Methyltrioxorhenium: A new catalyst for the activation of hydrogen peroxide to the oxidation of lignin and lignin model compounds

Article abstract of DOI:10.1016/j.bmc.2005.01.049

The oxidative degradation of lignin under totally chlorine free conditions is one of the most relevant targets for the design of environmental friendly pulping and bleaching industrial processes. Methyltrioxorhenium was found a powerful and promising catalyst for the oxidation of both phenolic and non-phenolic lignin model compounds by use of hydrogen peroxide as primary oxidant. Three different technical lignins, hydrolytic sugar cane lignin (SCL), red spruce kraft lignin (RSL) and a hardwood organosolvent lignin (OSL), that are representative examples of widely diffused para-hydroxyphenyl-guaiacyl, guaiacyl and guaiacyl-syringyl lignins, were also extensively degraded under similar experimental conditions.

Full text of DOI:10.1016/j.bmc.2005.01.049

Bioorganic & Medicinal Chemistry 13 (2005) 2569–2578
Methyltrioxorhenium: a new catalyst for the activation of
hydrogen peroxide to the oxidation of lignin and
lignin model compounds
Claudia Crestini,a,* Paola Pro,b Veronica Nerib and Raffaele Saladinob,*
a
`
Dipartimento di Scienze e Tecnologie Chimiche, Universita di Tor Vergata, Via della ricerca Scientifica, 00133 Rome, Italy
b
`
Dipartimento di Agrobiologia e Agrochimica, Universita della Tuscia, Via San Camillo de Lellis, 01100 Viterbo, Italy
Received 4 May 2004; accepted 21 January 2005
Abstract—The oxidative degradation of lignin under totally chlorine free conditions is one of the most relevant targets for the design
of environmental friendly pulping and bleaching industrial processes. Methyltrioxorhenium was found a powerful and promising
catalyst for the oxidation of both phenolic and non-phenolic lignin model compounds by use of hydrogen peroxide as primary
oxidant. Three different technical lignins, hydrolytic sugar cane lignin (SCL), red spruce kraft lignin (RSL) and a hardwood organo-
solvent lignin (OSL), that are representative examples of widely diffused para-hydroxyphenyl–guaiacyl, guaiacyl and guaiacyl–
syringyl lignins, were also extensively degraded under similar experimental conditions.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
compounds,6 phosphines,7 Bayer–Villiger rearrange-
ment8 and oxygen insertion into C–H bonds.9 Accord-
ingly with this high reactivity, MTO is able to catalyze
the oxidation of aromatic derivatives.10 In this latter
case the reaction proceeds through the formation of
reactive arene epoxide intermediates that further rear-
range and are oxidized to corresponding benzoquinones.
para-Benzoquinones are usually obtained by oxidation
of alkyl phenols that lack of substituents on the C-4 po-
sition of the aromatic ring. In the presence of bulky sub-
stituents, as in the case of cardanols (3-n-pentadecyl
phenol derivatives), ortho-benzoquinones can be ob-
tained in high yields because of steric hindrance to the
approach of the catalyst to the aromatic ring.11 Alk-
oxyl-substituted benzenes are less reactive than the cor-
responding phenolic derivatives towards MTO.12 To
date, a concerted mechanism for the oxygen transfer
from MTO monoperoxo and diperoxo intermediates
by a ÔbutterflyÕ transition state is the accepted reaction
pathway for MTO, while radical processes have not
been reported.4 To the best of our knowledge, there
are no reports in the literature dealing with the use of
MTO in the oxidation of polymeric phenolic derivatives
like lignin. With the aim to design new catalytic systems
for environmental friendly pulping and bleaching pro-
cesses, we started to study the use of MTO in the oxida-
tive degradation of lignin and lignin model compounds
with H2O2 as primary oxidant. We selected an array
In the paper production processes, environmental con-
cerns prompted to design pulping and bleaching se-
quences avoiding the use of chlorinated compounds.
Totally chlorine free (TCF) processes have been devel-
oped, as for example by the use of oxygen, hydrogen
peroxide (H2O2) and ozone as oxidants.1 However their
major drawback consists in a lack of selectivity in the
oxidation of lignin, which leads to the partial degrada-
tion of the cellulose contained in pulps, and ultimately
in a lower final product yield. This lack of selectivity is
mainly due to the formation of radical intermediates,
such as hydroxyl radicals that are able to attack both
cellulose and lignin.2 Selective catalytic processes based
on the activation of environmental friendly H2O2 might
solve these problems.
A novel catalyst potentially useful for this purpose is
methyltrioxorhenium (VII) (MeReO3, MTO).3 MTO,
in combination with H2O2, has became in recent years
an important catalyst for a variety of synthetic transfor-
mations, such as oxidation of olefins,4 alkynes,5 sulfur
*
Corresponding authors. Tel.: +39 06 7259 4734; fax: +39 06 7259
4328; e-mail addresses: crestini@uniroma2.it; saladino@unitus.it
0968-0896/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2005.01.049

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C. Crestini et al. / Bioorg. Med. Chem. 13 (2005) 2569–2578
of lignin model compounds resembling the main bond-
ing patterns in native and technical lignins and studied
their reactivity with MTO. Our attention was next
turned to more complex lignin polymers, hydrolytic su-
gar cane lignin (SCL), red spruce kraft lignin (RSL) and
hardwood organosolvent lignin (OSL), that are repre-
sentative examples of widely diffused para-hydroxyphen-
yl–guaiacyl, guaiacyl and guaiacyl–syringyl lignins.
Their oxidation was studied by means of advanced 31P
NMR techniques that allow the quantitative determina-
tion of all labile OH groups on the polymer—that is, ali-
phatic, different phenolic OH groups, carboxylic acids—
after phosphitylation of the sample.13,14
(5.0 mL) at room temperature during 6.0 h. Acetic acid
was selected as solvent because MTO is reported to dis-
play in this solvent its maximum catalytic activity.6 Irre-
spective from the experimental conditions used for the
oxidation, a low mass-balance with respect to isolated
products was observed showing an high efficiency for
the reaction. In accordance with results previously re-
ported in the literature, the loss of material from the
reaction mixture might be due to formation of polar
hydrophilic over-oxidation products not recovered by
usual work-up procedures.15
Reaction products were characterized by means of gas
chromatography–mass spectroscopy (GC–MS) analysis
after derivatization with bis(trimethylsilyltrifluoroacet-
MTO showed to be a powerful and efficient catalyst for
the oxidation of phenolic and non-phenolic lignin model
compounds affording both side chain oxidations and
aromatic ring cleavage reactions. Diphenylmethane
models that are usually recalcitrant to oxidation, were
also found extensively degraded mainly by cleavage of
the interunit bonding. MTO was also able to catalyze
the extensive degradation of all technical lignins,
increasing their degree of solubility and reducing the
content in condensed subunits.
