Differences in the Mechanisms of MnO2Oxidation between Lignin Model Compounds with the p-Hydroxyphenyl, Guaiacyl, and Syringyl Nuclei

Article abstract of DOI:10.1021/acs.jafc.0c01956

The purpose of this study was to examine how the rate and mechanism of MnO2 oxidation differ between the p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) types of simple nonphenolic lignin model compounds as well as the p-ethylphenyl (E) type compounds. The oxidation was conducted using an excess amount of MnO2 in a sulfate buffer solution at a pH value of 1.5 at room temperature. MnO2 oxidized at least the G and S nuclei, although it commonly oxidizes alcohols present at the benzyl position. The oxidation rates of the benzyl alcohol derivatives were in the order of G- > S- ? H- > E-type, which suggests that the rates are determined by the electronic effects of their methoxy and ethyl functional groups on not only their benzyl positions but also their aromatic π-electron systems. The kinetic isotope effect was observed in the MnO2 oxidations of the same derivatives deuterated at their benzyl hydroxymethyl groups. The observed magnitudes were in the order of E- ? H- > G- ? S-type, suggesting that the contribution of oxidation of their aromatic nuclei, which is another reaction mode of the oxidation of their benzyl positions, increases in the reverse order.

Full text of DOI:10.1021/acs.jafc.0c01956

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Differences in the Mechanisms of MnO2 Oxidation between Lignin
Model Compounds with the p‑Hydroxyphenyl, Guaiacyl, and
Syringyl Nuclei
Shirong Sun, Takuya Akiyama, Tomoya Yokoyama,* and Yuji Matsumoto
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ABSTRACT: The purpose of this study was to examine how the rate and mechanism of MnO2 oxidation differ between the p-
hydroxyphenyl (H), guaiacyl (G), and syringyl (S) types of simple nonphenolic lignin model compounds as well as the p-ethylphenyl
(E) type compounds. The oxidation was conducted using an excess amount of MnO2 in a sulfate buffer solution at a pH value of 1.5
at room temperature. MnO2 oxidized at least the G and S nuclei, although it commonly oxidizes alcohols present at the benzyl
position. The oxidation rates of the benzyl alcohol derivatives were in the order of G- > S- ≫ H- > E-type, which suggests that the
rates are determined by the electronic effects of their methoxy and ethyl functional groups on not only their benzyl positions but also
their aromatic π-electron systems. The kinetic isotope effect was observed in the MnO2 oxidations of the same derivatives deuterated
at their benzyl hydroxymethyl groups. The observed magnitudes were in the order of E- ≫ H- > G- ≫ S-type, suggesting that the
contribution of oxidation of their aromatic nuclei, which is another reaction mode of the oxidation of their benzyl positions, increases
in the reverse order.
KEYWORDS: aromatic nucleus, benzyl alcohol, guaiacyl, p-hydroxyphenyl, manganese dioxide, syringyl
nonrecyclable permanganate (MnO4¯) oxidation of lignin has
been reported fairly widely.4−14 It is generally known that
MnO2 oxidizes allyl and benzyl alcohols selectively among
various types of alcohols to afford the corresponding conjugate
and aromatic carbonyls, respectively, with the oxidation rates
being higher in nonpolar organic solvents than in polar
solvents.15−22 MnO2 oxidation of other alcohols progresses
only under severe conditions.17,23 However, if MnO2 had
oxidized only benzyl alcohols in the residual lignin, and
consequently the corresponding α-carbonyl groups had just
formed in our previous study,1 in accordance with this general
knowledge, sufficient delignification must not have been
attained. Just introducing α-carbonyl groups does not
contribute to delignification at the employed pH and
temperature.
The aim of this study is to examine the mechanism of MnO2
oxidation of lignin using model compounds. This paper is the
first in a series and focuses mainly on the mechanism of MnO2
oxidation of simple nonphenolic lignin model compounds
consisting of the p-hydroxyphenyl (H), guaiacyl (G), and
syringyl (S) nuclei. Scheme 1 shows the H-, G-, and S-type
lignin model compounds as well as p-ethylphenyl type (E-type)
compounds employed in this study. They are analogues of
INTRODUCTION
■
Lignin is commonly depolymerized and degraded to isolate a
portion with inevitable structural alteration, to promote
delignification and allow isolation of the other components
(the carbohydrates), or to obtain lignin-based fine chemicals in
a biomass conversion process of woody raw materials,
including chemical pulping for paper making. Oxidation is
one of the most common methods for these purposes, together
with acidic and alkaline treatments. In an oxidation process,
lignin should be selectively oxidized while minimizing
degradation of the carbohydrates as far as possible. Establish-
ment of a process to realize this requirement is desirable; the
oxidation processes currently in practical use do not show
satisfactory selectivity.
