Molecules with shape memory based on restricted rotation

Article abstract of DOI:10.1021/ol0167098

Figure presented Diimide 1 and octaimide 2 both adopt two stable conformations at room temperature as a result of restricted rotation about two C_aryl-N_imide single bonds, a compact "folded" and an open "unfolded" structure. Predictable ratios of folded and unfolded rotamers can be achieved by heating in solvents of appropriate polarity as measured by the Reichardt's parameter (E_T30). On cooling to room temperature, the resulting conformational changes are "locked in" as restricted rotation is reestablished.

Full text of DOI:10.1021/ol0167098

ORGANIC
LETTERS
2001
Vol. 3, No. 23
3757-3760
Molecules with Shape Memory Based on
Restricted Rotation
Dong-Sook Choi, Yong S. Chong, Daniel Whitehead, and Ken D. Shimizu*
Department of Chemistry and Biochemistry, UniVersity of South Carolina,
Columbia, South Carolina 29208
shimizu@mail.chem.sc.edu
Received September 6, 2001
ABSTRACT
Diimide 1 and octaimide 2 both adopt two stable conformations at room temperature as a result of restricted rotation about two Caryl−N
imide
single bonds, a compact “folded” and an open “unfolded” structure. Predictable ratios of folded and unfolded rotamers can be achieved by
heating in solvents of appropriate polarity as measured by the Reichardt’s parameter (ET30). On cooling to room temperature, the resulting
conformational changes are “locked in” as restricted rotation is reestablished.
One of the most attractive aspects of polymers is their ability
to be cast and recast into different forms and shapes at the
bulk level. Our goal has been to achieve this level of control
on the molecular level. Conformational control can be an
effective strategy to introduce new or tune existing properties
in molecular and macromolecular systems. One route is to
identify systems that adopt different conformations in dif-
ferent solvents.1 However, in most cases, these solvent-
induced conformations are fragile and disappear upon
removal from their promoting environments. We set out to
design a class of polymers based on restricted rotation that
could be shaped at elevated temperatures but would become
rigid and shape-persistent on cooling, thereby preserving the
solvent-induced conformations.2 In this manner, they could
be cast and recast into different shapes simply by heating
and cooling in different environments and would retain these
new shapes even upon transfer to a different environment.
To demonstrate the feasibility of this strategy diimide 1
and octaimide 2 were synthesized and studied. Both diimide
1 and octaimide 2 adopt two stable conformations: conver-
gent “folded” and divergent “unfolded” states that are well-
differentiated in terms of shape, size, and polarity (Scheme
1). These conformations arise from restricted rotation about
connecting Caryl-Nimide bonds within the rigid 1,4,5,8-
naphthalene diimide framework.2a,3 Restricted rotation about
the Caryl-Nimide bonds was imposed by the steric presence
of ortho-ether substituents.4 The long octadecyl ortho-ethers
serve to endow the rigid polyimide framework with excellent
organic solubility and also accentuate the differences in size
and shape between the two conformers. Finally, the systems
contain pendent nitrile groups that serve as strong polarizing
groups,5 by conferring very different dipole moments on the
(1) For recent examples of solvent-induced conformational changes,
see: (a) Leis, J.; Klika, K. D.; Karelson, M. Tetrahedron 1998, 54, 7497-
7504. (b) Palyulin, V. A.; Emets, S. V.; Chertkov, V. A.; Kasper, C.;
Schneider, H. J. Eur. J. Org. Chem. 1999, 3479-3482. (c) Gardner, R. R.;
McKay, S. L.; Gellman, S. H. Org. Lett. 2000, 2, 2335-2338. (d) Paliwal,
S.; Geib, S.; Wilcox, C. S. J. Am. Chem. Soc. 1994, 116, 4497-4498. (e)
Clegg, W.; Gimenez-Saiz, C.; Leigh, D. A.; Murphy, A.; Slawin, A. M.
Z.; Teat, S. J. J. Am. Chem. Soc. 1999, 121, 4124-4129. (f) Leigh, D. A.;
Moody, K.; Smart, J. P.; Watson, K. J.; Slawin, A. M. Z. Angew. Chem.
Int. Ed. Engl. 1996, 35, 306-310. (g) Janout, V.; Lanier, M.; Regen, S. L.
J. Am. Chem. Soc. 1996, 118, 1573-1574. (h) Hoger, S.; Meckenstock, A.
D.; Muller, S. Chem. Eur. J. 1998, 4, 2423-2434. (i) Moore, J. S. Acc.
Chem. Res. 1997, 30, 402-413.
(2) (a) Chong, Y. S.; Smith, M. D.; Shimizu, K. D. J. Am. Chem. Soc.
2001, 123, 7463-7464. (b) Degenhardt, C., III; Shortell, D. B.; Adams, R.
D.; Shimizu, K. D. J. Chem. Soc., Chem. Commun. 2000, 929-930.
(3) (a) Curran, D. P.; Geib, S.; DeMello, N. Tetrahedron 1999, 55, 5681-
5704. (b) Verma, S. M.; Singh, N. D. Aust. J. Chem. 1976, 29, 295-300.
(4) Shimizu, K. D.; Dewey, T. M.; Rebek, J. J. Am. Chem. Soc. 1994,
116, 5145-5149.
10.1021/ol0167098 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/23/2001

