8712
J. Am. Chem. Soc. 1997, 119, 8712-8713
Communications to the Editor
Upon visible light irradiation6 of the reaction mixture
containing cyclohexene (0.1 M), K2PtCl6 (5 × 10-4 M), and
[SbVTPP(OH)2]PF6 (3.3 × 10-5 and 1.0 × 10-5 M for the
irradiations at 550 and 420 nm, respectively) in acetonitrile-
water (2/1% (v/v)) for 60 min, K2PtCl6 completely disappeared,
while the porphyrin remained almost unchanged. The quantum
yield of disappearance of K2PtCl6 was 0.2, which was identical
with that of the oxygenated products. The major products were
cyclohex-2-enol (34%), 1,2-dichlorocyclohexane (36%), 2-chlo-
rocyclohexanol (cis, 12%; trans, 8%), cyclohexene oxide (2%),
and small amounts of 3,3′-bicyclohexenyl, 3-acetaminocyclo-
hexene, and cyclohexanone.7 Both K2PtCl6 and [SbVTPP(OH)2]-
PF6 were necessary for the photochemical oxygenation of
cyclohexene. ZnIITPP as a sensitizer instead of SbVTPP did
not afford any products. The net chemistry of reaction could
be expressed as in eqs 1-4.
Photochemical P-450 Oxygenation of Cyclohexene
with Water Sensitized by Dihydroxy-Coordinated
(Tetraphenylporphyrinato)antimony(V)
Hexafluorophosphate
Shinsuke Takagi, Minako Suzuki, Tsutomu Shiragami, and
Haruo Inoue*
Department of Industrial Chemistry, Graduate Course
of Engineering, Tokyo Metropolitan UniVersity
1-1 Minami-ohsawa, Hachiohji, Tokyo 192-03, Japan
ReceiVed April 30, 1997
Recently, many studies on water splitting, such as hydrogen
or oxygen evolution by visible light in relation to artificial
photosynthesis have been extensively reported.1 In particular,
highly efficient hydrogen evolution systems have been success-
fully achieved.2 Much attention is now shifted to the oxidation
of water and is focused on how a water molecule can be
incorporated into the oxidation terminal end of photoredox
systems.3,4 We have been examining photochemical redox
reaction systems that could actually incorporate a water molecule
upon visible light irradiation.4 An efficient photochemical
oxygenation reaction of cyclohexene sensitized by dimethoxy-
coordinated antimony(V) porphyrin [SbVTPP(OMe)2]Br has
already been reported.5 The key step of the reaction was a single
electron transfer to cyclohexene from the cation radical of [SbV-
TPP(OMe)2]Br generated by oxidative quenching of the triplet
porphyrin by K2PtCl6. Since K2PtCl6 was revealed to be an
efficient electron acceptor for excited SbVTPP, a substitution
of the axial methoxyl ligands by hydroxyl groups is expected
to generate another key intermediate metal-oxo complex for
the oxygenation reaction through deprotonation of the axial
hydroxyl group of the porphyrin cation radical.4 In this paper,
we report the photochemical oxygenation of cyclohexene
coupled with two-electron oxidative activation of water as an
axial porphyrin ligand. A metal-oxo type complex is supposed
to be a key intermediate in the oxygenation reaction which could
be called a “photochemical P-450 reaction”.
Light irradiation of the reaction mixture containing AgNO3
(5 × 10-3M) induced striking light scattering due to formation
of AgCl particles. The addition of AgNO3 suppressed the
formation of dichlorocyclohexane (15%) and enhanced the
formation of oxygenated products such as cyclohexene oxide
(6%) and cyclohexanone (34%). An experiment using H218O
revealed that an 18O atom was quantitatively incorporated into
cyclohex-2-enol, 2-chlorocyclohexanol, cyclohexene oxide, and
cyclohexanone under the degassed conditions. This clearly
indicates that a water molecule serves as the oxygen donor in
the photochemical oxygenation reaction. The turnover number
of the reaction based on the initial concentration of sensitizer
was 9.7, while the number based on the decomposed porphyrin
reached 5 × 102. The product distribution drastically changed
from that in the case of [SbVTPP(OMe)2]Br as a sensitizer.5
The oxygenation reaction sensitized by [SbVTPP(OMe)2]Br
afforded only cyclohex-2-enol and small amounts of 3,3′-
bicyclohexenyl and 3-acetaminocyclohexene, when the reaction
simply proceeded through cyclohexene cation radical formed
by hole transfer from the porphyrin cation radical upon oxidative
quenching of the excited triplet porphyrin by K2PtCl6 (hole
transfer mechanism, HTM).8 The results obtained above
obviously indicate that the oxygenation reaction sensitized by
dihydroxy-coordinated [SbVTPP(OH)2]PF6 proceeded by a
mechanism other than HTM. Higher formation of the cis than
the trans isomer of 2-chlorocyclohexanol suggests that some
stereoregulative process, such as reaction on the porphyrin plane,
was involved in the reaction. Since fluorescence of SbVTPP-
(1) (a) Kalyanasundaram, K.; Gratzel, M. Photosensitization and Pho-
tocatalysis Using Inorganic and Organometallic Compounds; Kluwer
Academic Publishers: Dordrecht, The Netherlands, 1993 and references
therein. (b) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141 and
references therein.
