Biomimetic conversion of α-pinene with H2O2 to sobrerol over V2O5: Dihydroxylation by a peroxo vanadium peracid vectoring gentle synergistic oxidation

Article abstract of DOI:10.1016/j.catcom.2020.106041

In this communication, we report the gentle preparation of sobrerol from dihydroxylation of α-pinene synergistically catalyzed by V₂O₅-H₂O₂ under benign conditions. It was proposed that a “peroxo vanadium acid”, V^VO(OH)(OOH), was formed by HOO^? insertion and proton transfer between V₂O₅ and H₂O₂. Theoretical DFT calculations that using the dimer?vanadium peracid as a model of the catalytically active species revealed that peroxo vanadium acid exhibited bifunctional catalytic capabilities resembling epoxidation of α-pinene by peracetic acid and then open-ring hydration with an acetic media.

Full text of DOI:10.1016/j.catcom.2020.106041

CatalysisCommunications142(2020)106041
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Biomimetic conversion of α-pinene with H2O2 to sobrerol over V2O5:
Dihydroxylation by a peroxo vanadium peracid vectoring gentle synergistic
oxidation
Qiang Liu, Geng Huang, Huiting He, Qiong Xu⁎, Hui Li, Jian Liu, Xianxiang Liu, Liqiu Mao,
Steven Robert Kirk, Shengpei Su, Dulin Yin⁎
National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, Key Laboratory of Chemical Biology and Traditional
Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, Hunan, PR
China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Dihydroxylation
Sobrerol
V2O5
Hydrogen peroxide
In this communication, we report the gentle preparation of sobrerol from dihydroxylation of α-pinene sy-
nergistically catalyzed by V2O5-H2O2 under benign conditions. It was proposed that a “peroxo vanadium acid”,
VVO(OH)(OOH), was formed by HOO− insertion and proton transfer between V2O5 and H2O2. Theoretical DFT
calculations that using the dimer‑vanadium peracid as a model of the catalytically active species revealed that
peroxo vanadium acid exhibited bifunctional catalytic capabilities resembling epoxidation of α-pinene by per-
acetic acid and then open-ring hydration with an acetic media.
1. Introduction
sulfoxidation, and cyclization of organic substrates, particularly, ter-
penes, indoles, and other biotransformations [6]. It was found that
The excellent oxidative catalysis properties of vanadium pentoxide
(V2O5) have been extensively recognized. It is widely known that V2O5
is the essential ingredient of industrial catalysts in the production of
bulk chemical raw materials, such as inorganic sulfuric acid and organic
phthalic anhydride. The catalytic oxidation performance of V2O5 is
undoubtedly outstanding among many vanadium oxides in fine synth-
esis applications. Gopinath et al. reported that V2O5 directly catalyzed
the conversion of aromatic aldehydes and alcohols to esters in the
presence of hydrogen peroxide, and proposed an intermediate state of
hemiacetals [1]. In subsequent explorations, acetals were used as sub-
strates to be oxidized to methyl esters in methanol by V2O5 [2]. Singhal
et al. have described the first observation for the oxidative cyanation of
tertiary amines with sodium cyanide in presence of V2O5 using mole-
cular oxygen as oxidant under mild reaction conditions [3]. Given these
reports, one can justifiably look forward to developing an efficient and
green catalytic oxidation system with V2O5.
V2O5 nanowires surprisingly exhibited an intrinsic peroxidase-like ac-
tivity that efficiently converts the classical peroxidase substrates such as
2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and
3,3,5,5,-tetramethylbenzdine in the presence of hydrogen peroxide [7].
The commercial V2O5 also showed bromoperoxidase - like performance
at regioselective bromination of organic substrates by a halide anion.
The selective monobromination of aromatic compounds by Mg-Al-
layered double hydroxide- CO32−-Br− (LDH-CO32−-Br−) with V2O5
and hydrogen peroxide as the oxidant, with high para-selectivity, was
reported [8]. Specific vanadium species, as catalytically active centers
could perform like enzymes, but the catalytic mechanism has to date
not been clarified. Attention has also been given to the bio-
transformation between terpenoids [9], so the effects of vanadium
oxide species on them has also been investigated.
Forests emit large quantities of volatile organic compounds (VOCs),
mainly terpenes [10], to the atmosphere. Their condensable oxidation
products can form secondary organic aerosol, and contribute to the
atmospheric particle burden with implications for air quality and cli-
mate [11]. Terpenes and terpenoids are one of the precious components
of the biomass widely existing in pine tree and other plants [12], on
which further exploration of fine utilization may well be justified, for
improving the atmospheric environment. Per year, 250,000–300,000
Vanadium is a trace element in living organisms, especially marine
life, which plays an important role in regulating the life appearance of
animals, such as vanadium in insulin mimics [4]. After vanadium-
containing haloperoxidases (V-HPO) were found in many marine or-
ganisms, the structure and catalytic performance were investigated,
including the mechanism of V-HPO-catalyzed bromination [5],
⁎ Corresponding authors.
E-mail addresses: xuqiong@126.com (Q. Xu), dulinyin@126.com (D. Yin).
https://doi.org/10.1016/j.catcom.2020.106041
Received 24 February 2020; Received in revised form 6 May 2020
Availableonline11May2020
1566-7367/©2020PublishedbyElsevierB.V.

