Angewandte
Chemie
Helical Structures
Four- and Sixfold Tandem-Domino Reactions Leading to Dimeric
Tetrasubstituted Alkenes Suitable as Molecular Switches**
Lutz F. Tietze,* Bernd Waldecker, Dhandapani Ganapathy, Christoph Eichhorst,
Thomas Lenzer, Kawon Oum, Sven O. Reichmann, and Dietmar Stalke
Abstract: A highly efficient palladium-catalyzed fourfold
tandem-domino reaction consisting of two carbopalladation
synthesis of functional materials like fluorescent dyes and
sterically overcrowded alkenes.[6]
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and two C H-activation steps was developed for the synthesis
The latter type of compounds was first prepared by
Feringa and Wynberg[7] and the use of domino reactions for
their synthesis was also described by Lautens et al.[8] and Zhu
et al.[9] Sterically overcrowded alkenes have intriguing prop-
erties, since they can act as light-driven molecular switches
and motors making them suitable for their application as
nanodevices in material sciences and biomolecular chemistry
as well.
of two types of tetrasubstituted alkenes 3 and 6 with intrinsic
helical chirality starting from substrates 1 and 4, respectively. A
sixfold tandem-domino reaction was also developed by
including a Sonogashira reaction. 20 compounds with different
substitution patterns were prepared with yields of up to 97%.
Structure elucidation by X-ray crystallography confirmed
helical chirality of the two alkene moieties. Photophysical
investigations of some of the compounds showed pronounced
switching properties through light-controlled changes of their
stereochemical configuration.
For the synthesis of the novel unusual dimeric tetrasub-
stituted alkenes 3a and 6a with inherent helical chirality we
developed a palladium-catalyzed fourfold tandem-domino
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reaction consisting of two carbopalladation and two C H-
P
seudosymmetric dimeric compounds, which are also found
activation reactions starting from substrates 1a and 4a,
respectively (Scheme 1). Our goal in this field was the
design of novel molecules containing two overcrowded
alkene moieties, for which the light-driven switching process
should be either autonomous or should allow a propeller-like
movement where the helicity of the two alkene moieties
depends on each other. The concept might also be of interest
in view of natural nanomotors.[10]
in nature, are interesting substances that very often show
dramatic differences in their bioactivity and other properties
compared to the corresponding monomeric unit.[1] Their
synthesis is usually performed by dimerization of the mono-
mers.[2] On the other hand, a tandem approach in which both
parts of the molecule are built up in parallel seems to be much
more elegant and efficient. However, only very few examples
utilizing this synthetic strategy have been reported to date.[3]
Herein we present a straightforward combination of
a tandem approach with our concept of domino reactions[4,5]
for the synthesis of dimeric tetrasubstituted alkenes such as 3
and 6 using palladium as catalyst. Transition-metal-catalyzed
domino reactions have recently been used by us in the
The first type of molecules would be represented by
structure 3a whereas the second type would be met by
structure 6a. We assume that in the formation of 3a and 6a
intermediates such as 2a and 5a occur, which are formed by
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two carbopalladation reactions. They then undergo two C H-
activation reactions, probably via transition structures 2a°
and 5a°, with two molecules of acetate.[11] However, exact
information about the time sequence cannot be given, though
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the terminal C H-activation reactions should be slower due
to a higher energy of activation.
[*] Prof. Dr. L. F. Tietze, M. Sc. B. Waldecker, Dr. D. Ganapathy,
Dr. C. Eichhorst
For the synthesis of 3a a combination of Pd(OAc)2 and
PPh3 in a ratio of 1:5 with (nBu)4NOAc as base and the
solvent DMF as catalytic system and a catalyst loading of
10 mol% at 1008C under microwave irradiation for 6 h gave
the highest yields. Thus, under these conditions 3a, containing
two overcrowded alkene moieties in an anti orientation, was
obtained from the dialkyne 1a with 94% yield. Lower catalyst
loading with 5 mol% and 1 mol% palladium led to decreased
yields as 68% and 2%, respectively. Similarly, for the
synthesis of the second type of helical bisalkenes 6a with
a syn orientation of the two overcrowded alkene moieties the
catalytic system described for 3a could also be used; however,
the reaction time had to be increased to 12 h, which is
probably due to much higher strain in molecules of type 6a.
Thus, the process employing dialkyne 4a again consists of two
Institute of Organic and Biomolecular Chemistry
Georg-August University of Gçttingen
Tammannstrasse 2, 37077 Gçttingen (Germany)
E-mail: ltietze@gwdg.de
Prof. Dr. T. Lenzer, Priv.-Doz. Dr. K. Oum
Physical Chemistry 2, University of Siegen
Adolf-Reichwein-Strasse 2, 57076 Siegen (Germany)
M. Sc. S. O. Reichmann, Prof. Dr. D. Stalke
Institute of Inorganic Chemistry
Georg-August University of Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
[**] We thank the Bundesland Niedersachsen, the Volkswagen Foun-
dation, and the Deutsche Forschungsgemeinschaft for their
generous support. In addition, our gratitude goes to the staff of the
mechanical workshop at the Department Chemistry–Biology, Uni-
versity of Siegen, for excellent technical assistance.
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carbopalladation and two C H-activation reactions with
10 mol% Pd(OAc)2 and PPh3 in a ratio of 1:5, (nBu)4NOAc
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 10317 –10321
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10317