6708
J. Am. Chem. Soc. 2001, 123, 6708-6709
of the product (20% based on the dicopper complex) has precluded
the kinetic and mechanistic investigation on the reaction between
the peroxo intermediate and the phenolate.8 As such, the mech-
anism for the catechol formation via intermolecular reactions
between the peroxo intermediate and phenol derivatives has yet
to be clarified.10
We report herein that efficient conversion of phenol derivatives
to the corresponding catechols is achieved for the first time by
intermolecular reactions of a (µ-η2:η2-peroxo)dicopper(II) com-
plex, supported by tridentate ligand LPy2Bz (N,N-bis[2-(2-pyridyl)-
ethyl]-R,R-dideuteriobenzylamine),11 with lithium salts of phenols.
The mechanistic studies on the catechol formation have been
performed to provide valuable mechanistic insight into the
phenolase activity of the enzyme.
Treatment of the copper(I) complex, [CuI(LPy2Bz)](PF6), with
dioxygen in anhydrous acetone at -94 °C afforded a brown color
solution which exhibited a strong absorption band at 364 nm
(ꢀ ) 26400 M-1 cm-1) together with a small one at 530 nm (1500
M-1 cm-1) and a resonance Raman band at 737 cm-1 that shifted
to 697 cm-1 upon 18O-substitution.12,13 The frozen acetone solution
of the intermediate was ESR silent at 77 K, and a Cu:O2 ) 2:1
stoichiometry was obtained for formation of the intermediate by
manometry. These results unambiguously indicate that the
oxygenated intermediate is a (µ-η2:η2-peroxo)dicopper(II) com-
plex as suggested previously by Karlin et al.11 This compound is
quite stable (no self-decomposition) at the low-temperature
enabling us to examine the reaction with external substrates.
Addition of lithium salts of p-substituted phenols (p-X-C6H4-
OLi; X ) Cl, Me, and CO2Me) to the solution of the peroxo
complex resulted in a spectral change shown in Figure 1A, where
the absorption bands due to the peroxo species decreased obeying
pseudo-first-order kinetics (see the inset of Figure 1A).14 From
the final reaction mixture in a preparative-scale was obtained the
catechol derivatives in fairly good isolated yields (90, 69, and
60% based on the peroxo intermediate for X ) Cl, Me, and CO2-
Me, respectively), but neither the corresponding o-quinone
Oxygenation of Phenols to Catechols by A
(µ-η2:η2-Peroxo)dicopper(II) Complex: Mechanistic
Insight into the Phenolase Activity of Tyrosinase
Shinobu Itoh,*,† Hideyuki Kumei,‡ Masayasu Taki,‡
Shigenori Nagatomo,§ Teizo Kitagawa,*,§ and
Shunichi Fukuzumi*,‡
Department of Chemistry, Graduate School of Science
Osaka City UniVersity, 3-3-138, Sugimoto
Sumiyoshi-ku, Osaka, 558-8585, Japan
Department of Material and Life Science
Graduate School of Engineering, Osaka UniVersity
CREST, Japan Science and Technology Corporation
2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
Institute for Molecular Science
Myodaiji, Okazaki 444-8585, Japan
ReceiVed February 26, 2001
Tyrosinase is a copper monooxygenase that catalyzes oxygen-
ation of phenols to catechols (phenolase activity) and the
subsequent two-electron oxidation of catechols to the correspond-
ing o-quinones (catecholase activity).1 Chemical and spectroscopic
studies have indicated that the enzyme has a dinuclear copper
active site nearly identical to that found in hemocyanin,1,2 where
a side-on type (µ-η2:η2) peroxo species3 is generated by the
reaction of the reduced dicopper(I) form and O2.1 As a pioneering
work by Karlin and co-workers in Cu/O2 chemistry, aromatic
ligand hydroxylation in a dinuclear Cu(I) complex by O2 was
first reported in early 1980s.4 The mechanistic studies have
indicated that the aromatic ligand hydroxylation reaction involves
an electrophilic attack on the arene ring by a (µ-η2:η2-peroxo)-
dicopper(II) intermediate.5 After their finding, several examples
of the aromatic ligand hydroxylation have been reported using
similar type of m-xylyl dinucleating ligands.6 With respect to the
intermolecular reactions between phenols and the peroxo inter-
mediate, however, most of the reactions so far reported afford a
C-C coupling dimer as a major product.7 Casella and co-workers
have recently reported the first synthetic (µ-η2:η2-peroxo)dicopper-
(II) complex which can react with an exogenous phenolate to
yield the corresponding catechol.8,9 Unfortunately, the low yield
(8) Santagostini, L.; Gullotti, M.; Monzani, E.; Casella, L.; Dillinger, R.;
Tuczek, F. Chem. Eur. J. 2000, 6, 519-522.
(9) There have been also a number of reports that demonstrated the catechol
and/or o-quinone formation from phenol derivatives in the reaction with a
copper complex under aerobic conditions. However, no direct information
about the active oxygen intermediate was presented: (a) Capdevielle, P.;
Maumy, M. Tetrahedron Lett. 1982, 23, 1573-1576. (b) Capdevielle, P.;
Maumy, M. Tetrahedron Lett. 1982, 23, 1577-1580. (c) Re´glier, M.; Jorand,
C.; Waegell, B. J. Chem. Soc., Chem. Commun. 1990, 1752-1755. (d)
Chioccara, F.; Di Gennaro, P.; La Monica, G.; Sebastiano, R.; Rindone, B.
