2 and 1c-(R)C2 were polymerized at con-
centrations ranging from that used for all polymerizations
described above (200 mM) to 2mM, and after 2 days of reaction
(when all traces of monomer were absent) the polymers were
isolated and characterized by GPC and CD. The GPC data
revealed that the molecular weights or sizes of the polymers
produced at different concentrations were essentially the same
for a given monomer. Contrastingly, the CD spectra of the
resulting polymers are strikingly different. Spectra for four of
the polymers 5b-(S)C2 and 5c-(R)C2 generated under these
conditions are shown in Figure 3.
The monomers 4 induce Sc* phases with helicities in full
agreement with a rigorous application of the odd-even rules
to smectogens.17,48 Indeed, when a methyl group is one of the
groups attached to the stereogenic center, in 4a-d, an alternance
is seen in the helices for a given configuration. The sense of
the helix induced in the Sc phase when a chlorine atom is
attached to the stereogenic center (4e) is opposite to that
resulting from that of the corresponding monomer with a methyl
(43) This nematic phase has been used previously in the study of induced
cholesteric phases, see: Nacari, J.; Spada, G. P.; Gottarelli, G.; Weiss, R.
G. J. Am. Chem. Soc. 1987, 109, 4352-4357.
(44) Smectic C host phase MX8056 from Displaytech Inc., Boulder,
Colorado.
(45) Significantly, the sign of the Cotton effect arising from the aromatic
chromophores in the CD spectra of the polymers and induced cholesteric
and smectic C liquid crystalline phases are the same, and defines their helical
sense, which has been assigned M or P on the basis of comparison with
previous studies of polymers and induced phases. See ref 40.
(46) Iglesias, R.; Serrano, J. L.; Sierra, T. Liq. Cryst. 1997, 22, 37-46.
(47) Barbera´, J.; Marcos, M.; Serrano, J. L. Mol. Cryst. Liq. Cryst. 1987,
149, 225.
The CD spectra clearly reveal that as the polymers are formed
under increasingly dilute conditions, so the induction from the
stereogenic center in the monomer to the growing polymer
backbone is reduced. This fact is witnessed most strikingly for
the CD spectra of the polymers derived from 5c-(R)C2, in which
the Cotton effect at 282 nm arising from the carbonyl group
(48) (a) Goodby, J. W.; Leslie, T. M. Mol. Cryst. Liq. Cryst. 1984, 110,
175-203. (b) Terashima, K.; Ichihashi, M.; Kikuchi, K.; Furukawa, K.;
Inukai, T. Mol. Cryst. Liq. Cryst. 1986, 141, 237-249.

9132 J. Am. Chem. Soc., Vol. 120, No. 36, 1998
Amabilino et al.
Table 2. Comparison of the Differential Molar Absorptivities from CD in CHCl3 for the Polymers 5b-(S)C2 Prepared in Different Solvents
hydrogen-bond
hydrogen-bond
dipolarity-polarizability
term52 (π*)a
cavitational parameter
∆ꢀ363
solvent
donor term (R)52
acceptor term (â)52
(Ω/100)53
(L mol-1 cm-1
)
n-C6H14
C6H14
(i-Pr)2O
CCl4
n-C4H9OH
EtOAc
MeOH
MeCOEt
Me2CO
C6H5Cl
CH2Cl2
DMF
0.00
0.00
0.00
0.00
0.79
0.00
0.93
0.06
0.08
0.00
0.30
0.00
0.00
0.00
0.00
0.00
0.49
0.00
0.88
0.45
0.62
0.48
0.48
0.07
0.00
0.69
0.76
0.89
-0.08
0.00
0.27
0.28
0.47
0.55
0.60
0.67
0.71
0.71
0.82
0.88
1.00
1.01
0.50
0.65
0.47
0.71
1.25
0.77
1.98
0.82
0.88
0.91
0.93
1.33
1.64
1.18
-0.93
-1.36
-0.74
-1.11
-0.33
-1.53
-0.60
-1.56
-1.82
-2.22
-3.03
-0.56
-0.91
-1.11
DMSO
C6H5NO2
a The correction factor δ for this term in the LSER expression was 0.001 except for CH2Cl2 and CCl4 (0.5) and C6H5Cl and C6H5NO2 (1.0).