1
amide) (BSTFA) and, when necessary by H and 13C
NMR determinations. In the absence of the catalyst less
than 5% conversion of substrate took place under other-
wise identical experimental conditions.
The simplest lignin model compound studied was vanil-
lyl alcohol 1. The oxidation of 1 proceeded with >95%
conversion of substrate to give products of alkyl side-
chain oxidation, vanillin 2 and 4-hydroxy-3-methoxy-
benzoic acid 3, the muconolactone 4 and the para-
benzoquinone derivative 5 (Scheme 1, here and else-
where products yields are reported in parentheses).
2. Results
2.1. Oxidation of lignin model compounds
The presence of compounds 2 and 3 are in accordance
with data previously reported on the reactivity of simple
aromatic hydrocarbons, such as toluene, with H2O2 and
MTO, in which case the oxidation of the benzylic group
was the main observed process. Muconolactone 4 was
obtained by an extensive oxidative ring opening of 1
to a muconic acid intermediate (not recovered under
our experimental conditions) followed by formation of
the lactone moiety. Muconolactone derivatives have
been previously recovered as minor products in the oxi-
dative degradation of lignin model compounds.16 The
isolation of the para-benzoquinone 5 is in accordance
with the known reactivity of methoxybenzenes with
MTO.12 In this latter case, the reaction proceeds by ini-
tial oxygen atom insertion on the para-position with re-
spect to the methoxy substituent followed by
demethylation and formation of the quinone moiety.
A similar behaviour was observed in the oxidation of
the non-phenolic model compound 6. Upon treatment
with H2O2 and MTO a quantitative conversion of sub-
strate was observed (>98%) and products of alkyl side-
chain oxidation, 3,4-dimethoxybenzaldehyde 7 and 3,4-
The reactivity of aromatic model compounds resembling
the most representative lignin bonding patterns is of
pivotal interest in order to rationalize the oxidative
behaviour of the polymer. Thus, an array of monomeric
and dimeric, phenolic and non-phenolic lignin model
compounds was selected in order to clarify the reactivity
of lignin subunits with H2O2 and MTO as
catalyst.
Vanillyl alcohol 1, veratryl alcohol 6, 1-(4-hydroxy-3-
methoxyphenyl)-2-(2,6-dimethoxyphenoxy)propane-1,3-
diol 10, 1-(4-ethoxy-3-methoxyphenyl)-2-(2,6-dimeth-
oxyphenoxy)propane-1,3-diol 14, 1-(4-ethoxy-3-meth-
oxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol 23,
2,20,3,30-tetramethoxy-5,50-dimethyl-diphenyl methane
28 and 2,20-dihydroxy-3,30-dimethoxy-5,50-dimethyl-di-
phenyl methane 35 were studied as substrates. Reactions
were carried out treating the appropriate substrate
(1.0 mmol) with H2O2 (35% aqueous solution,
1.5 mmol) and MTO (1.0% w/w) in CH3COOH
CHO
COOH
CH2OH
CH2OH
O
O
O
MTO/H2O2
AcOH
OCH3
OCH3
OCH3
O
COOR
OR
OR
OR
OH
1, 6
4 (1.8), 9 (1.2)
5 (1.4)
2 (3.7), 7 (3.4)
3 (4.9), 8 (3.9)
1,2,3,4
R=H
6,7,8,9 R=CH3
Scheme 1.

C. Crestini et al. / Bioorg. Med. Chem. 13 (2005) 2569–2578
2571
H3CO
HO
γ
OH
O
O
CH2OH
O
B
O
OH
HO
O
α
β
MTO/H2O2
AcOH
H3CO
OCH3
H3CO
HO
A
OCH3
OCH3
OCH3
O
OH
OH
OH
3 (16.3)
13 (9.3)
11 (14.9)
12 (11.9)
10
Scheme 2.
dimethoxybenzoic acid (veratric acid) 8, and the mucon-
olactone 9 were obtained (Scheme 1).
patterns (Scheme 3). Even if non-phenolic lignin model
compounds are usually less reactive than the corre-
sponding phenolic ones,12 in the presence of MTO a
high conversion of substrate was obtained in both cases
(>95%). The GS-lignin model compound 14 yielded
products of alkyl side-chain oxidation and cleavage at
the b-position, compound 15, followed in some cases
by dealkylation of the alkyl–aryl ether moieties, as in
cathecols 16–18, or by fragmentation at a-position and
partial hydroxylation of the aromatic ring, compounds
19 and 20 (Scheme 3).
Our attention was next focused on the oxidation of di-
meric lignin model compounds. The b-O-4 dimeric com-
pound 10 represents the most diffuse bonding pattern in
hardwood lignins (guaiacyl–syringyl lignins, GS-lig-
nins). The oxidation of compound 10 showed an exten-
sive degradation of the substrate (>98% conversion) and
a reactivity pattern close to that observed for 1 (Scheme
2).
Low amount of products of alkyl side-chain oxidation
and successive cleavage at the a-position, 4-hydroxy-3-
methoxybenzoic acid 3, or at the b-position, the hydroxy-
ketone 11 (deriving from the aromatic A-ring of the
substrate) and 2,6-dimethoxyphenol 12 (deriving from
the aromatic B-ring of the substrate) were recovered in
the reaction mixture. The muconolactone 13 was also
recovered in appreciable yield. However, a high molecu-
lar weight (>550) compound, probably a trimer or tetra-
mer, that showed in GC–MS a molecular peak fragment
corresponding to benzyl alcohol substructure, could not
be unambigously identified (see next).
On the basis of the reactivity showed by MTO in the
oxidation of anisole derivatives,6 it is reasonable to
suggest that the para-benzohydroquinone 20 can be
formed by initial oxidation of the aromatic ring to a
reactive arene oxide. Successive acidic rearrangement
of this intermediate to a c-hydroxy cyclohexadiene,
followed by hydrolytic C-a side-chain degradation, deal-
kylation and hydroxylation on the activated para-posi-
tion (with respect to the methoxy substituent) afforded
20. Supporting evidence for this hypothesis is provided
by previously reported results on the oxidation of
b-O-4 non-phenolic lignin model compounds with
dimethyldioxirane (DMD). DMD is a stoichiometric
oxidant characterized by a butterfly diperoxo like
transition state similar to that observed for MTO
peroxo intermediates in the oxidation of arenes.17 In
this latter case a para-benzohydroquinone derivative,
The oxidation of compounds 14 and 23 that are hard-
wood (guaiacyl–syringyl lignin, GS-lignin) and soft-
wood
(guayacyl)
non-phenolic
lignin
model
compounds, respectively, showed interesting reactivity
R4
OH
HO
HO
OH
O
R3
OR2
OH
OR1
OH
15 (5.6): R1=C2H5, R2=CH3, R3=H, R4=CH2OH
16 (2.2): R1=R2=R3=H, R4=CH2OH
18 (3.7): R1=R4=H, R2=CH3, R3=OH
CH3
O
17 (55.4)
HO
HO
O
MTO/H2O2
AcOH
O
CH3
COOH
OH
OH
OH
HO
HO
HO
OCH3
OC2H5
CH3
HO
OMe
OC2H5
O
COOH
CO2C2H5
COOH
COOH
14
19 (4.7)
21 (3.3)
22(4.0)
20 (5.8)
Scheme 3.

