The unusual characteristics of the aerobic oxidation of 3,4-dimethoxytoluene with metal/bromide catalysts

Article abstract of DOI:10.1002/adsc.200404139

DMtoluene (3,4-dimethoxytoluene; DM = 3,4-dimethoxy), a model compound for lignin oxidation, can be autoxidized in acetic acid using a Co/Mn/Br catalyst to its benzaldehyde in 51 mol % yield, and to its acid in 85-92 mol % yield. This synthesis is unusual because a number of unprecedented phenomena occur. 1) The rate of molecular oxygen reacting with the substrate is bi-phasic, i.e., two maximum in the rate of reaction are observed. 2) In the first phase, all of the 3,4-dimethoxytoluene is converted to intermediates, but very little to the carboxylic acid. 3) During the second maximum of activity, virtually all the intermediates are converted to the carboxylic acid with 95-100% mass accountability. 4) The rate of carbon monoxide and carbon dioxide formation is considerably higher during the second phase during which 5) 9-12% of methyl 3,4-dimethoxybenzoate (the methyl ester of the benzoic acid) is formed. Mechanisms are suggested for these unusual phenomena.

Full text of DOI:10.1002/adsc.200404139

UPDATES
The Unusual Characteristics of the Aerobic Oxidation of
,4-Dimethoxytoluene with Metal/Bromide Catalysts
3
Walt Partenheimer
Central Research and Development, E. I. DuPont de Nemours & Co., Inc., Experimental Station, Wilmington, Delaware
9880-0262, USA
1
Fax: (þ1)-302-695-8347, e-mail: delparts@att.net
Received: May 15, 2004; Accepted: August 16, 2004
Abstract: DMtoluene (3,4-dimethoxytoluene; DM¼
sources of lignin are spent liquors from the wood-pulp
industry. Its exact structure is unknown and varies de-
pending on the source of the lignin. It is known to be
composed of substituted aromatic derivatives held to-
gether by aliphatic chains. It is primarily composed of
p-hydroxybenzene, guaiacyl (4-alkyl-2-methoxyphe-
3
,4-dimethoxy), a model compound for lignin oxida-
tion, can be autoxidized in acetic acid using a Co/Mn/
Br catalyst to its benzaldehyde in 51mol % yield,
and to its acid in 85–92 mol % yield. This synthesis
is unusual because a number of unprecedented phe-
nomena occur. 1) The rate of molecular oxygen re-
acting with the substrate is bi-phasic, i.e., two maxi-
mum in the rate of reaction are observed. 2) In the
first phase, all of the 3,4-dimethoxytoluene is con-
verted to intermediates, but very little to the carbox-
ylic acid. 3) During the second maximum of activity,
virtually all the intermediates are converted to the
carboxylic acid with 95–100% mass accountability.
nol), 1,2a] nd syringyl (4-alkyl-2,5-dimethoxyphenol)
[
units.
The syringyl group, for example, can be oxi-
dized to vanillin and vanillic acid:
4) The rate of carbon monoxide and carbon dioxide
formation is considerably higher during the second
phase during which 5) 9–12% of methyl 3,4-dime-
thoxybenzoate (the methyl ester of the benzoic
acid) is formed. Mechanisms are suggested for these
unusual phenomena.
ACo/Br-catalyzed autoxidation of lignin in acetic acid
has produced vanillin and vanillic acid in yields of 1.9
[
3]
Keywords: autoxidation; catalytic hydrocarbon oxi-
dation; CÀH activation; cobalt; manganese; oxygen
and 3.0%, respectively. The Co/Mn/Br/Zr-catalyzed
autoxidation of lignin dissolved in acetic acid has result-
ed in p-hydroxybenzaldehyde, p-hydroxybenzoic acid,
vanillin, vanillic acid, syringaldehyde, and syringic
acid, i.e., the expected products from the oxidation of
[
4]
Introduction
all three of the above mentioned groups. Consequently
[
5]
we and others have oxidized 3,4-dimethoxytoluene as
Lignin, a polymeric component of woody plants, is the a model compound. The closely related compound, 4-
second most abundant natural material. Commercial methoxytoluene, has been autoxidized with metal/bro-
Table 1. Comparison of the reactivity of 3,4-dimethxytoluene to selected toluenes (in acetic acid solvent from Hammett rela-
[6]
tionships )
Type of Catalyst
[Co], M
Mol Ratio
Rho Value
std. dev.)
