Salen complexes with bulky substituents as useful tools for biomimetic phenol oxidation research

Article abstract of DOI:10.1016/S0968-0896(01)00053-0

The catalytic properties of bulky water-soluble Co-, Cu-, Fe- and Mn-salen complexes in the oxidation of phenolic lignin model compounds have been studied in aqueous water-dioxane solutions (pH 3-10). Mn catalysts were found to oxidize coniferyl alcohol in a same reaction time as horseradish peroxidase (HRP) enzyme and Mn and Co catalysts showed different regioselectivity suggesting a different substrate to catalyst interaction in the oxidative coupling. When the oxidation of material more relevant to plant polyphenolics was studied, the results indicated that the complexes catalyze one- and two-electron oxidations depending on the bulk of the substrate. Copyright

Full text of DOI:10.1016/S0968-0896(01)00053-0

Bioorganic & Medicinal Chemistry 9 (2001) 1633–1638
Salen Complexes with Bulky Substituents as Useful
Tools for Biomimetic Phenol Oxidation Research
Anssi Haikarainen,a Jussi Sipila,a,* Pekka Pietikainen,a
Aarne Pajunenb and Ilpo Mutikainenb
aUniversity of Helsinki, Department of Chemistry, Laboratory of Organic Chemistry,
PO Box 55, FIN-00014 University of Helsinki, Finland
bUniversity of Helsinki, Department of Chemistry, Laboratory of Inorganic Chemistry,
PO Box 55, FIN-00014 University of Helsinki, Finland
Received 11 December 2000; accepted 22 February 2001
Abstract—The catalytic properties of bulky water-soluble Co-, Cu-, Fe- and Mn-salen complexes in the oxidation of phenolic lignin
model compounds have been studied in aqueous water–dioxane solutions (pH 3–10). Mn catalysts were found to oxidize coniferyl
alcohol in a same reaction time as horseradish peroxidase (HRP) enzyme and Mn and Co catalysts showed different regioselectivity
suggesting a different substrate to catalyst interaction in the oxidative coupling. When the oxidation of material more relevant to
plant polyphenolics was studied, the results indicated that the complexes catalyze one- and two-electron oxidations depending on
the bulk of the substrate. # 2001 Elsevier Science Ltd. All rights reserved.
Introduction
and aziridination5b of olefins and oxidation of sulphides
to sulfoxides.5b,6 Salen complexes have also been used in
medicinal studies for example as mimics for superoxide
dismutase7 and as DNA scission catalysts.8 One of the
great advantages of the salen catalysts is that the struc-
ture of the ligand as well as the coordinated metal ion
can be easily varied. For example, introducing chiral
ligands has provided possibilities for enantioselective
epoxidations5,6 and recently for asymmetric addition of
cyanide to aldehydes.9
Plant polyphenols constitute an important source of
renewable carbon in the biosphere. Although research
on the oxidative bioconversion of lignocellulosic mat-
erials has been intensive for decades, our knowledge of
either the relevant enzymes participating in such pro-
cesses or the oxidative mechanisms they induce is still
rather limited. At present it is known that at least three
enzymes are involved in biodegradation of lignin-like
materials, namely lignin peroxidases,1 manganese per-
oxidases2 and laccases.3 The two former ones are two-
electron oxidants using hydrogen peroxide whereas lac-
cases are one-electron oxidants and use dioxygen. Their
direct use in the mechanistical studies is, however,
problematic. They are difficult to isolate from natural
sources in pure form and in biological systems they also
seem to use mediators, such as Mn ions or veratryl alco-
hol, in the oxidative conversions. Therefore, more easily
obtainable and readily oxidizing catalysts, especially
HRP but also porphyrins, salens and phthalocyanines,
have commonly been used in plant phenolic research.4
In polyphenol oxidation research, most of the studies
involving salen-type catalysts have included the use of
structurally simple Co-salens in organic solvents and
oxidations are usually performed under an over-pres-
sure of dioxygen.10 The results have indicated that these
conditions lead to a significant formation of quinonoic
structures. In biomimetic work, however, the preferred
system consists of oxidation in aqueous conditions.
