Efficient and exceptionally selective semireduction of alkynes using a supported gold catalyst under a CO atmosphere

Article abstract of DOI:10.1039/c4cc01595a

A new efficient method for chemo- and regio-selective semireduction of alkynes using CO/H₂O as the hydrogen source catalyzed by gold supported on high surface area TiO₂ was developed. A facile and practical synthesis of 1,2-dideuterioalkenes was also realized by using CO/D₂O as the reducing agent. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01595a

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Efficient and exceptionally selective
semireduction of alkynes using a supported
gold catalyst under a CO atmosphere†
Cite this: Chem. Commun., 2014,
50, 5626
Received 3rd March 2014,
Accepted 25th March 2014
Shu-Shuang Li, Xiang Liu, Yong-Mei Liu,* He-Yong He, Kang-Nian Fan and
Yong Cao*
DOI: 10.1039/c4cc01595a
www.rsc.org/chemcomm
A new efficient method for chemo- and regio-selective semireduc- the key active species for selective reduction of unsaturated polar
tion of alkynes using CO/H2O as the hydrogen source catalyzed by groups. Given the high efficiency and unique chemoselectivity of the
gold supported on high surface area TiO2 was developed. A facile Au–CO/H2O-mediated reduction, we were inspired to apply this
and practical synthesis of 1,2-dideuterioalkenes was also realized catalytic methodology to semireduction of alkynes. Herein, we report
by using CO/D2O as the reducing agent.
that Au NPs (mean size ca. 2.0 nm) deposited on high surface area
TiO2 (Au/HSA-TiO2) efficiently catalyze selective semireduction of
The catalytic semireduction of alkynes to the corresponding alkenes alkynes to their corresponding alkenes with CO/H2O as the reduc-
is of great significance in the manufacture of fine chemicals as tant under mild conditions. To the best of our knowledge, this study
well as in the polymerization industry.1 Such reactions are usually also reports semireduction of alkynes to the corresponding
carried out using molecular hydrogen in the presence of Pd-based 1,2-dideuterioalkenes with the CO/D2O couple in the presence
catalysts,2,3 with two main drawbacks:4–6 toxic additives are mostly of a gold catalyst for the first time.
required to improve the selectivity, and hydrogen uptake in these
Initially, we investigated the catalytic activities of a series of
systems should be strictly monitored to prevent over-reduction to supported Au NPs of approximately 2–3 nm size in the semireduc-
alkanes. An attractive alternative is catalysts based on supported gold tion of phenylacetylene (1a) under a mild CO atmosphere (10 atm) at
nanoparticles (NPs), which have shown interesting catalytic proper- 60 1C. These Au catalysts promoted the semireduction of 1a to the
ties in the chemoselective transformation of functional groups corresponding styrene (2a) with 499% selectivities (Table 1, entries
including partial hydrogenation of carbon–carbon multiple bonds.7,8 1–7 and 9–12). Of the Au catalysts tested, the use of high surface area
In this connection, a nanoporous gold-catalyzed selective semireduc- TiO2 as a support gave the highest activity to give 2a quantitatively
tion of alkynes to Z-olefins was recently reported.9 The hydrogen within 1 h (entry 1). This result is remarkable, and becomes more
source used in this process was organosilanes, which however relevant as the catalyst can be reused, at least five times, while
may not be economically suitable and also poses safety concerns. maintaining a 96% conversion and a yield up to 96% (entry 2). In a
Thus, the development of new alternative Au-based semireduction gram-scale reaction of 1a (20-fold scale up) for 19 h, 2a was obtained
methods pertaining to the use of safer and more expedient in 97% yield (entry 3). Identical gold NPs supported on commercial
hydrogen sources is highly desirable.
