Cobalt-catalyzed Aerobic Oxidation of Eugenol to Vanillin and Vanillic Acid

Article abstract of DOI:10.1002/jccs.201500357

A novel, simple, one-step method of synthesizing vanillin and vanillic acid from eugenol has been developed. The method uses ligand- and additive-free Co(OAc)₂·4H₂O as catalyst and molecular oxygen as oxidant to achieve catalytic process without isomerizing eugenol. Extensive screening efforts were used to optimize eugenol to obtain vanillin and vanillic acid. Under optimal conditions, the reaction provided vanillin and vanillic acid with 68.5% and 15.2% yields, respectively. Apart from the desired products, coniferyl alcohol 9-methyl ether and 4-hydroxy-3-methoxycinnamaldehyde as two intermediates were also detected in the reaction process. Level changes of all compounds over time were presented in the reaction. The reaction pathway from eugenol to vanillic acid was validated by conducting several control experiments. Furthermore, a possible reaction mechanism dominated by a circular redox reaction from Co(III) and Co(II) species was proposed. This method offers a potentially practical alternative for manufacturing vanillin and vanillic acid efficiently.

Full text of DOI:10.1002/jccs.201500357

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Cobalt-catalyzed Aerobic Oxidation of Eugenol to Vanillin and Vanillic Acid
Haifang Mao, Lizhi Wang, Feifei Zhao, Jianxin Wu, Haohua Huo and Jun Yu*
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, P.R. China
(Received: Sept. 1, 2015; Accepted: Dec. 10, 2015; Published Online: Jan. 26, 2016; DOI: 10.1002/jccs.201500357)
A novel, simple, one-step method of synthesizing vanillin and vanillic acid from eugenol has been devel-
oped. The method uses ligand- and additive-free Co(OAc)2×4H2O as catalyst and molecular oxygen as oxi-
dant to achieve catalytic process without isomerizing eugenol. Extensive screening efforts were used to
optimize eugenol to obtain vanillin and vanillic acid. Under optimal conditions, the reaction provided
vanillin and vanillic acid with 68.5% and 15.2% yields, respectively. Apart from the desired products,
coniferyl alcohol 9-methyl ether and 4-hydroxy-3-methoxycinnamaldehyde as two intermediates were
also detected in the reaction process. Level changes of all compounds over time were presented in the re-
action. The reaction pathway from eugenol to vanillic acid was validated by conducting several control
experiments. Furthermore, a possible reaction mechanism dominated by a circular redox reaction from
Co(III) and Co(II) species was proposed. This method offers a potentially practical alternative for manu-
facturing vanillin and vanillic acid efficiently.
Keywords: Eugenol; Vanillin; Molecular oxygen; Oxidation; Catalysis.
INTRODUCTION
Synthesis of vanillin from guaiacol
Scheme 1
Vanillin (4-hydroxy-3-methoxybenzaldehyde) and
vanillic acid (4-hydroxy-3-methoxybenzoic acid) are two
types of popular flavoring materials used worldwide.1-2
Both are widely employed in the pharmaceutical industry.3
Moreover, vanillin has a wide range of applications in the
manufacture of food, beverages, flavors, and fragrances.4
The market demand for vanillin in 2010 was over 15000
tons, and this demand increases year by year. However, the
market supply of vanillin is only 10000 tons per year.5
Nowadays, most vanillin is synthesized via nitrose
method (Scheme 1a) and glyoxylic acid method (Scheme
1b).6-8 With dwindling petroleum resources, renewable
methods for producing vanillin should be investigated.9-10
Recently, eugenol has been used as a substrate for manu-
facturing vanillin because of its economic and commercial
availability.11 The normal chemical synthesis of vanillin
from eugenol requires two steps (Scheme 2). First, the
eugenol is isomerizated to isoeugenol under basic condi-
tions.12 Then, the isoeugenol is oxidized into vanillin by us-
ing oxidants, such as potassium permanganate,13 iodo-
benzene diacetate,14 or hydrogen peroxide.15 However, the
above process usually requires high temperatures and gen-
erates excessive waste materials.8,13 Thus, new alternative
and environmentally friendly chemical processes are ne-
cessary.