In our previous study, we applied an oxidation process
employing manganese dioxide (MnO2) as an oxidant at a pH
value of 2 (in a sulfate buffer solution) and temperature of 70
°C to the prebleaching (oxygen delignification) stage of
chemical pulping.1 The latter half of the oxygen delignification
stage was substituted with the MnO2 oxidation process, which
promoted delignification while suppressing the degradation of
the carbohydrates relative to that observed in the common
oxygen delignification stage without the substitution. The
MnO2 oxidation process therefore has potential to satisfy the
above-described requirement. MnO2 is a recyclable oxidant
because it is reduced to Mn2+ in the oxidation process and
Mn2+ can be reconverted to MnO2 by oxygen oxidation under
alkaline conditions. This recyclability is another advantage of
the MnO2 oxidation process.
Received: March 26, 2020
Revised: April 30, 2020
Accepted: May 27, 2020
Published: June 10, 2020
MnO2 oxidation of lignin has been examined in only a few
studies, at least from a pure chemistry viewpoint,2,3 although
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Scheme 1. Nonphenolic Lignin Model Compounds (the H-, G-, and S-Types) and p-Ethylphenyl Type Compounds (the E-
Types) Employed in This Study
Scheme 2. Quantified Reaction Products as well as Another Possible One without Detection (Product BH) in MnO2 Oxidation
respectively, and purified by flash chromatography for the same
purposes. Product BH was purchased and purified by flash
chromatography for the same purposes. Products A′ were obtained
from the corresponding compounds III by oxidizing them with MnO2
in CH2Cl2 at room temperature and purified by flash chromatography
for the same purposes.
benzene (methoxybenzene (H-type, IH), 1,2-dimethoxyben-
zene (G-type, IG), 1,2,3-trimethoxybenzene (S-type, IS), and
ethylbenzene (E-type, IE)), benzyl alcohol (4-methoxybenzyl
alcohol (H-type, IIH), 3,4-dimethoxybenzyl alcohol (G-type,
IIG), 3,4,5-trimethoxybenzyl alcohol (S-type, IIS), and 4-
ethylbenzyl alcohol (E-type, IIE)), and benzyl alcohol
deuterated at the methylene of the benzylic hydroxymethyl
group (1-hydroxy(2H2)methyl-4-methoxybenzene (H-type,
IIIH), 4-hydroxy(2H2)methyl-1,2-dimethoxybenzene (G-type,
IIIG), 5-hydroxy(2H2)methyl-1,2,3-trimethoxybenzene (S-type,
IIIS), and 1-ethyl-4-hydroxy(2H2)methylbenzene (E-type,
IIIE)).
All the reaction products shown in Scheme 2 except for product BH
1
were identified on the basis of their H- and 13C-NMR (JNM-A500,
500 MHz, JEOL Ltd., Tokyo, Japan) and GC/MS (GC2010/
PARVUM2, Shimadzu Co., Kyoto, Japan) spectra. These assignments
and the MS spectra are shown in the Supporting Information.
Preparation of MnO2 for the Reaction. To 700 mL of a H2O
solution containing 0.20 mol MnSO4·H2O was added dropwise 100
mL of 4.0 mol L−1 NaOH. The resultant solution was bubbled with
O2 for 60 min with stirring at room temperature. The suspension was
neutralized with 150 mL of 1.0 mol L−1 H2SO4 with stirring for 30
min. The dropwise addition of 4.0 mol L−1 NaOH and successive O2
bubbling were then repeated to further advance the oxidation. The
obtained precipitates (mostly MnO2) were collected by filtration
under reduced pressure, washed with H2O until the filtrate became
neutral, and air-dried for further use. To examine the oxidation
powers, the precipitates were ground into powder in a mortar and
iodometrically titrated. The content was 84.9% of the theoretical
value.
MnO2 Oxidation Reaction. All reactions were conducted in a
round-bottom glass flask (200 mL volume) equipped with a magnetic
stirrer. A sulfate buffer solution (0.50 mol L−1, pH 1.5) was prepared
in advance by mixing Na2SO4 and H2SO4 solutions (0.50 mol L−1
each).
The precipitates of MnO2 obtained above were ground into
powder in a mortar. The powder (1.2 mmol (oxidation power basis:
1.2 × 84.9/100 = 1.02 mmol), oven-dry basis) was aged in sulfate
buffer solution (50 mL) for 120 min in a glass flask at room
temperature. Another sulfate buffer solution (10 mL) consisting of the
same sulfate components and containing one of the compounds
shown in Scheme 1 or 2 (12 μmol) as a starting material was added to
the sulfate buffer solution containing the MnO2 powder to initiate the
reaction at room temperature. The initial concentrations of the
compound and MnO2 (insoluble solid) were 0.20 and 20 mmol L−1,
respectively. Each reaction was conducted three times to confirm the
reproducibility.
