Synthesis and solid-state dynamics of molecular dirotors

Article abstract of DOI:10.1016/j.tet.2008.05.143

We report the design, synthesis, thermal properties, and dynamic NMR characterization of four new molecular dirotors, each containing two rotary 1,4-diethynyl phenylene units, a bridging 1,3-bis(diphenylmethyl) benzene stator, and two capping triphenylmet

Full text of DOI:10.1016/j.tet.2008.05.143

Tetrahedron 64 (2008) 8336–8345
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and solid-state dynamics of molecular dirotors
*
Stephanie L. Gould, Richard B. Rodriguez, Miguel A. Garcia-Garibay
Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the design, synthesis, thermal properties, and dynamic NMR characterization of four new
molecular dirotors, each containing two rotary 1,4-diethynyl phenylene units, a bridging 1,3-bis(diphe-
nylmethyl) benzene stator, and two capping triphenylmethyl groups. These compounds are variations of
the well-known wheel and axle structures. Ambient temperature quadrupolar echo 2H NMR spectra of
phenylene-deuterated samples revealed line shapes consistent with dynamic processes having exchange
rates >108 sꢀ1, and more than the two minima typically observed for most phenylene rotators. Rea-
sonable line-shape simulations were obtained with a model that involves four minima related by angular
displacements of 0ꢁ, 30ꢁ, 180ꢁ, and 230ꢁ. Cooling the samples to 260 K revealed spectra consistent with
samples that contain two or more components. These results indicate that novel wheel and axle
structures do create a certain amount of free volume that allows for the desired mobility of the rotating
units.
Received 14 March 2008
Received in revised form 15 May 2008
Accepted 30 May 2008
Available online 6 June 2008
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
anisotropic parameters,17 force-field models,18 and dielectric
spectroscopy with fluorinated analogs,19,20 we have shown that the
Most recent progress in the field of artificial molecular ma-
chinery has been based on the design and testing of functional
molecules in solution.1–3 Recognizing that macroscopic and bi-
ological machineries are part of complex assemblies with a well-
defined and external frame of reference, recent efforts in the
community have shifted toward the construction of artificial mo-
lecular machinery supported on surfaces4–6 and three-dimensional
crystalline aggregates.7–9 To describe the coexistence of rigid and
mobile elements in ordered arrays we recently suggested the term
‘amphidynamic crystals’.10,11 With hopes of emulating machines in
the macroscopic and biomolecular realms, we suggested that
amphidynamic materials could be built with structural designs
based on: (a) free volume compartments, (b) volume-conserving
motions, such as the rotation of a cylinder along its principal axis,
and/or (c) the concerted motions of two or more components.
Our initial ventures into molecular machinery focused on the
design and study of crystals built with molecules that mimic some
elements of macroscopic gyroscopes (Fig. 1). These compounds
have a phenylene rotator with a virtually barrierless dialkyne
linkage12 (shown in red), and two bulky triarylmethyl13–15 (or
triptycyl)16 groups that provide steric shielding and define a static
frame of reference in the crystal (shown in blue). Using variable
temperature solid-state NMR spectroscopy,13–16 X-ray thermal
internal dynamics of molecular gyroscopes with phenylene rotators
depend on the structure of the stator.10,11 Crystalline solids with
* Corresponding author. Tel.: þ1 310 825 3159; fax: þ1 310 825 0767.
E-mail address: mgg@chem.ucla.edu (M.A. Garcia-Garibay).
Figure 1. An illustration of a macroscopic toy gyroscope (top), and molecular gyro-
scopes with a single rotator having open (bottom) topology.
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.05.143

S.L. Gould et al. / Tetrahedron 64 (2008) 8336–8345
8337
molecular gyroscopes having various triarylmethyl (trityl) stators
showed ambient temperature rotational frequencies varying from
ca. 103 sꢀ1 (Fig. 1, R1¼R2¼H) to ca. 109 sꢀ1 (R1¼R2¼t-Bu). Their
packing properties are consistent with those of molecules with
a long and rigid rod capped with bulky ends. With shapes that
resemble a wheel and an axle, these molecules, are known to form
relatively low-density crystals with a tendency to trap solvent of
crystallization.21,22
blue and rotators in red. Among them are structures with two
stators and several internal rotators in the same rotary axle
(Fig. 2b), structures with two (or more) rotators sharing a common
central stator (Fig. 2c), and cyclic structures with two or more
stators that are either covalent linked (Fig. 2d), or shared (Fig. 2e).
Notably, multirotor connectivities such as that illustrated in
Figure 2d can be extended in extended 3D dimensional lattices by
taking advantage of metal organic frameworks (MOFs).31
Analyzing wheel and axle structures, Soldatov22 recently noted
that in addition to structures with two bulky ends linked by a rigid
rod,23–25 low-density crystals are also formed by structures with
shapes that consist of rigid platforms penetrated by a rod,1d,26,27 or
several platforms penetrated or interconnected by one or more
rods.28,29 The common feature of these structural motifs is that
close packing structures with complementary contact for all their
surfaces are impossible, so that they tend to require solvent of
crystallization.30
Recognizing that these low-density structures may be condu-
cive to various types of amphidynamic materials, and with the goal
of increasing the structural and dynamic complexities of potential
molecular machinery components, we decided to investigate the
synthesis and characterization of several types of covalently linked
molecular rotors. As illustrated in Figure 2, several topologies and
rotor connectivities may be envisioned with stators represented in
The realization of structures corresponding to Figure 2b–e rep-
resents an interesting synthetic challenge and pose several ques-
tions regarding the crystallization and dynamics of molecules with
multiple rotators. Since phenylene librations and internal rotations
are the result of coupled vibrations along the rotary axis and the
local environment, it will be interesting to identify the structural
attributes that may cause different rotators to have the same or
different frequencies, and whether they can act independent of
each other, or are correlated. As a first step to answer these and
other questions we decided to start with the structures based on
structural elements previously studied in our group such as trityl
and triptycyl stators, and diethynyl phenylene rotators (Fig. 2).
Structure I is based on a central 1,3-bis(diphenylmethyl)benzene
stator that permits changes in orientation of the two rotors that
may explored to form cyclic structures such that in Fig. 2d. Struc-
tures II and III are based on a central pentiptycene that maintains
(a)
(b)
(c)
(d)
(e)
I
II
III
Figure 2. (Top) Schematic representation of multirotor structures expected to have low packing density and rapid rotary dynamics. (Bottom) Potential dirotor structures analogous
to (c), with trityl and triptycyl groups as stators and phenylene groups as rotators.

