Efficient synthesis of helical tetrasubstituted alkenes as potential molecular switches: A two-component palladium-catalyzed triple domino process

Article abstract of DOI:10.1002/anie.201209510

Fast and easy: Various helical tetrasubstituted alkenes were synthesized by a palladium-catalyzed domino process. The domino process consists of a Sonogashira reaction, a carbopalladation, and a direct C-H functionalization. Copyright

Full text of DOI:10.1002/anie.201209510

Angewandte
Chemie
DOI: 10.1002/anie.201209510
Domino Reaction
Efficient Synthesis of Helical Tetrasubstituted Alkenes as Potential
Molecular Switches: A Two-Component Palladium-Catalyzed Triple
Domino Process**
Lutz F. Tietze,* Tim Hungerland, Christoph Eichhorst, Alexander Dꢀfert, Christian Maaß, and
Dietmar Stalke
Dedicated to Professor Manfred Reetz on the occasion of his 70th birthday
The development of highly efficient ecologically and eco-
nomically advantageous procedures for the synthesis of
natural products and materials is a very important topic in
organic chemistry. For this purpose we have utilized the
domino concept, which allows the preparation of complex
molecules starting from simple substrates in only a few
steps.[1] The quality of a domino process can be correlated
with the increase of complexity per step. Our research group
and many others have used domino processes in the synthesis
of numerous different compounds;[2] currently, the focus is on
the use of catalytic reactions in these domino processes,
light-absorption properties.[9] A general and fast route to
these compounds that allows diverse modifications is neces-
sary for optimization of their properties; in this respect,
domino reactions would be very suitable methods.
The first syntheses of tetrasubstituted alkenes by
À
a domino process involving a C H activation were described
by the groups of Zhu[10] and Lautens.[11] We have prepared
diastereo- and enantiopure tetrasubstituted alkenes (1) by
a domino process consisting of a combination of a carbopalla-
dation with a Heck[12] or a Stille reaction;[13] substrates of type
2a were used for the domino process involving a Stille
À
especially
transition-metal-catalyzed
transformations,
reaction. On the other hand, a carbopalladation/C H activa-
because classical cross-coupling reactions tolerate a variety
of functional groups and reaction conditions.[3] However, one
major drawback is the prerequisite for functionalization of
one or both coupling partners. This issue could be avoided by
tion domino process of substrates of type 2b did not lead to
the desired compounds but to acenaphthylenes 4 in excellent
yield.[5b] This result can be explained by a proximity effect,
that is, the hydrogen atom on the naphthalene moiety in 2b is
closer to the intermediately formed vinyl palladium moiety
than the two ortho-hydrogen atoms on the phenyl ether
group. By using substrates of type 2c, in which the naphtha-
lene moiety has been replaced by a phenyl ring, a carbopalla-
À
using direct C H functionalization of readily available
substrates.[4] Thus, the design of novel domino reactions
involving palladium-catalyzed transformations of unfunction-
alized aromatic scaffolds is a promising approach.[5,14]
Herein, we describe a novel two-component Pd-catalyzed
À
dation/C H activation domino process could be performed to
give 3 in up to 94% yield (Scheme 1).[14] But the obtained
alkenes are not suitable as potent molecular switches as their
absorption spectrum does not cover a broad range of the
wavelengths and they do not show bistability, owing to the
lack of the naphthalene moiety.
À
triple domino process, which includes a direct C H function-
alization as the last step, for the synthesis of helical
tetrasubstituted alkenes. Helical alkenes, first described by
Feringa and Wynberg,[6] represent a new class of compounds
that has attracted great interest in the past decade owing to
their high potential to act as analogues of mechanical
machines and motors, as well as electronic devices, on the
molecular level.[7] The reported structures are also of interest
in the design of ligands for transition metals.[8] With respect to
further applications as nanodevices the naphthalene part
seems to be a key feature owing to its steric demand and its
We therefore investigated the use of substrates of type 5,
for which the undesired C H activation cannot take place.
