Synthesis and photochemical investigations of tetrasubstituted alkenes as molecular switches-the effect of substituents

Article abstract of DOI:10.1002/chem.201003559

Molecular switches based on helical tetrasubstituted alkenes, substituted with either electron-withdrawing (CF₃, F, CN; 2 a-c, 3 a,c) or -donating substituents (Me, OMe; 2 d,e), have been synthesized from acyclic precursors 4 and 5 in a domino carbopalladation/Stille reaction. This palladium-catalyzed process allowed the rapid assembly of two C-C bonds, two six-membered rings, and the tetrasubstituted double bond in a completely diastereoselective fashion. The electronic effects of the substituents on the overall switching process were investigated by alternating irradiation of two different wavelength regions. Although the substituents had only a small influence on the absorption maxima, drastic differences in the switching behavior were observed. Copyright

Full text of DOI:10.1002/chem.201003559

DOI: 10.1002/chem.201003559
Synthesis and Photochemical Investigations of Tetrasubstituted Alkenes as
Molecular Switches—The Effect of Substituents
Lutz F. Tietze,*[a] M. Alexander Dꢀfert,[a] Tim Hungerland,[a] Kawon Oum,[b] and
Thomas Lenzer[b]
Dedicated to Professor Gerhard Bringmann on the occasion of his 60th birthday
ꢀ
Abstract: Molecular switches based on
helical tetrasubstituted alkenes, substi-
tuted with either electron-withdrawing
(CF3, F, CN; 2a–c, 3a,c) or -donating
substituents (Me, OMe; 2d,e), have
been synthesized from acyclic precur-
sors 4 and 5 in a domino carbopallada-
tion/Stille reaction. This palladium-cat-
alyzed process allowed the rapid as-
sembly of two C C bonds, two six-
membered rings, and the tetrasubstitut-
ed double bond in a completely diaste-
reoselective fashion. The electronic ef-
fects of the substituents on the overall
switching process were investigated by
alternating irradiation of two different
wavelength regions. Although the sub-
stituents had only a small influence on
the absorption maxima, drastic differ-
ences in the switching behavior were
observed.
Keywords: alkenes · domino reac-
tions · molecular switches · substitu-
ent effects · UV/Vis spectroscopy
Introduction
eculesꢀ functionality and potential applications.[5] To attain
this goal, both a short and modular assembly of these com-
pounds is needed. One way to ensure this is by making use
of domino reactions in the overall synthetic process.[6] This
would not only facilitate a rapid increase in molecular com-
plexity but would also allow the reduction of time, reagents,
and waste.
Within the last 30 years the field of materials science in
chemistry has progressed steadily. In particular, molecular
machines, logic elements, and switches are a burgeoning
field of research.[1] Molecular switches can be used for data
storage, which is particularly relevant when considering the
aspect of miniaturization.[1c–f] With the bistability of two dis-
tinct states being the prerequisite for a molecular switch, dif-
ferent external stimuli, such as light, electron transport,
complexation and chemical reactions, can be used to inter-
convert between the two states.[2] Thus, the success of digital
optical data systems, in which the recording and read-out of
information is carried out by using light, has led to a shift
from classic electronic semiconductor elements to light-
driven molecular switches in academia. Although rotaxanes
and certain transition-metal complexes featuring ambiden-
tate ligands may be triggered electrochemically by oxidation
or reduction, light is the preferred trigger for inducing rapid
switching between states.[3,4] One of the key aspects of these
molecular devices is the possibility of controlling the molec-
ular motion at the smallest possible level. This allows for a
subtle and distinct modification and governing of these mol-
The chiroptical switches developed by Feringa and co-
workers (e.g., 1) containing a helical tetrasubstituted alkene
moiety are particularly promising because their stable states
can be accessed by using circularly polarized light.[7] More-
over, due to an additional thermal isomerization following
photoinduced P/M isomerization, unidirectional motors
have been effectively realized.[8] Herein we describe our ap-
proach to the synthesis of tetrasubstituted alkenes 2 and 3
containing electron-donating and -accepting substituents.
Based on our previous successful approach[9] to the synthesis
of unsubstituted aromatic helical alkenes (2 f, 3 f, R=H),
the substituted switches 2 and 3 (R=CF3, F, CN, Me, OMe)
can be retrosynthetically traced back to the acyclic precur-
sors 4 and 5, which lead to the alkynes 8 and either oxygen-
or methylene-bridged aldehydes 6 or 7, respectively. The
key step of the synthetic route is the diastereoselective
domino carbopalladation/Stille reaction of 4 and 5, respec-
tively, in which the two six-membered rings and the tetra-
substituted double bond are formed in one process
(Scheme 1).
[a] Prof. L. F. Tietze, Dr. M. A. Dꢁfert, T. Hungerland
Institut fꢁr Organische und Biomolekulare Chemie
Georg-August University Gçttingen
Tammannstr. 2, 37077 Gçttingen (Germany)
Fax : (+49)551-39-9476
Acyclic tetrasubstituted alkenes have also recently been
prepared stereoselectively by transition-metal-mediated re-
actions in the research groups of Lautens, Florent, Shi, Yu,
Murakami, Tsuji, and Nakamura.[10]
E-mail: ltietze@gwdg.de
[b] Dr. K. Oum, Prof. T. Lenzer
Physikalische Chemie, Universitꢂt Siegen
Adolf-Reichwein-Str. 2
In addition to the investigation of the effect of substitu-
ents (R=CF3, F, CN, Me, OMe) on the domino reaction,
the shape of the absorption spectra of 2 and 3 and their
57076 Siegen (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003559.
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Chem. Eur. J. 2011, 17, 8452 – 8461

FULL PAPER
ene-bridged analogue 7 was accessed from commercially
available substituted naphthalene 12. Following the quanti-
tative substitution reaction with freshly prepared allylmag-
nesium bromide the terminal double bond in 13 was oxida-
tively cleaved by using potassium osmate in the presence of
2,6-lutidine to give 7 in a yield of 80%.
The substituted alkynes 8a–e were synthesized according
to the general reaction presented in Scheme 3. Depending
on whether direct coupling of the different phenols 15 was
Scheme 1. Chiroptic molecular switches 1–3 and the retrosynthetic ap-
proach to their synthesis.
switching behavior under irradiation with light at two differ-
ent wavelengths have also been examined.
Scheme 3. General scheme for the synthesis of alkynes 8.
Results and Discussion
Synthesis of substituted alkenes 2a–e and 3a,c: For the syn-
thesis of the substituted molecular switches 2 and 3, we first
synthesized the oxygen- or methylene-bridged aldehydes 6
and 7, respectively, and the donor- and acceptor-substituted
alkynes 8a–e. The two fragments underwent coupling fol-
lowed by a domino carbopalladation/Stille reaction. Alde-
hyde 6 was synthesized in three steps starting from 2-naph-
thol. The selective iodination was performed following a lit-
possible either the direct reaction of 15 with 2-iodophenol
or a three-step procedure starting from 14 and 2-fluoronitro-
benzene was used. After the formation of 16, the alkynes 8
were obtained by Sonogashira reaction with TMS-acetylene,
stannylation, and subsequent removal of the TMS group.