Q. Liu, et al.
CatalysisCommunications142(2020)106041
80
60
40
20
0
80
60
40
20
0
α -pinene
α -pinene oxide
Sobrerol
0
20
40
60
80
100
120
140
Time/min
Fig. 1. Effect of time on oxidation of α-pinene by V2O5 with H2O2.
tons of turpentine are produced worldwide from wood pulp pa-
permaking and resin tapping, from which about 100,000 tons are used
for the production of terpenes and terpenoids for pharmaceutical, food,
agricultural, and chemical industries that have developed them for their
potentials and effectiveness as drugs, flavor enhancers, pesticides, and
fine chemicals, respectively [13]. In particular, sobrerol, a mono-
terpenoid and well-known mucolytic agent (marketed as SOBREPIN),
exhibited excellent physiological activity against obstructive re-
spiratory diseases and has been widely investigated and applied [14]. In
addition, it has certain application value in perfume, essential oils and
other crude industries, and can also be used as an unsaturated monomer
material for sustainable renewable polymers [15]. Sobrerol also has
certain preventive and therapeutic effects on cardiovascular diseases
which also play an important role in the treatment of secret otitis media
in childhood [16,17]. Sobrerol was used as an adjuvant in combination
with anesthetic drugs, inhibiting propofol metabolism to prolong an-
esthesia time [18]. It has even been reported in frontier research that
sobrerol had demonstrated chemopreventive activity against mammary
cancer at the initiation and development stages of carcinogen induction
[19]. With the advancement of medical standards and detection means,
sobrerol was found to be a novel and promising therapeutic agent for
human colorectal cancer; it can restore the activity and efficacy of ce-
tuximab in ras mutated tumors [20]. It is particularly desirable to de-
velop higher value drugs as a combination treatment with sobrerol.
Hydrogen peroxide is a well-known green oxidant, which can effi-
ciently promote olefin epoxidation in conjunction with transition-metal
complexes [21] and, silicodecatungstate compounds [22], respectively.
Kapoor et al. [23] discovered that titanium containing in-
organic–organic hybrid mesoporous materials possessing catalytic ac-
tivity in epoxidation of α-pinene using hydrogen peroxide, and recently
another group presented a titanium-based silicalite epoxidation catalyst
for the reaction [24]. Corina et al. [25] evaluated the catalytic per-
formance of active vanadium nanospecies on MCM-41 for olefins in α-
pinene oxidation with H2O2, in which α-pinene conversion only was
12.8%, and sobrerol selectivity was 36.0%; thus, direct investigation of
V2O5-H2O2 for α-pinene oxidation is meaningful in academic
knowledge and industrial application.
The products were analyzed using GC-2010 PLUSAF gas chromato-
graphy (Shimadzu, Japan) equipped with a capillary column and an FID
detector. The GC mass balance was based on the substrate charged. The
organic solvent was removed by rotary evaporation, and then the crude
sobrerol separated out at room temperature. Sobrerol crystals were
obtained by recrystallization from ethyl acetate, and its structure was
determined by 1H NMR, 13C NMR, and 2D-NMR (HSQC). The re-
presentative GC spectra of crude reaction mixtures and the measured
NMR spectra of sobrerol can be viewed in the Supporting Information.
3. Results and discussion
Although sobrerol can be isolated from the extracts of some plants
(the content was 7 mg/kg from the fruits of Illicium lanceolatum [26]),
there is also a documented chemical synthesis of sobrerol starting from
methyl 3,5-dihydroxy-4-methyl benzoate in eight steps with an overall
yield of 26% [27]. Alternatively, it can be prepared through converting
α-pinene by microorganisms with biological enzymes, such as Armil-
lariella mellea, which was incubated in 0.2% α-pinene for 4–5 days with
a yield of sobrerol of 20% [28]. Generally, the raw materials for pre-
paring sobrerol were mostly α-pinene oxide, which is an epoxidation
product of α-pinene. First, α-pinene was oxidized to intermediate α-
pinene oxide by peroxy organic acids, and then α-pinene oxide was
converted to sobrerol through heating and hydration in an acidic
medium.
Coincidentally, the vanadium content of Armillariella mellea was
found to be around 0.2 mg/kg [29]. Vanadium was considered to be the
component in the active center in the enzyme that converted α-pinene
to sobrerol. Experiments were designed to verify this conjecture. α-
Pinene oxide was generally considered an intermediate to sobrerol,
which can be easily observed from Fig. 1. The effects of different per-
oxides on α-pinene oxidation with V2O5 were investigated, and the
results are listed in Table S1. It is indicated that α-pinene was difficultly
oxidized into sobrerol by other peroxides. This reaction process was
split and then explored. Firstly, a peroxyacetic acid solution was pre-
pared by acetic acid with 50% H2O2. Then, peroxyacetic acid solution
efficiently oxidized α-pinene into α-pinene oxide below 15 °C with a
yield of up to 85%. Finally, the conversion results of α-pinene and the
intermediate product α-pinene oxide under different conditions were
investigated. As shown in Table 1, α-pinene oxide was almost com-
pletely converted with the presence of acetic acid with a selectivity of
sobrerol of 58.5%. In addition, V2O5 also can efficiently catalyze the
conversion of α-pinene oxide to sobrerol in the presence of hydrogen
peroxide which gave results similar to those of acetic acid. Therefore, in
2. Experimental
V2O5 (0.15 mmol) was added to acetone (7 mL), and then an excess
of H2O2 solution (30%, 20 mmol) was slowly added dropwise to obtain
the vanadium solution at 293 K. α-Pinene (10 mmol) was added to the
vanadium solution, and biomimetic hydroxylation reaction occurred.
2

