Self-promoted vanadium-catalyzed oxidation of pyridinemethanol with molecular oxygen

Article abstract of DOI:10.1016/j.catcom.2020.106114

Catalytic oxidation of alcohols containing heteroatoms with molecular oxygen is usually rather challenging, as transition metal catalysts are easily deactivated by heteroatoms. In contrast to conventional results, herein facile oxidation of 2-pyridinemethanol with dioxgyen was observed over simple vanadium catalysts including VOSO₄ or VO(acac)₂, which seemed much easier than that of benzyl alcohol. 2-Pyridinemethanol could coordinate with the vanadium center, which would promote oxidation of 2-pyridinemethanol itself, rather than deactivate vanadium catalysts. This study would provide a new clue to develop efficient catalysts for transformations of heterocyclic compounds.

Full text of DOI:10.1016/j.catcom.2020.106114

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Self-promoted
vanadium-catalyzed
oxidation
of
pyridinemethanol with molecular oxygen
Yangyang Tang, Zhongtian Du, Meijin Li, Wanhui Wang,
Yonghou Xiao
PII:
S1566-7367(20)30190-4
DOI:
https://doi.org/10.1016/j.catcom.2020.106114
CATCOM 106114
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
10 June 2020
7 July 2020
8 July 2020
Please cite this article as: Y. Tang, Z. Du, M. Li, et al., Self-promoted vanadium-catalyzed
oxidation of pyridinemethanol with molecular oxygen, Catalysis Communications (2020),
https://doi.org/10.1016/j.catcom.2020.106114
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© 2020 Published by Elsevier.

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Self-promoted vanadium-catalyzed oxidation of pyridinemethanol
with molecular oxygen
Yangyang Tanga,b, Zhongtian Dua,b,* duzhongtian@dlut.edu.cn, Meijin Lib, Wanhui Wanga,b,*
chem_wangwh@dlut.edu.cn, Yonghou Xiaoc
aState Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116023, China
bSchool of Chemical Engineering, Dalian University of Technology, Panjin 124221, China
cPanjin Institute of Industrial Technology, Dalian University of Technology, Panjin 124221, China
*Corresponding authors.
Abstract
Catalytic oxidation of alcohols containing heteroatoms with molecular oxygen is
usually rather challenging, as transition metal catalysts are easily deactivated by
heteroatoms. In contrast to conventional results, herein facile oxidation of
2-pyridinemethanol with dioxgyen was observed over simple vanadium catalysts
including VOSO4 or VO(acac)2, which seemed much easier than that of benzyl
alcohol. 2-Pyridinemethanol could coordinate with the vanadium center, which would
promote oxidation of 2-pyridinemethanol itself, rather than deactivate vanadium
catalysts. This study would provide a new clue to develop efficient catalysts for
transformations of heterocyclic compounds.
Keywords
Catalytic oxidation; vanadium; alcohol; molecular oxygen; heterocyclic; deactivation.
1. Introduction
Heterocyclic compounds of commercial interest, such as pyridine derivatives,
find wide applications as intermediates in the chemical and pharmaceutical industries

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[1]. Selective oxidation of heterocyclic alcohols is one of the most important routes to
obtain aromatic heterocyclic aldehydes [2]. Stoichiometric inorganic oxidants, such as
manganese and chromium reagents, are often used both at the laboratory and
industrial levels, which deliver considerable amounts of deleterious waste [2,3].
Molecular oxygen is relatively inexpensive and produces water as the only by-product,
and the use of oxygen as the terminal oxidant has great benefits from both
environmental and economic viewpoints [4]. Thus, researchers have been making
extensive efforts to develop efficient catalytic alcohol oxidation with molecular
oxygen [5,6].
However, the pyridine ring is more electronegative than the benzene ring,
making the two-, four-, and six-ring carbons more electropositive. Oxidation of
pyridine analogues generally shows lower activity than that of benzyl alcohols under
the same reaction condition. And more importantly, many transition metal catalysts
are easily deactivated due to strong coordination/absorption of the nitrogen atom to
metal active center, thus oxidation of those alcohols containing heteroatoms is rather a
tough problem [7-11]. Only a few reports specifically discussed this issue. For
example, Sheldon and co-workers revealed that [Ru(PPh3)3Cl2] was highly efficient
for catalytic oxidation of a variety of primary and secondary alcohols into the
corresponding aldehydes and ketones. Nevertheless, poor yields were observed during
oxidation of some alcohols due to partial coordination of the heteroatoms to the
ruthenium center [10]. Gadi Rothenberg et al reported that various benzylic and
allylic alcohols gave moderate yields over transition-metal oxides supported on
nitrogen-doped carbon, meanwhile lower conversion of 2-pyridinemethanol was
observed under the same reaction conditions [11]. On the basis of these previous
reports, continuous efforts should be made to explore catalytic oxidation of alcohols