For the fluorine-substituted compounds, the three-step route
to 16b was chosen to avoid issues of regioselectivity in the
primary coupling step.
The coupling of the fluoro- (14b), methyl- (14d), and
methoxy-substituted (14e) phenols with 2-fluoronitroben-
zene proceeded smoothly to give 17b,d,e in almost quantita-
tive yields. For the reduction of the nitro group in 17 zinc in
strongly acidic media was used to afford the aromatic
amines 18b,d,e in yields of up to 96%. Under these condi-
tions debromination of the substrates and products could be
avoided. However, 18b still contained small amounts
erature procedure[11] to give 10 in
a
yield of 91%
(Scheme 2). After the reaction with 2-bromoethanol and
NaH to afford 11 in a yield of 74%, the appended hydroxy
group was oxidized by Parikh–Doering oxidation to afford
the corresponding aldehyde 6 in a yield of 51%. The use of
alternative oxidation reagents, for example, Dess–Martin pe-
riodinane or TPAP, did not lead to the desired product
owing to the sensitivity of the iodide moiety. The methyl-
AHCTUNGTREG(NNNU <5%) of the starting material 17b, which could not be sep-
arated chromatographically. Therefore the mixture was used
directly in the next step and gave the desired bromo-iodo-
substituted ether 16b in good yield (82%). The methyl-
(18d) and methoxy-substituted (18e) amines were allowed
to react under the modified Sandmeyer conditions devel-
oped by Krasnokutskaya et al.[12] and gave the desired di-
phenyl ethers in a yield of 77% (16d) and quantitatively
(16e), respectively. By employing the same coupling condi-
tions (K2CO3, DMSO, 958C, 20–22 h) for the fluorobenzenes
15a and 15c bearing CF3 and CN groups, respectively, the
substituted ethers 16a and 16c were accessed in excellent
yields (95% in both cases). Except for the fluorine-substitut-
ed derivative 16b, which is substituted at C-5, all other com-
pounds bear the donor/acceptor substituent at the para posi-
tion with respect to the ether bridge (Scheme 4).
Scheme 2. Synthesis of the aldehydes 6 and 7.
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L. F. Tietze et al.
Table 1. Formation of alkynes 7a–e.[a–c]
R (sub-
strate)
Yield Sonogashira reac-
Yield stanny-
lation [%]
Yield TMS
cleavage [%]
tion [%] (T [8C])[d]
1
4-CF3
96 (RT)
89
91
(16a)
2
3
5-F (16b) 83 (RT)
62
30
97
quant.
4-CN
(16c)
4-Me
83 (RT)
4
5
88 (55)
49
46
97
95
(16d)
4-OMe
(16e)
94 (70)[e]
[a] Sonogashira coupling conditions: Substrate (1.0 equiv), TMS-acety-
lene (1.1 equiv), [PdCl2A(PPh3)2] (0.02 equiv), CuI (0.04 equiv). [b] Stanny-
lation conditions: Substrate (1.0 equiv), tBuLi (2.5 equiv), Bu3SnCl
(2.0 equiv). [c] TMS cleavage conditions: Substrate (1.0 equiv), K2CO3
(0.5 equiv). [d] Reaction temperature of the Sonogashira cross-coupling
reaction. [e] NEt3/DMF (1:1) was used as the solvent system.
Scheme 4. Synthesis of diphenyl ethers 16a–e and the substitution pat-
terns for each system.
CTHUNGTRENNUNG
Following the synthesis of the aromatic ethers 16a–e, the
alkynes 8a–e were obtained in three further steps. In the So-
nogashira cross-coupling reaction, the acceptor-substituted
arenes (16a–c) reacted smoothly at room temperature to
give 19a–c in high yields (83–96%). In contrast, the donor-
substituted iodoarenes 16d and 16e reacted only sluggishly
and elevated temperatures of 55–708C were required to
bring the reaction to completion, again with good yields of
88 and 94%, respectively. Apparently the inductive effect of
the R group is sufficient to significantly influence the elec-
tronic properties of the right-hand aromatic ring, which re-
sults in a much reduced reactivity of the donor-substituted
substrates. In the next step, the stannane residue was intro-
duced by metal halide exchange with tBuLi[13] and subse-
quent reaction of the in situ formed organolithium com-
pound with Bu3SnCl. The stannanes 20a–e were obtained in
varying yields of 30–89%. The stannylation proved to be the
critical step of the reaction sequence due to partial cleavage
of the ether bridge. However, in the case of the cyano-sub-
stituted substrate 19c, nucleophilic addition of tBuLi on the
nitrile group led to a further reduction of the yield and gave
the desired stannane 20c in only 30%. The final TMS cleav-
age was affected by using methanol/CH2Cl2 in the presence
of 50 mol% K2CO3. The desilylation was accomplished to
give the desired alkynes 8a–e in excellent yields of at least
91% in all cases (Table 1).
tained in average yields of 60–70%. In addition to the
oxygen-bridged alcohols bearing an iodide, the correspond-
ing bromides 22a–e were also synthesized to investigate the
effect of the halide on the ensuing domino carbopalladation/
Stille reaction (Table 2).[14]
With these compounds in hand, the domino reaction was
studied. The desired alkenes 2a–e and 3a,c were obtained
by using either the conditions for Stille reactions of Grasa
and Nolan[15] (Pd
ACTHNUTRGNEN(UG OAc)2, IPr·HCl (23), TBAF, dioxane,
1008C, 38 h) or Fu and co-workers[16] ([Pd2dba3],
PtBu3·HBF4, CsF, dioxane, 808C, 18 h). Neither the condi-
tions recently developed by Naber and Buchwald using
XPhos (24) nor the Herrmann–Beller palladacycle 25 led to
the formation of the desired products in substantial
amounts.[17,18] As expected, the iodo compounds 4a–d gener-
ally gave higher yields than the corresponding bromo com-
pounds 22a–d. Subsequently all domino precursors were
subjected to either the original[9] or the improved reaction
conditions of Grasa and Nolan (Table 3). Apparently, the in-
troduction of an acceptor moiety into 4 leads to an overall
drop in the reactivity in the domino step because only the
conditions introduced by Grasa and Nolan could effect the
conversion to the desired alkenes 2a–c. In contrast, the
donor-substituted substrates 4d,e gave the desired products
2d,e by using either IPr or PtBu3 as the ligand system. How-
ever, the latter methodology was superior with regards to
the yields and purity of the products and henceforth was fa-
vored for electron-rich substrates. The formation of 3b,d,e
from 5b,d,e was not possible using the conditions employed
To prepare the substrates 4a–e and 5a–e for the domino
reaction, the alkynes 8a–e were coupled with the aldehydes
6 and 7. The reaction was performed by deprotonating the
alkynes 8 with LiHMDS as base followed by the very slow
addition of the aldehyde in toluene through a syringe pump
over 15 h. In this way, the propargylic alcohols could be ob-
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Tetrasubstituted Alkenes as Molecular Switches
FULL PAPER
Table 2. Synthesis of propargylic alcohols 4a–e, 5a–e, and 22a–e.[a]
Table 3. Domino reactions of 4a–e, 5a–f, and 22a–d,f.
