Facile assembly of bifunctional, magnetically retrievable mesoporous silica for enantioselective cascade reactions

Article abstract of DOI:10.1039/c9cc07123g

An integrated immobilization to encapsulate the Pd/C-coated magnetic nanoparticles within the chiral Ru/diamine-functionalized silica shell for the construction of a bifunctional magnetic catalyst is developed. This catalyst realizes a synergistic Suzuki cross-coupling/asymmetric transfer hydrogenation and a successive reduction/asymmetric transfer hydrogenation for the preparation of chiral aromatic alcohols.

Full text of DOI:10.1039/c9cc07123g

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Facile assembly of bifunctional, magnetically
retrievable mesoporous silica for enantioselective
cascade reactions†
Cite this: Chem. Commun., 2019,
5, 13578
5
Received 12th September 2019,
Accepted 10th October 2019
Zhongrui Zhao,‡ Fengwei Chang,‡ Tao Wang, Lijian Wang, Lingbo Zhao,
Cheng Peng and Guohua Liu
*
DOI: 10.1039/c9cc07123g
rsc.li/chemcomm
An integrated immobilization to encapsulate the Pd/C-coated
In the fabrication of bifunctional catalysts, yolk–shell–
6
magnetic nanoparticles within the chiral Ru/diamine-functiona- mesostructured silicas possess a particular distinguishing fea-
lized silica shell for the construction of a bifunctional magnetic ture, since the salient morphological structure is beneficial to
catalyst is developed. This catalyst realizes a synergistic Suzuki anchor dual-species through the detachable yolk and shell,
cross-coupling/asymmetric transfer hydrogenation and a succes- forming catalytically active site-isolated bifunctional catalysts. In
sive reduction/asymmetric transfer hydrogenation for the prepara- particular, the outer shell layer in these yolk–shell-mesostructured
tion of chiral aromatic alcohols.
silicas not only prevents magnetic aggregation, but also avoids the
loss of the magnet, overcoming the traditional drawbacks of
Increasing interest in the use of magnetically recoverable general magnetic catalysts. Thus, inspired by the constructions
nanomaterials as heterogeneous catalysts has led to a wide of yolk–shell-mesostructured heterobifunctional catalysts, it is
range of applications in various efficient chemical processes being reasonable to expect that a bifunctional magnetic catalyst could
1
reported in the literature. Numerous monofunctional magnetic be easily assembled through the incorporation of magnetic Fe
O
3 4
catalysts have been focused on in single-step catalytic reactions, into a yolk–shell-mesostructured silica network.
2
which involve the preparation of a variety of nonchiral and
Based on our continued interest in the development of
3
7
chiral organic molecules. Despite these significant achievements, magnetic catalysts, together with our recent experiences in
the development of magnetically recyclable catalysts is still the assembly of yolk–shell-mesostructured heterogeneous
lagging behind compared to general nonmagnetic heterogeneous catalysts, here we use an integrated immobilization strategy
8
4
catalysts. The main limitations lie in the challenging task of to encapsulate Pd/C-coated magnetic nanoparticles within a
characterization, which brings a series of subsequent problems, chiral Ru/diamine-functionalized silica shell, to construct a
for example, the deep understanding of the interactions of yolk–shell-mesostructured, site-isolated bifunctional magnetic
the active species and detailed elucidation of the role of the catalyst. Our aim lies in the exploration of a feasible enantio-
catalytic mechanism. In particular, the construction of a chiral selective cascade reaction. In this contribution, through utilization
9
bifunctional magnetic catalyst has not yet been explored, off the coupling or reductive ability of classic Pd/C nanoparticles,
especially for application in a more complicated multi-step together with an extensively studied asymmetric transfer hydro-
10
sequential enantioselective organic transformation. Reviewing the genation (ATH) reaction, we performed two types of sequential
recent explorations reported to date, only a mesoporous silica- organic transformations, synergistic Suzuki cross-coupling/ATH
supported acid–base-bifunctional magnetic catalyst has been used and successive reduction/ATH cascade reactions. As presented in
to prepare nonchiral organic molecules via a deacetalization-Henry Scheme 1, a simple three-step procedure was used to assemble the
5
cascade reaction of benzaldehyde dimethyl acetal. There is no bifunctional magnetic catalyst, abbreviated as MesRuArDpen@Pd/
11
doubt that the development of a chiral bifunctional magnetic C@Fe
3 4
O (5) (MesRuArDpen: Mes = mesitylene, and ArDpen =
catalyst and realization of an enantioselective cascade reaction are (S,S)-4-(((trimethoxysilyl)ethyl)phenylsulfonyl-1,2-diphenylethylene-
of great significance.
