2,6-Pyridodicarboxamide-Bridged Triptycene Molecular Transmission Devices: Converting Rotation to Rocking Vibration

Article abstract of DOI:10.1021/acs.joc.5b01778

A series of N²,N⁶-bis(triptycene-9-yl)pyridine-2,6-dicarboxamides 1-4 were designed and synthesized. Due to rotational constraint of the 2,6-diamidopyridine bridge, the triptycene components in the systems are held together. X-ray structures of 1-4 show that the molecules adopt a gear-like geometry in the solid states. DFT (B3LYP/6-31G(d)) calculations predict the gear-like C₂ conformation as global minimum structures for 1 and 2 and suggest that, through a slippage transition process, rotation of one triptycene component would give rise to a rocking vibration of the counter component due to the barrier for rotation of the triptycene components. VT NMR studies on 1-4 show that the pair of triptycene components undergo ceaseless slippage at room temperature but nearly freeze at temperatures as low as 183 K. Decreasing the temperature freezes the slippage between triptycene components as well, thus producing the appearance of phase isomers of 3 and 4. The dynamic features of the studied molecules indicate that this kind of molecule is able to function as a kind of molecular transmission device for transforming the mode of motion from rotation to rocking vibration.

Full text of DOI:10.1021/acs.joc.5b01778

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Article
2,6-Pyridodicarboxamide-bridged Triptycenes Molecular
Transmission Devices: Converting Rotation to Rocking Vibration
Guangxia Wang, Lishuang Ma, JunFeng Xiang, Ying Wang, Xuebo Chen, Yanke Che, and Hua Jiang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b01778 • Publication Date (Web): 21 Oct 2015
Downloaded from http://pubs.acs.org on October 21, 2015
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2,6-Pyridodicarboxamide-bridged Triptycenes
Molecular Transmission Devices: Converting
Rotation to Rocking Vibration
Guangxia Wang,†, §, ‡ Lishuang Ma,§ Junfeng Xiang, † Ying Wang,*, § Xuebo Chen,*, § Yanke Che,*,† and
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Hua Jiang*,†, §
† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
§ Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education; Chemistry
College, Beijing Normal University, Beijing 100875, China
‡ University of Chinese Academy of Sciences, Beijing 100049, China
Email: jiangh@bnu.edu.cn, ywang1@bnu.edu.cn, xuebochen@bnu.edu.cn and ykche@iccas.ac.cn
RECEIVED DATE (will be automatically inserted after manuscript is accepted).
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ABSTRACT: A serial of N2,N6ꢀbis(triptyceneꢀ9ꢀyl)pyridineꢀ2,6ꢀdicarboxamides 1–4 were designed and
synthesized. Due to rotational constraint of 2,6ꢀdiamidopyridine bridge, the triptycene components in
the systems are held together. Xꢀray structures of 1–4 show that the molecules adopt a gearꢀlike
geometry in the solid states. DFT (B3LYP/6ꢀ31G(d)) calculations predict the gearꢀlike C2 conformation
as global minimum structures for 1 and 2, and suggest that, through a slippage transition process,
rotation of one triptycene component would give rise to a rocking vibration of the counter one due to the
barrier for rotation of the triptycene components. VT NMR studies on 1–4 show that the pair of
triptycene components undergo ceaseless slippage at room temperature, but nearly freezes at
temperature as low as 183 K. Decreasing the temperature freezes the slippage between triptycene
components as well, thus producing the appearance of phase isomers of 3–4. The dynamic features of
the studied molecules indicate that this kind of molecules is able to function as a kind of molecular
transmission devices for transforming the mode of motion from rotation to rocking vibration.
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INTRODUCTION
Artificial molecular machines1, 2 have attracted considerable interest due to their attractive potential in
construction of ultraminiaturized machines. During the past decades, a variety of molecular machines,
such as molecular gears3, molecular shuttles4, molecular brakes5, molecular turnstiles6, molecular
motors7, 8 and molecular gyroscopes9, 10, have been rationally designed and constructed to mimic their
macroscopic counterparts. However, few of them adapts mechanical principles from the macroscopic
transmission devices used for transforming the mode of movements, although these kinds of machinery
components are indispensable in all macroscopic machines.
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On the other hand, as a kind of molecular rotary devices, molecular gears couple two or more
submolecular rotatory components in one molecule, so as to rotate correlatively, to mimic the
cogwheeling rotation of macroscopic gears. The threeꢀbladed 9ꢀtriptycene (Tp) is well known as an
excellent moiety to build up molecular gears due to its rigid and shapeꢀpersistent structure.1 Since the
pioneering studies of Mislow and Iwamura,11ꢀ16 a variety of molecular bevel gear systems based on
derivatives of Tp have been designed and synthesized, in which the intermeshed Tp components are
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coupled by various static atoms or groups, such as CH211, 13, 15, O12, 13, 15, S15, CHOH15, CO13a, SiH2 ,
NH14, 15, HC=CH17, GeCl16c, 16d and SiF218 (Figure 1). Due to the small axle–axle distance between Tp
groups, these molecular bevel gears (Tp2X or Tp3X) possess a low energy barrier (1~2 kcal molꢀ1) for
disrotatory cogwheeling (geared rotation) but exhibit a comparatively much higher barrier (> 30 kcal
molꢀ1) for correlated conrotations (take place in the process of gear slippage),11a indicating that these
tightly meshed gears undergo unhindered rotational motions with high gearing fidelity19. Accordingly,
the ground and the transition states (with C2 and Cs conformation, Figure S1) presenting in geared
rotation are almost isoenergetic, while the gearꢀclashed conformation (C2v) has a much higher energy.11a
Figure 1. Macroscopic bevel gear and a model for the structure of the reported molecular bevel gears
Tp2X (X = O, CH2, S, CHOH, CO, SiH2, NH, HC=CH or SiF2. The groups C3H2 and GeCl are also used
to assemble a cyclic gear with the structure which can be expressed as Tp3X).
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The energy gap between gearꢀclashed conformation and the ground state, which can be estimated from
the activation energy for gearꢀslippage process, highly depends on the distance between Tp groups.
Longer interaxle distance would typically give rise to a lower barrier. For example, the activation
energy for slippage was measured to be 42~43 kcal molꢀ1 in chloroꢀsubstituted Tp2O12c (with C(9)–O
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bond length (dC –O) of 1.41~1.42 Å and C(9)–O–C(9') bond angle (φC –O–C9') of 136°), 32~33 kcal molꢀ1
13c
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in that of Tp2CH2 (with dC –C of 1.53~1.57 Å and φC –C–C9' of 129°) and ~21 kcal molꢀ1 in Tp2SiH2
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(with dC –Si of 1.86 Å and φC –Si–C of 129°). Along this line, one may be curious how the dynamic
properties of such molecular gears change when the interaxle distance increases further. It has been
demonstrated (and is also easy to image) that gearing would lose the efficiency completely when the
gears are declutched.18 But what if the distance keeps increasing but still within a point where the two
rotators remain interaction? And, is that possible to develop a new molecular devices derived from the
bevel gears at this state?
