An integrated immobilization strategy manipulates dual active centers to boost enantioselective tandem reactions

Article abstract of DOI:10.1039/C8CC07841F

Manipulation of dual active centers via an integrated immobilization strategy can overcome the restriction of homogeneous catalysis in a sequential organic transformation. Herein, by utilizing hollow-shell-structured silica, hydrogen-bonding immobilization via a ship-in-a-bottle synthesis locks a Pd(carbene) center in a nanocage and covalent-bonding immobilization tethers chiral Ru(diamine) centers within the nanochannels, constructing a hetero-bifunctional catalyst. The benefit of this dual center manipulation enables a challenging Suzuki coupling-asymmetric transfer hydrogenation tandem reaction, and the advantage of this process provides various chiral biarylols with enhanced reactivity and enantioselectivity.

Full text of DOI:10.1039/C8CC07841F

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An integrated immobilization strategy manipulates
dual active centers to boost enantioselective
tandem reactions†
Cite this: Chem. Commun., 2018,
54, 13244
Received 1st October 2018,
Accepted 23rd October 2018
Xiaomin Shu, Ronghua Jin, Zhongrui Zhao, Tanyu Cheng
and Guohua Liu
*
DOI: 10.1039/c8cc07841f
rsc.li/chemcomm
Manipulation of dual active centers via an integrated immobilization via a ship-in-a-bottle synthesis, whereas the advantage of nano-
strategy can overcome the restriction of homogeneous catalysis in a channels is that they can anchor another type of small steric
sequential organic transformation. Herein, by utilizing hollow-shell- active centre through bonding organometallics by ligands incor-
structured silica, hydrogen-bonding immobilization via a ship-in- porated in the nanochannels, thereby manipulating the positions
a-bottle synthesis locks a Pd(carbene) center in a nanocage and of dual active centers that may provide a synergistic effect of site-
covalent-bonding immobilization tethers chiral Ru(diamine) centers isolated dual species to enhance reactivity and selectivity.
within the nanochannels, constructing a hetero-bifunctional catalyst.
Optically pure biarylols are valuable motifs in the preparation
The benefit of this dual center manipulation enables a challenging of polymers, fluorescent brighteners and chiral ligands.5 Theore-
Suzuki coupling-asymmetric transfer hydrogenation tandem reaction, tically, two single-step reactions, Suzuki coupling and asymmetric
and the advantage of this process provides various chiral biarylols with transfer hydrogenation (ATH), can be used for the synthesis of
enhanced reactivity and enantioselectivity.
chiral biarylols through the Suzuki coupling of haloacetophe-
nones and arylboronic acids, followed by an ATH process.6
Utilization of a ship-in-a-bottle strategy for the fabrication of Practically, a direct one-pot synthesis via this tandem reaction
silica-supported catalysts has attracted great interest, as this is quite difficult because of the intrinsic incompatibility of
strategy ensures homogeneous catalysis and heterogeneous the two common dual species under homogeneous catalysis.7
separation.1 However, their intrinsic drawbacks, such as leaching Although two methodologies, chemocatalysis followed by
during catalytic processes and cross-interactions in the assembly biocatalysis and active site-isolated catalysis, can perform
of dual active centers, greatly restricts their applications, espe- this two-step organic transformation, both have limitations.
cially in processes involving multiple organic transformations. For chemocatalysis followed by biocatalysis, this is actually a
Therefore, an integrated strategy, combining a ship-in-a-bottle one-pot two-step process,8 where a sensitive biocatalysis system
synthesis with well-established immobilization methods, including and complicated product isolations hinder its application. For
covalent bonding, ion-pair formation and encapsulation,2 not only active site-isolated catalysis,9 the drawback lies in low catalytic
exploits the respective advantages in the fabrication of silica- efficiency, since only highly active iodoacetophenones being used
supported bifunctional catalysts, but also allows the manipulation as substrates result in the reaction proceeding, where the
of dual active centers during the immobilization process, thereby transformation of chloroacetophenone, which is not very active,
making some complicated organic transformations that are diffi- is impossible because of the innate deficiency of heterogeneous
cult and impossible under homogeneous conditions feasible.
catalysts when only an immobilization strategy is used alone.