Entry
R (substrate)
Aldehyde (X,Y)
Yield [%] (product)
1
2
3
4
4-CF3 (8a)
4-CF3 (8a)
4-CF3 (8a)
5-F (8b)
6 (O,I)
7 (CH2,Br)
21 (O,Br)
6 (O,I)
77 (4a)
24 (5a)
66 (22a)
61 (4b)
5
6
7
8
5-F (8b)
5-F (8b)
7 (CH2,Br)
21 (O,Br)
6 (O,I)
7 (CH2,Br)
21 (O,Br)
6 (O,I)
7 (CH2,Br)
21 (O,Br)
6 (O,I)
77 (5b)
64 (22b)
82[b] (4c)
46[b] (5c)
38[b,c] (22c)
65 (4d)
63 (5d)
71 (22d)
56 (4e)
4-CN (8c)
4-CN (8c)
4-CN (8c)
4-Me (8d)
4-Me (8d)
4-Me (8d)
4-OMe (8e)
4-OMe (8e)
Entry
R (substrate)
X,Y
Conditions[a]
Yield [%] (product)
1
2
3
4
5
6
7
8
4-CF3 (4a)
4-CF3 (4a)
4-CF3 (22a)
4-CF3 (22a)
4-CF3 (5a)
5-F (4b)
O,I
O,I
O,Br
O,Br
CH2,Br
O,I
O,Br
CH2,Br
O,I
O,Br
CH2,Br
O,I
A
B
A
B
A
A
B
A
A
A
A
B
A
B
A
A
B
A
B
B
30 (2a)
9
traces (2a)
10
11
12
13
14
10–20 (2a)[b]
ACHTUNGTNER(NUNG 2a)
29 (3a)[c]
33 (2b)[b][d]
7 (CH2,Br)
58 (5e)
5-F (22b)
5-F (5b)
(2b)[e]
ACHTUNGTRENNUNG
[a] Reaction conditions: Alkyne (1.5 equiv), LiHMDS (1.5 equiv), NEt3
(10 equiv), toluene, ꢀ788C!RT, 2 h; then at ꢀ788C: aldehyde
(1.0 equiv) in toluene, 15 h. [b] The entire reaction was conducted at
ꢀ788C. [c] Reaction conditions: Alkyne (1.0 equiv), LiHMDS
(1.0 equiv), aldehyde (1.0 equiv), NEt3 (10.0 equiv).
(3b)
55 (2c)
24 (2c)
9
4-CN (4c)
4-CN (22c)
4-CN (5c)
4-Me (4d)
4-Me (22d)
4-Me (22d)
4-Me (5d)
4-OMe (4e)
4-OMe (4e)
4-OMe (5e)
H (22 f)
10
11
12
13
14
15
16
17
18
19
20
19 (3c)[f]
67 (2d)[c]
<10 (2d)
<10 (2d)[b]
O,Br
O,Br
CH2,Br
O,I
due to the decomposition of the starting materials. The rate-
determining step of the entire sequence seems to be the oxi-
dative addition reaction because neither the compounds
stemming from only a carbopalladation or an intermolecular
Stille reaction between two substrate molecules nor dehalo-
genated derivatives were detected as side-products. This was
further underlined by the fact that when the corresponding
bromine-derived substrates 22a–d or 5a–e were used, either
decidedly lower yields or the decomposition of the sub-
strates was observed, regardless of the conditions employed.
Destannylation was observed in only one case (entry 8). The
reactions of the unsubstituted bromo compounds 5 f and
22 f, which were synthesized in a prior study, are also includ-
ed.
The reason for the observed differences in reactivity in
the palladium-catalyzed domino step is not entirely clear.
The steric hindrance of the substituents seems to be small
and can probably be excluded. Because, as already men-
tioned, the oxidative addition reaction seems to be the rate-
determining step one has to assume that the reason for the
differences should be electronic in nature. A direct electron-
ic influence through hyperconjugation can probably be pre-
cluded. However, a p–p interaction between the naphtha-
lene moiety and the donor/acceptor-substituted phenyl rings
might account for the observed differences in reactivity.
Earlier studies of the stereoselectivity of the domino reac-
tions of 2 f and 3 f showed them to be completely diastereo-
selective.[9] The induced diastereoselectivity can be ex-
plained by an interaction between the palladium atom and
AHCTUNGTERG(NNUN 3d)
traces (2e)
O,I
47 (2e)[b][c]
CH2,Br
O,Br
CH2,Br
ACHTUNGTNER(NUNG 3e)
70 (2 f)
55 (3 f)
H (5 f)
Abbreviations: dba: dibenzylideneacetone; TBAF: tetra-n-butylammoni-
um fluoride. [a] Conditions A: Pd(OAc)2, IPr·HCl (23), TBAF, dioxane,
AHCTUNGTRENNUNG
1008C, 36–38 h. Conditions B: [Pd2dba3], PtBu3·HBF4, CsF, dioxane,
808C, 16–20 h. [b] Decomposition of the substrate. [c] The product was
isolated as a 1:1 E/Z mixture. [d] The product was isolated as a 1:0.8 E/Z
mixture. [e] Destannylation. [f] The product was isolated as a 10:1 E/Z
mixture.
the hydroxy group in the primarily assembled palladium in-
termediate 26, which forces the alkyne moiety to lie below
the naphthalene skeleton and thus controls the selective in-
sertion into the triple bond via intermediate 27 (Scheme 5).
In contrast, several of the newly synthesized compounds,
which are depicted in Scheme 6, were obtained not in a dia-
Scheme 5. Proposed intermediates to explain the stereoselectivity in the
domino carbopalladation/Stille reaction.
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L. F. Tietze et al.
Scheme 7. Stepwise mechanism for the light-induced unidirectional
switching process. Depiction of the half-chair conformers of the M,Z and
P,Z forms showing the 1,3-allylic strain in (M,Z)-2/3 and (M,E)-2/3.