Q. Liu, et al.
CatalysisCommunications142(2020)106041
Table 2
Atomic charges of active substances for oxidation of α-pinene by NBO analysis.
Table 1
Conversion of α-pinene and α-pinene oxide by V2O5 with H2O2 in different
media.
Entry
Molecule
-COOH
-COOH
-OH
-OH
Entry Mono-terpene
H2O2/
mmol
V2O5 /mmol Add. Conv. /% Sel.b /%
1
−0.326
−0.449
1
2
3
4
α-pinene
20
20
20
20
20
–
0.15
0.15
0.15
0.15
–
None 89.7
49.0
50.2
0
50.9
38.4
0
HAc
TEA
88.9
0
2
3
–
–
−0.721
0.491
α-pinene
oxide
None 99.4
None 15.0
5
−0.280
−0.442
6a
7a
0.15
–
None
HAc
0
97.7
–
58.5
a
b
Added the same amount of deionized water as hydrogen peroxide.
For sobrerol.
4
–
–
−0.717
−0.723
0.491
0.493
the process of dihydroxylation of α-pinene catalyzed by V2O5-H2O2, the
addition of acetic acid had almost no effect on the formation of so-
brerol, with a selectivity of 50.2%. However, with the introduction of
the organic alkali triethanolamine, the reaction was difficult to achieve
and the conversion of α-pinene was 0%. All the above experiments were
performed at 20 °C.
XRD pattern and TEM spectrum of V2O5 were shown in Fig. S1, S2
respectively. It turned out to be the most common amorphous crystal-
line powder. V2O5-H2O2 catalytic system is recyclable, and the activity
of vanadium does not change much. In this system, V2O5 was dissolved
in H2O2 solution and the catalytic species was peroxo vanadium peracid
in the system. Once finishing the reaction, V2O5 can be physically se-
parated directly from the reaction system, with only a little vanadium
remained in the aqueous solution which caused some mass loss of va-
nadium. The recycled V2O5 has similar catalytic activity for conversion
of α-pinene to sobrerol as commercial V2O5.
Previously, various catalytic mechanisms of V2O5-H2O2 were pro-
posed. Peroxovanadate can be formed and this provides a very notable,
stable and excellent oxidation effect and was considered to be perox-
idase-like active species [30]. The environment of the α-pinene trans-
formation by V2O5-H2O2 was similar to the precursor solution of the
V2O5-nH2O2 gel. So far, sol-gel syntheses of vanadium pentoxide gels
have been grouped into three convenient routes [31]: (1) acidification
of NaVO3 using an ion-exchange process and polymerization of the
resultant HVO3 in water, (2) hydrolysis and condensation of vanadium
alkoxide, and (3) reaction between H2O2 and amorphous V2O5. The
formation of peroxovanadate occurred during the formation of V2O5-
nH2O2 gel. Under acidic conditions, the formation of peroxovanadate
was maintained in a solution with higher molar ratio of H2O2: V2O5. In
situ 51V NMR spectroscopy was used to monitor the formation process
of vanadium gel [32] and it was found that vanadate oligomers were
the main substance in the vanadium solution in the presence of an
excess of H2O2. In these sol-gel syntheses routes, peroxovanadate was
unstable and easily decomposed, showing high activity, but the
monoperoxo cation had been observed to be a better oxidant than the
diperoxo anion. The vanadate oligomers were oxidized by hydrogen
peroxide, and then easily converted to peroxo vanadate peracid by
hydrogen transfer [33]. The peroxo vanadate acid, VVO(OH)(OOH), in
these processes was considered to have participated in the dihydrox-