To visualize the dynamic properties of such looselyꢀcoupled molecular bevel gears are challenging,
considering the uncertainty and diversity in the steric repulsion between Tp groups and the interaxle
distance in those unknown systems. A little trick is treating the systems as spur gears first, with a
presupposition that the Tp rotators are connected with a springꢀlike tension. In this model, given the
rigid structure of rotators, the length of spring under its equilibrium position is very important. A short
spring with the length close to that of the radii of Tp rotators would provide a tightly meshed molecular
gear, like most of the reported ones11ꢀ18, with ceaseless dynamic interconvertion between C2 and Cs
states (Figure S1). The C2 conformation is stabilized by π stacking and orbital interactions, whereas the
Cs one by orbital interactions only. With the increase of the length of spring, the orbital interactions are
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expected to weaken gradually. To a point, the Cs conformation might not be a local energy minimum
any more. In this process, the gearing fidelity19 would decrease, which means C2v (represents gear
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*
slippage) and C2v conformations20 might be two important transition states in the process of gearing
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(Figure S1). The maximum axle–axle distance for C2v orientation is larger than that of the C2v one.
*
Consequently, if the interaxle distance is between the two maximum axle–axle distance for C2v and C2v
orientations, there should be no interaction between two rotators when the gear adopts C2v conformation,
which means that there would be a break in the process of rotation transmission from one rotator to the
counter one. This is interesting because if the counter rotator could rotate back in every single break
with certainty, the rotation of the driving rotator would give rise to a vibration of the counter one.
Consequently, the system would not be a molecular gear with low gearing fidelity any more, but a
device which could be used to transform the mode of motion. Nevertheless, these speculations are only
based on a very simplified mechanical model. Without a concrete molecular model, what on earth are
the transitional states as well as the changes of energy gaps between different states under such
conditions compared to that in tightly meshed mode are hard to predict, and those will be more
complicated when the model is propagated to real molecule under different conditions.
To date, no molecular bevel gears with large interaxle distance are reported. Bryan has previously
reported that the incorporation of Tp units into crown ether created a spurꢀgearꢀlike structure, but the
rotators in it are too far from each other to intermesh.21 Recently, molecular spur gears with Tp groups
bridged with bibenzimidazole were rationally designed and described by Siegel.20 However, in despite
of considerable energy barriers for gear slippage (calculated to be ~8 kcal molꢀ1), the interconversion
between phase (dl and meso) isomers11a could not be experimentally observed.
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In this contribution, we describe the design, synthesis, Xꢀray structure and the dynamic properities of
2,6ꢀpyridinedicarboxamidesꢀbridged Tp derivatives 1–4, which are derived from the conventional
molecular gears. Due to the appropriate length of the static bridge as well as the large rotation barrier of
Tp components, these molecules behave as a kind of molecular transmission devices for transforming
the mode of motion from rotation of one Tp component to rocking vibration of the counter one. To
theoretically figure out the most energetically favourable conformational transformation path in the
process of intramolecular movement transmission, the minimum energy profiles (MEPs) as a function of
the dihedral angle between the planes of one benzene ring in a rotator and the adjacent amide were
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calculated for 1 and 2 using density functional theory (DFT). Besides, dynamic H and 13C NMR
measurements were also carried out to study the dynamic features of 1–4 in solutions.
RESULTS AND DISCUSSION
Molecular Design. A molecule that suitable for studying the rotation of the molecular bevel gear
should at least: (i) be structurally rigid and shapeꢀpersistent under gearing; (ii) have static group with
suitable length thus keeping the two rotators at an appropriate distance; (iii) maintain the two rotation
axles of rotators coplanar while rotating and (iv) allow for experimental observation of the
intramolecular motions.
Previous studies22 have shown that 2,6ꢀpyridinedicarboxamides prefer to adopt a stable, cisꢀ
conformation ascribed to the intramolecular hydrogenꢀbonding between amide and pyridineꢀN,
providing a good motif to construct bent supramolecules such as foldamers. Meanwhile, due to the sp2
hybridization of the nitrogen atom in amides, the entire 2,6ꢀpyridinedicarbox amides group is planar,
which renders the two rotators rotating around two separate axles within the same plane if this moiety is
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used as the static in the molecular gears. Furthermore, in contrast to the small axle–axle distance in the
molecular bevel gears those reported previously11ꢀ18, the distance between two amido nitrogen (dN·ꢁꢁN) in
2,6ꢀpyridinedicarboxamide group is as large as ca. 4.6 Å according to the molecular structure, which is
about 3.5 fold longer than that of typical double bonds (dC=C = 1.3 Å in ethylene) and about twice as the
width of a benzene ring (2.4 Å)23. All these concepts inspired us to design molecules 1–2 based on 2,6ꢀ
pyridinedicarboxamidesꢀbridged Tps, in which the methyl substituents on 2 were designed to increase
the size of gear blade thus being useful for studying the effect of the blade size of rotators on the
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dynamic properties.
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In the conventional Tpꢀbased molecular gears, the subsitituents on the bridgehead carbons were
typically designed to produce negligible steric interference with the rotary of rotators.20, 24 By this way,
the effect of stereodynamics of subsitituents themselves on that of gearing could be ignored. This
method could simplify the stereochemistry analysis of the gearing behaviors, but will give rise to rapid
and frictionless rotation of the Tp groups (no matter with or without cogwheel rotation) which are not
detectable on the NMR time scale. For the designed molecules, the introduction of amide would destroy
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the symmetry of the Tp group and produce an extra rotation energy barrier arising from the steric effect
of carbonyl oxygen atom of the amide, in which the extra rotational barrier might increase the
coalescence temperature in dynamic NMR experiments thus rendering the rotation of Tp to be
technically measurable. More importantly, this extra barrier might be able to define an equilibrium
position of the Tp components, by which the Tp groups keep a trend to rotate back as long as it deviates
from the equilibrium position.
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Molecules 3–4, which have two and four methyl subsitituents on each Tp rotators, respectively, and
could be regarded as a hybrid of 1 and 2, were designed for better understanding the relationship
between the size of rotators and the dynamics for the studied systems. Besides, it is noteworthy that,
different from molecules 1 and 2, the subsititution of methyl groups on 3–4 will break the equivalence
of three benzene rings on each Tp groups, giving rise to phase isomers.