Tetrahedron 1991, 47, 4429-4434. (e) Casella, L.; Gullotti, M.; Radaelli,
R.; Gennaro, P. D. J. Chem. Soc., Chem. Commun. 1991, 1611-1612. (f)
Sayre, L. M.; Nadkarni, D. V. J. Am. Chem. Soc. 1994, 116, 3157-3158. (g)
Casella, L.; Monzani, E.; Gullotti, M.; Cavagnino, D.; Cerina, G.; Santagostini,
L.; Ugo, R. Inorg. Chem. 1996, 35, 7516-7525. (h) Mandal, S.; Macikenas,
D.; Protasiewicz, J. D.; Sayre, L. M. J. Org. Chem. 2000, 65, 4804-4809.
(10) Blackman, A. G.; Tolman, W. B. In Metal-Oxo and Metal-Peroxo
Species in Catalytic Oxidations; Meunier, B. Ed.; Springer: Berlin, 2000; pp
179-211.
† Osaka City University.
‡ Osaka University.
§ Institute for Molecular Science.
(1) Solomon, E. I.; Sundaram, U. M.; Machonkin, T. E. Chem. ReV. 1996,
96, 2563-2605.
(2) (a) Magnus, K. A.; Hazes, B.; Ton-That, H.; Bonaventura, C.;
Bonaventura, J.; Hol, W. G. J. Proteins: Struct., Funct., Genet. 1994, 19,
302-309. (b) Magnus, K. A.; Ton-That, H.; Carpenter, J. E. Chem. ReV. 1994,
94, 727-735.
(3) Kitajima, N.; Fujisawa, K.; Moro-oka, Y. J. Am. Chem. Soc. 1989, 111,
8975-8976.
(4) (a) Karlin, K. D.; Dahlstrom, P. L.; Cozzette, S. N.; Scensny, P. M.;
Zubieta, J. J. Chem. Soc., Chem. Commun. 1981, 881-882. (b) Karlin, K.
D.; Hayes, J. C.; Gultneh, Y.; Cruse, R. W.; McKown, J. W.; Hutchinson, J.
P.; Zubieta, J. J. Am. Chem. Soc. 1984, 106, 2121-2128.
(5) (a) Nasir, M. S.; Cohen, B. I.; Karlin, K. D. J. Am. Chem. Soc. 1992,
114, 2482-2494. (b) Karlin, K. D.; Nasir, M. S.; Cohen, B. I.; Cruse, R. W.;
Kaderli, S.; Zuberbu¨hler, A. D. J. Am. Chem. Soc. 1994, 116, 1324-1336.
(c) Pidcock, E.; Obias, H. V.; Zhang, C. X.; Karlin, K. D.; Solomon, E. I. J.
Am. Chem. Soc. 1998, 120, 7841-7847.
(11) Sanyal, I.; Mahroof-Tahir, M.; Nasir, M. S.; Ghosh, P.; Cohen, B. I.;
Gultneh, Y.; Cruse, R. W.; Farooq, A.; Karlin, K. D.; Liu, S.; Zubieta, J.
Inorg. Chem. 1992, 31, 4322-4332.
(12) When excited at 406.7 nm, the Raman band at 581 cm-1 (553 cm-1
upon 18O-substitution) due to a bis(µ-oxo)dicopper(III) complex was also
detected in addition to the Raman band of the peroxo intermediate [νO-O
18
(16O2) ) 737 cm-1, ν
( O2) ) 697 cm-1]. Those Raman bands due to the
O-O
(6) (a) Kitajima, N.; Moro-oka, Y. Chem. ReV. 1994, 94, 737-757. (b)
Mahapatra, S.; Kaderli, S.; Llobet, A.; Neuhold, Y.-M.; Palanche´, T.; Halfen,
J. A.; Young, V. G., Jr.; Kaden, T. A.; Que, L., Jr.; Zuberbu¨hler, A. D.;
Tolman, W. B. Inorg. Chem. 1997, 36, 6343-6356 and references therein.
(7) (a) Kitajima, N.; Koda, T.; Iwata, Y.; Moro-oka, Y. J. Am. Chem. Soc.
1990, 112, 8833-8839. (b) Paul, P. P.; Tyekla´r, Z.; Jacobson, R. R.; Karlin,
K. D. J. Am. Chem. Soc. 1991, 113, 5322-5332. (c) Obias, H. V.; Lin, Y.;
Murthy, N. N.; Pidcock, E.; Solomon, E. I.; Ralle, M.; Blackburn, N. J.;
Neuhold, Y.-M.; Zuberbu¨hler, A. D.; Karlin, K. D. J. Am. Chem. Soc. 1998,
120, 12960-12961. (d) Halfen, J. A.; Young, V. G. Jr.; Tolman, W. B. Inorg.
Chem. 1998, 37, 2102-2103. (e) Mahadevan, V.; Henson, M. J.; Solomon,
E. I.; Stack, T. D. P. J. Am. Chem. Soc. 2000, 122, 10249-10250.
bis(µ-oxo)dicopper(III) complex became negligibly smaller when the solution
was excited at 514.5 nm. According to the Solomon’s paper on a similar
tridentate ligand system (Pidcock, E.; DeBeer, S.; Obias, H. V.; Hedman, B.;
Hodgson, K. O.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 1999, 121,
1870-1878), the content of the bis(µ-oxo) species in solution may be less
than a few percent.
(13) See Supporting Information.
(14) To prevent side reactions (autoxidation of the products), excess
O2 was completely removed by bubbling Ar gas into the solution of the
(µ-η2:η2-peroxo)dicopper(II) complex for 15 min before addition of the
phenolate substrates. During the bubbling of Ar gas, no spectral change of
the peroxo complex was observed at -94 °C.
10.1021/ja015702i CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/13/2001