Only a few examples of water-soluble salen complexes
have been reported and they have mainly been used as
DNA cleavage catalysts.8,11
Metal salen compounds have been investigated as catalysts
for several different reactions, for example epoxidation5
Results
In the present work, we have investigated the catalytic
properties of salen complexes by chancing the metal
*Corresponding author. Tel.: +4-358-191-50380; fax: +4-358-191-
50366; e-mail: jussi.sipila@helsinki.fi
0968-0896/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(01)00053-0

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A. Haikarainen et al. / Bioorg. Med. Chem. 9 (2001) 1633–1638
ions in the active center and varying steric demand of
the ligand backbone.12 For this purpose a new set of
water-soluble salen complexes containing bulky sub-
stituents were prepared (1–8 in Fig. 1) and their reac-
tivity compared to that of unsubstituted salen
compounds 9 and 10. All new ligands and complexes
were characterized with 1H NMR, 13C NMR, elemental
analysis, MS (ESI-TOF) or crystal structure determina-
tion.13
hydrogen peroxide giving mixtures of products in about
1:1:1 b-0-4, b-5 and b-b coupling ratios (Table 1). The
ratio of the couplings obtained with unsubstituted salen
9 was notably different, about 1:2:3, whereas iron com-
plexes 3 and 7 as well as HRP gave values practically
the same as in bulky Mn complexes. The monitoring of
the disappearance of CA by TLC showed that Mn
complexes catalyzed oxidations proceeded as fast as
those catalyzed by HRP. In cases of the iron complexes,
the reactions were slightly slower and the unsubstituted
Mn catalyst 9 was clearly the least active. The 1-D-
NMR studies of the materials revealed that in all cases
the oxidation produced similar types of oligomeric
materials. The possible enantioselectivity of the cou-
pling was checked by using the chiral manganese com-
plex 8 and analyzing the dimeric b-5 product formed by
HPLC equipped with a chiral column. The product was
racemic.
The first reaction system studied consisted of a coniferyl
alcohol (CA) and hydrogen peroxide in 2:1 buffer–
dioxane solution at pH 3, and the activity of different
catalysts in CA oxidation was monitored by following
the disappearance of CA by TLC. In dilute solutions,
the system predominantly gives structures shown in
Figure 2. The limited number of reaction products
allows a convenient way to study the possible interac-
tions between the catalyst and the substrate simply by
following the regioselectivity of the coupling reactions
by 1- and 2-D-NMR.
The activities of cobalt complexes were studied using a
slight overpressure of dioxygen. In cases of bulky cobalt
complexes 1 and 5, the coupling ratios were about 1:2:3
and this was also the case with catalyst 10. The dis-
appearance of CA in case of complexes 1 and 5 was also
The results indicated that the bulky Mn complexes 4
and 8 catalyzed the oxidation of coniferyl alcohol with
Figure 1. Complexes 1–10 used as oxidation catalysts.
Figure 2. Coupling products obtained on the oxidation of coniferyl alcohol.

A. Haikarainen et al. / Bioorg. Med. Chem. 9 (2001) 1633–1638
1635
faster than in case of simple Co-salen 10. The copper
complexes 2 and 6 were found to be inactive with
dioxygen. When the oxidant was changed to hydrogen
peroxide, they slowly gave coupling products in a 1:1:1
ratio. With cobalt and copper complexes the material
obtained was mainly dimeric.
15. At pH 3, the reactivity pattern was the same but the
conversions were lower. When oxidation experiments
on 15 were performed using cobalt complexes 1, 5 or 10
no oxidation products were found, neither at pH 3 nor
at pH 10.
To mimic the effect of the axially coordinating histidine
residue found in the peroxidase enzymes,14 we also per-
formed the reactions in the presence of imidazole. Imi-
dazole is known to coordinate to the metal ion center of
the salen complexes yielding complexes of increased
activities in oxidations.15 We found that imidazole
accelerated the oxidation of CA with all Fe- and Mn-
complexes, but had no effect on product distribution
(Table 2). On the other hand, Co- and Cu-salen cata-
lyzed oxidations with imidazole gave back intact start-
ing material.