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C. Crestini et al. / Bioorg. Med. Chem. 13 (2005) 2569–2578
2-methoxy-1,4-dihydroxy benzene, similar to 20, was
recovered in the reaction mixture.18 Reactive cyclohexa-
dienyl intermediates were identified by electrospray ion-
ization mass spectrometry during the biomimetic one-
In order to further investigate the structure of the high
molecular weight products produced during the oxida-
tion of b-O-4 lignin model compounds, the LC–MS
analysis of the high molecular weight product from
model 14 was performed. The molecular ion peak
M = 618 showed a value corresponding to twice the
molecular weight of substrate with loss of a water mol-
ecule. In accordance with the GC–MS molecular ion
fragmentation, and since MTO is considered to catalyze
oxidation with H2O2 without formation of radical spe-
cies, this tetramer was tentatively identified as a product
of nucleophilic addition of one of the free hydroxy
groups on the side-chain of 14 on the benzylic cation
derived by acidolysis of a second molecule of substrate.
This hypothesis is in accord with the above reported
presence of other products from the acidolytic
pathway.
electron oxidations of b-O-4 non-phenolic lignin model
compounds with heteropolyanion [PMo7V5O40]ꢀ8
.
It
19
is interesting to note that 20 was previously obtained
by reaction of vanillin with the UV-alkaline peroxide
system. In this latter case, the addition of the hydroper-
oxide anion by a Dakin transformation was suggested as
a plausible reaction pathway.20 Compound 19 is proba-
bly an over-oxidized derivative of initially formed 4-eth-
oxy-3-methoxybenzaldehyde. Benzaldehyde derivatives
such as 3-methoxy-4-hydroxybenzaldehyde (vanillin)
and 3,4-dimethoxybenzaldehyde (veratraldehyde) are
typical products of biomimetic degradation of b-O-4
phenolic and non-phenolic lignin model compounds
with porphyrins resembling the active site of lignin per-
oxidases and manganese dependent peroxidases.21
Diphenylmethane subunits are formed during lignin pul-
ping processes, their amounts varies upon the specific
wood species and pulping procedure used.24 Diph-
enylmethanes present in lignin show high recalcitrance
to oxidation and their efficient degradation requires se-
vere reaction conditions in bleaching processes.18 In
order to study the reactivity of such lignin subunits
towards the H2O2/MTO system two diphenylmethane
lignin model compounds 28 and 35 were oxidized. The
oxidation of 28 performed under previously described
experimental conditions gave products showing a differ-
ent degree of oxidation at the benzylic positions, com-
pounds 29, 30 and 31, alkyl side-chain oxidation and
cleavage of the diphenylmethane bridge, compound 32,
or deriving from the oxidative cleavage of one of the
aromatic ring as showed by the presence of a two carbon
atom alkyl side-chain, compounds 33 and 34 (Scheme
5).
Products of alkyl side-chain oxidation and aromatic ring
cleavage, the muconic acid derivatives 21 and 22, were
also recovered in the reaction mixture. Compound 22 that
shows apparently a reduced structure, was probably ob-
tained by an internal redox process. Signals referring
to reduced lignin subunits were previously observed in
1
the H and 13C NMR spectra of extensively oxidized
lignins.22 In a similar way, the oxidation of compound
23 afforded products of alkyl side-chain oxidation and
cleavage at the a- and b-positions deriving both from the
A-aromatic ring of substrate, products 24, 26 and 27,
or from the B-aromatic ring, compound 25 (Scheme 4).
In this latter case, the occurrence of acid-catalyzed (acid-
olytic) cleavage of b-O-4 linkages due to the acidity of
MTO was concluded on the basis of the presence of 2-
methoxyphenol 25 as a typical acidolysis product. Com-
pound 25 was probably obtained by formation of a ben-
zyl cation followed by b-hydrogen elimination and
hydrolytic cleavage. The effective formation of a benzyl
cation under acidolytic cleavage of b-O-4 lignin model
compounds was previously detected by means of ESI/
MS analysis.23
The presence in the reaction mixture of compounds 32–
34 is noteworthy. In fact, on the basis of these data, it is
possible to suggest that MTO can effectively catalyze the
oxidative degradation of lignin containing high amount
of recalcitrant diphenylmethane subunits by decreasing
their reticulation grade. When the non-phenolic diph-
enylmethane lignin model compound 35 was submitted
to oxidation under the same experimental conditions
several products of side-chain oxidation to benzyl alco-
hol 36 and 37, aldehyde and carboxylic acid, compound
38, were detected. Moreover, products of demethylation
at the alkyl–aryl ether moieties, 39, 40 and 41, and deriv-
ing from the oxidative cleavage of one of the aromatic
rings, compound 42, were also found (Scheme 6, prod-
In the case of compound 26 and 27, the alkyl side-chain
was found dehydrated in accord with the acidic Lewis
and Bro¨nsted character of MTO.4 In analogy with the
oxidation of 10 reported above, a high molecular weight
(>550) compound that showed in GC–MS a molecular
peak fragment corresponding to 4-ethoxy-3-methoxy
benzyl alcohol substructure, was recovered.
HO
HO
CHO
HO
CHO
O
OH
OCH3
MTO/H2O2
AcOH
O
Me
OCH3
OCH2CH3
OCH3
OCH3
OCH3
OCH2CH3
OCH2CH3
OCH2CH3
24 (3.4)
25 (24.4)
26 (5.8)
27 (7.3)
23
Scheme 4.

C. Crestini et al. / Bioorg. Med. Chem. 13 (2005) 2569–2578
2573
CH2OH
R
CH2OH
H3CO
H3CO
OCH3
COOH
CH3
OH
CH3
OH
OH
OH
OH
29 (2.6). R= CH3
30 (1.5). R= CH2OH
31 (trace). R= CHO
MTO/H2O2
AcOH
32 (2.4)
R
OCH3
H3CO
COOH
H3CO
OH
28
33 (0.8). R= CH3
34 (3.8). R= CH2OH
Scheme 5.