Scheme 6. Overview of the synthesized tetrasubstituted alkenes, reaction
conditions, and diastereomeric ratios. Compounds 2 f and 3 f are taken
from reference [9].
meric forms. In the case of the unsubstituted switches, the P
and M diastereomers show a distinct difference between the
two diastereomeric forms, providing good contrast and sen-
sitivity for monitoring the switching process. In general, this
was also the case with the substituted switches, even though
large differences in the overall change of the spectral shape
were observed from system to system. The E and Z diaste-
reomers of 2c and 3c showed no difference in their absorp-
tion spectra. This was confirmed by comparing the absorp-
tion spectrum of diastereopure (P,E)-2c and (P,E)-3c with a
spectra of the corresponding E/Z mixtures. Also, no change
in the isosbestic points was observed during the photoin-
duced switching and ensuing thermal isomerization (see
below). Moreover, the spectra of the E and Z diastereomers
of the other alkenes 2 and 3 showed only a small or no dif-
ference.
In our investigations of the substituted alkenes 2 and 3,
first the effect of the electron-withdrawing and -donating
groups on the absorption spectra was determined. For com-
parison, the previously synthesized unsubstituted switches
2 f and 3 f were included.[9] Each spectrum was normalized
to the global maximum at around 360 (2a–f) or 325 nm
(3a,c,f). Both the positions of the absorption maxima and
the overall shapes of the spectra were correlated with the
electronic nature of the substituents (Figure 1 and Figure 2,
also see Table S1 and Figure S1 in the Supporting Informa-
tion).
stereomerically pure form but rather as a mixture of E and
Z stereoisomers. However, this seems not to be due to a loss
of selectivity in the domino reaction but rather due to the
ability of these compounds to undergo a fast E/Z isomeriza-
tion even in daylight.
Summarizing the synthetic work, the synthesis of 2a–f
and 3a,c,f could be accomplished in six (2a,c and 3a,c) or
eight steps (2b,d–f and 3 f). In all cases the final step is the
domino carbopalladation/Stille reaction, which yields the de-
sired tetrasubstituted alkenes through the formation of two
ꢀ
C C bonds, two six-membered rings, and a tetrasubstituted
double bond in a completely diastereoselective fashion. The
procedure is highly flexible with regard to the substituents
and allows rapid access to the desired alkenes with different
substituents at different positions to allow structure–activity
studies.
Photochemical investigation: The irradiation of (P,E)-2/3
leads to a fast P/M and E/Z isomerization in the primary
step that is followed by a slower thermal M/P isomerization
(Scheme 7). The thermal isomerization of the M,Z to the
P,Z stereoform is driven by an unfavorable 1,3-allylic strain
in the M,Z form between the equatorial OH group and the
lower part of the molecule, as can be seen from the depic-
tion of the M,Z half-chair conformer in Scheme 7. An inver-
sion of the half-chair results, leading to M/P isomerization
to the P,Z isomer. The same is true for the thermal M,E iso-
merization to the P,E isomer after photochemical isomeriza-
tion of the P,Z form. Thus, as has been shown before, al-
kenes of type 1–3 can be used as unidirectional molecular
rotors.[8] Unsubstituted alkenes (R=H) show only two iso-
mers whereas substituted switches can adopt four stereoiso-
It was shown that the strongest influence on the absorp-
tion spectra stems from the nature of the bridging group in
the upper part of the molecule. The oxygen-bridged com-
pounds 2a–f show a global maximum at around lmax
=
355 nm with some smaller, additional peaks at around
320 nm. For the methylene-bridged alkenes 3a,c,f, however,
only one main band near lmax =325 nm is visible, probably
due to an overlap with other absorption bands at even short-
er wavelengths.
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Tetrasubstituted Alkenes as Molecular Switches
FULL PAPER
is similar to that of the unsubstituted compound (2 f) and
those of compounds with electron-donating substituents at
the 2-position (2d,e). In future studies it will therefore be
worthwhile investigating how an electron donor at the 3-po-
sition of the lower part affects the spectral properties. For
the alkenes 3, compared with the unsubstituted congener
(3 f), changes in the absorption spectra of the main bands
are considerably smaller, but the tail region at around
375 nm also appears to shift somewhat towards longer wave-
lengths when an electron-withdrawing substituent (3a,c) is
introduced.
In addition, the effect of the substituent on the switching
properties of each alkene was studied qualitatively. For this,
each compound was irradiated with light in the wavelength
range 310<l1 <365 nm (see the Experimental Section for
details) and the absorption spectra were recorded until the
photostationary state was reached (the changes are summar-
ized in Table 4). The wavelength range chosen includes the
Figure 1. UV/Vis absorption spectra of the oxygen-bridged alkenes 2a–f.
A colored version is provided in the Supporting Information.
Table 4. Quantitative change in absorbance DA of UV/Vis spectra after
irradiation.
Switch (X,R)
Absorbance change[a]
la [nm] DA
lb [nm] DA
lc [nm] DA
[b]
[b]
[b]
2a (O,CF3)
2b (O,F)
2c (O,CN)
–
320
324
–
–
–
–
–
ꢀ0.21 356
ꢀ0.28 389
ꢀ0.06 379
407
+0.758
+0.06
+0.17
+0.76
+0.77
–
ꢀ0.03 363
2d (O,Me)
2e (O,OMe)
3a (CH2,CF3) 326
3c (CH2,CN) 328
323
–
ꢀ0.19 356
ꢀ0.36 394
[c]
–
359
–
ꢀ0.37 400
[c]
[c]
ꢀ0.13
–
–
ꢀ0.02 350
ꢀ0.02 380
+0.01
[a] The absorption of the pure P isomer was normalized. The probe was
irradiated until the photostationary state was reached. la corresponds to
characteristic changes in the absorption spectra in the region 315–
330 nm, lb corresponds to 350–365 nm, and lc to 375–400 nm. [b] No
change in the absorption was observed under irradiation. [c] No charac-
teristic changes were observed in this region.
absorption maxima of all the investigated compounds 2 and
3.[19] Each spectrum was normalized to the maximum at
around 360 (2a–e) or 325 nm (3a,c) prior to light irradiation
and all later spectra were referenced to the first measure-
ment. Figure 3 shows a representative example for 2d (fur-
ther spectra can be found in the Supporting Information).
The absorption spectrum of the pure P form has a shoulder
at around 420 nm. Upon irradiation at 310<l1 <365 nm, a
band centered at around 400 nm grows up with time, which
arises from the M form produced by the switching process.
At the same time, the characteristic band of the P form,
with a peak at around 360 nm, decays. After a characteristic
irradiation time, a photostationary equilibrium is reached.
We note that there is a clean isosbestic point at around
370 nm, which indicates the transformation between the two
species P and M.
Figure 2. UV/Vis absorption spectra of the methylene-bridged alkenes
3a,c,f.
Systematic trends can be observed when comparing the
positions and the changes in the shapes of the steady-state
absorption bands of the compounds with either donor or ac-
ceptor substituents on the lower ring. For instance, in the
case of alkenes 2, the main band (330–400 nm) in the UV
spectra of compounds 2a,c, which feature electron-with-
drawing substituents at the 2-position of the lower part,
shows a double-peak structure and is shifted to longer wave-
lengths by around 20 nm relative to the unsubstituted com-
pound (2 f). On the other hand, with electron-donating sub-
stituents (2d,e) a structureless broad band is observed and
the absorption maxima and the band width are only slightly
different from 2 f. Note that 2b is a special case, because it
is the only compound with substitution at the 3-position of
the lower part: Interestingly, although it bears the electron-
withdrawing fluorine as substituent, its absorption spectrum
Interestingly, the switching behavior also appears to
change systematically with substitution on the lower part.