8338
S.L. Gould et al. / Tetrahedron 64 (2008) 8336–8345
1a (R=H)
1b (R=tBu)
1c (R=nBu)
1d (R=OMe)
R
R
R
R
Figure 3. Line formula of dirotor 1, with two phenylene rotators sharing a bistrityl stator, and a corresponding space filling model with a 4 Å van der Waals surface.
the two rotators in a rigid and opposite orientation. With the two
rotators linked at the sp2 carbons of the central pentiptycene ring,
structure III maintains the two rotators co-linear and conjugated.
As a first step to explore the synthesis and physical properties of
these structures we decide to start with compounds 1a–1d, which
are representative of structure I (Fig. 3). By linking the two rotators
with a bridging stator we hope to take advantage of synthetic
strategies recently developed in our group. Based on space filling
models, we expected the modified 1,3-bis(diphenylmethyl) group
at the center of the structure to provide the rotators with a large
amount of free volume, which should be manifested as a very fast
rotation. While the single rotators studied previously (molecular
gyroscopes) were found to interdigitate upon crystallization, the
central platform of compound 1 has the potential of preventing
adjacent molecules from occupying the space surrounding their
central phenylene.
We report here the synthesis and characterization of natural
abundance and the phenylene-d4 derivative of the unsubstituted
compound (1a), as well as three derivatives with para-tert-butyl
(1b), para-n-butyl (1c), and para-methoxy (1d) groups attached to
the bridging stator. While we have been unable to obtain single
crystals for X-ray diffraction characterization, we have established
that they have significantly faster rotary dynamics than analogous
monorotors.
Significantly higher yields were obtained for the tert-butyl (3b) and
n-butyl compounds (3c) (83% and 61% yields, respectively). The
required 4-(30,30,30-triphenylpropynyl)-1-iodobenzene (4) was
prepared by the literature procedures32 and submitted to the final
coupling reaction with 3a–3d to give dirotors 1a–1d in approxi-
mately 50% yield. The solubility of each compound varied consid-
erably. Unsubstituted dirotor 1a was surprisingly soluble in cold
aromatic solvents, while tert-butyl dirotor, 1b, was only highly
soluble in acetone. n-Butyl and methoxy dirotors 1c and 1d were
highly soluble. Compound 1c formed a glassy solid from most
solvents and a powder from dichloromethane/methanol. Dirotors
with deuterated central phenylene rotators (1a-d8–1d-d8) for 2H
NMR studies were prepared by coupling 4-(30,30,30-triphenylpro-
pynyl)-1-bromobenzene-d4,32 4-d4, with the appropriate dialkynes.
The dirotors and their precursors were characterized by FTIR, 1H
NMR, 13C NMR, and powder XRD. 1H and 13C NMR spectra of the
dirotors 1a–1d were consistent with a dynamically averaged
structure reflecting symmetry about the meta-phenylene group
that bridges the two rotators. The aromatic region of each dirotor
has multiple signals due to the non-equivalent nature of the ter-
minal trityl groups and the bridging bis(bistrityl) structure at the
center of the structure. The 1,4-substituted aromatic ring of the
rotator gave a signal corresponding to the expected AB pattern,
which are shifted upfield from the rest of the signals of the trityl
groups. The alkynyl and sp3 carbons on each side of the rotator are
not chemically equivalent, giving resolvable signals in the 13C NMR
spectrum. The deuterated dirotors (1a-d8–1d-d8) have identical
NMR spectra to their natural abundance counterparts, with the
exception of the rotator signals, which are very small or absent due
to lack of an internal NOE and the splitting due to coupling with the
2H nucleus.
2. Results and discussion
2.1. Synthesis and characterization
Molecular dirotors 1a–1d were synthesized by a convergent
synthesis centered on a double coupling reaction analogous to
the one used for preparation of the single rotor ‘gyroscopes’
(Scheme 1).14 A para-substituted bromobenzene was reacted with
lithium metal to form the corresponding anion, which was sub-
sequently reacted with dimethyl isophthalate to form diols 2a–2c
in isolated yields of ca. 40%. The methoxy diol 2d was obtained in
similar manner in 62% yield. Dialkynyl compounds 3a–3d were
obtained by reaction of 2a–2d with acetyl chloride followed by
a substitution reaction with ethynyl magnesium bromide in
refluxing benzene. The unsubstituted dialkyne 3a and the methoxy
derivative 3d, formed in 52% and 41% yields, respectively.
A detailed description of the local environment of each rotator
requires analysis of the molecular and packing structures. While
methoxy dirotor 1d showed the greatest promise, diffraction
quality single crystals would not be formed for any of the four
derivatives. Examination of the X-ray powder diffraction patterns of
1a, 1b, and 1d showed that powders formed by evaporation of
solvent are indeed crystalline. The unsubstituted dirotor 1a, the
tert-butyl dirotor 1b, and the methoxy dirotor 1d have relatively
sharp signals up to 2
featureless component at 2
Q
values of 25–30ꢁ, overlaying a broad
z20, which suggests the presence of
Q

S.L. Gould et al. / Tetrahedron 64 (2008) 8336–8345
8339
R
R
R
Li
OMe
+
MeO
R
R
THF
OH
O
R
OH
O
Br
2a-d
R
1. AcCl
2. HCCMgBr
Benzene
R
R
H
H
R
3a-d
I
R
4
R
Pd(PPh3)2Cl2
CuI
Benzene/i-Pr2NH
R
1a-d
Scheme 1. Synthesis of molecular dirotor compounds 1a–1d.
small crystallites with an amorphous component. The n-butyl
dirotor 1c shows only a featureless pattern consistent with an
amorphous solid.
