Thermoregulated phase-transfer iridium nanoparticle catalyst: Highly selective hydrogenation of the CO bond for α,β-unsaturated aldehydes and the CC bond for α,β-unsaturated ketones

Article abstract of DOI:10.1039/c6cy01137c

In the same catalytic system, thermoregulated ligand Ph₂P(CH₂CH₂O)₂₂CH₃-stabilized iridium nanoparticles exhibited a totally different orientation for the hydrogenation of unsaturated carbonyl compounds, namely, highly selective hydrogenation of the CO bond for α,β-unsaturated aldehydes and the CC bond for α,β-unsaturated ketones.

Full text of DOI:10.1039/c6cy01137c

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Thermoregulated phase-transfer iridium nanoparticle catalyst:
highly selective hydrogenation of C=O bond for α, β-unsaturated
aldehydes while C=C bond for α, β-unsaturated ketones
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Wenjiang Li, Yanhua Wang*, Pu Chen, Min Zeng, Jingyang Jiang, Zilin Jin
www.rsc.org/
In the same catalytic system, thermoregulated ligand theory (DFT) and the Langmuir-Hinshelwood (LH) mechanism,
Ph P(CH CH -stabilized iridium nanoparticles exhibited a the reactive bonds in the case of catalytic hydrogenation are
2
2
2 3
O)22CH
totally different orientation for the hydrogenation of unsaturated the ones involved in chemisorption on metal surface.
carbonyl compounds, namely, highly selective hydrogenation of Therefore, the problem of selective hydrogenation can be
the C=O bond for α, β-unsaturated aldehydes while the C=C bond ascribed in a first rough approximation to the factors which
for α, β-unsaturated ketones.
control the adsorption mode of substrate molecules on the
metal surface. In terms of catalytic hydrogenation of α, β-
unsaturated aldehydes and ketones, the key factors, such as
Catalytic hydrogenation of α, β-unsaturated aldehydes and
1
ketones is very important for the synthesis of fine chemicals.
4
5
the nature of metallic catalyst, support material, particle
6
size, presence of a second metal, preparation method, or
7
8
The hydrogenated products are different (Fig. 1) by the
presence of two functional groups with similar reactivity: C=O
and C=C bond, thereby the chemoselective hydrogenation of
α, β-unsaturated aldehydes and ketones has been a very
prolific research field over the past few decades. Among these
intensive studies, although the highly chemoselective
hydrogenation remains as a challenging task, more difficulty is
that one can orientate the different hydrogenated tendency
for α, β-unsaturated aldehydes and ketones under the
condition as similar as possible. For example, using different
hydrogen donors and Ir (I) complexes, Farnetti et al. realized
that unsaturated aldehydes could be hydrogenated to the
unsaturated alcohols, while unsaturated ketones were
9
temperature, have been proposed to influence the
hydrogenation selectivity, but there is no report about the
totally different hydrogenation orientations in case the
catalytic system is the same.
The design of new catalytic process with both recyclability
and chemoselectivity is desired because it does not require
tedious separation procedures and yields fewer by-products.
Based on the concept of thermoregulated phase-transfer
catalysis (TRPTC), our group synthesized thermoregulated
ligand Ph P(CH CH O) CH (n = 16 or 22)-stabilized noble metal
2
2
2
n
3
(Rh, Pt, Ru, Ir, Pd, and Au) nanoparticles (NMNPs) and realized
their thermoregulated phase-transfer function in the
2
1
0
selectively hydrogenated to the corresponding ketones. In
aqueous/alcoholic biphasic system. The general principle of
this catalytic process can be simply described as follows: The
system consists of the upper 1-pentanol phase and the lower
water phase. The as-prepared NMNPs are in the water phase
at room temperature. Afterward, the water/1-pentanol
mixture is heated gradually to a higher temperature under
stirring, and then the as-prepared NMNPs are transferred into
addition, Himeda reported that selective hydrogenations of
the C=C bond of enone were observed under basic conditions
and the ketone moieties can be hydrogenated under acidic
3
conditions. However, these studies mainly focused on the
metal complex catalysis. Moreover, the major drawbacks of
these catalytic systems are the use of supplementary ligands
or reagents associated with difficulties such as product
separation and metal catalyst recycling.