Discussion
Mn-salen catalyzed oxidations are known to occur
through a metal–oxo complex15,16 while cobalt com-
plexes activate dioxygen forming a 1:1 metal to oxygen
superoxo or a 2:1 metal to oxygen peroxo complexes as
the active oxidant.10a,17 The oxidative coupling of CA
catalyzed by simple manganese and cobalt salens (Table
1) show practically no differences, neither in b-0-4, b-5
and b-b coupling ratios nor in the rate of CA dis-
appearance. However, in cases of bulky Mn (4 and 8)
and Co catalysts (1 and 5) the regiochemistry of cou-
plings is markedly different to those observed with 9
and 10 whereas the rate of CA disappearance was again
about the same (Table 1). Interestingly, the oxidations
of CA catalyzed by bulky Mn, Cu and Fe complexes
show practically the same regiochemistry obtained with
HRP. Since HRP oxidations are assumed to proceed
without any special interaction between the phenoxy
radical and the catalyst during the oxidative coupling,18
it can be concluded that the bulky catalysts show similar
lack of interaction. On the other hand, the different
regiochemistry of couplings found with the unsub-
stituted salens 9 and 10 to that observed with bulky
catalysts (and with HRP) suggest a stronger interaction
with the phenoxy radicals formed from CA with the
catalysts, and that this interaction persists also in the
transition state of the coupling.
To examine the catalysts in the oxidation of the phe-
nolics representative for lignocellulosic material, we
then studied the oxidation of monomeric benzyl alco-
hols 11, 12 and a dimer 15 (Fig. 3) with complex 4,
which appeared to be the most active catalyst in CA
oxidation experiments. The reactions were first per-
formed in 1:1 water–methanol solution at pH 10. In the
oxidation of compounds 11 and 12, we found that they
were converted to 13 and 14, respectively, in a yield of
70%. When the substrate was changed to the more
bulky erythro-15, the reaction gave quantitatively a
diastereoisomeric mixture of compound 16, tetrameric
products formed in an oxidative coupling of the dimer
Table 1. Results of the oxidations of coniferyl alcohol
Catalyst Oxidant b-O-4a b-5a b-ba Reaction Polymerizationc
timeb
The beneficial effect of the added imidazole to the Fe-
and Mn-salen catalyzed H2O2 oxidations is a known
phenomenon.19 The partial crystal structure of the Mn
complex 17 with two imidazole ligands at apical posi-
tions suggests that the initially formed complex before
the addition of oxidant is of the octahedral type (Fig.
4).20 In the case of Co- and Cu-salen catalysts, on the
other hand, imidazole seem to form more stronger
octahedral complex and no oxidation is observed. The
result further suggest that this interaction of imidazole
is different than that of pyridine which is known to
facilitate the Co-salen catalyzed oxidations.10b
1 or 5
2 or 6
3 or 7
4 or 8
9
O2
1
1
1
1
1
1
1.5
23
1
1
1
23
3
1 h
Dimeric
Dimeric
Oligomeric
Oligomeric
Oligomeric
H2O2
H2O2
H2O2
H2O2
O2
1
1
1
3 d
2h
40 min
18 h
10
HRP
3
1
12h
1 h
Dimeric
Oligomeric
H2O2
1
aThe relative amounts of coupling products were determined by
NMR.
bDisappearance of coniferyl alcohol (TLC).
cThe degree of polymerization was estimated from the 13C NMR
spectra.