EXPERIMENTAL SECTION
■
Materials. All chemicals used in this study except for the organic
compounds described below were purchased from FUJIFILM Wako
Pure Chemical Co. (Osaka, Japan), Tokyo Chemical Industry Co.,
Ltd. (Tokyo, Japan), or Sigma-Aldrich Japan K. K. (Tokyo, Japan)
and used without further purification. Ultrapure H2O prepared using a
generator, Puric-Z (Organo Co., Tokyo, Japan), was used in all the
experiments.
Preparation of Model Compounds and Authentic Com-
pounds of Reaction Products. Compounds I (all the H-, G-, S-,
and E-types) and II were commercially available and purified by flash
chromatography (Isolera, Biotage Japan Ltd., Tokyo, Japan).
Compounds IIIH, IIIG, and IIIE were synthesized from commercially
available 4-methoxybenzoic acid, 3,4-dimethoxybenzoic acid, and 4-
ethylbenzoic acid, respectively, by direct reduction with LiAlD4 in dry
THF and purified by flash chromatography. Compound IIIS was
synthesized from commercially available 3,4,5-trimethoxybenzoic acid
by ethyl esterification in EtOH containing H2SO4 and successive
reduction with NaBD4 and LiCl in dry THF and then purified by flash
chromatography.
Scheme 2 shows the quantified reaction products and another
reaction product possibly generated in spite of no detection. The
formers are analogues of benzaldehyde (H-type (AH), G-type (AG), S-
type (AS), and E-type (AE)), 1,4-benzoquinone (G-type (BG) and S-
type (BS)), and benzaldehyde deuterated at the benzylic formyl group
(H-type (AH′), G-type (AG′), S-type (AS′), and E-type (AE′)). The
latter is 1,4-benzoquinone (H- (or E-) type (BH)). Products A were
purchased and purified by flash chromatography for use in authentic
compounds for identification, for preparing their calibration lines for
quantification, and for reacting them as starting materials in the MnO2
oxidations described below. Products BG and BS were isolated from
reaction solutions of the MnO2 oxidations of compounds IIG and IIS,
Quantification. A specific amount of the reaction solution was
withdrawn at prescribed reaction times and poured into a glass tube to
which a saturated NaHCO3 solution had already been added for
neutralization. The mixture was extracted with CH2Cl2 containing an
internal standard compound, 1,2,3-trimethoxybenzene or 3,4-
dimethoxybenzaldehyde (veratraldehyde), and twice further (without
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the internal standard compound) in the glass tube. The organic layers
were combined, dried over anhydrous Na2SO4, filtered with a
membrane filter, and injected into an HPLC instrument with an
groups on the aromatic nucleus in a chemical reaction
commonly correlates well with their Hammett’s substituent
constants (σ values), when they are located at the para and/or
meta of the benzyl carbon and the reactivity is relatively
compared with that of an analogous aromatic compound with
other or no functional groups present at both positions. The E-
type compounds were employed in this context. The total σ
value of two methoxy groups of the G-type compounds is
−0.153 (= −0.268 (para) + 0.115 (meta)), which is close to
that of the ethyl group of the E-type compounds (−0.151).
Each E-type compound can thus be expected to have similar
reactivity to the corresponding G-type compound in a reaction
occurring at their benzyl positions, provided that the reaction
is affected only by the electronic factors that originate in the
functional groups and appear locally at the benzyl positions.
Thus, the reactivities of the G- as well as the other types are
compared with those of the E-types in the following sections.
MnO2 Oxidation of Benzene Analogues (Compounds
I). Although MnO2 is an oxidant that selectively oxidizes an
analogue of allyl or benzyl alcohol, as described in the
Introduction section, it was examined by employing com-
pounds I whether or not MnO2 can oxidize the aromatic
nucleus of an aromatic compound without benzylic hydroxy
group and a substructure corresponding to the side-chain
portion in lignin and whether the progress of this oxidation is
dependent on the type of aromatic nucleus.
Compound IH disappeared in MnO2 oxidation and remained
with a recovery yield ( standard deviation calculated from
three duplicated runs) of 81.0 2.0% at a reaction time of 660
min, although no other peaks of any size appeared on the
HPLC chromatogram. These results were considered to
indicate that the aromatic nucleus of compound IH was
degraded to afford reaction products that do not exhibit an
absorbance at around 280 nm and/or that it just volatilized.
