Direct observation of the molecular structural changes during the claisen rearrangement including the transition state

Article abstract of DOI:10.1246/cl.2010.374

The detailed processes in the Claisen rearrangement were observed. The process was vibrationally excited in the electronic ground state by a stimulated Raman process using a 5-fs pulse. The Claisen rearrangement was found to follow a three-step pathway. At first, the C⁴-O bond is weakened to generate a bisallyl-like intermediate. Next, the formation of a weak C ¹-C⁶ bond results in the generation of an aromatic-like intermediate. Finally, C⁴-O breaking and C¹-C⁶ formation occur simultaneously to generate the product.

Full text of DOI:10.1246/cl.2010.374

doi:10.1246/cl.2010.374
374
Published on the web March 6, 2010
Direct Observation of the Molecular Structural Changes
during the Claisen Rearrangement Including the Transition State
Izumi Iwakura,*1 Atushi Yabushita,2 and Takayoshi Kobayashi2,3,4,5
1Innovative use of light and materials/life, PREST, JST, 4-1-8 Honcho, Kawaguchi 332-0012
2Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
3University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585
4ICORP, JST, 4-1-8 Honcho, Kawaguchi 332-0012
5Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0971
(Received February 1, 2010; CL-100099; E-mail: iwakura@ils.uec.ac.jp)
The detailed processes in the Claisen rearrangement were
observed. The process was vibrationally excited in the electronic
ground state by a stimulated Raman process using a 5-fs pulse.
The Claisen rearrangement was found to follow a three-step
pathway. At first, the C4-O bond is weakened to generate a bis-
allyl-like intermediate. Next, the formation of a weak C1-C6
bond results in the generation of an aromatic-like intermediate.
Finally, C4-O breaking and C1-C6 formation occur simulta-
neously to generate the product.
The Claisen rearrangement (Figure 1) is one of the most
useful and common sigmatropic rearrangements in organic
synthesis known for its high stereoselectivity. [3,3]-Sigmatropic
rearrangements of allyl aryl ethers reported by Claisen1 helped
spur the development of various other reactions.2 The Claisen
rearrangement is thought to proceed through a six-membered
transition state (TS) by a supra-supra facial reaction following
Woodward-Hoffmann rules3 and frontier orbital theory.4 Exper-
imental stereochemical outcomes and theoretical calculations
implicated a six-membered chair-form TS.2e-2i A more detailed
mechanism of the Claisen rearrangement (Figure 1) was studied
by theoretical calculations and the kinetic isotope effect (KIE)
using allyl vinyl ether (AVE)2a in previous work.5 As a result, a
bis-allyl-like TS was suggested.5e-5j In this work, using a 5-fs
laser pulse (Figures S1 in Supporting Information (SI)6) devel-
oped in our group,7 we observed detailed molecular structural
information in the molecular structure change states during
rearrangement, including the TS,8 via instantaneous vibration
frequencies.9
AVE has an absorption peak located at a wavelength shorter
than 220 nm, which cannot be reached either by one-photon or
two-photon absorption of the visible 5-fs laser pulses (from 525
to 725 nm, Figure S1 in SI). At least three-photon absorption
corresponding to the fifth-order nonlinearity is needed. There-
fore, the 5-fs laser pulse triggers coherent molecular vibrations
via the stimulated Raman process10 corresponding to the third-
order nonlinearity in the ground state.11
Figure 2. (a) FFT power spectrum of AVE. (b) Raman spectrum of AVE.
The induced absorbance difference ¦A oscillates around 0
within «5 © 10¹5, while the vibrational modulation (¤¦A) is
3 © 10¹4 (Figure S2 in SI). This indicates that there are no slow
dynamics due to the excited electronic state population. The fast
Fourier transform (FFT) power spectra of the real-time traces
from 200 to 800 fs (Figure 2a) agree well with the Raman data
of AVE (Figure 2b), which confirms that the pump-probe
observations closely reflect molecular vibration dynamics of the
electronic ground state. These observed frequencies can be
assigned to the C=C stretch (¯C=C) of the allyl and vinyl groups
(1650 cm¹1), C-H deformation (¤C-H) of the allyl and vinyl
groups (1286 and 1326 cm¹1, respectively), and the C-O-C
symmetric stretch (¯s C-O-C) of the ether group (911 cm¹1).
A spectrogram12 was obtained by time-gated Fourier trans-
form with a Blackman window of 400 fs-FWHM (Figure 3). In
the spectrogram, the molecular vibration modes immediately
after photoexcitation are only due to the reactant AVE. However,
new bands appear at a delay of 2 ps. These new bands at 1750,
¹1
1030, and 1150 cm are assigned to the C=O stretch (¯C=O),
the C-C-C symmetric stretch (¯s C-C-C), and the C-C-C
asymmetric stretch (¯as C-C-C), respectively. The frequencies of
these new modes correlate well with the frequencies of the
Raman spectrum of synthesized allylacetaldehyde (Figure S3 in
SI) by the oxidaton of 4-penten-1-ol. Therefore, the appearance
of the new modes verifies that allylacetaldehyde was generated.
Furthermore, the NMR spectrum of AVE after the pump-probe
experiment also proves the generation of allylacetaldehyde
(Figure S4 in SI). The quantum yield of the photoinduced
process was estimated to be about 0.01. As described above, the
time-resolved conformation changes during the Claisen rear-
rangement were observed via molecular vibration change.
The detailed mechanism of the reaction was thus clarified
and can be described as follows. Disappearance of the ¯s C-O-C
Figure 1. Three proposed TS. Aromatic like TS appears in
a
synchronous concerted case. Bis-allyl-like TS in which C4-O bond
breaking takes place in the first step of the reaction and 1,4-diyl-like TS
in which C1-C6 bond formation takes place in the first step of the reaction
appear in an asynchronous case.
Chem. Lett. 2010, 39, 374-375
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