Synthesis. Synthetic routes for molecular bevel gears 1–4 are outlined in Scheme 1. The key issue in
these approaches is to obtain the 9ꢀtriptycylamine derivatives, that is, the methylꢀsubstituted 9ꢀ
triptycylamines.
Tp derivatives are generally synthesized by the DielsꢀAlder reaction from the corresponding
anthracenes and benzynes.25 However, as pointed out by Yamamoto,26 this method is unfeasible for 9ꢀ
triptycylamine derivatives because of the presence of undesirable hydroamination of the amine group
with benzynes. The preparation was thus initiated with the 9ꢀnitroanthracene or its derivatives. Addition
of excess of benzenediazoniumꢀ2ꢀcarboxylate in portions to the solution of 9ꢀnitroanthracene while
refluxing provides benzyne in situ by thermal decomposition, which then undergo DielsꢀAlder reaction
to give the 9,10ꢀadduct, 9ꢀnitrotriptycene (10), in 46% yield. The low yield is ascribed to the strong
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Scheme 1. Synthesis of 2,6ꢀPyridinedicarbox amidesꢀbridged Triptycene and its Derivatives 1–4a.
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N2+Cl-
R1
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R2
10 R1=R2=H (46%)
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8 R2=H
R1=R2=CH3 (7%)
12 R1=H,R2=CH3 (35%)
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R1=CH3
9 R2=CH3
R1=CH3,R2=H (15%)
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R2
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R1=R2=H (92%)
15 R1=R2=CH3 (90%)
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R1=R2=H (56%)
2 R1=R2=CH3 (51%)
3
R1=H,R2=CH3 (92%)
17 R1=CH3,R2=H (90%)
R1=H,R2=CH3 (52%)
4 R1=CH3,R2=H (47%)
a
a
Reagents and conditions: Reagents and conditions: (i) 1,2ꢀepoxypropane, CH2ClCH2Cl, reflux; (ii)
SnCl2, HCl, CH3COOH, reflux; (iii) 2,6ꢀPyridinedicarboxylic dichloride, DIEA, CH2Cl2, rt (for 1, 3 and
4); or 2,6ꢀpyridinedicarboxylic acid, PyBOP, DMAP, DMF, reflux (for 2).
electron withdrawing of nitro group, which is not favorable for the DielsꢀAlder addition thus giving
considerable amount of 1,4ꢀadduct, 11ꢀnitroꢀ5,12ꢀdihydroꢀ5,12ꢀethenonaphthacene. The multiple methyl
group substituted 9ꢀnitrotriptycenes were prepared from the corresponding derivatives of 9ꢀ
nitroanthracene and benzenediazoniumꢀ2ꢀcarboxylate. For example, while 2,3,6,7,14,15ꢀhexamethylꢀ9ꢀ
nitrotriptycene (11) could be prepared from 2,3,6,7ꢀtetramethylꢀ9ꢀnitroanthracene (7) and 4,5ꢀdimethylꢀ
benzenediazoniumꢀ2ꢀcarboxylate (9), reaction of 7 with 4,5ꢀdimethylꢀbenzenediazoniumꢀ2ꢀcarboxylate
(8) gave 2,3,6,7ꢀtetramethylꢀ9ꢀnitrotriptycene (13). It was found methyl substitution is to the
disadvantage of producing desirable 9,10ꢀadducts, which is probably due to the increased steric
hindrance of the methyl group. Careful isolation of the additive productions gave 11 in only 7% yield,
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but a bit higher for 12 and 13 as 35% and 15%, respectively. Reduction27 of 10–13 with SnCl2 in the
acidic condition provided the corresponding aminotriptycenes 14–17. Amidation of 14 with 2,6ꢀ
pyridinedicarboxylic acid chloride, using N,Nꢀdiisopropylethylamine (DIEA) as the base, gave the target
compound 1 with the yield of 56%. However, the analogous convergent method to compound 2 was not
successful, in which the reaction was found to be hard to proceed even under refluxing in toluene.
Fortunately, 2 can be obtained in acceptable yield by coupling of 15 and 2,6ꢀpyridinedicarboxylic acid
with the help of peptide coupling reagent benzotriazolꢀ1ꢀylꢀoxytripyrrolidinophosphonium
hexafluorophosphate (PyBOP). The final two target compounds 3 and 4 were prepared according to the
method described for 1. Structures of 1–4 are confirmed by NMR and high resolution mass spectra. The
assignments of their experimental 1H and 13C NMR spectra were performed based on the corresponding
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1H–13C HSQC, H–13C HMBC, H–1H NOESY and H–1H COSY experiments (see Figure S2–S11 in
the Supporting Information (SI)).
X-ray structures of 1 and 2. Crystal structure of 1 was obtained by slowly evaporating the solvent of
mixture of dichloromethane (DCM) and methanol at ambient temperature (Figure 2a). The crystal is an
orthorhombic, solventꢀcontained system, with space group of P2(1)2(1)2(1). Each unit cell contains four
molecules of 1 and four of DCM, packed with high disorder. As expected, in the structure, the central
bridge 2,6ꢀpyridinedicarboxamido group adopted a preferred cis conformation ascribed to the hydrogen
bonding between pyridineꢀN and amidoꢀNH (the N−H···N(pyridine) distance of 2.43 and 2.22 Å),
holding the two Tp groups together to assemble a molecular bevel gearꢀlike structure in C2 conformation.
The structure is not entirely symmetrical. While one amido group maintains a planar geometry with the
core pyridyl group, the other is slightly twisted (with dihedral angles of 6°) to accommodate the sterics.
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Meanwhile, despite there is one blade (one phenyl ring) directed endo in both rotators, the inwardꢀ
facing ring in one rotator is more planar to the plane of pyridine (intersecting at an angle of 16°), but the
counter one on the other Tp group is significantly more torsional (dihedral angles of 59°). The distances
between the (9−9′) and (10−10′) bridgehead atoms of the Tp groups are 6.64 and 10.16 Å, respectively,
providing the distance between two center of the Tp axles of about 8.4 Å, which is larger than the
minimum axle−axle distances (7.0 Å) calculated by Siegel for a Tpꢀbased molecular spur gear in C2
conformation.20 Notably, although by wireframe representation two rotators seem separated apart, a
spaceꢀfilling mode (Figure 2a, right) reveals that they in fact partially stack with each other, suggesting
they are able to behave correlated motion at least when 1 adopts ground state conformation.
