A Pentiptycene-Derived Light

Emergence and Development of
Artificial Molecular Brake
Yang, J. -S. et al.
Org. Lett. 2008, 10, 2279.
J. Org. Chem. 2006, 71, 844.
Tobe lab.
Kazuhiro Ikuta
1
Contents
・Introduction
Molecular Machines
Molecular Brake
Purpose of This Work
・Results and Discussion
Synthesis of Molecular Brake 1
Results of NMR Spectra
Results of DFT Calculations
・Summary
2
Molecular Machines
Examples of molecular machines
ピンセット
Molecular switch
Irie, M. et al. Chem. Commun. 2005, 3895.
Molecular shuttle
Stoddart, J. F. et al. Acc. Chem. Res. 19978 31, 405.
Molecular tweezers
Lehn, J. –M. et al. J. Am. Chem. Soc. 2004, 126, 6637.
Molecular motor
3
Feringa, B. L. et al. J. Org. Chem. 2005, 3, 4071.
Molecular Brake
The first example of molecular brake
Kelly, T. R. et al. J. Am. Chem. Soc. 1994, 116, 3657.
Desired rotary motion
・operating at room temperature
⇒applicable to machines
・photocontrollable system
⇒clean reaction
Metal ion
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Purpose of This Work
To date, an effective room-temperature photocontrollable molecular brake has
yet to be demonstrated.
Purpose
Synthesis and characterization of a room-temperature light-driven molecular brake.
Image of molecular brake
compound in this work
5
Synthesis of Molecular Brake 1
Synthesis of compound 5
NH2OH HCl
THF
K2CO3, KI
C8H17Br
acetone
NBS
DMF, 80 ゜C
Yang, J. -S et al. J. Org. Chem. 2006, 71, 844.
SnCl2
1) H3PO2 aq.
THF
HCl
CH2Cl2
2) Me3CONO
CuCN
NMP, 200 ゜C
1) DIBAL-H
CH2Cl2
2)HCl
5
6
Results of NMR Spectra
Only one set of signals for trans-1
⇒free rotation about the Cvinyl-Caryl single
bonds
Two sets of signals for pentiptycene group
⇒rotation of the rotator is slower than
the NMR time scale
(a) 1H and (b) 13C NMR spectra of trans-1 and cis-1 in
DMSO-d6 at 298 K (500 MHz).
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VT NMR Spectra-(1)
Rotational barriers and rates for the pentiptycene rotator in cis-1 is obtained from VT
(variable-temperature) NMR.
融合
coalescence
cis-1
Pentiptycene peripheral phenylene (blade) region of the (a) experimental proton and (b) carbon and (c) simulated
carbon VTNMR spectra of cis-1 (9 and 60 mM for proton and carbon, respectively, DMSO-d6, 500 MHz).
A coalescence temperature (Tc) near 348 K is found for protons H3 and H3’,
corresponding to an energy barrier of ΔG‡ (348K) = 16.9 ± 0.2 kcal mol-1.
Hoever the multiplicity of proton signals in the phenylene blades of pentiptycene rotator
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⇒VT 13C NMR was carried out (b) and simulated (c).
VT NMR Spectra-(2)
cis-1
Pentiptycene peripheral phenylene (blade) region of the (a) experimental proton and (b) carbon and (c) simulated
carbon VTNMR spectra of cis-1 (9 and 60 mM for proton and carbon, respectively, DMSO-d6, 500 MHz).
The results suggest that the rotation is nearly blocked at 298 K, and the rate constant
(k) for interconversion between the two isoenergetic conformers of cis-1 is only 6 s-1.
The activation parameters
were obtained by Arrhenius
and Eyring plots.
Ea[a]
14.8 ± 0.5
ΔH‡[a]
14.1 ± 0.5
ΔS‡[b]
-7.6 ± 1.4
ΔG‡298 K[a]
16.4
ΔG‡348 K[a]
16.8
[a] kcal mol-1 [b] cal K-1 mol-1
Rotational barrier is mainly due to an enthalpic factor.
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DFT Calculations-(1)
The calculation results are justified by the good agreement of the calculated
(16.75 kcal mol-1) and the NMR-determined ΔG‡ value (16.4 kcal mol-1) at 298 K.
U-shaped cavity
V-shaped cavity
DFT-derived structures for cis-1 : (a) the optimized
conformation and (b) the transition structure along
the pentiptycene rotation coordinate.
cis-1
Brake moiety (dinitrophenyl group) is located at the U-shaped cavities.
⇒ V-shaped cavities are inaccessible to the brake moiety as a result of
severe steric interactions with H1, H1’, and bridgehead hydrogen atom.
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DFT Calculations-(2)
The rotational barriers for the pentiptycene rotator in trans-1 and the brake moiety in cis-1
could not evaluated because of their low energy (decoalescence of the signals could not be
observed even at 183 K in CD2Cl2).
DFT calculations have been applied to retrieve the corresponding information for that in
trans-1 (4.45 kcal mol-1) and the brake rotation in cis-1 (6.85 kcal mol-1).
With a calculated ΔG‡ value differing by 12.3 kcal mol-1 (※) for the pentiptycene rotation
in trans-1 versus cis-1 at 298 K, the difference in rotation rate is in the order of 109.
(※) ΔG‡cis is 16.75 kcal mol-1
krot
1
:
～109
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Photoswitching between trans-1 and cis-1
Wavelength
(nm)
Ratio of
[trans]/[cis]*)
trans → cis
306
25/75
cis → trans
254
45/55
*) photostationary states
光定常状態
Photoswitching between the two
photostationary states is quite robust.
robust : 強固である
Absorption spectra of trans-1 (curve a) and cis-1
(curve d) and their photostationary states irradiated
with alternating 306- (curves c) and 254-nm (curves
b) UV light irradiation in dichloromethane. Inset
shows the changes in absorbance at 322 nm starting
from trans-1 (10 μM) for 7 switching cycles
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Conclusion
The pentiptycene-derived stilbene 1 has been prepared and investigated as a
photocontrollable molecular brake that functions at room temperature.
Both experimental and computational results reveal that at 298 K rotation of
the four-bladed pentiptycene (the rotator) is “free” in trans-1 but is nearly
blocked in cis-1.
The brake-on (cis-1) and brake-off (trans-1 ) states differ by a rotation rate of
～109-fold and can be interconverted through the ethylene trans-cis
photoisomerization reactions.
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