Graphene-enhanced platinum-catalysed hydrosilylation of amides and chalcones: A sustainable strategy allocated with in situ heterogenization and multitask application of H2PtCl6

Article abstract of DOI:10.1039/c7ra10541j

We describe a new sustainable strategy for the comprehensive utilization of a platinum catalyst in different organic transformations, in which an organosilicon/graphene-supported platinum catalyst prepared from a simple hydrosilylation-type reduction could be further used in the 1,4-hydrosilylation of chalcones. The rationally designed and in situ formed Pt@G@Si nanocatalyst is demonstrated to be highly effective in the 1,4-hydrosilylation of α,β-unsaturated enones, allowing for the facile synthesis of a variety of otherwise inaccessible substituted silyl enolates. In addition, with the aid of platinum catalyst residue and TBAF, the one-pot downstream Michael addition of substituted silyl enolates to alkyl acrylates is also reported in this work.

Full text of DOI:10.1039/c7ra10541j

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Graphene-enhanced platinum-catalysed
hydrosilylation of amides and chalcones:
a sustainable strategy allocated with in situ
heterogenization and multitask application of
H2PtCl6†
Cite this: RSC Adv., 2017, 7, 50729
Ning Li,‡ab Xiao-Yun Dong,‡ab Jing-Lei Zhang,a Ke-Fang Yang,b Zhan-Jiang Zheng,b
a
a
ab
*
*
Wei-Qiang Zhang, Zi-Wei Gao
and Li-Wen Xu
We describe a new sustainable strategy for the comprehensive utilization of a platinum catalyst in different
organic transformations, in which an organosilicon/graphene-supported platinum catalyst prepared from
a simple hydrosilylation-type reduction could be further used in the 1,4-hydrosilylation of chalcones. The
rationally designed and in situ formed Pt@G@Si nanocatalyst is demonstrated to be highly effective in the
1,4-hydrosilylation of a,b-unsaturated enones, allowing for the facile synthesis of a variety of otherwise
inaccessible substituted silyl enolates. In addition, with the aid of platinum catalyst residue and TBAF, the
one-pot downstream Michael addition of substituted silyl enolates to alkyl acrylates is also reported in
this work.
Received 23rd September 2017
Accepted 22nd October 2017
DOI: 10.1039/c7ra10541j
rsc.li/rsc-advances
hypothesized that further exploration of the catalytic functions
of a recovered metal catalyst in new and downstream organic
Introduction
transformations would be killing two birds with one stone,
which would be benecial to the development of new reactions
and sustainable catalyst materials. In this regard, we have
previously reported a multitask and maximum reuse approach
in which the recovered palladium catalyst/residue could be
successfully applied in different types of one-by-one and
downstream reactions, from catalytic hydrogenation to cross-
coupling reactions, including the Suzuki and Sonogashira
reactions, and to Knoevenagel condensations.5 Similar to the
concept of tandem catalysis,6 a multitask strategy with the same
catalyst precursor could be applied in multiple reactions, which
is quite valuable for green and sustainable chemistry.7 None-
theless, such simple new catalyst preparations by a multitask
strategy are infrequently reported, and methods making use of
certain organic reaction-accessible deactivated metal catalysts
in organic transformations are also rare. Inspired by this
preliminary result, we believed that a multitask strategy based
on one-by-one downstream organic reactions allocated to the in
situ formation of transition-metal catalysts with different
activities would nd wide application in organic synthesis, in
which the synthetic reactions could be used as a springboard for
the facile and in situ construction of sustainable catalyst
materials.
It is of great necessity to explore new strategies for the
construction of sustainable catalyst materials from general
transition-metal-based heterogeneous catalysts in the prepara-
tion of ne chemicals and numerous industrial intermediates
for a variety of valuable compounds and materials.1 One of the
key driving forces of heterogeneous catalysis is the increased
demand for an environmentally friendly method, with a high
level of efficiency and chemoselectivity as well as an atom-
economic synthetic procedure.2 It is well recognized that an
ideal catalyst, including a heterogeneous supported metal
catalyst, should remain at least somewhat active until the last
molecule of the reaction substrate has been consumed.
However, a decrease in catalytic activity of recovered catalysts is
unavoidable in both heterogeneous catalysis and homogeneous
catalysis.3 Interestingly, there are few examples focused on the
reuse of a recovered and deactivated metal catalyst from metal
wastes in another new organic reaction.4 Therefore, we
aKey Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education
(MOE), School of Chemistry and Chemical Engineering, Shaanxi Normal University,
No. 620, West Chang'an Avenue, Xi'an 710062, P. R. China. E-mail: liwenxu@snnu.
edu.cn; liwenxu@hznu.edu.cn
bKey Laboratory of Organosilicon Chemistry and Material Technology of Ministry of
Education, Hangzhou Normal University, No.1378 Wenyi West Road, Hangzhou
311121, P. R. China
Platinum catalysts capable of Si–H activation and hydro-
silylation of carbon–carbon/heteroatom multiple bonds have
inspired intense research efforts within the polymer chemistry,
organosilicon material and synthetic chemistry communities.8
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra10541j
‡ N. Li and X. Y. Dong contributed equally to this work.
