J. Am. Chem. Soc. 1996, 118, 6311-6312
6311
phyrinatoantimony(V), ([SbVTPP(OCH3)2]Br), more efficiently
sensitizes the photooxygenation of cyclohexene in the presence
of both water as an oxygen donor and potassium hexachloro-
platinate(IV) (K2PtCl6) as an electron acceptor by visible light
irradiation.
Efficient Photochemical Oxygenation of
Cyclohexene with Water as an Oxygen Donor
Sensitized by Dimethoxy-Coordinated
Tetraphenylporphyrinatoantimony(V)
The photoreaction was performed by irradiating visible light
of λ ) 420 or 550 nm to an aqueous acetonitrile solution (33%
H2O) containing the SbVTPP12 (1.1 × 10-5 M), K2PtIVCl6 (5 ×
10-4 M) and cyclohexene (0.1 M) as typical conditions. The
oxygenation products were analyzed by GC-MS spectroscopy
(Shimadzu QP-5000). The practical detection limits of the
possible products were around 10-7 M.
Tsutomu Shiragami, Kyouichi Kubomura,
Daisuke Ishibashi, and Haruo Inoue*
Department of Industrial Chemistry
Faculty of Engineering
Tokyo Metropolitan UniVersity, 1-1 Minami-ohsawa
Hachiohji, Tokyo 192-03, Japan
The photoreaction induced formation of cyclohex-2-enol and
3,3′-bicyclohexenyl. Other oxygenation products such as cy-
clohexene oxide, cyclohexane-1,2-diol, and cyclohex-2-enone
were not detected at all in this system. The quantum yields for
the formation of cyclohex-2-enol and 3,3′-bicyclohexenyl under
the above conditions were 0.17 and 0.02, respectively. The
quantum yield was easily improved up to 0.42 for cyclohex-
2-enol formation by increasing the concentration of K2PtCl6 to
2 × 10-3 M as a practical solubility limit in the solvent system.
From the analysis of Stern-Volmer plots, the limiting quantum
yield was estimated to be 0.64 at 420 nm (Soret band of SbV-
TPP) irradiation for the formation of cyclohex-2-enol and 0.8
for the total oxidation of cyclohexene involving the dimer
formation. An experiment using H218O revealed that 18O atom
was quantitatively incorporated into cyclohex-2-enol under
strictly degassed conditions.13 This clearly indicates that water
molecule serves as an oxygen donor in the photochemical
oxygenation reaction. The decomposition of SbVTPP was
scarcely observed in the visible absorption spectrum during the
irradiation, indicating that the photoreaction proceeds catalyti-
cally. Turnover of the catalytic cycles based on SbVTPP was
more than 20 under the typical condition. The photooxygenation
did not proceed at all without K2PtIVCl6. Other porphyrins such
as zinc(II) tetraphenylporphyrin (ZnIITPP) or free base tetra-
phenylporphyrin also did not sensitize the reaction. Absorbance
at 320 nm due to K2PtCl6 (PtIV) disappeared completely during
the photoreaction, and no photooxygenation was observed when
K2PtCl4 (Pt(II)), a two-electron-reduced product from K2PtCl6,
was employed as an electron acceptor. The sum of quantum
yields for the formation of oxidation products (0.19) is almost
consistent with that for the disappearance of K2PtCl6 (0.17)
under the above typical conditions. This result strongly indicates
that the redox cycles at both the oxidative terminal end and the
reductive one are well balanced in the present case. The net
chemistry of the photoredox reaction in this system, thus, could
be expressed as eq 1.
ReceiVed January 3, 1996
We have found that visible light irradiation of a reaction
mixture of dimethoxy-coordinated tetraphenylporphyrinatoan-
timony(V) as a sensitizer, potassium hexachloroplatinate(IV)
as an electron acceptor, and cyclohexene in aqueous acetonitrile
induced formation of cyclohex-2-enol and 3,3′-bicyclohexenyl
with a high quantum yield of around 0.4. The limiting quantum
yield was estimated to be 0.8.
Photochemical redox reactions sensitized by metal complexes
have been extensively studied.1-5 Sensitizers with higher
oxidation potential are expected to incorporate more types of
electron donors such as phosphine, sulfide, alkene, and even
water molecules into the photoredox system.6,7 In general, high-
valent metalloporphyrins have been known to have high
oxidation potentials, though there have been few reports on their
photochemical behaviors.8-10 From these viewpoints, we have
recently focused our attention on some high-valent metallopor-
phyrins having antimony(V), phosphorus(V), germanium(IV),
and tin(IV) as a central metal atom and how oxidation reactions
could be coupled at the oxidation terminal end.7 We have
already reported photochemical epoxidation and oxygenation
of alkenes sensitized by antimony(V) porphyrins.7,11 The water
molecule serves as an oxygen donor in each reaction. In this
paper, we report that dimethoxy-coordinated tetraphenylpor-
(1) Kalyanasundaram, K.; Gra¨tzel, M. Photosensitization and Photoca-
talysis Using Inorganic and Organometallic Compounds; Kluwer Academic
Publishers: Dordrecht, The Netherlands, 1993.
