Efficient and selective reduction of α-pinene to cis-pinane by NaBH4 using NiCl2?6H2O/PEG-800/ethanol as the catalytic system

Article abstract of DOI:10.1080/00397911.2018.1560875

The reduction of α-pinene by NaBH₄ was achieved using NiCl₂?6H₂O in PEG-800/ethanol system under room temperature. Under the optimized conditions, the conversion of α-pinene and the selectivity of cis-pinane reached 97% and 98%, respectively. On the basis of TEM and a series of poisoning experiments, the nature of the active catalytic species for the reaction was discussed.

Full text of DOI:10.1080/00397911.2018.1560875

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
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Efficient and selective reduction of α-pinene to
cis-pinane by NaBH using NiCl •6H O/PEG-800/
4
2
2
ethanol as the catalytic system
Kaoxue Li, Kang Wang, Chuanchao Liu, Xuhao Yang, Xia Chen, Yang Ren,
Shuhua Cao, Meifang Liu, Xishi Tai & Fahui Li
To cite this article: Kaoxue Li, Kang Wang, Chuanchao Liu, Xuhao Yang, Xia Chen, Yang Ren,
Shuhua Cao, Meifang Liu, Xishi Tai & Fahui Li (2019): Efficient and selective reduction of α-pinene
to cis-pinane by NaBH using NiCl •6H O/PEG-800/ethanol as the catalytic system, Synthetic
4
2
2
Communications, DOI: 10.1080/00397911.2018.1560875
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https://doi.org/10.1080/00397911.2018.1560875
Efficient and selective reduction of a-pinene to cis-pinane
by NaBH using NiCl
ꢀ
6H O/PEG-800/ethanol as the
4
2
2
catalytic system
Kaoxue Li, Kang Wang, Chuanchao Liu, Xuhao Yang, Xia Chen, Yang Ren,
Shuhua Cao, Meifang Liu, Xishi Tai, and Fahui Li
Chemistry and Chemical and Environmental Engineering College, Weifang University,
Weifang 261061, China
ABSTRACT
ARTICLE HISTORY
The reduction of a-pinene by NaBH
4
was achieved using NiCl
2
ꢀ
2
6H O
Received 31 October 2018
in PEG-800/ethanol system under room temperature. Under the opti-
mized conditions, the conversion of a-pinene and the selectivity of
cis-pinane reached 97% and 98%, respectively. On the basis of TEM
and a series of poisoning experiments, the nature of the active cata-
lytic species for the reaction was discussed.
KEYWORDS
a-pinene; selective
reduction; NaBH4; Ni
nanoparticles; PEGs
GRAPHICAL ABSTRACT
Introduction
Conventionally, cis-pinane, as an important intermediate for the preparation of a num-
ber of perfume, pharmaceuticals and adhesives, is produced by hydrogenation
of a-pinene or b-pinene (Scheme 1a). In this process, a mixture of cis-pinane and trans-
[
1]
pinane is yielded. To obtain cis-rich pinane, the challenge is to develop novel stereo-
[
2]
selective catalytic systems with high selectivity in the formation of cis-pinane. For
[
3]
[4]
[5]
decades, several noble metal catalyst systems, based on Ru, Pt, and Pd have been
applied to the hydrogenation of pinene. However, the high cost and leaching of metals
CONTACT Kaoxue Li
likaoxue@wfu.edu.cn; Fahui Li
fahuili@163.com
Chemistry and Chemical and
Environmental Engineering College, Weifang University, Weifang 261061, China.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
K. LI ET AL.
Scheme 1. Strategies for the reduction of pinene. (a) Traditional hydrogenation process.
b) This work.
(
are the main disadvantages of these systems. Thus, procedures for the hydrogenation of
pinene based on Ni catalysts have been developed. For example, amorphous Ni-P/Al O
2
3
[
6]
[7]
[8]
[9]
catalyst, Ni/c-Al O catalysts, Ni/SFCC catalyst, Ni/AlPO catalyst have been
2
3
4
reported. However, these catalytic systems always required harsh conditions like high
hydrogen pressure and high reaction temperature. In addition, hydrogenation process
often makes use of external molecular hydrogen as a hydrogen donor, which must
[
10]
require special apparatuses to operate with flammable and explosive H gas.
2
Sodium borohydride is known to be a safe, stable and ease of handling hydride. It
has been tried to utilize for the reduction of a-pinene. However, the procedure did not
[
11]
[12]
undergo hydrogenation in CoCl /NaBH /THF
or RuCl /NaBH /THF/H O system.
3 4 2
2
4
Using NaBH /RuCl in aqueous DMAC for the reduction of a-pinene, there was a slight
4
3
[
13]
improvement in the activity of the catalyst and with a 13% yield of the cis-pinane.
Obviously, different catalytic systems have a significant influence on the reduction of
a-pinene by NaBH Very recently, we found the additive of PEG-400 in NiCl ꢁ6H O-
4
.
2
14]
2
[
EtOH system could significantly promote the reduction of alkenes by NaBH .
Thus,
4
we expected to further explore the effect of PEGs in various solvents on the reduction
of a-pinene by NaBH . Herein, we reported the application of PEG-800 in NiCl ꢁ6H O/
4
2
2
ethanol medium as an effective and selective catalytic system for the reduction of
a-pinene to cis-pinane by NaBH under room temperature (Scheme 1b).
4
Results and discussion
Effect of various chain length PEGs on the reduction of a-pinene
The effect of various chain length PEGs on the reduction of a-pinene was initially
investigated, and the reaction was carried out in ethanol/PEGs mixture (3 mL/2 g) in the
presence of 5 mol% NiCl2ꢀ6H O using NaBH4 as a hydrogen donor at room
2