XYZ ) XYZo + aR + bâ + s(π* + dδ) + cΩ/100 (1)
adjacent to the stereogenic carbon atom in the side-chain is
virtually constant, while those at 252 and 363 nm, which
correspond to the backbone of the polymer, vary dramatically.
In an effort to ascertain whether this effect was just a result of
monomer aggregation prior to the polymerization, the isocyanide
from LSER theory, where XYZ is the observable (here the ∆ꢀ363
of the polymer) and XYZo is the value of this observable in the
absence of any solute-solvent interaction, and hydrogen-bond
donor (R) and acceptor abilities (â), dipolarity-polarizability
(π*, and associated correction factor δ), and cavitational53 (Ω)
terms of the solvents are associated with the coeficients a, b, s,
d, and c, repectively, afforded a fit with regression coefficient
(r) ) 0.948 and a significance level (P) ) 0.001. The most
important terms determining the ∆ꢀ363 of polymer 5b-(S)C2 are
those associated with the dipolarity-polarizability and hydrogen-
bond acceptor terms (s ) -2.6, b ) +2.1), followed by those
associated with the cavitational and hydrogen bond donor terms
(c ) +1.2, a ) -1.0), while the dipolarity-polarizability term
correction factor is not significantly influential (d ) 0.2). The
1
1b-(S)C2 was subjected to a variable concentration H NMR
study in CD2Cl2, which showed no chemical shift changes
consistent with an aggregation phenomena.49 The noncovalent
interactions driving the stereoselectivity in these reactions are
therefore not a result of intrinsic recognition between unreacted
monomers, but apparently require covalent modification of the
isocyanide moiety to imine in the growing polymer, perhaps
being mediated by the metal ion catalyst.
B. Solvent. A noncovalent contribution to the transfer of
chirality in the polymerization process should also be influenced
by the solvent in which the reaction is performed. We therefore
carried out the polymerization of 1b-(S)C2 in some 14 different
solvents,50,51 evaluating the chiroptical properties of the resulting
polymer 5b-(S)C2. The ∆ꢀ363 values from the CD spectra of
the polymers are collected in Table 2, along with relevant
physicochemical characteristics of the solvents in which the
polymerizations were performed. At first sight, there was no
obvious relation between the polarity of the solvent and the
chiral induction observed. However, a linear solvation energy
relationship (LSER)52 analysis of the results reveals the phys-
icochemical features of the solvents which influence the
induction. Treatment of the results with the multiparameter
expression (eq 1)
value of XYZo (equivalent to (∆ꢀ363)o) is -1.88 L mol-1 cm-1
.
This result indicates that as the dipolarity-polarizability term
increases, greater induction of chirality is observed, in support
of the proposed influence of noncovalent interactions (aggrega-
tion) between polymer and incoming monomer. Contrastingly,
as the hydrogen-bond accepting ability of the solvent increases,
so induction of chirality is disfavored. The significance of this
term may be a result of competing coordination of the solvent
and the isocyanide monomers with the nickel ion which
catalyzes the polymerization. The counteracting influences of
the hydrogen-bond donating and cavitational terms are unclear
at this stage. The latter might be related with the large molar
volume associated with the aggregate between the polymer and
incoming monomer or with the nature of the intermolecular
forcessdispersive and van der Waalssin the assembly.
Therefore, overall, the stereoselectivity of the polymerization
involves a subtle balance in the intermolecular interactions
where, on one hand, high dipolarity-polarizability and hydrogen-
bond donating terms and, on the other hand, low hydrogen-
bond accepting and cavitational terms favor a high chiral
induction.