For alkenes 2, the compounds with electron-donating sub-
stituents (2d,e) or without substitution (2 f) at the 2-position
Chem. Eur. J. 2011, 17, 8452 – 8461
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8457

L. F. Tietze et al.
is, however, important to note that the “irreversible” switch-
ing led, under either thermal conditions or irradiation, to an
incomplete reversion from the photostationary state back to
the pure P stereoisomer. An intermediate of so far unknown
structure is formed that thermally reverts back to the corre-
sponding P stereoisomer overnight. In all cases, no degrada-
tion of the molecules during the switching process was ob-
served (Figures 4–6 and Table 5, additional spectra in the
Supporting Information).
Figure 3. Evolution of the absorption spectra of 2d in acetonitrile be-
tween 300–500 nm upon irradiation at 310<l1 <365 nm. The black lines
represent the start and end states with the grey lines being intermediate
measurements recorded every 0.4 min. The arrows depict the increase or
decrease in absorbance over time.
Figure 4. Fully reversible switching of 3 f under alternating irradiation
with light of 310<l1 <365 and 400<l2 <445 nm.[9]
show pronounced switching behavior. In contrast, electron-
withdrawing substituents lead to either slightly less pro-
nounced changes, as in the case of 2c, or no change, as for
2a. Factors influencing the relative absorption ratios are the
specific concentrations of the two forms in the photostation-
ary equilibrium and their difference in oscillator strength.
The reason for the special behavior of 2a is as yet unclear:
One possible explanation for the apparently inefficient
switching in 2a or 2c could be the intramolecular interac-
tion between the electron-withdrawing group and the upper
part and further experiments will be required to address this
issue, for example, spectroscopic studies of the switching
process on short timescales. For instance, it could be that
isomerization is suppressed by a competing nonradiative
pathway after electronic excitation. Alternatively, the time-
scale for the thermal back-reaction after switching could be
considerably faster than for the other compounds such that
the concentration of the isomer in the photostationary state
of the current experiments is very low. The switch with a
substituent at the 3-position (2b) behaved very much like
2d–f, consistent with the absorption band characteristics
above. Alkenes 3 displayed only small changes with and
without substitution.
Figure 5. Photochemical reversible switching of 2b under alternating irra-
diation with light of 310<l1 <365 and 400<l2 <445 nm. The lower
graph shows the incomplete reversion in the absence of light.
After the qualitative investigation, the kinetics of the
switching and usually more than one switching cycle were
studied to monitor the fatigue resistance of the systems. The
monitoring under thermal conditions involved recording the
time-dependent absorption changes of the sample at 300 K
in the absence of light. According to their photochemical
and thermal switching behavior, we have grouped the com-
pounds into three categories: 1) Completely photochemical-
ly and thermally reversible (“normal” switching), 2) photo-
chemically reversible, thermally only partially reversible,
and 3) neither photochemically nor thermally reversible. It
Summarizing, the introduction of substituents led to a
drastic change in the switching behavior of the helical al-
kenes. The most obvious aspect is the reversibility of the
overall process and in the case of 2a the absence of any
changes. Alkenes 2b, 2c, and 3c show full reversibility upon
alternating irradiation with light of two different wave-
lengths (310<l1 <365 nm, 400<l2 <445 nm). The photoin-
duced P/M isomerization occurred under the chosen condi-
tions in the order of minutes, however, depending on the
choice of substituent, the photostationary state was reached
between 1.9 (2b) and 8 min (2d). No clear trend in the time
8458
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Chem. Eur. J. 2011, 17, 8452 – 8461

Tetrasubstituted Alkenes as Molecular Switches
FULL PAPER
ments will be required to give clues as to the nature of that
state.
In any case, variation of the type and position of the sub-
stituents on the lower part of these compounds leads to an
impressive variability of the steady-state and dynamic ab-
sorption properties. The effect of substituents on the photo-
chemical properties make this class of compound an attrac-
tive target for in-depth studies of the switching process on
very short timescales. Such experiments are currently under-
way in our laboratories. These will provide information re-
garding the molecular switching-mechanism and the sub-
stituent-dependence of the ultrafast photoinduced kinetics.
Such mechanistic information will be prerequisite to the tai-
loring of chiroptical switches with higher optical contrast
and improved thermal stability.
Figure 6. Photochemical irreversible switching of 2c under alternating ir-
radiation with light of 310<l1 <365 and 400<l2 <445 nm. The lower
graph shows the incomplete reversion in the absence of light.
Conclusion
We have described the successful expansion of the diaste-
reoselective domino carbopalladation/Stille reaction for the
construction of various acceptor- (CF3, F, CN) and donor-
substituted (Me, OMe) helical alkenes 2 and 3. It has been
shown that, depending on the electronic nature of the sub-
stituent, the reaction proceeds by two complementary meth-
ods using either PtBu3·HBF4 or IPr·HCl (23) as the ligand.
In addition to oxygen-bridged alkenes, two methylene-
bridged systems were synthesized.
The photophysical properties of all the substituted helical
alkenes were investigated. The UV/Vis absorption spectra
were mainly influenced by the bridging group. The electron-
donating or -withdrawing properties of the lower part of the
molecule led to some differences in the steady-state spectra
and significant changes in the switching behavior. Thus, the
time to reach photostationary equilibrium, the reversibility
of the process, and possibly also the probability of compet-
ing nonradiative pathways greatly depend on the type of
substituent.
Table 5. Measured kinetics and overall switching behavior.
Switch (X,R)
l
[a] [nm]
t[b] [min]
Switching
2a (O,CF3)
2b (O,F)
–
–
1.9
no spectral changes observed
normal switching behavior under
irradiation, thermally irreversible
normal switching behavior under
irradiation, thermally irreversible
neither photochemically
389
410
394
400
2c (O,CN)
2d (O,Me)
2e (O,OMe)
3.2
ca. 8
3.6
nor thermally reversible
neither photochemically
nor thermally reversible
2 f (O,H)
3a (CH2,CF3)
3c (CH2,CN)
400
325
328
6.0
3.6
2.8
normal behavior[c,d]
normal behavior[c]
normal switching behavior under
irradiation, thermally irreversible
normal behavior[c,d]
3 f (CH2,H)
380
6.0
[a] Wavelength chosen for monitoring of the switching. [b] Time until the
photostationary state was reached. [c] Completely photochemically and
thermally (T=300 K) reversible. [d] The data is taken from ref. [9].
Thus, the peculiar substituent effect on both the domino
reaction and the photochemical properties makes this class
of compound very attractive for further studies.
required to reach the photostationary state could be found
with respect to the nature of the substituent.