The oxidation of lignin models 11 and 12 yield the
respective carbonyl compounds 13 and 14 indicating
that in the case of simple benzyl alcohols the oxidation
gives two-electron oxidation products. The reactions
thus resemble 2,3-dichloro-5,6-dicyanobenzoquinone
(DDQ) oxidations of phenols, probably involving a
hydride abstraction from the benzylic position.21 One
possible reason for the different regioselectivity in oxi-
dations of 11 and CA may be that CA with a low oxi-
dation potential preferentially forms phenoxy radicals
prone to rapid coupling reactions. Interestingly, with a
dimeric erythro-15 the regioselectivity of the oxidation is
markedly different. The reaction gives now quantita-
tively a diastereoisomeric mixture of coupling product
Table 2. The results of the oxidation of coniferyl alcohol with added
imidazole
Catalyst
Oxidant
b-O-4a
b-5a
b-ba
Reaction timeb
c
1 or 5
2 or 6
3 or 7
4 or 8
9
O2
—
—
1
1
1
—
—
—
1
1
2
—
—
—
1
1
3
—
—
—
c
H2O2
H2O2
H2O2
H2O2
O2
30 min
30 min
1 h
c
10
—
aThe relative amounts of coupling products were determined using
NMR.
bDisappearance of coniferyl alcohol (TLC).
cNo reaction.

1636
A. Haikarainen et al. / Bioorg. Med. Chem. 9 (2001) 1633–1638
Figure 3. Benzylic alcohols and their oxidation products.
Figure 4. Crystal structure of the complex cation of the imidazole adduct 17.
16, not a a-carbonyl structure. The DDQ oxidation of
lignin models also forms these structures, however, as
minor products. The observed preferential formation of
coupling products 16 in salen catalyzed oxidation is
most probably due to the difficulties of Mn-salen cata-
lyst to form a complex with the bulky substrate that is
required for the hydride (or electron) abstraction from
the benzylic position.
77.1 ppm in 13C for CDCl3 and d 2.04 in 1H and
29.8 ppm in 13C NMR for acetone-D6) as references.
The mass spectra were recorded using PerSeptive Bio-
systems Mariner Biospectrometry Workstation ESI-
TOF spectrometer. HPLC analysis was carried out
using Waters 600 Controller and pump, Waters 996
Photodiode Array Detector and Chiralcel OF column.
TLC was conducted on precoated Merck Silica Gel 60
F254 plates and visualized with UV light and
molybdatophosphoric acid–Ce(SO4)2–H2SO4 with sub-
sequent heating at 120 ꢀC. Solvents were analytical or
HPLC grade. Dioxane was distilled from sodium before
use. Coniferyl alcohol was synthesized according to the
literature procedure.22 The syntheses of the bulky salen
complexes are described elsewhere.13 The unsubstituted
salen complexes were prepared using the literature
methods.23 Lignin model compound erythro-15 was
prepared according to the standard literature proce-
dure.24 The 13C NMR signals for acetylated b-O-4, b-5
and b-b structures used in product characterizations
were found at 80.1, 88.0 and 85.0 ppm, respectively.
Corresponding 1H NMR signals were found at 4.69,
5.51 and 4.75 ppm. The degree of the oxidation of the
benzyl alcohols 11 and 12 was calculated from 1H NMR
spectra by comparing the characteristic peaks of alde-
hyde protons and benzylic methylene protons. The dia-
stereoisomeric mixture of compound 16 has similar NMR
spectra when compared to 15, but the peaks are wider.
Peracetate of compound 16 gave an identical 13C NMR
spectrum to that reported earlier.25 The identification of
Conclusion
The present results demonstrate that bulky metal salen
compounds are useful biomimetic oxidation catalysts in
plant polyphenol research. Our experiments show that
the substrate to catalyst interaction may vary sub-
stantially between dioxygen and hydrogen peroxide
oxidations and that the bulk of the substrate affects
dramatically to the regioselectivity of the oxidation. The
results also demonstrate that the understanding of the
role of water in biomimetic oxidation systems seems to
be of the utmost importance.
Experimental
The 1- and 2-D NMR spectra were recorded on Varian
Gemini 2000 (200 MHz) and Varian Inova (300 MHz)
spectrometers in CDCl3 or acetone-D6 (for acetylated
1
16) using the residual solvent signals (d 7.27 in H and

A. Haikarainen et al. / Bioorg. Med. Chem. 9 (2001) 1633–1638
1637
16 was also based on the TLC analysis and comparison
with the authentic sample.26
temperature=193(2) K, wavelength=0.71073 A, crystal
system: monoclinic, space group: P2(1), unit cell dimen-
sions: a=13.017(8) A, b=12.631(6) A, c=48.72(3) A,
a=90ꢀ, b=98.01(5)ꢀ, g=90ꢀ, volume=7932(8) A3,
Z=4, absorption coefficient=0.353 mmÀ1, F(000)=3192.
