Sodium borohydride-nickel chloride hexahydrate in EtOH/PEG-400 as an efficient and recyclable catalytic system for the reduction of alkenes

Article abstract of DOI:10.1039/c8ra00905h

An efficient, safe and one-pot convenient catalytic system has been developed for the reduction of alkenes using NaBH₄-NiCl₂·6H₂O in EtOH/PEG-400 under mild conditions. In this catalytic system, a variety of alkenes (including trisubstituted alkene α-pinene) were well reduced and the Ni catalyst could be recycled.

Full text of DOI:10.1039/c8ra00905h

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Sodium borohydride-nickel chloride hexahydrate
in EtOH/PEG-400 as an efficient and recyclable
catalytic system for the reduction of alkenes†
Cite this: RSC Adv., 2018, 8, 7761
Kaoxue Li, * Chuanchao Liu, Kang Wang, Yang Ren and Fahui Li*
Received 29th January 2018
Accepted 5th February 2018
An efficient, safe and one-pot convenient catalytic system has been developed for the reduction of alkenes
DOI: 10.1039/c8ra00905h
4 2 2
using NaBH –NiCl $6H O in EtOH/PEG-400 under mild conditions. In this catalytic system, a variety of
rsc.li/rsc-advances
alkenes (including trisubstituted alkene a-pinene) were well reduced and the Ni catalyst could be recycled.
ꢀ
12
The reduction of alkenes is an important transformation in NaBH in hexane at 30 C. Unfortunately, heterogeneous
4
organic synthesis, widely used particularly in petrochemical, catalysts of NPS on solid supports are oen more inert than
19
pharmaceutical, and ne chemical processes. Traditionally, corresponding soluble NPs catalysts. In view of the above, we
1
–4
5–7
direct hydrogenation,
hydride reduction methods
reduction of alkenes. Among the reported methods, the utility agent and stabilizer for soluble metal NPs.
of sodium borohydride for the reduction of simple alkenes rst Mohamed et al. found that iron metal NPs formed in an
catalytic hydrogen transfer
and wanted to explore the use of soluble NPs generated in situ for
8
–10
have been employed for the this process. PEG-400 is known to be an excellent dispersion
20,21
Abdul Rahman
11
described by Brown in 1962 is well known. In recent years, ethanol-PEG-400 solution displayed a more uniform distribu-
several related catalytic systems on modication of Brown's tion. In this work, we introduce NaBH
22
4 2 2
/NiCl $6H O in EtOH/
approach have been developed. These catalytic systems include PEG-400 to the reduction of alkenes, to the best of our knowl-
1
2
NaBH
4
/NiCl
2
$6H
2
O/moist alumina in hexane, InCl
3
–NaBH
4
14
edge, the system is novel for the reaction. The novel system was
13
reagent system, NaBH /RuCl under aqueous conditions,
expected to show the following advantages (Scheme 1): (a)
4
3
15
2+
NaBH /CH COOH in the presence of Pd/C and NaBH4- NaBH not only reduces Ni to soluble Ni (0) NPs in situ, but
4
3
4
16
RANEY® nickel system in water. Nevertheless, most of these also serves as hydrogen donation for the reduction of alkenes
systems require costly transition metal catalyst, long reaction with ethanol; (b) the in situ generated soluble Ni (0) NPs catalyst
time and a large excess of NaBH
4
. In addition to this, little has stabilized by PEG-400 is stable, efficient and recyclable for the
been done to recycle the catalyst for the reduction of alkenes reduction of alkenes.
using NaBH
system for the reduction of alkenes by NaBH
desirable.
4
. Thus, a very simple, efficient and recyclable
We rst chose the reduction of styrene as model reaction
would be highly (Table 1). Initially, the effect of VEtOH/VPEG-400 (volume ratio of
ethanol to PEG-400) on the reaction was investigated. Curiously,
4
In the reduction of alkenes by NaBH , the in situ generated using pure ethanol and PEG-400 as the solvent gave lower yields
4
metal nanoparticles (NPs) from the combination of appropriate
15
metal salts and NaBH
4
catalyze the reduction of alkene. In
general, metal NPs tend to agglomerate during the catalytic
17
processes and therefore need to be protected by stabilizers.
Castro et al. ever noticed the aggregation of NPs aer just one
time in the reduction of alkenes by NaBH without use of
4
18
a stabilizer. Immobilized nanoparticles (NPs) on insoluble
solid supports were generally used for this process in the past
12,15,16
literature.
Morimoto and coworkers reached 90% yield of ethylbenzene
within 3 h using NiCl $6H O on moist alumina reducted by
One signicant example was that Takashi
2
2
Chemistry & Chemical and Environmental Engineering College, Weifang University,
Weifang 261061, China. E-mail: likaoxue@wfu.edu.cn; fahuili@163.com; Tel:
+
†
1
86-536-8785283
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra00905h
4 2 2
Scheme 1 Reduction of alkenes using NaBH –NiCl $6H O in EtOH/
PEG-400 system.