(49) Small chemical shift changes in the resonances arising from the
hydrogen atoms attached to the aromatic ring adjacent to the isocyanide
group were observed. It is likely that these changes resulted from the
presence of small quantities residual water (also present in the polymeriza-
tions from the Ni(II) complex). Isocyanides are known to act as a hydrogen-
bond acceptors. See: (a) Schleyer, P. v. R.; Allerhand, A. J. Am. Chem.
Soc. 1962, 84, 1322-1323. (b) Ferstandig, L. L. J. Am. Chem. Soc. 1962,
84, 1323-1324. (c) Ferstandig, L. L. J. Am. Chem. Soc. 1962, 84, 4, 3553-
3557. (d) Allerhand, A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1963, 85,
866-870. In addition to the 1H NMR results, UV-vis spectra of the
monomers at various concentrations and temperatures showed no significant
differences.
Conclusion
(50) (a) Ramos, E. Ph.D. Thesis, Universitat Autonoma de Barcelona,
Spain, 1996. Selection of the 14 solvents, representative of the most relevant
solute-solvent interactions, was made on the basis of solvent classification
guidlines. See: (b) Ventosa, N. Ph.D. Thesis, Universitat Ramon Llull,
Spain, 1997. (c) Ventosa, N.; Ruiz-Molina, D.; Rovira, C.; Toma´s, X.;
Andre´, J.-J.; Veciana, J. Manuscript in preparation.
(51) As far as we are aware, only King and Borodinsky (ref 11c) have
performed a limited study of the effects of solvent on the polymerization
of achiral isocyanides.
Induction of chirality over large distances in the formation
of covalent bonds is remarkable and rare.54 The stereogenic
group present in the monomers is able to transmit its chiral
message over at least 14 atoms (∼16 Å) to the polymerizable
carbon atom. The results presented here are therefore important
for the understanding of chiral induction in general and, in
(52) Taft, R. W.; Abboud, J.-L. M.; Kamlet, M. J.; Abraham, M. H. J.
Soln. Chem. 1985, 14, 153-186.
(53) Hildebrand, J. H.; Scott, R. L. The Solubility of Non-Electrolytes,
3rd ed.; Dover Publications: New York, 1964.

Long-Range Chiral Induction
J. Am. Chem. Soc., Vol. 120, No. 36, 1998 9133
Experimental Section
particular, in the diastereoselective formation of synthetic
polymers and helical liquid crystalline phases.
Materials and Methods. Chemicals were used as purchased from
the Aldrich Chemical Co. and Fluka Chemie AG. The enantiomeric
purity of the enantiomers of 2-octanol (Flukabrand) was assayed by
polarimetry prior to use. Solvents (SDS) were purified according to
literature procedures. (S)-3-Methylpentanol,24 and (S)-(-)-4-methyl-
hexanol25 were prepared according to literature procedures. Column
chromatography was performed using silica gel (60 ACC Chromagel
(SDS), with particle diameters of 70-200 µm for atmospheric pressure,
and 35-70 µm for medium pressure). Melting points were determined
using an Electrothermal melting point apparatus and are uncorrected.
Mass spectra were obtained on a VG TS-250 mass spectrometer using
the electron impact method (70 eV) operating in the positive mode.