Relative Reactivity
(
4
-NO2
H
4-CH3
3,4-di-CH O
4-CH O
3
3
1
2
4
5
6
7
8
Co/Br
0.02
2/2
À0.85 (0.08)
À1.28 (0.13)
1.33 (0.12)
1.38 (0.12)
1.47 (0.21)
À1.81
1.00
1.00
1.00
1.00
1.00
1.00
1.00
4.70
8.63
25.6
29.0
33.0
41.4
97.9
100
19.7
88.2
105
125
172
564
584
21.7
102
122
147
203
695
720
Co/Mn/Br
Co/Mn/Zr/Br
Co/Ce/Br
Co/Mn/Cl/Br
Co
0.004
0.01
0.04
0.019
–
1/1/2
1/1/0.19/2
4/0.4/1
1.9/.016/5.5/1.8
–
–
10.3
11.2
12.3
14.5
26.9
27.4
bromide
–
À1.82
Adv. Synth. Catal. 2004, 346, 1495–1500
DOI: 10.1002/adsc.200404139
ꢀ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1495

UPDATES
Walt Partenheimer
[
6,7]
mide and other catalysts. The author has reviewed the highly reactive substrate, see Table 1. It is 88 times
metal/bromide autoxidation of hydrocarbons up to the more reactive than 4-nitrotoluene, 10 times more reac-
[6]
year 1995. 251different reagents have been oxidized
tive than toluene and slightly less reactive than 4-me-
in this manner in which 84% are alkylaromatic com- thoxytoluene with a Co/Mn/Br catalyst. The rapid rate
pounds. One can predict, and is substantiated herein, of reaction at 808C reflects this reactivity. The tempera-
that the reactivity of 3,4-dimethoxytoluene is very high ture is chosen so that the gases exiting the reaction have
compared to that of toluene and other selected methyl- an oxygen content of at least 3–5%. This is done be-
aromatic compounds, see Table 1.
cause the rate of reaction of radicals with dioxygen is dif-
fusion controlled. If the oxygen content in the reactor is
too low, radical coupling occurs which leads to by-prod-
ucts, often intensely colored, and a reduction in the rate
of reaction since the steady state concentration of the
free radicals is being decreased.
The initial conditions used in the experiments are
given on Table 2 and the yields in Table 3. The reaction
sequence and intermediates formed are given on Fig-
Discussion
On the Synthesis of 3,4-Dimethoxybenzaldehyde and
3,4-Dimethoxybenzoic Acid via Autoxidation
The oxidation reaction occurs via a metal-catalyzed free ure 1. Intermediate concentrations as a function of con-
[
6]
radical chain mechanism. 3,4-Dimethoxytoluene is a version of 3,4-DMtoluene are given in table 4. 3,4-Di-
methoxybenzaldehyde is obtained in 51% yield at a con-
version of 97%. This yield is unusually high during au-
[
6]
toxidation of methylaromatic compounds since the
usual maximum yields range from 15–35%. However,
a 52% yield of the benzaldehyde has been reported for
[
7f]
4
-methoxytoluene at a conversion of 67%. Trying to
isolate this aldehyde from the other products would, of
course, not be trivial.
Yields to the DMbenzoic acid are 75–80%. Since
methyl DMbenzoate is the methyl ester of DMbenzoic
acid (Figure 1) and the acid can be obtained by hydrol-
ysis of the ester, it is possible to obtain 85–92% of 3,4-di-
methoxybenzoic acid via subsequent hydrolysis. These
yields are comparable to the oxidation of many other
methylaromatic compounds such as toluene, p-xylene,
[
6]
etc. using metal/bromide catalysts.