General procedure for the oxidation of coniferyl alcohol
Coniferyl alcohol (180 mg, 1.0 mmol) was dissolved in
dioxane (1.5 mL). Into this mixture, the catalyst
(0.05 mmol, 5 mol%) and imidazole (34 mg, 0.5 mmol,
when used) was added followed by the introduction of
the buffer (3.5 mL, pH 3, 0.01 M citrate-phosphate).
When HRP was used, 5 mg was dissolved in the buffer
and added to the CA–dioxane solution. Hydrogen per-
oxide (1.0 mmol of 10% solution in the buffer) was then
added to the resulting mixture in three portions (15 min
intervals) or the mixture was stirred under an O2 atmo-
sphere. After the disappearance of coniferyl alcohol
(TLC monitoring, ethyl acetate as the eluent) the mix-
ture was diluted with water (10 mL) and extracted with
ethyl acetate (3Â15 mL). The combined extracts were
washed with 0.01 M HCl (10 mL) and brine (10 mL) and
dried with Na2SO4. The solvent was evaporated (recov-
eries in all cases over 90%) and the residue acetylated in
1:1 pyridine–acetic anhydride overnight. The acetylated
material was then analyzed by NMR. The degree of
polymerization was estimated from the signal widths of
Due to the limited quality of the data, only a partial
structure determination is given here. Bond angles and
lengths could not be determined, but the general con-
nectivity was clearly established.
Acknowledgements
We thank Technology Development Center of Finland
(TEKES) for financial support of this investigation.
References
1. (a) Tien, M.; Kirk, T. K. Science 1983, 221, 661. (b) Ham-
mel, K. E.; Jensen, K. A., Jr.; Mozuch, M. D.; Landucci, L. L.;
Tien, M.; Rease, E. A. J. Biol. Chem. 1993, 268, 12274.
2. (a) Wariishi, H.; Valli, K.; Gold, M. H. Biochem. Biophys.
Res. Commun. 1991, 176, 269. (b) Kishi, K.; Wariishi, H.;
Marquez, L.; Dunford, H. B.; Gold, M. H. Biochemistry 1994,
33, 8694.
3. (a) Morohoshi, N.; Wariishi, H.; Muraiso, C.; Nagai, T.;
Haraguchi, T. Mokuzai Gakkaishi 1987, 33, 218. (b) Ducros,
V.; Davies, G. J.; Lawson, D. M.; Wilson, K. S.; Brown, S. H.;
Ostergaard, P.; Pedersen, A. H.; Schneider, P.; Yaver, D. S.;
Brzozowski, A. M. Acta Cryst. D 1997, D53, 605.
1
the H and 13C NMR together with the information
obtained from TLC of the raw materials. HPLC analy-
sis of the non-acetylated material was carried out using
hexane–isopropanol (9:1) as an eluent, flow rate 1.0 mL
minÀ1. When the reactions using H2O2 or O2 were per-
formed without the catalyst, no oxidation of CA was
evident after 3 days.
4. (a) Joo, H.; Chae, H. J.; Yeo, J. S.; Yoo, Y. J. Process
Biochem. 1997, 32, 291. (b) Kurek, B.; Artaud, I.; Pollet, B.;
Lapierre, C.; Monties, B. J. Agric. Food Chem. 1996, 44, 1953.
(c) Okusa, K.; Miyakoshi, T.; Chen, C.-L. Holzforschung
1996, 50, 15. (d) Dordick, J. S.; Marletta, M. A.; Klibanov,
A. M. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 6255. (e) Kurek,
B.; Artaud, I.; Pollet, B.; Lapierre, C.; Monties, B. J. Agric.
Food Chem. 1996, 44, 1953. (f ) Matsumoto, M.; Kuroda, K.