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a
Table 1. Effect of various chain length PEGs on the reduction of a-pinene.
Entry
PEGs
Conversion (%)
Selectivity (%)
b
1
None
No reaction
—
98
98
98
98
98
98
98
96
2
3
4
5
6
7
8
9
a
PEG-200
PEG-400
PEG-600
PEG-800
PEG-1000
PEG-1500
PEG-2000
PEG-4000
90
93
94
97
72
40
17
1
PEGs 2 g, ethanol 3 mL, NiCl
room temperature, 5 h.
2
ꢀ6H
2
O 0.25 mmol, a-pinene 5 mmol, NaBH
4
2.5 mmol, sealed tube,
b
In the absence of PEGs.
temperature for 5 h. As shown in Table 1, it was surprising that the reaction did not
occur in the absence of PEGs (entry 1). In comparison, the lower molecular weight of
PEGs such as PEG-200, PEG-400, PEG-600 and PEG-800 showed significantly improved
conversion (entries 2–5). Specially, PEG-800 could deliver 97% conversion and 98% cis-
pinane selectivity (entry 5). In contrast, the reaction proceeded sluggishly in higher
molecular weight PEGs including molecular weights of 1000, 1500, 2000, and 4000
(
entries 6–9), which might be attributed to the higher viscosity of the reaction medium,
[
15]
as similar reported by Zhou et al. in PEGs/water system.
With these observations, we
selected PEG-800 as the suitable additive for our research.
Effect of the combination of PEG-800 with various solvents on the reduction
of a-pinene
To compare the effect of PEG-800 in various solvents for the reduction of a-pinene by
NaBH4, the reaction was performed at the same conditions with an array of solvents
combined with PEG-800. The results are summarized in Table 2. The combination
of PEG-800 with various alcohols, such as methanol, ethanol, 1-propanol, 2-propanol,
1-butanol, ethylene glycol (EG) and glycerin, afforded different a-pinene conversion to
the reduction products. The solution of PEG-800/methanol and PEG-800/ethanol both
delivered >90% conversion and 98% cis-pinane selectivity (entries 1, 2), whereas PEG-
800/1-butanol furnished pinane with 21% conversion and 96% cis-pinane selectivity
(entry 5). The very low conversions (<10%) were obtained in PEG-800/1-propanol,
PEG-800/2-propanol, PEG-800/EG and PEG-800/glycerin (entries 3, 4, 6, 7). Similarly,
PEG-800/DMF, PEG-800/THF, PEG-800/EtOAc and PEG-800/benzene also delivered
lower conversions (entries 9–11, 13). Surprisingly, no conversion was observed in PEG-
8
1
00/H O, PEG-800/toluene, PEG-800/HOAc and PEG-800/HCOOH (entries 8, 12, 14,
5). These results indicated that solvent effect is profound to the reaction, and ethanol
2
is the best solvent for the reaction in the presence of PEG-800.
Effect of different Ni source catalyst on the reduction of a-pinene
The effect of different Ni source catalyst on the reduction of a-pinene was also investi-
gated (Table 3). From Table 3, using NiCl2ꢀ6H O and NiBr2ꢀxH O as Ni source, catalyst
2 2
offered a substantial boost in a-pinene conversion. Specially, NiCl2
ꢀ
6H O catalyst
2