The parallels and differences between the odd-even ef-
fects16,17,55 concerning the helical preferences in the polymers
and LC phases indicate that the steric influence of the chiral
tail accounts for the diastereoselectivities observed in the
polymerizations. The relative conformation of the chiral tails
with respect to the promesogenic phenyl benzoate core is
decisive in the induction of a given helical sense in the polymers,
overcoming the influence of the electronic environment of the
stereogenic center, which also contributes in governing the
thermodynamically determined helicities in mesophases. The
helical preference in the kinetically controlled polymerization
reactions15 reported here appears to be influenced by stereo-
selective noncovalent interactions between the promesogenic
cores. Indeed, the influence of noncovalent interactions in
stereoselective reactions is precedented.56 This effect has been
identified by performing the polymerizations at various con-
centrations and in different solvents. We hypothesize that in
the absence of dipolar effects these same interactions result in
the observed induction of N* and Sc* phases57 and which may
be active in the transmission of chiral messages throughout
mesophases and related organized media.58
1
NMR spectra were recorded on a Bruker ARX300 (300 MHz H, 75
MHz 13C) spectrometer using the deuterated solvent as lock and
tetramethylsilane as the internal reference.
Polarimetry was performed using a Dr. Kernchen Optik+Electronik
Propol polarimeter in a 1 cm cell. Circular dichroism spectra were
recorded using spectrometric grade solvents (Romel) in a 1 cm cell at
sample concentrations of 10-3 to 10-5 M using a Jasco J-720
Spectropolarimeter and were analyzed using the associated J700
software. GPC was performed using two TOSOHAAS TSK-gel
columns, G4000HXL and G3000HXL (7.8 mm id × 30 cm) from
Supelco Inc., in series protected by a TSK guard column. Injections
of 50 µL of the polymers dissolved in THF (1 mg/mL) were introduced
and the THF eluant was pumped at 1 mLmin-1 with a Perkin-Elmer
410LC pump. The eluant was monitored simultaneously using a Perkin-
Elmer LC-235 diode array detector and the polarimeter described above
equipped with an HPLC flow cell.
Mesophase Preparation and Characterization. The induced
twisted nematic phases studied by circular dichroism were prepared
by dissolving the monomers (in the form of either their isocyanide 1
or their formamides 4) in either MBBA (Aldrich) or phase ZLI-1052
(Merck, phase sequence X-15-N-48-I) in their isotropic states at 60
°C, then introducing the sample to a 4 µm cell, which had been
pretreated with aligning agent, by capillary action. The mixtures were
composed of 1-2 wt % of the chiral dopant for MBBA and 4-6% in
ZLI-1052. When the mixture had cooled, the presence of cholesteric
phases was confirmed outside the cells by the presence of the typical
oily streak texture under a microscope equipped with cross-polarizers.
The CD spectra of the resulting phase was performed by placing the
sample in the spectropolarimeter cavity perpendicular to the direction
of the source light.
The induced chiral smectic C phases were prepared by dissolving
3-5 wt % of the monomers 4 in a commercial phenylpyrimidine Sc
host (MX8056, phase sequence: X-0.5-Sc-57-SA-76-N)44 in its isotropic
state at 150 °C. The mixtures exhibit three different mesophases: upon
cooling the isotropic liquid a fingerprint texture appeared indicating
the cholesteric phase; further cooling resulted in the formation of a
smectic A phase in a homeotropically oriented form with small areas
of fan-shaped texture; finally, the room temperature smectic C phase
was identified by the appearance of gray-blue marbled and Schlieren
textures which correspond to a pseudohomeotropically oriented sample.
In addition, dechiralization lines appeared in the fan shape texture
indicating the helical organization of the smectic C. All of the mixtures
were studied in 5 µm ITO cells with polyimide coating for a planar
alignment (purchased from LINKAM), and showed optical switching
when an a.c. external field was applied to the chiral smectic C phase,
confirming its ferroelectric nature. The Ps values could not be
accurately determined as they were below the sensitivity of the
measuring system. The CD spectra were recorded on free-standing
films38 of the mixtures in the smectic C phase.