P25 which has much smaller surface area were also found to be
We have recently discovered an excellent Au-catalyzed transfer effective but inferior to Au/HSA-TiO2 (entry 1 vs. 4), whereas Au/CeO2,
reduction strategy that can allow rapid and chemoselective reduction Au/ZnO, Au/Al2O3 and Au/ZrO2 gave 2a in low yields (entries 9–12).
of substituted nitro and carbonyl compounds under very mild Further investigation into the reaction parameters revealed that the
conditions.10 This process can circumvent the limited hydrogen catalytic activity was sensitive to the solvent employed in this
delivery capacity that has plagued previously reported Au-catalyzed reaction. Among the solvents tested, ethanol was the best (see
hydrogenation processes.11 The reductants used in this green Table S1 in ESI†). Notably, without organic solvent, this semireduc-
methodology were CO and H2O (rather than H2), where the transient tion can still proceed to end in neat water (Table 1, entry 5). The
Au0–H formed by the CO-induced H2O reduction is believed to be most noteworthy aspect of the Au/HSA-TiO2–CO/H2O reduction
system, however, is that the exclusive selectivity of 2a can be
maintained even after prolonged reaction time following the complete
consumption of 1a (Fig. S3, ESI†). This distinct feature, together with
the fact that the semireduction could proceed smoothly even at 25 1C
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and
Innovative Materials, Fudan University, Shanghai 200433, P. R. China.
E-mail: yongcao@fudan.edu.cn, ymliu@fudan.edu.cn; Fax: +86-21-65643774
(entry 7), offers practical and real advantages over the existing methods
to establish a more sustainable and industrially-viable process.
† Electronic supplementary information (ESI) available: Experimental section,
TEM and XPS data. See DOI: 10.1039/c4cc01595a
5626 | Chem. Commun., 2014, 50, 5626--5628
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Table 1 Semireduction of phenylacetylene (1a) to styrene (2a) with CO/
H2O using various catalystsa
Table 2 Semireduction of various alkynes with CO/H2Oa
Temp. Time Conv.b Sel.b
Entry Substrate
(1C)
(h)
(%)
(%) Z/Ec
1
60
1
499
499
499
—
—
Entry Catalyst
Temp. (1C) Time (h) Conv.e (%) Sel.e (%)
1
Au/HSA-TiO2 60
Au/HSA-TiO2 60
Au/HSA-TiO2 60
1
1
19
1
9
4
10
1
1
1
1
499
96
97
72
99
499
499
n.r.
39
5
3
24
Trace
Trace
Trace
Trace
499
499
499
499
499
499
499
—
499
499
499
499
—
2b
3c
4
2
60
0.5 499
0.3 499
Au/TiO2
60
5d
6
7
Au/HSA-TiO2 60
Au/HSA-TiO2 40
Au/HSA-TiO2 25
3
4
60
60
499
499
—
—
8
9
HSA-TiO2
Au/CeO2
Au/ZnO
Au/Al2O3
Au/ZrO2
Pd/C
Pt/C
Ru/Al2O3
Ir/TiO2
60
60
60
60
60
60
60
60
60
1
3
499
499
10
11
12
13
14
15
16
1
1
1
1
5
60
499
499
—
—
—
—
—
1
a
Reaction conditions: 0.5 mmol 1a, metal (1 mol%), CO (10 atm),
6
7
8
60
100
80
4
16
14
499
93
b
c
ethanol (4.5 mL), H2O (0.5 mL). Fifth run. Conditions: substrate
(10 mmol), Au (0.1 mol%), CO (20 atm), ethanol (18 mL), H2O (2 mL).
d
e
H2O (5 mL), no organic solvent. The conversion and selectivity were
499 96/4
499 97/3
determined by GC using anisole as the internal standard.
