Catalytic conversion of vanillic acid to catechol by palladium acetate/bis(aminomethyl)-nido-dicarba-undecaborane (11) system

Article abstract of DOI:10.1016/j.jorganchem.2018.02.008

The vanillic acid was converted to catechol using a catalyst system of PdAc₂/[Na]⁺[7,8-bis(aminomethyl)-nido-C₂B₉H₁₀]. The 90.3% yield of catechol was achieved from vanillic acid in 4 h using the system at 240 °C. The reaction was also compared using catalyst system of PdAc₂/TMEDA (N,N,N′,N′-tetramethylethylenediamine), PdAc₂/dppe (1,2-bis(diphenylphosphine)ethane) and Pd/AC (activated carbon) under the same conditions. The product, catechol, was characterized by ¹H and ¹³C NMR spectra and its yield was determined by a high-performance liquid chromatography (HPLC).

Full text of DOI:10.1016/j.jorganchem.2018.02.008

Journal of Organometallic Chemistry xxx (2018) 1e7
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Catalytic conversion of vanillic acid to catechol by palladium
acetate/bis(aminomethyl)-nido-dicarba-undecaborane (11) system
,
Yinghuai Zhu a *, Zhiyu Bai b, Wen Chuen Phuan b, Fatima Abi Ghaida c,
Narayan S. Hosmane c, Jun Ding b
a School of Pharmacy, Macau University of Science and Technology, Avenida Wai Long, Taipa 999078, Macau, China
b Department of Materials Science & Engineering, National University of Singapore, Engineering Drive 1, 117575, Singapore
c Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115-2862, USA
a r t i c l e i n f o
a b s t r a c t
The vanillic acid was converted to catechol using a catalyst system of PdAc2/[Na]þ[7,8-bis(aminomethyl)-
nido-C2B9H10]. The 90.3% yield of catechol was achieved from vanillic acid in 4 h using the system at
240 ꢀC. The reaction was also compared using catalyst system of PdAc2/TMEDA (N,N,N0,N0-tetramethy-
lethylenediamine), PdAc2/dppe (1,2-bis(diphenylphosphine)ethane) and Pd/AC (activated carbon) under
the same conditions. The product, catechol, was characterized by 1H and 13C NMR spectra and its yield
was determined by a high-performance liquid chromatography (HPLC).
Article history:
Received 10 December 2017
Received in revised form
5 February 2018
Accepted 6 February 2018
Available online xxx
© 2018 Elsevier B.V. All rights reserved.
Keywords:
Carborane
Organometallics
Vanillic acid
Catechol
Biomass
1. Introduction
the depletion of fossil fuels and increased environmental pollution
because of their rapid consumption, the discovery of green and
Catechol is a naturally occurring chemical, which can be isolated
directly from several plants including beech wood and mimosa
catechu [1e4]. Synthetic catechol is commercially available and,
currently, it has been used as a precursor to pesticides, flavors and
fragrances [5e7]. It has also been used as a common building block
in organic synthesis to prepare pharmaceutical compounds [5e7].
For industrial use, the catechol is prepared by (1) the hydrolysis of
ortho-halogen-substituted phenols or ortho dihalobenzenes under
suitable conditions with hot aqueous basic solutions; (2) the
dehydroxylation of salicyaldehyde using hydrogen peroxide; (3)
the replacement of sulfonic groups by alkali fusion, and (4) the
hydroxylation of phenol using hydrogen peroxide [5,8,9]. However,
these methods have some inherent limitations of considerably
lower product yields and the use of highly corrosive reagents. On
the other hand, all direct synthetic approaches involve fossil fuels
as the original starting materials. Fossil fuels have been used as one
of the essential chemical resources for many centuries. Considering
sustainable chemical and energy resources is therefore warranted.
The biomass is well recognized as a sustainable source of fuels
and chemicals [10e12]. In case of chemical production using
biomass, new processes need to be developed to produce high
value-added chemicals and fuel additives. Lignin is increasingly
drawing attention as a suitable renewable resource to produce ar-
omatic chemicals, particularly phenolic compounds. For example,
phenolics comprising mainly vanillic acid, syringic acid and vanillin
have been produced from organosolv beech wood lignin in a
reasonable yield of 33% [13,14]. In addition, separation and purifi-
cation technology has been well developed for biomass conversion
processes and >99 wt% of the phenolics can be recovered [13e17].
