Redox-regulated rotary motion of a bis(9-triptycyl)-TTFV system

Article abstract of DOI:10.1021/ol403295q

A tetrathiafulvalene vinylogue (TTFV) unit was covalently linked to two benzyltriptycene molecular rotors to form a molecular gearset. Dynamic NMR studies showed that reversible redox reactions at the TTFV central unit exerted regulation over the rotational properties of the 9-triptycyl rotors.

Full text of DOI:10.1021/ol403295q

Letter
pubs.acs.org/OrgLett
Redox-Regulated Rotary Motion of a Bis(9-triptycyl)-TTFV System
Guang Chen and Yuming Zhao*
Department of Chemistry, Memorial University, St. John’s, NL, Canada A1B 3X7
S
* Supporting Information
ABSTRACT: A tetrathiafulvalene vinylogue (TTFV) unit was covalently linked to two benzyltriptycene molecular rotors to
form a molecular gearset. Dynamic NMR studies showed that reversible redox reactions at the TTFV central unit exerted
regulation over the rotational properties of the 9-triptycyl rotors.
Scheme 1. Relationship of Concerted Rotations in Diphenyl-
TTFV and Its Dication
he study of molecular rotary devices has attracted
considerable attention, driven by the challenging goal of
T
attaining ultraminiaturized molecular machines.1 The past few
years have witnessed a large array of elegantly designed
molecular rotary devices, including gears,2 rotors,3 motors,4
clutches,5 brakes,6 and vehicles,7 while the ambition to build
more complex molecular machines continues to tantalize the
mindset of both experimentalists and theoreticians. For a truly
functioning molecular rotary machine, precise control over
both rotational speed and direction is a feature highly desirable
but very challenging to achieve. So far, suitable design
principles and concepts for such kinds of molecular devices
are far from being reliably established. Therefore, as the first
step, it is imperative to design and investigate some new
prototype models to expand our knowledge base.
supported by the fact that both the steric crowding and electron
density distribution of dipenyl-TTFV are substantially altered
when it is switched from neutral to dicationic state. As such, the
diphenyl-TTFV unit may act as a “molecular rotating regulator”
under the control of redox stimuli.
Aiming at prototyping a diphenyl-TTFV-regulated molecular
rotary device, we have designed and synthesized compound 1
(Scheme 2). The molecular structure of 1 is composed of two
9-triptycyl end groups and a diphenyl-TTFV central unit. The
9-benzyltriptycene segment was chosen because it has been
well studied as a molecular gear, in which the phenyl and 9-
triptycyl moieties favor geared (or concerted) rotation.1a,10
Unlike many analogous molecular gear systems, in our design, a
hydroxyl group is attached to the benzylic position. This feature
was designed for a two-fold purpose. First, its synthesis can be
easily undertaken reliably from aldehyde addition reaction.
In our recent studies, diphenyl-substituted tetrathiafulvalene
vinylogues (TTFV)8 have been identified as a class of
potentially useful building blocks for making molecular rotary
devices with controllability over rotational speed and/or
direction. Our reasoning comes from the unique redox
properties of diphenyl-TTFV, which generally shows a
reversible two-electron transfer process on oxidation.8,9
Associated with this redox reaction is a dramatic conforma-
tional change, as depicted in Scheme 1. Analysis of the purely
concerted rotations among the aryl groups crowding together
reveals interesting relationships; that is, the two phenyl groups
favor disrotation in the neutral state and conrotation in the
dicationic state. If such is the case, rotary components
covalently linked to the two phenyl groups can thus be
regulated by the oxidation state of the TTFV moiety in terms of
both rotational speed and direction, assuming that the
concerted rotational barriers in TTFV and [TTFV2+] are
distinctly different. In theory, such an assumption is reasonably
Received: November 14, 2013
Published: January 14, 2014
© 2014 American Chemical Society
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Scheme 2. Synthesis of Compound 1 via Two Different
Routes
1
Figure 1. H NMR (500 MHz, CDCl3) spectra and assignments of
signals in the aromatic regions for (A) compound 1 and (B)
compound 5a.
