Molecular switches flipped by oxygen

Article abstract of DOI:10.1002/anie.200700443

(Chemical Equation Presented) Singlet oxygen fuels anthracenes and forces an axial rotation of aryl substituents during the oxidation. The molecular switch can be synthesized in only one step from commercially available starting materials. Thermal cleavag

Full text of DOI:10.1002/anie.200700443

Angewandte
Chemie
DOI: 10.1002/anie.200700443
Molecular Devices
Molecular Switches Flipped by Oxygen**
Daniel Zehm, Werner Fudickar, and Torsten Linker*
Molecular machines are of current interest in biology and
chemistry.[1] Nature provides the archetype—elegant motor
proteins driven by adenosine triphosphate (ATP).[2] Artificial
motors[3] and switches,[4] which represent central devices of
molecular machines, have been synthesized by various
strategies during the past few years, with special emphasis
on the question of energy supply.[5] Many systems are
powered by light, electrons, protons, or heat; more recently,
a chemically driven unidirectional motor has been de-
scribed.[6] However, singlet oxygen has not been used to fuel
synthetic motors or to flip molecular switches until now.
The sensitized photoreaction of oxygen with visible light
generates singlet oxygen (1O2), which conveniently oxidizes
organic compounds under mild conditions.[7] During our
investigations on stereoselective reactions[8] and photolithog-
raphy[9] with this reagent, we became interested in the
photooxygenation of anthracenes. Herein we describe our
results on the construction of a molecular rotary switch,
flipped by singlet oxygen and thermal isomerization. The
stator and rotor are connected in only one step from
commercially available starting materials, the switching
process proceeds with good to high yields, and oxygen is
formed as the only waste product.
high yields, and many repetitive cycles of cycloaddition and
cycloreversion are possible.[10] If ortho substituents on the aryl
ꢀ
ring are present (R ¼ H), the rotation around the C C single
bond should be sterically hindered, affording cis and trans
isomers. At higher temperatures random thermal (Brownian)
isomerizations might take place, but after oxidation the
endoperoxide structure 2 should block the axial rotation
(Scheme 1). Furthermore, the two stations of the switch could
be distinguished by simple optical readout.
Interestingly, only few examples for the preparation of
9,10-bisarylanthracenes 1 from anthraquinone have been
reported.[11] Therefore, we applied the Suzuki coupling[12] to
obtain the 9,10-bisarylanthracenes 1b–f from 9,10-dibro-
moanthracene (3)and the boronic acids 4 in good yields
(Scheme 2). With the sterically more demanding derivatives
Our system is based on the photooxygenation of 9,10-
bisarylanthracenes 1 to give the corresponding 9,10-endoper-
oxides 2 (Scheme 1). For the phenyl-substituted compound 1a
(R = H), this reaction is known to proceed reversibly with
Scheme 2. Synthesis of 1 bySuzuki coupling.
4c and 4d, monocoupling occurred as a side reaction. Each
cis/trans isomer 1b–e could be separated by column chroma-
tography or crystallization; only the fluoro derivative 1 f was
isolated as a mixture. The structures of the products and their
configurations were proven by distinct NMR signals and X-
ray analysis.[13] In summary, we connected the rotor (ortho-
substituted benzene)with the stator (anthracene)of the
newly designed molecular switch in only one step from
commercially available starting materials, which makes the
synthesis much simpler than those of other known systems.
Scheme 1. Concept of the reversible reaction of 9,10-bisarylanthracenes
1 with singlet oxygen (1O2) to give the 9,10-endoperoxides 2 with
ꢀ
rotation around a C C single bond.
[*] Dipl.-Chem. D. Zehm, Dr. W. Fudickar, Prof. Dr. T. Linker
Department of Chemistry
Universityof Potsdam
Karl-Liebknecht-Strasse 24–25, 14476 Potsdam/Golm (Germany)
Fax: (+49)331-977-5056
E-mail: linker@chem.uni-potsdam.de
Homepage: http://www.chem.uni-potsdam.de/ochome
ꢀ
Possible rotations around the C C single bonds were
investigated by heating anthracenes 1 to various temper-
atures. The fluoro derivative 1 f undergoes thermal (Brown-
ian) cis/trans isomerization in solution at 708C within 100 min,
which was easily followed by 19F NMR spectroscopy.[13]
However, all other anthracenes 1b–e are configurationally
[**] This work was generouslysupported bythe DFG.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7689