Tetrahedron Lett. 1981, 22, 4437.
5. (a) Katsuki, T. Recent Res. Dev. Pure Appl. Chem. 1997, 1,
35. (b) Ito, Y. N.; Katsuki, T. Bull. Chem. Soc. Jpn. 1999, 72,
603. (c) Jacobsen, E. N. In Catalytic Asymmetric Synthesis;
Ojima, I., Ed.; Wiley-VCH: New York, 1993; pp 159–202.
6. (a) Nakajima, K.; Kojima, K.; Fujita, J. Chem. Lett. 1986,
1483. (b) Kokubo, C.; Katsuki, T. Tetrahedron 1996, 52,
13895. (c) Bunce, S.; Ronald, R. J.; Farrugia, L. J.; Kun-
chandy, S.; Meason, L. L.; Muir, K. W.; O’Donnel, M.; Pea-
cock, R. D.; Stirling, D.; Teat, S. J. Polyhedron 1998, 17, 4179.
7. (a) Henke, S. L. Expert Opin. Ther. Pat. 1999, 9, 169. (b)
Baker, K.; Marcus, C. B.; Huffman, K.; Kruk, H.; Malfroy,
B.; Doctrow, S. R. J. Pharmacol. Exp. Ther. 1998, 284, 215. (c)
Baudry, M.; Etienne, S.; Bruce, A.; Palucki, M.; Jacobsen, E.;
Malfroy, B. Biochem. Biophys. Res. Commun. 1993, 192, 964.
8. (a) Routier, S.; Vezin, H.; Lamour, E.; Bernier, J.-L.; Cat-
teau, J.-P.; Bailly, C. Nucleic Acids Res. 1999, 27, 4160. (b)
Kumbhar, A. S.; Damle, S. G.; Dasgupta, S. T.; Rane, S. Y.;
Kumbhar, A. S. J. Chem. Res., Synop. 1999, 98. (c) Sakamoto,
F.; Sumiya, T.; Fujita, M.; Tada, T.; Tan, X. S.; Suzuki, E.;
Okura, I.; Fujii, Y. Chem. Lett. 1998, 1127.
General procedure for the oxidation of benzyl alcohols
Substrate (1.0 mmol) and catalyst (0.05 mmol, 5 mol%)
were dissolved in 1:1 MeOH–buffer solution (10 mL, pH
10 or 3, 0.01 M carbonate or citrate-phosphate). To the
resulting mixture was added hydrogen peroxide
(3 mmol, 30% solution) in three portions at 10 min
intervals or the mixture was stirred under an O2 atmo-
sphere. After stirring overnight at room temperature,
the mixture was diluted with brine (20 mL) and extrac-
ted with ethyl acetate (3Â15 mL). The combined
extracts were washed with 0.01 M HCl (20 mL) and
brine (20 mL). After drying with Na2SO4 the solvent
was evaporated. Recoveries were always over 90%. The
residue was analyzed with NMR spectrometry.
X-ray crystallography
Preparation of 17. Compound 4 was dissolved in
CH2Cl2. A ten-fold excess of imidazole was added
resulting in the change of color from brown to greenish
brown. To this mixture was added a solution of an
excess of NH4PF6 in ethanol-isopropanol. The solution
was filtered and allowed to evaporate slowly at room
temperature. Crystals of the imÀidazole adduct
[Mn(C62H62N2O2P2)(C3H4N2)2]3+ 3PF6 2(C3H8O) 17
.
.
9. Belokon, Y. N.; Caveda-Cepas, S.; Green, B.; Ikonnikov,
N. S.; Khrustalev, V. N.; Larichev, V. S.; Moscalenko, M. A.;
North, M.; Orizu, C.; Tararov, V. I.; Tasinazzo, M.; Timo-
feeva, G. I.; Yashkina, L. V. J. Am. Chem. Soc. 1999, 121,
3968.
as a hexafluorophosphate salt were obtained after 2days.