Soluble transition-metal nanoparticles have drawn much
attention because of their high catalytic efficiency and unique
physicochemical properties. Based on density functional
Fig.
1
Reaction pathways for the chemoselective
hydrogenation of α, β-unsaturated aldehydes and ketones
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Table 1 The chemoselective hydrogenation of CAL using the as- Table 2 The reusability of the as-prepared Ir-NPs for the
a
a
prepared Ir-NPs.
chemoselective hydrogenation of CAL.
b
b,c
Entry
Temp.
Time
(min)
60
P
H2
Conv.
b
(%)
Sel.
Entry
Time(h)
Conv.(%)
Sel.(%)
b,c
(℃
)
(MPa)
(%)
1
2
3
4
5
6
7
1
1
>99
>99
87
>99
98
97
98
98
95
95
1
2
3
4
5
6
7
8
9
50
60
1
1
73
93
>99
>99
60
1
70
80
90
70
70
70
70
70
70
60
60
60
20
40
80
60
60
60
1
1
>99
>99
>99
68
>99
95
3
90
4
93
1
87
6
93
1
>99
>99
96
10
97
1
80
a
Reaction conditions: 1-pentanol 4 mL, water 4 mL containing
-3
.7×10 mmol Ir-NPs (Ph
1
>99
94
6
2 2 2 3
P(CH CH O)22CH /Ir = 1 (molar ratio)),
0.5
1.5
2
>99
96
CAL/Ir = 100 (molar ratio), 50 mg of n-decane as internal
b
1
0
1
>99
>99
standard, T = 70
c
Selectivity for cinnamyl alcohol and the main by-product was
℃, 1 MPa H
2
.
Determined by GC and GC-MS.
1
93
a
Reaction conditions: 1-pentanol 4 mL, water 4 mL containing
3
-phenyl-1-propanol.
-3
6
2 2 2 3
.7×10 mmol Ir-NPs (Ph P(CH CH O)22CH /Ir = 1 (molar ratio)),
CAL/Ir = 100 (molar ratio), 50 mg of n-decane as internal
b
standard.
c
Determined by GC and GC-MS. Selectivity for
2
reaction conditions (1 MPa H , 70 ℃ , 60 min) for the
cinnamyl alcohol and the main by-product was 3-phenyl-1-
propanol.
chemoselective hydrogenation of CAL in the water/1-pentanol
biphasic system.
Considering that the lifetime is an important aspect for a
catalyst, the reusability of Ir-NPs was examined. After each
reaction, the upper organic phase was separated from the
lower catalyst-containing phase. And the lower catalyst-
containing phase was directly reused in the next reaction run.
The results were summarized in Table 2. Although the
conversion began to decrease in the third cycle (Table 2, entry
the upper 1-pentanol phase, in which the reaction proceeds
homogeneously. As soon as the reaction is completed and the
system is cooled to room temperature, the as-prepared
NMNPs could return to the aqueous phase. Therefore, by
simple phase separation the as-prepared NMNPs can be easily
separated from products. It is noteworthy that the phase
transfer efficiency of NMNPs from 1-pentanol to aqueous or
vice versa isn’t less than 99.9%. Herein, we employed the as-
prepared Ir-NPs as catalyst for the hydrogenation of α, β-
unsaturated aldehydes and ketones. The catalytic activity,
selectivity and reusability of the catalyst, the substrate scope
as well as the possible hydrogenation mechanism were
explored.
3
9
), prolonging the reaction time can make the conversion ≥
0%. (Table 2, entries 4-7).
In the reusability experiments, we also investigated the
factors, which resulted in the decrease of catalyst activity.