4
K. LI ET AL.
Table 2. Effect of solvent on the reduction of a-pinene.
Entry
Solvent
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
Methanol
Ethanol
1-Propanol
2-Propanol
1-Butanol
EG
91
97
6
2
21
9
5
98
98
83
97
96
88
56
—
91
84
94
—
84
—
—
Glycerine
H
2
O
No reaction
DMF
THF
EtOAc
Toluene
Benzene
HOAc
3
3
1
0
1
2
3
4
5
No reaction
6
No reaction
No reaction
HCOOH
PEG-800 2 g, Solvent 3 mL, NiCl
tube, room temperature, 5 h.
2 2
ꢀ6H
O 0.25 mmol, a-pinene 5 mmol, NaBH
4
2.5 mmol, sealed
Table 3. Effect of different Ni source on the reduction of a-pinene.
Entry
Ni source
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
NiCl
NiBr
NiF
NiSO
Raney-Ni
2
ꢀ
6H
xH
2
O
O
97
94
5
98
98
94
57
98
98
60
90
2
ꢀ
2
2
4
ꢀ6H
2
O
5
51
72
4
NiC
Ni(NH
Ni(NO
H
4 6
O
SO
4
ꢀ4H
2
O
4H
O
2
3
)
2
ꢀ
2
O
3
)
2
ꢀ
6H
2
7
4
PEG-800 2 g, ethanol 3 mL, Ni catalyst 0.25 mmol, a-pinene 5 mmol, NaBH 2.5 mmol
sealed tube, room temperature, 5 h.
Table 4. Effect of different hydrogen source on the reduction of a-pinene.
Entry
Hrogen source
Conversion (%)
Selectivity (%)
1
2
3
4
a
H
N
2
(4 MPa)
No reaction
—
70
76
98
2
H
2
ꢀH O
2
3
1
97
Citric acid
NaBH
4
PEG-800 2 g, ethanol 3 mL, NiCl
2 2
ꢀ6H O 0.25 mmol, a-pinene 5 mmol, hydrogen source
2.5 mmol, sealed tube, room temperature, 5 h.
delivered the highest conversion to the products. Raney-Ni and NiC H O ꢀ4H O
4 2
4
6
furnished
a
moderate reaction conversion, whereas NiF2, NiSO4
ꢀ6H O,
2
Ni(NH SO )
ꢀ
4H O and Ni(NO ) 6H O delivered much lower a-pinene conversion.
ꢀ
3 2
2
3 2
2
2
These results illustrated that NiCl2ꢀ6H O is a good Ni source catalyst for the reduction
2
of a-pinene.
Effect of hydrogen donors on the reduction of a-pinene
Hydrogen donors have a prominent influence on the reduction of a-pinene and the
results are listed in Table 4. Amazingly, the reaction did not occur with H as a hydro-
2
gen donor even under 4 MPa pressure. In the case of N H2ꢀH O or citric acid, the
2
2

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Table 5. Poisoning experiments.
Entry
Poison additive
Conversion(%)
Selectivity(%)
1
2
3
4
None
Hg (500 equiv)
97
90
79
3
98
98
92
90
CS
CS
2
(0.5 equiv)
(1 equiv)
2
PEG-800 2 g, ethanol 3 mL, NiCl
2 2 4
ꢀ6H O 0.25 mmol, a-pinene 5 mmol, NaBH 2.5 mmol,
sealed tube, room temperature, 5 h.
Figure 1. TEM image of Ni nanoparticles in situ generated in PEG-800/ethanol system.
reaction could be proceeded sluggishly. However, when NaBH was used, a high conver-
4
sion of 97% was obtained with 98% cis-pinane selectivity.
TEM and poisoning experiments
After reaction, TEM was used to investigate the morphology of Ni catalyst. As depicted
in Figure 1, the TEM image revealed that Ni nanoparticles in situ generated and display
a homogeneous distribution in PEG-800/ethanol mixtures with an average particle size
of 3.2 nm. In order to gain a better understanding of the catalysis nature of the reaction,
series of poisoning experiments probing for heterogeneous catalysis were carried out.
Hg (0) test is most commonly used to distinguish homo/heterogeneous catalysts due to
[
16]
the surface amalgamation of the mercury with heterogeneous catalyst.
As can be
seen from Table 5 when 500 equiv Hg (0) (relative to Ni) was added to the reaction
mixtures at the beginning of the reaction, no significant effect on the catalytic activity
was observed. Furthermore, the effect of quantitative CS poison on the reaction was
2
studied. When 0.5 equiv of CS (relative to Ni) was added to the reaction mixtures at
2
t ¼ 5 min, the reaction still gave 79% conversion with 92% cis-pinane selectivity, whereas
when we added 1.0 equiv of CS to the reaction mixtures under the similar conditions,
2
the reaction was almost completely inhibited (entry 4). Accordingly, these results indi-
cated that the nature of active catalyst species for the reaction is perhaps the homoge-
neous Ni complex, which Ni nanoparticles generated in situ only act as the reservoir of
the catalytic species.

6
K. LI ET AL.
Conclusions
In summary, we developed a novel efficient and selective catalytic system for the reduc-
tion of a-pinene to cis-pinane by NaBH using NiCl2ꢀ6H O catalyst in PEG 800/ethanol
2
4
under room temperature. On the basis of TEM and series of poisoning experiments, the
role of the Ni nanoparticle generated in situ probably act as a reservoir for active cata-
lyst species, since our studies suggest that the reaction process is very likely to be
homogeneous.
Experimental section
General procedure for the reduction of a-pinene by NaBH4
In a typical experiment, a solution of a-pinene (5 mmol), Ni salt (0.25 mmol), PEGs
(
2 g) in solvent (3 mL) was added 2.5 mmol NaBH . The reaction tube was immediately
4
ꢂ
sealed and stirred at 25 C for 5 h. After reaction, the result mixture was extracted with
5
mL n-heptane, and the crude products of the reaction were determined by GC and
GC-MS analysis.
Funding
This work was supported by Shandong Provincial Natural Science Foundation (ZR2018QEM003),
Project of Shandong Province Higher Educational Science and Technology Program (J17KA117)
and Weifang Science & Technology Development Plan (2018GX002).
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