Crystal data and structure refinement for 17.20 Empirical
formula C74H86F18MnN6O4P5, formula weight=1675.26,

1638
A. Haikarainen et al. / Bioorg. Med. Chem. 9 (2001) 1633–1638
10. (a) Bolzacchini, E.; Canevali, C.; Morazzoni, F.; Orlandi,
M.; Rindone, B.; Scotti, R. J. Chem. Soc., Dalton Trans. 1997,
4695. (b) Bozell, J. J.; Hames, B. R.; Dimmel, D. R. J. Org.
Chem. 1995, 60, 2398.
15. Srinivasan, K.; Michaud, P.; Kochi, J. K. J. Am. Chem.
Soc. 1986, 108, 2309.
16. Feichtinger, D.; Plattner, D. A. Angew. Chem., Int. Ed.
Engl. 1997, 36, 1718.
11. (a) Shearer, J. M.; Rokita, S. E. Bioorg. Med. Chem. Lett.
1999, 9, 501. (b) Cheng, C.-C.; Lu, Y.-L. J. Chin. Chem. Soc.
1998, 45, 611. (c) Tanaka, T.; Tsurutani, K.; Komatsu, A.; Ito,
T.; Iida, K.; Fujii, Y.; Nakano, Y.; Usui, Y.; Fukuda, Y.;
Chikira, M. Bull. Chem. Soc. Jpn. 1997, 70, 615. (d) Mandal,
S. S.; Kumar, N. V.; Varshney, U.; Bhattacharya, S. J. Inorg.
Biochem. 1996, 63, 265. (e) Routier, S.; Bernier, J.-L.; Waring,
M. J.; Colson, P.; Houssier, C.; Bailly, C. J. Org. Chem. 1996,
61, 23262. (f ) Botsvali, M.; Evans, D. F.; Missen, P. H.;
Upton, M. W. J. Chem. Soc., Dalton Trans. 1985, 1147. (g)
17. (a) Floriani, C.; Calderazzo, F. J. Chem. Soc. (A) 1969,
946. (b) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem.
Rev. 1979, 79, 139. (c) Punniyamurthy, T.; Bhatia, B.; Reddy,
M. M.; Maikap, G. C.; Iqbal, J. Tetrahedron 1997, 53, 7649.
(d) Deng, Y.; Busch, D. H. Inorg. Chem. 1995, 34, 6380.
18. Chioccara, F.; Polia, S.; Rindone, B.; Pilati, T.; Brunow,
G.; Pietikainen, P.; Setala, H. Acta Chem. Scand. 1993, 47,
610.
19. Pietikainen, P. Tetrahedron Lett. 1994, 35, 941.
20. To be published later.
Mukherjee, A. J.; Ra
ˆ
y, PJ.. Indian Chem. Soc. 1955, 32, 633.
21. Becker, H.-D.; Bjork, A.; Adler, E. J. Org. Chem. 1980,
45, 1596.
22. Quideau, S.; Ralph, J. J. Agric. Food Chem. 1992, 40, 1108.
23. (a) Pajunen, A.; Brunow, G.; Haikarainen, A.; Pietikai-
nen, P.; Sipila, J. Z. Kristallogr. 1998, 213, 441. (b) Diehl, H.;
Hach, C. C. Inorg. Synth. 1950, 3, 196.
12. Sipila, J.; Pietikainen, P.; Haikarainen, A.; Brunow, G.
Poster Presentations, 9th International Symposium on Wood
and Pulping Chemistry, Montreal, Canada, 9–12June 1997;
Marriot Chaˆ
1997; p105–1.
teau Champlain Montrael: Quebec, Canada,
´ ´
13. Haikarainen, A.; Sipila, J.; Pietikainen, P.; Pajunen, A.;
Mutikainen, I. Dalton Trans. 2001, 991.
14. English, A. M.; Tsaprailis, G. Adv. Inorg. Chem. 1995, 43,
79.
24. Sipila, J.; Syrjanen, K. Holzforschung 1995, 49, 325.
25. Landucci, L. L.; Ralph, S. A.; Hammel, K. E. Holz-
forschung 1998, 52, 160.
26. Pew, J. C.; Connors, W. J. J. Org. Chem. 1969, 34, 580.