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was in situ formed and immobilized on graphene-dispersed
organosilicon material during the platinum (H2PtCl6)-cata-
lyzed hydrogenation/hydrosilylation of amide, and then the in
situ formed graphene-containing organosilicon material
(G@Si)-supported platinum catalyst (Pt@G@Si) is used in the
catalytic hydrosilylation of chalcones (Fig. 2). On the basis of the
multitask strategy, the rationally designed and in situ formed
Pt@G@Si nanocatalyst material that is achieved from the
H2PtCl6-catalyzed hydrosilylation-type reduction of amides in
the
presence
of
graphene-dispersed
PMHS
(poly-
methylhydrosiloxane) is demonstrated to be effective in the 1,4-
hydrosilylation of a,b-unsaturated enones, allowing for the
facile synthesis of a variety of otherwise inaccessible substituted
silyl enolates in a straightforward manner.
Fig. 1 Possible products from the hydrosilylation of a,b-unsaturated
carbonyl compounds.
Results and discussion
Initially, we wanted to develop a new method for the catalytic a-
silylation of a,b-unsaturated ketones by platinum catalysis
because there are few reports on the catalytic a-silylation of a,b-
unsaturated ketones. However, when we tried to apply
commercially available platinum catalysts in this reaction we
found it really challenging because the desired a-silylated
product was not obtained in these preliminary explorations
(Fig. 1). In this case, the 1,4-conjugate reduction was a major
reaction process when chalcone was used as a model substrate.
Fortunately, in this work, we presented an unprecedented new
platinum catalyst-controlled 1,4-silylation of chalcones based
on the multitask process, in which the supported platinum
catalyst is quite effective for the 1,4-hydrosilylation of chalcones
to prepare a variety of otherwise inaccessible silyl enolates. It is
well known that silyl enolates are extremely valuable and useful
organosilicon compounds in organic synthesis.12 In the past few
decades, transition-metal-catalyzed 1,4-hydrosilylation of a,b-
unsaturated carbonyl compounds has become a powerful
method for the preparation of silyl enolates.13 However, the
stereoselectivities are not satisfactory in most of these reports.
Subsequently, several improved methods have also been re-
ported in past years. For example, Yorimitsu and Oshima re-
Although many advances have been made in the eld of
hydrosilylation reactions of alkenes or a,b-unsaturated alde-
hydes or ketones, there has been no report on the platinum-
catalyzed hydrosilylation of chalcones.9 Notably, this type of
hydrosilylation reaction continues to present a challenge
because it usually generates mixtures of reductive carbonyl
compounds, ketene silyl acetals, a- and b-adducts, silyl ethers,
and other side-products depending on the catalyst systems
(Fig. 1).10,11
Herein, we conrm the powerful potential of the multitask
strategy in platinum-catalyzed organic reactions, in which the
new organosilicon/graphene-supported platinum(II) catalyst
ported
a palladium-catalyzed 1,4-hydrosilylation of a,b-
unsaturated ketones with hydrosilanes for the preparation of
silyl enolates with high Z selectivity.14 Unfortunately, the
substrate scope was not enough good because of the limitation
of the hydrosilanes. In 2014, Takeuchi et al. investigated
a cationic rhodium complex-catalyzed 1,4-hydrosilylation of
a,b-unsaturated ketones with triethylsilane (Et3SiH) to prepare
various silyl enolates in good yields.15 All these homogeneous
catalyst systems seem actually to be complementary. For the 1,4-
hydrosilylation of chalcones, there is only one example reported
very recently, where Shishido et al.16 developed an Nb2O5-sup-
ported Pd–Au alloy catalyst which could be used as a selective
catalyst for the hydrosilylation of a,b-unsaturated ketones.
These works suggested that considerable attention has been
focused on the transition-metal catalysed hydrosilylation of a,b-
unsaturated ketones.
Fig. 2 Our hypothesis for a multitask strategy with an in situ formed
organosilicon-supported platinum catalyst: from hydrosilylation/
hydrogenation of amides (eqn (1)) to hydrosilylation of chalcones
(eqn (2)).