essentially the same as that previously observed for the analogous
molecular gyroscopes (370 ꢁC).33
2.3. Solid-state 13C CPMAS and NQS NMR spectroscopies
2.2. Thermal analysis
Line-shape changes occurring as a function of temperature in
the 13C CPMAS spectra can be used to measure exchange processes
associated with rotary dynamics of phenylene groups in the kilo
hertz regime.34–36 The method takes advantage of the magnetic
non-equivalence that exists in the solid state for phenylene nuclei
that are related by an 180ꢁ rotation along the dialkyne axis. With
a dynamic range that depends on the difference between their
resonance frequencies, two signals are observed at lower temper-
atures when the exchange is slow. Broadening and coalescence
occur as the exchange rate (kex) approaches and supersedes the
difference in resonance frequencies in hertz (Dn), kexꢂ2pdn, until
a single peak is observed in the fast exchange regime. Exploratory
analysis of the 13C CPMAS spectra revealed a complex aromatic
region for all four dirotors. This is illustrated by the spectrum of
unsubstituted dirotor 1a in Figure 4. From the 14 non-equivalent
aromatic signals distinguishable in solution, only two overlapping
groups are observed between 140–150 ppm and 130–120 ppm in
the solid state. The lack of resolution for the signals corresponding
to rotator makes it impossible to use VT 13C NMR spectroscopy as
a dynamic characterization method. While we have shown that
a strategy based on the deuteration of all the stator signals could
make it possible, we decided to explore the use of 2H NMR
spectroscopy on samples where only the rotator is deuterated.
Differential scanning calorimetry (DSC) and thermogravimetric
analyses (TGA) were carried out with the same powder samples,
which were used for dynamic studies using NMR spectroscopy
(vide infra). As summarized in Table 1, endothermic peaks consis-
tent with a melting transition were observed for compounds 1a, 1b,
and 1d in both natural abundance and deuterated forms. These
were relatively broad, in agreement with the powder diffractogram,
which had previously suggested a non-uniform sample phase.
Samples of the n-butyl dirotor 1c showed no melting transitions,
which is consistent with visual observations of an opaque powder
slowly softening and becoming increasingly translucent over
a broad temperature range. The onset of decomposition for all eight
dirotor samples occurred in the range of 350–430 ꢁC, which is
Table 1
Melting point and decomposition temperatures for dirotor compounds
Compound
Mp (ꢁC)
Decomp. (ꢁC)
1a (–H)
273–286
221–234
da
420
430
395
410
1b (–t-Bu)
1c (–n-Bu)
1d (–OMe)
220–239
1a-d8 (–H)
279–289
214–224
da
430
390
350
410
2.4. Rotational dynamics by quadrupolar 2H NMR
spectroscopy
1b-d8 (–t-Bu)
1c-d8 (–n-Bu)
1d-d8 (–OMe)
225–240
Quadrupolar echo 2H NMR spectroscopy is a well-defined
powerful tool used to determine internal molecular dynamics in
a
DSC shows no sharp melting point, consistent with the visual softening and
liquifying over a large temperature range.

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S.L. Gould et al. / Tetrahedron 64 (2008) 8336–8345
simulations using the program described by Nishikiori et al.44 as-
suming a two-fold (180ꢁ) flipping model with a quadrupolar-cou-
pling constant (QCC) of 180 kHz representative of the slow
(kꢃ103 sꢀ1), intermediate (104ꢃkꢃ107 sꢀ1), and fast exchange re-
gimes (kꢂ108 sꢀ1).
Measurements with compounds 1a-d8–1d-d8 were carried out
with a quadrupolar echo sequence on static powdered samples.
Spectra were obtained at 46.07 MHz with 90ꢁ pulse width of 2.5
ms,
echo and refocusing delays of 35 and 25
ms, respectively, and a de-
lay of 5–10 s between pulses. The spectra acquired in the temper-
ature range between 162 and 298 K are illustrated in Figure 6 with
a line broadening function of 3 kHz. In spite of the different sub-
stituents and the varying degrees of crystallinity determined by
XRPD, all four compounds gave qualitatively similar spectra with
features characteristic of structurally heterogeneous samples. The
lower temperature spectra (162–208 K) are consistent with
exchange processes involving sites in the intermediate and fast
regimes. Measurements between 231 and 243 K resulted in spectral
data characteristic the fast exchange regime. And finally, the 298 K
spectra for the substituted dirotors were significantly narrower
than that expected for a dynamic process involving only two sites
related by 180ꢁ rotation.
While the spectra derived from structurally complex samples
are often comparable to the sum of spectra representing environ-
ments in the slow, intermediate, and fast components, a rigorous
characterization in terms of a discrete number or a wide distribu-
tion of sites is very difficult.45 Complexities arise not only from
ambiguities in the number and type of spectral components, but
also from the fact that their individual intensities depend both on
their mole fraction and on their spin lattice relaxation times. Aware
of these challenges, we decided to carry out simple simulations
with the unique purpose of determining (a) whether or not a two-
fold flipping process reasonably represents the data, and (b) the
approximate regimes that best represent the rotary dynamics of the
phenylene rotator.
As illustrated in the left column in Figure 7, the lower tem-
perature spectra can be qualitatively simulated with components
that experience two-fold flipping processes with exchange rates
of 105 and 108 sꢀ1. Symmetric sharp peaks at the edges and the
center of the experimental spectrum, with a separation of 25 and
90 kHz, are consistent with those of the slower and faster com-
ponents, respectively. The ambient temperature spectra have the
inner peaks at 35 kHz, which are consistent with rotation of
the phenylene along a relatively static axis, but the width of the
spectrum is significantly narrower than that expected for a sim-
ple two-fold flipping process. It is well known that spectral
narrowing occurs when the exchange process occurs with rota-
tions involving more than two minima. Spectral simulations in-
volving axially symmetric three-site or four-site processes with
angular displacements of 120ꢁ or 90ꢁ in the fast exchange regime
result in very narrow spectra with central peaks separated by
a distance of 16 kHz, rather than the 25 kHz observed in our
experiment.46
Figure 4. 13C CPMAS spectrum of dirotor 1a.