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Figure 2. The ball and stick diagrams (Left, top view) as well as the stick diagrams overlaid by
Diamond 3.0 generated spaceꢀfilling representation (Right, side view) for molecular structure of (a) 1
and (b) 2 which crystallized from the mixture of dichloromethane and methanol. The contained
dichloromethane molecules in the cell unit of 1 were omitted for clarity.
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Single crystal of 2 can also be obtained from the solution in the mixture of dichloromethane and
methanol (Figure 2b). The crystal is a monoclinic, with space group of P2(1)/n. Each unit cell contains
only four molecules of 2. Similar to that of 1, the intramolecular hydrogen bonding constrains the
methylꢀsubsitituted Tp groups, providing a gearꢀlike structure in C2 conformation, wherein the two
amides are twisted and tilted up and down along the plane of pyridyl group with the dihedral angles of
17° and 19°. The diminish in planarity compared to that of 1 is ascribed to the bigger steric exclusion
from two methylꢀsubsitituted Tp groups when compared to that of unsubsitituted ones. The distances
between the (9−9′) and (10−10′) bridgehead atoms of the Tp groups are 6.40 and 9.72 Å, which are a bit
smaller than those in the structure of 1. This probably dues to the stronger π stacking and orbital
interactions between two Tp groups of 2, as suggested by the fact that the distance between the planes of
two proximal benzene rings on one Tp group and the closest methyl protons on the counter Tp one are
just about 3.05 Å. A spaceꢀfilling mode (Figure 2b, right) clearly shows that the two Tp groups
intermesh more tightly in comparison with that of 1, providing hope of possessing a higher efficiency of
motion transmission.
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Calculations. While some of the prevoius computational studies on molecular bear or spur gears
directly focus on the molecular geometries and the relative energies of C2, Cs and C2v conformations,11ꢀ
14, 20
identification of the motion profile as well as the ground and the transition states within it for the
studied system needs to determine the most energetically favourable conformational transformation
pathway, i.e., MEP, considering the motions in the studied systems might be considerably different from
those of the molecular gears. Therefore, starting from the Xꢀray structures, the unconstrained geometries
of 1–2 were first optimized and confirmed to the global minima at DFT level of theory with the B3LYP
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hybrid functional and the 6ꢀ31G* basis set. Then, from this point, the MEPs were obtained by relaxed
potential energy surface scan procedures28, 29 implemented in the GAUSSIAN 03 program package30 as
a function of the dihedral angle (denotes ϕ) of C17(carbonyl)–N14(amide)–C51(bridgehead)–C70(ring α)
for 1 and C17(carbonyl)–N14(amide)–C47(bridgehead)–C71(ring α) for 2 (could be approximately
regarded as the dihedral angle between the planes of benzene ring α and the adjacent carbonyl group)
(Figure S17). At each point on the MEPs, all the degrees of freedom other than the fixed dihedral angle
were allowed to be fully optimized. Given the threeꢀfold symmetry of Tp group, the dihedral angle was
scanned totally by 120° with a step size of 1°. To verify the transition states obtained by scan strategy
(scanned TSs), we reꢀoptimized two transition states by using the conventional TS optimization method
(to give optimized TSs) along the reaction pathways of the rotational isomerization of 1 and 2. In this
case, all the degrees of freedom are completely relaxed without any predefined reaction coordinates. All
computational results were summarized in Figure 3 and Figure 4.
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DFT calculation confirms the C2 conformation as the global minimum for 1 by vibrational analysis
(Figure 3). The optimized geometry bears a close resemblance to the Xꢀray structure (Figure 2a). For
example, the distances between the (9−9′) and (10−10′) bridgehead atoms of the Tp groups are predicted
to be 6.88 and 10.62 Å, and the central bridged moiety 2,6ꢀpyridinedicarboxamide is also almost planar.
The dihedral angles between the planets of two benzene rings directed endo (α and αʹ, Figure 3) and the
plane of pyridyl group are measured to be 17.7° and 17.6°, indicative of slightly more symmetrical
structure when compared to that in the crystal. The motion of 1 is predicted to start with a gearing.
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Figure 3. Minimum energy profile of rotational isomerization of 1 along C17ꢀN14ꢀC51ꢀC70 (ϕ)
dihedral angle calculated by relaxed potential energy surface scan at the B3LYP/6ꢀ31G (d) level of
theory. The transition states were calculated by scan strategy (TSs) and direction method of
conventional TS optimization (TSo), respectively.
Rotating the Tp moiety where benzene ring α located antiꢀclockwise from ϕ dihedral angle of −181°
gives rise to a slight distortion of pyridylamides (Table S2, SI) and, more importantly, a disrotation of
the counter Tp group, accompanying by a considerable increase in energy ascribed to the steric
hindrance of carbonyl oxygen. The highest energy comformer of the energy curve, which shows up at
the point where ϕ dihedral angle is −129° and is ca. 7.4 kcal molꢀ1 higher in energy than that in the
minimum conformation, adopts an approximately gearꢀclashed C2v orientation, suggesting that the
gearing will lose fidelity soon. Notably, a temporary C=O···H−C hydrogen bond between the carbonyl
oxgen and the proton H38 on benzene ring β is predicted to be present in process of that ring β passes
over the carboxyl group, and the strongest bonding (with H···O distance of 1.98 Å and H−C···O bond
angle of 114.5°) occurs when these two groups are nearly coplanar, wherein ϕ = −129°. This hydrogenꢀ
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bonding might be able to stabilize the C2v transition state, thus, to some extent, lowering the rotational
barrier of the Tp group.31 Besides, the scanned TS shows negligible differences in geometric structure
(deviation: 0.02ꢀ1.26°/dihedral angles; 0.0ꢀ0.002Å/bond lengths) and relative energy (deviation: 0.05
kcal molꢀ1) compared with the one obtained by the direct method of the conventional TS optimizations
(see Figure 3 and Table S4 in SI). Frequency calculation on this scanned TS yields one and only one
imaginary frequency (41.67i cmꢀ1) corresponding to the rotational motion of triptycene along the axle of
C(triptycene)–N(amide) bond, which is very closed to the optimized TS (41.75i cmꢀ1) with the same
vibration mode. Intrinsic reaction coordinate (IRC) calculations starting from optimized or scanned TSs
confirm that these energy maxima unambiguously connect with the corresponding reactant of ground
state minima along the reaction paths of triptycene rotation along the axles of CꢀN bond. These suggest
that there are no principal differences in describing the rotational isomerization of triptycene by using
the scan strategy and the direct method of transition state optimization, and also indicate that it is
reasonable to choose C17ꢀN14ꢀC51ꢀC70 dihedral angle as the reaction coordinate in the MEP
calculations of this rotational isomerization. As expected, with just a slightly continuous rotation of the
driving Tp group from −129° to −128°, the gear slips and the counter Tp group rotates back (antiꢀ
clockwise), accompaning with a drastic reduction (3.3 kcal molꢀ1) in energy. The reverse rotation is
ascribed to the relatively too large interaxle distance as well as the rotation barrier for the counter Tp
rotator. Another important transition geometry, which adopts an approximate Cs conformation, is
observed at ϕ = −128°, in which the dihedral angle between the planes of ring αʹ and pyridyl group is
13°, pretty close to that in the minimum conformation. With further rotation of the driving Tp rotator to
the final where ϕ = −61°, the counter Tp moiety keeps almost stable, while the energy moderately
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decreases. These computational results clearly show that rotation of one Tp rotator on 1 would give rise
to a vibration of the counter one, indicating that rotationꢀdrived vibration is the main mode of the
intramolecular motion of 1 as we designed.