In our previous studies, we have found that poly-
methylhydrosiloxane (PMHS) and related organosilicon
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materials were useful multifunctional supports for the prepara- number of benzene rings and with double carbon–carbon (–C]
tion of supported transition-metal catalysts.5,10f,g And in this C–) bonds,19 could be linked with PMHS under cobalt-catalyzed
regard, we have also noticed that the platinum-catalyzed silicon-carbon bond-forming reaction conditions (Fig. 3, step 1).
hydrosilylation/reduction
of
carboxamides
with
poly- In fact, the hydrosilanes, including pentamethyldisiloxane
methylhydrosiloxane (PMHS) was reported by Nagashima et al.17 (TMSOSiMe2H) or tris(trimethylsilyl)silane (TTMSS), have been
in 2009, in which the synergetic effect of two Si–H groups was reported by Bandermann20 and Lalevee21 for use as an effective
´
found to be crucial to the reduction of amides to amines under initiator for the radical polymerization of activated olens. And
mild reaction conditions. In addition, such a hydrosilylation we also found that the catalytic polymerization of activated
reaction was accompanied by the removal of both organosilicon olens with PMHS could be completed by two different cobalt
wastes and platinum as insoluble silicone resin. Therefore the catalyst systems ([Co(OAc)2/TsOH] and [Co(acac)2]), in which
hydrosilylation of amides could act as a springboard for the the PMHS-derived semi-IPNs was formed by a coupling reaction
construction of an organosilicon material (silicone)-supported involving hydrosilane.22
platinum catalyst. However, we found that the recovery and
reuse of a platinum catalyst immobilized on PMHS is really not results on the cobalt-catalyzed radical polymerization of acti-
easy due to the formation of polysiloxane gel in this reaction.18
vated alkenes (Fig. 4, eqn (1)), especially with the decomposition
Thus on the basis of previous reports and our experimental
Therefore, inspired by our previous ndings that PMHS analysis of these PMHS-based SIPNs by TBAF, we believed that
could be used to initiate the cobalt-catalyzed polymerization of the PMHS-involved cobalt-catalyzed polymerization of activated
acrylates and subsequent multitask processes,5,10g we wanted to alkenes was a controlled radical reaction which could be used in
construct
a
graphene-containing organosilicon material the preparation of graphene-modied PMHS (Fig. 4, eqn (2)).23
(G@Si)-supported platinum catalyst (Pt@G@Si) accompanied In fact, we found that graphene-modied PMHS was easily ob-
by the platinum-catalyzed reduction of amides to amines tained with good dispersion under the reported reaction
(Fig. 3). As shown in Fig. 3, we hypothesized that graphene that conditions. However, the simple mixture of graphene and
is accepted as specic alkenes that are built with an innite PMHS resulted in phase separation because of their inherent
liquid and solid properties even at high temperature. This
phenomenon indirectly supported the hypothesis that gra-
phene and PMHS would be linked under cobalt-catalyzed
radical reaction conditions. In addition, similar to our
previous report,22 the cobalt catalyst used in this step could be
easily washed by water and organic solvent and did not have an
impact on the next application in the platinum-catalysed
hydrosilylation of amides. Then, satised with the successful
preparation of dispersed graphene-modied PMHS material, we
Fig. 3 Construction of a graphene-containing organosilicon material Fig. 4 Synthesis of structurally diverse insoluble PMHS-based semi-
(G@Si)-supported platinum catalyst (Pt@G@Si) accompanied by the interpenetrating networks (PMHS-SIPNs) via cobalt-catalyzed radical
platinum-catalyzed reduction of amides to amines.
polymerization of terminal olefins.
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believed the graphene could be an ideal supporter and additive
for the immobilization of a platinum catalyst according to the
microencapsulation method, that was rst introduced by
Kobayashi in 1998.24 Therefore, we prepared the cross-linked
graphene-containing PMHS by microencapsulation of gra-
phene with PMHS (G@PMHS, see ESI†) for further application
in the construction of a so-hard-so sandwich-type graphene-
containing organosilicon material (G@Si)-supported platinum
catalyst, accompanied by the platinum-catalyzed reduction of
amides to amines. As shown in Fig. 4, the construction of
a
graphene-containing organosilicon material (G@Si)-
supported platinum catalyst (Pt@G@Si) was proved to be
successfully completed in the platinum-catalyzed reduction of
amides to amines as in the following exploration.
With the novel graphene-modied PMHS (G@PMHS) in
hand, we then focused on the utilization of G@PMHS with the
dual function of a novel hydrosilane (Si–H) reagent and catalyst
support for the further evaluation of a multitask process by an
in situ formed organosilicon-supported platinum catalyst in
different hydrosilylation reactions. Initially, the platinum cata-
lyst precursor (H2PtCl6, 1 mol% of Pt)-catalyzed hydrosilylation
of amides with the aid of graphene-containing PMHS
(G@PMHS) was carried out according to Nagashima's reduction
reaction conditions (Fig. 5). Fortunately, it was found that yields
of the desired product (2) were quite high in the presence of
G@PMHS, which were better than those with graphite-
containing PMHS. And we found that the use of 5 mol% of
H2PtCl6 gave complete conversion without any side reactions in
the presence of G@PMHS. More interestingly, the novel plat-
inum catalyst attached to the carbon surface here was assisted
by the cross-linked silicone material that has a high binding
affinity for Pt ions in order to create a possibly recyclable hybrid
platinum-graphene catalyst that exhibited minimal metal
leaching. Notably, we have investigated the recovered platinum
catalyst aer the reduction of amide with PMHS, and, unfor-
tunately, the resulting organosilicon material-supported plat-
inum catalyst (Pt@G@Si) did not exhibit catalytic activity in the
hydrosilylation-type reduction of amide under the optimized
reaction conditions. These results suggested that the recovered
platinum catalyst is different from the catalyst precursor or
H2PtCl6 aer the reaction. Therefore, further investigation of
the recovered organosilicon material-supported platinum cata-
lyst in new organic transformations is a valuable topic (Fig. 6).
