Phenylene-coated magnetic nanoparticles that boost aqueous asymmetric transfer hydrogenation reactions

Article abstract of DOI:10.1002/chem.201302797

Phenylene-coated organorhodium-functionalized magnetic nanoparticles are developed through co-condensation of chiral 4-(trimethoxysilyl)ethyl) phenylsulfonyl-1,2-diphenylethylene-diamine and 1,4-bis(triethyoxysilyl)benzene onto Fe₃O₄ followed complexation with [{CpRhCl ₂}₂]. This magnetic catalyst exhibits excellent catalytic activity and high enantioselectivity in asymmetric transfer hydrogenation in aqueous medium. Such activity is attributed to the high hydrophobicity and the confined nature of the chiral organorhodium catalyst. The magnetic catalyst can be easily recovered by using a small external magnet and it can be reused for at least 10 times without loss of its catalytic activity. This characteristic makes it an attractive catalyst for environmentally friendly organic syntheses. Magnetic moments: Phenylene-coated organorhodium-functionalized magnetic nanoparticles (NPs) are developed through co-condensation of chiral 4-(trimethoxysilyl)ethyl)phenylsulfonyl-1,2-diphenylethylene-diamine and 1,4-bis(triethyoxysilyl)benzene onto Fe₃O₄ followed by complexation with [{CpRhCl₂}₂]. The magnetic NPs exhibit excellent catalytic activity and high enantioselectivity in asymmetric transfer hydrogenation in water, owing to high hydrophobicity and the confined chiral organorhodium, and can be easily recovered with an external magnet and reused. Copyright

Full text of DOI:10.1002/chem.201302797

DOI: 10.1002/chem.201302797
Communication
&
Magnetic Nanoparticles
Phenylene-Coated Magnetic Nanoparticles that Boost Aqueous
Asymmetric Transfer Hydrogenation Reactions
[
a]
Xiaoshuang Gao, Rui Liu, Dacheng Zhang, Meng Wu, Tanyu Cheng, and Guohua Liu*
[7]
general mesoporous materials, PMO materials also possess
large surface area/pore volume ratios, tunable pore dimen-
sions, and well-defined pore arrangements, all of which benefit
the formation of site-isolated active species and enhance cata-
Abstract: Phenylene-coated organorhodium-functional-
ized magnetic nanoparticles are developed through co-
condensation of chiral 4-(trimethoxysilyl)ethyl)phenylsul-
fonyl-1,2-diphenylethylene-diamine and 1,4-bis(triethyoxy-
[6c]
lytic activity. Thus, it is reasonable to expect that the design
of an organosilica-coated magnetic catalyst would not only en-
hance catalytic efficiency but would also realize magnetic sep-
aration, the applicability of which in asymmetric reactions has
not been explored.
silyl)benzene onto Fe O4 followed complexation with
3
[
{Cp*RhCl } ]. This magnetic catalyst exhibits excellent cat-
2 2
alytic activity and high enantioselectivity in asymmetric
transfer hydrogenation in aqueous medium. Such activity
is attributed to the high hydrophobicity and the confined
nature of the chiral organorhodium catalyst. The magnetic
catalyst can be easily recovered by using a small external
magnet and it can be reused for at least 10 times without
loss of its catalytic activity. This characteristic makes it an
attractive catalyst for environmentally friendly organic syn-
theses.
To develop silica-based mesoporous heterogeneous catalysts
[
8,9]
for asymmetric catalysis,
we recently discovered two inorga-
[9a,b]
nosilica-coated magnetic catalysts
with high recycling effi-
ciency. However, their reaction rates hardly matched those of
their corresponding homogeneous systems. Interestingly, two
kinds of chiral organonickel-functionalized periodic mesopo-
rous organosilicas show significantly enhanced catalytic perfor-
mance in the asymmetric Michael addition of 1,3-dicarbonyl
compounds to nitroalkenes owing to the high hydrophobicity
[9c,d]
The use of magnetic materials as immobilized supports for het-
erogeneous catalysis has attracted much interest because of
of their organosilicate walls.
Thus, in this contribution, we
utilize the high hydrophobicity of periodic mesoporous orga-
nosilica and design a phenylene-coated magnetically separable
heterogeneous catalyst. As demonstrated in the present
study, the [Cp*RhArDPEN]-functionalized magnetic catalyst
[
1]
the merits of magnetic separation. However, intrinsic disad-
[
2]
vantages, such as magnetic aggregation and magnetic loss,
still hinder the potential applications of such materials. Recent-
ly, various surface-coated or otherwise surface-modified mag-
netic nanoparticles have opened up a variety of possibilities
[10]
5
(Cp*RhArDPEN =[(h -C Me )RhC1{(lS,2S)-N-4-ethylenephenyl-
5
5
sulfonyl-1,2-diphenylethylenediamine}], where Cp*=pentame-
thylcyclopentadienyl and ArDPEN=N-4-ethylenephenylsulfon-
yl-1,2-diphenylethylenediamine) exhibits excellent catalytic ac-
tivity and high enantioselectivity in asymmetric transfer hydro-
[3]
for the immobilization of homogeneous chiral complexes. In
particular, many inorganosilica-coated nanoparticles, which are
designed to overcome magnetic aggregation and to avoid
magnetic loss, have been applied successfully to various asym-
[10,11]
genation in aqueous medium.