The thermal reversion, whether complete or incomplete,
occurs for the systems 2a–e and 3a,c in the order of mi-
nutes. Even though some alkenes show good contrast be-
tween the pure P stereoisomer and the P/M mixture in the
photostationary state (2b,d,e), the partial irreversibility and
the fast thermal isomerization step prevents their usage in
optical data storage devices. Although alkene 3a might po-
tentially be used, the maximum contrast is, however, smaller
than for the unsubstituted alkenes 2 f and 3 f.
As already mentioned, the intermediate observed in the
back-switching of 2b–e and 3c has not been characterized
so far. A metastable electronic state is unlikely due to the
longevity of the observed intermediate. It is possible that a
metastable conformer, for example, a boat conformer, could
account for the recorded data. However, further measure-
Experimental Section
Full synthetic details and general procedures for all the synthesized com-
pounds 2a–e, 3a,c, 4a–e, 5a–e, 6, 7, 8a–e, 9–13, 16a–e, 17b,d,e, 18b,d,e,
19a–e, 20a–e, 21, and 22a–d can be found in the Supporting Information.
Representative procedure for the alkynylation in the synthesis of 4d:
NEt3 (46 mg, 63 mL, 0.456 mmol, 10.00 equiv) was added to a solution of
LiHMDS (68 mL, 0.068 mmol, 1.50 equiv, 1.00m in toluene) at ꢀ788C fol-
lowed by the addition of a solution of the alkyne 8d (34 mg, 0.068 mmol,
1.50 equiv) in toluene (0.6 mL). The reaction solution was stirred at
ꢀ788C for 15 min, at ꢀ608C for 15 min, allowed to reach RT over
30 min, and stirred at RT for another hour. After cooling back to ꢀ788C,
a solution of the aldehyde 6 (19 mg, 0.071 mmol, 1.00 equiv) in toluene
(1.5 mL) was added through a syringe pump over 10 h and the mixture
was stirred at ꢀ788C for another 5 h. Afterwards, the solution was al-
lowed to reach RT over 3 h and the reaction was stopped by the addition
Chem. Eur. J. 2011, 17, 8452 – 8461
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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8459

L. F. Tietze et al.
of a sat. aq. solution of NH4Cl (5 mL) and H2O (5 mL). The aqueous
phase was extracted with EtOAc (3ꢄ10 mL), the combined organic
phases were dried over MgSO4, and the solvent removed in vacuo. Flash
column chromatography (SiO2, PE/EtOAc, 20:1) afforded the alcohol 4d
as a yellow oil (24 mg, 0.030 mmol, 65%).
J=7.7, 1.5 Hz, 1H; 8’-CH), 7.63 (d, J=8.1 Hz, 1H; 10-CH), 7.72 ppm (d,
J=8.8 Hz, 1H; 6-CH).
Optical experiments: A commercial absorption spectrometer (Varian
Cary 50 Bio) was employed to record the steady-state absorption spectra
and the photoinduced switching processes. In a typical switching experi-
ment a sample of (P,E)-2/3 was subjected to light at alternating wave-
lengths. A high-pressure Hg/Xe Lamp (Osram HBO, 200 W) combined
with appropriate optical glass filters (Schott) was used. For irradiation in
the wavelength range 310<l1 <365 nm we employed a UG11 (thickness:
1 mm) and WG320 (2 mm) filter combination. They acted as a bandpass
filter so that only the Hg lines at 313.2, 334.1, and 365.0 nm were used
for illumination. The light was focused on the sample cell by a Suprasil I
lens with a focal length of 100 mm. The new absorption band emerging
at longer wavelengths was attributed to the absorption of the switched
form. At the same time the original band at short wavelength decreases.
Back-switching was induced by using the same lamp in combination with
a GG400 (1 mm) filter. This filter provides a sharp cut-off below 400 nm
and therefore only the Hg lines at 404.6 and 435.8 nm can pass, which are
located in a spectral region in which the switched form predominantly
absorbs. The power of the filtered light was measured at the sample posi-
tion by using a calibrated power meter (Melles Griot 13PEM001): The
power was 23 mW for the UG11/WG320 filter combination and 190 mW
for the GG400 filter.
1H NMR (600 MHz, CDCl3): d=0.81 (t, J=7.3 Hz, 9H; Sn-
A
ACHUTNTGRNE[NUG CH2ACHTUNTGREN(NUGN CH2)2CH3]3), 1.20–1.29 (m,
ACHTUNGTRENNUNG
2.31 (s, 3H; 4“’-CH3), 2.80 (s, 1H; 2-OH), 4.19 (ddd, J=17.2, 9.5, 5.5 Hz,
2H; 1-CH2), 5.01 (dd, J=7.7, 3.2 Hz, 1H; 2-CH), 6.67 (d, J=8.2 Hz, 1H;
3”-CH), 6.78 (dd, J=8.3, 0.8 Hz, 1H; 6“’-CH), 6.99 (td, J=7.6, 1.0 Hz,
1H; 4”-CH), 7.04 (dd, J=8.2, 1.5 Hz, 1H; 5“’-CH), 7.13 (d, J=8.9 Hz,
1H; 3’-CH), 7.21 (ddd, J=9.1, 7.5, 1.7 Hz, 1H; 5”-CH), 7.26 (d, J=
1.8 Hz, 1H; 3“’-CH), 7.40 (ddd, J=8.0, 6.9, 1.0 Hz, 1H; 6’-CH), 7.48 (dd,
J=7.7, 1.6 Hz, 1H; 6”-CH), 7.54 (ddd, J=8.3, 6.8, 1.2 Hz, 1H; 7’-CH),
7.73 (d, J=8.1 Hz, 1H; 4’-CH), 7.77 (d, J=8.8 Hz, 1H; 5’-CH), 8.11 ppm
(d, J=8.5 Hz, 1H; 8’-CH); 13C NMR (125 MHz, CDCl3): d=10.0 (Sn-
A
2ACHTUNGTRENNUNG(CH2)2CH3]3), 13.8 (SnAHCUTNGTNERN[UGN (CH2)3CH3]3), 20.8 (4’’’-Me), 27.4 (Sn-
ACHTUNGTRENNUNG
82.6 (C-4), 89.5 (C-3), 90.1 (C-1’), 113.6 (C-1“), 115.0 (C-3’), 116.8 (C-3”),
117.6 (C-6“’), 122.4 (C-4”), 124.7 (C-6’), 128.1 (C-4’, C-7’), 129.9 (C-5“),
130.2 (C-5”’), 130.3 (C-5’), 131.2 (C-8’), 132.4 (C-8’a), 132.6 (C-2“’), 133.9
(C-6”, C-4“’), 135.4 (C-4’a), 137.8 (C-3”’), 155.3 (C-2’), 158.4 (C-2“),
159.0 ppm (C-1”’); IR (film): n=3418, 2955, 2924, 2870, 2852, 1594, 1464,
1252, 1211, 1074, 749 cmꢀ1; UV (CH3CN): lmax (loge/mꢀ1 cmꢀ1)=203.5
(4.7753), 233.0 (4.8661), 285.0 (3.9660), 323.0 (3.3766), 335.5 nm (3.3816);
MS (ESI, MeOH): m/z (%): 1641.3 (77) [2M+Na]+, 833.1 (100)
[M+Na]+; HRMS (ESI): m/z calcd for C39H47IO3Sn [M+Na]+: 833.1492;
found 833.1498.