Diimide 1 is the shortest model system in which restricted
rotation can be easily observed. The presence of restricted
rotation in diimide 1 was immediately apparent by the
isolation of two spectroscopically similar compounds that
interconverted on heating. A particularly useful method for
identifying rotamers was the utilization of two-dimensional
TLC. The isomers were first separated on a square TLC plate.
The plate was heated to re-equilibrate the rotamers and then
run in the perpendicular direction. The appearance of new
spots off of the diagonal identified the compounds that are
conformational rotamers. The individual syn and anti con-
formers of 1 were sufficiently stable at room temperature to
be separated and isolated by flash chromatography (1.5%
EtOAc in CH2Cl2, Merck silica gel 60). The rotational
barriers about the Caryl-Nimide bonds were measured from
the equilibration rates of the pure syn and anti isomers at 64
°C in CHCl3. Both isomers concur on a rotational barrier of
27 kcal/mol, which corresponds to a half-life of 11 h at 64
°C or 93 days at 23 °C.6
Scheme 1
Assignments of the syn and anti isomers of model
compound 1 were initially complicated by the high sym-
metries of the isomers, which thwarted their structural
assignment by NMR spectroscopy. However, the differences
in symmetry of the isomers did enable differentiation and
assignment by their different dipole moments, as measured
by TLC and by their nonequivalent equilibrium ratios. The
use of elution order in TLC has been successfully applied
elsewhere in the atropisomers of tetraarylporphyrins and
N,N′-diaryl-1,4,5,8-perylenediimides.4,7 The more polar syn
isomer was assigned as the lower Rf compound. Further
support for the TLC-based assignments was provided by the
observed differences in equilibrium ratios in different
solvents. For example, the rotational barrier studies revealed
that diimide 1 attains a 43:57 equilibrium ratio in chloroform
with the higher Rf less polar anti rotamer being favored. We
assigned the major isomer as the anti conformer, which is
presumably preferred in nonpolar solvents as a result of its
smaller dipole moment. These differences in dipole moment
were expected to be expressed to different extents depending
upon the medium, and accordingly in a more polar solvent
such as DMSO, a 1:1 equilibrium ratio was observed.
Octaimide 2 was then synthesized to demonstrate the ease
with which this platform can be extended to larger systems.
In the case of oligomer 2, restricted rotation is present only
about the central naphthalene diimide surface, again yielding
two conformational isomers, as only the four central of the
eight Caryl-Nimide bonds show restricted rotation. The larger
octaimide 2 was assembled in a stepwise fashion from the
appropriate amine and anhydride components and could be
accomplished without the necessity of protecting groups
(Scheme 3). For example, diamine 9 was formed selectively
by condensation of aniline 8 with 1,4,5,8-naphthalenetetra-
carboxylic at 150 °C. Condensation reactions carried out at
higher temperatures (160 °C) led to oligomerization, whereas
syn and anti conformers. This enables control over the syn/
anti equilibrium, and therefore shape, by choice of the
dielectric constant of the surrounding medium.
An attractive feature of this rigid shape-persistent frame-
work is the ease with which it can be assembled. For
example, diimide 1 was synthesized in 86% yield by simply
condensing the appropriate aniline and anhydride compo-
nents, 6 and 1,4,5,8-naphthalenetetracarboxylic dianhydride
(NTDA), in DMF (Scheme 2). The requisite ortho-aniline 6
Scheme 2a
a (a) K2CO3, Br(CH2)17CH3, acetone, ∆ (98%). (b) fuming HNO3,
23 °C (93%). (c) cyclohexene, 10% Pd/C, EtOH, reflux (99%). (d)
DMF, 130 °C, 12 h (86%).
(5) Bezborodov, V. S.; Petrov, V. F. Liq. Cryst. 1997, 23, 771-788.
(6) Calculated from the Arrhenius equation assuming an ideal value of
A ) 2.08 × 1010 s-1 deg-1. The ratio of isomers was monitored by HPLC.
(7) Sanders, G. M.; Van Dijk, M.; Machiels, B. M.; van Veldhuizen, A.
J. Org. Chem. 1991, 56, 1301-1305.
was synthesized in three steps from 4-cyanophenol 3 via
alkylation, nitration, and reduction.
3758
Org. Lett., Vol. 3, No. 23, 2001