the solid state.37–39 As deuterium NMR spectroscopy is largely
dominated by the orientation-dependent interaction between the
nuclear spin and electric quadrupole moment at the nucleus, the
characteristically broad spectrum from molecules in static pow-
dered samples is strongly affected by internal molecular motions in
the solid state. A single crystal with only one type of C–2H bond
would give a doublet with a quadrupolar splitting Dn that depends
on the orientation angle
b that the bond makes with respect to the
external field.40
ꢀ
ꢁꢀ
ꢁ
ꢀ
ꢁ
Dn ¼ 3=4 e2qzzQ=h 3cos2
b
ꢀ 1 ¼ 3=4QCC 3cos2
b
ꢀ 1
Q represents the electric quadrupole moment of the deuteron, e and
h are the electric charge and Planck constant, and qzz is the magni-
tude of the principal component of electric field gradient tensor,
which lies along the C–2H bond. The frequency difference for signals
of aromatic C–2H bonds, which have a QCCz180 kHz, changes from
135 kHz when
b
¼0ꢁ and 67.6 kHz for
b
¼90ꢁ, to 0 kHz when the C–2H
bond is oriented at the magic angle of
b
¼54.7ꢁ. A collection of
doublets in all possible orientations gives rise to a broad symmetric
spectrum with two maxima and two shoulders known as a Pake or
powder pattern. Variations in the powder pattern of a solid sample
occur when the C–2H bonds experience reorientations, and line-
shape analysis of the deuterium NMR spectra allows for the
characterization of the type of motion, in terms of discrete jumps
between different sites or continuous rotation, and for the de-
termination of exchange rates in a range of ca. 103 to 108 sꢀ1
.
For our studies, we have taken advantage of the well charac-
terized spectral changes occurring upon rotation of a phenylene
group about its 1,4-axis.41–43 Figure
5 illustrates spectral
A qualitative model that accounts for our observations is based
on contributions from two components in fast exchange (Fig. 7,
right column). One component, involving a normal two-fold flip-
ping process, accounts for the width in the lower part of the
spectrum and at the baseline. The second component accounts for
the narrow width on the top of the spectrum and requires more
than two sites. For our simulation, we used a model that involves
four energy minima at 0ꢁ, 30ꢁ, 180ꢁ, and 220ꢁ (Fig. 7h), which may
be viewed as a combination of two two-fold processes (Fig. 8). A
physical interpretation of this model could be related to local
fluctuations in the environment of a two typical two-fold site. We
believe this model describes the phenomenon observed at room
temperature.
Figure 5. Simulated spectra for a dynamic process involving 180ꢁ jumps of a para-
phenylene with exchange rates in hertz indicated on the left of each spectrum.

S.L. Gould et al. / Tetrahedron 64 (2008) 8336–8345
8341
Figure 6. Experimental 2H NMR spectra for dirotors 1a-d8–1d-d8 acquired between 162 and 298 K.
0° (360°)
D
180°
D
D
D
0
180
360
30°
220°
Figure 8. A hypothetical site exchange model involving two two-fold sites that are
offset by 30ꢁ.
3. Conclusions
With four test samples, we have implemented a simple pro-
cedure for the synthesis of molecular dirotors of the wheel and axle
types. As expected, challenges were found in the crystallization of
these compounds, which tend to form finely powdered solids.
X-ray diffraction and thermal analyses (DSC) data showed the
structures of the parent compound (1a), the tert-butyl (1b), and
the para-methoxy (1d) derivatives to be partially crystalline, and
the structure of the n-butyl compound (1c) to be amorphous. While
solid-state 13C CPMAS NMR measurements gave spectra with in-
sufficient resolution for dynamic studies in the kilo hertz regime,
quadrupolar echo 2H NMR spectra of phenylene-deuterated sam-
ples revealed a highly dynamic, if heterogeneous system. Spectral
measurements between 162 and 230 K indicated sample domains
Figure 7. Quadrupolar echo 2H NMR spectra of dirotor 1c-d8 at 208 K: (a) experi-
mental spectrum, (b) simulated spectrum as the sum of contributions of two-fold
flipping models with exchange rates of (c) 108, and (d) 105 sꢀ1 2H NMR spectrum of
.
dirotor 1c-d8 at 298 K: (e) experimental spectrum, (f) simulated spectrum as the sum
of contributions with a two-fold flipping site with an exchange rate of (g) 108, and (h)
a four-site model with rotational minima at 0ꢁ, 30ꢁ, 180ꢁ, and 220ꢁ with exchange rates
with phenylene rotations varying from ca. 104 to 108 sꢀ1
.
Measurements at ca. 240 K were suggestive of rotations in the fast
of 108 sꢀ1
.
exchange regime, at ꢂ108 sꢀ1. Finally, ambient temperature spectra

8342
S.L. Gould et al. / Tetrahedron 64 (2008) 8336–8345
were narrower than those expected for a two-fold flipping process
in the fast exchange regime, but not narrow enough for continuous
rotations. A simulated spectrum that accounts for the experimental
one was based on a model that involves four sites, with two energy
minima at 0ꢁ and 180ꢁ and two more at 30ꢁ and 220ꢁ. In conclusion,
while these novel structures are clearly conducive to low-density
materials that allow for fast internal dynamics, it will be desirable
to find modifications that lead to crystalline materials. Studies with
more rigid structures that prevent the reorientation of the two
rotors and structures with photochemically active stators are
currently in progress.
stirred for 48 h after which the reaction was quenched with satu-
rated aqueous ammonium chloride. The aqueous layer was
extracted with Et2O (ꢄ3). The combined organic layers were dried
with Na2SO4 and concentrated. Compound 2b was purified by
column chromatography (ethyl acetate/toluene; gradient of 0:1 to
1:20 by volume) to afford 3.2 g (42% yield) as a white solid. Anal-
ysis: mp¼174–176 ꢁC; 1H NMR (500 MHz, CDCl3) 1.29 (s, 36H),
7.08–7.42 (m, 20H); 13C NMR (125 MHz, CDCl3) 31.24, 34.32, 81.57,
124.57, 124.96, 126.27, 127.00, 127.47, 143.89, 146.46, 194.74; FTIR
3418 (br), 2959, 2903, 2867,1724,1662,1604,1508,1475, 1460,1401,
1362, 1269, 1201, 1171, 1108, 1017, 930, 894, 830, 796, 758, 715,
678 cmꢀ1. MS (MALDI-TOF) calculated for C48H58O2 666.4 [M]þ;
found 689.0 [MþNa]þ.