Doubling the catalyst concentration in examples 1and
2
resulted in a 34% increase in the rate of reaction of 3,4-
dimethoxytoluene, similar to that observed with other
[
10,8]
substrates such as p-xylene
and hydroxymethylfur-
[
8,9]
Figure 1. Suggested reaction sequence during the autoxida- fural.
The increased catalyst concentration also re-
duces, by more than 50%, the observed carbon dioxide
formed, see Table 3. The increased bromide, however,
tion of 3,4-dimethoxytoluene to 3,4-dimethoxybenzoic acid
(DM¼3,4-dimethoxy).
Table 2. Initial conditions for the autoxidation of 3,4-dimethoxytoluene.
Catalyst Type
[DMtoluene], M
[catalyst], M
Time of rxn.
min]
Temp [ 8C]
Rate Constant
5 [d]
À1
[
[s Â10 ]
1
2
3
4
5
Co/Mn/Br
Co/Mn/Br
Co/Mn/Br
Co/Mn/Br
Co
0.66
0.66
0.010/0.010/0.020
0.020/0.020/0.040
524
0.020/0.020/0.040
0.12
812
499
60,80
351
80
80
11.9(0.4)
16.0(0.8)
[
[
[
a]
b]
c]
0.610.020/0.020/0.040
0.66
0.66
9.04(0.82)
80
80, 95
13.2(1.5)
–
592
[
[
a]
b]
Initial reaction temperature was 608C. The temperature was increased to 808C at time 313 min.
An additional, equimolar amount of HBr was added at 221min. An additional equimolar amount of Co(II) acetate was
added at 291min.
[
[
c]
Increments of acetaldehyde were added to initiate the reaction at 21, 348, 499 min. The temperature was increased to 958C
at 280 min.
From first order regression analysis for the disappearance of 2,4-dimethoxytoluene.
d]
1496
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Unusual Characteristics of the Aerobic Oxidation of 3,4-Dimethoxytoluene
UPDATES
Table 3. Final yields from the autoxidation of 3,4-dimethoxytoluene.
Yield [mol %]
DMtoluene
DMaldehyde
Methyl
DMbenzoate
DMbenzoic
acid
DMbenzyl
acetate
DMbenzyl
alcohol
Total
0.56
96
105
70
CO /DMtoluene,
mol/mol
x
[
a]
1
2
3
4
5
0
6.1
8.9
76
2.6
80
75
4.7
9.3
0
1.8
0.07
4.2
95
0
0
15
28
1.9
17.4
44
12
0
0
0
0.22
0.15
–
9.5
0.05
0
310
60 5. 1
–
[
a]
CO is the sum of the carbon dioxide and carbon monoxide integrated over the entire reaction. The carbon dioxide content
x
is 5–8 times higher than that of the carbon monoxide.
Table 4. Selected samples of the GC analysis of products dur-
ing the aerobic oxidation of 3,4-dimethoxytoluene (from Ex- matic rings having high electron density.
ample 2).
Ring-brominated compounds are formed from aro-
[
6,10,11]
The bro-
mo-3,4-DMtoluene and bromo-3,4-DMbenzaldehyde
occur with yields around 1% representing about 37%
of the bromine added as catalyst to the system. Although
almost certainly present, but not analyzed for, is bromo-
DMbenzoic acid. The mass accountability of 95–100%
at the end of the experiments indicates relatively low
yields of Br-DMbenzoic acid.
1
2
3
4
5
Time, min
,4-DMtoluene
bromo-3,4-DMtoluene
,4-DMbenzaldehyde
methyl 3,4-DMbenzoate
15 135 240 321 457
3
67.6 32.9
0.7 1.6
3.2
1.2
0.0
0.1
0.0
0.0
2.1
7.6
2.4
0.0
0.4
3
2.9 27.5 51.2 19.9
0.0
0.6
1.3
0.0
2.7
1.5
0.2
0.9
3.4
0.0
1.1
3.9
1.8
0.0
1.3
3
3
,4-DMbenzyl acetate
,4-DMbenzyl alcohol
Unusual Phenomena during the Oxidation of
2.2 19.5 62.6 83.3 DMtoluene
bromo-3,4-DMbenzaldehyde 0.0
,4-DMbenzoic acid 0.0
74.3 68.6 80.7 89.6 95.8
11 30 37 22
3
Total
Organobromine
6.2 There are five features that, in the authorꢁs experience,
are unique to the metal/bromide autoxidation of 3,4-di-
methoxytoluene.
