Synthesis of chiroptical molecular switches by Pd-catalyzed domino reactions

Article abstract of DOI:10.1021/ja906260x

New photochromic switches based on helical alkenes can quickly and efficiently be accessed by Pd-catalyzed domino reactions using a modular approach; this allows a wide variability in product formation with the advantages of a convergent synthetic route.

Full text of DOI:10.1021/ja906260x

Published on Web 11/13/2009
Synthesis of Chiroptical Molecular Switches by Pd-Catalyzed
Domino Reactions
Lutz F. Tietze,*,† Alexander Du¨fert,† Florian Lotz,† Lars So¨lter,‡ Kawon Oum,‡
Thomas Lenzer,‡ Tobias Beck,§ and Regine Herbst-Irmer§
Institut fu¨r Organische und Biomolekulare Chemie, Georg-August-UniVersita¨t Go¨ttingen,
Tammannstrasse 2, D-37077 Go¨ttingen, Germany, Institut fu¨r Physikalische Chemie, Georg-August-
UniVersita¨t Go¨ttingen, Tammannstrasse 6, D-37077 Go¨ttingen, Germany, and Institut fu¨r
Anorganische Chemie, Georg-August-UniVersita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany
Received August 6, 2009; E-mail: ltietze@gwdg.de
Abstract: New photochromic switches based on helical alkenes can quickly and efficiently be accessed
by Pd-catalyzed domino reactions using a modular approach; this allows a wide variability in product
formation with the advantages of a convergent synthetic route. The alkenes have been synthesized in
excellent enantioselectivity and their switching properties assessed by stimulation with nanosecond laser
pulses at two different wavelengths in over 1000 switching cycles.
Introduction
The basic requirement for a molecular switch is bistability,
in which both forms can be interconverted by means of external
stimuli such as light, heat, pressure, magnetic or electric fields,
pH change or chemical reactions.2a,4 Moreover, one must be
able to selectively address both forms individually and detect
them separately, preferably on very short time scales to allow
their potential use in modern computing machines.5
The chiroptical switches developed by Feringa et al. (e.g., 1)
containing a helical tetra-substituted alkene moiety are particu-
larly promising since their stable states can be accessed using
circularly polarized light (Scheme 1).6
The development of organic compounds that allow a con-
trolled motion at the molecular level is an important prerequisite
for the construction of nanoswitches and nanomotors.1,2 Based
on today’s requirements in the miniaturization of data storage
devices and the concomitant success of digital optical data
systems, in which recording and read-out of information is
carried out by light, the shift from classic electronic semicon-
ductor elements to light-driven molecular switches has gained
great impetus. This is underlined by the development of new
information storage techniques, fabrication processes and novel
synthetic methodologies in the past years.3
Quite recently, tetra-substituted alkenes have also been
prepared by Lautens, Florent and Yu.7
The advantage in using inorganic compounds for data-storage
devices is the long-term knowledge and experience of their
fabrication, but they lack some desirable properties such as. e.g.
the fine-tuning of a large variety of physical properties or the
characterization of single isolated structures. On the other hand,
organic compounds can overcome such deficiencies with the
embedded advantage of solving problems such as decreased thermal
and photochemical stability by small structural modifications.
Here we describe an efficient and general access to novel
compounds of type 2 with a helical backbone using Pd-catalyzed
(4) (a) Sienicki, K., Ed.; Molecular Electronics and Molecular Electronic
DeVices; CRC Press: Boca Raton, FL, 1994; Vol. III. (b) Emmelius,
M.; Pawlowski, G.; Vollmann, H. W. Angew. Chem. 1989, 101, 1475–
1502. Angew. Chem., Int. Ed. Engl. 1989, 28, 1445-1471.
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Chem. 2008, 120, 1480–1483. Angew. Chem., Int. Ed. 2008, 47, 1458-
1461.
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B. L.; Wynberg, H. J. Am. Chem. Soc. 1977, 99, 602–603. (c) Jager,
W. F.; de Lange, B.; Schoevaars, A. M.; Feringa, B. L. Tetrahedron:
Asymmetry 1993, 4, 1481–1497.
† Institut fu¨r Organische und Biomolekulare Chemie.
‡ Institut fu¨r Physikalische Chemie.
§ Institut fu¨r Anorganische Chemie.
(1) (a) Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and
Machines: A Journey into the Nanoworld, Wiley-VCH: Weinheim,
2003. (b) Acc. Chem. Res. 2001, 34, 409-522 (Molecular Machines
special issue).