The resulting graphene-containing organosilicon material
(G@Si)-supported platinum catalyst (Pt@G@Si) and other
supported platinum catalysts (for example, Pt@graphite@Si
generated from graphite-modied PMHS) evaluated in this
work were characterized using X-ray photoelectron spectroscopy
(XPS), scanning electron microscopy (SEM), transmission elec-
tron microscopy (TEM), Raman spectroscopy, and/or X-ray
diffraction (XRD). The detailed spectra for these characteriza-
tions are provided in the ESI (Fig. S1–S13†). In addition,
graphite- and graphene-modied PMHS material and the
recovered Pt catalyst for reuse have also been adequately char-
acterized by ICP-MS analysis (Table S3 of ESI†). All these char-
acteristic data supported the nding that the regular structure
and morphology of Pt@G@Si are different from those of
Fig. 5 Platinum (H2PtCl6)-catalyzed hydrosilylation of amides with the
aid of graphene-containing PMHS: (1) the determination of the
superiority of graphene-modified PMHS (G@PMHS) over graphite-
modified PMHS (graphite@PMHS) and commercially available PMHS in
this reaction. (2) The recovery of in situ formed graphene-containing
organosilicon material (G@Si)-supported platinum catalyst (Pt@G@Si)
and graphite-containing organosilicon material (graphite@Si)-sup-
ported platinum catalyst (Pt@graphite@Si).
Pt@graphite@Si. Thus it could indirectly provide a reason why
graphene-modied PMHS (G@PMHS) is better than graphite-
modied PMHS (graphite@PMHS) in the platinum-catalyzed
hydrosilylation of amides (Fig. 5).
Accompanying the change in the structure of the surface, the
morphology of Pt@G@Si and Pt@graphite@Si is shown in the
SEM and TEM images (Fig. S1–S6†). From the TEM images of
the catalyst, one can conclude that the distribution of platinum
nanoparticles on the graphene-containing silicone material
surface, with the hard–so interface of carbon-based 2D mate-
rial and silicon-based cross-linked polymer, creates a conned
space for platinum nanoparticles (Fig. 7).
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(Pt 4f7/2) and 75.5 (Pt 4f5/2) are assigned to Pt(II), which probably
anchored with the Si–O groups on the surface of the G@Si
material. Of course, the functional species on the graphene
surface have a great inuence on the distribution of Pt(II) species.
Therefore, aer this reaction, the graphene-containing organo-
silicon material (G@Si) reduced a certain amount of the Pt(IV)
(H2PtCl6) to Pt(II), which was strongly chemisorbed at the surfaces
of the G@Si material. Meanwhile, returning to the H2Pt(IV)Cl6-
catalyzed hydrosilylation of amides, it was clearly determined
that the Pt(IV) intermediate plays a crucial role in this reaction,
and the recovered organosilicon material-supported platinum
catalyst (Pt@G@Si) did not exhibit any catalytic activity in the
hydrosilylation-type reduction of amide, which gives indirect
support to the previous report on the theoretical study of the
reaction mechanism of platinum-catalyzed reduction of amides
with hydrosilanes bearing dual Si–H groups. In 2015, Nakatani
et al. reported that a key to accomplishing the reduction of
amides via the present mechanism is the formation of a ve-
coordinate Pt(IV) species (Pt(IV)-disilyl-dihydride intermediate).25
On the other hand, the XPS analysis for Pt@G@Si supports
the presence of Pt(II) species (Fig. S11†). Thus it is obvious that
the platinum nanoparticles are possibly not metallic Pt but exist
as an aggregation of Pt(II) on the surface of the graphene-
containing organosilicon material, which differs from previous
reports on the preparation of Pt nanoparticles.26 Of course, it is
difficult to exclude the possibility that Pt(IV) or Pt(II) species
aggregate to metallic platinum nanoparticles in the reductive
hydrosilylation of amides.26 However, the XPS spectrum of
Pt@G@Si (Pt 4f region) shows a typical contribution from Pt(II).
This fact indicates that the Pt nanoparticles are possibly
oxidized.27 According to the large decrease in Si–H groups on the
graphene-modied PMHS (G@PMHS) during the reductive
hydrosilylation of amides, it is a rational to suppose that the Si–H
groups transformed into Si–O groups, thus forming Pt(II) groups
in this case. Based on the distribution of Si–O groups on the
graphene-containing organosilicon material (G@Si), the Pt(II)
groups may occur on the edge plane or basal plane of the G@Si
material.27a Another possible explanation for this apparent
contradiction is that the graphene-containing organosilicon
material (G@PMHS) partially reduces Pt(II) to Pt(0), in which case
mixed valence nanoparticles form. Alternatively, the presence of
“oxidized” Pt(II) species may reect the strong chemisorption
between the surface atoms of the Pt nanoparticles and the
graphene-containing organosilicon material. Such chemical
interactions can strongly change the binding energy of Pt. In both
scenarios, the Pt(II) species are undoubtedly formed in situ during
the induction period.26a Therefore, the novel graphene-modied
PMHS (G@PMHS) appears to serve two crucial roles: one as
a support for the Pt catalyst during the reduction of amides and
the other as a nanostructured matrix responsible for controlling
platinum nanoparticle growth.