R1
R2
CH3
CH3
OH
H3CO
OCH3
OCH3
H3CO
OCH3
OCH3
OCH3
36 (trace). R1= Me, R2=CH2OH
37 (3.6). R1= R2=CH2OH
39 (trace)
CH3
CH3
38 (24.3). R1= COOH, R2=CHO
CH3
MTO/H2O2
R1
R2
OCH3
OCH3
H3CO
AcOH
COOH
OCH3
H3CO
H3CO
OCH3
OCH3
OH
OH
35
40 (trace). R1= R2=CH2OH
41 (trace). R1= Me, R2=CHO
42 (trace).
Scheme 6.
ucts yields are reported in parentheses). It is interesting
to note that the methyl ester of 42 (not shown) was pre-
viously recovered in low amount after the ozonation of
35. In accordance with the reactivity showed by MTO,
this derivative was formed by initial oxidative ring open-
ing of one of the aromatic rings to give the correspond-
ing muconic acid derivative followed by a stepwise
oxidative degradation.25
2.2. Oxidation of lignins
Once clarified the reactivity of MTO towards lignin
model compounds, our attention was focused on the
oxidation of the lignin polymer. Technical lignins are
samples of lignin recovered after pulping processes. As
such they are chemically modified and contain significa-
tive amounts of condensed subunits heavily recalcitrant
to further oxidation treatments. In order to verify the
ability of MTO to degrade the complex phenolic poly-
meric structures present during bleaching processes, we
selected three different technical lignins, from hydrolytic,
kraft and organosolvent processes, respectively.
Products of oxidation of the activated benzylic position
and oxidative cleavage of one of the aromatic moieties
were recently observed in the treatment of phenolic
and non-phenolic diphenylmethane lignin model com-
pounds with polyoxometalate26 and by hydrogen perox-
ide biomimetic oxidation with manganese and iron
porphyrins.27 In this latter case, the site of oxidation
was determined by the distribution of the electronic den-
sity on the aromatic ring, which was in turn modulated
by the electronic effect of the substituents.
More specifically, a hydrolytic sugar cane lignin (SCL),
a red spruce kraft lignin (RSL) and a hardwood organo-
solvent lignin (OSL) were selected as representative
examples of p-hydroxyphenyl–guaiacyl, guaiacyl and
guaiacyl–syringyl lignins.
From the data above reported it is possible to conclude
that the catalytic system H2O2/MTO is able to exten-
sively oxidize monomeric and dimeric, phenolic and
non-phenolic, lignin model compounds. In all the cases
studied the cleavage of the alkyl side-chain, or both
alkyl side-chain oxidation and aromatic ring cleavage
were operative processes. Recalcitrant diphenylmethane
units were also efficiently degraded.
The lignins were submitted to oxidation with H2O2 and
MTO as catalyst. After recovery, the samples were
derivatized with 2-chloro-4,4,5,5-tetramethyl-1,3,2-
dioxaphospholane in the presence of a known amount
of cholesterol as internal standard.13,14 In Table 1 the
amounts of the different labile OH groups expressed
as millimole per gram of lignin as measured by 31P
NMR of phosphytilated samples are reported. Since

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C. Crestini et al. / Bioorg. Med. Chem. 13 (2005) 2569–2578
Table 1. MTO/H2O2 oxidative degradation of lignins analyzed by 31P NMR
Sample/treatment
Aliphatic OH
(mmol/g)
Condensed/syringyl OH
(mmol/g)
Guaiacyl OH
(mmol/g)
p-Hydroxy phenyl OH
(mmol/g)
COOH
(mmol/g)
SCL/CH3COOH
SCL/CH3COOH, H2O2, MTO
OSL/CH3COOH
1.7
0.82
0.29
0.50
0.560
0.548
—
0.807
1.26
0.921
1.052
0.904
1.587
0.528
0.354
0.755
0.606
0.278
0.173
1.822
0.984
0.713
0.289
0.367
0.535
0.882
1.504
OSL/CH3COOH, H2O2, MTO
RSL/CH3COOH
RSL/CH3COOH, H2O2, MTO
—
—
—
Data are reported as millimoles of each functional group per gram of lignin; RSL: red spruce kraft lignin MW 28,000; SCL: sugar cane hydrolysis
lignin; OSL: Organosolvent hardwood lignin (50% maple, 35% birch, 15% aspen).
the oxidations were carried out in glacial acetic acid, the
occurrence of acidolytic side processes should be consid-
ered.28 For this reason data about reference experiments
carried out treating the samples with acetic acid are also
reported in Table 1. The oxidative treatments induced
an appreciable decrease in the content of aliphatic OH
groups (43%, 14% and 67% reduction in SCL, OSL
and RSL, respectively), thus indicating the occurrence
of side-chain oxidation reactions. Syringyl OH units in
OSL were found to be reduced of 46%. All the lignins
showed guaiacyl OH groups reduction after MTO cata-
lyzed oxidation, while para-hydroxyphenyl (p-OH)
groups in SCL were only slightly modified. Besides the
general decrease of guaiacyl units, a reduction of the
condensed units by 67% and 60% in SCL and RSL,
respectively, was observed. All the lignin samples dis-
played an increase in COOH content after MTO/H2O2
treatment. The modifications occurred on the lignin
samples are in accord with the above reported reactivity
of model compounds. The decrease of aliphatic OH
groups is indicative of side-chain oxidation processes.
The decrease of non-condensed phenolic groups could
be explained either by aromatic ring cleavage or by oxi-
dative coupling reactions. However, the decrease of con-
densed OH groups is noteworthy since it indicates that
coupling processes—that result in more condensed and
insoluble structures—are not operative to a significative
extent.29 Furthermore the reactions of side-chain and
aromatic ring cleavage yield the formation of carboxylic
acid groups. This reaction pathway is confirmed by the
increase of COOH content after MTO catalyzed oxida-
tion of all the lignin samples studied.
Experiments performed on technical lignins, hydrolytic
sugar cane lignin (SCL), red spruce kraft lignin (RSL)
and hardwood organosolvent lignin (OSL), showed that
MTO is highly efficient also in the oxidative degradation
of complex polymeric substrates, yielding to the forma-
tion of more soluble lignins fragments with a high
degree of degradation as indicated by low contents in
aliphatic and condensed OH groups and high amounts
of carboxylic acid moieties.
These data indicate MTO as an interesting candidate for
the development of alternative totally chlorine free
(TCF) delignification processes. Further work is there-
fore in progress in order to exploit further on the reac-
tivity of lignin and lignin model compounds with
heterogeneous MTO catalysts.