As expected, the high hy-
[
4]
metric reactions. However, most of them still suffer from
lower catalytic efficiency compared with their homogeneous
drophobicity of the phenylene coating significantly enhances
the catalytic activity while retaining high enantiomeric excess
(ee) values. The catalyst can be recovered easily by using
a small magnet; neither filtration nor extraction is necessary.
Furthermore, the catalyst can be reused for at least 10 times
without loss of its catalytic activity. This characteristic makes it
an environmentally friendly catalyst in organic synthesis.
[5]
counterparts owing to the intrinsic nature of inorganosilica.
Thus, how to adjust magnet-coat type and to improve catalytic
efficiency are still unmet challenges in the design of magnet-
supported heterogeneous catalysts for asymmetric catalysis.
Periodic mesoporous organosilica (PMO) materials are
known to possess high hydrophobicity owing to their intrinsic
The phenylene-coated chiral organorhodium-functionalized
magnetic catalyst, abbreviated as Fe O @Cp*RhArDPEN-PMO
[
6]
organosilicate walls, a property that may facilitate asymmetric
reactions in aqueous catalytic systems. More importantly, like
3
4
(3), was synthesized as outlined in Scheme 1. First, magnetic
nanoparticles (Fe O ) were obtained according to reported
3
4
[3a,12]
methods.
The phenylene layer was then coated onto Fe O
3 4
[
a] Dr. X. S. Gao, R. Liu, D. C. Zhang, M. Wu, T. Y. Cheng, Prof. G. H. Liu
Key Laboratory of Resource Chemistry of Ministry of Education
Shanghai Key Laboratory of Rare Earth Functional Materials
Shanghai Normal University
by co-condensation of (S,S)-4-(trimethoxysilyl)ethyl)phenylsul-
fonyl-1,2-diphenylethylene-diamine (1) and 1,4-bis(triethyoxysi-
lyl)benzene by using cetyltrimethylammonium bromide (CTAB)
as a template. This step afforded ArDPEN-functionalized PMO
Shanghai 200234 (P. R. China)
E-mail: ghliu@shnu.edu.cn
(
2) in the form of a black powder. Finally, 3 was obtained by
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201302797.
the direct complexation of (Cp*RhCl ) with 2 followed by fur-
2
2
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Communication
the ArDPEN moiety, respectively. The peak at
13
9
4.8 ppm in the C MAS NMR spectrum of 3’ was
due to the carbon atoms in the Cp* ring, and that at
.4 ppm originated from the carbon atoms of the
8
CH groups attached to the Cp* ring. These peaks
3
were absent in the spectrum of 2’, suggesting the
formation of the Cp*RhArDPEN complex in 3’. In
sharp contrast to the spectra of 2’ and the homoge-
neous counterpart of 3, Cp*RhTsDPEN (TsDPEN= N-4-
methylphenylsulfonyl-1,2-diphenylethylenedia-
[11d]
mine),
the chemical shifts in the spectrum of 3’
were very similar to those of Cp*RhTsDPEN, demon-
strating that 3 and 3’ had the same well-defined
single-site active species as those in its homogene-
ous counterpart. Other peaks, denoted by asterisks in
Figure 1, were assigned to the rotational sidebands,
which could be distinguished through comparison
with peaks of pure phenylene-bridged PMO (Ph-
PMO).
Scanning electron microscopy (SEM) demonstrated
that catalyst 3 was composed of uniformly dispersed
nanospheres with an average size of about 370 nm
Scheme 1. Synthesis of the magnetic catalyst, 3. CTAB=cetyltrimethylammonium bro-
mide.
(
Figure 2a). Transmission electron microscopy (TEM)
confirmed that the core–shell-structured magnetic
nanospheres in 3 were encapsulated by a thin orga-
ther Soxhlet extraction to clear its nanochannels (see the Sup-
porting Information for additional experimental details, and
Figures S1 and S2). To characterize the active species of the
magnetic nanoparticles, non-magnetic ArDPEN-functionalized-
PMO (ArDPEN-PMO, 2’) and organorhodium-functionalized-
PMO (Cp*RhArDPEN-PMO, 3’) were also prepared by in situ re-
[
12b]
moval of Fe O4
(see the Supporting Information, Figures S1,
3
S3–S6).