375
energy of the component molecules distributed over the
Boltzmann distribution.
The present experimental results show that stimulated
Raman excitation of vibrations drives the Claisen rearrangement
in the ground state, and detailed reaction processes were clarified
for AVE. It demonstrates that the observation of transition states
by vibrational real-time spectroscopy opens a new methodology
to study reactions in the ground state via the stimulated Raman
excitation of coherent vibration using ultrashort visible laser
pulse.
The authors thank Prof. Hiroshi Ikeda of Osaka Prefecture
Univ. and Prof. Shigehiko Hayashi of Kyoto Univ. for their
helpful discussions, Dr. Satoko Kezuka of Tokai Univ. for her
assistance with NMR spectra, and JASCO Inc. for their
assistance with Raman spectra.
Figure 3. Spectrogram using a Blackman window function.
(890 cm¹1) and the ¤CH (1500 cm¹1) bands around 800 fs shows
2
References and Notes
that the C4-O bond is weakened or broken in the first step of the
reaction. The frequency shift of the ¯C=C also suggests the
weakening of the C4-O bond as follows. The ¯C=C of the vinyl
1
2
L. Claisen, Ber. Dtsch. Chem. Ges. 1912, 45, 3157.
a) C. D. Hurd, M. A. Pollack, J. Am. Chem. Soc. 1938, 60, 1905. b) F. E.
Ziegler, Chem. Rev. 1988, 88, 1423. c) U. Nubbemeyer, Synthesis 2003,
961. d) A. M. M. Castro, Chem. Rev. 2004, 104, 2939. e) W. v. E.
Doering, W. R. Roth, Tetrahedron 1962, 18, 67. f) P. Vittorelli, T.
Winkler, H.-J. Hansen, H. Schmid, Helv. Chim. Acta 1968, 51, 1457. g)
J. J. Gajewski, J. Jurayj, D. R. Kimbrough, M. E. Gande, B. Ganem,
B. K. Carpenter, J. Am. Chem. Soc. 1987, 109, 1170. h) R. L. Vance, N.
G. Rondan, K. N. Houk, F. Jensen, W. T. Borden, A. Komornicki,
E. Wimmer, J. Am. Chem. Soc. 1988, 110, 2314. i) H. Hu, M. N.
Kobrak, C. Xu, S. Hammes-Schiffer, J. Phys. Chem. A 2000, 104, 8058.
R. Hoffmann, R. B. Woodward, Acc. Chem. Res. 1968, 1, 17.
K. Fukui, Acc. Chem. Res. 1971, 4, 57.
a) O. Wiest, K. A. Black, K. N. Houk, J. Am. Chem. Soc. 1994, 116,
10336. b) J. G. Hill, P. B. Karadakov, D. L. Cooper, Theor. Chem. Acc.
2006, 115, 212. c) M. J. S. Dewar, E. F. Healy, J. Am. Chem. Soc. 1984,
106, 7127. d) M. J. S. Dewar, C. Jie, J. Am. Chem. Soc. 1989, 111, 511.
e) J. J. Gajewski, N. D. Conrad, J. Am. Chem. Soc. 1979, 101, 2747.
f) L. Kupczyk-Subotkowska, W. H. Saunders, Jr., H. J. Shine, W.
Subotkowski, J. Am. Chem. Soc. 1994, 116, 7088. g) M. M. Davidson,
I. H. Hillier, J. Phys. Chem. 1995, 99, 6748. h) J. J. Gajewski, Acc.
Chem. Res. 1997, 30, 219. i) H. Y. Yoo, K. N. Houk, J. Am. Chem. Soc.
1997, 119, 2877. j) M. P. Meyer, A. J. DelMonte, D. A. Singleton, J. Am.
Chem. Soc. 1999, 121, 10865.
¹1
and allyl groups, observed at 1650 cm just after photoexcita-
tion, is separated in a blue-shifted mode toward 1690 cm¹1 and a
¹1
red-shifted mode toward 1570 cm from 500 to 800 fs. These
frequency shifts suggest that C4-O bond weakening or breaking
caused the electronic density in the vinyl and allyl groups to
increase and decrease, respectively.
After the C4-O bond weakening, electrons are transferred
from the vinyl group to the allyl group to form a weak C1-C6
bond. This reduces the electronic density along the C1=C2 in the
vinyl group resulting in the red shift of ¯C=C. In the allyl group,
the electronic density along the C5=C6 bond is increased to
