Ligand-free nickel-catalyzed semihydrogenation of alkynes with sodium borohydride: A highly efficient and selective process for: Cis -alkenes under ambient conditions

Article abstract of DOI:10.1039/c7cc02140b

We report a low-cost and efficient catalytic system, involving in situ generated ligand-free Ni NPs, methanol and sodium borohydride, for the semihydrogenation of alkynes under ambient conditions. This catalytic system exhibits remarkably high activity, satisfactory cis-selectivity for internal alkynes, good stability and general applicability.

Full text of DOI:10.1039/c7cc02140b

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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DOI: 10.1039/C7CC02140B
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Ligand-free nickel-catalyzed semihydrogenation of alkynes with
sodium borohydride: a highly efficient and selective process for
cis-alkenes under ambient conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Xin Wen,a,† Xiaozhen Shi,a,† Xianliang Qiao,a Zhilei Wu,a Guoyi Baia,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report a low-cost and efficient catalytic system, involving in- also be used as catalysts in combination with a nitrile
situ generated ligand-free Ni NPs, methanol and sodium functionalised imidazolium based ionic liquid.12 A copper-
borohydride, for the semihydrogenation of alkynes under ambient catalyzed system using a N-heterocyclic carbene ligand under
conditions. This catalytic system exhibits remarkably high activity, molecular hydrogen enabled the semihydrogenation of
satisfying cis-selectivity for internal alkynes, good stability and internal alkynes to cis-alkenes.13 However, these catalytic
general applicability.
systems always make use of organic ligands, intermetallic
compounds or functional solvent, to achieve relatively high
selectivity for alkenes, which often leads to low catalytic
activity and hence lengthy reaction times. In addition, such
reactions are usually conducted using molecular hydrogen as
the hydrogen source, which must be strictly monitored to
prevent over-reduction to alkanes and also require special
equipments to operate with flammable hydrogen gas.14 In this
regard, the use of ammonia-borane as hydrogen donor has
alleviated this, but the high cost limits its further applications.
The use of the inexpensive borohydrides as reductants in the
semihydrogenation of alkynes is rather limited. Sodium
borohydride has been used as a reducing agent for the
transformation of 1,2-diphenylacetylene (DPA) into stilbene
catalyzed by a Ni(0) complex, but under forcing conditions of
80oC for 72 h.15 Thus, a reaction system involving a simple and
non-precious metal catalyst, a cheaper hydrogen source, and a
practical procedure, operating with high efficiency would be
highly desirable.
The selective semihydrogenation of alkynes to the
corresponding alkenes is of great importance in the polymer
industry as well as in the synthetic organic chemistry.1 For
decades, Pd-based catalysts have dominated this
transformation.2 For example, Pd/Al2O3 exhibited good
catalytic performance in the removal of even small amounts of
acetylene from ethylene streams which was a key requirement
in the polyethylene industry, because its presence would
decrease the polyethylene purity as well as deactivate the
polymerization catalyst.3 The most famous Pd-based catalyst
for this transformation is Lindlar’s catalyst. However
necessary poisons and quinoline always lead to harmful wastes
and heavy metal residues
n the products.4 In addition, over-
reduction to the alkane, a requirement for complex ligand,
long reaction times, poor catalyst shelf-life and hence
reproducibility, are features of such systems.5
, the
i
As a cheaper alternative to the Pd-based catalysts, non-
precious metal catalysts, based on Ni,6 Co,7 Cu,8 and Fe,9 have
been applied to the selective semihydrogenation of alkynes to
the corresponding alkenes. For example, Richmond and co-
workers reported that the cis-/trans-selectivity of nickel-
catalyzed semireduction of alkynes can be controlled by a
Herein, we report
a
ligand-free nickel-catalyzed
semihydrogenation of alkynes, which operates under ambient
conditions, for the transformation of internal and even
terminal alkynes to the corresponding alkenes with great
efficiency and high selectivity in the presence of borohydrides.
In this work, the formation of the Ni NPs catalyst and the
semihydrogenation of alkynes were completed in a one-pot
process at room temperature within a few minutes. Notably,
the borohydride exhibits dual functionalities: reduction of Ni(II)
to Ni NPs and hydrogen donation for the semihydrogenation.