observed for the cis and trans isomers of photoresponsive
azobenzene containing polymers.8 The differences in size
were experimentally apparent from gel permeation chroma-
tography (GPC) (Figure 1). Despite the identical molecular
Scheme 3a
Figure 1. GPC traces of anti-2 and syn-2. Conditions: Waters
Styragel HMW 2 column at 1 mL/min, in chloroform.
weight of the isomers, GPC was able to differentiate syn-
and anti-2, yielding estimated molecular weights (Mn) of
2665 and 2457, respectively, on the basis of polystyrene
standards. The unfolded anti isomer eluted first, demonstrat-
ing that it had a larger hydrodynamic volume and giving
additional support for the initial dipole-based assignments.
In addition, the GPC measured molecular weights of both
syn-2 and anti-2 were inflated in comparison to their absolute
molecular weights (2286), which is consistent with what has
been observed for other shape-persistent systems.9
a (a) 90% HNO3, 23 °C (49%). (b) H2, 10% Pd/C, EtOH (92%).
(c) 0.5 equiv of 1,4,5,8-naphthalenetetracarboxylic dianhydride,
CH2Cl2, 23 °C then neat, 150 °C in vacuo, 3 days (44%). (d) CH2Cl2
23 °C, then neat 170 °C in vacuo, 12 h (36%). (e) H2, 10% Pd/C,
EtOH (97%). (f) phthalic anhydride, DMF, reflux 12 h (59%). (g)
6 equiv of 1,4,5,8-naphthalenetetracarboxylic dianhydride, DMF,
reflux.
The unique aspect of octaimide 2 is its dynamic shape-
persistent and shape-adaptable qualities. Control of the anti/
syn equilibria at elevated temperatures would enable the
oligomers to be “molded” into one shape or the other and
then to hold that shape. A possible strategy was suggested
from studies of the model compound 1 of using differences
in dipole moment of the conformers to control morphology.10
These differences should be expressed to a different extent
depending upon the dielectric constant of the medium. To
test this strategy, oligomer 2 was equilibrated in a range of
different solvents, by heating for greater than 10 half-lives.
On cooling to room temperature, the anti/syn equilibria ratios
were preserved as a result of the re-establishment of restricted
rotation even when removed from the promoting environ-
ment. The anti/syn ratios were then measured by integration
of the central naphthalene proton by 1H NMR in CDCl3. The
heating experiments demonstrated that in more polar solvents
such as acetonitrile the differences in dipole are minimized,
and the anti/syn ratio approaches 1:1. In nonpolar solvents
at lower temperatures (140 °C) no reaction was observed.
Reaction of the first amine of 8 apparently reduces the
nucleophilicity of the second such that oligomerization is
suppressed, eliminating the need for protecting groups. The
final condensation reaction between diamine 9 and monoan-
hydride 10 was carried out under more forcing conditions
(170 °C, neat, in vacuo) because of lower reactivity of the
amines in 9, and the product, octaimide 2, was isolated in
36% yield.
Like model system 1, oligoimide 2 was isolated as a
mixture of stable syn and anti conformers. The syn and anti
conformers were separated by preparative TLC and assigned
on the basis of polarity arguments and also by differences
in size and shape (vide infra). The rotational barrier of 2
(26.5 kcal/mol) was similar to that of model system 1 (27
kcal/mol) and was calculated from the rate of reequilibration
of the pure syn and anti isomers in acetonitrile at 80 °C.
Molecular models predicted that the two conformational
isomers would have drastically different shapes. The rigid
framework coupled with the limited conformational freedom
enabled an accurate measure of the dimensions of syn- and
anti-2. The syn isomer has a curved compact shape; whereas,
the anti isomer has a larger extended shape (Scheme 1).
Similar differences in hydrodynamic volume have been
(8) (a) Junge, D. M.; McGrath, D. V. J. Am. Chem. Soc. 1999, 121,
4912-4913. (b) Izumi, A.; Teraguchi, M.; Nomura, R.; Masuda, T.
Macromolecules 2000, 33, 5347-5352. (c) Beattie, M. S.; Jackson, C.;
Jaycox, G. D. Polymer 1998, 39, 2597-2605.
(9) (a) Rader, H. J.; Spickermann, J.; Mullen, K. Macromol. Chem. Phys.
1995, 196, 3967-3978. (b) Rader, H. J.; Spickermann, J.; Kreyenschmidt,
M.; Mullen, K. Macromol. Chem. Phys. 1996, 197, 3285-3296.
(10) Dipole moments of anti- and syn-2 were calculated as 0.66 and
6.05 D using MacSpartan (semiemperical, AM1).
Org. Lett., Vol. 3, No. 23, 2001
3759