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Figure 4. Minimum energy profile of rotational isomerization of 2 along C17ꢀN14ꢀC47ꢀC71(ϕ) dihedral
angle calculated by relaxed potential energy surface scan at the B3LYP/6ꢀ31G (d) level of theory. The
transition states were calculated by scan strategy (TSs) and direction method of conventional TS
optimization (TSo), respectively.
In contrast to 1, the DFT calculation on molecule 2 predicts a slightly more complicated
intramolecular movement profile (Figure 4). The global minimum of 2 possesses a C2 conformation and
closely resembles its single crystal geometry (Figure 2b), wherein the central 2,6ꢀpyridine
dicarboxamide moiety is planar and the dihedral angles between this moiety and both the plane of
benzene ring α and αʹ are 23°. At this state, the ϕ dihedral angle was measured to be 182°. Different
from that of 1, clockwise rotation of the Tp group where benzene ring α is located initailly gives a
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distortion of the adjacent amide and a gradual increase in energy, but does not induce perceptible antiꢀ
clockwise rotation of the counter Tp group. This is probably ascribed to the big methylꢀsubstituted Tp
rotators, which is larger in the radius than the minimum axleꢀaxle distance required for the Cs
orientation so that the pyridylamide connected to the driving rotator should at first deform to
accommodate the crescent sterics. The highest energy comformer, which lies ca. 12.0 kcal molꢀ1 higher
in the energy level than that of the global minium, shows up at ϕ = 122°, where the pyridylamide
connected with the driving rotator is seriously distorted while the counter Tpꢀamide is almost still under
the equilibrium position. Similar to the case of 1, the highest energy conformer obtained by scan
strategy (i.e., TSs) is very close to the one computed by conventional TS optimizations along the
unbiased reaction coordination of 2 (Table S5). This reveals that the rotational isomerization proceeds
along the axle of C(triptycene)–N(amide) bond is the predefined reaction coordinate in our scan strategy.
Continuous rotation of the driving rotator leads to an apparent disrotatory motion of the counter Tp
group, through a way of the antiꢀclockwise rotation of the counter Tp group around C23(Tp)–N(amide)
bonds as well as the deformation of the pyridylamide connected (Table S3, SI). The strain in the whole
molecule thus gets released, giving a sharp decrease in the energy. Surprisingly, in the range of ϕ =
121°−97°, rotation of the driving rotator once again produces only the deformation of the connected
amide, which gives rise to a transient increase in the interaxle distance and a slight decrease in energy.
Subsequently, through a gearꢀclashing transition (C2v/96°, Figure 4), the position of benzene ring α and
αʹ reverses, providing a new transition structure in which the counter Tp almost returns back to its
equilibrium position. This is distinctly different from what we hypothesized, as we expected that the
methyl substituents would increase the size of rotator’s blade thus benefiting gearing. Obviously, the
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rotation barrier of Tp group was underestimated in our design. In fact, this barrier is so high that the
molecule prefers to weaken the N−H···N (pyridine) hydrogenꢀbonding and distorting the amide group
to behave slippage rather than overcoming an additional rotation barrier from the counter rotator.
Further rotation of the driving Tp group from ϕ = 96° releases the strain arising from the torsion of
pyridylamides, providing a moderate decrease in energy until finally a global minimum geometry is reꢀ
obtained. All these predict that the methyl substituents would not change the basic mode of motion of 1
but will increase the energy barrier of slippage thus raising the free energy of activation for the overall
motion transmission.
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VT-NMR experiments of 1 and 2. Experimentally studies on the motions of 1 and 2 in solution were
carried out with NMR spectroscopy rather than other methods, such as timeꢀresolved emission
spectroscopy32 (internal rotation in the studied molecules would be too slow on the timescale of
fluorescence emission, giving the fluorescence lifetimes are generally on the order of 1ꢀ10 nsec). At
ambient temperature, the Tp groups rotate fast around C9 (Tp)–N(amide) bonds on the NMR time scale,
1
giving the six benzyl rings on Tp moieties magnetically equivalent. Consequently, the H NMR
spectrum of 1 in CD2Cl2 at 298 K shows only one set of sharp signals of benzyl rings (i.e., two doublets
at 7.48 and 7.54 ppm those are assigned to protons H1 and H4 as well as two triplets at 7.02 and 7.12
ppm for H2 and H3, respectively). In addition, the amide protons exhibit one single signal at 9.61 ppm as
1
well. In the corresponding H−1H NOESY spectrum (Figure S2), the crossꢀpeaks between amide
protons and their neighboring pyridinyl protons are completely absent, demonstrating that the central
bridge 2,6ꢀpyridinedicarboxamides unit is mainly rotationally constrained in solution as that in the solid
structure.
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To explore more dynamic features of 1 in CD2Cl2, variableꢀtemperature (VT) 1H NMR investigations
were then carried out (Figure 5a and Figure S18). While the pyridyl protons signals of 1 show no
changes at all upon the decrease of the temperature in the range examined (Figure S18), the Tp protons,
especially the protons H1 and H2 close to counter Tp rotator, undergoes slight upfield and downfield
shift, respectively, at the beginning of this process. The changes in the chemical shift are ascribed to the
increase in the fraction of 1 in the ground state conformation. Decreasing the temperature further
provides considerable broadening of H1 and H2, clearly indicating that the TpꢀN rotations slow down
upon lowing the temperature. The decoalescence of H1 and H2 were observed at 203 K. This
comparatively high coalescence temperature in comparison with all those tightly meshed molecular
gears12a is due to the steric hindrance of carbonyl oxygen atom as well as the slippage process, arising
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1
13
Figure 5. a) Partial VT H NMR spectra (500 MHz) of 1 (2 mM) in CD2Cl2. b) Partial VT C NMR
spectra (125 MHz) of 1 (40 mM) in CD2Cl2. c) Simulated VT 13C NMR spectra at the region of carbon
C1, which were obtained with DNMR Line shape Analysis procedure implemented in the Topspin 2.1
program33. The temperature (K) and calculated interconversion rate constants (kr, sꢀ1) are given for each
trace.