Producing catechol from lignin is a relatively straightforward pro-
cess, and thus more efforts are warranted for this potentially
promising technology. Lignin can be depolymerized by oxidation or
reduction reactions to generate the aromatics, and the major
products are methoxyl group-functionalized phenolics, such as
phenol, guaiacol and vanillic acid [18e20]. A deep oxidation
depolymerization of lignin often produces phenolic acids, including
vanillic acid [13,21]. We have already demonstrated that lignin-
based sources can produce terephthalic acid by a cascade fix-bed
* Corresponding author.
E-mail address: yhzhu@must.edu.mo (Y. Zhu).
https://doi.org/10.1016/j.jorganchem.2018.02.008
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2
Y. Zhu et al. / Journal of Organometallic Chemistry xxx (2018) 1e7
process including hydrogenation demethylation and carboxylation
reactions [22]. Accordingly, the vanillic acid and syringic acid were
converted to terephthalic acid in two steps via an intermediate of p-
hydroxybenzoic acid. In this work, the methoxyl group-
functionalized phenolics, such as vanillic acid, were used as
model compounds to demonstrate this concept as illustrated in
Scheme 1.
bis(aminomethyl)-ortho-carboranyl
hydrochloride,
[1,2-
(NH3CH2)2-1,2-closo-C2B10H10
]
2þ[Clꢁ]2 (2), was obtained in 81%
yield. Compound 2 was reacted further with sodium ethoxide in
ethanol to produce the corresponding sodium compound (3),
[Na]þ[7,8-(NH2CH2)2-7,8-nido-C2B9H10]-, in 75% yield as a colorless
salt. Compounds 2 and 3 showed the typical infrared absorptions of
methylene and amine functional groups in the IR spectra, as well as
the B-H group in the 1H NMR spectra measured in the solvent of
D2O and DMSO‑d6, respectively as presented in Fig. 1. Compound 2
2. Results and discussion
showed NMR peaks of CH2 at
d
¼ 4.38 ppm and of BH at a range of
1.10e2.90 ppm, respectively; whereas the NMR spectrum of 3
exhibited two types of BH peaks at ꢁ0.80e3.20 ppm
and ꢁ2.90 ~ ꢁ2.20 ppm for cage BH and bridge BH, respectively.
The presence of NMR peak of bridge BH in 3 confirmed the suc-
cessful decapitation of 2. The chemical shifts of CH2 groups
attached to the carborane cage shifted to higher field from
4.38 ppm to 3.10 and 2.92 ppm. The NH groups showed broad
resonances at a range of 4.80e7.60 ppm due to the H-D interaction.
The 13C NMR spectra exhibited the peaks for methylene groups and
for carbons of the polyhedral carborane cage at 39.72 (CH2), 74.13
(Ccage) ppm for 2; and 35.21(CH2), 54.93 (Ccage) ppm for 3, respec-
tively, in methanol-d4. The resonances of both CH2 and cage car-
bons shifted to higher field. The 11B NMR spectra showed peaks
at ꢁ5.42 (4B) and ꢁ12.77 (6B) ppm for 2, and -11.36 (3B), ꢁ19.44
(4B), ꢁ37.51(1B), ꢁ39.70 (1B) ppm for 3 in methanol-d4, respec-
tively. The peaks of ꢁ37.51, ꢁ39.70 ppm are as a result of the
decapitation of closo-carborane cage 2. Unfortunately, our attempts
to grow single crystals of these compounds were not successful.
The FT-IR spectra also exhibited strong absorption peaks at
2520 cmꢁ1 and 2519 cmꢁ1 for 2 and 3, respectively, which are
attributed to B-H stretching modes of vibrations (nBH).
