C O M M U N I C A T I O N S
Scheme 1. Photochemical and Thermal Isomerizations for 3
of hours, is a major advance and offers perspectives for future
applications of these light-driven motors.
Supporting Information Available: Synthetic procedures and the
experimental and analytical details, spectral and kinetic data, X-ray
structural information (PDF) and the crystallographic information file
(CIF). This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) Special Issue of Sci. Am.: Nanotech: The Science of Small Gets Down
to Business, September 2001.
(2) Molecular Motors; Schliwa, M., Ed.; Wiley-VCH: Weinheim, Germany,
2003.
(3) (a) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem.,
Int. Ed. 2000, 39, 3349-3391. (b) Feringa, B. L.; van Delden, R. A.;
Koumura, N.; Geertsema, E. M. Chem. ReV. 2000, 100, 1789-1816. (c)
Brouwer, A. M.; Frochot, C.; Gatti, F. G.; Leigh, D. A.; Mottier, L.;
Paolucci, F.; Roffia, S.; Wurpel, G. W. H. Science 2001, 291, 2124-
2128. (d) Jime´nez, M. C.; Dietrich-Buchecker, C.; Sauvage, J. P. Angew.
Chem., Int. Ed. 2000, 39, 3284-3287.
(4) (a) Tashiro, K.; Konishi, K.; Aida, T. J. Am. Chem. Soc. 2000, 122, 7921-
7926. (b) Zheng, X. L.; Mulcahy, M. E.; Horinek, D.; Galeotti, F.;
Magnera, T. F.; Michl, J. J. Am. Chem. Soc. 2004, 126, 4540-4542. (c)
Joachim, C.; Gimzewski, J. K. Struct. Bonding 2001, 99, 1-18. (d) Jian,
H.; Tour, J. M. J. Org. Chem. 2003, 68, 5091-5103. (e) Dominguez, Z.;
Dang, H.; Strouse, M. J.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2002,
124, 2398-2399. For an extensive review on rotor systems, see: (f)
Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. ReV. 2005, 105,
1281-1376.
(5) For other unidirectional molecular motors, see: (a) Kelly, T. R.; de Silva,
H.; Silva, R. A. Nature 1999, 401, 150-152. (b) Leigh, D. A.; Wong, J.
K. Y.; Dehez, F.; Zerbetto, F. Nature 2003, 424, 174-179. (c) Fletcher,
S. P.; Dumur, F.; Pollard, M. M.; Feringa, B. L. Science 2005, 310, 80-
82.
define a full 360° rotary cycle of the upper half with respect to the
lower half, as shown in Scheme 1, a methoxy substituent was
introduced, and the rotation cycle of olefin 3 was followed with
1
low-temperature H NMR spectroscopy.
Upon irradiation of a racemic mixture of cis-3 (365 nm, 5 h,
-80 °C), new signals corresponding to the unstable trans isomer
appeared.11 The signals for the methyl substituent (doublets at 0.06
and 0.19 ppm), which adopts an axial orientation in the stable cis-3
isomer, shift downfield (0.67 and 0.82 ppm) as a result of the
equatorial orientation which it adopts in this unstable trans-3
isomer.17 Also, the signals for the methoxy group (singlets at 2.47
and 2.63 ppm) shifted to considerably lower field (2.92 and 3.22
ppm) due to the cis-to-trans isomerization. On standing for 30 min
at 20 °C in the dark, conversion of the unstable trans-3 to the
expected stable trans-3 isomer with an axial methyl group (0.13
and 0.18 ppm) was observed. Examination of the integrals reveals
that not all unstable trans-3 is converted to stable trans-3: notably,
20% is thermally converted back to stable cis-3. Similar experiments
starting with racemic trans-3 indicate a corresponding trans-to-cis
isomerization.11
(6) Koumura, N.; Zijlstra, R. W. J.; van Delden, R. A.; Harada, N.; Feringa,
B. L. Nature 1999, 401, 152-155.
(7) (a) Koumura, N.; Geertsema, E. M.; Meetsma, A.; Feringa, B. L. J. Am.
Chem. Soc. 2000, 122, 12005-12006. (b) Koumura, N.; Geertsema, E.
M.; van Gelder, M. B.; Meetsma, A.; Feringa, B. L. J. Am. Chem. Soc.
2002, 124, 5037-5051.
(8) ter Wiel, M. K. J.; van Delden, R. A.; Meetsma, A.; Feringa, B. L. J. Am.
Chem. Soc. 2003, 125, 15076-15086.