4. Experimental
1H NMR, 13C NMR and 31P NMR spectra were re-
corded on a Bruker AM 400, or Bruker 200 spectrome-
ter. Mass Spectroscopy (MS) was performed with a GC
Shimadzu GC-17A and a mass-selective detector QP
6000. All solvents were ACS reagent grade and were
redistilled and dried according to standard procedures.
Chromatographic purifications were performed on col-
umns packed with Merck silica gel 60, 230–400 mesh
for flash technique. Thin layer chromatography was car-
ried out using Merck platten Kieselgel 60 F254. Lignin
samples were purchased from Aldrich and used without
further purification. Quantitative 31P NMR spectra were
obtained using methods identical to those described by
Argyropoulos et al. The chemical shifts were referenced
to phosphoric acid. LC–MS analyses were performed by
means of a TSQ Quantum Ultra AM Thermo Finnigan
instrument.
3. Conclusions
MTO was an efficient catalyst for the H2O2 oxidation of
lignin model compounds. Its reactivity towards mono-
meric and dimeric, phenolic and non-phenolic lignin
model compounds is displayed at the level of the alkyl
side-chain with oxidation and fragmentation reactions
and at the level of the aromatic moieties by hydroxyl-
ation, demethylation and oxidative ring-opening cleav-
age reactions. The efficiency of MTO was also high
towards recalcitrant condensed lignin models such as
phenolic and non-phenolic diphenylmethanes. In this
latter case, both alkyl side-chain oxidation and cleavage
of the diphenylmethane bridge and of the aromatic ring
were observed.
4.1. Starting materials
Lignin model compounds 1-(4-hydroxy-3-methoxyphen-
yl)-2-(2,6-dimethoxyphenoxy)-3-hydroxypropanol 10,
1-(4-ethoxy-3-methoxyphenyl)-2-(2,6-dimethoxyphen-
oxy)propane-1,3-diol 14, 1-(4-ethoxy-3-methoxyphe-
nyl)-2-(2-methoxyphenoxy)propane-1,3-diol 23, 2,20-
methylenebis(6-methoxy-4-methylphenol) 28, 1,10-meth-
ylenebis(2,3-dimethoxy-5-methylbenzene)
35
were
synthesized according to literature procedures.30,31

C. Crestini et al. / Bioorg. Med. Chem. 13 (2005) 2569–2578
2575
4.2. Typical procedure for the oxidation of lignin model
compounds
(hydroxymethyl)hex-2-enedioic acid 21, 6-ethoxy-3-
(hydroxymethyl)-6-oxohexanoic acid 22, 4-hydroxy-3-
[2-hydroxy-5-(hydroxymethyl)-3-methoxybenzyl]-5-meth-
oxybenzaldehyde 31, 4-(hydroxymethyl)-2-[5-(hydroxy-
methyl)-2,3-dimethoxybenzyl]-6-methoxyphenol 40, 3-
(2,3-dimethoxy-5-methylbenzyl)-4-hydroxy-5-methoxy-
benzaldehyde 41 and (2,3-dimethoxy-5-methylphe-
nyl)acetic acid 42 were identified on the basis of the
fragmentation spectra. The fragmentation patterns are
shown in Table 2.
The model compounds (1.0 mmol) to be oxidized and
H2O2 (1.5 mmol, 35% water solution) were added to a
solution of the catalyst (1.0% w/w) in CH3COOH
(5 mL), and the mixtures were stirred at room tempera-
ture. At the end of the reaction the catalyst was filtered
off and the solvent was evaporated under reduced pres-
sure. The residues were dissolved in 20 lL of pyridine in
the presence of 3,4-dimethoxytoluene as an internal
standard for GC–MS analysis. The mixture were than
derivatized with bis-trimetylsilyl trifluoroacetamide
(BSTFA) and analyzed.
Selected data for compounds 3, 4, 7, 8, 9, 15, 17, 18, 19,
20, 26, 27, 29, 30, 32, 33 and 34.
4.2.1. 4-Hydroxy-3-methoxybenzoic acid (3). dH
(CDCl3 + DMSO-d6): 7.50 (2H, m, PhH), 6.80 (1H, m,
PhH), 3.80 (3H, s, OCH3). dC (CDCl3): 166.6 (CO),
150.4 (C), 146.5 (C), 123.9 (CH), 121.3 (C), 114.3
(CH), 111.7 (CH), 51.7 (CH3); m/z (EI) 168 (M+).
Gas chromatography and gas chromatography–mass
spectrometry of the reaction products were performed
using a DB1 column (30 m · 0.25 mm and 0.25 mm film
thickness), and an isothermal temperature profile of
100 ꢁC for the first two minutes, followed by a 20 ꢁC/
min temperature gradient to 300 ꢁC and finally an iso-
thermal period at 300 ꢁC for 10 min. The injector tem-
perature was 280 ꢁC. Chromatography grade helium
was used as the carrier gas.
4.2.2. (2Z)-3-(5-Oxo-2,5-dihydrofuran-3-yl)acrylic acid
(4). Characterized as methyl ester. dH (CDCl3): 6.75
(dd, 1H, J = 12.4 Hz, J = 2 Hz, CH), 6.30 (m, 1H,
HCO), 6.12 (d, 1H, J = 12.4 Hz, CH), 5.25 (m, 2H,
CH2OOC), 3.78 (s, 3H, CH3COO) m/z (EI) 168 (M+).
The identification of vanillin 2, 2,6-dimethoxyphenol 12,
4-ethoxy-3-methoxybenzaldehyde 24, (2E)-3-(4-ethoxy-
3-methoxyphenyl)aceylaldehyde 25 was carried out by
comparison with sample of authentic products. 2-hydr-
oxy-5-(hydroxymethyl)benzo-1,4-quinone 5, 2-hydroxy-
1-(4-hydroxy-3-methoxyphenyl)ethanone 11, (5-oxo-
2,5-dihydrofuran-3-yl)acetic acid 13, 5-methoxybenz-
ene-1,2,4-triol 20, (2E)-3-(4-ethoxy-3-methoxyphenyl)-
acrylaldehyde 26, (2E)-3-(4-ethoxy-3-methoxyphenyl)-
prop-2-en-1-ol 27, [3-(2,3-dimethoxy-5-methylbenzyl)-
4,5-dimethoxyphenyl]methanol 36, 4-(hydroxymethyl)-
2-[5-(hydroxymethyl)-2,3-dimethoxybenzyl]-6-methoxy-
phenol 37, 3-(5-formyl-2,3-dimethoxybenzyl)-4,5-dime-
thoxybenzoic acid 38, 2-(2,3-dimethoxy-5-methylben-
zyl)-6-methoxy-4-methylphenol 39 were assigned by
comparison with the GC–MS fragmentation spectra of
original compounds.16,32,33
4.2.3. 3,4-Dimethoxybenzaldehyde (7). dH (CDCl3): 9.86
(1H, s, CHO), 6.90–7.42 (3H, m, PhH), 3.86 (3H, s,
OCH3), 3.82 (3H, s, OCH3). dC (CDCl3): 190.4 (CO),
134.3 (C), 149.1 (C), 129.7 (C), 126.5 (CH), 110.4
(CH), 108.4 (CH), 55.7 (CH3), 55.5 (CH3); m/z (EI)
166 (M+).