2 3 4
diamine (2)). Firstly, readily available SiO -coated Pd/C-Fe O
12
nanoparticles (1) were coated via the condensation of 2 and 1,2-
bis(triethoxysilyl)ethane to afford ArDpen@SiO @Pd/C@Fe O (3).
Key Laboratory of Resource Chemistry of Ministry of Education, Key Laboratory of
Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234,
China. E-mail: ghliu@shnu.edu.cn
2
3 4
Selective etching of the sandwiched SiO -coated layer then
2
gave ArDpen@Pd/C@Fe
3 4
O (4). Finally, the reaction of 4 and
†
Electronic supplementary information (ESI) available: Experimental procedures
(
MesRuCl , followed by strict Soxhlet extraction, led to the
2 2
)
and analytical data of chiral products. See DOI: 10.1039/c9cc07123g
‡
These authors contributed equally to this work.
yolk–shell-mesostructured catalyst 5 as a dark-gray powder,
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Scheme 1 Preparation of catalyst 5.
Fig. 1 (a) Wide-angle powder XRD patterns of Fe
(1) and catalyst 5. (b) Magnetization curves of Fe
and catalyst 5 measured at 300 K. (c) Separation-redispersion images of
catalyst 5 using an external magnet.
3
O
4
, SiO
2 3 4
@Pd/C@Fe O
0
3
O
4
, SiO @Pd/C@Fe O (1)
2
3 4
where a similar catalyst 5 without Fe
3
O
4
on the yolk was also
0
prepared by the reaction of 4 (obtained by the removal of Fe
3
O
4
from 4) and (MesRuCl
Fig. S1 and S2 in the ESI†).
Through analysis of solid-state cross-polarization (CP)/magic (Fig. 1c). In addition, the scanning electron microscopy (SEM)
2 2
) following a similar procedure (see
0
0
angle spinning (MAS) spectra of 4 and 5 as representatives, the images revealed the uniformly dispersed nanoparticle with an
data clearly reveal the well-defined single-site active species in average size of B450 nm, whereas transmission electron micro-
0
catalyst 5 (see Fig. S3 in the ESI†). It was found that material 4
scopy (TEM) demonstrated each silica yolk with B250 nm in
0
and 5 have similar carbon signals for the chiral ligands, where diameter separated by a thin silica shell of B80 nm in thickness
the peaks at d = 64–68 ppm can be ascribed to alkyl carbon atoms, (see Fig. S5 in the ESI†). Nitrogen adsorption–desorption isotherm
and those at d = 121–162 ppm correspond to aromatic carbon of catalyst 5 also disclosed its mesoporous structure owing to the
atoms in the –NCHC H moiety. Different from 4 , the spectrum existence of a typical IV character with an H1 hysteresis loop
of catalyst 5 shows the appearance of new characteristic carbon (see Fig. S5 in the ESI†).
0
ꢀ
6 5
0
signals at d = 20 and 105 ppm, which are attributed to the methyl
With this magnetic catalyst 5 in hand, we initially investigated
carbon atoms bonded to mesitylene and the aromatic carbon the single-step Suzuki cross-coupling reaction of 4-iodoaceto-
0
atoms in mesitylene. These signals in 5 are similar to those of its phenone and phenylboronic acid to test the coupling ability of
1
1a
homologue MesRuTsDpen,
thereby confirming the well- the Pd/C nanoparticles according to the optimal reaction condi-
14a,15
defined single-site Ru/diamine centers in catalyst 5. The solid- tions (see Table S1 in the ESI†).
state Si MAS NMR spectrum of catalyst 5 further demonstrates mixed PrOH/H
that catalyst 5 possesses an organic silica network with the coupling intermediate 1-([1,1 -biphenyl]-4-yl)ethan-1-one in a
strongest T species (R–Si(OSi) : R = ethylene-bridged groups quantitative yield. Then, this reaction system was also found to
Firstly, we found that the
2
O-system (3 : 1, v/v) was able to produce the
29
0
i
0
3
3
and/or alkyl-linked chiral functionalities) as its main silica wall be compatible for the second-step ATH transformation, where the
0
of the outer silica shell, since the signals of the organic silica’s ATH of 1-([1,1 -biphenyl]-4-yl)ethan-1-one catalyzed by 5 provided
0
0
T-series content in catalyst 5 were markedly higher than those of (S)-1-([1,1 -biphenyl]-4-yl)ethan-1-ol) with a slightly enhanced ee
13
the inorganic silica’s Q-series signals (see Fig. S4 in the ESI†).