(2) (a) Lehn, J.-M.; Sauvage, J.-P. NouV. J. Chim. 1977, 1, 449. (b)
Moradpour, A.; Amouyal, E.; Keller, P.; Kagan, H. NouV. J. Chim. 1978,
2, 547. (c) Harriman, A.; Porter, G.; Richoux, M.-C., J. Chem. Soc. Trans.
1981, 2, 77, 833. (d) Johansen, O.; Mau, A. W. H.; Sass, W. H. F. Chem.
Phys. Lett. 1983, 94, 107, 113. (e) Gratzel, M. Energy Resources through
Photochemistry and Catalysis; Academic Press: New York, 1983.
(3) (a) Lehn, J.-M.; Sauvage, J.-P.; Ziessel, R. NouV. J. Chim. 1979, 3,
423. (b) Erbs, W.; Kiwi, J.; Gratzel, M. Chem. Phys. Lett. 1984, 110, 648.
(c) Erbs, W.; Desilvestro, J.; Borgarello, E.; Gratzel, M. J. Phys. Chem.
1984, 88, 4001. (d) Nahor, G. S.; Mosseri, S.; Neta, P.; Harriman, A. J.
Phys. Chem. 1988, 92, 4499. (e) Nahor, G. S.; Neta, P.; Hambright, P.;
Thompson, A. N., Jr.; Harriman, A. J. Phys. Chem. 1989, 93, 6181. (f)
Kaneko, M.; Yao, G.-J.; Kira, A. J. Chem. Soc., Chem. Commun. 1989,
1338. (g) Harriman, A. J. Photochem. Photobiol. A: Chem. 1990, 51, 41.
(h) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (i) Geselowitz, D.; Meyer,
T. J. Inorg. Chem. 1990, 29, 3894.
(6) The sample was irradiated with visible light (λ ) 420 or 550 nm
which correspond to Soret band or Q-band of SbVTPP) from a 500 W Xe
lamp (Ushio UXL 500DKO) through filters (420 nm, sharp cut filter Toshiba
L-39 + interference filter Toshiba KL-42; 550 nm, sharp cut filter Toshiba
Y-52 + interference filter Toshiba KL-55).
(4) (a) Inoue, H.; Sumitani, M.; Sekita, A.; Hida, M. J. Chem. Soc., Chem.
Commun. 1987, 1681. (b) Inoue, H.; Okamoto, T.; Kameo, Y.; Sumitani,
M.; Fujiwara, A.; Ishibashi, D.; Hida, M. J. Chem. Soc., Perkin Trans. 1
1994, 105. (c) Takagi, S.; Okamoto, T.; Shiragami, T.; Inoue, H. J. Org.
Chem. 1994, 59, 7373. (d) Okamoto, T.; Takagi, S.; Shiragami, T.; Inoue,
H. Chem. Lett. 1993, 687.
(7) The oxygenated products were analyzed by GC-MS (Shimadzu QP-
5000). Quantitative analysis was carried out by SIM (selected ion monitoring
method) detection mode. A GC column of TC-17 (GL Sciences Inc., 30
m) was used at temperatures 60-250 °C.
(5) Shiragami, T.; Kubomura, K.; Ishibashi, D.; Inoue, H. J. Am. Chem.
Soc. 1996, 118, 6311.
(8) Shono, T.; Ikeda, A. J. Am. Chem. Soc. 1972, 94, 7892.
S0002-7863(97)01371-1 CCC: $14.00 © 1997 American Chemical Society