Technomimetic molecules: Synthesis of a molecular wheelbarrow

Article abstract of DOI:10.1016/S0040-4039(03)01519-3

A molecular analog of a wheelbarrow is synthesized following a strategy based on sequential double Knoevenagel and Diels-Alder reactions.

Full text of DOI:10.1016/S0040-4039(03)01519-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6261–6263
Technomimetic molecules: synthesis of a molecular wheelbarrow
Gorka Jimenez-Bueno and Gwe´nae¨l Rapenne*
NanoSciences Group, CEMES-CNRS, 29 rue Jeanne Marvig BP 4347, F-31055 Toulouse Cedex 4, France
Received 16 June 2003; revised 19 June 2003; accepted 19 June 2003
Abstract—A molecular analog of a wheelbarrow is synthesized following a strategy based on sequential double Knoevenagel and
Diels–Alder reactions.
© 2003 Elsevier Ltd. All rights reserved.
Recent advances in the imaging and manipulation of
single molecules1 has stimulated much interest in the
synthesis of molecules exhibiting unique electronic
properties such as amplification or rectification,2 but
also very special mechanical properties like the possibil-
ity to grow atomic-sized wires of gold on a surface of
report here the design and synthesis of a molecular
analog of a wheelbarrow represented in Figure 1. Its
skeleton is made of polycyclic aromatic hydrocarbons
(PAHs)6 which, due to their rigidity, are easily manipu-
lated by the tip of the scanning tunneling microscope
(STM). In addition they are relatively resistant to the
deposition techniques.7
copper3 or to observe
supramolecular bearing.4
a molecular rotor in a
The wheelbarrow is constituted of two legs (3,5-di-tert-
butyl phenyl groups) and two wheels (ethynyl trip-
tycene groups) connected to a polycyclic aromatic
hydrocarbon platform.
In the field of single molecule manipulation it is a
challenge, in a bottom-up approach, to design, synthe-
size and manipulate a molecule which would at the
same time undergo translation and rotation motions.5
In the case of a macroscopic wheelbarrow, pushing the
wheelbarrow results in the rotation of the wheel. We
The left side is equipped with two legs: it has been
shown that the 3,5-di-tert-butyl phenyl groups are held
in a conformation in which the phenyl groups are
nearly perpendicular to the main aromatic board.
Moreover, the tert-butyl groups connected to the PAHs
are used to increase organic solubility and are easily
observed by STM techniques, inducing a good contrast
in the image. The two 4-tert-butyl phenyl groups play
the role of handles for subsequent manipulation with
the tip of the microscope. The right side corresponds to
the axle with two 9-triptycenyl groups acting as wheels.
We opted for two wheels instead of one for obvious
synthetic reasons. Figure 2 shows one of the possible
conformations of
1
obtained by semiempirical
calculation8 and the two three-cogged wheels which can
freely rotate along the axle due to the acetylenic
spacers.
Figure 1. Chemical structure of a molecular wheelbarrow and
its macroscopic analog.
The synthesis of the molecular wheelbarrow 1 is out-
lined in Scheme 1. Our strategy is based on the repeti-
tion of a double Knoevenagel–Diels–Alder reaction
sequence on an a-diketo fragment. The first sequence
allows the connection of the two 3,5-di-tert-butyl
phenyl legs, while the second sequence provides the
Keywords: polyaromatic hydrocarbons; triptycene; single molecule;
Sonogashira coupling.
* Corresponding author. Tel.: +33 (0)5 62 25 78 41; fax: +33 (0)5 62
25 79 99; e-mail: rapenne@cemes.fr
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01519-3

6262
G. Jimenez-Bueno, G. Rapenne / Tetrahedron Letters 44 (2003) 6261–6263
ratio between the different types of tert-butyl groups.
Oxidation of the ethane bridge of the pyracene in 3
with benzeneseleninic anhydride (step c) yielded the
diketo fragment 4 necessary for the connection of the
second axle.10 This is the key step of our strategy.
Halogens are introduced at this stage for subsequent
coupling in order to connect the triptycene wheels. The
double Knoevenagel condensation of 4 with 1,3-di(4-
iodophenyl)-propan-2-one (step d) gave the diiodide
derivative of cyclopentadienone 5 with a quantitative
yield, due to the strong acidic character of the protons
involved in the reaction. The Diels–Alder reaction of 5
with di-(4-tolyl)acetylene provided, after CO extrusion
and aromatization, the precursor 6 with 30% yield (step
e). This low yield may be due to steric hindrance
between the overcrowded alkyne and the tert-butyl
groups of the substrate but the methyl groups are
necessary to improve the solubility of this family of
molecules in organic solvents.
Figure 2. The CPK model showing the minimum energy
conformation of the molecular analog 1. Top view (left) and
side view (right). Methyl groups have been omitted for clarity.
precursor for wheels connection. Finally, a double
Sonogashira coupling yields the molecular wheelbar-
row. The starting cyclopentadienone 2 was obtained via
a first double Knoevenagel reaction of 1,3-bis(3,5-di-
tert-butyl phenyl) propan-2-one with diketopyracene
(step a) following a described procedure.9 The Diels–
The two wheels were then simultaneously covalently
attached to the axle by a double coupling of 9-ethynyl-
triptycene with 6 under classical Sonogashira conditions
(step f).11 Pd(PPh3)4, CuI and 6 were suspended under
argon in a 1:1 degassed mixture of freshly distilled THF
and piperidine. Two equivalents of 9-ethynyltriptycene,
obtained following a published procedure,5 were added
Alder
reaction
of
2
with
di-(4-tert-butyl
phenyl)acetylene (step b) provided, after CO extrusion
and aromatization, ethane-bridged 3 with a 97% yield.
The 1H NMR spectrum of 3 clearly showed the 2:1
Scheme 1. Reagents and conditions: (a) EtOH, 20 h, Ar, 20°C, 90%; (b) di(4-tert-butylphenyl)acetylene, diphenylether, 16 h, Ar,
reflux, 97%; (c) (C6H5SeO)2O, chlorobenzene, 62 h, Ar, reflux, 60%; (d) 1,3-bis(4-iodophenyl)propan-2-one, KOH, EtOH, Ar,
reflux, 100%; (e) di(4-tolyl)acetylene, diphenylether, 16 h, Ar, reflux, 30%; (f) 9-ethynyltriptycene, Pd(PPh3)4 10 mol%, CuI 20
mol%, piperidine–THF (1:1), 24h, Ar, 20°C, 46%.