Initially, the semihydrogenation of DPA was chosen as the
model reaction to optimize the reaction conditions, including
different solvents and hydrogen donors, towards a unique
triphos ligand.10 More recently,
a
NiGa intermetallic
nanocrystalline material with uniform particle size and
controlled composition was found to be highly selective in the
semihydrogenation of alkynes.11 Ni nanoparticles (NPs) can
a Key Laboratory of Chemical Biology of Hebei Province, College of Chemistry and
Environmental Science, Hebei University, Baoding 071002, China. Phone: +86-312-
5079359; Fax: +86-312-5937102. E-mail address: baiguoyi@hotmail.com.
†These authors contributed equally to this work.
††Electronic Supplementary Information (ESI) available: [XXX].
See DOI: 10.1039/x0xx00000x
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stilbene in methanol were much higher than those in ethanol.
View Article Online
Table 1 Semihydrogenation of DPA catalyzed by in-situ
Changing the hydrogen source to poDtaOsIs: i1u0m.103b9/oCr7oChCy0d2r1i4d0eB,
afforded a 99% conversion within 15 minutes and displayed a
slightly lower activity than sodium borohydride, yet the
selectivities roughly equivalent (Table 1, entry 11). Ammonia
borane exhibited moderate reactivity compared with
borohydrides in methanol in the presence of Ni NPs (Table 1,
entries 12-14). Note that the ratio of cis- and trans- (Z/E)
selectivity for stilbene decreased from 1.6 to 0.4 with the
increase of the molar equiv. of ammonia borane from 2 to 8,
but was much lower than that obtained using sodium
borohydride (Table 1, entry 3; Z/E = 19), which might be
attributed to the isomerization of cis-stilbene, as recently
reported by Fu using a less sterically hindered Co-based
catalyst in the presence of ammonia borane.7 Furthermore,
the semihydrogenation of DPA did not work while using
hydrogen gas under 1 atm pressure (Table 1, entry 15). In the
absence of Ni NPs, the controlled experiment gave a
conversion of only 3% in 100 minutes (Table 1, entry 16),
demonstrating the crucial role of Ni NPs as the active catalyst.
Combining the results mentioned above, sodium borohydride
(2.0 molar equiv.) and methanol were chosen as the best
reductant and solvent in the semihydrogenation of DPA in the
presence of appropriate amount (0.4 mmol%) of Ni catalyst
(Table S1, ESI††), and most importantly, the turnover
frequency (TOF) value under such reaction conditions was
2940 h-1, which is much higher than those of the noble metal-
free catalysts and even most noble metal examples (Table S2,
ESI††). Thus, we can conclude that the one-pot process
including in-situ generation of Ni NPs and semihydrogenation
of DPA was a remarkably efficient and selective system for cis-
stilbene synthesis.
generated Ni NPs in one-pot processa
Entry Solvent Reductant
Molar Time Conv.c (Z/E)/alkaned
equiv.b (min)
(%)
(%)
1
2
MeOH
MeOH
NaBH4
NaBH4
0.5
1.0
115
10
42
96
(87/12)/1
(91/8)/1
3
4e
5e
6
MeOH
MeOH
MeOH
EtOH
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
KBH4
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
4.0
8.0
-
5
5
98
68
99
34
8
(94/5)/1
(96/4)/0
(90/7)/3
(66/30)/4
(70/26)/4
(47/37)/16
(69/26)/5
(52/39)/9
(93/6)/1
(59/37)/4
(29/65)/6
(27/65)/8
-
30
150
150
150
150
150
15
7
dioxane
t-BuOH
EtOAc
DMF
8
6
9
3
10
11
12
13
14
15
16f
9
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
99
52
86
94
0
NH3BH3
NH3BH3
NH3BH3
H2
3
5
3
240
100
NaBH4
2.0
3
(88/12)/0
aReaction conditions: DPA (1.0mmol), 0.4mol% Ni catalyst, solvent (8 mL), room
temperature. bThe molar equivalent of reductant to DPA. cDetermined by GC.