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2009, 121, 1475–1479. Angew. Chem., Int. Ed. 2009, 48, 1447-1451.
(b) Arthuis, M.; Pontikis, R.; Florent, J.-C. J. Org. Chem. 2009, 74,
2234–2237. (c) Yu, H.; Richey, R. N.; Mendiola, J.; Adeva, M.;
Somoza, C.; May, S. A.; Carson, M. W.; Coghlan, M. J. Tetrahedron
Lett. 2008, 49, 1915–1918.
(2) (a) Feringa, B. L., Ed.; Molecular Switches; Wiley-VCH: Weinheim,
2001. (b) Crano, J. C.; Guglielmetti, R. J.; Organic Photochromic and
Thermochromic Compounds; Springer: New York, 1999, Vols. 1-3.
(c) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema, E. M.
Chem. ReV. 2000, 100, 1789–1816. (d) Huang, Y.-L.; Hung, W.-C.;
Lai, C.-C.; Liu, Y.-H.; Peng, S.-M.; Chiu, S.-H. Angew. Chem. 2007,
119, 6749–6753. Angew. Chem., Int. Ed. 2007, 46, 6629-6633.
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A R T I C L E S
Tietze et al.
Scheme 1. Chiroptic Molecular Switches 1 and 2
Scheme 3. Synthesis of 4a and 4ba
a Reagents and conditions: (a) Me3SiCCH, PdCl2(PPh3)2, CuI, NEt3, RT,
18 h, 92%; (b) tBuLi, Bu3SnCl, THF, -78 °CfRT, 1.5 h, 92%; (c) K2CO3,
MeOH/CH2Cl2, RT, 12 h, 97%; (d) LiHMDS, NEt3, toluene, -78 °C, 18 h;
(e) chromatography on a chiral stationary phase support (Daicel IB), >99% ee.
domino processes8 consisting either of a carbopalladation of
an alkyne and a Mizoroki-Heck reaction or a combination of
a carbopalladation and a Stille reaction.
The synthesis of the substrates 4a and 4b for the preparation
of the tetra-substituted alkenes 2c and 2d containing an aromatic
A ring by a domino carbopalladation/Stille reaction11 was carried
out employing the known diphenyl ether 13 as starting material
that is accessible from o-bromophenol and o-fluoronitrobenzene
in three steps.12 Sonogashira coupling of 13 with TMS acetylene
gave 14, which by a lithium-halide exchange and subsequent
transmetalation and desilylation in basic methanol led to 6 in
three steps in 82% yield (Scheme 3). The coupling of 6 with
the aldehydes 7 and 89,13 to give 4a and 4b was achieved in
80% and 77% yield, respectively, employing LiHMDS as base
and NEt3 as complexing additive in toluene and slow addition
of the aldehyde. The use of nBuLi, LDA and KHMDS as bases
only caused a polymerization of the aldehyde.
There is a high solvent dependence in this reaction which is
attributed to the aggregation behavior of organolithium com-
pounds,14 while the slow addition of the aldehyde prevents an
aldol condensation.
For the preparation of the enantiopure helical alkenes 2c-d
the racemic alcohols 4a and 4b were resolved by HPLC on a
chiral stationary phase with >99 ee;10 however, they were also
Results and Discussion
Synthesis of the Substrates 3a,b and 4a,b. As precursors for
the formation of the tetra-substituted alkenes 2a-d compounds
3a and 3b with a cyclohexene moiety as ring A as well as 4a
and 4b with an aromatic ring A were employed, which could
easily be obtained by addition of the metalated alkynes 5a and
5b as well as 6 to the aldehydes 7 and 8, respectively.