Fig. 6 Multitask processes of recovered platinum catalyst in different
types of downstream reactions.
Fig. 7 TEM images for a Pt@graphite@Si catalyst (left) and Pt@G@Si
catalyst (right, for Pt nanoparticles, 30–40 nm).
XRD patterns of Pt@G@Si and Pt@graphite@Si are shown in
Fig. S9 and S10.† The main peak at 26.3ꢀ is ascribed to the (002)
plane of the graphitic-type lattice. No obvious diffraction peak
assigned to Pt particles and graphene is found in the XRD
patterns, indicating that platinum nanoparticles (NPs) and gra-
phene exist with good dispersion or with very small size. In
addition, Raman, IR, and solid-state NMR analysis of the
Pt@G@Si and graphene material also gave useful structural
information of the corresponding organosilicon-supported
graphene-based platinum catalyst (Fig. S7, S8 and S13†). This is
also attributed to the residual platinum NPs being encapsulated
into the graphene-containing organosilicon material (G@Si) aer
catalytic hydrosilylation of amides.
XPS was employed to investigate the electronic interaction
between platinum and supports based on the binding energy
shi of Pt 4f. In the XPS spectra of Pt@G@Si material (see
Fig. S11 and S12 of ESI†), the Pt 4f XPS spectra of the hybrid
catalyst present two main peaks at about 72.5 and 75.5 eV, cor-
Following characterization of the in situ generation of plati-
num(II) catalysts, the efficiency of heterogeneous Pt@G@Si or
Pt@graphite@Si were evaluated as catalysts for the hydro-
silylation of chalcone and triethylsilane under mild reaction
conditions (Fig. 8 and Table S1, see ESI†). It should be noted that
chalcone is a common, simple and privileged scaffold in
responding to the spin–orbit split doublet of Pt 4f7/2 and 4f5/2
,
respectively. It is well known that Pt 4f XPS spectra are decon-
voluted into three different components, including the predom-
inant metallic Pt(0), oxide states Pt(II), and Pt(IV). Peaks at 72.5 eV
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Next, having the optimized conditions for the hydrosilylation
of chalcone in hand, we determined the privileged role of
graphene-derived Pt@G@Si in this reaction, and then exam-
ined the scope of the 1,4-hydrosilylation reaction of various
chalcones bearing a substituent on the aromatic ring.
As can be seen from Table 1, the scope of Pt-catalyzed 1,4-
hydrosilylation of various chalcones with triethylsilane appears
to be very broad, and the silicon–oxygen bond-forming 1,4-
hydrosilylation reaction is satisfactory in isolated yield and Z
selectivity even with versatile hydrosilanes. Since the ability of
hydrosilane to hydrogenate enones, including chalcones, to
saturated ketones or alcohols has been recognized in the
past,28,29 this is a rare example where a heterogeneous platinum
catalyst (Pt@G@Si) could be used to promote the silicon–
oxygen bond-forming 1,4-hydrosilylation of aromatic enones.
As shown in Table 1, a number of substituted chalcones were
Fig. 8 The catalytic activity of different platinum catalysts in the 1,4-
hydrosilylation of chalcone with triethylsilane.
numerous naturally occurring compounds,28 and, unexpectedly,
there has been no report on the catalytic hydrosilylation of initially exposed to the 1,4-hydrosilylation under the optimized
enones up to the present. We initially screened the reaction reaction conditions. Substrates bearing a variety of functional
parameters (temperature and solvent effect) with Pt@graphi- groups on the aryl ring (for example, Cl, Br, F, MeO, Me) on the
te@Si as a model catalyst in this reaction. While Pt@graphite@Si meta-, para-, or ortho-position of the chalcones were tolerated in
was slightly less efficient compared to Pt@G@Si, the hydro- this reaction. And there is almost no difference in reactivity for
silylation reaction still reached 56% yield of silyl enolate 5a with
moderate chemoselectivity (the ratio of Z/E selectivity is 70 : 30,
see Fig. 8). As expected, the graphene-derived Pt@G@Si catalyst
was a highly efficient catalyst, with the reaction reaching
Table 1 Pt@G@Si-catalyzed 1,4-hydrosilylation of chalcones with
various hydrosilanesa
complete conversion and an excellent isolated yield (94%) in 3 h
ꢀ
at 60 C, and there are almost no other side products shown in
Fig. 5. Notably, in a series of control experiments at 60 ꢀC (Fig. 8),
it was found that the Karstedt catalyst or H4PtCl6 also exhibited
no activity in this silicon-oxygen bond-forming hydrosilylation
reaction, which further conrmed the privileged role of the in situ
formed Pt@G@Si nanocatalyst in the synthesis of a silyl enolate
product. An important feature commonly seen in the well-
dispersed and conned Pt@G@Si in silicone material is the
Entrya
R1/R2
[Si]–H
Ratio of Z/Eb
Yieldc (%)
moderate size of the platinum nanoparticles (30–40 nm, see the
TEM image of Fig. 7). Interestingly, the recovered Pt@Si that was
directly obtained from the H2PtCl6-catalyzed hydrosilylation of
amide without graphene or graphite exhibited almost no catalytic
activity in the hydrosilylation reaction of chalcone (Fig. 8).