(R)-4-(1-(Hexyloxy)ethyl)phenol (3a-(R)C1). A solution of (R)-
4-(1-(hexyloxy)ethyl)acetophenone (100 mg, 0.4 mmol) in dry CHCl3
(0.5 mL) was added to a solution of m-chloroperbenzoic acid (142 mg,
0.79 mmol) in dry CHCl3 (2 mL), and the mixture was stirred for 10
days. The mixture was diluted with CHCl3, and this solution was
extracted with NaHCO3 (aqueous, saturated) and water and dried
(MgSO4), and the solvent was removed. The crude product was
subjected to flash column chromatography (SiO2, hexanes:ethyl acetate
All our evidence presented points to polymers with stable
optically active helical conformations which are rather rigid and
are determined largely during their creation, not being free to
reorient once they are formed, in contrast to other more flexible
helical polymers.
To shed more light on the steroselective mechanism through
which such long-range chiral induction is produced, we are
currently exploring the effect of changing the spacer between
the benzene rings in the monomer unit. In addition, poly-
(isocyanide)s have been shown to exhibit interesting material
properties,59 and we are presently studying the unique charac-
teristics of the variety of polymers presented here. For now,
they stand as a rare and extraordinary example of long-range
chiral induction in covalent bond formation in polymers and
highlight the parallels of this phenomenon with long-range chiral
induction propagated by noncovalent interactions in liquid
crystals.
(54) For two recent articles, see: (a) Heinemann, C.; Demuth, M. J.
Am. Chem. Soc. 1997, 119, 1129-1130. (b) Linwane, P.; Magnus, N.;
Magnus, P. Nature 1997, 385, 799-801. The latter example is a result of
a folded conformation brought about by cation binding. For a very recent
report has detailed long distance transfer of chirality in the formation of
poly(isocyanide)s derived from helicenes, see: Chen, J. P.; Gao, J. P.; Wang,
Z. Y. Polym. Int. 1997, 44, 83-87.
(55) Odd-even effects in aliphatic chains have also been observed in
amphiphilic assemblies, see: (a) Yamada, N.; Okuyama, K.; Serizawa, T.;
Kawasaki, M.; Oshima, S. J. Chem. Soc., Perkin Trans. 2 1996, 2707-
2713. (b) Shimizu, T.; Mitsutoshi, M. J. Am. Chem. Soc. 1997, 119, 2812-
2818.
(56) The influence of π-π interactions in synthesis has been discussed.
For a review, see: (a) Jones, G. B.; Chapman, B. J. Synthesis 1995, 475-
497. For concrete examples, see: (b) Corey, E. J.; Becker, K. B.; Varma,
R. K. J. Am. Chem. Soc. 1972, 94, 8616-8618. (c) Norrby, P.-O.; Becker,
H.; Sharpless, K. B. J. Am. Chem. Soc. 1996, 118, 35-42. (d) Quan, R.
W.; Li, Z.; Jacobsen, E. N. J. Am. Chem. Soc. 1996, 118, 8156-8157. (e)
Corey, E. J.; Noe, M. C. J. Am. Chem. Soc. 1996, 118, 11038-11053.
(57) Dipole-dipole interactions leading to helical structures induced in
nematic liquid crystals has been proposed. See: Khachaturyan, A. G. J.
Phys. Chem. Solids 1975, 36, 1055-1061.
(58) Walba, D. M.; Stevens, F.; Clark, N. A.; Parks, D. C. Acc. Chem.
Res. 1996, 29, 591-597.
(59) (a) Teerenstra, M. N.; Klap, R. D.; Bijl, M. J.; Schouten, A. J.;
Nolte, R. J. M.; Verbiest, T.; Persoons, A. Macromolecules 1996, 29, 4871-
4875. (b) Teerenstra, M. N.; Hagting, J. G.; Schouten, A. J.; Nolte, R. J.
M.; Kauranen, M.; Verbiest, T.; Persoons, A. Macromolecules 1996, 29,
4876-4879.

9134 J. Am. Chem. Soc., Vol. 120, No. 36, 1998
Amabilino et al.
