Photoresponsive molecular switch to control chemical fixation of CO2 [10]

Full text of DOI:10.1021/ja983960i

J. Am. Chem. Soc. 1999, 121, 2325-2326
2325
Scheme 1
Photoresponsive Molecular Switch to Control
Chemical Fixation of CO2
Hiroshi Sugimoto, Takayuki Kimura, and Shohei Inoue*
Department of Industrial Chemistry, Faculty of Engineering
Science UniVersity of Tokyo
Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
ReceiVed NoVember 17, 1998
Photocontrol of chemical or physical functions has attracted
continuous interest in connection with the photoresponsive
systems in nature, such as vision1 and photomorphogenesis in
plants.2 However, chemists trying to elucidate the mechanism of
photoresponsive natural systems have devoted their efforts to
investigate the physicochemical properties of light-absorbing
molecules, such as photochromism and electron-transfer, but have
hardly dealt with connection of these properties with biosynthetic
chemical reactions eventually affording the products of the
processes induced by photoresponsive molecules. On the other
hand, interests in artificial systems exhibiting functions in response
to light have increased, as exemplified by the photoregulated
interaction between photoresponsive hosts and guests.3 However,
there have been a few studies demonstrating reactions regulated
by photoresponsive molecules.4 Therefore, we have been prompted
to construct a light-driven switching-system, a “photoresponsive
molecular switch”, which acts as an on-off switch to start and
stop chemical reactions based on a novel concept. Herein we
report the photoresponsive molecular switch, consisting of
aluminum porphyrin and an olefin, to control the reaction of
carbon dioxide and an epoxide through the light-induced structural
change, photoisomerization, of the olefin.5
To realize a photoresponsive molecular switch, three parts, such
as reaction site, reaction control site, and photocontrol site, should
be included in the system. The combination of aluminum
porphyrin and stilbazole, a nitrogen-containing base, was chosen
as the components of a photoresponsive molecular switch (Scheme
1), since the photoswitchable complexation of a base to metal-
loporphyrin was expected to provide a step toward a light-driven
on-off switch to control chemical reactions on metalloporphyrins.
For the construction of the photoresponsive switch, stilbazole (2-
phenylethenylpyridine), a derivative of stilbene which is one of
the representative compounds related to the natural photorespon-
sive molecules, such as retinal, is very interesting, since it
undergoes isomerization from the trans-form to the cis-form upon
UV irradiation and reversion by visible light via the complexation
of the pyridine group to the metal center of metalloporphyrins.6
Actually, we have recently found that photoisomerizable 2-stil-
bazole coordinates to zinc tetraphenylporphyrin to different
extents, depending on the structure of the geometric isomers of
stilbazole due to steric repulsion between zinc porphyrin and
stilbazole.7 Furthermore, the complexation of zinc tetraphenylpor-
phyrin with 2-stilbazole is reversibly photoswitchable by UV and
visible lights through the trans-cis isomerization of the stilbazole.
On the other hand, complexation of metalloporphyrin with a
nitrogen-containing base is particularly important, since some
reactions on metalloporphyrins require the coordination of a base
to the central metal of metalloporphyrin, as observed both in
natural and artificial systems. For example, aluminum porphyrin
with an aluminum alkoxide group produces five-membered cyclic
carbonate by the catalytic reaction of carbon dioxide and epoxide
only when the nitrogen-containing base, such as 1-methylimida-
zole, coexisting in the reaction system coordinates to the metal
center of metalloporphyrin.8 Therefore, the aluminum porphyrin-
stilbazole system is expected to serve as a novel photoresponsive
molecular switch to control the reaction of carbon dioxide and
epoxide.