(2) (a) Connolly, J. S. Photochemical ConVersion and Storage of Solar
Energy; Academic Press: London, 1981. (b) Kalyanasundaram, K. Pho-
toelectrochemistry, Photocatalysis and Photoreactors; Fundamentals and
DeVelopments; Schiarello, M., Ed.; Reidel: Dordrecht, The Netherlands,
1985; p 239 and references therein.
(3) (a) Lehn, J. M.; Sauvage, J. P. NouV. J. Chim. 1977, 1, 449. (b)
Moradpour, A.; Amoyal, E.; Keller, P.; Kagan, H. NouV. J. Chim. 1978, 2,
547. (c) Kalyanasundaram, K.; Kiwi, J.; Gra¨tzel, M. HelV. Chim. Acta 1978,
61, 2720. (d) Brown, G.; Brunschwig, B.; Creutz, C.; Endicott, J.; Sutin,
N. J. Am. Chem. Soc. 1979, 101, 1298. (d) DeLaive, P.; Sullivan, B.; Meyer,
T.; Whitten, D. J. Am. Chem. Soc. 1979, 101, 4007.
(4) (a) Haweker, J.; Lehn, J. M.; Ziessel, R. HelV. Chim. Acta. 1986,
69, 1990. (b) Willner, I.; Maidam, R.; Mander, D.; Durr, H.; Dorr, G.;
Zrangerele, K. J. Am. Chem. Soc. 1986, 109, 1009. (c) Ishida, H.; Tanaka,
K.; Tanaka, T. Chem. Lett. 1988, 339.
(5) (a) McLendon, G.; Miller, D. S. J. Chem. Soc., Chem. Commun. 1980,
553. (b) Kalyanasundram, K.; Gra¨tzel, M. HelV. Chim. Acta. 1980, 63, 478.
(c) Harriman, A.; Porter, G.; Richoux, M. C. J. Chem. Soc., Faraday Trans.
2 1981, 77, 833.
(6) Harriman, A.; Neta, P.; Richoux, M. C. Homogeneous and Hetero-
geneous Photocatalysis; Pelizzetti, E., Serpone, N., Eds.; Reidel: Dordrecht,
1986; p 123 and references cited therein.
(7) (a) Inoue, H.; Sumitani, M.; Sekita, A.; Hida, M. J. Chem. Soc., Chem.
Commun. 1987, 1681. (b) Inoue, H.; Okamoto, T.; Komiyama, M.; Hida,
M. J. Photochem. Photobiol. A. 1992, 65, 221. (c) Inoue, H.; Okamoto, T.;
Kameo, Y.; Sumitani, M.; Fujiwara, A.; Ishibashi, D.; Hida, M. J. Chem.
Soc. Perkin Trans. 1 1994, 105.
(8) Furhop, J. H.; Kadish, K. M.; Davis, D. G. J. Am. Chem. Soc. 1975,
95, 514.
(9) Hopf, F. R.; Whitten, D. G. Porphyrins and Metalloporphyrins; Smith,
K. M., Ed.; Elsevier: New York, 1975; p 667 and references cited therein.
(10) Inoue, H.; Chandrasekaran, K.; Whitten, D. G. J. Photochem. 1985,
30, 269.
(11) (a) Takagi, S.; Okamoto, T.; Shiragami, T.; Inoue, H. J. Org. Chem.
1994, 24, 7373. (b) Okamoto, T.; Takagi, S.; Shiragami, T.; Inoue, H. Chem.
Lett. 1993, 687.
The equation indicates that hydrochloric acid is generated
during the photooxygenation sensitized by SbVTPP. After light
(12) The sensitizer was synthesized by a reaction of antimony(III)
tribromide (SbIIIBr3) and free base tetraphenylporphyrin (H2TPP) in pyridine,
followed by oxidation to antimony(V) by bromine. The crude antimony
porphyrin was further refluxed in methanol to replace two axial ligands
with methoxy groups and then purified by alumina column chromatography
(eluent CH3CN:CH2Cl2 ) 20:1). The obtained porphyrin complex was
identified as the dimethoxy-coordinated species ([SbVTPP(OCH3)2]Br) by
UV-vis, FAB-MS, and 1H-NMR spectroscopies and elemental analysis.
[SbVTPP(OCH3)2]Br‚2H2O (yield 11%); λmax (CH3CN)/nm 418 (log ꢀ 5.72),
550 (4.26), and 590 (4.05); FAB-MS (M+) 795 and 797; 1H-NMR δH
(CDCl3)/ppm -2.18 (6H, s, -OCH3 (axial ligand)), 7.95-8.35 (20H, m,
benzene ring), and 9.57 (8H, s, pyrrole â); Anal. (C46H38N4O4SbBr) calcd
C, 60,92; H, 4.09; N, 6.14; found. C, 60.54; H, 4.20; N, 6.14.
(13) GC-MS SIM analysis indicated that 18O content in the obtained
cyclohex-2-enol was 91% after the photoreaction when 96% H218O was
used in the reaction mixture. This indicates that 95% of oxygen atom in
the product came from water molecules.
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