dRelative percent ratio of alkene (Z/E) to the corresponding alkane. eA two-steps
method was used (ESI††). fNi NPs was absent in the semihydrogenation.
combination of high activity and good selectivity for cis-
stilbene. In this one-pot process, the Ni NPs were generated in
the solution, whilst the DPA molecule was hydrogenated to cis-
stilbene over such Ni NPs catalysts with Ni loadings as low as
0.4 mol% (Table 1). Using 0.5 molar equiv. of NaBH4, low
conversion (42%) with 87% selectivity for cis-stilbene was
obtained within 115 minutes (Table 1, entry 1). When the
amount of NaBH4 was increased to 1 equiv. (Table 1, entry 2),
the conversion of DPA was 96% within 10 minutes. A further
increase of the amount of NaBH4 to 2 equiv. (Table 1, entry 3),
the conversion of DPA increased to 98% in 5 minutes while the
selectivity for cis-stilbene increased to 94% with a high ratio of
cis- and trans- ratio (Z/E = 19). In contrast, when the
transformation proceeded in a two step procedure (see the
ESI†† for details), the conversion of DPA was only 68% in 5
minutes (Table 1, entry 4), but reached 99% if the reaction
time was prolonged to 30 minutes (Table 1, entry 5),
demonstrating that the in-situ generated Ni NPs was a highly
active catalyst in the semihydrogenation of DPA. Among the
solvents examined (Table 1, entries 6-10), the
semihydrogenation proceeded sluggishly in non-protic
solvents such as dioxane, THF, DMF, EtOAc and a weak polar
solvent, t-BuOH. In contrast, the conversion increased to 34%
in ethanol and markedly to 98% within 5 minutes in methanol,
demonstrated the crucial role of solvents in this reaction.
Particularly, the conversion of DPA and selectivity for cis-
The semihydrogenation of DPA under the optimized
reaction conditions was also monitored by in-situ IR
spectroscopy (see ESI†† for details, Fig. S1), suggesting the
generation of cis-stilbene and NaB(OCH3)4. Trans-stilbene and
1,2-diphenylethane were not detected due to their low
concentrations. Transmission electron microscopy (TEM)
revealed the in-situ generated, narrowly-dispersed Ni NPs with
an average size of 6.2±0.3 nm, as determined after the
volatilization of methanol. These tiny Ni NPs can be isolated by
simple centrifugation and reused 5 times with >91%
conversion in less than 20 minutes and >91% selectivity for cis-
stilbene (Fig. S2, ESI††).
Fig. 1 TEM images of in-situ generated Ni NPs in the
semihydrogenation of DPA.
2 | J. Name., 2017, 00, 1-5
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The general applicability of this catalytic system involving the
ligand-free Ni catalyst, sodium borohydride and methanol was
investigated with various terminal and internal alkynes under
ambient conditions. The results are summarized in Table 2 and
reveal that the cis-semihydrogenation pathway observed for DPA is
general. Tested substrates bearing a series of functional groups,
such as fluoro, chloro, bromo, cyano, ethyl, methoxyl, alkyl, ester
and trifluoromethyl provided the corresponding cis-alkene products
in good conversion with satisfying selectivity. Aldehydes or ketones
that do not tolerate the reaction conditions16 were not included in
the above functional groups. Substrates bearing a pyridine
substituent afforded lower Z/E selectivity probably due to
coordination with metal catalyst.7 Nevertheless, it should be noted
that terminal alkynes with either electron-withdrawing or electron-
donating groups can be smoothly hydrogenated to the
corresponding alkenes with 99% conversion and >94% selectivity.
View Article Online
DOI: 10.1039/C7CC02140B
Scheme
1
Deuterium-labeling experiments of cis-
semihydrogenation of DPA.