When compound IH was reacted in the absence of MnO2
under otherwise exactly the same conditions where it must
stably have existed, it disappeared and remained with the same
recovery yield as for MnO2 oxidation. Compound IH must
therefore have volatilized and not have been oxidized by
MnO2. Because exactly the same phenomena were observed in
the reactions of compound IE with and without MnO2, it must
also have volatilized and not been oxidized by MnO2.
Incidentally, because product BH was afforded when
compound IH was subjected to the MnO2 oxidation applying
1.0 mol L−1 H2SO4 under otherwise the same conditions, the
oxidation power of MnO2 is dependent on the system acidity.
It can be excluded from an explanation for these observed
phenomena that polymers formed but they were not detected
by the HPLC analysis, when the employed conditions are
taken into consideration.
ultraviolet−visible (UV−vis) absorption detector (LC-2010CHT
,
Shimadzu Co.) for quantification based on an absorbance at 280 nm.
In HPLC analyses, an HPLC column, Luna 5 u C18(2) 100 Å
(length: 150 mm; inner diameter: 2.0 mm; particle size: 5.0 μm;
Phenomenex, Inc., Torrance, CA, USA), was used at an oven
temperature of 40 °C with a solvent flow rate of 0.2 mL min−1. The
types of solvent and gradients were as follows: For the reactions of the
H-, G-, and S-type compounds, the gradient of MeOH/H2O (v/v)
was from 30/70 to 40/60 for 30 min and then maintained for 10 min.
For the reactions of the E-type compounds except for compound IE,
the gradient of MeOH/H2O (v/v) was from 30/70 to 50/50 for 10
min, from 50/50 to 60/40 for 15 min, and maintained for 10 min. For
the reactions of compound IE, the gradient of MeOH/H2O (v/v) was
from 40/60 to 50/50 for 10 min, from 50/50 to 85/15 for 20 min,
and maintained for 10 min.
RESULTS AND DISCUSSION
■
Preparation of MnO2. Commercially available active
MnO2 (FUJIFILM Wako Pure Chemical Co.) was ground
into powder. The oxidation power was iodometrically titrated
to be 90.2% of the theoretical value by the method described
above. The MnO2 powder was applied to the oxidation of
compound IIG under conditions identical to those described
above. When the logarithmic plot for the disappearance of
compound IIG was approximated to a pseudo-first-order
reaction, the approximation was rather bad. The disappearance
was gradually accelerated when compared with that which was
supposed to follow a pseudo-first-order reaction from the
initial stage. The rate reached a ceiling level at a reaction time
of about 120 min. In contrast, the disappearance followed a
pseudo-first-order reaction well from the initial stage at a rate
similar to the above ceiling level, when the MnO2 powder had
primarily been aged following the procedure described above.
The observed acceleration of the oxidation using the MnO2
powder without the pre-aging thus suggests that the MnO2
powder was aged and became more active during the
oxidation. This clarified that pre-aging is necessary for the
commercial MnO2.
MnO2 was synthesized from Mn2+ by the method described
above, and the obtained precipitates were ground into powder.
This synthesized MnO2 powder was applied to the oxidation of
compound IIG with or without pre-aging. The disappearance
approximated well to a specific pseudo-first-order reaction
from the initial stage, regardless of conducting pre-aging. It was
faster than that observed when the above-described commer-
cial MnO2 powder was used, which was surprising because the
synthesized MnO2 powder had less oxidation power (84.9%)
than the commercial powder (90.2%). This clarified that pre-
aging is not necessary for the synthesized MnO2 powder.
The observed difference between the commercial and the
synthesized MnO2 powders must have arisen from their
physical properties. Physical properties of commercially
available MnO2 are often dependent on the vender. Thus,
the synthesized MnO2 powder, whose physical properties are
possibly prepared to be constant anytime, was employed in this
study. Because any aging and the consequent activity change
during the MnO2 oxidation process interfere with ready
analysis, pre-aging was applied to the synthesized MnO2
powder in spite of its unnecessity.