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containing heteroatoms with dioxygen under mild reaction conditions.
In transition metal catalyzed reactions in liquid-phase, solvent molecules, ligands,
and substrates usually compete for the metal active coordination sites. Various
transitional metal complexes with effective ligands have been broadly investigated by
researchers for decades, however, we wonder whether the positive effect of nitrogen
atom exists for transition metal-catalyzed alcohol oxidation, rather than the generally
discussed metal deactivation. Vanadium has a wide variety of chemistry, and
oxovanadium easily forms complexes with heteroatom (e.g. O or N) [12,13]. Many
vanadium complexes and heterogeneous vanadium catalysts were reported for
efficient catalytic oxidation with peroxide or oxygen [12-21]. Recently, we also
described several vanadium-catalyzed oxidation reactions with molecular oxygen.
Simple vanadyl compounds exhibited unique activity for alcohol oxidation and C-C
bond cleavage under optimized reaction conditions [22-25]. Based on these reports
above, herein we performed comparative oxidation of 2-pyridinemethanol and benzyl
alcohol, and revealed self-promoted vanadium-catalyzed oxidation of
pyridinemethanol with molecular oxygen.
2. Experimental
2.1 Materials and apparatus
2-Pyridinemethanol, 4-pyridinemethanol and benzyl alcohol were purchased
from Aladdin Chemical Co. Ltd. 2-Picolinic acid was obtained from Shanghai
Macklin Biochemical Co. Ltd. 4-Methylbenzyl alcohol and pyridine was provided by
Energy Chemical Co. Ltd. 2-(Methoxymethyl)pyridine was obtained from Ark pharm.
Acetonitrile (HPLC grade) was supplied by Tianjin Kermel Chemical Reagent
Development Center.VOSO4·xH2O was obtained from Alfa Aesar, and dried at 120 °C
for 24 h before used. VO(acac)2 (95%) was provided by Aladdin Chemical Co. Ltd.

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Unless otherwise stated, other reagents were analytic grade and used as received.
The UV-Vis spectra were recorded on an Agilent Cary 60 spectrometer (USA)
with quartz cells at 25 °C, and the wavelength range was 190-800 nm. On-line
ATR-IR spectra were recorded on the Mettler Toledo React IR15 spectrometer
(Switzerland) with a MCT detector and a diamond probe. The DiComp (diamond)
probe was connected via AgX 9.5 mm×1.5 m fiber (silver halide). Data manipulation
was carried out with iC IRTM (v. 4.2.26) software. For more experimental details, see
supporting information.
2.2 Typical procedure for alcohol oxidation
The catalytic alcohol oxidation was carried out in a stainless autoclave reactor
equipped with a magnetic stirring and automatic temperature control apparatus.
The typical procedure for catalytic oxidation: 2-pyridinemethanol (5 mmol,
545.0 mg) and VOSO4 (5 mol%, 43 mg) were put into the autoclave followed by
acetonitrile (4 mL). After the autoclave was sealed, dioxygen (0.2 MPa) was charged.
o
Then the autoclave was heated to 80 C within 20 min. After 4 h, the reactor was
cooled down to room temperature, and depressurized carefully. Whereafter, a sample
of the reaction mixture was taken for GC analysis.
2.3 Products analysis
GC analysis was performed on GC SP-6890 equipped with SE-54 capillary
column (30 m×0.32 mm×0.50 µm) and a flame ionization detector. Conversion and
selectivity were determined based on area normalization without any purification.
Identification of main products was based on GC-MS and the comparison with
authentic samples. GC-MS was performed on a Thermo Fisher Trace
1300GC-ISQ_LT with electron ionization (EI) mass spectrometry.
3. Results and discussion