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie. M.A.D. thanks the Studienstiftung
des deutschen Volkes and the Fonds der Chemischen Industrie for a doc-
toral scholarship.
Representative procedure for the domino carbopalladation/Stille reaction
in the synthesis of 2d: A mixture of [Pd2dba3] (0.6 mg, 0.001 mmol,
0.05 equiv), PtBu3·HBF4 (0.4 mg, 0.001 mmol, 0.10 equiv), and CsF
(4.0 mg, 0.027 mmol, 2.20 equiv) in dioxane (0.2 mL) was stirred at RT
for 5 min and then a solution of 4d (9.5 mg, 0.012 mmol, 1.00 equiv) in
dioxane (0.6 mL) was added. The reaction vessel was placed in a preheat-
ed oil bath at 808C and stirred for 18 h. Afterwards, the mixture was
cooled to RT and the reaction quenched by the addition of a sat. aq. solu-
tion of NH4Cl (5 mL) and H2O (5 mL). After the extraction with EtOAc
(3ꢄ10 mL), the organic phase was dried over MgSO4 and the solvent re-
moved in vacuo. Purification by flash column chromatography (SiO2, PE/
EtOAc 20:1) yielded 2d as a yellow resin as a 1:1 mixture of the E and Z
stereoisomers (3.0 mg, 0.008 mmol, 67%).
[1] a) V. Balzani, M. Venturi, A. Credi, Molecular Devices and Ma-
chines:
A Journey into the Nanoworld, Wiley-VCH, Weinheim,
2003; b) special issue on Molecular Machines, Acc. Chem. Res. 2001,
34, 409–522; c) M. Irie, Chem. Rev. 2000, 100, 1685–1716; d) Y. Yo-
koyama, Chem. Rev. 2000, 100, 1717–1740; e) G. Berkovic, V. Kron-
gauz, V. Weiss, Chem. Rev. 2000, 100, 1741–1754; f) S. Kawata, Y.
Kawata, Chem. Rev. 2000, 100, 1777–1788.
[2] H. Dꢁrr in Photochromism: Molecules and Systems (Eds.: H. Dꢁrr,
H. Bouas-Laurent), Elsevier, Amsterdam, 1990, 1–14.
13C NMR[20] (125 MHz, CDCl3): d=20.1 (Z-CH3), 21.2 (E-CH3), 65.1 (E-
C-2, Z-C-2), 74.3 (E-C-3, Z-C-3), 113.8 (E-C-10b), 113.9 (Z-C-10b), 115.9
(Z-C-3’), 116.4 (Z-C-5), 116.6, 116.9 (Z-C-5’), 118.0, 118.0, 122.4 (E-C-7’),
122.9, 122.9, 123.1 (Z-C-3’), 123.2, 124.1, 124.2, 124.2, 124.3, 124.5, 125.1,
125.5, 125.6, 125.7 (Z-C-8), 125.8 (E-C-8), 127.3 (Z-C-8’), 127.4, 127.5 (E-
C-8’), 127.6, 127.8, 128.0 (Z-C-1’), 128.0 (E-C-6’), 128.6 (Z-C-4’, Z-C-6’),
129.2, 129.2, 129.4, 130.2 (E-C-6), 130.4 (Z-C-6), 130.6, 130.7, 131.9,
132.5, 151.1, 152.4, 153.3, 154.0, 154.0, 154.5 ppm; UV (CH3CN): lmax
(loge/mꢀ1 cmꢀ1)=226.0 (4.5158), 307.0 (3.5843), 322.5 (3.6628), 356.0 nm
(3.7936); MS (ESI, MeOH): m/z (%): 807.3 (100) [2M+Na]+, 415.1 (97)
[3] a) K. A. McNitt, K. Parimal, A. I. Share, A. C. Fahrenbach, E. H.
Witlicki, M. Pink, D. K. Bediako, C. L. Plaiser, N. Le, L. P. Heeringa,
D. A. Vander Griend, A. H. Flood, J. Am. Chem. Soc. 2009, 131,
1305–1313; b) J. M. Spruell, W. F. Paxton, J.-C. Olsen, D. Benitez,
E. Tkatchouk, C. L. Stern, A. Trabolsi, D. C. Friedman, W. A. God-
dard, J. F. Stoddart, J. Am. Chem. Soc. 2009, 131, 11571–11580;
c) J. F. Stoddart, Chem. Soc. Rev. 2009, 38, 1802–1820.
[4] a) O. Johansson, L. O. Johannissen, R. Lomoth, Chem. Eur. J. 2009,
15, 1195–1204; b) T. E. Bitterwolf, Coord. Chem. Rev. 2006, 250,
1196–1207; c) P. Coppens, I. Novozhilova, A. Kovalevsky, Chem.
Rev. 2002, 102, 861–883; d) B. A. McClure, E. R. Abrams, J. J. Rack,
J. Am. Chem. Soc. 2010, 132, 5428–5436.
3ACTHNUTRGNEUNG
[M+Na]+; HRMS (ESI): m/z calcd for C27H20O [M+Na]+: 415.1305;
found: 415.1295.
E stereoisomer: 1H NMR (600 MHz, CDCl3): d=1.74 (d, J=7.0 Hz, 1H;
2-OH), 2.41 (s, 3H; 2’-CH3), 4.70–4.80 (m, 2H; 3-CH2), 5.83 (s, 1H; 2-
CH), 6.29 (ddd, J=8.1, 7.4, 1.3 Hz, 1H; 7’-CH), 6.52 (dd, J=7.9, 1.4 Hz,
1H; 8’-CH), 6.91–6.96 (m, 2H; 6’-CH, 8-CH), 7.10 (mc, 1H; 9-CH), 7.15
(dd, J=8.2, 1.0 Hz, 1H; 5’-CH), 7.16 (d, J=8.9 Hz, 1H; 3’-CH), 7.19–
7.24 (m, 4H; 1’-CH, 4’-CH, 7-CH, 5-CH), 7.63 (d, J=8.1 Hz, 1H; 10-
CH), 7.71 ppm (d, J=8.8 Hz, 1H; 6-CH). Z stereoisomer: 1H NMR
(600 MHz, CDCl3): d=1.52 (s, 3H; 2’-CH3), 1.77 (d, J=6.6 Hz, 1H; 2-
OH), 4.70–4.80 (m, 2H; 3-CH2), 5.85 (s, 1H; 2-CH), 6.28 (d, J=1.7 Hz,
1H; 1’-CH), 6.73 (ddd, J=8.3, 2.1, 0.6 Hz, 1H; 4’-CH), 6.91–6.96 (m, 1H;
8-CH), 7.04 (d, J=8.2 Hz, 1H; 3’-CH), 7.10 (mc, 1H; 9-CH), 7.17 (d, J=
8.8 Hz, 1H; 5-CH), 7.19–7.24 (m, 2H; 7’-CH, 7-CH), 7.29 (dd, J=8.1,
1.2 Hz, 1H; 5’-CH), 7.35 (ddd, J=8.3, 7.2, 1.5 Hz, 1H; 6’-CH), 7.43 (dd,
[5] a) Molecular Switches (Ed.: B. L. Feringa), Wiley-VCH, Weinheim,
2001; b) Organic Photochromic and Thermochromic Compounds,
Vol. 1–3 (Eds.: J. C. Crano, R. J. Guglielmetti), Springer, New York,
1999; c) B. L. Feringa, R. A. van Delden, N. Koumura, E. M. Geert-
sema, Chem. Rev. 2000, 100, 1789–1816; d) Y.-L. Huang, W.-C.