The synthesis of 5a was accomplished by coupling 3-bro-
mocyclohexene (9) and o-iodophenol (10) under basic conditions
followed by a Sonogashira reaction with propargylic alcohol
and subsequent oxidative removal of the hydroxymethyl group
employing MnO2 (Scheme 2). In a similar way, 5b was
synthesized by reaction of 11 with 9 followed by the introduction
of the alkyne moiety. After metalation of 5a and 5b and
subsequent coupling with aldehyde 79 compounds 3a and 3b
were obtained in 91% and 82% yield, respectively, as an almost
1:1 mixture of diastereomers, which could be separated by
chromatography on silica gel. For the preparation of the
enantiopure helical alkenes 2a-b the enantiomers of 3a and
3b were separated by HPLC on a chiral stationary phase.10
Scheme 2. Synthesis of Substrates 3a,ba
a Reagents and conditions: (a) K2CO3, DMF, 80 °C, 1 h, 98%; (b) propargylic alcohol, PdCl2(PPh3)2, CuI, HNiPr2, RT, 19 h; (c) MnO2, KOH, Et2O, RT,
1 h, 75% over 2 steps; (d) nBuLi, 7, THF, -60 °CfRT, 4 h; (e) Mg, 9, Et2O, RT, 12 h, 63%; (f) tBuLi, I2, Et2O, -78 °Cf0 °C, 2 h, 80%; (g) propargylic
alcohol, PdCl2(PPh3)2, CuI, NEt3, THF, RT, 24 h; (h) MnO2, KOH, Et2O, RT, 1 h, 64% over 2 steps; (i) chromatography on silica gel and chromatography
on a chiral stationary phase (Daicel IB), >99% ee.
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Synthesis of Chiroptical Molecular Switches
A R T I C L E S
Scheme 4. Enantioselective Synthesis of 4a and 4ba
a (a) Dess-Martin periodinane, CH2Cl2, RT, 15 h; (b) 17, iPrOH, MeCN, RT.
Scheme 5. Synthesis of Ketone 15 via Weinreb Amide 19a
prepared in an enantioselective way by reduction of the
corresponding ketones 15 and 16, respectively, using the chiral
Noyori catalyst 1715 in a transfer hydrogenation to give (S)-4a
and (R)-4b with excellent yield and very good enantioselectivity
of 90 and 96% ee (Scheme 4). For the enantioselective reduction
of 15 we also investigated DIP-Chloride16 and Me-CBS.17
Whereas DIP-Chloride only led to a decomposition of 15, CBS-
reduction gave enantioenriched 4a with moderate 40% ee.
a Reagents and conditions: (a) 2-chloroacetic acid, K2CO3, DMF, 120
°C, 3 d, 50%; (b) DCC, DMAP, EtNiPr2, HN(OMe)Me ·HCl, CH2Cl2, RT,
18 h, 67%; (c) 6, LiHMDS, NEt3, toluene, -78 °C, 18 h, 55%.
The ketones 15 and 16 were obtained by oxidation of the
racemic alcohols 4a and 4b using Dess-Martin periodinane with
50% and 63%, respectively. Both with IBX and pyridine ·SO3
according to Parikh-Doering18 an oxidation was not possible;
employing Swern conditions a decomposition of the substrates
and with MnO2 only partial conversion was observed.
Shibasaki20 show an especially broad substrate scope. Unfor-
tunately, the two aldehydes 7 and 8 are exceedingly base
sensitive, and thus, a product formation was not observed using
these procedures. Although the alkyne could be reisolated in
almost quantitative yield, the aldehyde decomposed under the
employed reaction conditions.
Besides the oxidation of 4a to give the ketone 15 we also
developed a more straightforward approach in which 15 was
directly formed by coupling of 6 with the corresponding
Weinreb amide 19 derived from 18 in 55% yield (Scheme 5).
Synthesis of the Tetra-Substituted Alkenes 2a-d by a
Pd-Catalyzed Domino Process. The Pd-catalyzed transformation
of the enantiopure syn-diastereomers of (2S,1′′S)-3a and (2S,1′′R)-
3b using catalytic amounts of the Pd-catalyst 2121 led to the
helical alkenes (2S,P)-2a and (2S,P)-2b respectively in 80 and
92% yield as single diastereomers. However, under the reaction
conditions a partial isomerization of the primarily formed double
bond was observed to give an approximately 1:1.2 mixture of
the corresponding double bond isomers in ring A (Scheme 6).
In contrast, the reaction of the anti-diastereomers of 3a and
3b in presence of catalytic amounts of 21 did not give the tetra-
substituted alkenes but solely led to the acenaphthylenes 20 by
CH activation.8
The domino reaction of (S)-4a and (R)-4b with Pd2dba3 in
the presence of PtBu3 ·HBF422 afforded the helical alkenes (S,P)-
2c and (R,P)-2d in 70% and 55% yield, respectively, again as
single diastereomers (Scheme 7). Thus, the configuration of the
helical structure in the products 2 of the Pd-catalyzed domino
reaction of 3a and 3b as well as of 4a and 4b is controlled by
the stereogenic center in the starting materials, whereby the (S)-
enantiomer exclusively leads to (P)-helicity and the (R)-
enantiomer to (M)-helicity.