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energy barrier to TpꢀN rotations. With even further decrease in the temperature, the rotation of Tp
groups becomes frozen and thus either of the H1 and H2 signals split into two peaks, with an intensity
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1
ratio of 2:1. The H NMR spectra in this case mainly correspond to 1 in ground state conformation,
where the protons H1 and H2 show small peaks at 7.58 and 6.91 ppm, respectively, which can be
assigned to the relevant protons on the inwardꢀfacing benzene rings (labelled as α and αʹ in Figure 3),
and larger peaks at 7.36 and 7.11 ppm, respectively, to the corresponding protons on the other four rings.
The smaller peak of H2 signal is upfield from the bigger one, which is ascribed to ringꢀcurrent effects
arising from πꢀstacking interaction between rings α and αʹ. In contrast, the signals of H1 on ring α and αʹ
locates downfield which is probably ascribed to the deshielding of pyridineꢀN (3.57 Å for the distance
of C1−N(pyridyl) in the crystal structure). Expectedly, the benzene rings β and γ are not chemically
equivalent in case of 1 in ground state as predicted by DFT calculation (Figure 3), as evidenced by that
the signals of H1 at 7.36 ppm and H2 at 7.11 ppm split further when the temperature is as low as 183 K.
It is noteworthy that, in the whole process, the chemical shift of amide proton remains almost
unchanged (ꢂδ = 0.07 ppm) (Figure S18), which indicates that, throughout the entire range of
temperature examined, the intramolecular Hꢀbonding between pyridineꢀN and amide always presents
and that the N−H···N (pyridine) bonding pattern (including the bond angle and bond distance) does not
change much upon the change of temperature. This is important because it experimentally once again
demonstrate that, the molecule keeps adopting a gearꢀlike conformation in the motion, as previously
indicated by the 1H−1H NOESY spectrum.
The mulitiplicity and overlap of some important protons signals of 1 prevents a direct lineꢀshape
1
analysis of the H NMR spectra for elucidating the kinetic parameters for the motions of 1. To
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circumvent this problem, VT 13C NMR experiment was carried out (Figure 5b and Figure S19).34
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gradually broaden and decoalesce at about 213 K. At 183 K, two pair of sharp peaks with a 2:1 ratio in
intensity for C1 and C2 were observed. The rate constant corresponding to the process that any one of
the benzene rings passes over the carbonyl group were derived from the line shape analyses of the C1
resonances (Figure 5c). From these constants, the free energy of activation (ꢂG#) as well as the enthalpy
(ꢂH#) and entropy (ꢂS#) were derived from the Eyring plots35 (Table 1, Figure S20), in which ꢂG# is
calculated to be 9.0 kcal molꢀ1 that is pretty close to DFTꢀcalculated energy gap between the transition
state (TSs and TSo) and the minimum conformers.
Table 1. The Summary of the thermodynamic Parameters for 1–4.
ꢂG#
ꢂH#
ꢂS#
a
298 K
Compd.
Nmethyl
(kcal molꢀ1)
9.0 ± 1.0
(kcal molꢀ1)
12.3 ± 0.4
(cal molꢀ1 Kꢀ1)
11.0 ± 2.0
1 b
2 c
2 b
3 b
4 c
0
12
12
4
11.0 ± 0.2
11.0 ± 0.2
9.0 ± 0.7
9.2 ± 0.1
9.2 ± 0.1
12.3 ± 0.3
8.9 ± 0.2
ꢀ5.9 ± 0.4
ꢀ5.9 ± 0.3
11.0 ± 1.2
ꢀ6.3 ± 0.7
8
10.8 ± 0.4
a
b
Number of methyl substituents on the molecule. These values were obtained from VT 13C NMR
experiments. c These values were obtained from VT 1H NMR experiments.
We next performed VT 1H and 13C NMR experiments on 2 in CD2Cl2 (Figure 6, Figure S21, S23 and
S24). Similar to that of 1, Decreasing the temperature slows down and finally freezes the rotation of
methylꢀsubstituted Tp groups, giving rise to broadening followed by splitting of the signals of the
protons and carbons which are spatially close to the counter Tp group. In this process, the chemical shift
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of amidoꢀNH just slightly downshifts (ꢂδ = 0.15 ppm) (Figure S21), which suggests that, similar to that
of 1, the amides are Hꢀbonded with pyridineꢀN all the time but might be a slightly more distorted at
ambient temperature compared to that of 1 statistically. A decoalescence is observed at ca. 213 K for
both proton H1 and carbon C1, wherein the signals split into two peaks with a 2:1 ratio in intensity.
Another decoalescence is found to be around 193 K and 183 K for H1 and C1, respectively, in which the
resonances split further and exhibit three distinguishable peaks as expected. Lineꢀshape analysis of the
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VT H NMR spectra provides the rate constant of the rotation (Figure 6), which shows that, while the
rotation of 2 is as frozen as that of 1 at 183 K, increasing the temperature gives rise to an increase (but
less dramatic than that of 1) in the rotational speed. All these observations indicate that the rotational
barrier of the substitutedꢀTp groups in 2 is a bit higher than that of Tp groups in 1. Two sets of
1
corresponding thermodynamic parameters were thus derived independently from the VT H and 13C
NMR spectra (Figure S22 and S26). Both approaches provide very consistent results of 11.0 kcal molꢀ1
(Table 1)36, which is again consistent with that of the DFTꢀcalculated results (TSs and TSo).
Figure 6. a) Experimental and b) simulated VT 1H NMR spectra (500 MHz) of 2 (2 mM) at the region
of H1 and H4 in CD2Cl2. The temperature (K) and calculated interconversion rate constants (kr, sꢀ1) are
given for each trace.