[a]
(
HBr basis) [%]
[
a]
Does not include bromo-3,4-DMbenzoic acid which was
not measured.
1. Biphasic behavior: Normally, the rate of molecular
oxygen uptake increases to a maximum, as the benzalde-
hyde concentration rises to its highest level and then de-
creases to zero as the intermediates (aldehyde, alcohol,
also results in higher yields of the ring brominated com- acetate) are converted to the acid. Figure 2 illustrates
pounds. The selectivity increase, i.e., reduction in carbon that for DMtoluene the dioxygen rate decreases from
[
9]
dioxide, has been previously rationalized. The free 9 mL/min to 5 mL/min but then the rate of oxygenation
radical chain mechanism is significantly modified in again increases to 8 mL/min before a final, rapid de-
the presenceof the catalystso that the reaction pathways crease to zero. Bi-phasic behavior has been observed
[
9]
become predominately metal catalyzed. The cobalt se- by us in the autoxidation of benzyl alcohol and oscillat-
[12]
lectively and rapidly reacts with the intermediate peroxy ing behavior has been observed for benzaldehyde but
radicals, which by-passes the propagation step, and also never for an alkylaromatic compound. The two rate
st
nd
reacts with the peroxy acid to directly produce the car- maxima will be referred to as the 1 and 2 stages of
boxylic product.
the reaction.
This biphasic behavior is not being caused by the for-
mation of non-active bromine species. This is because
addition of hydrogen bromide near the end of the first
stage of reaction, example 4, had no affect on the overall
ð1Þ
In experiment #3 we lowered the temperature to 608C. results. Also the high mass accountability at the end of
Conducting an autoxidation with highly reactive sub- the reaction indicates that inordinate amounts of bro-
stances often benefits from lower temperatures since mo-DMbenzoic acid are not forming.
the reaction rates are reduced allowing for more radicals
2. Rate of formation of the carboxylic acid: Normally in
to react with the dioxygen in solution. No improvement metal/bromide-catalyzed autoxidation the carboxylic
in yields or change in the amount of the methyl DMben- acid starts to form almost immediately and rises rapidly
zoate occurred, however.
as the conversion of the methylaromatic compound in-
Adv. Synth. Catal. 2004, 346, 1495–1500
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1497

UPDATES
Walt Partenheimer
temperature usually further stimulates the rate of reac-
tion but the reaction still resulted in low yields of inter-
mediates with none of the desired benzoic acid being
formed. Acetaldehyde is added because it is rapidly oxi-
dized to peroxyacetic acid which, in turn, rapidly oxidiz-
es the Co(II) dimer to Co(III). Co(III) is the active cat-
alyst species because it is capable of oxidizing the alkyl-
aromatic to the benzylic radical.
Some Possible Mechanisms for the Unusual Behavior
This is the first reported instance of biphasic behavior
for the autoxidation of an alkylaromatic compound.
The biphasic behavior seen here is similar to that report-
ed for benzyl alcohol to benzoic acid using metal/bro-
mide catalysts in acetic acid. In the first phase the benzyl
alcohol almost completely reacts to form benzaldehyde
Figure 2. Rate of reaction of dioxygen and rate of formation
of carbon monoxide, carbon dioxide, and methyl 3,4-DMben-
zoate. From example 2 in Table 2.
[
13,14]
creases to 100%.
tion normally occurs as the conversion approaches
00%. The reason for this is that the benzaldehydes
and benzylic alcohols are 5 and 15 times, respectively,
Maximum benzoic acid concentra-
nd
in 80% yield. In the 2 phase the predominately benzal-
dehyde solution then rapidly reacts to form benzoic acid
1
[8]
in 88% yield. 4-methoxybenzyl alcohol, an alcohol of
similar reactivity to 3,4-dimethoxytoluene (see Table 1)
also undergoes a biphasic reaction giving a maximum
[
8]
more reactive than the methylaromatic compound.