If the obtained product 1 is indeed a mixture of two
enantiomers and one meso isomer, the only reasonable
1
explanation for the clean H NMR spectral features is that
Second, the chirality of the benzylic carbon desymmetrizes the
triptycyl unit, which in turn facilitates the study of rotational
properties by dynamic H NMR.
the chemical shift of each aromatic proton in the enantiomers is
coincidentally identical to that of the corresponding proton in
the meso isomer. The probability of such a scenario is obviously
very low. On the other hand, if the dimerization of 5a takes
1
The synthesis of compound 1 was undertaken by two
different routes, as illustrated in Scheme 2. The first route
began with the oxidative dimerization of dithiafulvenyl (DTF)-
substituted phenylbromide 2 promoted by iodine in CH2Cl2 at
room temperature. The reaction afforded dibromo-TTFV 3 in
56% yield. Compound 3 was then subjected to a double
lithium−halogen exchange with t-BuLi at low temperature. The
in situ generated phenyl carbanion then underwent a
nucleophilic addition with triptycene-9-carbaldehyde 4 to give
the target compound 1 in a relatively low yield. Purification of
compound 1 turned out to be extremely difficult, due to the
formation of some inseparable byproducts. MS analysis
revealed that the major byproduct(s) resulted from dethio-
methylation of compound 1, likely via a sulfur−lithium
exchange mechanism.11
Alternatively, a second route was explored, in which
compound 2 was first treated with t-BuLi, followed by an
addition reaction with aldehyde 4, to give compound 5a. Again,
the disfavored dethiomethylation reaction occurred noticeably
in this reaction, leading to the formation of a byproduct 5b.
Fortunately, compounds 5a and 5b could be easily separated by
column chromatography. Oxidative dimerization of pure 5a
with iodine then yielded compound 1 in a very good yield of
62%.
1
place via a stereoselective pathway, then the H NMR results
are in line with the reasoning that either the pair of enantiomers
or the meso isomer is selectively formed. In conceiving the
possible transition states involved in the oxidative dimerization
of 5a, one can easily find the preference for the meso product
based on the hydrogen-bonding stabilization, as rationalized in
Scheme 3. At this juncture, however, conclusive evidence for
the optical purity of product 1 still awaits further investigation.
Nevertheless, the clean and well-resolved spectral features
observed in the aromatic region of the 1H NMR spectrum of 1
indicate a two-fold symmetry, while such spectral simplicity
indeed offers a great benefit to the subsequent dynamic NMR
studies.
The electronic properties of compound 1 were studied by
UV−vis absorption spectroscopy (see Figure S-26 in the
Supporting Information). Both the UV−vis spectra of 1 and 5a
show the same the maximum absorption wavelength at λmax
=
358 nm, suggesting little differences in the degree of π-electron
delocalization. The electrochemical redox properties of
compound 1 and precursor 5a were examined by cyclic
voltammetry (CV), and the data are shown in Figure 2.
In the cyclic voltammogram of 5a, a prominent anodic peak
(Epa = +0.82 V) is observed in the forward scan, which can be
assigned to the single-electron oxidation on the DTF unit that
leads to the formation a DTF radical cation. In the reverse scan,
a cathodic peak (Epc = +0.49 V) emerges, which is consistent
with the two-electron reduction of a typical TTFV dication.8,9
The CV results thus point to an EC mechanism wherein the
DTF radical cation is converted into [TTFV2+] at the working
electrode through a rapid electrochemical dimerization
process.8 The cyclic voltammogram of 1 shows a quasi-
reversible redox wave pair (Epa = +0.67 V, Epc = +0.44 V),
which closely resembles the CV features of other known
diphenyl-TTFVs.8,9 The results suggest that the presence of
triptycyl and benzylic alcohol groups has little effect on the
Since the structure of 1 carries two asymmetric benzylic
carbons, it is possible that the product 1 resulting from
oxidative dimerization of 5a is composed of a pair of
enantiomers (RR,SS) and a meso (RS) isomer, provided that
the dimerization reaction proceeds without particular stereo-
1
selectivity. If such is the case, the H NMR spectrum of 1
should be convoluted by these isomers and hence exhibit
somewhat complex spectral patterns. Nevertheless, the actual
1H NMR of 1 shows patterns much simpler than the expected
mixture scenario. As can be seen from Figure 1A, all the
aromatic protons of 1 are well-resolved and the spectral features
appear to be similar to those of precursor 5b (Figure 1B).
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Scheme 3. Proposed Transition States for the Oxidative
Dimerization of 5a Leading to Three Different
Stereoisomers
1
Figure 3. H NMR (300 MHz, DMSO-d6) spectra of compound 1
recorded at varying temperatures and the spectrum of [12+] at 298 K.
program12 allows the kinetic constants related to rotation to be
calculated, which through the Erying plot give the activation
energy for rotation as ΔH⧧ = 22.0 1.2 kcal mol−1 and ΔS⧧ =
17
4 cal mol−1 K−1 (see the Supporting Information for
details). These values are very close to results of DTF precursor
5a (ΔH⧧ = 18.8 1.3 kcal mol−1, ΔS⧧ = 3 3 cal mol−1 K−1),
suggesting that the TTFV linker does not add a significant
amount of “torque” or “friction” to the rotation of the two
benzyl-triptycene gears.