In addition to the preparation of the enantioenriched alcohols
4a and 4b by the two-step oxidation-reduction process, we also
looked into their direct synthesis by an enantioselective addition
of the alkyne 6 on the aldehydes 7 and 8. There are several
ways to perform an enantioselective alkynylation of aldehydes,
whereby the two procedures developed by Carreira19 and
(9) Tietze, L. F.; Lotz, F. Eur. J. Org. Chem. 2006, 4676–4684.
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diameter 2.0 cm; length 25 cm, eluent: n-hexane/iPrOH, 90:10.
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The induced diastereoselectivity can be explained by an
interaction of the Pd-atom and the hydroxyl group in the
(16) Corey, E. J.; Shibata, S.; Bakshi, R. K. J. Org, Chem. 1988, 53, 2861–
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Scheme 6. Domino Reaction of syn-3a,b and anti-3aa
a Reagents and conditions: (a) 21, LiOAc ·2 H2O, tBu4NOAc, MeCN/DMF/H2O, 120 °C; syn-3a,b: 15 h; anti-3a: 2 h.
Scheme 7. Domino Carbopalladation/Stille Reaction of 4a and 4ba
a Reagents and conditions: (a) Pd2dba3, PtBu3 ·HBF4, CsF, 1,4-dioxane,
80 °C, 18 h.
Scheme 8. Proposed Intermediates Explaining the Stereoselectivity
in the Domino Carbopalladation/Stille Reaction
Figure 1. Molecular structure of 2c in the crystal.
primarily assembled Pd-intermediate (22),23 which forces the
alkyne moiety to lie below the naphthalene skeleton and thus
controls the selective insertion into the triple bond via inter-
mediate 23 (Scheme 8).
For the elucidation of the structures of the helical alkenes 2
compound 2c was crystallized and its configuration determined
by X-ray crystallography (Figure 1, see the Supporting Informa-
tion for details). The twisted structure of the xanthene moiety
in 2c probably results from a steric hindrance in the Fjord-
region.24
The CD-spectrum of 2c with its high molar ellipticity of Θ
) 5 × 105 deg cm2 dmol-1 indicates that the tetra-substituted
alkenes of type 2 contain a chiral chromophore as expected
(Figure 2).
Figure 2. CD spectrum of 2c in acetonitrile at room temperature.
show that they can exist in two stable configuration which can
be interconverted by irradiation with light. Thus, the two
diastereomers show drastic differences with respect to their
UV-vis absorption spectra in the 300-450 nm range, as shown
in Figure 3. For instance, the UV-absorption of the pure P-form
of 2d (solid line) shows a λmax only at 325 nm. In contrast, the
M-form of 2d (dashed line) is characterized by two absorption
bands centered at 320 and 380 nm.
Reversible optical switching was confirmed for both 2c and
2d (Scheme 9). First, photophysical investigations were con-
ducted by irradiation of 2c and 2d at two wavelength ranges
(310 < λ1 < 365 nm and 400 < λ2 < 445 nm) by employing a
high-pressure Hg-Xe lamp combined with corresponding
Photochemical Investigations. For the determination of the
usability of the tetra-substituted alkenes 2 is was necessary to
(23) Machotta, A. B.; Straub, B. F.; Oestreich, M. J. Am. Chem. Soc. 2007,
129, 13455–13463.
(24) The term, Fjord-region, was originally utilized solely for ben-
zo[c]phenanthrene. It describes sterically overcrowded molecular
regions that cause a deviation from planarity: (a) Silvermann, B. D.
Cancer Biochem. Biophys. 1982, 6, 23–29. (b) Silvermann, B. D.; La
Placa, S. J. J. Chem. Soc., Perkin Trans. 2 1982, 415–417. (c)
Utermoehlen, C. M.; Singh, M.; Lehr, R. E. J. Org. Chem. 1987, 52,
5574–5582.
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A R T I C L E S
Figure 3. UV-vis spectra of 2c (upper curve) and 2d (lower curve). Solid
line: P-form. Dashed line: M-form.
Scheme 9. Light-Induced Interchange Between the Respective P-
and M-Forms of the Aromatic Optical Switches 2c,d
Figure 4. Photochemical switching experiment for 2d employing alternate
15 ns laser pulses at 308 and 390 nm. Spectra were recorded after each
laser shot (repetition frequency 1 Hz) and integrated in the regions 314-340
nm (A) and 360-377 nm (B). The inset shows a magnification of the late
part of curve (B) after the establishment of the photostationary equilibrium.