4. Experimental
4.1. General
4.2.3. 1,3-Bis[1,1-bis(40-n-butylphenyl)hydroxymethyl]benzene (2c)
Diol 2c was synthesized following the same procedure de-
scribed above for compound 2b to give 3.0 g (39% yield) of a yellow
oil. 1H NMR (500 MHz, CDCl3) 0.93 (t, 12H, J¼7.5 Hz), 1.35 (septet,
8H, J¼7.5 Hz), 1.55–1.61 (m, 8H), 2.36 (s, 2H), 2.58 (t, 8H, J¼8 Hz),
The 1H and 13C NMR spectra were acquired on Bruker NMR
spectrometer at 500 MHz and 125 MHz, respectively. Spectra were
dissolved in CDCl3 or (CD3)2CO as indicated. Solid-state FTIR spectra
were obtained on a PerkinElmer Spectrum 100 instrument. DSC
and TGA analyses were recorded on PerkinElmer Pyris Diamond
TG/DTA and DSC instruments. All reactions were run under inert
atmosphere (argon). Mass spectrometry data was obtained with
MALDI-TOF at the University of California, Riverside, Mass Spec-
trometry Center or at the University of California, Los Angeles,
Molecular Instrumentation Center.
7.04 (d, 8H, J¼6.5 Hz), 7.09 (d, 8H, J¼6.5 Hz), 7.16–7.28 (m, 4H); 13
C
NMR (125 MHz, CDCl3) 13.85, 22.31, 33.43, 35.11, 81.70, 126.30,
127.65, 127.70, 128.12, 128.93, 141.57, 144.151, 146.53; FTIR 3454 (br)
3024, 2956, 2927, 2857, 1913, 1797, 1659, 1605, 1574, 1509, 1465,
1458, 1411, 1378, 1327, 1183, 1162, 1141, 1020, 2928, 891, 830, 795,
778, 729, 710, 695 cmꢀ1. MS (MALDI-TOF) calculated for C48H58O2
667.0 [M]þ; found 649.0 [MꢀOH]þ and 689.0 [MþNa]þ.
4.2. Materials
4.2.4. 1,3-Bis[1,1-bis(40-methoxyphenyl)hydroxymethyl]-
benzene (2d)
All reagent chemicals were purchased from Aldrich and used
without further purification. All solvents were purchased from
VWR. Reaction solvents were distilled prior to use. EMD Silica gel-
60 (particle size 40–63 nm) purchased from VWR was used for
column chromatography.
4-(30,30,30-Triphenylpropynyl)-1-iodobenzene (4) and the deu-
terated equivalent 4-(30,30,30-triphenylpropynyl)-1-bromobenzene-
d4 (4-d4) were prepared according to previously published
procedures.32
Diol 2d was obtained following a modified procedure similar to
that of 2b. Lithium metal (0.63 g, 91 mmol) was stirred in 100 mL
THF at ꢀ78 ꢁC. To the metal suspension, 12 mL (91 mmol) 4-bro-
moanisole was added. Stirring continued for 5 min then the cold
bath was removed; formation of the anion as indicated by the dark
color occurred within 20 min. The solutionwas cooled to ꢀ78 ꢁC and
2.2 g (11 mmol) dimethyl isophthalate was added. The remaining
procedure was same as for compound 2a. Purification by column
chromatography (methanol/dichloromethane; 1:99) afforded 4.7 g
of a white solid (62% yield). Analysis: mp¼139–141 ꢁC; 1H NMR
(500 MHz, CDCl3) 2.63 (s, 3H), 3.79 (s, 12H), 6.78 (d, 8H, J¼10 Hz),
7.10 (d, 8H, J¼10 Hz), 7.16–7.25 (m, 4H); 13C NMR (125 MHz, CDCl3)
55.18, 81.42, 113.00, 126.40, 127.29, 127.52, 129.04, 139.35, 146.86,
158.51; FTIR 3454(br), 3024, 2956, 2927, 2857,1913,1797,1659,1605,
1574, 1509, 1483, 1465, 1458, 1411, 1378, 1327, 1183, 1162, 1141, 1020,
928, 891, 830, 795, 778, 729, 710, 695, 661 cmꢀ1. MS (MALDI-TOF)
calculated for C36H34O6 562.7 [M]þ; found 545.0 [MꢀOH]þ.
4.2.1. 1,3-Bis[1,1-diphenyl-1-hydroxymethyl]benzene (2a)
Lithium metal (0.63 g, 91 mmol) was stirred in 80 mL freshly
distilled THF. After 20 min of stirring, 9.6 mL (91 mmol) bromo-
benzene was added. After 10 min, the dark brown solution was
cooled to ꢀ78 ꢁC. Dimethyl isophthalate (2.2 g, 11 mmol) was
added. The solution was stirred for 48 h during which time the cold
bath was allowed to naturally warm to room temperature. The
reaction mixture was washed with satd aq NH4Cl (ꢄ2). The aqueous
layer was extracted with Et2O (ꢄ2). The combined organic layers
were dried with Na2SO4 and concentrated. Purification was done
with column chromatography (ethyl acetate/toluene; 2.5:97.5) to
give 2.1 g (42% yield) of a white solid. Analysis: mp¼85–89 ꢁC; 1H
NMR (500 MHz, CDCl3) 3.87 (s, 2H), 7.18–7.47 (m, 24H); 13C NMR
(125 MHz, CDCl3) 81.76, 127.10, 127.67, 127.73, 127.88, 128.11, 131.66,
132.36; FTIR 3405 (br), 3057, 3025, 2950, 1953, 1896, 1812, 1700,
1614, 1597, 1490, 1446, 1312, 1277, 1205, 1159, 1140, 1112, 1094, 1022,
914, 891, 797, 756, 674, 696 cmꢀ1. MS (MALDI-TOF) calculated for
4.2.5. 1,3-Bis[1,1-diphenylprop-2-ynyl]benzene (3a)
Compound 2a (1.5 g, 3.4 mmol) was refluxed in 20 mL acetyl
chloride for 1 h. After cooling, excess acetyl chloride was removed
by rotary evaporation. The white paste was dissolved in 10 mL dry
benzene and rotary evaporated to remove all acetyl chloride twice.