90:10). Complete separation of the acetate from the starting material
proved impractical. The impure material (70 mg) was dissolved in a
mixture of MeOH (3 mL) and H2O (1 mL), LiOH‚H2O (70 mg, 1.7
mmol) was added, and the mixture was stirred for 2 h. The mixture
was poured onto a mixture of ice and HCl (aq), and the mixture was
extracted with ethyl acetate. The organic phase was washed with water
and brine and dried over MgSO4, and the solvent was removed. The
crude phenol was purified by flash column chromatography (SiO2,
hexanes:ethyl acetate 85:15): white solid, 90%; 1H NMR (CDCl3, 300
MHz) δ 0.84 (t, J ) 6.6 Hz, 3H, CH2CH3), 1.14-1.48 (m, 6H, CH2),
1.39 (d, J ) 6.5 Hz, 3H, CHCH3), 1.52 (m, 2H, CH2), 3.25 (t, J ) 6.6
Hz, 2H, OCH2), 4.32 (q, J ) 6.6 Hz, 1H, CHCH3), 5.40 (s, 1H, OH),
6.79 (d, J ) 8.5 Hz, 2H, Ho to OH), 7.16 (d, J ) 8.5 Hz, 2H, Ho to
10%) and H2O, dried (Na2SO4), filtered, and concentrated in vacuo.
The crude products were purified by flash column chromatography
(SiO2, hexane:EtOAc, 3:1). The products were stable in CH2Cl2
solution under Ar at reduced temperatures for months, but in general
were unstable at room temperature in their pure state as isolated.
Representative analytical data is given for (R)-4-(1-(methylheptyl)oxy)-
phenyl 4-isocyanobenzoate (1b-(R)C2): off-white solid, 78%; EIMS
351 [M+]; IR (NaCl plate) 2123 (NC), 1736 (CO) cm-1 1H NMR
;
(CDCl3, 300 MHz) δ 0.89 (t, J ) 7 Hz, 3H, CH2CH3), 1.1-1.8 (m,
13H, CHCH3, and (CH2)5), 4.33 (q, J ) 6 Hz, 1H, CHCH3), 6.92 (d,
J ) 9 Hz, 2H, CHo to OCHCH3), 7.10 (d, J ) 9 Hz, 2H, CHm to
OCHCH3), 7.51 (d, J ) 9 Hz, 2H, CHo to NC), 8.24 (d, J ) 9 Hz, 2H,
CHm to NC); [R]25 (CH2Cl2) -5.2 deg cm2 g-1. The (S) enantiomer
546
CH); 13C NMR (CDCl3, 75.47 MHz) δ 14.0, 22.6, 24.0, 25.8, 29.8,
(69% yield) has identical physical and spectroscopic characteristics and
25
25
546
[R]
(CH2Cl2) +5.2 deg cm2 g-1
. The corresponding racemic
31.6, 68.6, 77.4, 115.2, 127.5, 136.1, 155.0; [R] (CHCl3) +54.8 deg
546
cm2 g-1
.
material (87% yield) has identical physical and spectroscopic charac-
teristics and exhibits no rotation of plane-polarized light.
General Procedure for Synthesis of Phenols 3b-g. The appropri-
ate benzyl-protected phenol was dissolved in CH2Cl2, and 10% Pd/C
(Degussa type, 20 wt % of substrate) was added. An atmosphere of
H2 (1 atm) was introduced, and the mixture was allowed to stir at
ambient temperature for 4 h. The crude mixture was filtered through
Celite, the solvent was stripped, and the product was subjected to flash
column chromatography (SiO2, hexane:ethyl acetate, using a composi-
tion from 97:3 to 9:1 by volume depending on the product). Repre-
sentative analytical data is given for (R)-4-(1-(methylheptyl)oxy)phenol
General Procedure for the Preparation of Polymers 5a-g. The
appropriate isocyanide 1a-g was dissolved in dry CH2Cl2 at a
concentration of freshly prepared isocyanide monomer of approximately
200 mM, and 0.01 mol equiv of NiCl2‚6H2O in MeOH (42 mM) was
added, air being present. The mixture, which immediately turned dark
brown, was stirred at ambient temperature in a sealed vial for 2 days.
