Heterogeneous Gold-Catalyzed Selective Semireduction of Alkynes using Formic Acid as Hydrogen Source

Article abstract of DOI:10.1002/adsc.201501183

A convenient and robust protocol for the selective transfer semireduction of alkynes was developed, using bio-renewable formic acid as the hydrogen source and easily handled supported gold nanoparticles as the catalyst. The catalytic system showed several attractive features such as high activity and selectivity, recyclability, scalability and adaptability to continuous operation under mild reaction conditions, thus providing a practical alternative to current methods for alkyne semireduction.

Full text of DOI:10.1002/adsc.201501183

COMMUNICATIONS
DOI: 10.1002/adsc.201501183
Heterogeneous Gold-Catalyzed Selective Semireduction of
Alkynes using Formic Acid as Hydrogen Source
a
a
a
a
a,
Shu-Shuang Li, Lei Tao, Fu-Ze-Rong Wang, Yong-Mei Liu, and Yong Cao *
a
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University,
Shanghai 200433, Peopleꢀs Republic of China
Fax: (+86)-21-6564-3774; e-mail: yongcao@fudan.edu.cn
Received: December 31, 2015; Revised: March 8, 2016; Published online: April 25, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201501183.
Abstract: A convenient and robust protocol for the
selective transfer semireduction of alkynes was de-
veloped, using bio-renewable formic acid as the hy-
drogen source and easily handled supported gold
nanoparticles as the catalyst. The catalytic system
showed several attractive features such as high ac-
tivity and selectivity, recyclability, scalability and
adaptability to continuous operation under mild re-
action conditions, thus providing a practical alterna-
tive to current methods for alkyne semireduction.
there is a pressing need to develop new recoverable
and reusable catalysts that can enable rapid, ligand-
free and chemoselective TSR of alkynes.
Heterogeneous catalysis by gold is a key emerging
area with promise to enable the development of inno-
vative methods for chemo- and regiocontrolled reduc-
[9]
tions. As a continuation of our efforts to exploit the
[10]
potential of Au-catalyzed reductions,
we recently
discovered the outstanding catalytic ability of sup-
ported Au nanoparticles (NPs) for the reduction of ni-
[10f]
troaromatics to anilines
as well as quinoline deriva-
Keywords: alkynes; continuous operation; formic
acid; semireduction; supported gold nanoparticles
tives to their corresponding 1,2,3,4-tetrahydroquino-
[10g]
lines
using FA as a terminal reductant. Given the
clear benefits of adopting the Au-based transfer re-
duction protocols, we became attracted to the possi-
bility of utilizing the Au-FA combination to develop
new catalytic processes that enable selective alkyne
Olefinic compounds constitute one of the largest semireduction under mild and heterogeneous condi-
classes of industrially synthesized organic chemicals tions. Herein, we report an efficient, versatile, and
and are important intermediates for a broad spectrum scalable approach to the FA-mediated reductive
of products varying from petrochemicals to fine transformation of alkyne compounds using a simple
[
1]
chemicals. Semireduction of alkynes is arguably the supported gold catalyst system under mild and clean
simplest and most straightforward approach to con- conditions. The present Au-catalyzed, FA-mediated
[
2]
struct the olefinic linkage.
The metal-catalyzed alkyne TSR system has the following noteworthy fea-
is attracting much tures: (i) unprecedented high catalytic efficiency
[
3–5]
transfer semireduction (TSR)
attention because of its critical advantage over the toward alkene production under very mild and
[
6]
conventional semihydrogenation method, which suf- ligand-free conditions, (ii) high selectivity over very
fers from problems with over-reduction as well as sensitive competing groups, and (iii) robust durability
[
7]
safety concerns. Of special interest in this regard is and easy compatibility with continuous flow systems.
the use of bio-renewable formic acid (FA) as safe and At the outset of our studies we choose the reduc-
convenient hydrogen source, essentially owing to its tion of phenylacetylene (1a) as the reaction model.
inherent hydrogen donating capabilities as compared We initially applied the procedures elaborated for the
[
8]
[10f]
to most other TSR agents. In this context, TSR of base-free transfer reduction of nitro compounds
to
alkynes in the presence of FA represents an area of anilines at 608C in the presence of 5 equiv. of FA
growing importance and a variety of homogeneous using Au deposited on single-phase rutile titania (Au/
[5a–f]
metal-based TSR systems have been developed.