360-377 nm (“M region”). The results are depicted in Figure
4, A and B, respectively.
Starting from a solution containing predominantly the P-
form,25 it takes approximately 150 alternate laser shots to reach
a photostationary equilibrium under the selected conditions: The
concentration of P decays (curve A) at the expense of M (curve
B). The inset shows a magnification at long time scales after
establishment of the photostationary equilibrium. Characteristic
up- and down-steps are observed, which indicate alternate
switching from P to M and vice versa, which is induced by the
two different lasers. After 1000 shots no fatigue of the optical
switching was observed. Residual drifts of the lines are likely
due to an imperfect correction of shot-to-shot fluctuations in
the laser energies. The nanosecond laser experiments demon-
strate that the light-induced interconversion process between
the two diastereomeric isomers is instantaneous and reversible
and that 2d can, in principle, be employed as a reversible optical
switch. The distinct change in absorbance between the two
diastereomeric isomers of 2d provides good contrast and
sensitivity for monitoring the switching process. However, it is
not yet clear which process is responsible for the undulatory
structure of the curves A and B at early times before reaching
the photostationary equilibrium. This will be dealt with in more
detail in future laser spectroscopic investigations.
optical glass filters (see the Supporting Information for figures
and experimental details). To assess the possibility of a fast
optical switching, 2c and 2d were subjected to alternate
nanosecond laser excitation at wavelengths of 308 and 390 nm
employing a repetition frequency of 1 Hz. An instantaneous P
to M conversion was achieved by an excimer laser (308 nm,
15 ns pulse length, 37 mJ ·cm-2 ·pulse-1) and the reverse process
was initiated by an excimer-pumped dye laser (390 nm, 15 ns
pulse length, 4.6 mJ ·cm-2 ·pulse-1).
The two laser beams were overlapped in a 1 cm quartz cuvette
containing a solution of 2c and 2d in acetonitrile with an initial
optical density of 0.6 for the pure P-form. A transient broadband
absorption spectrum in the range 314-377 nm was then
recorded after each laser shot by monitoring the sample
absorption employing a xenon lamp/fiber-coupled spectrograph/
gated ICCD combination (with the detection starting 1 µs after
the laser shot using a 200 µs integration time). “Non-destructive
readout conditions” were achieved by using a fast (∼2 ms
opening time) shutter and a suitable glass filter combination
placed behind the xenon lamp. This guaranteed that the lamp
intensity did not have an impact on the switching process. The
thermal MfP back-conversion of 2d was found to be negligible
during the experiment. It was confirmed that the crossing point
at 348 nm (Figure 3, lower panel) is indeed an isosbestic point.
Thus, the thermal back-conversion of 2d takes several hours.
In contrast, we observed that 2c shows inferior thermal stability
compared to that of 2d, and thermal MfP back-switching
occurred on the order of minutes. This is an interesting result,
considering the fact that the structural difference between 2c
and 2d is fairly small.
Conclusions
In conclusion we have described a generally applicable
synthetic strategy for the formation of helical tetra-substituted
alkenes using a domino reaction that consists of a carbopalla-
dation and a Mizoroki-Heck reaction or a carbopalladation and
a Stille reaction. The helical chirality of the tetra-substituted
alkenes is controlled by the stereogenic center in the substrates.
(25) The first experiment started with a sample containing only the P-form.
On subsequent runs the sample had to be converted back to the P-form
from the P/M-equilibrium which was achieved by illuminating the
sample using a continuous blue-light LED (center wavelength 405
nm, 200 mW) for 30 s prior to the laser experiments.
Each spectrum was normalized to the laser energy and
integrated in the regions 314-340 nm (“P region”) and
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A R T I C L E S
Tietze et al.
ASAP November 13, 2009. The corrected version was published
November 19, 2009.
Broadband transient absorption detection after alternate laser
excitation at 308 and 390 nm demonstrates that especially 2d
in principle can be used as an optical switch.
Supporting Information Available: Description of prelimi-
nary optical switching experiments, experimental procedures,
spectral data, crystallographic information files (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft. A.D. thanks the Studienstiftung des
deutschen Volkes and the Fonds der Chemischen Industrie for a
doctoral scholarship.
Note Added after ASAP Publication. The compound numbers
corresponding to Scheme 3 were incorrect in the text published
JA906260X
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