Compound IG or IS disappeared in MnO2 oxidation and
remained with a recovery yield of 60.8 1.1% or 60.0 1.2%,
respectively, at a reaction time of 660 min, although neither
compound disappeared in the reaction without MnO2. Product
BG or BS was afforded as the exclusive major reaction product
with a yield of 20.9 1.1% or 13.2 0.1%, respectively, at the
same reaction time. Many small peaks in addition to the large
peak of product BG or BS appeared on the HPLC
chromatogram of the reaction solution withdrawn at this
reaction time, indicating the formation of many minor reaction
products. The proportion of the amount of afforded product
BG or BS to that of disappearing compound IG or IS,
respectively, did not vary largely during the reaction. Product
Choice of the E-Type Compounds. Reactivity at the
benzyl position of an aromatic compound with functional
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Table 1. Observed Pseudo-First-Order Reaction Rate Constants (kobs), Squares of the Correlation Coefficients (R2) in the
Approximations, and Ratios of kobs Values between Compounds II and III (kobs(II)/kobs(III)) for Estimation of the Magnitudes
of the Kinetic Isotope Effects
a
b
c
d
b
c
e
b
c
C.I
kobs
R2
C.II
kobs
R2
C.III
kobs
3.13 0.09
R2
k
obs(II)/kobs(III)
IH
IG
IS
IIH
IIG
IIS
IIE
14.7 0.1
911 23
343 18
2.09 0.06
0.995
0.997
0.995
0.999
0.985
0.988
0.962
0.968
0.994
0.995
0.992
0.991
IIIH
0.995
0.986
0.987
1.00
1.00
1.00
0.983
0.985
0.983
0.961
0.958
0.941
kobs(IIH)/kobs(IIIH) = 4.7
7.96 0.05
8.08 0.14
0.986
0.972
0.998
0.987
0.983
0.991
IIIG
IIIS
IIIE
227
127
8
2
kobs(IIG)/kobs(IIIG) = 4.0
kobs(IIS)/kobs(IIIS) = 2.7
IE
0.244 0.008
kobs(IIE)/kobs(IIIE) =8.6
a
b
c
Compounds I. Unit: ×10−4 min−1. The values after the “ ” marks are the standard deviations of three duplicated runs. The value obtained from
d
e
each of three duplicated runs. Compounds II. Compounds III.
BG or BS was stable under the employed conditions when
subjected to MnO2 oxidation as a starting compound. The
disappearance of compound IG or IS and concomitant
formation of product BG or BS, respectively, are shown in
Figure S1 in the Supporting Information.
was thus 77.3%, which was lower than those of compounds IIH
and IIG. The oxidation of the aromatic nucleus (S nucleus)
may thus increase reactions affording unidentified reaction
products. Because few small peaks appeared on the HPLC
chromatogram, the oxidation of the aromatic nucleus must
have afforded aliphatic reaction products accompanying the
degradation of the S nucleus. Product AS or BS was stable
under the employed conditions when subjected to MnO2
oxidation as a starting compound.
All the observed phenomena in the MnO2 oxidation of
compound IIE were the same as those of compound IIH except
for the slower oxidation. The recovery yield and yield of
product AE were 87.0 0.1% and 13.2 0.1%, respectively, at
a reaction time of 660 min. The total of these was 100.2%,
indicating that this oxidation reaction was quantitative.
Product AE was stable under the employed conditions when
subjected to MnO2 oxidation as a starting compound. Each
disappearance of compounds II and concomitant formation of
the reaction products are shown in Figure S2 in the Supporting
Information.
Each disappearance of compounds II was approximated to a
pseudo-first-order reaction, and the value of kobs is listed in
Table 1. The approximations were good in most of the 12 runs
(four compounds × three duplications; see the R2 values in
Table 1). The rates were in the order of compounds IIG > IIS
≫ IIH > IIE. If the MnO2 oxidations had been affected only by
the electronic effects originating in the functional groups on
their aromatic nuclei and appearing locally only at their benzyl
positions, the rates should have been in the order of
compounds IIH > IIG ≥ IIE > IIS on the basis of their
Hammett’s σ values, which are −0.268, −0.153, −0.151, and
−0.038, respectively. This inconsistency suggests that the
MnO2 oxidations are affected by not only the electronic effects
on the benzyl positions but also some other factors.
These other factors are discussed here. Previous studies
proposed that allyl or benzyl alcohols can form the π-complex
type of interaction with Lewis acid sites on the surface of
MnO2 aggregates, and hence these alcohols are oxidized by
MnO2 much more readily than other types of alcohols.18,24,25
On the basis of this proposal, MnO2 is presumed to readily
oxidize an analogue of benzyl alcohol with high electron
density in its aromatic nucleus owing to the ready formation of
the π-complex type of interaction with it. In this context, the
The above observations confirm that MnO2 cannot oxidize
the H- and E- but can oxidize the G- and S-type aromatic
nuclei to afford products BG and BS, respectively, as the
exclusive major reaction products under the employed
conditions in spite of the slow progress. The labilities of the
G- and S-type aromatic nuclei do not seem to be different.
The disappearance of compound IG or IS was approximated
to a pseudo-first-order reaction to obtain the pseudo-first-order
reaction rate constant kobs, which is listed in Table 1. The
approximations were fairly good in most of the six runs (two
compounds × three duplications; see the R2 values in Table 1).