Firstly, the leaching of Ir in the upper 1-pentanol phase was
studied by ICP-AES. The results were shown in Table S1. We
can know that the leaching of Ir, in each cycle, gradually
decreased as the number of cycle experiments increased. After
three cycles, the amount of Ir leaching was 10 wt. %. In order
to investigate the influence of Ir leaching, the fresh catalyst (90
wt.%, relative to the initial Ir-NP catalyst) was used to catalyze
the hydrogenation of CAL in the same reaction conditions, and
the conversion was 89% (The real conversion for the Ir-NP
catalyst after three cycles was only 68% within 1 h). So, we
further investigated the size and the morphology of Ir-NPs by
TEM test after the third cycle (see Fig. S1). Careful analysis of
the diameter revealed that the size of Ir-NPs changed to 2.1
For the study reported here, cinnamaldehyde (CAL) was
firstly chosen as the model substrate for the hydrogenation of
α, β-unsaturated aldehydes and ketones in the water/1-
pentanol biphasic system. Various reaction conditions were
studied and the results were summarized in Table 1(see ESI†
for full experimental details). The effect of reaction
temperature was investigated in the range of 50-90
MPa H for 60 min (Table 1, entries 1-5). It was noteworthy
that the conversion of CAL reached a maximum value (>99%)
as the reaction temperature increased to 70 , whereas the
selectivity to C=O group began to decrease when the reaction
temperature exceeded 70 . As can be observed therein
Table 1, entries 3, 6-8), longer reaction time hindered the
℃ at 1
2
℃
nm (2.1±0.2 nm, Fig. S1) with respect to the freshly prepared
Ir-NPs (1.9±0.3 nm, Fig. S2). Within the range of allowable
℃
error, the size remained almost the same. However, the
morphology of Ir-NPs was different and became more
agglomerated, indicating that the stabilizer could not maintain
the stabilization of Ir-NPs very well. So we evaluated the loss of
thermoregulated ligand after one cycle with the aid of ICP-AES.
The result showed that the loss of thermoregulated ligand was
up to 12 wt.%. Subsequently, we added fresh thermoregulated
ligand (12 wt. %) to the solution of catalyst after one cycle. The
result showed that the selectivity to C=O bond was restored
(
enhancement of the selectivity towards the cinnamyl alcohol.
Generally, increasing pressure may lead to the decrease of the
selectivity in the selective catalytic hydrogenation. As shown in
Table 1 (entries 3, 9-11), Ir-NPs exhibited the best selectivity
towards cinnamyl alcohol at 1 MPa H
2
along with comparative
high conversion. As a consequence, we established the optimal
2
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to > 99%. In addition, we also recycle the catalyst nearby 75%
conversion (see Table S2). Similar results were observed
compared with >99% conversion. From the above-mentioned
experiment data, we could infer that the reason for
deactivation of the catalyst may result from the loss of Ir and
the morphology change of Ir-NPs. Moreover, thermoregulated
ligand played an important role on the catalyst’s reactivity and
selectivity.
Once the best reaction conditions were established, we
applied it to the catalytic hydrogenation of chalcone,
surprisingly, a completely different hydrogenation orientation
towards the C=C group occured. Through simply prolonging
the reaction time, the conversion of chalcone was up to 98 %
together with a very high chemoselectivity (> 99%) (Table 3,
entry 6). Inspired by this exciting preliminary result, we further Fig. 2 Two different hydrogenation orientations for α, β-
unsaturated aldehydes and ketones.
Table 3 The chemoselective hydrogenation of different α, β-
a
unsaturated aldehydes and ketones.
En
try
explored the catalytic hydrogenation of different α, β-
unsaturated aldehydes and ketones. From Table 3, we knew
that α, β-unsaturated aldehydes were selectively
hydrogenated to the unsaturated alcohols (Table 3, entries 1-
5), while α, β-unsaturated ketones were hydrogenated to the
corresponding ketones (Table 3, entries 6-10). It was
noteworthy to mention that chemoselectivity for unsaturated
alcohols/saturated ketones was not less than 96%, except that
trans-2-octenal and benzylideneacetone showed a relatively
low selectivity but still had a value of 94% and 90%,
respectively (Table 3, entries 5 and 7).
Time Conv.
(h)
Sel.