The residue was dissolved in 350 mL dry benzene and refluxed for
1 h. Ethynyl magnesium bromide (70 mL, 0.5 M in THF, 35 mmol)
was added to the refluxing solution. Stirring continued at refluxing
temperatures for 18 h. The cooled solution was quenched with satd
aq NH4Cl. The aqueous layer was extracted with Et2O (ꢄ2). The
organic layers were combined and dried with Na2SO4. Purification
of compound 3a by column chromatography (toluene/hexanes;
30:20) resulted in 0.8 g (52% yield). Analysis: mp¼143–144 ꢁC; 1H
NMR (500 MHz, CDCl3) 2.63 (s, 2H), 7.01–7.322 (m, 24H); 13C NMR
(125 MHz, CDCl3) 44.42, 73.52, 89.38, 126.79, 127.17, 127.92, 128.22,
129.33, 130.51, 144.33, 144.62; FTIR 3305, 3283, 1596, 1490, 1445,
1420, 1261, 1182, 1156, 1099, 1032, 1002, 883, 797, 755, 660,
697 cmꢀ1. MS (MALDI-TOF) calculated for C36H25 457.2 [MꢀH]þ;
found 457.2 [MꢀH]þ.
C
32H26O2 442.2 [M]þ; found 425.2 [MꢀOH]þ.
4.2.2. 1,3-Bis[1,1-bis(40-tert-butylphenyl)hydroxymethyl]-
benzene (2b)
Lithium metal (0.63 g, 91 mmol) was stirred in 200 mL freshly
distilled THF. After 20 min, 16 mL (91 mmol) bromo-4-tert-butyl-
benzene was added. Upon the formation of the anion as indicated
by a dark brown color the reaction mixture was cooled to ꢀ78 ꢁC
and 2.2 g (11 mmol) dimethyl isophthalate was added. The cold
bath was allowed to warm naturally and the reaction mixture was

S.L. Gould et al. / Tetrahedron 64 (2008) 8336–8345
8343
4.2.6. 1,3-Bis[1,1-bis(40-tert-butylphenyl)prop-2-ynyl]benzene (3b)
Compound 3b was synthesized following the same procedure as
described above for compound 3a using 1.2 g (1.8 mmol) 2b to give
1.0 g (83% yield) of white solid. Analysis: mp¼138–142 ꢁC; 1H NMR
(500 MHz, CDCl3)1.29(s,36H), 2.57(s,2H),7.07(d, 8H, J¼8.5 Hz), 7.18–
7.36 (m, 12H); 13C NMR (125 MHz, CDCl3) 31.33, 34.35, 54.63, 73.06,
89.67,124.71,127.02,127.33,128.32,128.52,141.83,144.40,149.39;FTIR
3301, 2960, 2903, 2867, 1981, 1598, 1506, 1477, 1460, 1395, 1363, 1268,
1202, 1109, 1018, 925, 888, 827, 792, 753, 729, 713, 679 cmꢀ1. MS
(MALDI-TOF) calculated for C36H34O6 682.5; found 682.5.
chloride. The aqueous layer was extracted with Et2O (ꢄ3). The
combined organic layers were dried with Na2SO4 and concentrated.
The product was purified by column chromatography (ethyl ether/
hexanes; gradient of 0:1 to 1:49) to afford 0.5 g (50% yield) of
a white solid. Analysis: mp¼200–228 ꢁC; 1H NMR (500 MHz,
(CD3)2CO) 1.26 (s, 36H), 7.20 (d, 8H, J¼8.5 Hz), 7.26–7.36 (m, 42H),
7.45 (d, 4H, J¼8.5 Hz), 7.52 (d, 4H, J¼8.5 Hz); 13C NMR (125 MHz,
CDCl3) 30.61, 33.93, 55.25, 56.02, 84.79, 84.86, 97.16, 97.23, 122.96,
123.14, 124.75, 126.75, 126.87, 127.49, 127.98, 128.41, 128.83, 131.05,
131.40, 141.91, 145.00, 145.15, 149.42; FTIR 3058, 2960, 2925, 2854,
1734, 1596, 1490, 1462, 1445, 1363, 1261, 1180, 1156, 1079, 1032, 907,
840, 800, 772, 754, 733, 719, 696, 659 cmꢀ1. MS (MALDI-TOF) cal-
culated for C106H94 1366.8 [M]þ; found 1391.0 [MþNa]þ.
4.2.7. 1,3-Bis[1,1-bis(40-n-butylphenyl)prop-2-ynyl]benzene (3c)
Compound 3c was synthesized following the same procedure as
described above for compound 3a using 2.1 g (3.1 mmol) com-
pound 2c to give 1.3 g (62% yield) of yellow oil. 1H NMR (500 MHz,
CDCl3) 0.92 (t, 12H, J¼7.5 Hz), 1.31–1.37 (m, 8H), 1.55–1.61 (m, 8H),
2.57 (t, 8H, J¼8.0 Hz), 2.59 (s, 2H), 7.03 (d, 8H, J¼8.5 Hz), 7.08 (d, 8H,
J¼8.5 Hz), 7.20–7.23 (m, 3H), 7.26 (s, 1H); 13C NMR (125 MHz,
CDCl3) 13.86, 22.30, 33.41, 35.05, 54.72, 73.05, 89.70, 127.09, 127.36,
127.75, 128.67, 130.43, 141.11, 142.01, 144.45; FTIR 3308, 3307, 3023,
2955, 2927, 2857, 1909, 1794, 1664, 1599, 1509, 1481, 1466, 1457,
1412, 1377, 1283, 1190, 1122, 1102, 1021, 929, 887, 829, 791, 777, 739,
706, 691 cmꢀ1. MS (MALDI-TOF) calculated for C36H34O6 683.0
[M]þ; found 705.0 [MþNa]þ.