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Table 1 Optimization of reaction conditions for the reduction of RANEY® nickel catalyst. The results revealed that the in situ
a
styrene
generated Ni (0) NPs exhibited a higher activity for the reduction
b
of styrene (entries 4, 20). Thus, we can conclude that the EtOH/
Entry
Solvent (v/v)
NaBH
4
(equiv.)
Yield (%)
PEG-400 using NiCl $6H O–NaBH is a very simple and efficient
2
2
4
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
EtOH
PEG-400
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.75
1.0
1.25
0.5
76
41
84
99
92
79
98
80
60
18
26
23
21
26
<1
<1
89
79
68
system for the reduction of styrene.
The in situ generated Ni (0) NPs in EtOH/PEG-400 were
characterized by UV-vis, XPS aer model reaction. Fig. 1a
showed the UV-vis spectrum of nickel chloride hexahydrate in
EtOH/PEG-400 before and aer reaction. Apparently, a broad
band at 250–270 nm appeared aer reaction, which indicated
EtOH-PEG400(4/1)
EtOH-PEG400(3/2)
EtOH-PEG400(2/3)
EtOH-PEG400(1/4)
MeOH-PEG400(3/2)
1-Propanol-PEG400(3/2)
1-Butanol-PEG400(3/2)
2-Propanol-PEG400(3/2)
H O-PEG400(3/2)
2
Ethyl acetate-PEG400(3/2)
Toluene-PEG400(3/2)
Cyclohexane-PEG400(3/2)
HCOOH-PEG400(3/2)
CH COOH-PEG400(3/2)
3
26
the formation of Ni (0) NPs. XPS spectra (Fig. 1b) showed that
Ni 2p3/2 peak at approximately 852.8 eV and Ni 2p1/2 peak at
0
1
2
3
4
5
6
7
8
9
0
27
870.9 eV, respectively, indicating the generation of Ni (0) NPs.
The motive to use PEG-400 as a stabilizer in ethanol was the
possibility to protect and recycle the Ni (0) NPs catalyst. Aer
model reaction, the catalyst could be separated by simple
extracting with n-heptane followed by decantation and reused
directly without further purication. The results for the reuse of
the Ni catalyst were shown in Table 2. As can be seen from Table
2, in the rst run, the yield of ethylbenzene reached 99% for
15 min. In subsequent consecutive runs the catalytic activity
decreased, similar observations have also been made about the
EtOH-PEG400(3/2)
EtOH-PEG400(3/2)
EtOH-PEG400(3/2)
EtOH-PEG400(3/2)
c
54
a
ꢀ
Reaction conditions: N
2
atmosphere, 30 C, NiCl
2
$6H
2
O 0.25 mmol,
b
c
solvent (5 mL), styrene 5 mmol. GC yield. Catalyzed by RANEY®
nickel.
(entries 1, 2), while adding certain amounts of PEG-400 to the
ethanol led to an enhancement in activity rst (entries 3, 4),
then further addition of PEG-400 retarded the reaction (entries
5, 6), and a best yield was obtained when using 3/2 ratio of
ethanol and PEG-400 (entry 4), which might be due to the good
dispersion and stabilization of the Ni (0) NPs in the mixture.
Subsequently, the inuence of PEG-400 in different solvent on
the reduction of styrene was also investigated. It was found that
the combination of PEG-400 with MeOH also gave excellent
yield (entry 7), whereas PEG-400 in 1-propanol, 1-butanol, 2-
propanol, H
lower yields of product (entries 8–14). The very low yields were
obtained in HCOOH-PEG-400 and CH COOH-PEG-400 system
entries 15, 16). These results indicated that EtOH and MeOH
2
O, ethyl acetate, toluene, cyclohexane afforded the
3
(
were the suitable solvents for the reduction of styrene in the
presence of PEG-400. Studies on the effects of the amounts of
NaBH in EtOH/PEG-400 (3/2 ratio) showed that the yield of
4
ethylbenzene decreased with the increase of the molar equiva-
lents of NaBH (entries 4, 17–19). It is possible that excess
4
amounts of boron products strongly bound the nanocatalyst
surface that passivates the active sites of the Ni catalyst.
Importantly, the appropriate amounts of the NaBH
4
were only
0
.5 molar equivalents (entry 4), which were lower than those in
12,15,23,24
the earlier reports.