À
[*] Prof. Dr. L. F. Tietze, T. Hungerland, C. Eichhorst, Dr. A. Dꢀfert
Institute of Organic and Biomolecular Chemistry
University of Gçttingen
Tammannstrasse 2, 37077 Gçttingen (Germany)
E-mail: ltietze@gwdg.de
C. Maaß, Prof. Dr. D. Stalke
Institute of Inorganic Chemistry, University of Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
[**] We would like to thank the State of Lower Saxony, the VW
Foundation, the DFG and the Fonds of the Chemical Industry for
generous support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209510.
Scheme 1. Domino processes for the synthesis of 1, 3, and 4 from
substrates 2.
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However, treatment of 5 with Pd(OAc)2 in the presence of
PtBu3·HBF4 led to the ketone 6; the vinyl palladium
intermediate underwent a ß-hydride elimination and not the
the alkene 11b in 67% yield, and substrates 7a and 8d led to
alkene 11j in 77% yield (Scheme 3).
To determine the scope and limitations of this reaction we
used various aryliodides 7 bearing either a phenoxy (7a), a 2-
nitrophenoxy (7b), or a 4-nitrophenoxy (7c) moiety to
examine the influence of electronic and steric properties on
the process (Scheme 4). Moreover, we also changed the
substituents on the alkyne moiety (8a–d) and we even
introduced a heterocyclic component (8e) in this part. In all
cases we got good to excellent yields with up to 96%, as found
for 11k and 11l. In most of the examples the reaction
mixtures were heated to 1208C in a microwave reactor but, in
a few cases, that is for the synthesis of 11a, 11d, and 11j, the
reaction mixtures were heated at 1008C in an oil bath.
Although it was our intention to keep the catalytic system as
simple as possible, we also investigated the influence of other
ligands; only in the reaction of aryliodide 7b and alkyne 8a to
give 11b was a slightly better yield achieved when 1,1’-
bis(diphenylphosphanyl)ferrocene (dppf) was used as the
ligand instead of PPh3.
Products 11d–o were obtained as mixtures of E and
Z isomers, owing to fast isomerization of the double bond.[18]
In most of the cases, the Z configuration, where the naph-
thalene moiety and the substituent on the lower part are
opposite each other, is slightly preferred. Furthermore, the
influence of the substituents on the lower and upper part of
the substrates is not highly pronounced, although the use of
substrates with electron-withdrawing groups, such as NO2 and
CN, seems to improve the yields, whereas the position of the
À
desired C H activation (Scheme 2).
Scheme 2. Palladium-catalyzed transformation of 5 to give ketone 6.
The problem could be solved by using alkynes of type 9 as
À
substrates for the carbopalladation/C H activation domino
process, where a b-hydride elimination of the proposed vinyl
palladium intermediate 10 could not take place. Thus, the
desired helical alkene 11b could be obtained in 31% yield
from alkyne 9a in the presence of Pd(OAc)2, PPh3, and
K2CO3. However, initial attempts to generate substrate 9a by
a Sonogashira reaction of the aryliodide 7b and the alkyne 8a
under standard reaction conditions ([PdCl2(PPh3)2]/CuI/
NEt3) led exclusively to the homocoupling product (Table 1,
entry 1). Finally, by using a different base/solvent system we
were able to obtain the desired product 9a in good yield
(Table 1, entry 2). But, when these reaction conditions were
applied to substrates 7a and 8d the alkyne 9b was formed in
only 42% yield; when standard reaction conditions were used
the yield was 28% (Table 1, entries 3 and 4). Fortunately, by
applying copper-free conditions including a catalytic system
consisting of Pd(OAc)2/PPh3 in the presence of nBu4NOAc
the alkyne 9b was formed in 88% yield (Table 1, entry 5).
As the yield of the domino process of 9a was not
satisfactory and to improve the efficiency of the whole
À
reaction sequence we combined the carbopalladation/C H
activation domino process with the Sonogashira reaction to
give a two-component triple domino process.[15–17] Pleasingly,
reaction conditions similar to those employed for the
Sonogashira reaction of 7a and 8d could be used for the
domino transformations; the only changes required were
a higher temperature and the use of DMF as the solvent.
Under these reaction conditions a mixture of 7b and 8a gave
Table 1: Optimization of the coupling of aryl iodides 7 and alkynes 8 by
a Sonogashira reaction.