Normal synthesis of vanillin from eugenol
Scheme 2
ing molecular oxygen as oxidant offer attractive industrial
prospects.16-17 A variety of transition-metal catalysts, such
as copper, cobalt, nickel, iron, and manganese, has been
employed for benzylic oxidation of aromatic compounds.16
Commercially available cobalt salts, which are inexpensive
and have low toxicity, are widely used in straightforward
benzylic C(sp3)-H oxyfunctionalization.7 Li et al. synthe-
sized a Co-[Salen-Py][PF6]2 catalyst for the oxidation of
Transition-metal catalyzed oxidation reactions utiliz-
* Corresponding author. Tel: +86-021-6087223; Fax: +86-021-60877231; Email: yujun@sit.edu.cn
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4-methyl guaiacol to vanillin by using ethylene glycol as
the solvent. A circular redox reaction from Co(III) and
Co(II) species is the key for oxidation.6 However, this cata-
lyst is rather complex and hard to synthesize. In addition,
despite ethylene glycol being an ideal solvent, its high boil-
ing point makes it difficult to recycle. Hence, industrial
production of these products is constrained by the use of
impractical metal-ligand-coordinated catalysts and high-
boiling ethylene glycol.
perature improves oxidation efficiency. Obviously, the oxi-
dation reaction could not proceed with the absence of
NaOH (entry 4). However, a 5.0 equiv. of NaOH turned out
to be sufficient to produce the highest yields for 2 and 3 at
66.7% and 16.2%, respectively (entry 5). However, mini-
mal change was observed in these yields when NaOH con-
tent was increased to 6.0 equiv. (entry 6). It is indicated that
NaOH is important in triggering the oxidation. The large
excess of NaOH, offering strong alkaline conditions, was
necessary to maintain high selectivity for the desired prod-
ucts.21 The same yields were achieved using 1.5 or 1.0
mol% Co(OAc)2×4H2O (entries 5 and 7; entry 7 with opti-
mal reaction condition). When using a lower catalyst load-
ing of 0.5 mol%, the yields of 2 and 3 dropped sharply to
61.4% and 11.9%, respectively, and most impurities were
formed (entry 8). One possible reason for this observation
was that a small amount of catalyst limited the generation
of products. In addition, only trace amounts of 2 and 3 were
observed under nitrogen atmosphere without oxygen (en-
try 9).
In this study, Co(OAc)2 without ligand and additive
was used as the reaction catalyst, and its catalytic perfor-
mance for aerobic oxidation of eugenol to vanillin and
vanillic acid was investigated for the first time (Scheme 3).
The reaction was performed under the mediation of NaOH
and methanol without isomerizing eugenol. The effects of
main reaction parameters, NaOH amount, temperature, and
catalyst loading were also analyzed. The desired products
and two intermediates were separated in the reaction pro-
cess. Level changes of all compounds as a function of time
were clearly presented. Then, the reaction pathway from
eugenol to vanillic acid was determined by several control
experiments. In addition, a possible reaction mechanism
was proposed through further studies.
Level changes of all compounds over time are labeled
in Figure 1. It shows that, apart from the desired products 2
and 3, coniferyl alcohol 9-methyl ether (4) and 4-hydroxy-
3-methoxycinnamaldehyde (5) were also isolated in the re-
action process (the structures of all compounds are shown
One-step synthesis of vanillin and vanillic
acid from eugenol
Scheme 3
RESULTS AND DISCUSSION
Selected optimizations for oxidation
Table 1 shows the effects of NaOH content, reaction
temperature, and catalyst loading on the catalytic perfor-
mance of eugenol during aerobic oxidation. It can be seen
that Co(OAc)2×4H2O proved to be an effective catalyst for
producing vanillin (2) and vanillic acid (3) with the yields
of 51.5% and 9.6% at 70 °C, respectively (entry 1). Fur-
thermore, an enhanced reaction temperature of 80 °C re-
markably increased the yields of 2 and 3 to 64.3% and
12.8%, respectively (entry 2). However, an increased tem-
perature of 90 °C resulted in minimal change, indicating
that high temperatures are not necessary for screening (en-
try 3). The reason may be that properly elevating the tem-
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Level Changes of All Compounds over Time
mesh). Several control experiments were then conducted to
investigate the reaction pathway (Scheme 5). Under opti-
mal conditions, using 1 as reaction substrate produced in-
termediates 4 (41% yield) and 5 (14% yield), as well as the
desired oxidation products 2 and 3 (13% and 2% yields, re-
spectively) at 80 °C for 6 h (Scheme 5a). Using 4 instead of
1, the reaction provided compounds 5, 2, and 3 with 31%,
45%, and 9% yields, respectively (Scheme 5b). 5 produced
the desired products 2 and 3 with yields of 53% and 12%
(Scheme 5c). Furthermore, a small amount of 3 (15% yield)
also could be obtained using 2 as the reaction substrate
(Scheme 5d). The results show that substrate 1 gradually
generated two intermediates and two desired products.
When 4 was employed as the substrate, compounds 5, 2,
and 3 were produced in the reaction. By contrast, 5 was
converted to 2 and 3 without any 4 being detected in the
process. In addition, no 5 was observed in the oxidation of
2 to 3. In conclusion, substrate 1 initially generated 4,
which was transformed into 5 followed by the subsequent
transformations of 5 to 2 and 2 to 3 in the process, forming
the reaction pathway (Scheme 6).