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stable up to 2508C. This observation can be rationalized by
indicate a radical pathway.[14] This would explain the observed
rotation resulting from steric (R = Me, Et, iPr, 1b–d)and
electrostatic (R = OMe, F, 1e,f)repulsions between the
incoming oxygen and the substituents R in the reactive
radical intermediate 5 (Scheme 4). The activation energies on
the steric demand of the substituents R and unfavorable
interactions with the adjacent hydrogen atom of the anthra-
cene system. This is in good accordance with the rotational
barrier of 104 kJmolꢀ1 determined for the methyl derivative
1b by gas chromatography.[11c] Finally, we forced a thermal
axial rotation of the methyl- and methoxy-substituted anthra-
cenes 1b and 1e neat at 3208C without decomposition. In all
cases, the thermodynamically more stable trans isomers are
formed in excess; only the sterically most hindered deriva-
tives 1c and 1d cannot rotate at all. Thus, we set up the
prerequisites for a molecular switch with Brownian 1808
rotations of an aryl rotor around an anthracene stator for
distinct temperatures and substituents R.
The crucial experiments were the reactions of the
1
anthracenes 1 with O2, which was conveniently generated
Scheme 4. Radical intermediates 5 and rotation to endoperoxides 2.
at ꢀ208C from molecular oxygen by irradiation with two
sodium lamps in the presence of catalytic amounts of
methylene blue (MB)as the sensitizer. We oxidized the cis
and trans isomers 1 independently to investigate possible
rotations (Scheme 3). The photooxygenations proceeded
smoothly to give the 9,10-endoperoxides 2 by [4+2] cyclo-
the pathway to such radicals exceed 120 kJmolꢀ1, [14] energies
higher than the rotation barriers of anthracenes 1. Thus,
singlet oxygen functions not only as the fuel for the 1808
rotation but also as the brake for the molecular switch,[15]
since the products 2 cannot rotate back to the trans isomers.
To exclude a rotation by a photochemical process in the
excited state of the anthracenes, 1O2 was generated from
sodium molybdate and hydrogen peroxide in the dark. Again,
the endoperoxides 2 are formed exclusively as cis isomers, and
thus the hindered axial rotation is powered by a chemical
oxidation and not by light. Therefore, the anthracenes 1 are
1
fueled by O2, and our system is well adapted for a simple
molecular switch flipped by oxygen.
The next station of the rotor was easily attained by
thermolysis of the endoperoxides 2 at 1108C (Figure 1). This
reaction afforded the anthracenes 1 in quantitative yield, and
the products were isolated in analytically pure form without
further purification.[13] We distinguished the two stations of
Scheme 3. Photooxygenations of 1.
addition; only the sterically more demanding anthracenes 1c
and 1d afforded oxidation by-products.[13] Additionally, the
reactions could be followed easily by optical readout, since
the products are nonfluorescent in contrast to the anthracenes
1. Finally, the endoperoxides 2b, 2e, and 2 f were isolated in
good to high yields and in analytically pure form by column
chromatography.[13]
Interestingly, the peroxides 2 are formed exclusively as cis
isomers, irrespective of the cis or trans configuration of the
starting materials 1, which is confirmed by NMR studies and
X-ray analysis.[13] Thus, the trans isomers undergo a selective
ꢀ
1808 rotation around the C C single bonds during oxidation
at ꢀ208C, whereas anthracenes 1b–e are thermally stable up
to 2508C. The mechanism of the photooxygenation of acenes
is still a matter of debate, but very recent theoretical studies
Figure 1. a) Thermolysis of the 9,10-endoperoxides 2 and b) UV/Vis
spectra of 2 f and 1 f in solution (CHCl3) along with photographs
(insets).
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Angewandte
Chemie
the rotor easily by optical readout, owing to the characteristic
fluorescence of the anthracenes 1 in contrast to the non-
fluorescent endoperoxides 2 (Figure 1). Furthermore, molec-
ular oxygen is the only waste product, and thus the
thermolysis represents the back reaction of the photooxyge-
nation.
sole waste product, making our molecular switch environ-
mentally friendly.
Received: January 31, 2007
Revised: May 22, 2007
Published online: August 23, 2007
To complete the switching process and to set up the final
molecular device for repetitive cycles, we chose the methoxy
derivative trans-1e, since of the relevant trans isomers it
provides the best yield in the reaction with 1O2. Indeed,
heating anthracene cis-1e to 3208C forces the rotation around
Keywords: acenes · axial rotation · molecular switches ·
oxidation · peroxides
.
ꢀ
the C C single bond resulting in the thermodynamically more
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Scheme 5. Three stations of the molecular switch 1e with stator (blue)
and rotor (green).
1
cene trans-1e (station A), reaction with O2 in the key step
yields the endoperoxide 2e (station B). Cycloreversion by
thermolysis affords selectively the cis isomer 1e in quantita-
tive yield (station C). During the first two steps, the rotor
(green)undergoes a selective 180 8 rotation around the stator
(blue), with oxygen as the only waste product. Finally, heating
to 3208C returns the molecular switch to its initial state with
high yield and high trans selectivity (station A), and thus
repetitive switching is possible.
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can be flipped by molecular oxygen in its singlet state and by
thermal energy. The stator and rotor are easily connected in
only one step from commercially available starting materials.
High energetic barriers for the rotations of the aryl substitu-
ꢀ
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found, which were overcome during the oxidation with singlet
oxygen. The rotation of the switch proceeds with good yields,
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Angew. Chem. Int. Ed. 2007, 46, 7689 –7692
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7691

Communications
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Angew. Chem. Int. Ed. 2007, 46, 7689 –7692