TiO -R, see details in the Supporting Information) as
2
Despite their usefulness, the recovery of the costly a benchmark system. However, the reduction hardly
and potentially toxic transition metals remains as occurred and no trace of styrene (2a) could be detect-
a critical issue toward the development of a greener ed (Table 1, entry 1), despite an effort to optimize
and more sustainable chemical synthesis. Therefore, these screening assays by extending the reaction time
1
410
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COMMUNICATIONS Heterogeneous Gold-Catalyzed Selective Semireduction of Alkynes
[
a]
Table 1. Semireduction of phenylacetylene (1a) to styrene (2a) using various catalysts.
[
b]
[c]
o
[d]
[d]
Entry
Catalysts
Hydrogen Source
FA
Temperature [ C]
Time [h]
Conversion [%]
Selectivity [%]
1
2
3
0.5% Au/TiO -R
0.5% Au/TiO -R
0.3% Au/TiO -R
0.3% Au/TiO -R
0.3% Au/TiO -R
0.3% Au/TiO -R
0.3% Au/TiO -R
0.8% Au/TiO -R
0.1% Au/TiO -R
0.5% Au/TiO -R
0.5% Au/TiO -R
0.5% Au/TiO -R
0.5% Au/TiO -A
0.5% Au/TiO -P25
0.5% Au/ZrO2
0.5% Au/CeO2
0.5% Au/Al O3
60
60
60
60
60
25
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
0.5
0.5
0.5
0.5
0.7
2
trace
56
77
75
99
99
98
52
76
–
2
FA/NEt (3:2)
>99
>99
>99
>99
>99
>99
>99
>99
94
2
3
FA/NEt (3:2)
2
3
[e]
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
FA/NEt (3:2)
2
3
FA/NEt (3:2)
2
3
[
[
f]
FA/NEt (3:2)
2
3
g]
FA/NEt (3:2)
20
2
3
FA/NEt (3:2)
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
2
3
FA/NEt (3:2)
2
3
[
h]
0
1
2
3
4
5
6
7
8
9
0
1
2
3
H2
12
2
H PO2
trace
trace
28
47
11
–
–
2
3
2-propanol
2
FA/NEt (3:2)
>99
>99
>99
>99
>99
–
–
–
–
–
2
3
FA/NEt (3:2)
2
3
FA/NEt (3:2)
3
FA/NEt (3:2)
5
8
3
FA/NEt (3:2)
2
3
0.5% Au/SiO2
AuNPore
0.5% Pd/TiO -R
0.5% Pt/TiO -R
0.5% Ru/TiO -R
FA/NEt (3:2)
trace
trace
trace
trace
trace
n.r.
3
FA/NEt (3:2)
3
FA/NEt (3:2)
2
3
FA/NEt (3:2)
2
3
FA/NEt (3:2)
2
3
TiO -R
FA/NEt (3:2)
–
2
3
[
[
[
[
[
[
[
[
a]
Reaction conditions: 1a (0.5 mmol), metal (0.5 mol%), hydrogen source (5 equiv.), acetone (2 mL).
The values given before the catalysts represent the weight content of the active metal deposited on solid supports.
b]
c]
d]
e]
f]
Ratio of FA/NEt is based on molar ratio.
3
Conversion and selectivity determined by GC using p-xylene as an internal standard. n.r.=no reaction.
Fifth run.
Catalyst (Au: 1 mol%), FA (15 equiv.).
g]
h]
1
a (10 mmol), catalyst (Au: 0.05 mol%), FA (5 equiv.), acetone (10 mL).
2
H introduced by bubbling.
or elevating the reaction temperature (Supporting In- the preparation procedure, which has proven quite in-
[10e,h]
formation, Table S1, entries 1–3). Alternative ap- fluential for supported Au catalysts.
Finally, the
proaches by adding a small amount of base additives optimized Au/TiO -R with a gold content of 0.3 wt%
2
were investigated, and while a modest improvement was carefully prepared by using a modified deposi-
was observed with triethanolamine or diethanolamine, tion–precipitation (DP) with NaOH procedure, desig-
NEt was identified as a particularly effective base nated as 0.3% Au/TiO -R. With this catalyst, it is pos-
3
2
promoter (Supporting Information, Table S1, en- sible to accomplish quantitative 1a-to-2a conversion
tries 4–7). Subsequent investigation focused on the in- within only 0.7 h at 608C (Table 1, entry 5). Notably,
fluence of the NEt concentration and led to the dis- the selective transformation of 1a to 2a can still pro-
3
covery of a 3:2 FA/NEt mixture is capable of afford- ceed smoothly to the end at temperatures as low as
3
ing by far the most efficient conversion of 1a to 2a 258C (entry 6). More relevant is that with this particu-
(
Supporting Information, Table S1, entries 7–12). lar Au/TiO -R-FA combination the reaction was oper-
2
Among the solvents tested, acetone was found to be ative even upon decreasing the Au loading to
the most suitable medium for desired TSR (Support- 0.05 mol% (entry 7). Thus, in a gram-scale reaction of
ing Information, Table S2). Importantly, no conver- 1a (20-fold scale-up) for 20 h, a 98% yield of 2a was
sion was observed in the absence of the Au NPs obtained (Supporting Information, Figure S1). In this
(
Table 1, entry 23).