Hollow-shell-structured silicas,3 as an important member Thus, by exploiting the advantage of an integrated immobiliza-
of the silica-support family used in catalysts,4 have inherent tion strategy to overcome the low catalytic efficiency, together
superiority in the formation of ship-in-a-bottle catalysts because with the outstanding site-isolation and synergic effects10 of the
their morphological structures contain unique double hollow silica in the multiple organic transformation,11 the rational design
cavities, nanochannels and nanocages. The benefit of nanocages of the supported catalyst has the ability to solve the incompatible
is that they can immobilize one type of large steric active centre disadvantage of dual active centers, to realize a challenging Suzuki
coupling-ATH tandem reaction with chloroacetophenones that are
Key Laboratory of Resource Chemistry of Ministry of Education, Key Laboratory of
not very active.
Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234,
In this contribution, based on our previous efforts,12 we entrap a
China. E-mail: ghliu@shnu.edu.cn
large steric Pd(carbene) species on the inner surface of a nanocage
† Electronic supplementary information (ESI) available: Experimental procedures
and analytical data of the chiral products. See DOI: 10.1039/c8cc07841f
via a hydrogen-bonding strategy using a ship-in-a-bottle synthesis,
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silanols was also proven by solid-state 19F MAS NMR, since 5 gave
the same characteristic fluorine signals at ꢀ145 ppm as its
homogeneous counterpart (see Fig. S4 in the ESI†). Moreover,
the 29Si MAS NMR spectra showed the main hydrophobic silica
network of the organic silica in 5 because the contents of the
organic silica (T signals) were markedly higher than those of the
inorganic silica (Q signals)17 due to the removal of the inorganic
SiO2 core during the etching process (see Fig. S5 in the ESI†).
Fig. 1 shows the morphology of catalyst 5. The scanning
electron microscopy (SEM) image in Fig. 1a shows uniformly
dispersed silica nanospheres with an average size of around
230 nm, whereas the transmission electron microscopy (TEM)
image in Fig. 1b shows the hollow-shell structure with a thin
silica shell of 30 nm in thickness, confirmed by a broken
nanosphere, as shown in Fig. 1c. It was notable that the TEM
image with chemical mapping showed that the palladium and
ruthenium centers in 5 are uniformly distributed (see Fig. S6 in
Scheme 1 Preparation of catalyst 5.
where such a hydrogen-bonding approach13 can efficiently com- the ESI†). Moreover, the small-angle X-ray diffraction (XRD)
pensate for the leaching drawbacks of the ship-in-a-bottle synthesis pattern (see Fig. S7 in the ESI†) exhibited a well-resolved peak at
during the catalytic process. Meanwhile, chiral Ru(diamine) species 2y = 0.8–1.01 and the nitrogen adsorption–desorption isotherm
are located within the nanochannels via a covalent-bonding presented a typical IV-type isotherm with a H2 hysteresis loop (see
strategy using tethered chiral diamine ligands. Scheme 1 shows Fig. S8 in the ESI†), demonstrating the mesoporous structure.
the three-step immobilization procedure for the fabrication of
At the start of the investigation into the catalytic properties
these hollow-shell mesoporous silica nanospheres, abbreviated under the optimal reaction conditions (see Table S1 in the ESI†),
as catalyst 5, which is Me@IPrPdBF4@mesityleneRuArDPEN@ we found that the Suzuki coupling-ATH one-pot reaction catalysed
HSMSNs (IPr14 = 1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H- by 5 proceeded smoothly using 4-chloroacetophenone, which
imidazolium chloride, mesityleneRuArDPEN:15 mesitylene = is not very active, instead of highly active 4-iodoacetophenone
1,3,5-trimethylbenzene and ArDPEN = (S,S)-4-((trimethoxysilyl)- or 4-bromoacetophenone, where the tandem reaction of
ethyl)phenylsulfonyl-1,2-diphenylethylene-diamine (2)). In the 4-chloroacetophenone and phenylboronic acid afforded the
first step, the co-condensation of 1,2-bis(triethoxysilyl)ethane chiral product (S)-1-(biphenyl-4-yl)ethanol in 97% yield with
and 2 onto a silica core (1) led to ethylene-coated silica,16 where 96% ee, which was comparable to that attained with 4-iodo-
the exterior silanols were protected with trimethysilyl (–SiMe3) acetophenone or 4-bromoacetophenone as substrates (entry 1
groups to form a hydrophobic silylated ArDPEN-functionalized versus entries 4 and 5), and was markedly better than that with
silica, Me/Shell@ArDPEN@SiO2 (3). In the second step, this SiO2 mixed dual homogeneous counterparts (entry 1 versus entry 1 in
core was etched to afford hollow-shell mesoporous Me/shell@ parenthesis). Both comparisons indicate the superiority of 5 in
ArDPEN (4). In the third step, hydrogen-bonding immobilization the trimethysilyl-protection of exterior silanols, as it not only
via a ship-in-a-bottle process was performed by the in situ entrap-
ment of IPrPdCl2(3-chloropyridine) followed by anion exchange
with AgBF4 to afford hydrogen-bonded IPrPdBF4 on the inner
surface of the nanocage, which goes through a complexation with
(mesityleneRuCl2)2 in the nanochannels to produce catalyst 5 as a
light-yellow powder (see the ESI† experimental, Fig. S1 and S2).