To establish the photoswitching of the reaction by the aluminum
porphyrin-stilbazole system as the photoresponsive molecular
switch through the photoisomerization of 2-stilbazole (Scheme
1), the rate of the reaction must differ to a large extent depending
upon the isomer structure of stilbazole. When the reaction was
carried out in the dark at 25 °C, the yields of the product in the
presence of cis- and trans-3′,5′-di-tert-butyl-2-stilbazole (2) were
different from each other enough to realize a photoswitching
system. In the case of using cis-2, which is expected to readily
coordinate to methoxyaluminum 5,10,15,20-tetraphenylporphine
((TPP)AlOMe, 1),9 the reaction of 1,2-epoxypropane (propylene
oxide, PO) (20 equiv with respect to 1) and CO2 by the 1-cis-2
(3 equiv with respect to 1) system took place as evidenced by
the appearance and increase of a peak at 1797 cm-1 due to the
CdO group of propylene carbonate (PC) in the periodically
measured IR spectra of the reaction mixture, where the yield of
(1) Langer, H., Ed.; Photochemistry and Physiology of Visual Pigments;
Springer-Verlag: Berlin, Heidelberg, and New York, 1973.
(2) Smith, H., Ed.; Phytochrome and photomorphogenesis; McGraw-Hill:
London, 1975.
1
PC, as estimated by H NMR, was 7 and 23% in 6 and 18 h,
respectively (Figure 1; 9).10 In the presence of trans-2, which is
(3) See, for example: (a) Shiga, M.; Takagi, M.; Ueno, K. Chem. Lett.
1980, 1021-1022. (b) Shinkai, S.; Nakaji, T.; Nishida, Y.; Ogawa, T.; Manabe,
O. J. Am. Chem. Soc. 1980, 102, 5860-5865. (c) Shinkai, S.; Minami, T.;
Kusano, Y.; Manabe, O. J. Am. Chem. Soc. 1983, 105, 1851-1856. (d)
Shinkai, S.; Minami, T.; Kusano, Y.; Manabe, O. Tetrahedron Lett. 1984,
23, 2581-2584. (e) Shinkai, S.; Ogawa, T.; Nakaji, T.; Kusano, Y.; Manabe,
O. Tetrahedron Lett. 1979, 47, 4569-4572. (f) Ueno, A.; Yoshimura, H.;
Saka, R.; Osa, T. J. Am. Chem. Soc. 1979, 101, 2779-2780. (g) Ueno, A.;
Saka, R.; Osa, T. Chem. Lett. 1979, 841-844.
(4) For reviews, see: Willner, I. Acc. Chem. Res. 1997, 30, 347-356.
Willner, I.; Rubin, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 367-385.
(5) A part of the present work was presented at the 1st International
Conference on Supramolecular Science and Technology, October, 1998,
Poland.
(7) Iseki, Y.; Inoue, S. J. Chem. Soc., Chem. Commun. 1994, 2577-2578.
(8) Aida, T.; Inoue, S. J. Am. Chem. Soc. 1983, 105, 1304-1309.
(9) To a round-bottomed flask (50 mL) equipped with a three-way stopcock
containing 5,10,15,20-tetraphenylporphyrin (TPPH2) (0.5 mmol), CH2Cl2 (20
mL) and Me3Al (0.048 mL, 1 equiv) were successively added by a hypodermic
syringe in a nitrogen stream, and the mixture was stirred for 1 h in a nitrogen
atmosphere at room temperature. The volatile fractions were removed from
the reaction mixture under reduced pressure to leave methylaluminum
tetraphenylporphyrin ((TPP)AlMe) as a purple powder. Methoxyaluminum
tetraphenylporphyrin ((TPP)AlOMe, 1) was prepared by the reaction of (TPP)-
AlMe with MeOH. To a CH2Cl2 (20 mL) solution of (TPP)AlMe (0.5 mmol),
MeOH (5 mL) was added by a syringe in a nitrogen stream, and the mixture
was stirred for 15 h in a nitrogen atmosphere at room temperature. The solvent
and unreacted MeOH were removed from the reaction mixture under reduced
pressure to leave 1 as a purple powder: Takeda, N.; Inoue, S. Bull. Chem.
Soc. Jpn. 1978, 51, 3564.
(6) Whitten, D. G.; Wildes, P. D.; DeRosier, C. A. J. Am. Chem. Soc. 1972,
94, 7811-7823.