4.2.8.3. Dirotor (1c). Molecular rotor 1c was prepared following the
same procedure as for compound 1b using 0.65 g (0.95 mmol)
compound 3c, 1.2 g (2.6 mmol) 4-(30,30,30-triphenylpropynyl)-1-
iodobenzene, 4, 67 mg (0.095 mmol) bis(triphenylphosphine)palla-
dium dichloride and 39 mg (0.19 mmol) copper iodide in 80 mL
freshly distilled benzene and 1 mL diisopropylamine. Compound 1c
was obtained in 50% yield (0.65 g) as a glassy solid. 1H NMR
(500 MHz, (CD3)2CO) 0.84 (t, 12H, J¼5 Hz), 1.25–1.32 (m, 8H), 1.48–
1.56 (m, 8H), 2.55 (t, 8H, J¼5 Hz), 7.09 (d, 8H, J¼10 Hz), 7.15 (d, 8H,
J¼10 Hz), 7.25–35 (m, 34H), 7.41 (d, 4H, J¼8.5 Hz), 7.51 (d, 4H,
J¼8.5 Hz); 13C NMR (125 MHz, (CD3)2CO) 13.23, 22.05, 33.33, 34.70,
55.42, 56.02, 84.65, 84.77, 97.14, 97.33, 122.91, 123.13, 126.86, 127.88,
127.98, 128.64, 128.84, 130.71, 130.75, 131.34, 131.41, 132.28, 141.25,
142.21, 145.00, 145.31; FTIR cmꢀ1. MS (MALDI-TOF) calculated for
4.2.8. 1,3-Bis[1,1-bis(40-methoxybutylphenyl)prop-2-ynyl]-
benzene (3d)
Dialkyne 3d was synthesized following the same procedure as
described above for compound 3a with a modification at the pu-
rification step. The reaction of 3.0 g (5.3 mmol) 2d afforded 1.3 g of
a white solid (41% yield) after purification by column chromatog-
raphy (dichloromethane/hexane; 50:50). Analysis: mp¼70–73 ꢁC;
1H NMR (500 MHz, CDCl3) 2.61 (s, 2H), 3.78 (s, 12H), 6.75 (d, 8H,
J¼9 Hz), 7.03–7.04 (m, 1H), 7.08 (d, 8H, J¼9 Hz), 7.22–2.26 (m, 3H);
13C NMR (125 MHz, CDCl3) 54.05, 55.20, 73.03, 89.85, 113.17, 127.29,
127.62, 129.93, 130.07, 137.11, 144.86, 158.24; FTIR 3283, 3001, 2954,
2932, 2904, 2835, 2548, 2051, 1892, 1676, 1606, 1582, 1506, 1481,
1461, 1440, 1422, 1297, 1246, 1175, 1116, 1031, 886, 825, 800, 737,
706, 689 cmꢀ1. MS (MALDI-TOF) calculated for C40H34O4 578.3
[M]þ; found 578.0 [M]þ.
C
106H94 1366.8 [M]þ; found 1366.7 [M]þ.
4.2.8.4. Dirotor (1d). Molecular rotor1dwasprepared following the
same procedure as for compound 1b using 1.2 g (2.1 mmol) com-
pound 3d, 2.5 g (5.3 mmol) 4-(30,30,30-triphenylpropynyl)-1-iodo-
benzene, 4, 150 mg (0.21 mmol) bis(triphenylphosphine)palladium
dichloride, and 800 mg (0.42 mmol) copper iodide in 80 mL freshly
distilled benzene and 2 mL diisopropylamine. Compound 1d was
obtained in 69% yield (1.8 g) as a white solid. Analysis: mp¼220–
239 ꢁC; 1H NMR (500 MHz, (CD3)2CO) 3.73 (s, 12H), 6.82 (d, 8H,
J¼10 Hz), 7.14 (d, 8H, J¼10 Hz), 7.19–7.40 (m, 34H), 7.51 (d, 4H,
J¼8.5 Hz); 13C NMR (125 MHz, CDCl3) 54.79, 55.19, 56.18, 84.61,
84.91, 97.20, 97.50, 113.24, 122.97, 123.23, 126.89, 127.27, 127.59,
128.07, 129.17, 130.05, 130.37, 131.44, 131.50, 137.42, 145.23, 145.55,
158.24; FTIR cmꢀ1. MS (MALDI-TOF) calculated for C94H70O4 1262.5
[M]þ; found 1262.5 [M]þ.
4.2.8.1. Dirotor (1a). Dialkyne, 3a (0.15 g, 0.33 mmol), and 0.45 g
(0.96 mmol) 4-(30,30,30-triphenylpropynyl)-1-iodobenzene, 4, were
dissolved in 10 mL freshly distilled THF and 0.5 mL triethyl amine.
The reaction mixture was degassed for 1 h prior to 98 mg
(0.14 mmol) bis(triphenylphosphine)palladium dichloride and
53 mg (0.28 mmol) copper(II) iodide being added. The solution was
stirred at 50 ꢁC for 48 h. After cooling the solution was washed with
satd aq NH4Cl. The organic layer was dried over Na2SO4 and con-
centrated. The product was purified by column chromatography
(benzene/hexanes; 30:70) to afford 90 mg (25% yield) of pure dir-
otor 1a as a white solid. Analysis: mp¼273–286 ꢁC; 1H NMR
(500 MHz, CDCl3) 7.20–7.35 (m, 62H); 13C NMR (125 MHz, CDCl3)
56.11, 56.15, 84.77, 84.95, 97.00, 97.29,122.86,126.78,126.87,127.43,
127.68, 127.98, 128.03, 129.01, 129.15, 130.66, 144.99, 145.19; FTIR
3056, 2956, 1726, 1740, 1696, 1490, 1446, 1263, 1182, 1157, 1103,
1079, 1032, 1002, 888, 837, 789,756, 698 cmꢀ1. MS (MALDI-TOF)
calculated for C106H94 1366.74 [M]þ; found 1367.7 [M]þ.
4.2.8.5. Dirotor (1a-d8). Dialkyne, 3a (0.8 g, 1.7 mmol), and 2.2 g
(5.2 mmol)
4-(30,30,30-triphenylpropynyl)-1-bromobenzene-d4,
4-d4, were dissolved in 10 mL freshly distilled THF and 10 mL diiso-
propylamine. The reaction mixture was degassed for 1 h prior to
240 mg (0.34 mmol) bis(triphenylphosphine)palladium dichloride
and 130 mg (0.68 mmol) copper(II) iodide being added. The solu-
tion was stirred at 50 ꢁC for 5 days. After cooling the solution was
washed with satd aq NH4Cl. The organic layer was dried over
Na2SO4 and concentrated. Dirotor 1a-d8 was purified by column
chromatography (benzene/hexanes; 30:70) to afford 90 mg (25%
yield) of a white solid. Analysis: mp¼279–289 ꢁC; 1H NMR
(500 MHz, (CD3)2CO) 7.20–7.35 (m, 58H); 13C NMR (125 MHz,
CDCl3) 56.11, 56.15, 84.77, 84.95, 97.00, 97.29, 122.86, 126.78, 126.87,
127.43, 127.68, 127.98, 128.03, 129.01, 129.15, 130.66, 144.99, 145.19;
FTIR 3056, 3032, 1597, 1490, 1446, 1424, 1321, 1182, 1157, 1080, 1033,
1002, 890, 853, 821, 786, 761, 756, 744, 728, 698 cmꢀ1. MS (MALDI-
TOF) calculated for C90H54D8 1150.54 [M]þ; found 1173.5 [MþNa]þ.