It has been reported that methoxyl-functionalized phenols,
derived from lignin resources, can undergo demethylation to form
the corresponding arenes [23e29]. However, there are very limited
reports associated with demethylation of the methoxyl-
functionalized phenols to produce catechol. Nevertheless, to
improve the valorization from lignin, it is necessary to use a process
that does not require the addition of hydrogen. In this work, the
phenolic compounds are prepared by a catalytic hydrothermal
demethylation reaction in aqueous media. The vanillic acid and
guaiacol were selected as the model compounds to investigate the
catalytic conversion to produce catechol. Palladium acetate has
been widely used in organic synthesis as an efficient catalyst and,
therefore it was used as the catalyst for the hydrothermal conver-
sion reported here. On the other hand, carborane-based multi-
dentate ligand, sodium salt of [7,8-bis(aminomethyl)-nido-C2B9H10
]
anion, was prepared and used as a catalyst component because of
its stability in both the acidic and basic media [30]. For comparison,
the reactions were also investigated with catalyst systems of PdAc2/
TMEDA, PdAc2/dppe and Pd/AC under the same conditions.
The ligand, [Na]þ[7,8-bis(aminomethyl)-nido-C2B9H10]-, was
prepared according to the reaction pathway shown in Scheme 2.
The precursor (1), 1,2-bis(phthalimidomethyl)-1,2-dicarda-closo-
dodecaborane (12), was prepared according to the procedure
published elsewhere [31]. After deprotection and acidification of
the functional moiety with hydrochloric acid, the resulting
Defunctionalization of phenolic compounds, vanillic acid,
guaiacol and syringic acid, to produce the catechol and pyrogallol
was carried out in a Parr reactor using a solvent mixture of water
and DMA (v/v ¼ 1:2) at 240 ꢀC. The reactions were not conducted
Scheme 1. Synthesis of catechol from lignin through phenolics.
Scheme 2. Synthesis of the sodium salt of bis(aminomethyl)-nido-carborane anion.
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Y. Zhu et al. / Journal of Organometallic Chemistry xxx (2018) 1e7
3
Fig. 1. 1H NMR spectra of 2 (a) in D2O and 3 (b) in DMSO‑d6.
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4
Y. Zhu et al. / Journal of Organometallic Chemistry xxx (2018) 1e7
under a hydrogen atmosphere, but under an inert atmosphere of
argon. In addition, an alkaline medium was prepared using an
aqueous solution of K2CO3 (1.0 M) to reach a pH value of approxi-
mately 10. Nonetheless, it has been consistently demonstrated that
ligand plays an important role in the reactions involving palladium-
based catalysts as they can alter the nature of the active palladium
species [32-34]. Consequently, the commonly used ligands,
catechol molecule with a trace of impurity. Accordingly, four aro-
matic hydrogen atoms in catechol molecule exhibit into two groups
of absorptions at
Fig. 3 (a). In the 13C NMR spectrum, shown in Fig. 3 (b), three peaks
were observed at
d
¼ 6.60 and 6.70 ppm, respectively, as shown in
d
¼ 114.5, 118.1 and 144.1 ppm corresponding to
three types of aromatic carbons of the catechol molecule,
respectively.
N,N,N0,N0-tetramethylethylenediamine
and
1,2-
While the reaction mechanism is still unknown, palladium-
catalyzed decarboxylation of the aromatic carboxylic acid [35]
and decarbonylation of aldehyde [36] have been reported. Some
active species, such as Ar-CO2-Pd(II)(L)-Ac, could also be applicable
in the defunctionalization process. Since the heterogeneous Pd/AC
catalyst also demonstrated a reasonable activity in the defunc-
tionalization reaction, shown in Table 1, entry 6, a heterogeneous
catalytic process involving in situ generated palladium nano-
particles could not be excluded at this point of investigation, but the
study of their reaction mechanism is currently underway in our
laboratory.
bis(diphenylphosphine)ethane, and our newly prepared anionic
ligand, [7,8-(NH2CH2)2-7,8-nido-C2B9H10]- (3), were used as catalyst
additives to investigate the defunctionalization reaction of the
phenolic compounds. The catalyst performance and the corre-
sponding results are listed in Table 1. The conversion of vanillic acid
and the yield of catechol were analyzed by HPLC (high performance
liquid chromatography). A typical HPLC spectrum, and the 1H and
13C NMR spectra of the product mixture were shown in Figs. 2 and
3, respectively.
Table 1 shows that (1) the conversions of the substrate are much
higher in the presence of PdAc2 catalyst when compared to the
commercially available Pd/AC catalyst under the same reaction
conditions; (2) different types of ligands led to varying product
3. Conclusions
yield indicating that the ligands are also important in the reaction
Phenolic compounds, such as vanillic acid and guaiacol, were
converted to catechol by the catalyst system of PdAc2/[Na]þ[7,8-
bis(aminomethyl)-nido-C2B9H10]- in the mixed solvent of DI-H2O
and DMA at 240 ꢀC. The catechol yields of 90.3% and 94.3% were
obtained from the defunctionalization reaction of vanilic acid and
guaiacol, respectively.