This finding also suggests that the sandwiched inorganic SiO
value relative to that catalyzed by its homologue MesRuTsDpen
(97% yield and 99% ee versus 98% yield and 97% ee). Finally,
2
-
coated layer has been mostly removed, guaranteeing the chiral when we combined the two single-step reactions into a one-pot
Ru/diamine-functionality in the outer silica’s nanochannels and Suzuki cross-coupling/ATH process, the reaction still provided
the Pd/C particles on the inner magnetic yolk.
the target chiral product in 96% yield with 99% ee, which is
0
Fig. 1 presents the wide-angle XRD patterns and the magne- comparable to that attained with its parallel catalyst 5 (Table 1,
3 4 2 3 4
tization curves of Fe O , SiO @Pd/C@Fe O (1) and catalyst 5. entry 1 versus entry 1 in brackets). It is notable that the obtained
The wide-angle XRD patterns (Fig. 1a) show the general diffrac- enantioselectivity was slightly better than that obtained with a
tion peaks (2y values of 30.1, 35.4, 43.1, 57.0 and 62.6 degrees) physical mixture of 4 and the homologue MesRuTsDpen, and
of the Fe
3 4
O nanoparticles, the characteristic diffraction peaks markedly higher than that achieved via the physical mixing of Pd/C
at (2y values of 40.0, 46.5 and 68.3 degrees) for the palladium nanoparticles and MesRuTsDpen as dual catalysts (Table 1, entry 1
14
composites in 1. Due to the use of an in situ coating procedure, versus entries 2 and 3). These comparisons suggest that this active
this comparison also suggests that the Fe O and palladium site-isolated feature in 5 enables the efficient elimination of the
3
4
14c
components in catalyst 5 could be retained.
In addition, negative cross-interaction of the dual species, leading to enhanced
their magnetization saturation values are 60.85, 35.89, and reactivity and enantioselectivity.
ꢀ
1
6
.06 emu g (Fig. 1b), disclosing that catalyst 5 can be readily
Table 1 summarizes the general feasibility of the 5-catalyzed
separated from the reaction system using an external magnet Suzuki cross-coupling/ATH cascade reactions of iodoacetophenones
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a
Table 1 The 5-catalyzed Suzuki coupling/ATH cascade reactions
b
b
Entry
I, 8
Ar
Yield (%)
ee (%)
c
1
2
3
4
5
6
7
8
9
4-I, 8a
4-I, 8a
4-I, 8a
4-I, 8b
4-I, 8c
4-I, 8d
4-I, 8e
4-I, 8f
4-I, 8g
4-I, 8h
4-I, 8i
4-I, 8j
3-I, 8k
3-I, 8l
3-I, 8m
3-I, 8n
3-I, 8o
3-I, 8p
3-I, 8q
3-I, 8r
3-I, 8s
4-I, 8t
Ph
Ph
Ph
96 (95)
91
93
93
97
97
93
93
95
95
95
89
93
92
92
90
95
94
92
97
84
96
99 (99)
d
97
89
e
4-FPh
4-ClPh
3-ClPh
4-CF
3-CF
4-MePh
3-MePh
3-MeOPh
3-Thienyl
Ph
4-FPh
4-ClPh
3-ClPh
3
4-CF Ph
4-MePh
95
94
94
94
94
95
96
93
95
96
95
95
95
95
95
95
94
95 f
99
3
Ph
Ph
3
10
11
12
13
14
15
16
17
18
19
20
21
22
Fig. 2 Time course for the cascade reaction of 4-iodoacetophenone and
phenylboronic acid catalyzed by 5.
synergistic Suzuki cross-coupling and ATH process in this cascade
reaction.