(%)
Substrate
Product
b
b,c
(%)
1
3
96
>99
>99
2
3
7
90
7
3
72
98
96
94
To our knowledge, among the reported catalytic
hydrogenation of α, β-unsaturated aldehydes and ketones in
11
>99
>99
4
5
the same catalytic system,
thermoregulated ligand
totally
Ph P(CH CH O) CH -stabilized Ir-NPs exhibited
2
a
2
2
22
3
3
.5
different hydrogenation orientation, namely, highly selective
hydrogenation of the C=O bond for α, β-unsaturated
aldehydes while the C=C bond for α, β-unsaturated ketones.
The excellent selectivity towards unsaturated alcohol to some
extent was not surprising, because relevant studies and
calculations had shown that Ir-NP catalyst could facilitate the
9
98
76
>99
90
6
7
C=O adsorption in the hydrogenation of α, β-unsaturated
12
2.5
carbonyl compounds.
Herein, we did not add any
supplementary ligands or reagents to change the properties of
Ir-NP catalyst, yet the hydrogenation orientations were totally
different, thus we supposed two different adsorption models
to illustrate the reason: one was the C=O adsorption model for
α, β-unsaturated aldehydes and the other was the C=C
adsorption model for α, β-unsaturated ketones (see Fig.2). In
the former, the C=O bond would be preferentially
hydrogenated over the C=C bond, leading to the high
selectivity to unsaturated alcohols.Several recent works had
directly or indirectly proposed the similar C=O adsorption
3
.5
>99
>99
>99
>99
8
9
4
1
10
>99
>99
a
model for the
3
chemoselective hydrogenation
of
Reaction conditions: 1-pentanol 4 mL, water 4 mL containing
1
-3
cinnamaldehyde. However, the situation would be different
in the case of α, β-unsaturated ketones. This may be due to
6
.7×10 mmol Ir-NPs (Ph
2
P(CH
2
CH
2
O)22CH
3
/Ir = 1 (molar ratio)),
-decane as internal
Determined by GC and GC-MS.
Substrate/Ir = 100 (molar ratio), 50 mg of
n
b
the fact that thermoregulated ligand Ph P(CH CH O)22CH
2 2 2 3
standard, T = 70
c
2
℃, 1 MPa H .
around the nanoparticle surface constrained the geometry of
metal active sites, permitting only the best favorable
The main by-product was the fully hydrogenated product.
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adsorption models to access. Assuming a simple geometric 12 (a) F. Delbecq and P. Sautet, J. Catal., 1995, 152, 217-236; (b)
V. Ponec, Appl. Catal., A, 1997, 149, 27–48; (c) P. Chen, J. Q.
model did not illustrate well the real reaction manner, but it
Lu, G. Q. Xie, G. S. Hu, L. Zhu, L. F. Luo, W. X. Huang and M. F.
Luo. Appl. Catal., A, 2012, 433-434, 236–242.
could guide our next work to get more information. So, further
studies are under way in our laboratory.
1
3 (a) K. B. Vu, K. V. Bukhryakov, D. H. Anjum and V. O.
Rodionov, ACS Catal., 2015,
, 2529−2533; (b) K. R. Kahsar,
5
D. K. Schwartz and J. W. Medlin, J. Am. Chem. Soc., 2014,
136, 520−526; (c) B. H. Wu, H. Q. Huang, J. Yang, N. F. Zheng
and G. Fu, Angew. Chem., Int. Ed., 2012, 51, 3440–3443.
Conclusions
In summary, thermoregulated phase-transfer iridium
nanoparticles stabilized
by thermoregulated ligand
2
2
2 3
Ph P(CH CH O)22CH were very active and easily recyclable for
the chemoselective hydrogenation of cinnamaldehyde in the
water/1-pentanol biphasic system. More encouragingly, the Ir-
NP catalyst, in the same catalytic system, exhibited a totally
different hydrogenation orientation, namely, α, β-unsaturated
aldehydes can be selectively hydrogenated to the unsaturated
alcohols, meanwhile α, β-unsaturated ketones can be
selectively hydrogenated to the corresponding ketones.
Acknowledgements
This study was supported financially by the National Natural
Science Foundation of China (21173031, 21373039).
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í
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eda, A. G mez-
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3
9
,
1
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Thermoregulated phase-transfer iridium nanoparticles exhibited a totally different
hydrogenation orientation for α, β-unsaturated aldehydes and ketones.
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