Considering the unique properties and high performance of the
ligand, bis(aminomethyl)-nido-dicarba-undecaborane (11), we
expect our prototype catalyst system to find broader applications
-
and thus with the ligand (3), [7,8-(NH2CH2)2-7,8-nido-C2B9H10
]
anion, the best performance was demonstrated in producing the
demethylation products; and (3) catechol has always been the main
product. This observation was further confirmed by HPLC and NMR
spectra of the product mixture. In the HPLC plot, shown in Fig. 2,
the main peak due to catechol absorption was observed at
tR z 20.20 min and is consistent with the 1H and 13C NMR spectra
of the product mixture showing the main peaks that are assigned to
Table 1
Catalyst performance for hydrothermal demethylation.a,b
Entry
Catalyst
Substrate
Conv.%
Catechol yield (%)
Mass balance (%)
1
2
3
4
5
6
7
8
No
PdAc2
VA
VA
VA
VA
VA
VA
Guaiacol
SA
18.6
37.8
71.2
87.0
99.5
46.7
98.5
97.7
1.7
9.8
45.7
48.6
90.3
34.5
94.3
61.4 (Pyrogallol)
94.5
86.8
90.3
93.6
95.2
91.4
96.1
87.8
PdAc2/dppe
PdAc2/TMEDA
PdAc2/3
Pd/AC
PdAc2/3
PdAc2/3
a
Reaction conditions: catalyst loading amounts: [Pd]/[ligand]/[PhOH] ¼ 1:1:20, 1 M aqueous K2CO3 (10 mL), 20 mL of dimethylacetamide (DMA); reaction tempera-
ture ¼ 240 ꢀC, reaction time ¼ 4hrs; TMEDA ¼ N,N,N0,N0-tetramethylethylenediamine, dppe ¼ 1,2-bis(dimethylamino)ethane, VA ¼ vanillic acid, SA ¼ syringic acid,
AC ¼ activated carbon.
b
Yield (%) is determined by HPLC, and mass balance was calculated based on the amount of added substrate.
Fig. 2. HPLC of the product mixture of the defunctionalization reaction.
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Y. Zhu et al. / Journal of Organometallic Chemistry xxx (2018) 1e7
5
Fig. 3. 1H (a) and 13C (b) NMR spectra of the product mixture of the hydrothermal demethylation reaction in DMSO‑d6.
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6
Y. Zhu et al. / Journal of Organometallic Chemistry xxx (2018) 1e7
both in academia and in the fine chemical industry, especially for
45.02, N 12.77 (B was analyzed by ICP-OES). 1H NMR (DMSO‑d6,
the defunctionalization reactions in biomass conversion.
ppm),
d
¼ 7.60e4.80 (br, 4H, 2NH2), 3.10, 2.92 (m, 4H, 2CH2),
3.20 ~ ꢁ0.80 (m, br, 9H, 9BH), ꢁ2.20 ~ ꢁ2.90 (br, 1H, BHbridge). 13
C
4. Experimental
NMR (CD3OD, ppm),
ppm),
d
¼ 54.93 (Ccage), 35.21 (CH2). 11B NMR (CD3OD,
d
¼ ꢁ11.36 (3B), ꢁ19.44 (4B), ꢁ37.51 (1B), ꢁ39.70 (1B). IR
Tetramethylethylenediamine
(tmeda),
1,2-
(KBr pellet, cmꢁ1), 3435 (s, br), 3332(s, s), 3277(s, s), 2955(m, br),
2562(s, s), 2545(s, s), 2519(vs, br), 2491(s, s), 1642(m, br), 1561(s, s),
1460(m, s), 1431(m, s), 1379(s, s), 1309(m, s), 1206(w, s), 1141(m, s),
1038(m, s), 971(m, s), 953(w, s), 922(w, s), 827(w, s), 756(w, s),
506(w, br).