Besides the utilization of the coupling ability of Pd/C nano-
particles, as shown in Table 1, the reducing ability of the Pd/C
8
a
3-MePh
nanoparticles in catalyst 5 was also examined. Based on the
obtained kinetic investigation of the successive reduction/ATH
transformation of (E)-1-(4-styrylphenyl)ethanone (see Fig. S7 in the
ESI†), Table 2 summarizes a series of 5-catalyzed cascade reac-
tions, showing that all of the tested one-pot processes steadily
3-MeOPh
3-Thienyl
4-AcetylPh
a
Reaction conditions: catalyst 5 (21.98 mg, 2.0 mmol of Ru, 8.57 mmol of
Na (1.0 mmol), iodoacetophenones
Pd, based on ICP analysis), HCO
0.10 mmol) and boronic acids (0.12 mmol), and 4.0 mL of ( PrOH/H
2
i
(
2
O, provided the corresponding alkylethyl-substituted chiral aromatic
v/v = 3/1) were added sequentially to a 10.0 mL round-bottom flask. The
mixture was then stirred at 60 1C for 10–16 h. Yields were determined
by H-NMR analysis and ee values were determined by chiral HPLC
alcohols in high yields with high enantioselectivities.
Fig. 3 exhibits the recycling of catalyst 5 in the Suzuki
analysis (see ESI in Fig. S8 and S11). Data were obtained by the use of coupling/ATH cascade reactions of 4-iodoacetophenone and
b
1
c
0
d
5
as a catalyst. Data were obtained from the use of a mixture of 4 and
e
MesRuTsDpen as dual reductive catalysts. Data were obtained from
the use of a mixture of Pd/C nanoparticles and MesRuTsDpen as dual
a
f
Table 2 The 5-catalyzed successive reduction/ATH cascade reductions
reductive catalysts. dr value = 86/14.
and a series of aryl boronic acids. It was found that the various
aryl boronic acids could efficiently react with 3- and 4-iodo-
acetophenone, providing the corresponding chiral biarylols in
high yields and enantioselectivities. Also, the cascade reaction of
acetylphenylboronic acid and 4-iodoacetophenone afforded Entry
b
b
I, 8
Ar
Yield (%)
Ee (%)
0 0 0 0
(
1S,1 S)-1,1 -([1,1 -biphenyl]-4,4 -diyl)bis(ethan-1-ol) in high yield
1
2
3
4
5
6
7
8
9
4-, 10a
4-, 10b
4-, 10c
4-, 10d
4-, 10e
4-, 10f
4-, 10g
3-, 10h
3-, 10i
3-, 10j
3-, 10k
3-, 10l
3-, 10m
3-, 10n
4-H
4-F
3-F
4-Cl
4-Me
3-Me
4-OMe
4-H
4-F
3-F
4-Cl
4-Me
3-Me
4-OMe
97
94
93
95
95
94
95
93
92
92
92
93
92
93
98
94
95
96
96
96
96
96
93
94
94
93
92
95
with 99% ee and 96/14 dr (Table 1, entry 22).
Fig. 2 shows the kinetic process of the 5-catalyzed Suzuki
cross-coupling/ATH of 4-iodoacetophenone and phenylboronic
acid. Firstly, the Suzuki cross-coupling of 4-iodoacetophenone
(6a) and phenylboronic acid (7a), and the asymmetric reduction
of 6a proceed simultaneously as the concentration of 6a decreases
sharply. During the first 2 h, a maximum transformation of 51% 10
1
1
1
1
2
3
for the formation of reductive (S)-1-(4-iodophenyl)ethan-1-ol (A)
and a maximum transformation of 20% for the formation of the
0
coupling with 1-([1,1 -biphenyl]-4-yl)ethan-1-one (B) are observed. 14
In the subsequent 2 h, the reaction proceeds rapidly to produce
a
Reaction conditions: catalyst 5 (21.98 mg, 2.0 mmol of Ru, 8.57 mmol of
Na (1.0 mmol), ketones (0.10 mmol), and
O v/v = 3/1) were added sequentially to a
0.0 mL roundꢀbottom flask. The mixture was then stirred at 50 1C for
0
the chiral product (S)-1-([1,1 -biphenyl]-4-yl)ethan-1-ol (8a). After Pd, based on ICP analysis), HCO
2
i
4
1
6
.0 mL of co-solvents ( PrOH/H
2
that, the coupling of A and 7a, and the asymmetric reduction of B
proceeds smoothly with the concomitant disappearance of A and
b
1
–12 h. The yields were determined by H-NMR analysis and ee values
B, providing 8a in 96% yield within 12 h. This time course shows a were determined by chiral HPLC analysis (see ESI in Fig. S8 and S11).
1
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