Although the active catalytic species of the Pt@G@Si nano-
catalyst is not observed under the rapid reaction conditions,
experimental results indicate that the Pt@G@Si nanocatalyst is
distinct from that of organosilicon-encapsulated platinum in
terms of the identity of the synergistic effect of Pt NPs on 2D
graphene surface combined with cross-linked silicone material
and the active Pt(II) nanoparticle species. In addition, the stability
of the Pt@G@Si catalyst formed during the hydrosilylation using
a mixture of H2PtCl6 and G@PMHS was also investigated, in
which the fresh Pt@G@Si catalyst obtained by hot ltration
exhibited the same level of high yield (94%) in the 1,4-hydro-
silylation of chalcone with triethylsilane. Unfortunately, although
the used catalysts could be easily recovered by simple ltration
for recycling experiments, an obvious drop in yield was observed
in the second cycle (20% yield). As shown in Table S3 (see ESI†),
the results of ICP-MS analyses indicated that a large majority of
the Pt species (about 81%) in Pt@G@Si catalyst were detached
from the supported platinum catalyst.
1
2
3
4
5
6
7
8
H/H
4-OMe/H
4-Br/H
2-OMe/H
4-Cl/4-Cl
3-OMe/4-F
4-Ph/H
H/4-Me
H/4-Br
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
4d
4e
4f
91 : 9
79 : 21
96 : 4
5a: 94
5b: 83
5c: 86
5d: 85
5e: 89
5f: 90
5g: 88
5h: 90
5i: 88
5j: 91
5k: 90
5l: 89
5m: 84
5n: 89
5o: 84
5p: 76
5q: 71
5r: 81
5s: 73
5t: 78
5u: 83
88 : 12
80 : 20
75 : 25
80 : 20
78 : 22
93 : 7
82 : 18
89 : 11
77 : 23
85 : 15
97 : 3
9
10
11
12
13
14
15
16
17
18
19
20
21
H/4-Cl
4-Cl/H
3-OMe/H
H/4-OMe
4-Me/H (X ¼ N)
3-Br/H (X ¼ N)
H/H
H/H
H/H
H/H
H/H
99 : 1
90 : 10
90 : 10
91 : 9
90 : 10
90 : 10
99 : 1
H/H
4g
a
Reactions were performed with chalcone (0.5 mmol), hydrosilane
b
([Si]–H, 1.5 mmol), Pt@G@Si catalyst (5 mol%), THF (1 mL). The
ratio of Z/E is determined by H-NMR. c Isolated yield.
1
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these chalcones because good to excellent yields (71–95% better Z-selectivity and achieved the same level of yields in
yields) and high ratio of Z selectivities (up to 99 : 1) could be comparison to that of a heterogeneous Pt@G@Si catalyst.
achieved (entries 1–13). Interestingly, the pyridine-containing Therefore, the (PPh3)2PtCl2-catalyzed 1,4-hydrosilylation of
chalcones are also reactive substrates: for example, 1-pyridin- chalcones provided direct evidence in support of the key catalyst
2-yl-3-p-tolyl-propenone and 3-(3-bromo-phenyl)-1-pyridin-2-yl- of Pt@G@Si being Pt(II). Meanwhile, this work also provides
propenone were smoothly converted to the corresponding silyl a strategy in which the structural analysis of heterogeneous
enolate products 5n and 5o with 89% and 84% yields, respec- catalyst materials would give a new opportunity to design new
tively (entries 14 and 15). When the hydrosilanes were catalysts, even for homogeneous catalysis.
substituted with other groups, such as methyl, ethoxyl, phenyl,
In addition, we have also investigated the possibility of the
tert-butyl, the hydrosilylation of chalcone 3a went smoothly and recovery and re-use of Pt@G@Si aer the catalytic 1,4-hydro-
furnished the desired silyl enolate products in good yields (71– silylation of chalcone. However, the recovered Pt@G@Si is not
83%), thus showing the high tolerance for structurally diverse the same platinum catalyst as that used freshly in this reaction,
and functional hydrosilanes as well as the high 1,4-silylation which was determined by TEM and XPS analysis (see Fig. S14†).