Fig. 1. Time-dependent changes for the levels of all
compounds. Reaction conditions: 1 (0.2 mol),
Co(OAc)2·4H2O (1.0 mol%), NaOH (5.0 equiv.),
CH3OH (150 mL), 80 °C, O2 (0.5 atm).
in Scheme 4). The quantity of 1 decreased sharply during
the initial 8 h and disappeared at 14 h. The yield of product
2 increased slowly during the first 4 h and rapidly in the
next 8 h, after which it increased slowly and achieved the
best yield of 68.5% at 20 h. Product 3 was produced at 4 h,
after which its yield increased slowly to 15.2% at 20 h. The
level of compound 4 rapidly increased in the beginning and
peaked with a 41% yield at 6 h before dropping gradually
and disappearing by 14 h. Similarly, compound 5 initially
increased and peaked (18.5% yield) at 8 h and gradually de-
creased before disappearing by 20 h. The disappearance of
compounds 4 and 5 at the end of the reaction indicated that
they are intermediates in the reaction. Figure 1 shows the
changes of all compound levels as a function of time in the
reaction process. However, these changes cannot explain
the specific pathway of the reaction.
Studies for oxidation process of eugenol
Scheme 5
The structures of all compounds of the reac-
tion
Scheme 4
Reaction pathway
All pure products and intermediates were isolated
through column chromatography on silica gel (200–300
Reaction mechanism and role of Co
Several control experiments were conducted to fur-
ther explore the mechanism of the oxidation reaction
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Oxidation process of eugenol
Mechanistic studies for intermediates
Scheme 7
Scheme 6
(Scheme 7). When the radical inhibitor 2, 2, 6, 6-tetra-
22
methylpiperidine-N-oxyl (TEMPO) (100 mol%) was
added into the reaction, only a trace amount of 2 was iso-
lated (Scheme 7a). No product was observed at the absence
of the catalyst (Scheme 7b). Only trace amounts of 2 and 3
were detected under nitrogen atmosphere (Scheme 7c).
Using dried Co(OAc)2 (1.0 mol%), dried methanol, and
CH3ONa (5.0 equiv.) instead of NaOH at 80 °C for 12 h, the
reaction produced 4 and 5 with 36% and 6% yields, respec-
tively (Scheme 7d). Additionally, no product resulted from
the reaction using pure H2O as solvent (Scheme 7e). The
results indicate that a free radical oxidation process exists,
in which TEMPO could efficiently suppress oxidation re-
action. No product could be produced without using a cata-
lyst or oxygen in the reaction. This finding indicates that a
radical oxidation process occurred, in which the catalyst
and oxygen acted as the initiator. A large amount of 4
wasn’t translated into 5 when all the dried reagents and
neat methanol were employed, indicating the direct in-
volvement of H2O in the transformation process. However,
a small amount of 5 was still produced in the control exper-
iment (Scheme 7d). The reason for this finding could be
that the reaction system containing Co(II) species can gen-
erate a small amount of H2O in the presence of oxygen.
That is to say, in the Co(II)/O2 system, Co(II) species re-
acted with 1/4O2 to form Co(III) species and 1/2H2O. The
transformation of 1 to 4 suggests the involvement of metha-
nol in the formation of intermediate 4 (Scheme 6). On the
other hand, the results also indicated that the reaction could
not proceed with the use of H2O as solvent (Scheme 7e).
These findings proved that mediation of methanol is indis-
pensable for this oxidation reaction.
anion is activated to generate phenoxy radical I by single-
electron transfer from the phenoxide to the oxidant Co(III)
species, which can be produced from the oxygenation of
Co(OAc)2×4H2O. The phenoxy radical I is subsequently
translated into radical III through resonance and rearrang-
ing.24-26 The radical III is then disproportionated to the pri-
mary substrate 1 and the transient, highly active para-
benzoquinone derivative A.17,23 This derivative further re-
acts with methanol to produce the intermediate 4 by nu-
cleophilic addition reaction. In the second phase, 4 is oxi-
dized to the transitory B, which produces intermediate 5
upon a rapidly spontaneous aromatization followed by the
fast release of methanol.27-29 Finally, the para-phenoxy
radical triggers the mild reaction through conveying reac-
tivity to the remote benzyl group. NaOH is essential to the
formation of phenoxy radical. The proposed mechanism is
characterized by (1) a powerful generation of the high re-
dox active Co(III) species derived from Co(II) species via
oxygenation and two phenoxy radical processes;30-31 and
(2) the mediation of methanol and two functional forma-
tions of para-benzoquinone derivatives.