(9) Sandstro¨m, J. Top. Stereochem. 1983, 14, 83-181.
(10) The N-methylated analogue decomposed during the irradiation experi-
ments, most likely via photoinduced electron transfer from the amine to
the quinoid part of the molecule, giving radical-derived side products.
For a related process, see: Gan, H.; Whitten, D. G. J. Am. Chem. Soc.
1993, 115, 8031-8037.
(11) See Supporting Information.
(12) Bond length of the central olefin was 1.352 Å (comparable to other
motors).
When the thermal and photoisomerizations of enantiomerically
pure 3 (cis and trans) were investigated with UV/vis and CD
spectroscopy, results similar to those found for 2 were observed.11
The following activation parameters were determined: ∆qG° ) 84.7
(13) Assignment of the absolute configuration was based on comparison of
CD data with those of related compounds (see ref 7).
(14) The positive solvatochromism of the long-wavelength band of the unstable
isomer indicates intramolecular charge transfer nature for the corresponding
transition. Related push-pull stilbene systems show similar CT bands.
See: Meier, H.; Gerold, J.; Kolshorn, H.; Baumann, W.; Bletz, M. Angew.
Chem., Int. Ed. 2002, 41, 292-295.
kJ‚mol-1 (∆qH° ) 67.4 kJ‚mol-1, ∆qS° ) -59.1 J‚mol-1‚K-1
,
t1/2(20 °C) ) 124 s) for the conversion of unstable (2′R)-(P)-trans-3
(15) Quantum yield ) 0.12 (determined in the 5-10% range of photoisomer-
to the stable isomers, ∆qG° ) 85.1 kJ‚mol-1 (∆qH° ) 59.3
kJ‚mol-1, ∆qS° ) -88.1 J‚mol-1‚K-1, t1/2(20 °C) ) 173 s) for the
conversion of unstable (2′R)-(P)-cis-3 to the stable isomers.18
The new molecular motor is significantly faster than all previ-
ously described systems. The two photochemical and two thermal
isomerizations (Scheme 1) observed for 3 by low-temperature CD
and 1H NMR spectroscopy confirm the four-stage rotary cycle and
the unidirectionality of the 360° rotary motion for new motors 2
and 3. While an N-Boc-protected amine has a smaller electron-
donating capability compared to that of the N-alkyl group initially
proposed, it is clearly still significant,19 giving greater single bond
character to the rotational axis in the unstable form.20,21 It should
be emphasized that the observation that the thermal steps do not
result in exclusively a “forward” helix inversion, but also involve
a 20% “backward” cis-trans isomerization, leaves the overall
unidirectionality intact, as steps 2 and 4 are strictly unidirectional.22
Compared to earlier generation motors,6-8 the increase in speed,
which allows for full rotation at 20 °C at the scale of minutes instead
ization).
(16) For previous systems, the photoequilibria favor the unstable isomer.
Whereas the rate of rotation is almost solely determined by the thermal
steps, the “photon-efficiency” is affected, as it is governed by a
combination of quantum yield and photostationary state equilibrium.
(17) At low temperature, all signals of all isomers of 3 are split up, due to the
fact that the rotation in the carbamate moiety is frozen out.
(18) Corrected for the thermal “backward” isomerization, t1/2(20 °C) ) 155 s
for the (2′R)-(P)-trans-3 conversion via step 2, t1/2(20 °C) ) 217 s for
the (2′R)-(P)-cis-3 conversion via step 4, and t1/2(20 °C) ) 50 s for the
(2′R)-(P)-2 to (2′R)-(M)-2 conversion via the “forward” helix inversion.
(19) Hansch, C.; Leo, A.; Unger, S. H.; Kim, K. H.; Nikaitani, D.; Lien, E. J.
J. Med. Chem. 1973, 16, 1207-1216.
(20) Remarkably, the thermal barrier is significantly increased by the introduc-
tion of the methoxy substituent. A reasonable explanation for this
observation is that the methoxy group is in conjugation with the carbonyl,
thereby lowering its electron-withdrawing capabilities.
(21) An increased rate of the thermal steps was observed in more polar solvents,
in accordance with a dipolar transition state. In CD3OD, 58% of the
unstable trans isomer was converted back to stable cis.
(22) The thermal helix inversion of stable (2′R)-(M)-cis-3 to unstable (2′R)-
(P)-cis-3 and stable (2′R)-(M)-trans-3 to unstable (2′R)-(P)-trans-3 has
never been observed (by NMR or CD).
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