4.2.4. 3,4-Dimethoxybenzoic acid (8). dH (CDCl3 +
DMSO-d6): 7.60–6.80 (3H, m, PhH), 3.80 (3H, s,
OCH3), 3.78 (3H, s, OCH3). dC (CDCl3 + DMSO-d6):
167.1 (CO), 151.7 (C), 147.8 (C), 124.5 (CH), 111.7
(CH), 110.4 (CH), 111.7 (CH), 54.3 (CH3), 54.1 (CH3);
m/z (EI) 182 (M+).
4.2.5. Methyl (2Z)-3-(5-oxo-2,5-dihydrofuran-3-yl)acryl-
ate (9). dH (CDCl3): 6.75 (dd, 1H, J = 12.4 Hz, J = 2 Hz,
CH), 6.30 (m, 1H, HCO), 6.12 (d, 1H, J = 12.4 Hz, CH),
5.25 (m, 2H, CH2OOC), 3.78 (s, 3H, CH3COO) m/z (EI)
168 (M+).
4-Hydroxy-3-methoxybenzoic acid 3, (2Z)-3-(5-oxo-2,5-
dihydrofuran-3-yl)acrylic acid 4, 3,4-dimethoxybenzal-
dehyde 7, 3,4-dimethoxybenzoic acid 8, methyl (2Z)-3-
(5-oxo-2,5-dihydrofuran-3-yl)acrylate (muconolactone
derivative) 9, 1-(4-ethoxy-3-methoxyphenyl)propane-1,
2,3-triol 15, (4-hydroxy-3-methoxyphenyl)(hydroxy)ace-
tic acid 17, 5-(1,2-dihydroxyethyl)-3-methoxybenzene-
1,2-diol 18, 5-methoxybenzene-1,2,4-triol 20, 2-[2-hy-
droxy-5-(hydroxymethyl)-3-methoxybenzyl]-6-methoxy-
4-methylphenol 29, 2,20-methylenebis[4-(hydroxymeth-
yl)-6-methoxyphenol] 30, 2-hydroxy-5-(hydroxymeth-
yl)-3-methoxybenzoic acid 32, (2-hydroxy-3-methoxy-
5-methylphenyl)acetic acid 33 and [2-hydroxy-5-
(hydroxymethyl)-3-methoxyphenyl]acetic acid 34, were
recovered in appreciable amount after flash chromato-
4.2.6. 1-(4-Ethoxy-3-methoxyphenyl)propane-1,2,3-triol
(15). dH (CDCl3 + DMSO-d6): 6.50–7.0 (3H, m, PhH),
4.50 (1H, m, CH), 4.10–4.20 (2H, m, OCH2), 3.80
(3H, s, OCH3), 3.62 (1H, m, CHOH), 3.30–3.50 (2H,
m, CH2OH), 1.40–1.50 (3H, m, CH3). dC
(CDCl3 + DMSO-d6): 149.89 (C), 148.78 (C), 134.64
(C), 118.35 (CH), 113.59 (CH), 110.06 (CH), 76.51
(CH), 74.79 (CH), 64.40 (CH2), 63.7 (CH2), 55.8
(CH3), 14.90 (CH3); m/z (EI) 242 (M+).
4.2.7. (3,4-Dihydroxyphenyl)(hydroxy)acetic acid (17).
dH (CDCl3 + DMSO-d6): 6.50–7.0 (3H, m, PhH), 5.0
(1H, s, CH). dC (CDCl3 + DMSO-d6): 176.46 (C@O),
145.70 (C), 143.30 (C), 131.60 (C), 120.90 (CH),
117.40 (CH), 115.80 (CH), 71.56 (CH); m/z (EI) 185
(M+).
1
graphy of the respective crude and characterized by H
NMR and 13C NMR analyses (see next).
1-(3,4-Dihydroxyphenyl)propane-1,2,3-triol 16, 4-eth-
oxy-2,5-dihydroxy-3-methoxybenzoic acid 19, (2Z)-4-

2576
C. Crestini et al. / Bioorg. Med. Chem. 13 (2005) 2569–2578
Table 2. Mass spectrometric data of compounds 9, 11–42
Product
Derivativea
MS (m/z) data (%)
9
—
–Si(CH3)3
—
168 (M+, 12), 136 (100), 124 (53)
11
12
13
15
16
17
18
19
20
21
22
24
25
26
27
29
30
31
32
33
34
36
37
38
39
40
41
42
254 (M+, 28), 212 (68), 197 (32), 182 (100), 73 (93)
154 (M+, 100), 139 (55), 93 (41), 96 (32), 65 (30), 111 (30)
214 (M+, 52), 213 (100), 169 (95), 139 (91), 111 (86), 75 (62), 73 (48)
458 (M+, 2), 328 (6), 313 (75), 145 (31), 132 (28), 117 (81), 73 (100)
416 (M+, 8), 401 (2), 329 (18), 328 (100), 160 (24), 73 (45)
472 (M+, 8), 430 (100), 207 (12), 165 (70), 73 (68)
–Si(CH3)3
3 · –Si(CH3)3
3 · –Si(CH3)3
4 · –Si(CH3)3
3 · –Si(CH3)3
3 · –Si(CH3)3
–Si(CH3)3
2 · –Si(CH3)3
2 · –Si(CH3)3
—
416 (M+, 8), 318 (100), 262 (63), 160 (32), 73 (43)
444 (M+, 3), 357 (12), 356 (100), 284 (2), 262 (58), 73 (46)
228 (M+, 4), 215 (19), 213 (100),169 (72), 139 (63), 11 (51), 73 (22)
318 (M+, 3), 245 (14), 147 (100), 73 (76)
348 (M+, 2), 275 (21), 147 (32), 111 (68), 73 (100)
180 (M+, 37), 151 (100), 109 (17), 95 (10), 81 (20), 65 (25)
124 (M+, 74), 109 (100), 81 (75), 77 (4), 65 (7)
—
—
206 (M+, 23), 191 (26), 176 (35), 161 (100), 147 (53)
280 (M+, 6), 206 (24), 103 (18), 73 (100)
–Si(CH3)3
2 · –Si(CH3)3
2 · –Si(CH3)3
3 · –Si(CH3)3
2 · –Si(CH3)3
2 · –Si(CH3)3
3 · –Si(CH3)3
–Si(CH3)3
2 · –Si(CH3)3
–Si(CH3)3
–Si(CH3)3
2 · –Si(CH3)3
2 · –Si(CH3)3
–Si(CH3)3
448 (M+, 3), 313 (78), 299 (49), 207 (43), 132 (48), 117 (83), 75 (98), 73 (100)
464 (M+, 4), 282 (8), 207 (100), 85 (61), 73 (98)
534 (M+, 2), 516 (8), 309 (17), 287 (24), 197 (85), 147 (52), 73 (100)
356 (M+, 12), 341 (23), 309 (18), 207 (100), 132 (31), 117 (61), 75 (88), 73 (81)
340 (M+, 8), 339 (22), 309 (24), 147 (38), 117 (32), 75 (54), 73 (100)
414 (M+, 36), 413 (100), 309 (38), 251 (46), 207 (75), 73 (79)
404 (M+, 12), 351 (6), 347 (8), 207 (43), 73 (100)
492 (M+, 21), 462 (16), 207 (100), 73 (87)
432 (M+, 12), 431 (38), 401 (8),281 (6), 207 (100), 96 (16), 73 (43)
374 (M+, 92), 344 (100), 313 (20), 193 (36), 162 (28), 151 (32), 73 (81)
464 (M+, 12), 404 (9), 207 (100), 73 (68)
446 (M+, 2), 431 (46), 207 (100), 73 (49)
282 (M+, 38), 252 (12), 223 (18), 208 (43), 193 (21), 73 (100)
a Underivatized; number of –Si(CH3)3 per molecule: trimethylsilylated with N,O-bis(trimethylsilyl)-acetamide.