bis(diphenylphosphino)ethane (dppe), palladium acetate (PdAc2),
palladium on activated carbon (Pd/AC, 5 wt%), dimethylacetamide
(DMA) and other chemicals were purchased from Sigma-Aldrich
Pte Ltd. The 1,2-bis(phthalimidomethyl)-1,2-dicarda-closo-dodec-
aborane (12) was prepared according to the procedure reported in
the literature [31]. The 1H, 13C and 11B NMR spectra were recorded
on a Bruker Fourier-Transform multinuclear NMR spectrometer at
400, 100.6 and 128.4 MHz, relative to external Me4Si (TMS) and
BF3$OEt2 standards, respectively. Infrared (IR) spectra were
measured using a BIO-RAD spectrophotometer with KBr pellets
technique and presented in the sequence of signal strength as
strong (s), medium (m) and weak (w), and peak pattern as single
(s), multiple (m) and broad (br). Elemental analyses were carried
out on a CHNSO Elemental Analyzer. Inductively coupled plasma-
optical emission spectroscopy (ICP-OES) analysis was determined
using a VISTA-MPX, CCD Simultaneous ICP-OES analyzer. After
workup procedures, the product mixtures were separated by the
high-performance liquid chromatography (Agilent, column: Ami-
nex HPX-87H Column, wash solution: 10% MeCN in 0.02 N H2SO4,
flow rate 0.6 mL/min, column temperature 60 ꢀC, UV detector
210 nm). Products from demethylation and decarboxylation of
vanillic acid were analyzed by 1H and 13C NMR spectra in DMSO‑d6.
4.3. General procedure of catalytic hydrothermal
defunctionalization of phenolics
The hydrothermal reactions were carried out in a 100 mL Parr
reactor. In a typical procedure, the reactor was fed with 10.0 mL of
1.0 M aqueous K2CO3, 20.0 mL of DMA, substrate 5.0 mmol (vanillic
acid 840.8 mg or catechol 620.7 mg or syringic acid 990.8 mg),
0.25 mmol catalyst and 0.25 mmol ligand. After addition of the
reactants, the reaction mixture was flushed three times with argon,
followed by heating to 240 ꢀC for 4hrs with constant stirring. The
reaction mixture was later cooled to room temperature using an ice
bath, and then transferred to a 100-mL round-bottom flask to dry
the contents under vacuum. The resulting residue was then dis-
solved in 20.0 mL de-ionized water and neutralized with 2 N
aqueous HCl solution to a pH value of approximately 3.5. The
mixture was then filtered using a syringe filter (0.2 mm) and sub-
jected to HPLC analysis. The analysis of the product mixture was
carried using an Aminex HPX-87H column eluted with a solution of
10% MeCN in 0.02 N H2SO4 at a flow rate of 0.6 mL/min and 60 ꢀC
column. The experimental results are summarized in Table 1.
ꢁ
4.1. Synthesis of [1,2-(NH3CH2)2-1,2-closo-C2B10H10
]
2þ[Cl]
(2)
2
A
solution containing 2.31 g (0.005mol) of bis(ph-
thalimidomethyl)-1,2-dicarba-closo-dodecarborane (1) in 50 mL
glacial acetic acid, 12 mL water and 15 mL concentrated HCl was
heated to 100 ꢀC for 4 h to produce a clear colorless solution. This
resulting solution was evaporated under reduced pressure to dry-
ness and the residue was stirred with 50 mL chloroform for 3 h to
dissolve the phthalide by-product and then remove it by filtration.
The remaining solid was crystallized from acetone to provide 1.12 g
Acknowledgements
We thank the Faculty Research Grants (GRFs), Macau University
of Science and Technology (FRG-17-050-SP) for financial support.
NSH thanks the National Science Foundation for the continuous
support through Single-PI grants of over $2.5 million from 1983
until 2013.
of bis(aminomethyl)-ortho-carborane hydrochloride (2) as
a
colorless solid in 81% yield. M.p. ¼158e160 ꢀC (decomposed).
Elemental analysis for C4H20B10N2Cl2: Calculated (%) C 17.46, H 7.32,
B 39.28, N 10.18; Found (%) C 17.62, H 7.10, B 39.14, N 10.25 (B was
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Please cite this article in press as: Y. Zhu, et al., Journal of Organometallic Chemistry (2018), https://doi.org/10.1016/j.jorganchem.2018.02.008