case, remarkable values of the turnover number
These results prompted us to further optimize the (TON=1960) and initial turnover frequency (TOF=
À1
Au/TiO -R system by altering the Au content during 300 h ) were calculated. Already, these results repre-
2
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Shu-Shuang Li et al.
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sent the best catalyst productivities ever reported for of 1a led to the observation of normal primary KIEs
previous FA-mediated TSR systems (Supporting In- (kHCOOH/kDCOOH =4.13, kHCOOH/kDCOOD =4.22). Taken
[
5]
formation, Table S3).
Besides TiO -R, other polymorphic forms of TiO
together, these results indicate that the FA dissocia-
tion with the CÀH bond cleavage at the catalyst sur-
2
2
as well as ZrO , Al O , CeO and SiO were also face is the rate-determining step of the present alkyne
2
2
3
2
2
tested as supporting substrates for the TSR reaction. semireduction reaction.
Whereas single-phase anatase TiO2 supported Au
gave quite a low yield of 2a (Table 1, entry 13), gold duction of nitroarenes to anilines using FA,
Consistent with the case identified for catalytic re-
[10f]
we
NPs loaded on commercial P25 (anatase/rutile=4/1) found that gold is uniquely active for TSR of alkynes
showed only moderate activity (Table 1, entry 14). with FA compared with other noble metals (Table 1,
Meanwhile, the performances of other supported Au entries 20–22). To gain an insight into the origin of
catalysts were very poor under identical conditions the enhanced TSR activity achieved by using rutile
(
entries 15–18), showing that the coupling of rutile TiO as support, the decomposition of FA in the ab-
2
TiO with Au NPs is essential for achieving a high sence of 1a has been investigated for Au NPs deposit-
2
TSR activity. Note that the use of the nanoporous Au ed on different supports (Supporting Information,
catalyst without a support resulted in only a trace of Figure S6). It is revealed that the lower performances
1
a conversion (entry 19), further confirming the im- observed with Au/TiO -A, Au/Al O , Au/CeO and
2 2 3 2
portance of the support in this efficient TSR system. Au/SiO are due to the fact that the FA decomposi-
2
After the reaction, the optimized 0.3% Au/TiO -R tion rate catalyzed by these catalysts is significantly
2
was readily separated from the reaction mixture by lower than that over the Au/TiO -R sample. This sce-
2
simple filtration and could be reused without loss of nario is reinforced by another set of control assays
its activity or selectivity (entries 3 and 4). Inductively showing that the FA decomposition over 0.3% Au/
coupled plasma (ICP-AES) analysis showed the ab- TiO -R also occurred with much higher rates than
2
sence of Au species in the filtrate after the reaction that over other Au/TiO -R-based materials with
2
(
detection limit of 0.1 ppm), confirming that no leach- higher Au loadings. At this juncture, it is interesting
ing occurred during the reaction. Transmission elec- to note that Au/ZrO can only afford very limited
2
tron microscopy (TEM) and X-ray photoelectron TSR activity under otherwise identical conditions, de-
spectroscopy (XPS) measurements of 0.3% Au/TiO2- spite the effectiveness of this material for FA dehy-
[11]
R after the reuse revealed almost no change in the drogenation. This could be rationalized by the fact
Au dispersion or metallic state of Au before and after that although a high FA activation activity is a prereq-
reuse, corroborating the durability of 0.3% Au/TiO2- uisite for TSR performance, the competitive pathway
R in the recycling experiments (Supporting Informa- that favors unproductive FA decomposition should be
tion, Figures S2–S5).
minimized.