4.2.8. 5-(1,2-Dihydroxyethyl)-3-methoxybenzene-1,2-diol
(18). dH (CDCl3 + DMSO-d6): 6.70–6.40 (2H, m, PhH),
4.90–4.65 (1H, m, CH), 3.81 (3H, s, OCH3), 3.70–3.50
(2H, m, CH2). dC (CDCl3 + DMSO-d6): 145.3 (C),
149.7 (C), 133.8 (C), 131.3 (C), 110.01 (CH), 105.3
(CH), 76.7 (CH), 68.1 (CH2), 56.2 (CH3); for mass spec-
trum (EI) see Table 2.
123.04 (CH), 112.82 (CH), 110.25 (CH), 64.40 (CH2),
55.73 (CH3), 14.73 (CH3); for mass spectrum (EI) see
Table 2.
4.2.12. (2E)-3-(4-Ethoxy-3-methoxyphenyl)prop-2-en-1-
ol (27). dH (CDCl3 + DMSO-d6): 6.91 (1H, s, PhH),
6.85–6.79 (2H, m, 2PhH), 6.54–6.49 (1H, m, CH),
6.30–6.24 (1H, m, CH), 4.83 (1H, s, OH), 4.20 (2H, m,
CH2), 4.10–4.05 (2H, m, CH2), 3.87 (3H, s, OCH3),
1.42 (3H, m, CH3). dC (CDCl3 + DMSO-d6): 149.89
(C), 148.57 (C), 131.28 (C), 130.11 (CH), 128.64 (CH),
119.18 (CH), 112.23 (CH), 109.73 (CH), 64.40 (CH2),
62.84 (CH2), 55.73 (CH3), 14.73 (CH3); for mass spec-
trum (EI) see Table 2.
4.2.9. 4-Ethoxy-2,5-dihydroxy-3-methoxybenzoic acid
(19). dH (CDCl3 + DMSO-d6): 7.67 (1H, broad s, OH),
6.72 (1H, s, PhH), 4.40–4.20 (2H, m, OCH2), 3.83
(3H, s, OCH3), 1.50–1.40 (3H, m, CH3). dC
(CDCl3 + DMSO-d6): 167.7 (CO), 151.6 (C), 150.7 (C),
135.4 (C), 134.5 (C), 112.6 (C), 107.7 (CH), 64.5
(OCH2), 60.9 (CH3), 14.8 (CH3); for mass spectrum
(EI) see Table 2.
4.2.13. 2-[2-Hydroxy-5-(hydroxymethyl)-3-methoxybenz-
yl]-6-methoxy-4-methylphenol (29). dH (CDCl3): 7.06
(1H, m, PhH), 6.97 (1H, m, PhH), 6.77 (1H, m, PhH),
6.63 (1H, m, PhH), 4.87 (2H, broad s, OH), 4.65 (2H,
s, CH2), 3.91 (2H, s, CH2), 3.79 (3H, s, OCH3), 3.76
(3H, s, OCH3), 2.31 (3H, s, CH3). dC (CDCl3): 148.7
(C), 146.8 (C), 145.4 (C), 142.8 (C), 138.1 (C), 134.2
(C), 126.3 (C), 125.1 (C), 124.0 (CH), 122.3 (CH),
110.9 (CH), 109.1 (CH), 63.6 (CH2), 56.21 (CH3), 26.9
(CH2), 20.9 (CH3); for mass spectrum (EI) see Table 2.
4.2.10. 4-Methoxybenzene-1,2,3,5-tetrol (20). dH
(CDCl3 + DMSO-d6): 6.30–6.50 (2H, m, PhH), 3.9
(3H, s, OCH3). dC (CDCl3 + DMSO-d6): 144.30 (C),
142.43 (C), 141.64 (C), 89.34 (CH), 84.31 (CH), 56.09
(CH3); m/z (EI) 156 (M+).
4.2.11. (2E)-3-(4-Ethoxy-3-methoxyphenyl)acrylaldehyde
(26). dH (CDCl3 + DMSO-d6): 9.61 (1H, s, CHO), 7.38–
7.34 (1H, m, CH), 7.03 (1H, m, PhH), 6.94 (1H, s, PhH),
6.93 (1H, m, PhH), 6.60–6.56 (1H, m, CH), 4.10–4.07
(2H, m, CH2), 3.87 (3H, s, CH3), 1.44–1.40 (3H, m,
CH3). dC (CDCl3 + DMSO-d6): 193.41 (CO), 152.63
(CH), 150.96 (C), 148.86 (C), 128.56 (C), 127.17 (CH),
4.2.14. 2,20-Methylenebis[4-(hydroxymethyl)-6-methoxy-
phenol] (30). dH (CDCl3): 7.05 (2H, m, PhH), 6.77 (2H,
m, PhH), 4.64 (4H, s, 2CH2), 4.40 (2H, broad s, OH),

C. Crestini et al. / Bioorg. Med. Chem. 13 (2005) 2569–2578
2577
3.91 (2H, s, CH2), 3.79 (6H, s, 2CH3). dC (CDCl3): 147.7
References and notes
(C), 145.4 (C), 139.8 (C), 126.3 (C), 122.4 (CH), 110.9
(CH), 63.6 (CH2), 56.2 (CH3), 26.9 (CH2); for mass
spectrum (EI) see Table 2.