MnO2 Oxidation of Benzyl Alcohol Analogues
(Compounds II). Compound IIH was oxidized in MnO2
oxidation, and product AH formed as the exclusive major
reaction product. The recovery yield and yield of product AH
were 52.7 0.1% and 39.3 0.7%, respectively, at a reaction
time of 660 min. The total of these was 92.0%, indicating that
side reactions contributed only slightly. Few small peaks
appeared on the HPLC chromatogram of the reaction solution
withdrawn at this reaction time. Compound IIH was stable in
the absence of MnO2 under otherwise the same conditions.
Product AH was stable under the employed conditions when it
was subjected to MnO2 oxidation as a starting compound.
All the observed phenomena in the MnO2 oxidation of
compound IIG were the same as those of compound IIH except
for the remarkably higher rate. The recovery yield and yield of
product AG were 4.3 0.4% and 87.2 8.7%, respectively, at a
reaction time of 30 min. The total of these was thus 91.5%.
Product AG was stable under the employed conditions when
subjected to MnO2 oxidation as a starting compound.
In contrast, compound IIS afforded products AS and BS as
the exclusive major and prominent secondary reaction product,
respectively, although all of the other phenomena were the
same as those of compound IIG except for the slower
oxidation. The S nucleus of compound IIS was oxidized in
MnO2 oxidation. The recovery yield and yields of products AS
and BS were 3.5
0.7%, 66.0
1.2%, and 7.8
0.2%,
respectively, at a reaction time of 120 min. The total of these
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a
Scheme 3. Three Possible Reaction Modes in the MnO2 Oxidations of the Benzyl Alcohol Analogues (Compounds II and III)
a
The former is representatively described, although the latter also undergoes these modes.
slowest MnO2 oxidation of compound IIE can be explained as
follows: Compounds IIH, IIG, and IIS with the methoxy group
substituent(s) have the aromatic nuclei with electron densities
higher than that of compound IIE with the ethyl group
substituent, resulting in rapid or ready formation of the π-
complex type of interaction with MnO2. The methoxy groups
resonantly donate their lone pairs to the π-electron systems of
their aromatic nuclei, which consequently increase their
electron densities higher than that of compound IIE elevated
only inductively by the ethyl group. The faster MnO2
oxidations of compounds IIG and IIS than that of compound
IIH can result from the high electron densities of their aromatic
nuclei, owing to the presence of plural methoxy groups.
Because the MnO2 oxidation of the benzyl position of
compound IIS is relatively suppressed by the electron-
withdrawal inductive effect originating in two meta-substituted
methoxy groups, the MnO2 oxidation of compound IIS must
have been suppressed and slower than that of compound IIG.
Instead, the oxidation of the aromatic S nucleus of compound
IIS progressed to afford product BS as the secondary prominent
reaction product, owing to the quite high electron density of
the S nucleus.
Although MnO2 can oxidize the aromatic nucleus of not
only the S-type (compound IS) but also G-type (compound
IG), as shown in the previous section, product BG was not
detected in the MnO2 oxidation of compound IIG. This is
probably because the oxidation of the benzyl position of
compound IIG is fast enough to be exclusive in competing with
the oxidation of the G nucleus.
MnO2 Oxidation of Deuterated Benzyl Alcohol
Analogues (Compounds III). The observed phenomena in
the MnO2 oxidation of each of compounds III were mostly the
same as those of the corresponding compounds II except that
the rate was lower due to the appearance of the primary kinetic
isotope effect (Table 1). The recovery yield of compound IIIH
and yield of product AH′ were 81.7 0.5% and 16.9 0.3%,
respectively, at a reaction time of 660 min. The total of these
was 98.6%. Those of compound IIIG and product AG′ were 1.9
0.2% and 86.6 0.2%, respectively, at a reaction time of 180
min. The total of these was 88.5%. In the MnO2 oxidation of
compound IIIS, products AS′ and BS formed as the exclusive
major and secondary prominent reaction products, respec-
tively. The recovery yield of compound IIIS and yields of
products AS′ and BS were 11.5 0.4%, 31.0 0.2%, and 18.1
0.0%, respectively, at a reaction time of 180 min. The total of
these was 60.6%. The recovery yield of compound IIIE and
0.0% and 1.3 0.0%,
respectively, at a reaction time of 660 min. The total of these
was 99.7%. Each disappearance of compounds III and
concomitant formation of the reaction products are shown in
Figure S3 in the Supporting Information.
yield of product AE′ were 98.4
Regarding the observed aspects other than the rates, it
should be noted that only the S-type compounds showed a
significant difference between compounds III and II. The
amount of afforded product AS′ was smaller than twice that of
product BS in the MnO2 oxidation of compound IIIS, while
product AS was afforded in an amount about 7−9 times as
large as that of product BS in the reaction of compound IIS.