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3.1 Oxidation of pyridinemethanols and benzyl alcohols.
Table 1. Catalytic alcohol oxidation with molecular oxygen[a].
Entry
Substrate
catalyst
-
Conv.[%]
N.D
N.D
< 1
99
Select.[%][b]
1
Benzyl alcohol
2
2-Pyridinemethanol
Benzyl alcohol
-
3
VOSO4
VOSO4
VOSO4
VOSO4
VO(acac)2
VO(acac)2
VO(acac)2
VO(acac)2
VOSO4
-
4
2-Pyridinemethanol
4-Methylbenzyl alcohol
4-Pyridinemethanol
Benzyl alcohol
99
-
5
< 1
77
6
98
82
99
84
95
7
33
8
2-Pyridinemethanol
4-Methylbenzyl alcohol
4-Pyridinemethanol
99
9
40
10
11[c]
70
Benzyl alcohol
13
99
86
96
2-Pyridinemethanol
[a] Reaction conditions: 5 mmol substrate, 5 mol% catalyst, 4 mL acetonitrile, 0.2 MPa O2, 80 oC ,
4 h. [b] Selectivity for the corresponding aldehyde. [c] 2.5 mmol benzyl alcohol and 2.5 mmol
2-pyridinemethanol.
Catalytic alcohol oxidation with dioxygen was performed in liquid phase. No
oxidation of benzyl alcohol or 2-pyridinemethanol was observed without a catalyst at
o
80 C within 4 h (Table 1, entries 1 and 2). Less than 1% of benzyl alcohol was
converted when VOSO4 was used as a catalyst (Table 1, entry 3), which is consistent
with our previous report [22]. Unexpectedly, when 2-pyridinemethanol was subjected
to oxidation, 99% conversion was facilely obtained under the same reaction
conditions (Table 1, entry 4). The α-C-H bond dissociation enthalpy (BDE) of

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2-pyridinemethanol is higher than that of benzyl alchohols (The α-C-H BDEs of
2-pyridinemethanol and benzyl alcohol are approximately 443 and 432 KJ/mol
respectively). Generally, pyridinemethanols were much more difficult to be oxidized
than benzylic alcohols using transition metal catalysts, or much harsher reaction
conditions are usually needed [7-11, 26]. On the contrary, 2-pyridinemethanol was
smoothly oxidized in this work. To further verify the results above, we carried out the
oxidation of 4-methylbenzyl alcohol with an electron donating group, and no obvious
oxidation was observed either (Table 1, entry 5); meanwhile 77% of the
4-pyridinemethanol was converted under the same reaction conditions (Table 1, entry
6). In order to further confirm the difference of activities, VO(acac)2 was used to
replace inorganic VOSO4, and oxidation of pyridinemethanol alcohols and benzyl
alcohols were performed. Only 33% and 40% conversions were observed when
benzyl alchohol/4-methylbenzyl alcohol was oxidized separately (Table 1, entries 7
and 9). Under the same reaction circumstances, 99% and 70% conversions of
2-pyridinemethanol/4-pyridinemethanol alcohols were obtained respectively (Table 1,
entries 8 and 10), which are much higher than those of benzylic alcohols. Additionally,
when the pressure of O2 was increased to 0.5 MPa, similar phenomenon was also
observed (Table S1). Therefore, pyridinemethanols were easier to be oxidized than
benzylic alcohols catalyzed by VOSO4 or VO(acac)2, which is obviously different
from previous reports [7-11, 26].
Furthermore, competitive oxidation of 2-pyridinemethanol and benzyl alcohol
was carried out. An equimolar mixture of benzyl alcohol and 2-pyridinemethanol was
subjected to oxidation in the same reactor (Table 1, entry 11). Nearly 99% conversion
of 2-pyridinemethanol could still be obtained within 4 h, meanwhile 13% of benzyl
alcohol conversion was oxidized. This result suggests that the presence of