95
Subsequent studies focusing on the comparison with other noble
metals (Table 1, entries 13–16) showed that the presence of gold is
essential for exclusive semireduction of 1a with CO/H2O. To better
understand the superior activity and selectivity, we conducted several
experiments to gain mechanistic insight into the present Au/HSA-
TiO2–CO/H2O semireduction system. First, in a control experiment
under identical conditions without 1a, H2 was not detected. Second,
by using H2 as a reducing agent instead of CO/H2O, we observe
substantial formation of the over-reduced product of ethylbenzene
even at low 1a conversion levels (Fig. S4, ESI†). These results rule out
the participation of H2 in the Au/HSA-TiO2 catalyzed semireduction
as described above. Third, in a separate experiment investigating the
influence of supports by using O2 as the hydrogen acceptor showed
that the liquid phase CO oxidation over Au/HSA-TiO2 occurred with
much higher rates than other catalysts (Fig. S5, ESI†), indicating that
the CO species adsorbed on the Au–HSA-TiO2 interface could be
more active for relevant transformation. Taken together, it is likely
that in situ gold-hydride species generated from the reaction of CO
with H2O over Au/HSA-TiO2 plays a key role in facilitating the
desired semireduction in a controlled manner,8a although the
precise origin of such exceptionally high selectivity remains to be
clarified at this stage.
9
60
60
2
1
499
499
499 499/1
499 499/1
10
11
90
60
18
8
92
499 96/4
499 91/9
12
13
499
80
12
12
96
90
499
499
—
—
14
100
a
Reaction conditions: substrate (0.5 mmol), Au/HSA-TiO2 (Au 1 mol%), CO
(10 atm), ethanol (4.5 mL), H2O (0.5 mL). b The conversion and selectivity
were determined by means of GC using anisole as the internal standard.
c
Z/E ratio was determined by GC and 1H NMR spectroscopy.
With these findings in hand, we then extended our studies to quantitative yields without detectable over-reduced alkanes.
various kinds of alkynes to establish the scope of this Au/HSA-TiO2– Furthermore, there seems to be a prominent electronic effect
CO/H2O-based semireduction system. The results depicted in Table 2 of the substituents in the benzene ring on the overall catalytic
confirmed that a broad range of sensitive and reducible functional efficiencies. For example, a much shorter reaction time is required
groups, including halide, ketone, ether and ester substituents for reaction completion when p-fluorophenyl-acetylene was used
(Table 2, entries 2, 3, 6 and 9–11), were tolerated in the semireduc- as a substrate (entry 3), in sharp contrast to the significantly
tion process. For terminal alkynes, which are too active to stay in prolonged reaction time needed for obtaining the desired product
the partially-reduced state in many conventional procedures,2,12 in quantitative yield when p-methoxy-phenyl-acetylene was used
the corresponding terminal alkenes were obtained in almost as a substrate (entry 6). The Au/HSA-TiO2–CO/H2O protocol is also
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through this mild and convenient Au–CO/D2O-based catalytic
procedure.16 These preliminary results clearly demonstrate that
this Au–CO/D2O protocol is amenable to deuterium-labeled
compound synthesis, and work on this topic is ongoing.
In summary, we have developed a new, practical and efficient
gold-catalyzed method for highly selective semireduction of a range of
alkynes to the corresponding alkenes with CO/H2O as a reducing
agent. An essential feature of the present methodology is the exclusive
alkene selectivity independent of alkyne conversion. Furthermore, the
CO/D2O couple can work as a deuterium transfer reagent in the
presence of a gold catalyst, opening up a new avenue for cheap,
convenient, and green synthesis of deuterium-labeled alkenes.
This work was supported by the National Natural Science
Foundation of China (21073042, 21273044), the State Key Basic
Research Program of PRC (2009CB623506), the Research Fund
for the Doctoral Program of Higher Education (2012007000011)
and Science & Technology Commission of Shanghai Municipality
(08DZ2270500, 12ZR1401500).
Fig. 1 Syntheses of 1,2-dideuterioalkenes via CO/D2O-mediated alkyne
semireduction. The yield was determined by GC and deuteration efficiency
was confirmed by 1H NMR.
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5628 | Chem. Commun., 2014, 50, 5626--5628
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