This significant difference in the S-type compounds suggests
that in the oxidation of compound IIIS, the S-nucleus oxidation
is competitively and alternatively enhanced by suppressing the
oxidation of the benzyl position due to the presence of the
deuteriums and appearance of the kinetic isotope effect.
Possible Reaction Modes in MnO2 Oxidation.
Although MnO2 commonly oxidizes alcohol groups present
at the benzyl position,18 it certainly oxidized the G and S nuclei
of compounds IG and IS, respectively, which do not have the
benzyl position and substructures corresponding to the side-
chain portion of lignin, under the employed conditions. On the
basis of this observation, three possible reaction modes can be
proposed for the MnO2 oxidation of compounds II and III, as
shown in Scheme 3.
Mode 1 is direct oxidation of the benzyl position to afford
the corresponding benzaldehyde-type products A or A′, which
is the general mode of MnO2 oxidation. Mode 2 begins with
the oxidation of the aromatic nucleus but finally results in the
oxidation of the benzyl position to afford products A or A′. In
this mode, the aromatic cation radical must be the primary
intermediate, which is generated by the one-electron oxidation
of the aromatic nucleus by MnO2. Mode 3 also begins with the
oxidation of the aromatic nucleus and finally results in the
oxidative degradation of the aromatic nucleus to afford product
B or B′ as well as other unidentified aliphatic reaction
products. It is discussed in the following paragraphs which is
the major reaction mode in the MnO2 oxidation of the H-, G-,
S-, and E-type compounds on the basis of the magnitudes of
the kinetic isotope effects, which are expressed by the ratios of
the kobs values of compounds II to those of the corresponding
compounds III (kobs(II)/kobs(III)) listed in Table 1, focusing
on how dependent the ratio is on the types of the H, G, S, and
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E nuclei of compounds II and III. On the other hand, it is not
discussed in this study which mechanism the MnO2 oxidation
follows, the consecutive two steps of one-electron oxidation or
one step of two-electron oxidation, and how primarily
generated intermediates are further oxidized by MnO2 and
rearrange to products A and B, although the aromatic cation
radical is drawn as the intermediate on the way in Scheme 3.
Other possible intermediates are not discussed in this study
because any data obtained in this study cannot sufficiently
contribute to distinguishing between the above-described two
mechanisms of MnO2 oxidation (two steps of one-electron
oxidation and one step of two electrons) and tracing further
rearrangements. However, it is valuable enough to clarify
which mode (1, 2, or 3) is the major in the MnO2 oxidation of
the H-, G-, S-, and E-type compounds.
compounds is based on its higher electron density as well as
relative resistance in the rearrangement of the aromatic cation
radical to the benzyl radical accompanying the deprotonation
from the benzyl position. The MnO2 oxidation of the benzyl
radical must readily progress to afford products A.
Final Statements. MnO2 oxidized the G and S nuclei of
compounds IG and IS, respectively, which do not have
substructures corresponding to the side-chain of lignin,
although MnO2 commonly oxidizes alcohol groups present at
the benzyl positions. MnO2 oxidized compounds II, the
analogues of benzyl alcohol, with the rate in the order G- > S-
≫ H- > E-type. This order suggests that their reactivity is
determined by the electronic effects of their methoxy and ethyl
groups on not only their benzyl positions but also their
aromatic π-electron systems. The magnitudes of the kinetic
isotope effects in MnO2 oxidations were estimated from the
ratios of the kobs values between compounds II and III, the
nondeuterated and deuterated analogues of benzyl alcohol,
respectively. Because the kinetic isotope effect did not appear
when the MnO2 oxidation began with their aromatic nuclei,
the ratio decreased with increasing contribution of the
oxidation of the aromatic nuclei. The observed ratios were in
the order of E- ≫ H- > G- ≫ S-type, which suggests that the
contribution of the oxidations of the aromatic nuclei increases
in the reverse order.
The ratio between the E-type compounds (kobs(IIE)/
obs(IIIE)) is 8.6 and the largest among all types. This ratio
k
seems to be a common magnitude as an expression of the
primary kinetic isotope effect. As described in the previous
section, MnO2 cannot oxidize compound IE, and the MnO2
oxidation of compound IIE or IIIE quantitatively affords
product AE or AE′, respectively. On the basis of these facts, the
MnO2 oxidations of the E-type compounds IIE and IIIE are
presumed to progress exclusively via mode 1.