Entry Substrates Reaction conditions
Result[a]
1
2
3
7b+8a
7b+8a
7a+8a
[PdCl2(PPh3)2], CuI, NEt3, RT, 18 h homocoupling
product
[PdCl2(PPh3)2], CuI, nBu4NOAc,
1,4-dioxane, 608C, 35 min
9a (86%)
[PdCl2(PPh3)2], CuI, NEt3, RT, 17 h 9b (28%) +
homocoupling
product (46%)
4
5
7a+8d
7a+8d
[PdCl2(PPh3)2], CuI, nBu4NOAc,
1,4-dioxane, 608C, 19 h
9b (42%) +
homocoupling
product
Scheme 3. Sonogashira reaction of aryl iodide 7b and alkyne 8a to
À
give 9a followed by a carbopalladation/C H activation domino reac-
Pd(OAc)2, PPh3, nBu4NOAc,
1,4-dioxane, 608C, 2.5 h
9b (88%)
tion (right). Triple domino process for the synthesis of helical alkenes
11 from aryl iodides 7 and alkynes 8 (left). DMF=N,N’-dimethylform-
amide.
[a] Yield of the isolated product is given in parentheses.
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Figure 1. Crystal structures of alkenes 11b (top) and 11j (bottom). The
hydrogen atoms are omitted for clarity and the anisotropic displace-
ment parameters are depicted at the 50% probability level. Com-
pounds 11b (M/P) and 11j (M/P,Z) crystallize as racemic mixtures in
the solid state. Torsion angle for 11b (C6-C7-C14-C30): 7.4(3)8. Torsion
angle for 11j (C9-C8-C19-C31): 6.9(3)8.
prepared by a copper-catalyzed Chan–Lam-type coupling[21]
of 2-iodo-1-naphthol and phenylboronic acid, whereas sub-
strates 7b and 7c were synthesized from 2-iodo-1-naphthol
and fluoronitrobenzene by a known aromatic substitution
protocol to yield the corresponding aryliodides 7 in just one
step.[22] The aromatic alkynes 8a–e were obtained using
procedures developed by us.[6b]
In conclusion we have developed a highly efficient and
short route to various helical tetrasubstituted alkenes through
a Pd-catalyzed domino process consisting of a Sonogashira
Scheme 4. Synthesis of alkenes 11a–o by a Pd-catalyzed domino
reaction of aryl iodides 7a–c and alkynes 8a–e. Reaction conditions:
Aryl iodide (1.2 equiv), alkyne (1.0 equiv), Pd(OAc)2 (20 mol%), PPh3
(1.0 equiv), nBu4NOAc (3.0 equiv), dimethylformamide (1.0 mL,
0.03 mmol), microwave irradiation, 1208C, 4 h. Yields given are of
isolated products. [a] Reaction conducted at 1008C in an oil bath.
[b] The ligand dppf (1,1’-bis(diphenylphosphino)ferrocene) was used.
À
reaction, a carbopalladation, and a direct C H functionaliza-
tion. Photochemical investigations of the synthesized alkenes
are currently underway.
NO2 group appears to be less important. Due to their
instability a separation of the E/Z isomers was not possible;
thus, by performing an EXSY-NMR experiment on com-
pound 11e the exchange rate between both isomers was
measured to be k = 0.06 sÀ1. With this data, the activation
barrier of the isomerization could be calculated to EA =
79.9 kJmolÀ1.
Figure 1 shows the crystal structures of alkenes (M)-11b
and (P,Z)-11j. Their folded conformation and helical shape
around the central double bond is confirmed by single crystal
X-ray analysis.[19,20]
Received: November 27, 2012
Published online: && &&, &&&&
À
Keywords: C H activation · domino reactions ·
molecular switches · Sonogashira reaction ·
tetrasubstituted alkenes
.
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Chemie
Communications
Domino Reaction
L. F. Tietze,* T. Hungerland, C. Eichhorst,
A. Dꢁfert, C. Maaß,
D. Stalke
&&&& — &&&&
Efficient Synthesis of Helical
Tetrasubstituted Alkenes as Potential
Molecular Switches: A Two-Component
Palladium-Catalyzed Triple Domino
Process
Fast and easy: Various helical tetrasub-
stituted alkenes were synthesized by
a palladium-catalyzed domino process.
The domino process consists of a Sono-
gashira reaction, a carbopalladation, and
À
a direct C H functionalization.
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