effects such as steric or attractive interactions.12 Molecular
models confirm that in the syn isomer there is the possibility
for significant interactions between the octadecyl chains.
Although either conformer can be favored, complete
control over the morphology of oligomer 2 was not observed,
as the differences in dipole moment were not sufficient to
shift the equilibria entirely to one conformation or the other.
As a consequence, this system is more akin to a two-
component polymer blend in which the ratios of each
component can be adjusted simply by heating in different
solvents. These ratios are then preserved on cooling. The
properties of this “blend” can be tuned back and forth without
reformulating the blend by addition of one or the other
component.
Figure 2. The observed energy differences between the anti-2 and
syn-2 in different solvents plotted against the solvent polarity scale
Octaimide 2 represents an example of a molecule that can
be shaped on the molecular level. This was accomplished
by designing a shape-persistent/shape-adaptable system,
which arises from restricted rotation about connecting
Caryl-Nimide single bonds. At elevated temperatures the
molecules can be predictably shaped into either a folded syn
or an unfolded anti conformation having unique shapes,
hydrodynamic volumes, and dipole moments by choice of a
particular solvent environment. On cooling to room temper-
atures, these conformational preferences are saved and
preserved as restricted rotation is reinstated. In this way, the
properties of the oligomer can be modulated or tuned by
controlling the percentage of the syn and anti conformations.
ET(30)
.
such as toluene and benzene, the less polar anti isomer is
favored (64:36). A regular relationship between conforma-
tional ratio and solvent polarity was observed by a roughly
linear relationship between ∆G° versus the solvent polarity
scale ET(30) (Figure 2). This predictable relationship enabled
precise control over the anti/syn ratio by choice of a solvent
of appropriate polarity.
Alternatively, a 35:65 anti/syn ratio was observed on
heating in the solid state (neat in vacuo at 80 °C for 24 h).
This preference for the syn isomer is possibly due to the
dominance of packing effects over dipole effects. The
influence of effects other than differences in dipole moments
on the syn/anti equilibrium is seen even in solution. If the
equilibrium ratio were controlled solely by solvent polarity,
then a linear relationship between ∆G° and the Kirkwood
parameter ((ꢀ - 1)/(2ꢀ + 1)) is predicted.11 However, this
plot shows considerably more scatter than the ∆G° versus
ET(30) plot. Thus, the relatively linear relationship between
∆G° and ET(30) suggests that the conformational equilibrium
is controlled not by dipole moment alone but also by other
Acknowledgment. This work was supported by research
grants from the NSF (9807470 and 9820502) and the
Research Corporation Research Innovation Award.
Supporting Information Available: Synthetic procedures
and characterization for all new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL0167098
(11) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry;
VCH: Weinheim, 1990.
(12) Leis, J.; Klika, K. D.; Karelson, M. Tetrahedron 1998, 54, 7497-
7504.
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