The solvent was evaporated, MeOH was added, and the resulting tan-
colored solid was filtered at the pump, washed with two aliquots of
MeOH, and dried in vacuo. The chiroptical data for the polymers are
collected in Table 1 in the text. Representative analytical data is given
for (+)-poly-{(R)-4-(1-(methylheptyl)oxy)phenyl 4-iminobenzoate} (5b-
1
(3b-(R)C2): colorless oil, 93%; H NMR (CDCl3, 300 MHz) δ 0.86
(t, J ) 7 Hz, 3H, CH2CH3), 1.23 (d, J ) 6 Hz, 3H, CHCH3), 1.29-
1.75 (m, 10H, (CH2)5), 4.17 (q, J ) 6 Hz, 1H, CHCH3), 4.69 (s, 1H,
OH), 6.72 (d, J ) 9 Hz, 2H, OArO), 6.76 (d, J ) 9 Hz, 2H, OArO).
General Procedure for Synthesis of Phenyl Benzoate Formamide
Derivatives 4a-g. The appropriate phenol 3a-g (1 mol equiv) was
combined in dry CH2Cl2 (typically 6 mL for 1.5 mmol of product)
with 4-formamidobenzoic acid (1 mol equiv), DCC (1 mol equiv), and
DMAP (catalytic quantity), and the mixture was stirred in dry at ambient
temperature for 16 h. Filtration of the reaction mixture (to remove
precipitated dicyclohexylurea) afforded a solution which was diluted
with CH2Cl2 and extracted with NaOH (aqueous, 2%, 2 × 50 mL) and
H2O (50 mL). Drying (Na2SO4) and removal of solvent afforded white
solids which were subjected to flash column chromatography (SiO2,
1:1 ethyl acetate:hexane by volume) leaving the products as white solids.
Representative analytical data is given for (R)-4-(1-(methylheptyl)oxy)-
phenyl 4-formamidobenzoate (4b-(R)C2): white solid, 85%; mp 128-
(R)C2): 87%; IR (KBr) 1740 (CO), 1656 (CdN) cm-1 1H NMR
;
(CDCl3, 300 MHz) δ 0.65-0.94 (bm, 3H, CH2CH3), 0.96-1.80 (bm,
13H, CHCH3 and (CH2)5), 3.95-4.33 (bm, 1H, CHCH3), 5.45-6.30
(bm, 2H), 6.31-7.25 (bm, 4H), 7.26-8.15 (bm, 2H); 13C NMR (CDCl3,
75.47 MHz) δ 14.1, 19.6, 22.6, 25.5, 29.4, 31.8, 36.7, 74.1, 116.1,
116.5 (vb), 122.3, 128.4 (b), 130.0 (vb) 143.8 (b), 150.9 (b), 155.6,
160.7 (b), 162.3 (b), 164.0 (b). The (-)-(S) diastereomer and the
corresponding racemic modification were isolated in 93 and 89% yields,
respectively, and had identical spectroscopic characteristics.
Acknowledgment. This work was supported by a project
from Fundacio´n Ramo´n Areces. E.R. thanks the Generalitat de
Catalunya (CIRIT) for a doctoral fellowship, and D.B.A.
received postdoctoral support from the Ministerio de Educacio´n
y Cultura (Modalidad A) and the CSIC in Spain. We warmly
thank Merck in Darmstadt for providing ZLI-1052. We also
warmly thank Dr. Nora Ventosa (Institut de Cie`ncia de Materials
de Barcelona, CSIC) for guidance concerning the LSER
analysis.