In contrast, when the conversion of the DMtoluene is
nearly 100%, the yield to DMbenzoic acid is only 8–
[15]
yield of 54% of 4-methoxybenzaldehyde. It has been
previously reported that the autoxidation of benzalde-
hyde is inhibited by increasing concentrations of benzyl
18%. In fact, the first time this reaction was performed
it was prematurely terminated because the GC indicated
close to 100% conversion and the rate of oxygen uptake,
which is continually monitored, was declining. It is dur-
[16]
alcohol. So it is possible that the biphasic behavior is
caused by the presence of alcohol moieties forming in
solution. However for 3,4-dimethoxytoluene only rela-
tively low amounts of the benzyl alcohol and acetates
are observed – a total of about 5–7%. Perhaps unusually
high amounts of the hydrate of 3,4-dimethoxybenzalde-
nd
ing the 2 stage that the rate of reaction again increases
and that>80% of DMbenzoic acid is formed, see Fig-
ure 2 and Table 3.
3. Formation of the methyl ester of the carboxylic acid
hyde, i.e., DMPhCH(OH) form but this compound has
2
in high yields (9–12%) has never been reported during
autoxidation: The esters are seen in trace amounts dur-
ing the commercial production of p-xylene to tereph-
not been detected in our GS/MS studies. We did detect
the corresponding acetate of the hydrate i.e.,
DMPhCH(OH)(OAc) so this is a distinct possibility.
[6]
thalic acid. All of the methyl DMbenzoate is produced
Two other unique phenomena that occur during the
nd
in the 2 stage of the reaction, see Figure 2.
nd
2
phase – the high rate of carbon dioxide formation
nd
4
. The 2 stage of high rate of oxygen uptake is accom-
and the formation of methyl DMbenzoate – are proba-
bly closely related. Isotopic labeling studies during the
autoxidation of p-xylene, a commercial process which
is quite similar to that described here, indicate that by-
products form from the methyl group of the acetic
acid. Of particular importance is that the majority of
the methanol and methyl acetate form from the oxida-
panied by a very high rate of formation of carbon dioxide,
see Figure 1: In example 2, during the first stage the val-
ues slowly increase from 0.08 to 0.23 mL/min but then
rapidly rise to a maximum value of 1.8 mL/min during
nd
the 2 stage. Carbon monoxide is seen as well but is
less by about a factor of 5.
5. Use of a cobalt catalyst, without manganese and bro-
[17]
tion of the acetic acid. There is a close association of
mine, at significantly higher catalyst concentrations than
used in metal/bromide catalysts, normally will oxidize a
methylbenzene compound to its corresponding benzoic
the rate of decomposition of the acetic acid to the rate
[18]
of formation of methanol and methyl acetate. The
mechanisms of formation of these species are rational-
ized by the following set of reactions:
[
6]
acid in high yields: Although a significant amount of
oxygen uptake initially occurred the final conversion
was only 72%. Virtually no DMbenzoic acid was
formed, see example 5 in Tables 1and 2. The deep green
coloration that normally occurs due to a high steady
state concentration of cobalt(III) was not seen in this ex-
periment. Addition of acetaldehyde or increasing the
ð2Þ
ð3Þ
1498
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Adv. Synth. Catal. 2004, 346, 1495–1500

Unusual Characteristics of the Aerobic Oxidation of 3,4-Dimethoxytoluene
UPDATES
ð4Þ
ð5Þ
X is either Co(III), a reduced bromide species (Br), or
some organic radical species that abstracts a hydrogen
Figure 4. Suggested mechanism for the formation of methyl
DMbenzoic acid from Co(III)
atom from acetic acid. The acetoxy radical undergoes
an exceedingly fast decomposition with a first-order
9
À1
[19]
rate constant of 1.6Â10 s at 608C. It is important
that no methanol and only traces of methyl acetate are
detected by GC in the experiments described here.