We confirmed in separate experiments that FA con-
On the basis of the above observations and the
version in the presence of alkyne was much higher known chemistry regarding the Au-mediated reaction
[12]
than the corresponding conversion in the absence of of alkynes,
we propose a catalytic cycle involving
alkyne over 0.3% Au/TiO -R (Supporting Informa- an unusual alkyne-assisted activation of the FA mole-
2
tion, Table S4). Moreover, when replacing FA with cules over the supported Au NPs (Scheme 1). Al-
H , much lower yields of 2a were achieved under oth- though the precise way by which the hydrogen trans-
2
erwise identical reaction conditions and a certain fers from HCOOH to alkyne remains to be clarified
amount of the over-reduced product is generated at at this stage, it is important to emphasize that an ini-
such low 1a conversion levels (Table 1, entry 10). tial fast adsorption of alkynes onto Au NPs (Step 1)
Meanwhile, variation of the hydrogen source revealed can be established based on the kinetic studies (Sup-
that FA is the most effective (entries 11 and 12). porting Information, Figure S7, an unusual broken
[8c,13]
Overall these results show that the present FA-medi- order concentration dependency of alkynes).
Sub-
ated reaction is a TSR process and mechanisms en- sequent adsorption of FA leads to the formation of
+
tailing semihydrogenation utilizing H gas in situ gen- Au-H NEt and Au-formate species, which are be-
2
3
erated from the FA decomposition are not likely op- lieved to be the key intermediates for the transfer re-
erating. To further understand the reaction pathways duction (Step 2). Eventually, the two hydrogen atoms
involved in the present Au/TiO -R-catalyzed semire- from these two different species were rapidly con-
2
duction, the kinetic isotope effect (KIE) was mea- sumed by the chemisorbed alkyne species generated
sured (Supporting Information, Table S5). When the in the first step, together with a final formation of the
substrate was allowed to react with HCOOD, the alkene products (Step 3). In the overall reaction, the
transformation of 1a at low conversion levels (< unique alkynophilicity of small Au NPs (Supporting
15%), with respect to that with HCOOH, gives Information, Figure S3, data for XPS) attached to
kHCOOH/kHCOOD =1.04. Meanwhile, the use of DCOOH TiO -R thus may be one key aspect in leading to
2
or DCOOD instead of HCOOH in the semireduction
1
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COMMUNICATIONS Heterogeneous Gold-Catalyzed Selective Semireduction of Alkynes
trolling the stereoselectivity of the reaction presents
[14]
a further challenge.
In line with the syn-addition
manner as prevailing in heterogeneous semireduction
[6]
processes,
(Z)-alkenes are mostly formed from
a wide range of simple and functionalized internal al-
kynes (Table 2, entries 12–16, 18–20). Interestingly, it
turns out to be an exception for 4-phenyl-3-butyn-2-
one that an exclusive E-stereoselectivity is achieved
(
entry 17), which might result from the low activation
energy of thee cis-trans isomerism for such a,b-ethyl-
enic ketone structures. Notably, the semireductions of
non-polar internal alkynes such as diphenylacetylene
and 1-phenyl-1-propyne proceed quite smoothly
under mild conditions in the present Au-FA system
(
entries 12 and 13), in contrast to the case with our
previous Au-CO/H O system wherein elevated tem-
2
perature and relatively long reaction time are re-
[10d]
quired to facilitate the reduction kinetics.
More-
Scheme 1. A proposed mechanism for the heterogeneous
gold-catalyzed FA-mediated TSR of alkynes.
over, (Z)-3-hexen-1-ol (leaf alcohol), which is a com-
mercially important fragrance material, can also be
readily obtained as a sole product in 86% isolated
yield (Table 2, entry 20).
a facile Au-formate decarboxylation for the highly se-
lective alkyne semireduction.