capacity with
(Pt@G@Si).
a
superior active Pt-based nanocatalyst The platinum catalyst is possibly dispersed into the inner
organosilicon material. Although the change in chemical
Notably, these results showed that the really active platinum structure of the catalyst support (graphene-containing organo-
is Pt(II) because the XPS spectrum of Pt@G@Si (Pt 4f region) silicon material) cannot be ruled out at this time, we favor the
shows a typical contribution from Pt(II). In fact, inspired by this deactivation of the platinum catalyst on the basis of the above
nding, we then investigated the catalytic effect of (PPh3)2PtCl2 results. We believe that a further evaluation of the multitask
on the 1,4-hydrosilylation of chalcones under similar reaction process could be a solution to the catalytic application of
conditions. Gratifyingly, (PPh3)2PtCl2 is proved to be a highly recovered Pt@G@Si catalyst. Therefore, in performing these
efficient catalyst in this reaction (Table 2), in which it exhibited reactions with the same catalyst precursor, we anticipated that
the multitask strategy would be a major advantage from the
point of view of the catalyst-economic process. We have shown
Table 2 The determination of homogeneous Pt(II)-catalyzed 1,4-
hydrosilylation of chalcones with various hydrosilanes: inspired by the
Pt@G@Si-catalyzed 1,4-hydrosilylationa
Table 3 The transformations of silyl enolates 5a
Entrya
SM
[Si]
R
Yieldb (%)
1
2
3
4
5
6
7
8
5a
5p
5q
5r
5s
5t
5u
5b
5c
5d
5e
5f
5g
5h
5i
SiEt3
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
7a: 89
7a: 83
7a: 81
7a: 80
7a: 76
7a: 78
7a: 81
7b: 85
7c: 82
7d: 80
7e: 81
7f: 82
7g: 83
7h: 81
7i: 83
7j: 89
7k: 90
7l: 88
7m: 89
7n: 89
7a: 91c
SiMePh2
SiMe2Ph
SiMe(OEt)2
SiMe2(OEt)
SiMe2t-Bu
Si(OEt)3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
SiEt3
Entrya
R/R2
[Si]–H
Ratio of Z/Eb
Yieldc (%)
1
2
3
4
5
6
7
8
H/H
4-OMe/H
4-Br/H
2-OMe/H
4-Cl/4-Cl
3-OMe/4-F
4-Ph/H
H/4-Me
H/4-Br
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
4f
98 : 2
95 : 5
96 : 4
95 : 5
99 : 1
99 : 1
97 : 3
>99 : 1
97 : 3
99 : 1
92 : 8
95 : 5
97 : 3
98 : 2
99 : 1
97 : 3
99 : 1
99 : 1
94
87
85
88
86
91
86
83
82
89
84
84
86
85
81
81
87
83
9
10
11
12
13
14
15
16
17
18
19
20
21
9
10
11
12
13
14
15
16
17
18
H/4-Cl
4-Cl/H
3-OMe/H
H/4-OMe
4-Me/H (X ¼ N)
3-Br/H (X ¼ N)
H/H
5j
5k
5l
5m
5n
5a
H/H
H/H
SiEt3
a
Reactions were performed with silyl enolate (0.5 mmol), alkyl acrylate
a
b
c
Reactions were performed with chalcone (0.5 mmol), hydrosilane (0.5 mmol), TBAF (0.1 eq.), THF (1 mL). Isolated yield. With the
([Si]–H, 1.5 mmol), (PPh3)2PtCl2 catalyst (5 mol%), THF (1 mL). The addition of TBAF in presence of Pt@G@Si catalyst, total yield for one-
b
ratio of Z/E is determined by H-NMR. c Isolated yield.
pot two-step.
1
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that there is no border between homogeneous catalysis and formed in the reduction of amides and its sustainable catalytic
heterogeneous catalysis, and the multitask strategy could be application in downstream 1,4-hydrosilylation reactions. In
considered as an environmentally benign process to reuse detail, we described an interesting nding that an
a transition-metal catalyst from a homogeneous reaction organosilicon/graphene-supported platinum nanocatalyst
system, and an investigation into the corresponding in situ could be in situ prepared from the simple H2PtCl6-catalyzed
formed supported metal catalyst in a heterogeneous reaction reduction reaction of amides, and the corresponding graphene-
system would give useful information for developing a new and organosilicon-supported Pt nanoparticles (NPs) were found
transition-metal catalyst system.