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2-pyridinemethanol could promote the oxidation of itself as well as benzyl alcohol.
Besides molecular oxygen, hydrogen peroxide was often used as an oxidant for
vanadium catalyzed oxidation [13, 19]. Oxidation of pyridinemethanols and benzylic
alcohols was performed respectively by using hydrogen peroxide instead of oxygen
(Table S2). Under the same reaction conditions, the conversion of 2-pyridinemethanol
is more than three times higher than that of benzyl alcohol (Table S2, entries 1 and 2),
and the conversion of 4-pyridinemethanol is also two times higher than that of
4-methylbenzyl alcohol. These results suggest that self-promotion effect of
2-pyridinemethanol should not be ignored for catalytic oxidation with both dixoygen
and peroxide in liquid phase.
Figure 1. Catalytic oxidation of benzyl alcohol by adding different additives.
3.2 The effect of pyridine ring on alcohol oxidation
We therefore wonder whether the interaction between vanadium species and the
nitrogen atom occurred, which could affect the catalytic activity of VOSO4. Control
experiments were designed to test the effect of N-heterocyclic compounds (Reaction
conditions: 5 mmol benzyl alcohol, 5 mol% VOSO4, 5 mol% heterocyclic compounds,
4 mL acetonitrile, 0.5 MPa O2, 80 oC , 4 h). Catalytic oxidation of benzyl alcohol with
molecular oxygen was carried out by adding 5 mol% N-heterocyclic compounds as an
additive (Fig. 1). Pyridine, 2-pyridinemethanol, 2-(methoxymethyl)pyridine, and
2-picolinic acid was investigated respectively under the same reaction conditions. No
conversion of benzyl alcohol was observed without an additive. As expected, 30%
conversion of benzyl alcohol was obtained in the presence of even 5 mol%
2-pyridinemethanol. When 5 mol% of pyridine was added, 25% of benzyl alcohol was
converted, indicating that the presence of pyridine ring could also promote the

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oxidation of benzyl alcohol. Furthermore, when 2-(methoxymethyl)pyridine was used,
56% benzyl alcohol was oxidized, which showed even higher conversion than those
using pyridine or 2-pyridinemethanol. However, when 2-picolinic acid was tested, the
conversion of benzyl alcohol remarkably decreased to 9%. Simultaneously, different
experimental phenomena were observed. In the presence of 2-pyridinemethanol, the
reaction mixture became green gradually, while it was blue when pyridine was added.
Therefore, the addition of pyridine derivatives affected the catalytic oxidation of
benzyl alcohol significantly, and it was the presence of pyridine rings, rather than the
hydroxymethyl group, which was important for vanadium-catalyzed alcohol
oxidation.
3.3 UV–vis and on-line ATR-IR investigation
In Pd(OAc)2/pyridine catalytic system [27], Shannon S. Stahl et al revealed that
pyridine would promote the aerobic oxidation of palladium(0) via coordination. Susan
K. Hanson and coworkers reported that pyridine could coordinate to dipicolinate
vanadium(V) alkoxide complexes as a base [28]. In this work, we speculated that the
pyridine ring of 2-pyridinemethanol could activate the vanadium catalyst through in
situ coordination to the vanadium center as a ligand, and promote oxidation of the
substrate itself.
Figure 2. The interaction of 2-pyridinemethanol and VO(acac)2 investigated by UV–vis
spectroscopy.
Figure 3. The interaction of benzyl alcohol and VO(acac)2 investigated by UV–vis spectroscopy.
Then we employed UV–vis spectroscopy to investigate coordination of

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2-pyridinemethanol (Fig.2). To obtain noticeable changes, VO(acac)2 with strong
absorption spectra was selected as the vanadium source. The peak at ca. 261 nm is
assigned to a π-π* electronic transition of 2-pyridinemethanol (Fig.2, red line). The
VO(acac)2 exhibited a high intensity band centered at ca. 300 nm with a shoulder at
ca. 270 nm (Fig.2, blue line). This peak at ca. 300 nm was assigned to intra-ligand
transitions originated from β-diketonato ligands, and the absorption at ca. 270 nm
corresponded to that of free acetylacetone in acetonitrile [29,30]. After
2-pyridinemethanol was stirred with VO(acac)2 in the acetonitrile for 4 h, the UV-vis
spectrum was collected. The absorption at 300 nm weakened significantly, while a
strong and broad peak appeared at ca.272 nm, which is the typical absorption band of
free acetylacetone (Fig.2, black line) [29-32]. We believe that coordination of
2-pyridinemethanol with the oxovanadium led to release of free acetylacetone,
accompanied by a ligand exchange reaction. Then the interaction of benzyl alcohol
and VO(acac)2 was studied. By contrast, the peak at ca. 300 nm originated from
β-diketonato ligands remained strong, and no significant absorption band associated
with free acetylacetone was strengthened (Fig. 3). This observation was quite different
from that of 2-pyridinemethanol, which should be related with the different oxidation
activity of benzyl alcohol and 2-pyridinemethanol.
On-line attenuated total reflection infrared (ATR-IR) spectroscopy is a powerful
approach to study chemical reactions [33]. We further investigated the interaction
between 2-pyridinemethanol and VOSO4 by on-line ATR-IR. In order to avoid the
disturbance of organic molecules, VOSO4 without ligands was selected as the
vanadium source. First, 2-pyridinemethanol was dissolved in acetonitrile, and the
absorption bands remained stable (Fig. S1). After the addition of 10 mol% VOSO4, a
strong absorption peaks at 1119 cm-1 appeared soon (Fig. 4), which could be assigned