Solid-state 13C CP/MAS NMR spectra (cross-polarization
(CP)/magic angle spinning (MAS) spectroscopy) (see Fig. S3 in
the ESI†) revealed that 5 produced characteristic carbon signals
at 155 ppm for the carbon atom bonded to the palladium in the
IPrPdBF4 complexes, whereas those at around 99 ppm were
assigned to the carbon atoms of the mesitylene ring and those at
19 ppm to the methyl carbon atoms bonded to mesitylene
in the mesityleneRuArDPEN-complexes. These characteristic
signals, together with the other observed general carbon peaks
in both species, suggested the formation of single-site Pd(carbene)
and chiral Ru(diamine) species in 5, because their chemical shifts
were similar to those attained with their homogeneous
counterparts.14a,15a Furthermore, the hydrogen-bonding of the
Fig. 1 (a) SEM image of 5, (b) TEM image of 5, and (c) SEM image of a
broken nanosphere.
Pd(carbene) species through the BF4ꢀ anion to the inner surface
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Table 1 Suzuki coupling-ATH one-pot tandem reactions of chloro-
provides a hydrophobic benefit to promote catalytic efficiency
but also ensures that the dual species cooperatively enhances
the enantioselectivity. To elucidate the hydrophobic effect of
the silylated shell and synergistic effect of the dual species in 5,
two parallel experiments were compared using the reaction
of 4-chloroacetophenone and phenylboronic acid as a model. In
the first case, the reaction catalyzed by its analogue, IPrPdBF4@
mesityleneRuArDPEN@HSMNs (50), (50 has no trimethysilyl pro-
tection of the exterior silanols, obtained by a similar procedure)
only gave (S)-1-(biphenyl-4-yl)ethanol in 83% yield, even in a
prolonged threefold reaction time (entry 2 versus entry 1). This
comparison (83% yield versus 97% yield) demonstrated that the
hydrophobic benefit of 5 promotes the catalytic performance
in an aqueous medium, proven by the water contact angles
acetophenones and various boronic acidsa
Entry
Ar
Products
Yieldb (%)
eeb (%)
1
2
3
4
5
6
7
8
9
4-ClPh, Ph
4-Cl, Ph
4-Cl, Ph
4-Br, Ph
4-I, Ph
8a
8a
8a
8a
8a
8b
8c
8d
8e
8f
8g
8h
8i
8j
8k
8l
97(64)
83
55
96(90)c
95d
90e
99(75)
99(86)
93(35)
92(89)
96(39)
91(70)
92(44)
94(26)
97(65)
94(60)
95(58)
91(61)
96(54)
94(64)
93(70)
88(68)
83(76)
77(65)
94(89)c
95(85)c
96(88)c
95(90)c
95(82)c
93(83)c
95(81)c
96(90)c
96(77)c
94(85)c
94(89)c
95(82)c
95(90)c
95(90)c
96(92)c
92(88)c
96(80)c
93(90)c
4-Cl, 4-FPh
4-Cl, 4-CF3Ph
4-Cl, 3-CF3Ph
4-Cl, 4-CNPh
4-Cl, 4-MePh
4-Cl, 3-MePh
4-Cl, 4-MeOPh
3-Cl, Ph
3-Cl, 4-FPh
3-Cl, 3-CF3Ph
3-Cl, 4-MePh
3-Cl, 4-MeOPh
4-Cl, 3-thieny
3-Cl, 3-thieny
4-Cl, styryl
3-Cl, styryl
(91.71 for 5 versus 50.41 for 50) (see Fig. S9 in the ESI†). In the 10
second case, the reaction catalyzed by the mixed 500 (Me@mesity-
leneRuArDPEN@HSMSNs (500) has no IPrPdBF4 species, obtained
11
12
13
by the complexation of 4 and (mesityleneRuCl2)2) plus homoge- 14
15
16
17
neous IPrPdBF4 as dual catalysts only provided (S)-1-(biphenyl-4-
yl)ethanol in 55% yield with a decreased ee value (entry 1 versus
8m
8n
8o
8p
8q
entry 3). This finding, together with a comparison of the homo- 18
19
20
21
geneous counterparts as dual catalysts (entry 1 in parenthesis),
suggested a positive synergistic effect, since the dual species in 5
efficiently eliminated the negative interference of the dual species
under the homogeneous conditions.