To test the redox-regulated rotational properties for
compound 1, dicationic [12+] was first generated by addition
of iodine (3 equiv) in a DMSO-d6 solution of 1 at room
temperature. The 1H NMR spectrum of the resulting [12+] was
immediately recorded (see the bottom trace in Figure 3).
Interestingly, the spectral pattern of the triptycyl protons in
[12+] bears close resemblance to the spectrum of neutral 1
measured above the coalescent temperature. VT-NMR analysis
of [12+], unfortunately, did not give meaningful data for exact
calculation of its rotational energy barriers due to significant
line shape broadening of signals. However, qualitative assess-
ment on the change in rotational properties before and after
TTFV oxidation can still be clearly made; that is, after the
central TTFV unit is oxidized, the energy barrier for triptycyl of
rotation is lowered and the rotation speed is considerably
accelerated at room temperature. Rationalization for this
oxidation-accelerated rotational property can be made based
on an anchimeric assistance effect elicited by the benzylic
hydroxyl group. As illustrated in Scheme 4, the interaction of
Figure 2. Cyclic voltammograms of (A) compound 1 and (B) 5a.
Experimental conditions: electrolyte, Bu4NBF4 (0.1 M); analyte (ca.
10−3 M); solvent, CH2Cl2; working electrode, glassy carbon; counter
electrode, Pt wire; reference electrode, Ag/AgCl; scan rate, 0.2 V s−1.
redox properties of the central TTFV unit, and the good
electrochemical reversibility of 1 is indeed amenable to the
study of redox-regulated rotatory motion within the molecular
system.
The rotational properties of compound 1 and precursor 5a
were studied by variable temperature (VT)-NMR experiments.
For compound 1, at room temperature (298 K), each proton
on the 9-triptycyl group appeared as distinctive signals as a
result of very slow or restricted rotation between the triptycene
wheel and the nearby phenyl paddle (see Figure 3). As the
temperature is increased, significant peak broadening and
merging can be seen due to faster rotation of the triptycene-
benzyl molecular gear. For instance, the protons corresponding
to Hd on the α and β phenyl rings of triptycene (for labeling,
see Figure 1) appear as two sets of doublet of doublets (δ 7.88
and 7.76) at 298 K, which gradually merge into one broad
singlet (δ 7.82) at temperatures above 338 K. Analysis of the
VT-NMR data of 1 by simulation with the WinDNMR
Scheme 4. Anchimeric Assistance Effect in [12+] Which
Facilates the Rotation of the Triptycyl Group
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Letter
(9) (a) Chen, G.; Mahmud, I.; Dawe, L. N.; Zhao, Y. Org. Lett. 2010,
12, 704. (b) Chen, G.; Mahmud, I.; Dawe, L. N.; Daniels, L. M.; Zhao,
Y. J. Org. Chem. 2011, 76, 2701. (c) Zhao, Y.; Chen, G.; Mulla, K.;
Mahmud, I.; Liang, S.; Dongare, P.; Thompson, D. W.; Dawe, L. N.;
Bouzan, S. Pure Appl. Chem. 2012, 84, 1005.
(10) (a) Yamamoto, G. Pure Appl. Chem. 1990, 62, 569.
(b) Yamamoto, G. Tetrahedron 1990, 46, 2761. (c) Yamamoto, G.;
Oki, M. J. Org. Chem. 1983, 48, 1233.
the hydroxyl electron lone pair with adjacent carbocation is
expected to cause a wider bond angle between the triptycyl
rotor and the adjacent phenyl paddle and hence results in much
easier rotation of the triptycyl group.
In summary, we have prepared a prototype molecular gearset
1 designed to give redox-regulated rotatory motion. Dynamic
NMR studies have shown that oxidation of the TTFV moiety in
compound 1 causes its two triptycyl rotors to rotate at a faster
rate than in the neutral state. At this juncture, the performances
of this molecular gearset are far from being well-tuned and
controlled since there are still many factors and fundamental
issues that need to be investigated and understood. The current
work is a promising small step contributing to the giant leap for
achieving controllable and truly functioning artificial molecular
rotary devices.
(11) Foubelo, F.; Yus, M. Chem. Soc. Rev. 2008, 37, 2620.
(12) Reich, H. J. J. Chem. Educ. 1995, 72, 1086.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic procedures and spectroscopic characterizations for all
1
new compounds and detailed dynamic H NMR analysis of 1
and 5a. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: yuming@mun.ca.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We acknowledge NSERC, Canada Foundation of Innovation
(CFI), and Memorial University for financial support.
■
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