1. Kuwahara, M.; Shimada, M. In Biotechnologyin the Pulp
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2. Gierer, J. Holzforshung 1997, 51, 34.
4.2.15. 2-Hydroxy-5-(hydroxymethyl)-3-methoxybenzoic
acid (32). dH (CDCl3 + DMSO-d6): 7.6–7.31 (5H, m,
2PhH + 3OH), 4.75 (2H, s, OCH2), 3.92 (3H, s, OCH3).
dC (CDCl3 + DMSO-d6): 169.8 (CO), 155.7 (C), 144.7
(C), 137.5 (C), 122.01 (CH), 112.94 (CH), 63.8 (CH2),
56.0 (CH3); for mass spectrum (EI) see Table 2.
3. Adam, W.; Herrmann, W. A.; Lin, J.; Saha-Mo¨ller, C. R.;
Fischer, R. W.; Correia, J. D. G. Angew. Chem., Int. Ed.
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4. (a) Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew.
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Abu-Omar, M. M. Catal. Today 2000, 55, 317–363.
5. Zhu, Z.; Espenson, J. H. J. Org. Chem. 1995, 60, 7728–
7732.
6. Adam, W.; Herrmann, W. A.; Saha-Mo¨ller, C. R.;
Shimizu, M. J. Mol. Catal. A 1995, 97, 15–20.
7. Abu-Omar, M. M.; Espenson, J. H. J. Am. Chem. Soc.
1995, 117, 272–280.
4.2.16. 2-(Hydroxy-3-methoxy-5-methylphenyl) acetic
acid (33). dH (CDCl3 + DMSO-d6): 8.64 (1H, broad s,
OH), 6.96 (1H, s, PhH), 6.60 (1H, s, PhH), 3.76 (3H,
s, OCH3), 3.69 (sH, s, CH2), 2.31 (3H, s, CH3). dC
(CDCl3 + DMSO-d6): 177.8 (CO), 147.6 (C), 141.2 (C),
133.1 (C), 125.8 (C), 121.3 (CH), 110.9 (CH), 56.2
(CH3), 36.6 (CH2), 21.5 (CH3); for mass spectrum (EI)
see Table 2.
8. (a) Herrmann, W. A.; Fischer, R. W.; Correira, J. D. G. J.
Mol. Catal. 1994, 94, 213–223; (b) Bernini, R.; Mincione,
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9. (a) Murray, R. W.; Iyanar, K.; Chen, J.; Wearing, J. T.
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Mandelli, D.; ShulÕpin, G. B. Tetrahedron Lett. 1996, 37,
6487–6490.
4.2.17. [2-Hydroxy-5-(hydroxymethyl)-3-methoxyphenyl]
acetic acid (34). dH (CDCl3 + DMSO-d6): 7.5–6.6 (3H,
m, 2PhH + OH), 4.6–4.4 (2H, m, CH), 3.79 (3H, s,
OCH3), 3.66 (2H, s, CH2). dC (CDCl3 + DMSO-d6):
177.8 (CO), 146.5 (C), 143.5 (C), 138.7 (C), 122.2 (C),
118.9 (CH), 111.2 (CH), 63.6 (CH2), 56.2 (CH3), 36.6
(CH2); for mass spectrum (EI) see Table 2.
10. Adam, W.; Hermann, W. A.; Lin, J.; Saha-Moller, C. R.
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4.3. Oxidation of lignin. General procedure
Oxidations of hydrolytic sugar cane lignin (SCL), red
spruce kraft lignin (RSL) and hardwood organosolvent
lignin (OSL) were carried out in acetic acid (5 mL), in
the presence of 100 mg of lignin, 1.0 mg of MTO (1%
w/w) and 500 lL of H2O2 (35% water solution). After
24 h the reaction mixtures were evaporated, washed with
water, centrifuged and freeze-dried.
15. Crestini, C.; Pastorini, A.; Tagliatesta, P. J. Mol. Catal. A
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4.4. Quantitative 31P NMR
16. Crestini, C.; Bernini, R.; Porri, A.; Giovannozzi-Ser-
manni, G. Holzforschung 1996, 50, 193–200.
17. For a general review on DMD chemistry see: Curci, R. In
Advances in Oxygenated Processes; Baumstark, A. L., Ed.;
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18. Argyropoulos, D. S.; Sun, Y.; Berry, R. M.; Bouchard, J.
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20. Wallis, A. F. A.; Sun, Y.-P.; Nguyen, K. L. J. Wood
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21. For examples of biomimetic degradation of non-phenolic
lignin model compounds with metallo porphyrins see: (a)
Crestini, C.; Tagliatesta, P.; Saladino, R. In Oxidative
delignification chemistry—Fundamental and Catalysis;
Argyropoulos, D. S., Ed.; ACS Symposium series 785,
Washington, DC, 2001; pp 213–225; (b) Artaud, I.; Ben-
Aziza, K.; Mansui, D. J. Org. Chem. 1993, 58, 3373; (c)
Cui, F.; Dolphin, D. Bioorg. Med. Chem. 1994, 2, 735.
22. (a) Crestini, C.; Giovannozzi-Sermanni, G.; Argyropou-
los, D. S. Bioorg. Med. Chem. 1998, 6, 967–973; (b)
Crestini, C.; Argyropoulos, D. S. Proceedings of the fifth
Derivatization of the lignin samples with 2-chloro-
4,4,5,5-tetramethyl-1,3,2-dioxaphospholane were per-
formed as previously described. Samples of lignin
(30 mg) accurately weighed, were dissolved in a solvent
mixture composed of pyridine and deuterated chloro-
form, 1.6:1 v/v ratio (0.5 mL). The phospholane
(100 lL) was then added, followed by the internal stan-
dard and the relaxation reagent solution (100 lL each).
The 31P NMR data reported in this effort are averages
of three phosphitylation experiments followed by quantit-
ative 31P NMR acquisitions. The maximum standard
deviation of the reported data was 2 · 10ꢀ2 mmol/g,
while the maximum standard error was 1 · 10ꢀ2 mmol/g.
Acknowledgements
Italian Murst, cofin 2003 ÔLa catalisi dei metalli di
transizione nello sviluppo di strategie sintetiche inno-
vative di eterociclciÕ is acknowledged for financial
support.

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