To
gain mechanistic insights, deuterium-labeling
experiments were conducted to verify the hydrogen source in
the alkene product. When NaBD4 was used as the reductant in
the semihydrogenation of DPA, the cis-stilbene was
Table 2 Nickel-catalyzed semihydrogenation of alkynes with monodeuterated in the presence of Ni NPs catalyst (Scheme
sodium borohydridea
1a). Likewise, it was monodeuterated when CH3OD was used
as the solvent, as presented in Scheme 1b. Conversely,
dideuterated cis-stilbene was isolated when the reaction was
carried out using NaBD4 as the reductant in CH3OD (Scheme
1c). These results therefore demonstrated that the two extrinsic
hydrogen atoms in cis-stilbene should derive from the B-H of
borane (hydride) and the O-H of methanol (proton),
respectively. Furthermore, the nature of the byproduct
containing boron can give significant information about the
Molar
equiv.
Time
(min)
Conv.c (Z/E)
Entry
Productsb
(%)
/Alkane
1
0.5
1.0
1.0
0.5
1.0
3
5
5
3
3
>99
(96)/4
(98)/2
(96)/4
(95)/5
(94)/6
2
3
>99
>99
>99
99
mechanism. Thus, the direc
t
11B NMR spectrum was then
4
recorded for a reaction mixture of DPA semihydrogenation, which
was carried out in methanol-d4 in the presence of NaBH4 as
reductant. Two peaks at 3.1 and -43.8 ppm, which were
respectively assigned to NaB(OCD3)4 and NaBH4, were observed in
the 11B NMR spectrum (see ESI††), revealed that NaBH4 was
converted into NaB(OCD3)4, in accordance with literature data.17
However, monomethoxy, dimethoxy and trimethoxy borate salts
were not detected in the reaction mixture. The formation of the
tetramethoxyl borate salt indicated that the hydroxyl group in
methanol was crucial in the protonation of triple bond. Combing
the results of deuterium-labeling experiments and 11B NMR data,
we propose that the catalytically active H, derived from NaBH4, was
first generated on the surface of the Ni catalyst. Subsequently, the
catalytically active, nucleophilic H would attack the triple bond of
alkyne, whilst the secondary protonation of the triple bond by the H
derived from -OH of methanol, would afford the cis-alkene, as
presented in Scheme 2. In turn, the residual H of B-H and the H of -
OH were delivered to the alkyne, whilst the methoxyl of methanol
reacted with B of B-H, producing another 3 molecules of alkene and
the by-product NaB(OCH3)4 finally, as verified by the 11B NMR
spectrum. As depicted in Scheme 2, ideally 1 molecule of NaBH4
can reduce 4 molecules of alkyne in methanol. Indeed, only part of
the NaBH4 was utilizable in the semihydrogenation due to the
generation of hydrogen gas that escaped from the solution, as
previously reported by other groups in the hydrogen production
from sodium borohydride in alcohol-containing solutions.18 It must
be pointed out that the generated hydrogen gas was not the
5
6
0.5
0.5
5
5
>99
99
(98)/2
(96)/4
7
8
8.0
2.0
40
20
61
99
(61)/39
(94)/6
9
,
10
1.5
2.0
5
98
99
(93/7)/0
(91/7)/2
11
15
12
13
3.0
2.0
15
35
98
(93/4)/3
(85/2)/1
>99
14
1.2
5
98
(87/4)/9
a Reaction conditions: substrates (1.0 mmol), 0.4 mol% Ni catalyst and methanol hydrogen source in the semihydrogenation, which was proven by
(8 mL) at room temperature. b Products were identified using GC–MS and the cis-
or trans-isomerism were determined by NMR.
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by the sodium borohydride in the presence of Ni NPs.
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In summary, we present a novel catalytic system for the
semihydrogenation of internal and terminal alkynes under
ambient conditions. The catalytic system involving ligand-free
Ni NPs, methanol and sodium borohydride, exhibited
remarkable high activity, satisfying cis-selective and general
applicability. This protocol can accomplish in a cheaper, more
practical manner and is a compelling alternative to the
semihydrogenation of alkynes over the existing catalytic
systems.
The authors thank Professor David W. Knight for his kind
assistance in preparing this article. Financial support by the
National Natural Science Foundation of China (21376060 and
21676068), the Natural Science Foundation of Hebei Province
(B2016418005 and B2014201024) and Hebei University
construction project for comprehensive strength promotion of
Midwest colleges and universities is gratefully acknowledged.
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