10.1021/ja983960i CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/26/1999

2326 J. Am. Chem. Soc., Vol. 121, No. 10, 1999
Communications to the Editor
) 290-360 nm) at 25 °C for 18 h (total reaction time: 24 h),
which induces isomerization of 2 from the trans-form to the cis-
form, the reaction was gradually accelerated. The faster reaction
than that in the dark proceeded up to 16 and 30% yield of PC
after 18-h irradiation and the following 6-h in the dark, respec-
tively (Figure 1; O). This corresponds to about 10-folds accelera-
tion. This observation indicates that irradiation with UV light
induces the trans-to-cis isomerization of 2 and thus 2 coordinated
to 1 more easily than before irradiation resulting in the enhanced
reactivity of 1 with CO2 and PO to produce PC more efficiently.
The fact that the reaction continued at an accelerated speed even
after the UV irradiation was stopped indicates that the UV light
did not serve as the direct energy for acceleration, to confirm the
activation of 1 by coordination with 2. In sharp contrast, the
subsequent irradiation with visible light (λ > 380 nm), which
induces the cis-to-trans isomerization of 2, of the reaction system
for 18 h slowed the reaction, where the rate of reaction became
close to that in the presence of trans-2 in the dark.11
In conclusion, the present study demonstrated an interesting
novel example of photoresponsive molecular switch, the light-
driven on-off switching system for chemical reaction, composed
of aluminum porphyrin and stilbazole, which can regulate the
reaction of carbon dioxide and propylene oxide to give propylene
carbonate catalyzed by aluminum porphyrin in the presence of
stilbazole by the UV or visible light through the photoisomer-
ization of stilbazole. The present results provide a novel concept
for the design of photoresponsive systems to regulate the chemical
and physical functions by using light as information.
Figure 1. Reaction of carbon dioxide and propylene oxide in a (TPP)-
AlOMe (1)-3′,5′-di-tert-butyl-2-stilbazole (O) system, initial isomer
structure of 2 is the trans-form, under irradiation; (b) isomer structure
of 2 is the trans-form, in the dark; (9) isomer structure of 2 is the cis-
form, in the dark.
Acknowledgment. H.S. thanks NOVARTIS Foundation (Japan) for
the Promotion of Science, Kurata Memorial Foundation, and Nissan
Science Foundation for financial support.
JA983960I
(10) A typical procedure for the reaction of carbon dioxide (CO2) and 1,2-
epoxypropane (propylene oxide, PO) by the methoxyaluminum 5,10,15,20-
tetraphenylporphine (1)-3′,5′-di-tert-butyl-2-stilbazole (2) system was as
follows: trans-2 (3 mmol) was added to a 100-mL round-bottomed flask
attached to a three-way stopcock containing a CDCl3 solution (20 mL) of 1
(1 mmol) under dry CO2. With vigorous stirring, CO2 was passed through the
solution for 5 min, and then PO (20 mmol, 1.4 mL) was added to the solution
by a hypodermic syringe at 25 °C. A small amount of the reaction mixture
was periodically taken out by a syringe in a nitrogen stream and subjected to
expected to hardly coordinate to 1, under otherwise similar
conditions, the reaction of CO2 and PO with the 1- trans-2 system
proceeded much more slowly to attain only 1, 2, and 5% in 6,
18, and 30 h, respectively (Figure 1; b).
1
IR and H NMR measurements to estimate the yield of propylene carbonate
(PC) formed. Ushio LX-300, a 300-W Xe lamp, with Kenko HA-50 filter for
visible light (λ > 380 nm) or Kenko U-330 filter for UV light (λ ) 290-360
nm) was used for irradiation at 25 °C.
(11) Although the coordination of stilbazole (2) to methoxyaluminum
porphyrin (1) could not be estimated quantitatively by the spectral analyses,
the coordination of a related compound, chloroaluminum porphyrin [(TPP)-
AlCl] with cis-2, but not with trans-2, was confirmed by UV/vis spectrum.
Furthermore, the coordination of 2 to (TPP)AlCl was reversibly controlled
by UV and visible light irradiation.
As expected from the above results, the (TPP)AlOMe (1)-
stilbazole (2) system under the irradiation with UV light was
confirmed to serve as an on-switch to speed up the reaction, while
under the visible light irradiation the system worked as an off-
switch to reduce the rate of the once accelerated reaction. When
the reaction mixture of CO2 and PO with the 1-trans-2 system
after the 6-h reaction in the dark was irradiated with UV light (λ