G. Jimenez-Bueno, G. Rapenne / Tetrahedron Letters 44 (2003) 6261–6263
6263
with a syringe under argon and the mixture was kept at
20°C for 24 hours. The double coupling afforded 112 in
46% yield after column chromatography (SiO2, eluent:
cyclohexane/0–20% CH2Cl2, Rf=0.31) as an orange
solid.
5. Joachim, C.; Tang, H.; Moresco, F.; Rapenne, G.;
Meyer, G. Nanotechnology 2002, 13, 330–335.
6. Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. Rev.
1999, 99, 1747–1786.
7. Moresco, F.; Meyer, G.; Rieder, K. H.; Tang, H.;
Gourdon, A.; Joachim, C. Phys. Rev. Lett. 2001, 86,
672–675.
The synthesis of a polyaromatic hydrocarbon designed
by analogy with a wheelbarrow has been achieved.
Studies are currently in progress to image and manipu-
late 1 with an STM tip. We are hoping to reproduce the
mechanical behavior of a wheelbarrow at the molecular
level, i.e. to convert the translation movement of the tip
into the rotation of the wheels.
8. Accelrys, Cerius 2 4.6, San Diego, 2001.
9. Gourdon, A. Eur. J. Org. Chem. 1998, 2797–2801.
10. Clayton, M. D.; Marcinow, Z.; Rabideau, P. W. J. Org.
Chem. 1996, 61, 6052–6054.
11. Sonogashira, K. In Metal-catalyzed Cross-coupling Reac-
tions; Diederich, F.; Stang, P. J., Eds; Cross-coupling
reactions to sp carbon atoms; Wiley-VCH: Weinheim,
1997; pp. 203–229.
Acknowledgements
12. All new compounds were fully characterized by 1H
NMR, 13C NMR, UV–vis and MS. Compound 1: Orange
solid umax (CH2Cl2)/nm (o dm3 mol−1 cm−1) 230 (55500),
246 (39250, sh), 276 (17550, sh), 319 (22100), 425 (8040),
This work was supported by the CNRS and the
European Community within the TMR ‘Atomic and
Molecular Manipulation’. We are also grateful to Drs
Andre´ Gourdon, Christophe Coudret and Christian
Joachim for fruitful discussions.
1
450 (9450). H NMR (400 MHz, CD2Cl2): l (ppm), 7.84
(m, 6H, Tript), 7.7 (d, 4H, Jb–a=8Hz), 7.49 (m, 6H,
Tript), 7.42 (d, 4H, Ja–b=8Hz), 7.19 (m, 4H), 7.14 (m,
12H, Tript), 7.08 (m, 4H), 6.9 (d, 4H, J=8 Hz), 6.8 (m,
6H), 6.7 (d, 4H, J=9 Hz), 6.4 (d, 2H, J=8 Hz), 6.18 (d,
2H, J=7Hz), 5.5 (s, 2H, Tript), 2.2 (s, 6H, Me), 1.8 (s,
36H, 3,5-di-tert-butyl-phenyl), 1.1 (s, 18H, 4-tert-butyl-
phenyl); 13C NMR (75 MHz, CD2Cl2) l (ppm), 150.13,
148.28, 148.01, 145.47, 144.69, 144.50, 140.77, 139.32,
138.17, 137.86, 137.30, 136.80, 136.67, 131.97, 131.19,
131.03, 130.44, 127.81, 126.24, 125.96, 125.59, 125.43,
125.24, 125.05, 124.86, 124.55, 123.84, 123.68, 123.45,
122.58, 122.49, 121.48, 120.04, 93.03 (alkynes), 34.81 (4
CMe3), 34.09 (2 CMe3), 31,23 (12 CMe3), 31,05 (6 CMe3),
21.00 (2 CH3); HR–FAB+−MS (m-NBA/m/z) 1801.9500
(M+H+, calculated for C140H121: 1801.9468).
References
1. Anderson, H. Angew. Chem. Int. Ed. 2000, 39, 2451–2453
and references cited.
2. Joachim, C.; Gimzewski, J. K.; Tang, H. Phys. Rev. B
1998, 58, 16407–16417.
3. Rosei, F.; Schlunack, M.; Jiang, P.; Gourdon, A.; Laegs-
gaard, E.; Stensgaard, I.; Joachim, C.; Besenbacher, F.
Science 2002, 296, 328–331.
4. Gimzewski, J. K.; Joachim, C.; Schlitter, R. R.; Langlais,
V.; Tang, H.; Johannsen, I. Science 1998, 281, 531–533.