This is possibly because the hydrogen
source for the reduction can be sufficiently derived from the
B–H of NaBH and the O–H of ethanol in our catalytic system
ideally 1 molar of NaBH can reduce 4 molar of alkenes with
ethanol), as recently reported by Bai et al. for the semi-
4
(
4
25
hydrogenation of alkynes with NaBH
activity of the in situ generated Ni (0) NPs catalyst in EtOH/PEG-
00 (3/2 ratio) had also been compared with the commercial
4
in methanol. The
Fig. 1 (a) UV-vis spectra of the solution of NiCl
00 before and after the reduction of styrene using NaBH
in situ generated Ni NPs.
2
$6H
2
O in EtOH/PEG-
4
4
. (b) XPS of
4
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a
Table 2 Recycle of the catalyst
Table 3 Reduction of alkenes with NaBH
4
/NiCl
2
$6H
2
O in EtOH/PEG-
a
400 system
b
Run
Time/min
Yield (%)
b
Entry
Substrate
Time (min)
Yield (%)
1
2
3
15
120
120
99
99
97
1
2
3
4
5
6
7
8
9
1
1
1
1
1-Hexene
1-Octene
1-Decene
1-Dodecene
Styrene
4-Methylstyrene
Allyl phenyl ether
trans-Anethole
b-Pinene
Norbornene
Cyclopentene
Cyclohexene
1,5-Cyclooctadiene
a-Pinene
15
15
15
15
15
15
15
120
240
15
98
96
95
94
a
ꢀ
Reaction conditions: N
EtOH/VPEG-400 ¼ 3 : 2 (5 mL), styrene 5 mmol, NaBH
yield.
2
atmosphere, 30 C, NiCl
2
$6H
4
2
O 0.25 mmol,
2.5 mmol. GC
b
V
99
100
100
94
96
100
96
28,29
decrease for in situ generated Nps catalyst during recycling.
0
1
2
3
However, prolonging the reaction time from 15 min to 120 min
could still keep a high yield of ethylbenzene. Notably, the
activities of Ni (0) NPs from second run still higher than the
corresponding heterogeneous Ni NPS on moist alumina re- 14
ported by Takashi Morimoto et al.
To explore the reason why the catalytic activity decreased
30
120
300
300
99
c
94(72 : 28)
91
12
a
ꢀ
Reaction conditions: N
EtOH/VPEG-400 ¼ 3 : 2 (5 mL), alkenes 5 mmol, NaBH
2
atmosphere, 30 C, NiCl
2
$6H
4
2
O 0.25 mmol,
b
V
2.5 mmol. GC
c
aer rst run, the leaching, size and distribution of Ni NPs were yield. Ratio of cyclooctene/cyclooctane.
characterized aer reaction. The leached Ni species were
checked by ICP-AES and found to be only 0.3%. HRTEM image
showed that Ni (0) NPs were well-dispersed with an average stabilization of the Ni (0) NPs in EtOH/PEG-400 system. There-
diameter of 3–5 nm (Fig. 2), indicating the good dispersion and fore, the reason for the decrease of catalyst could hardly be
explained by Ni leaching or the aggregate of the Ni (0) NPs.
Meanwhile, control experiments were performed between in
situ generated Ni (0) NPs in the second run and preformed Ni (0)
NPs catalyst (see the ESI† for details)for the reduction of styrene
(Fig. 3). Obviously, the activities of the two kinds of Ni (0) NPs
catalysts for the model reaction were almost equivalent. This
promoted us to infer Ni (0) NPs had been changed from in situ
generated to preformed catalyst, which may be the main reason
for the reduced activity of the catalyst in subsequent runs
during recycling. The details are under investigation.
The scope of this catalytic system was also examined for the
reduction of various olens (Table 3). Both aliphatic and acyclic
olens were reactive in the catalytic system, affording their
corresponding alkanes with excellent yields. For example,
monosubstituted terminal alkenes were efficiently reduced to
their corresponding alkanes within 15 min (entries 1–7).
Disubstituted olens were also reduced efficiently (entries 8–
Fig. 2 HRTEM image of in situ generated Ni NPs after model reaction.
13), but with longer reaction times compared to mono-
substituted terminal alkenes except for norbornene (entry 10).
Reduction of 1,5-cyclooctadiene afforded cyclooctene and
cyclooctane with 94% yield within 300 min (entry 13). Impor-
tantly, trisubstituted alkene a-pinene (in contrast with previous
14,31
4 2 4 3
studies of NaBH /CoCl (ref. 30) and NaBH /RuCl system,
which were inert for the reduction of a-pinene) was also reduced
without any difficulty (entry 14).
Conclusions
In summary, we have developed a novel system for the reduc-
tion of alkenes in one pot. The catalytic system involving
NaBH /NiCl $6H O/EtOH/PEG-400 exhibited good activity,
4
2
2
recyclability and general applicability. Further the protocol is
second run and preformed Ni (0) NPs catalyst for the model reaction. convenient, cheap and safe to carry out on a large scale.
Fig. 3 Control experiments between in situ generated Ni (0) NPs in the
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