1
129 °C; EIMS 369 [M+]; H NMR (CDCl3, 300 MHz) δ 0.89 (t, J )
7 Hz, 3H, CH2CH3), 1.1-2.0 (m, 13H, CHCH3 and (CH2)5), 4.32
(quintet, J ) 6 Hz, 1H, CHCH3), 6.90 (d, J ) 9 Hz, 2H, Ho to OR),
7.08 (d, J ) 9 Hz, 2H, Hm to OR), 7.10 (d, J ) 9 Hz, 0.7H, Ho to NH),
7.68 (d, J ) 9 Hz, 1.3H, Ho to NH), 8.13 (d, J ) 9 Hz, 1.3H, Hm to
NH), 8.18 (d, J ) 9 Hz, 0.7H, Hm to NH), 8.33 (s, 0.65H, HCO in
cis), 8.47 (s, 0.65H, NH in cis), 8.92 (d, J ) 11 Hz, 0.35H, HCO in
trans), 9.07 (d, J ) 11 Hz, 0.35H, NH in trans); 13C NMR (CDCl3,
75.47 MHz) δ 14.1, 19.7, 22.6, 25.5, 29.3, 31.8, 36.5, 74.6, 116.6,
Supporting Information Available: Complete experimental
procedures for the preparation of 4-formamidobenzoic acid,
1-(4-acetylphenyl)ethanol, (R)-1-(4-acetylphenyl)ethyl acetate,
(R)-1-(4-acetylphenyl)ethanol, (R)-4-(1-(hexyloxy)ethyl)aceto-
phenone, (R)-1-(1-(methylheptyl)oxy)4-(benzyloxy)benzene, (R)-
2-[(4-benzyloxy)phenoxy]-n-butyl propionate, and (R)-2-[4-
hydroxyphenoxy]-n-butyl propionate, and analytical data for 1a-
(R)C1, 1c-(R)C2, 1d-(S)C3, 1e-(S)C3, 1f-(S)C4, 1g-(S)C5, 4a-
(R)C1, 4c-(R)C2, 4d-(S)C3, 4e-(S)C3, 4f-(S)C4, 4g-(S)C5, 5a-
(R)C1, 5c-(R)C2, 5d-(S)C3, 5e-(S)C3, 5f-(S)C4, 5g-(S)C5, as
well as Figures S1-S4 showing the CD spectra of the induced
cholesteric and smectic C mesophases, and the dipoles respon-
sible for effects on the helical induction in the LCs (13 pages,
print/PDF). See any current masthead page for ordering
information and Web access instructions.
117.2, 119.3, 122.4, 125.2, 125.8, 131.4, 132.1, 141.8, 142.0, 144.1,
25
546
156.1, 159.6, 162.2, 165.1, 165.3; [R] (CH2Cl2) -5.2 deg cm2 g-1
.
Anal. Calcd. for C22H27NO4: C, 71.52; H, 7.37; N, 3.79. Found: C,
71.84; H, 7.54; N, 4.05. The (S) enantiomer (79% yield) has identical
25
physical and spectroscopic characteristics and [R] (CH2Cl2), +5.2
546
deg cm2 g-1
. The corresponding racemic material (82% yield) has
identical physical and spectroscopic characteristics and exhibits no
rotation of plane-polarized light.
General Procedure for Synthesis of Phenyl Benzoate Isocyanide
Derivatives 1a-g. The appropriate formamide 4a-g (1 mol equiv)
was combined in dry CH2Cl2 (typically 6 mL for 1.5 mmol of product)
with dry NEt3 (2 mol equiv), and the mixture was cooled in an ice/salt
mixture under an atmosphere of Ar. Trichloromethylchloroformate
(diphosgene, 0.55 mol equiv) in dry CH2Cl2 was added dropwise by
syringe, and the mixture was stirred while warming to ambient
temperature for 1 h. Aqueous NaHCO3 (10%) and CH2Cl2 were added,
and the separated organic phase was washed with NaHCO3 (aqueous,
JA980474M