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

3

Examples of molecular machines

Feringa, B. L. et al. J. Org. Chem. 2005, 3, 4071.

Molecular motor

Molecular tweezers

Lehn, J. –M. et al. J. Am. Chem. Soc. 2004, 126, 6637.

Molecular switch

Molecular shuttle

Stoddart, J. F. et al. Acc. Chem. Res. 19978 31, 405.

Irie, M. et al. Chem. Commun. 2005, 3895.

ピンセット

Molecular BrakeThe first example of molecular brake

Kelly, T. R. et al. J. Am. Chem. Soc. 1994, 116, 3657.

4

Desired rotary motion・ operating at room temperature⇒ applicable to machines・ photocontrollable system⇒ clean reaction

Metal ion

Purpose of This Work

5

Synthesis and characterization of a room-temperature light-driven molecular brake.

To date, an effective room-temperature photocontrollable molecular brake has yet to be demonstrated.

Image of molecular brake compound in this work

Purpose

Synthesis of Molecular Brake 1

Synthesis of compound 5

Yang, J. -S et al. J. Org. Chem. 2006, 71, 844.6

NH2OH HCl

THF

SnCl2

HClCH2Cl2

1) H3PO2 aq.THF

2) Me3CONO

K2CO3, KI

C8H17Bracetone

NBSDMF, 80 ゜ C

CuCN

NMP, 200 ゜ C

1) DIBAL-HCH2Cl2

2)HCl

5

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).

7

VT NMR Spectra-(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).

cis-1

Rotational barriers and rates for the pentiptycene rotator in cis-1 is obtained from VT (variable-temperature) NMR.

coalescence

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.

8

融合

Hoever the multiplicity of proton signals in the phenylene blades of pentiptycene rotator⇒VT 13C NMR was carried out (b) and simulated (c).

VT NMR Spectra-(2)

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).

cis-1

9

Ea[a] ΔH‡[a] ΔS‡[b] ΔG‡

298 K[a]

ΔG‡348 K

[a]

14.8 ± 0.5 14.1 ± 0.5 -7.6 ± 1.4 16.4 16.8

[a] kcal mol-1 [b] cal K-1 mol-1

The activation parameters were obtained by Arrhenius and Eyring plots.

Rotational barrier is mainly due to an enthalpic factor.

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.

DFT Calculations-(1)

DFT-derived structures for cis-1 : (a) the optimized conformation and (b) the transition structure along the pentiptycene rotation coordinate.

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.

cis-1

U-shaped cavity

V-shaped cavity

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.

10

DFT Calculations-(2)

11

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

Photoswitching between trans-1 and cis-1

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

12

trans → cis

cis → trans

Wavelength(nm)

Ratio of [trans]/[cis]*)

306

254

25/75

45/55

*) photostationary states

Photoswitching between the two photostationary states is quite robust.

robust : 強固である

光定常状態

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.

13

Our Work-Rotaxane Molecular Brake

14

A Shuttling and a Rocking Molecular Machines

with Reversible Brake Function

Keiji Hirose and Yoshito Tobe

Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan

RWTH Aachen 11.07.2008

16

A Molecular Machine with Reversible Brake Function

Machines at molecular level are …

in perpetual Brownian motion.These motions have to be stopped effectively.

Our reversible brake systems works quantitatively in response to external photochemical and thermal stimuli. The rate of shuttling and rocking motion are proved to be reduced to less than 1% by reducing the size of ring component.