Based on the above findings and Baik-Ji-type mecha-
nism beginning with a phenoxy radical,17,23 a similar radi-
cal reaction mechanism was proposed to rationalize the al-
kaline Co(OAc)2×4H2O-catalyzed oxidation of 1 (Scheme
8). A hydrogen ion is abstracted from the phenolic hydro-
xyl of 1 by NaOH to form a phenolate anion in the basic
system.21 During the first phase, the oxidation of phenolate
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Level Changes of All Compounds over Time
A possible mechanism for the formation of
Scheme 8
After neutralization of the reaction system, methanol was
removed in vacuo. The resulting solution was extracted with tolu-
ene (3 ´ 200 mL), and the organic layers were combined and con-
centrated in vacuo to give a yellow solid. vanillin (2) was ob-
tained by recrystallization from ethanol–water mixtures (150
mL). The mother liquor was concentrated in vacuo to give a yel-
low solid. After flash chromatography on silica gel (200–300
mesh), vanillic acid (3) and the remaining vanillin (2) were sepa-
rated.
intermediates
Procedure related to the mechanism: Another reaction
was conducted under similar conditions as the above to determine
the reaction mechanism. The reaction mixture was removed as a
function of time.
After neutralization of the reaction mixture, methanol was
removed in vacuo. The resulting solution was extracted with tolu-
ene (3 ´ 200 mL), and the organic layers were combined and con-
centrated in vacuo to give a yellow solid. After flash chromato-
graphy on silica gel (200–300 mesh), vanillin (2), vanillic acid
(3), coniferyl alcohol 9-methyl ether (4), and 4-hydroxy-3-me-
thoxycinnamaldehyde (5) were isolated.
CONCLUSIONS
A novel, simple, one-step method to synthesize vanil-
lin and vanillic acid from eugenol has been developed. The
process uses ligand- and additive-free Co(OAc)2×4H2O as
catalyst and molecular oxygen as oxidant to achieve the cat-
alytic process without isomerizing eugenol. The effects of
the main reaction parameters, such as NaOH amount, tem-
perature, and catalyst loading, were investigated to optimize
the oxidation of eugenol to vanillin and vanillic acid. Under
the optimal reaction condition of 80 °C, Co(OAc)2× 4H2O
(1.0 mol%) and NaOH (5.0 equiv.), the reaction produced
vanillin and vanillic acid with 68.5% and 15.2% yields, re-
spectively. Apart from these products, coniferyl alcohol
9-methyl ether and 4-hydroxy-3-methoxycinnamaldehyde
as two intermediates were also isolated in the reaction pro-
cess. Level changes of all compounds as a function of time
were then presented. Several control experiments were con-
ducted to investigate the reaction pathway. This pathway in-
volves generating coniferyl alcohol 9-methyl ether from
eugenol. The ether then produces 4-hydroxy-3-methoxycin-
namaldehyde, which subsequently transforms to vanillin
and vanillic acid. More efforts were employed to further ex-
plore the mechanism of the oxidation reaction. Finally, a
mechanism that involves a powerful generation of the high
redox active Co(III) species derived from Co(II) species via
oxygenation, two phenoxy radical processes, mediation of
EXPERIMENTAL
Materials and Instruments: All reagents and solvents
were purchased from Sinopharm Chemical Reagent Co., Ltd and
used without additional purification. 1H NMR spectra were re-
corded on a Bruker DPX 500 MHz spectrometer in CDCl3 and
DMSO-d6 with Me4Si (TMS) as internal standard at room tem-
perature. Chemical shifts are given in ppm. Mass spectra were re-
corded on a Waters Micromass GCT Premier mass spectrometer.
Melting points (m.p.) were determined by employing a Hanon
MP470 automated melting point instrument without further cor-
rection.
General reaction procedure: The oxidation reactions
were carried out in a 500 mL autoclave equipped with a thermo-
meter and a mechanical stirrer.
In a typical reaction, NaOH (40.0 g; 5.0 equiv.) and metha-
nol (150 mL) were added into the reactor and vigorously stirred at
room temperature. When the alkali had dissolved completely,
eugenol (32.8 g; 0.2 mol) was added to the system and stirred for
10 min to ensure the conversion of eugenol to sodium salt.
Co(OAc)2×4H2O (1.0 mol%) was then introduced quickly. Reac-
tions were performed at 80 °C (Psystem, 1.0 atm without O2) with
the reaction system (1.5 atm) filled with pure O2 (0.5 atm) and
concluded at 20 h.
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ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial support
from the started project of scientific research for the intro-
duction of talent of Shanghai Institute of Technology New
Process for Synthesis of Vanillin (No.1010k13608-YJ
2013-50), “Research center of analysis and measurement
of Shanghai Institute of Organic Chemistry (SIOC)” for
high-resolution mass spectrometry measurements and
Leading Academic Discipline Project of Shanghai Educa-
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