ylation of α-pinene. Density functional theory (DFT) calculations car-
ried out using Gaussian 09 W, also provided sufficient and beneficial
support for oligovanadate peracid. The B3LYP functional was employed
and all calculations, including optimizations, frequencies and charge
analysis, were performed using the 6–311 + G (d, p) basis set. All the
atomic charges of their catalytically active centers were calculated
employing the natural bond orbital (NBO) charge analysis and com-
pared. The calculated atomic charges of the catalytically active species
are listed in Table 2, showing values are close to the experimental re-
action results.
two steps. In the first step, epoxidation of olefins was a very important
oxygen transfer reaction, especially in commercial industry. There is no
doubt that the peroxy species were the active group of this step, whe-
ther it was metal peroxo complex [34], organic peroxy compound, or
inorganic peroxide. They behaved in the Lewis acid mechanism or the
main element oxidation [35]. Similar mechanisms were considered to
apply in the epoxidations of α-pinene by peracetic acid and such va-
nadium acid analogues. The oxygen atom, close to the hydrogen atom,
of the peroxy group on peroxo vanadium acid oligomers and perox-
yacetic acid had very similar charges and were both easily abstracted
which conveniently promoted the electrophilic epoxidation of the
olefin. The vanadic acid and acetic acid formed after oxygen transfer
exhibited a similar intrinsic acid function and continued to promote the
hydration reaction of the epoxide, as hydrogen atoms on their hydroxyl
group had notably similar electronegativity, and it was dissociated to
produce H+ protons conveniently. Their similarities were confirmed by
comparing the charge distribution values of the active species with
values from the simulations (Table 2). Futhermore, we proposed the
mechanism of dihydroxylation of α-pinene by V2O5-H2O2, as has been
described by a catalytic cycle in Scheme 1. V2O5 was dissolved in a low
concentration of hydrogen peroxide to obtain a vanadium (V) solution,
which contained peroxo vanadium acid and vanadic acid. Peroxo va-
nadium acid transfered oxygen to α-pinene, and combined with it to
form an ionic intermediate, followed by hydration to produce sobrerol.
Vanadic acid was oxidized to peroxo vanadium acid in the presence of
excess hydrogen peroxide.
4. Conclusions
In conclusion, peroxidase-like catalytic activity of commercial V2O5
in a mild environment with H2O2 was shown to have a better effect on
the conversion of biomass pinene beyond the level of biotransformation
with Armillariella mellea. The dihydroxylation of α-pinene by V2O5-
H2O2 was an efficient and biomimetic green route to prepare sobrerol.
The peroxo vanadium acid, VVO(OH)(OOH), was considered to be a
source of catalytic species for the dihydroxylation of α-pinene to so-
brerol.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Conventionally, the conversion of α-pinene into sobrerol required
3

Q. Liu, et al.
CatalysisCommunications142(2020)106041
Scheme 1. Dihydroxylation of α-pinene by peroxo vanadium acid
fractions (yields), J. Geophys. Res.-Atmos. 112 (2007).
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21878074 and 21975070).
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