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The fact that ꢂG# for the rotation in molecule 2 is just a slight larger than that in the case of 1 suggests
that both the molecules do not tend to gear, but motions in a way as predicted by DFT calculation. This
is because the methyl substituent would not affect the rotation of the whole Tp rotator around C9(Tp)–
N(amide) bonds but only the interaction between two Tp groups while motioning. Consequently, if both
the molecules keep gearing, their free energy of activation would be very close, giving the gearing itself
is almost unhindered. Besides, it is not likely that 2 keeps gearing while 1 undergoes continuous
slippage, since in such case, the molecule 2 needs to overcome the rotation barrier for one more Tp
rotators, which would give rise to considerable increase in the free energy of activation when compared
to that of 1. In both cases, ꢂH# is predominant over ꢂS# in its contribution to ꢂG#, indicating the rotation
is mainly enthalpically disfavorable. The negative value of ꢂS# in case of 2 suggests that molecule 2 in
the transition states have totally less degree of freedom when compared to that in the ground state,
probably in such cases, the vibration of some subunits are partially restricted by the torsional strains.
X-ray structures and dynamic behavior of 3 and 4. The crystal structure of 3 was obtained by
slowly evaporating the solvent of acetonitrile at ambient temperature (Figure S12a). The crystal is a
monoclinic system, and there are four molecules of 3 and four of acetonitrile in each unit cell. The
geometry is highly symmetrical, with the two benzene rings with methyl substituent locates flank. The
distances between the (9−9′) and (10−10′) bridgehead atoms of the Tp groups are 7.05 and 11.24 Å,
respectively, indicating the two Tp groups possess a larger axle−axle distance compared to that of 1
crystalized from the solution of the mixture of DCM and methanol. This larger axle−axle distance is
probably induced by cocrystallized acetonitrile molecules in the crystal package, as we later found that
crystal structure of 1 with acetonitrile cocrystallized can also be successfully obtained from solution of
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acetonitrile and the structure possess larger axle−axle distance than that of the one grown in mixture of
DCM and methanol (Figure S12c). The larger interaxle distance is favorable for releasing the steric
repulsion between Tp groups, which leads to an approximate C2v conformation in geometry of 3
wherein the dihedral angles between the planes of two benzene rings directed endo and the plane of
pyridyl group are as small as 2° and 7°, respectively.
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Crystals of 4 grown in the mixture of DCM and methanol shows also a symmetrical structure, in
which the two benzene rings without methyl substituents directly flank unexpectedly (Figure S12b).
There are two molecules of 4 and four of methanol in each unit cell. Nevertheless, the distances between
the (9−9′) and (10−10′) bridgehead atoms of the Tp groups are as short as 6.25 and 9.46 Å, respectively,
indicative of a slightly more compact conformation than those in the other studied crystal structures.
Since the Xꢀray structures of 3 and 4 skeletally resemble closely those of 1 and 2 (Figure S13 and
S14), we then carried out dynamic NMR studies for better understanding of the dynamic properties of
1
this type of molecules. Due to the fast rotation of Tp groups at ambient temperature, the H NMR
spectrum of 3 in CD2Cl2 at 298 K shows only one set of sharp signals of the amides and the benzene
rings without methyl substituents (Figure S27). Decreasing the temperature below 233 K gives rise to
broadening of all Tp signals and splitting of the signals of unsubstituted benzene rings, wherein the
decoalescences of H1 and H2 occur at ca. 203 K. Notably, in this process, decoalescences of amideꢀNH
and pyridonyl protons were observed at 193 K, which is ascribed to the appearance of phase isomers
(Figure S35 and S36). The splitting of Tp signals arising from the presence of phase isomers is hard to
distinguish due to the overlap of signals. Integration analysis shows two phase isomers exit in ca. 2:1
ratio, consistent with the observation for most molecular gear systems11b, 18. Similar phenomena could
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be also observed on the corresponding VT 13C NMR spectra, wherein decoalescences of several Tp
carbons such as C1 and C2 are observed at ca. 213 K and the phase isomers starts to show up at ca. 203
K (Figure S28 and S29). Clearly, in both the VT 1H and 13C NMR experiments, the signals of Tp groups
decoalesce at a bit higher temperature when compared to that of the appearance of the phase isomers.
This is understandable. Since that given the energy barrier for vibration is much lower than that for
rotation, even under certain conditions the rotation of Tp rotators is freezing, it is still possible for the
vibration to take place, providing possibility of slippage thus inhibiting the appearance of phase isomers.
Consequently, for the studied cases, decoalescence of Tp signals might not always synchronize with that
of amides, especially for system which possess rotators with short blades such as 3. On the other hand,
the fact that the phase isomers could not be observed above 203 K indicates the two Tp rotators keep
slipping in those cases, providing evidence of that the rotation of the driving rotator gives a vibration of
the counter one. Calculation of the thermodynamic parameters for Tp rotation was based on the changes
in signal line shape of carbon C11 upon the decrease of temperature (Figure S30 and S31), in which the
range of temperature was carefully chosen as 263–193 K to avoid the effect of the presence of phase
isomers. The resultant data are listed in Table 1.
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For molecule 4 possessing more methyl substituents, the decoalescences of amideꢀNH and pyridonyl
protons show up at temperature as high as ca. 213 K (Figure 7). The signal of methyl protons Hm2
decoalesces at ca. 223 K, indicating slipping caused by the vibration of Tp groups presents expectedly.
The decoalescence of Tp signals is hard to identify at 213 K due to the serious broadening but can be
clearly observed at 203 K, although those signals could not be precisely assigned in such case. The
nonequivalence of two benzene ring with methyl substituents as well as the appearance of phase isomers
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gives rise of a very complicated H NMR spectrum of 4 at low temperature such as 193 K and 183 K.
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Nevertheless, the ratio of two phase isomers could be still obtained by integration of splited amideꢀNH
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Figure 7. Partial VT 1H NMR spectra (500 MHz) of 4 (2 mM) in CD2Cl2. The circles in different colors
indicate signals of different set of isomers. The splitting of Tp signals arising from the appearance of
phase isomers is hard to distinguish due to the overlap of signals.
signals, which gives that of ca. 1.2:1 rather than 2:1, probably because the meso isomers have lower
potential energy in the ground state when compared to the dl ones (Figure S37 and S38). The changes in
signal of methyl protons Hm1 and Hm2 upon the decrease of temperature from 263 K to 203 K were used
to calculate the thermodynamic parameters for the Tp rotation (Figure S33 and S34). As shown in Table
1, the values of kinetic parameters in case of 4 are approximate to those of 2, while the values of 3 are
close to those of 1. This suggests that, in terms of the motion transformation between the two Tp
components in the studied systems, combination of unsubstituted Tp group and the one with six methyl
substituents might provide similar dynamic property as that of 2.
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Notably, despite both the DFT calculation and VTꢀNMR experiments indicate a rotationꢀdrove
vibration as the most energyꢀeconomical way for the motion of 1, for real molecules in solution, the
driving rotator would not be preselected and fixed, and a geared rotation might be still possible to occur
(especially at high temperature) if the molecule possess high enough energy for both Tp groups to
overcome their respective rotation barriers. Besides, the thermally stimulated motions should occur
without direction bias, giving the threeꢀfold symmetry of Tp groups and the absence of any processes of
chiral discrimination8b.