Hung, C.-C. Lai, Y.-H. Liu, S.-M. Peng, S.-H. Chiu, Angew. Chem.
2007, 119, 6749–6753; Angew. Chem. Int. Ed. 2007, 46, 6629–6633.
[6] a) L. F. Tietze, G. Brasche, K. M. Gericke, Domino Reactions in Or-
ganic Synthesis, Wiley-VCH, Weinheim, 2006; b) L. F. Tietze, Chem.
Rev. 1996, 96, 115–136; c) L. F. Tietze, U. Beifuss, U. Angew. Chem.
1993, 105, 137–170; Angew. Chem. Int. Ed. Engl. 1993, 32, 131–163;
Angew. Chem. Int. Ed. Engl. 1993, 32, 131–163; d) M. Leibeling,
D. C. Koester, M. Pawliczek, S. C. Schild, D. B. Werz, Nat. Chem.
8460
www.chemeurj.org
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Tetrasubstituted Alkenes as Molecular Switches
FULL PAPER
Biol. 2010, 6, 199–201; e) D. M. DꢀSouza, F. Rominger, T. J. J.
Mꢁller, Angew. Chem. 2005, 117, 156–161; Angew. Chem. Int. Ed.
2005, 44, 153–158.
[13] nBuLi in THF with or without TMEDA activation had previously
failed to provide the desired stannanes during our synthesis of the
unsubstituted molecular switches of type 2 and 3. See ref. [9].
[14] The bromo compound was prepared analogously by using the corre-
sponding bromo aldehyde. For its preparation, see: L. F. Tietze, F.
Lotz, Eur. J. Org. Chem. 2006, 4676–4684.
[15] G. A. Grasa, S. P. Nolan, Org. Lett. 2001, 3, 119–122.
[16] A. F. Littke, L. Schwarz, G. C. Fu, J. Am. Chem. Soc. 2002, 124,
6343–6348.
[17] J. R. Naber, S. L. Buchwald, Adv. Synth. Catal. 2008, 350, 957–961.
[18] a) W. A. Herrmann, C. Broßmer, K. ꢅfele, C.-P. Reisinger, T. Prier-
meier, M. Beller, H. Fischer, Angew. Chem. 1995, 107, 1989–1992;
Angew. Chem. Int. Ed. Engl. 1995, 34, 1844–1848; b) M. Beller, H.
Fischer, W. A. Herrmann, K. ꢅfele, C. Broßmer, Angew. Chem.
1995, 107, 1992–1993; Angew. Chem. Int. Ed. Engl. 1995, 34, 1848–
1849.
[7] a) B. L. Feringa, J. Org. Chem. 2007, 72, 6635–6652; b) B. L. Ferin-
ga, H. Wynberg, J. Am. Chem. Soc. 1977, 99, 602–603; c) W. F.
Jager, B. de Lange, A. M. Schoevaars, B. L. Feringa, Tetrahedron:
Asymmetry 1993, 4, 1481–1497.
[8] a) N. Koumura, E. M. Geertsema, M. B. van Gelder, A. Meetsma,
B. L. Feringa, J. Am. Chem. Soc. 2002, 124, 5037–5051; b) M. K. J.
ter Wiel, R. A. van Delden, A. Meetsma, B. L. Feringa, J. Am.
Chem. Soc. 2003, 125, 15076–15086; c) J. Vicario, M. Walko, A.
Meetsma, B. L. Feringa, J. Am. Chem. Soc. 2006, 128, 5127–5135;
d) A. A. Kulago, E. M. Mes, M. Klok, A. Meetsma, A. M. Brouwer,
B. L. Feringa, J. Org. Chem. 2010, 75, 666–679.
[9] L. F. Tietze, A. Dꢁfert, F. Lotz, L. Sçlter, K. Oum, T. Lenzer, T.
Beck, R. Herbst-Irmer, J. Am. Chem. Soc. 2009, 131, 17879–17884.
[10] a) K. M. Gericke, D. I. Chai, N. Bieler, M. Lautens, Angew. Chem.
2009, 121, 1475–1479; Angew. Chem. Int. Ed. 2009, 48, 1447–1451;
b) M. Arthuis, R. Pontikis, J.-C. Florent, J. Org. Chem. 2009, 74,
2234–2237; c) Z.-B. Zhu, M. Shi, Org. Lett. 2009, 11, 5278–5281;
d) H. Yu, R. N. Richey, J. Mendiola, M. Adeva, C. Somoza, S. A.
May, M. W. Carson, M. J. Coghlan, Tetrahedron Lett. 2008, 49,
1915–1918; e) N. Ishida, Y. Shimamoto, M. Murakami, Org. Lett.
2010, 12, 3179–3181; f) H. Tsuji, Y. Ueda, L. Ilies, E. Nakamura, J.
Am. Chem. Soc. 2010, 132, 11854–11855.
[19] Because a Hg/Xe lamp was used as the light source yielding a broad
wavelength range, filter combinations were chosen to select the de-
sired wavelengths. Monochromators would have lowered the light
intensity too much leading to a much slower switching and were
thus not used.
[20] The signals of the 13C NMR spectrum could not be fully assigned to
the respective E and Z stereoisomers due to multiplets in the
1H NMR spectrum. The given signals are those of both forms with
an assignment given if possible.
[11] J. Iskra, S. Stavber, M. Zupan, Synthesis 2004, 1869–1873.
[12] E. A. Krasnokutskaya, N. I. Semenischeva, V. D. Filimonov, P. Kno-
chel, Synthesis 2007, 81–87.
Received: December 9, 2010
Revised: April 17, 2011
Published online: June 10, 2011
Chem. Eur. J. 2011, 17, 8452 – 8461
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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