Given the recognized merits of continuous process-
ing, a flow system was implemented for 1a semire-
[15]
Building upon these results, we then extended our duction using the present Au-mediated TSR protocol
studies to various kinds of alkynes to establish the (reaction conditions: 200 mg 0.3% Au/TiO -R,
2
À1
À1
À1
scope of this Au-FA-based TSR system. As depicted 0.25 molL 1a, 1.25 molL FA, 0.83 molL NEt3,
in Table 2, various structurally diverse terminal al- acetone as solvent, see the Supporting Information
À1
kynes, including activated, inactivated, and heterocy- for details). At 608C with a flow rate of 4.5 mLh ,
clic ones, can be transformed into the corresponding the reaction proceeded smoothly to afford the prod-
alkenes in excellent yields without any over-reduced uct quantitatively, corresponding to a space-time yield
À1
À1
products being detected. These results are remarkable of 1.9 mol h g . Note that this value is considera-
Au
2
a
as the terminal alkynes are generally too active to bly higher than that obtained for the batch-type reac-
À1
À1
stay in the partially reduced state in most convention- tion under identical conditions (ca. 1.4 mol h g ,
Au
2
a
[
5]
al procedures. More significantly, a wide variety of see Table 1, entry 5), underscoring the potential of the
synthetically useful functional groups, both electron- continuous method for intensified processing. It was
withdrawing and electron-donating, were not affected shown that the exclusive selectivity toward 2a forma-
in the semireduction (entries 2–9). For example, halo- tion can be retained during the continuous operation,
gen-substituted phenylacetylenes are selectivity trans- albeit with a progressive decrease in 1a conversions
formed into the corresponding chloro- or fluorostyr- from 99% to 85% after 180 min on stream. It was
enes with no observation of dehalogenation (Table 2, noteworthy, however, that the activity of the spent
entries 3 and 4), which quite often occurs in the cases catalyst could be recovered after the treatment in ace-
À1
[16]
of other metal catalysts. Other reducible functional tone (0.5 mLmin for 30 min), without significant
moieties are also tolerated, including ester, ether and loss of 2a yields after the regeneration (Figure 1, see
ketone substituents (entries 5–9). Moreover, this Au- the Supportinbg Information for details). As such, the
FA-mediated TSR process is not limited to arylal- turnover number (TON) after 9 h under these condi-
kynes, the semireduction of aliphatic alkynes can also tions approached 3,200, a notably higher number as
be readily and efficiently achieved under the standard compared with typical batch-type FA-based TSR sys-
[5]
reaction conditions (Table 2, entry 10). The reaction tems. These results, while preliminary, suggest that
was also adaptable to the semireduction of substrates this Au-FA-based continuous-flow TSR procedure
bearing unprotected hydroxy groups to synthetically holds great promise to develop more efficient yet sub-
useful allyl alcohols (entry 11).
Furthermore, the Au-FA protocol also shows excel- tion of alkynes to the corresponding alkenes.
lent promise for the semireduction of internal alkynes. In summary, we have developed a practical and
stantially safer means for target-specific transforma-
It should be stressed that this transformation is gener- convenient protocol for selective semireduction of al-
ally more difficult than that of terminal alkynes owing kynes using green and bio-renewable FA as the termi-
to the steric hindrance of these molecules, and con- nal reductant. Driven by a simple and robust hetero-
Adv. Synth. Catal. 2016, 358, 1410 – 1416
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Shu-Shuang Li et al.
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[
a]
Table 2. Semireduction of various alkynes with Au-FA.
[a]
Yield and selectivity determined by GC; numbers in parenthesis refer to isolated yield.
geneous gold catalyst, which enables the transfer sem- a continuous-flow procedure, providing a valuable al-
ireduction by a remarkably facile and controlled ternative to currently used methods for the transfor-
alkyne-assisted FA activation, the protocol is efficient, mation of alkynes to the corresponding alkenes.
operationally simple, scalable and readily adapted to
1
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Adv. Synth. Catal. 2016, 358, 1410 – 1416

COMMUNICATIONS Heterogeneous Gold-Catalyzed Selective Semireduction of Alkynes
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Figure 1. Recycling experiments of the continuous-flow FA-
mediated TSR of 1a over 0.3% Au/TiO -R. Reaction condi-
2
À1
À1
À1
tions: 200 mg catalyst, 0.25 molL 1a, 1.25 molL FA, 0.83
molL NEt , flow rate=4.5 mLh , 608C.
À1
3
Experimental Section
General Procedure for the Transfer Semireduction of
Alkynes with Formic Acid
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W. Wang, M. Goto, L. B. Han, J. Am. Chem. Soc. 2011,
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0
.5 mol%), formic acid (5 equiv.) as a FA/NEt (3:2) mix-
3
Sprengers, M. Catellani, C. J. Elsevier, Angew. Chem.
ture, acetone (2 mL) was added to a flask (10 mL) equipped
with a reflux condenser. The resulting mixture was allowed
to reflux with vigorous stirring (800 rpm) at 608C. After
completion of the reaction, the reaction mixture was filtered
and the filtrate was washed with water and extracted by
ethyl acetate. The mixture was concentrated and dried
under reduced pressure to give the crude product, which
was purified by silica gel chromatography eluting with pe-
troleum ether (60–908C)/ethyl acetate mixture. The conver-
sion and selectivity were determined by GC-FID using p-
xylene as an internal standard.
2
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[
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[
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0
.3% Au/TiO -R material. Further investigation of the
2
associated catalyst deactivation is ongoing in our labo-
ratory.
1
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