4.2.8.2. Dirotor (1b). Dialkyne, 3b (0.5 g, 0.7 mmol), and 1.0 g
(2.1 mmol) 4-(30,30,30-triphenylpropynyl)-1-iodobenzene, 4, in
80 mL freshly distilled benzene and 1 mL diisopropylamine was
degassed for 1 h with argon. Bis(triphenylphosphine)palladium
dichloride (49 mg, 0.07 mmol) and 27 mg (0.14 mmol) copper(II)
iodide were added and the solution refluxed for 48 h. After cooling
the solution was quenched with saturated aqueous ammonium
4.2.8.6. Dirotor (1b-d8). Dialkyne, 3b (0.38 g, 0.56 mmol), and
0.60 g (1.4 mmol) 4-(30,30,30-triphenylpropynyl)-1-bromobenzene-
d4, 4-d4, were dissolved in 25 mL freshly distilled THF and 1 mL

8344
S.L. Gould et al. / Tetrahedron 64 (2008) 8336–8345
triethyl amine. The reaction mixture was degassed for 1 h. Bis-
(triphenylphosphine)palladium dichloride (77 mg, 0.11 mmol) and
43 mg (0.22 mmol) copper(II) iodide being added. The solution was
stirred at 50 ꢁC for 48 h. After cooling the solution was washed with
satd aq NH4Cl. The organic layer was dried over Na2SO4 and con-
centrated. Dirotor 1b-d8 was purified by column chromatography
(dichloromethane/hexanes; 30:70) to afford 90 mg (25% yield) of
a white solid. Analysis: mp¼268–272 ꢁC; 1H NMR (500 MHz,
(CD3)2CO) 1.25 (s, 36H), 7.01–7.34 (m, 50H); 13C NMR (125 MHz,
(CD3)2CO) 30.59, 33.94, 55.36, 56.05, 84.21, 84.64, 96.98, 97.25,
124.76, 124.89, 126.81, 127.93, 128.26, 128.35, 128.83, 141.90, 144.99,
149.45, 149.70; FTIR 3058, 2959, 2902, 2865, 1598, 1505, 1491, 1460,
1447, 1423, 1394, 1363, 1268, 1185, 1109, 1018, 889, 827, 792, 755,
727, 712, 697, 675, 660 cmꢀ1. MS (MALDI-TOF) calculated for
References and notes
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106H86D8 1374.78 [M]þ; found 1374.8 [M]þ.
4.2.8.7. Dirotor (1c-d8). Dialkyne, 3c (0.36 g, 0.53 mmol), and
0.50 g (1.2 mmol) 4-(30,30,30-triphenylpropynyl)-1-bromobenzene-
d4, 4-d4, were dissolved in 30 mL freshly distilled THF and 1 mL
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bis(triphenylphosphine)palladium
dichloride
and
19 mg
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(0.10 mmol) copper(II) iodide were added. The solution was stirred
at 50 ꢁC for 48 h. After cooling the solution was washed with satd
aq NH4Cl. The organic layer was dried over Na2SO4 and concen-
trated. Dirotor 1c-d8 was purified by column chromatography
(dichloromethane/hexanes; 10:90) to afford 330 mg (46% yield) of
a glassy solid. 1H NMR (500 MHz, CDCl3) 0.87–0.92 (m, 12H), 1.26–
1.38 (m, 8H), 1.52–1.59 (m, 8H), 2.46–2.57 (m, 8H), 6.93–7.31 (m,
50H); 13C NMR (125 MHz, CDCl3) 13.96, 22.42, 33.46, 35.14, 55.42,
56.02, 84.65, 84.77, 97.14, 97.33, 126.84, 126.87, 127.88, 128.01,
128.74, 128.84, 129.15, 141.15, 141.21, 141.71, 142.46, 145.18; FTIR
3084, 3057, 3054, 2955, 2827, 2857, 1947.6, 1904, 1800, 1597, 1508,
1491, 1465, 1447, 1424, 1377, 1322, 1296, 1249, 1186, 1121, 1102, 1080,
1034, 1020, 1002, 965, 929, 887, 853, 828, 777, 756, 727, 697,
658 cmꢀ1. MS (MALDI-TOF) calculated for C106H86D8 1374.78 [M]þ;
found 1374.8 [M]þ.
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4.2.8.8. Dirotor (1d-d8). Dialkyne, 3d (0.36 g, 0.60 mmol), and
0.78 g (1.8 mmol) 4-(30,30,30-triphenylpropynyl)-1-bromobenzene-
d4, 4-d4, were dissolved in 23 mL freshly distilled benzene and 1 mL
triethyl amine and then degassed for 1 h followed by the addition of
42 mg (0.06 mmol) bis(triphenylphosphine)palladium dichloride
and 23 mg (0.12 mmol) copper(II) iodide. The solution was refluxed
for 48 h. After cooling the solution was washed with satd aq NH4Cl.
The organic layer was dried over Na2SO4 and concentrated. Dirotor
1d-d8 was purified by column chromatography (dichloromethane/
hexanes; 40:60) to afford 180 mg (23% yield) of a white solid.
Analysis: mp¼225–240 ꢁC; 1H NMR (500 MHz, (CD3)2CO); 3.75 (s,
12H), 6.83 (d, 8H, J¼9 Hz), 7.14 (d, 8H, J¼9 Hz), 7.20–7.41 (m, 34H);
13C NMR (125 MHz, CDCl3) 54.78, 55.16, 56.15, 84.85, 85.01, 97.24,
97.55, 113.22, 122.76, 122.99, 126.88, 127.20, 127.60, 128.03, 129.15,
130.03, 137.45, 145.19, 145.54, 158.21; FTIR 3057, 2931, 2833, 1605,
1582, 1507, 1490, 1461, 1447, 1424, 1297, 1248, 1176, 1114, 1034, 889,
828, 800, 784, 755, 727, 698, 655 cmꢀ1. MS (MALDI-TOF) calculated
for C94H62D8O4 1270.58 [M]þ; found 1270.6 [M]þ.
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Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2008.05.143.

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