a
Reaction conditions: catalyst 5 (13.35 mg, 2.0 mmol of Ru and
1.20 mmol of Pd, based on ICP analysis), Cs2CO3 (0.20 mmol), HCO2Na
(1.0 mmol), haloacetophenones (0.10 mmol) and boronic acids
(0.12 mmol), and 2.0 mL of H2O/iPrOH (v/v = 1 : 3) were added sequentially
to a 10.0 mL round-bottomed flask. The mixture was then stirred at 80 1C
To provide insight into the catalytic nature, the kinetic process
for the tandem reaction of 4-chloroacetophenone and phenyl-
boronic acid was analyzed by monitoring the reaction time course
(see Fig. S10 in the ESI†). It was found that the ATH reduction of
4-chloroacetophenone (6a) and the Suzuki cross-coupling reaction
of 6a and phenylboronic acid occur simultaneously with a sharp
decrease in the concentration of 6a. During the first two minutes,
the reductive intermediate (S)-4-chlorophenylethanol (A) in a maxi-
mum yield of 38% and the coupling intermediate 1-(biphenyl-
b
for 0.5–1.5 h. The yields were confirmed by 1H NMR and the ee values
were determined by chiral HPLC analysis after purification by flash-column
c
chromatography (see Fig. S10 and S12 in the ESI). Data in parentheses
were obtained by the use of the mixed homogeneous IPrPdBF4 plus
d
mesityleneRuTsDPEN as dual catalysts. Data were obtained using 50 as
e
a catalyst within 1.5 h. Data were obtained using the mixed 500 plus
homogeneous IPrPdBF4 as dual catalysts within 1.5 h.
4-yl)ethanone (B) in a maximum yield of 30% were observed. bore electron-withdrawing and electron-donating substituents at
Meanwhile, the reaction also produced the chiral product the R group position or whether the chloroacetophenone had a
(S)-1-(biphenyl-4-yl)ethanol (8a) in 27% yield. Then, the Suzuki meta- or para-position (entries 6–12 versus 13–17, and entries 18
cross-coupling of A and phenylboronic acid, and the ATH of versus 19). More interestingly, the tandem reaction of a different
1-(biphenyl-4-yl)ethanone (B) proceeded smoothly, providing 8a in type of styrylboronic acid also proceeded with chloroaceto-
97% yield in 30 minutes. This kinetic investigation provided phenones, resulting in enhanced yields and enantioselectivities
evidence for the synergistic Suzuki cross-coupling and ATH process. (entry 20 versus 21).
Table 1 summarizes the applicability of 5 in the Suzuki
Besides the use of chloroacetophenones as substrates
coupling-ATH one-pot tandem reactions with chloroacetophe- in Table 1, the other chloro-substituted heterocyclic aromatic
nones and a series of boronic acids, providing further evidence rings, 1-(5-chlorothiophen-2-yl)ethan-1-one and 1-(2-chloro-
for the feasibility of the tandem reactions and the positive pyridin-4-yl)ethan-1-one, could also be converted into the chiral
synergistic effect. In the case of feasible tandem reactions using products with enhanced yields and enantioselectivities (eqn (1)
chloroacetophenones as substrates, which are not very active, of Scheme 2). Moreover, this one-pot organic transformation
we found that all the tested tandem reactions with chloroaceto- could be used to synthesize more challenging chiral biaryldiols,
phenones provided chiral biarylols in enhanced yields, con- where the tandem reactions of acetylphenylboronic acid and
firming the general feasibility. In the case of a positive synergistic meta- or para-substituted chloroacetophenone steadily afforded
effect of the dual species, all the tested 5-catalyzed tandem the chiral biaryldiols in enhanced yields with excellent dr
reactions had higher ee values than those attained with their and ee values (eqn (2) of Scheme 2). In addition, the effect of
homogeneous counterparts, demonstrating the positive synergistic size selectivity toward some substrates was also investigated,
effect. Also, it was found that their enantioselectivities were not where the reaction catalyzed by 5 with a large size pyren-1-ylboronic
significantly influenced by the structural and electronic properties acid was observed to have an obviously lower yield than that
of the substituents regardless of whether the arylboronic acids attained with its counterparts (eqn (3) of Scheme 2), suggesting
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Conflicts of interest
There are no conflicts to declare.
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