The ratio between the H- or G-type compounds is 4.7
(kobs(IIH)/kobs(IIIH)) or 4.0 (kobs(IIG)/kobs(IIIG)), respec-
tively, which is smaller than that between the E-type
compounds (kobs(IIE)/kobs(IIIE)). Because the rate-determin-
ing step is the oxidation of the aromatic nucleus and hence the
kinetic isotope effect does not appear when the MnO2
oxidation progresses only via mode 2, any contribution of
mode 2 to MnO2 oxidation must decrease the ratio. On the
basis of this fact, the MnO2 oxidations of the H-type
compounds IIH and IIIH must progress via not only mode 1
but also mode 2, although MnO2 cannot oxidize the H nucleus
in the oxidation of compound IH. Mode 2 can become
progressive in the MnO2 oxidations of compounds IIH and IIIH
owing to the presence of their benzyl hydroxymethyl groups as
an inductively electron-donating group. Similar discussion is
possible for the MnO2 oxidations of the G-type compounds
(IIG and IIIG). Because MnO2 can oxidize compound IG, the
contribution of mode 2 is greater than that in the oxidations of
compounds IIH and IIIH, resulting in kobs(IIG)/kobs(IIIG) being
smaller than kobs(IIH)/kobs(IIIH).
Because any carbon−carbon bond does not cleave in modes
1 and 2, these modes may not contribute to delignification in a
bleaching process of actual chemical pulp. Because mode 3
does not contribute to the oxidation of the G-type compounds,
the MnO2 oxidation may not be effective in a bleaching
process of softwood pulp in contrast to that of hardwood pulp
examined in our previous paper.1 We will examine these in the
next coming papers.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jafc.0c01956.
Assignments of 1H- and 13C-NMR spectra of com-
pounds III, products A, and products B; disappearances
of compounds I and concomitant formations of products
B in the MnO2 oxidations of compounds I (Figure S1);
disappearances of compounds II and concomitant
formations of products A and B in the MnO2 oxidations
of compounds II (Figure S2); and disappearances of
compounds III and concomitant formations of products
A′ and B′ in the MnO2 oxidations of compounds III
(Figure S3) (PDF)
The ratio between the S-type compounds (kobs(IIS)/
obs(IIIS)) is 2.7, which is much smaller than the others. As
k
described in the previous section, the MnO2 oxidations of
compounds IIS and IIIS afford products BS, which clearly
shows that MnO2 oxidizes their S nuclei to afford the products.
Thus, mode 3 contributes substantially to the MnO2 oxidations
of compounds IIS and IIIS. The kinetic isotope effect does not
naturally appear in mode 3, and hence the contribution of
mode 3 to MnO2 oxidation decreases the ratio. The electron-
withdrawal inductive effects of the two meta-methoxy groups
(σ value: +0.115 × 2) of compounds IIS and IIIS decrease the
contribution of mode 1, which further decreases the ratio.
These facts indicate that modes 1, 2, and 3 all contribute to the
MnO2 oxidations of compounds IIS and IIIS. Compounds IIS
and IIIS thus progress to mode 3 contrary to the lack of mode
3 in the MnO2 oxidations of compounds IIG and IIIG, although
both the S- and G-type compounds are oxidized to be the
cation radicals as the primary intermediates. This relative
preference of mode 3 in the oxidations of the S-type
AUTHOR INFORMATION
■
Corresponding Author
Tomoya Yokoyama − Laboratory of Wood Chemistry,
Department of Biomaterial Sciences, Graduate School of
Agricultural and Life Sciences, The University of Tokyo, Tokyo
113-8657, Japan; orcid.org/0000-0003-4202-1611;
Phone: +81-3-5841-5264; Email: yokoyama@
woodchem.fp.a.u-tokyo.ac.jp; Fax: +81-3-5841-2678
Authors
Shirong Sun − Laboratory of Wood Chemistry, Department of
Biomaterial Sciences, Graduate School of Agricultural and Life
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Article
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(14) Parkas, J.; Brunow, G.; Lundquist, K. Quantitative lignin
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oxidation. BioResources 2007, 2, 169−178.
Sciences, The University of Tokyo, Tokyo 113-8657, Japan;
orcid.org/0000-0002-1101-2571
Takuya Akiyama − Wood Chemistry Laboratory, Department of
Forest Resource Chemistry, Forestry and Forest Products
Research Institute, Ibaraki 305-8687, Japan; orcid.org/
0000-0003-0484-2291
Yuji Matsumoto − Wood Chemistry Laboratory, Department of
Forest Resource Chemistry, Forestry and Forest Products
Research Institute, Ibaraki 305-8687, Japan
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work was partly supported by grants from the Project of
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