induce the blue shift of ¯C=C, and the C4-C5 changes to a
C4=C5. This makes the three C=C bonds (C1=C2, C4=C5, and
C5=C6) equivalent and they have the same frequency of
3
4
5
¹1
¹1
1580 cm at about 1500 fs delay. The frequency of 1580 cm
agrees fairly well with the frequency of ¯C=C in benzene known
to be 1585 cm¹1 from the literature.13 Therefore, the appearance
6
Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
¹1
of a band around 1580 cm
implies that the generated
intermediate has an aromatic-like six-membered structure
including aromatic C=C bonds similar to those of benzene.
The generation of aromatic-like six-membered structure is also
supported from the appearance of ¤C-H and ¯s C-C-C at 1190 and
1000 cm¹1, respectively. Finally, C4-O bond breaking and C1-
C6 bond formation proceed simultaneously (i.e., in a synchro-
nous concerted process) to generate the product.
7
8
A. Baltuška, T. Fuji, T. Kobayashi, Opt. Lett. 2002, 27, 306.
In this work, many vibrational modes are excited via Raman excitation
because of broad bandwidth of the pump pulse. The relaxation of those
vibration modes proceeds in parallel with “reaction.” Being different
from T theoretical TS, the signal observed in this work shows vibration
frequency changes caused by not only reaction itself but also molecular
structural change etc. The molecular structure is thought to change
gradually between the reactant, the TS, and the product. The state, which
is continuously changing its structure between stable states (the reactant
and the product), is called as “molecular structure change states
including the TS.”
The existence of a six-membered intermediate are suggested
¹1
by the appearance of bands at 1000, 1190, and 1580 cm at
9
AVE stored in a glass cell (volume: 400 mm3) to be used as the solution
sample at 295 « 1 K. The experiments were performed at pump and
probe intensities of 150 and 25 ¯W, respectively.
¹1
1.5 ps followed immediately by a weak 1750 cm band. This
¯
band intensity has highest value at about 2.2 ps when the
C=O
five bands with frequencies 1030, 1150, 1320, 1520, and
10 a) N. C. Blais, M. Zhao, D. G. Truhlar, D. W. Schwenke, D. J. Kouri,
Chem. Phys. Lett. 1990, 166, 11. b) D. A. V. Kliner, D. E. Adelman,
E. N. Zare, J. Chem. Phys. 1991, 95, 1648. c) K. Mikulecky, K.-H.
Gericke, J. Chem. Phys. 1993, 98, 1244. d) F. F. Crim, J. Phys. Chem.
1996, 100, 12725.
11 a) I. Iwakura, A. Yabushita, T. Kobayashi, J. Am. Chem. Soc. 2009, 131,
688. b) A. T. N. Kumar, F. Rosca, A. Widom, P. M. Champion, J. Chem.
Phys. 2001, 114, 6795.
12 M. J. J. Vrakking, D. M. Villeneuve, A. Stolow, Phys. Rev. A 1996, 54,
R37.
13 G. Herzberg, Infrared and Raman Spectra, Van Nostrand Reinhold,
New York, 1945.
¹1
1650 cm appear. Therefore, the time constants of 2.2 ps does
not necessarily reflect the formation time of the allylacetalde-
hyde but the time required for the accumulation of allyl-
acetaldehyde to be detected by its vibration signals. Molecular
¹1
vibration modes of 1000, 1190, and 1580 cm were observed
for about 500 fs or longer as seen in Figure 3. The reason why
they could be observed for such a long period can be explained
as follows. It is because the occurrence time to generate six-
membered intermediate has variation depending on the thermal
Chem. Lett. 2010, 39, 374-375
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/