to be a highly efficient catalyst in the silicon-carbon bond-
Finally, in order to obtain more information on the synthetic forming 1,4-hydrosilylation of chalcones, which is clearly
utilization of the corresponding products, silyl enolates, that benecial to the development of a new multitask strategy with
were obtained in this work, TBAF-promoted conjugate addition multitask and comprehensive utilization of a recovered and
of silyl enolates 5 to ethyl acrylate 6 was investigated in THF at deactivated transition metal catalyst. Apart from the advantages
room temperature. It should be noted that there has been no generated from the novel multitask strategy, the corresponding
report on the conjugate addition of such silyl enolates to alkyl platinum catalyst systems that were in situ formed in the
acrylate in the past.30 As shown in Table 3, the conjugate addi- H2PtCl6-catalyzed reduction of amides provide the following
tion occurred smoothly to give the desired product in good to features: (i) a high-yield and enhanced deoxygenated reduction
excellent yields (76–90% isolated yields). Various silyl enolates of amides with the aid of carbon-based materials, such as gra-
with different silicon-based bulky groups could be used in this phene or graphite, was observed under mild reaction condi-
reaction and gave the a-branched products in good yields. And tions; (ii) an example of highly efficient platinum-catalyzed 1,4-
notably, the same level of conversion could be achieved by the hydrosilylation of a,b-unsaturated enones is characterized by
one-pot Pt@G@Si-catalyzed 1,4-hydrosilylation of chalcones exquisite chemoselectivity, allowing for the facile synthesis of
and then the TBAF-promoted conjugate addition of silyl eno- a variety of otherwise inaccessible substituted silyl enolates in
lates 5 to alkyl acrylates 6 (Table 3, entry 21). Thus, it is an a straightforward manner. And in view of the new catalytic
interesting and useful example that aromatic 5-oxohexanoate function of a rationally designed platinum catalyst supported
products (1,5-keto esters)31 could be prepared by a new way with on a graphene/organosilicon-based composite material in the
the conjugate addition of silyl enolates to ethyl acrylate.
downstream reaction, we anticipate that the multitask process
and the in situ formed or newly generated platinum catalyst as
well as the hydrosilylation reaction of chalcones will offer the
possibility for new application of waste or deactivated transition
metal catalysts for new downstream organic transformations.
Finally, in order to obtain more information of the synthetic
Experimental
General procedure for the platinum-catalyzed 1,4-
hydrosilylation reaction of chalcones
The triethylsilane (0.17 mL, 1.5 mmol, 3.0 equiv.), Pt@G@Si utilization of the corresponding products, silyl enolates, we
(10.2 mg, 25 mmol, 5 mol%), and chalcone (104 mg, 0.5 mmol, 1.0 found that the silyl enolates could be used as nucleophiles in
equiv.) were dissolved in anhydrous THF (1.0 mL). And then the the conjugate addition of alkyl acrylates, which proceeded
reaction mixture was heated to 60 ꢀC and monitored by TLC until smoothly to give a new type of 1,5-keto esters. Further investi-
total conversion of the starting materials (about 3 hours). The gations of related mechanism on platinum-promoted hydro-
reaction mixture was cooled to room temperature. Aer ltration silylation transformations and an asymmetric version are
under diminished pressure, the organic layer was diluted with currently underway in our laboratory.
10.0 mL EtOAc, and the aqueous layer was extracted with EtOAc
(5.0 mL ꢁ 2). The combined organic layer was dried over Na2SO4
Conflicts of interest
and concentrated. Purication by silica gel ash chromatography
(using petroleum : EtOAc ¼ 10 : 1) gave the desired products with There are no conicts to declare.
good yields. All the products were conrmed by MS, and the
usual spectral methods (1H-NMR, 13C-NMR) (see ESI†). For
example, (Z)-(1,3-diphenylprop-1-enyloxy)triethylsilane (5a): 94%
Acknowledgements
yield; 1H NMR (400 MHz, CDCl3) d 7.39 (d, J ¼ 6.9 Hz, 2H), 7.19 (t, This Project was supported by the National Natural Science
J ¼ 5.2 Hz, 3H), 7.17–7.15 (m, 3H), 7.15–7.05 (m, 2H), 5.22 (t, J ¼ Foundation of China (No. 21472031, 21503060, and 21773051),
7.2 Hz, 1H), 3.50 (d, J ¼ 7.2 Hz, 2H), 0.85 (t, J ¼ 7.9 Hz, 9H), 0.54 Shaanxi Provincial Natural Science Foundation of China
(q, J ¼ 7.9 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 150.3, 141.7, (2017JM2001), Zhejiang Provincial Natural Science Foundation
139.55, 128.5, 128.1, 127.7, 125.9, 109.8, 77.4, 77.1, 76.8, 32.4, 6.8, of China (LZ18B020001, LY16E030009, and LY17E030003), and
5.5, 5.2. HRMS (ESI): m/z: [M + H]+ calculated for C21H29OSi: Science and Technology Department of Zhejiang Province
325.1982, found: 325.1971.
(2015C31138). This work is also supported partially by the
Program of “One Hundred Talented People” of Shaanxi Prov-
ince. The authors also thank Dr Z. R. Qu, Dr C. Q. Sheng, Dr K. Z.
Jiang, and Dr Q. H. Pan (all at HZNU) for their technical and
Conclusions
In summary, we have conrmed the powerful potential of the analytical support. The authors also sincerely thank the
multitask process for a recovered platinum catalyst in situ reviewers for their help in the improvement of this manuscript.
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