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CONCLUSIONS
In summary, we have designed and synthesized a serial of N2,N6ꢀbis(triptyceneꢀ9ꢀyl)pyridineꢀ2,6ꢀ
dicarboxamides 1–4, which are derivative from conventional molecular gears. DFT and dynamic NMR
experiments have indicated that this kind of molecules possess a different dynamic properties from
those of the molecular gears, wherein one triptycene rotator rotates around the C(Tp)–N(amide) bonds
while the other one keeps associated rocking vibration through an intermediate slippage process.
Enlarging the blades of rotators with methyl substituents gives rise to the raise of the free energy of
activation, but would not change the mode of the motion. Our studies thus provide a conceptual
approach to design molecular transmission devices those are capable of transforming the mode of
motion from rotation to rocking vibration.
EXPERIMENT SECTION:
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All starting chemicals were obtained from commercial sources and used without further purification.
Anhydrous dichloromethane (DCM) was distilled over calcium hydride (CaH2) under inert atmosphere.
NMR spectra were obtained with a 400, 500 or 600 MHz spectrometer using chloroformꢀd (CDCl3),
dichloromethaneꢀd2 (CD2Cl2), methanolꢀd4 (CD3OD) and acetoneꢀd6 as the solvents. The chemical shift
references were as follows: (1H) chloroformꢀd, 7.26 ppm; (13C) chloroformꢀd, 77.16 ppm (chloroformꢀd);
(1H) dichloromethaneꢀd2, 5.32 ppm; dichloromethaneꢀd2, 53.52 ppm; (1H) methanolꢀd4, 3.31 ppm, (1H)
acetoneꢀd5, 2.05 ppm. High resolution mass spectra (EI and ESI) were acquired on GCT, FTꢀICR
spectrometer. IR spectra were recorded on FTꢀIR spectrometer with thin KBr disk.
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9-nitro-2,3,6,7-tetramethylanthracene (7). To a solution of 2,3,6,7ꢀtetramethylanthracene37 (190 mg,
0.8 mmol) in CH2Cl2 (19 mL), an aqueous HNO3 solution (20%, 19 mL) was added. The mixture was
refluxed for 4 h. The reaction mixture was diluted with water (10 mL) and extracted with CH2Cl2 (3×10
mL). The combined organic extracts were dried over Na2SO4, then concentrated under reduced pressure.
Compound 7 (212 mg, 95%) was obtained as a yellow solid. Rf = 0.6 (petroleum ether (PE)/CH2Cl2 =
2/1). Mp = 190–192 °C. IR (KBr): 2918, 2850, 1643, 1531, 1460, 1444, 1384, 1327, 1116, 898 cmꢀ1.
1H NMR (400 MHz, CDCl3), δ = 8.32 (s, 2H), 7.74 (s, 2H), 7.65 (s, 2H), 2.47 (s, 6H), 2.46 (s, 6H); 13C
NMR (100 MHz, CDCl3), δ = 139.4, 136.2, 130.1, 127.8, 127.2, 121.8, 120.2, 21.0, 20.3. HRMS (EIꢀ
TOF) m/z: [M]+ calcd for 279.1259 (100%, C18H17NO2), found 279.1263 (100%, 1.3 ppm).
4,5-dimethylbenzenediazonium-2-carboxylate (9). To a mechanically stirred suspension of 4,5ꢀ
dimethyl anthranilic acid (825 mg, 5 mmol) in ethanol (50 mL) in an iceꢀwater bath was added dropwise
concentrated hydrochloric acid (0.63 mL), following the addition of isopentyl nitrite (1.35 mL). After
the mixture was stirred for 15 min, diethyl ether (150 mL) was added. Then the reaction mixture was
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continued to stir for another 30 min. The product was filtered and washed with diethyl ether to give the
product, 975 mg (92%), which was used without further purification.
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General Methodology for the Synthesis of Compounds 10–13. The procedure for synthesis of
triptycene or derivatives followed as previously reported38: Benzynes (839 or 9) (7–10 mmol) was added
in portions to a mixture of 9ꢀnitroanthracene or its derivative 7 (1 mmol) and 1,2ꢀepoxypropane (2 mL)
in dichloroethane (30 mL) for refluxing 24 h. The reaction mixture was concentrated under reduced
pressure. Then the resultant solid was washed with methanol. The residue was purified by silica gel
column chromatography.
9-nitrotriptycene (10). Purification by silica gel column chromatography (PE/CH2Cl2 = 3/1) to give
compound 10 (137 mg, 46%) as a white solid. Rf = 0.5 (PE/CH2Cl2 = 3/1). Mp = 246–248 °C. IR
(KBr): 3072, 3038, 2968, 1543, 1459, 1446, 1365, 1295, 1156, 872, 803, 752, 620 cmꢀ1. 1H NMR (400
MHz, CDCl3), δ = 7.67ꢀ7.71 (m, 3H), 7.42ꢀ7.46 (m, 3H), 7.09ꢀ7.14 (m, 6H), 5.42 (s, 1H); 13C NMR
(100 MHz, CDCl3), δ = 143.8, 140.4, 126.9, 125.8, 123.7, 122.1, 96.9, 54.0. HRMS (EIꢀTOF) (m/z)
[M]+ calcd for 299.0946 (100%, C20H13NO2), found 299.0950 (100%, 1.2 ppm). Lit.26a Yield: 33%; 1H
NMR (CDCl3), δ = 7.68 (m, 3H), 7.44 (m, 3H), 7.07ꢀ7.15 (m, 6H), 5.41 (s, 1H).
2,3,6,7,14,15-hexamethyl-9-nitrotriptycene (11). Purification by silica gel column chromatography
(PE/CH2Cl2 = 3/1) to give compound 11 (27 mg, 7%) as a white solid. Rf = 0.5 (PE/CH2Cl2 = 3/1). Mp
= 258–260 °C. IR (KBr): 3010, 2958, 2920, 2853, 1548, 1465, 1384, 1362, 1271, 866, 809, 604 cmꢀ1.
1H NMR (400 MHz, CDCl3), δ = 7.41 (s, 3H), 7.16 (s, 3H), 5.17 (s, 1H), 2.17 (s, 9H), 2.16 (s, 9H); 13C
NMR (100 MHz, CDCl3), δ = 141.9, 138.6, 137.1, 135.5, 134.7, 133.4, 124.7, 123.0, 122.8, 96.7, 52.7,
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