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to free sulphate [22,34,35]. One reasonable interpretation is that coordination of
2-pyridinemethanol led to release of free sulphate. We also studied the interaction of
benzyl alcohol and VOSO4 by using on-line ATR-IR spectroscopy (Fig. S2 and S3).
As we predicted, no significant bands appeared, which was quite different from that of
2-pyridinemethanol. All these results are in good agreement with UV–vis studies and
catalytic activities mentioned above.
Figure 4. Interaction between 2-pyridinemethanol and VOSO4 monitored by online ATR-IR.
Nitrogen heteroatom could easily coordinate to oxovanadium form complexes
[12,13]. In previous reports on vanadium-catalyzed oxidation, Georgiy B. Shul’pin et
al described coordination chemistry of vanadium with pyrazinecarboxylic acid, which
was highly efficient for catalytic oxidation of various organic substrates [19]. Susan K.
Hanson and coworkers demonstrated pyridine could act as both a ligand and a base
with the latter directing a two-electron oxidation pathway [28]. Though many
examples for oxidation of pyridinemethanol have been described in previous
vanadium-catalyzed reports [16-18], self-promoted oxidation of 2-pyridinemethanol
was rarely discussed exclusively. We believe that 2-pyridinemethanol could play
double roles as a substrate and a ligand. 2-Pyridinemethanol could coordinate with the
vanadium center, and in situ activate the vanadium catalyst and then promote the
oxidation of 2-pyridinemethanol itself. In this context, the presence of N-heteroatom,
which could usually deactivate transition metal catalysts, would activate vanadium
catalysis instead. Therefore, benzyl alcohol became more difficult to be oxidized
compared with 2-pyridinemethanol under the same reaction conditions.
3. Conclusion

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In summary, we disclosed the self-promotion effect during vanadium-catalyzed
oxidation of pyridinemethanol with molecular oxygen. Pyridinemethanols exhibited
much easier to be oxidized than benzylic alcohols under the same reaction conditions.
2-Pyridinemethanol could play both roles as a substrate and a ligand. The
N-heteroatom of 2-pyridinemethanol would in situ coordinate to the vanadium center
and promote oxidation of 2-pyridinemethanol itself. This study would provide a new
clue to develop efficient catalysts for transition-metal catalyzed transformations of
heterocyclic compounds.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(21473184 and 21776028), Natural Science Foundation of Liaoning Province
(2019-MS-047) and the Fundamental Research Funds for the Central Universities
(DUT15RC(3)094).
CRediT author statement
Yangyang Tang: Investigation, Formal analysis. Zhongtian Du: Supervision,
Conceptualization,
Writing-Original
Draft. Meijin
Li:
Investigation,
Validation. Wanhui Wang: Supervision, Writing-Reviewing and Editing. Yonghou
Xiao: Resources, Methodology.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Supplementary data
Supplementary material

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Graphical abstract
Highlights
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Metal catalysts are easily deactivated by heteroatoms due to strong coordination.
Self-promoted vanadium-catalytic oxidation of 2-pyridinemethanols was observed.
2-Pyridinemethanol could play double roles as a substrate and a ligand/base.
2-Pyridinemethanol could activate vanadium catalysts, rather than deactivation.

Figure 1

Figure 2

Figure 3

Figure 4