This suggests that methyl DMbenzoate is forming via a
concerted mechanism within the coordination sphere
ð7Þ
nd
of the metal. Methyl DMbenzoate forms during the 2
phase which is initiated when the DMbenzaldehyde is
at its highest concentration. Since DMbenzaldehyde The cobalt(III) dimer can now simultaneously oxidize
forms the peroxybenzoic acid, we describe a possible both the coordinated acetic acid and DMbenzoic acid
mechanism starting with the peroxy acid.
to their respective radical moieties as shown in Figure 4.
The peroxy acid enters the coordination sphere of the The acetoxy radical forms the methyl radical within the
substitutionally labile Co(II). The ligand displaced is the coordination sphere which attacks the aromatic acid
[
20]
acetic acid since it is exceptionally weakly bound
only ligands necessary for the mechanism are included dioxide. The mechanism given below is similar to the for-
in the coordination sphere): mation of benzylic bromide from Co(III)/bromide mix-
radical generating the methyl-DMbenzoate and carbon
(
[
20,22,23]
tures for which there is kinetic evidence.
One still
needs to explain why this behavior is peculiar to DMto-
luene oxidation and not to 4-methoxytoluene or toluene
autoxidation. Perhaps the steric hindrance of the meta-
ð6Þ methoxy group in the coordination sphere allows the
methyl radical unusually good access to the DMbenzoic
The peroxy acid is known to very rapidly oxidize the acid.
[
21]
Co(II) dimer to Co(III).
Other mechanisms are feasible such as the hydrolysis
of the peroxybenzoic acid to DMbenzoic acid and hy-
drogen peroxide. Hydrogen peroxide can lead to hy-
droxyl radicals which can attack the acetic acid and
lead to a mechanism similar to that described above.
Experimental Section
Cobalt and manganese salts were the metal(II) acetate tetrahy-
drates. Bromine source was 48% aqueous hydrobromic acid.
[9]
The reactor is a glass autoclave as previously described.
The source of dioxygen was air (20.9%) at ambient pressure.
The flow rate of air through the reactor was 100 mL/min. Liq-
uid samples were periodically removed and analyzed by GC
and GC/MS. A continuous flow of the vent gases was passed
through a GC and analyzed for dioxygen, dinitrogen, carbon
monoxide and carbon dioxide.
The GC response factors for DMtoluene and DMbenzalde-
hyde were determined from known samples. Response factors
for DMbenzyl alcohol, DMbenzyl acetate, methyl DMben-
zoate were assumed to be the same as those of their corre-
sponding toluene derivatives. The response factors for Br-
DMtoluene and Br-DMbenzaldehyde were given a value of
1.5 similar to analogous compounds. The assumptions made
in the response factors do not result in significant errors be-
Figure 3. Product formation from the aerobic oxidation of
,4-dimethyltoluene. ꢂothersꢁ are Br-DMtoluene, Br-DMben-
zaldehyde, DMbenzyl alcohol, and DMbenzyl acetate
DM¼3,4-dimethoxy; from example 2 in Table 2).
3
(
Adv. Synth. Catal. 2004, 346, 1495–1500
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ꢀ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1499

UPDATES
Walt Partenheimer
cause these compounds form in only small amounts. alpha-
Bromodimethoxytoluene, the benzylic bromide, was not de-
tected. Its absence is expected for this highly reactive sub-
strate. The 3,4-dimethoxybenzoic acid yield was determined
by HPLC calibration with a authentic sample of 3,4-DMbenzo-
ic acid.
Solids of DMbenzoic acid precipitated from the reactor
when cooled to room temperature. A 43% yield of DMbenzoic
acid precipitated at room temperature in example 2 of Table 2.
Higher yields presumably could have been obtained by partial
evaporation of the filtrate. Washing the solids with water to re-
move the paramagnetic catalyst metals, and dissolution in deu-
tero-DMSO, gave proton and carbon NMR spectra identical to
those of an authentic sample of DMBenzoic acid. Samples for
HPLC were prepared by taking weighed aliquots from the re-
actor and diluting them with acetic acid so that precipitation of
DMbenzoic acid would not occur.
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