Incorporation of well-defined single-site Cp*RhArDPEN
active species within the phenylene layer could be confirmed
by comparison with ArDPEN-PMO (2’) and Cp*RhArDPEN-PMO
13
(
(
3’). As shown in their solid-state C MAS NMR spectra
Figure 1), both 2’ and 3’ present strong characteristic carbon
signals for SiC H Si and NCH groups around 128 and 47–
6
4
55 ppm, corresponding to phenylene-bridged organosilica and
Figure 2. a) SEM image, b) TEM image of catalyst 3.
nosilica layer 30 nm thick (Figure 2b). Wide-angle X-ray
powder diffraction patterns (Figure 3) clearly show that 3 pro-
duced peaks similar to those of pure Fe O nanoparticles, sug-
3
4
gesting that there was no obvious change after the surface
coatings. Figure 4 presents the magnetization curves and sepa-
ration–redispersion process of catalyst 3. The superparamag-
À1
netic behavior of catalyst 3 was 55.1 emug of the magnetiza-
tion saturation value (Figure 4a). These magnetic nanoparticles
could be separated easily by placing a small magnet near the
bottle (Figure 4b). As desired, the phenylene-coated magnetic
catalyst not only efficiently negated magnetic dipolar attrac-
1
3
Figure 1. C CP MAS NMR spectra of Ph-PMO, ArDPEN-PMO (2’) and catalyst
’.
3
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[a]
Table 1. Asymmetric transfer hydrogenation of aromatic ketones.
[
b]
[b]
[c]
Entry
R
Conv. [%]
ee [%]
[
c]
1
2
3
4
5
6
7
8
9
Ph
Ph
4-FPh
4-ClPh
4-BrPh
3-BrPh
4-MePh
4-OMePh
3-OMePh
4-CNPh
>99 (96 )
>99
96 (95 )
[
d]
96
94
93
93
92
96
95
96
86
93
>99
>99
>99
>99
>99
>99
>99
>99
10
Figure 3. Wide-angle powder XRD patterns of pure Fe
3
O
4
and catalyst 3.
11
4-CF
3
Ph
>99
[
a] Reaction conditions: catalyst 3 (56.20 mg, 2.00 mmol of Rh based on
ICP analysis), HCO Na (0.68 g, 10.0 mmol), ketone (0.2 mmol), and water
2.0 mL), 408C, 0.5–5.0 h. [b] Determined by chiral GC analysis (see the
2
(
Supporting Information, Figure S8). [c] With 0.2mol% of catalyst. [d] Data
were obtained by using 2 plus Cp*RhTsDPEN as a catalyst.
To rule out the effect of noncovalent adsorption, a parallel
experiment using ArDPEN-functionalized-PMO (2) plus the ho-
mogeneous catalyst, Cp*RhTsDPEN, was investigated. The
result was that the asymmetric reaction of acetophenone af-
forded (S)-1-phenyl-1-ethanol with over 99% conversion and
96% ee (entry 2), indicating that the homogeneous catalyst
added through noncovalent adsorption onto the material also
resulted in a good catalytic performance. However, when this
catalyst was subjected to Soxhlet extraction, the reused cata-
lyst gave only a small amount of product, demonstrating that
the noncovalent physically adsorbed catalyst could be elimi-
nated by Soxhlet extraction. This observation confirmed that
the nature of the active catalyst was indeed derived from the
magnetic nanoparticles as the catalyst 3 had also undergone
this process of Soxhlet extraction, yet remained active.
Figure 4. a) Magnetization curves of pure Fe
b) Separation–redispersion process of catalyst 3.
3 4
O and catalyst 3 at 300 K,
tion but also retained enough superparamagnetism for mag-
netic separation.
It is worth mentioning that the asymmetric reactions cata-
lyzed by 3 have markedly higher reaction rates relative to
those obtained with general inorganosilica-based mesoporous
Table 1 summarizes the catalytic performance of 3 in aque-
ous asymmetric transfer hydrogenation. Because of its phenyl-
ene-coating, catalyst 3 was able to catalyze the asymmetric re-
[8d]
heterogeneous catalysts,
and they are even higher than
those obtained with the homogeneous counterpart,
[10a]
action in the absence of Bu NBr, which is often necessary to
Cp*RhTsDPEN.
Taking 4-methylacetophenone as an exam-
4
promote catalytic performance in homogeneous catalysis sys-
ple, the asymmetric reaction of 4-methylacetophenone cata-
lyzed by 3 could be completed within 1.5 h, whereas 4.0 h was
needed with the homogeneous catalyst system. To confirm
this result, kinetic profiles of the asymmetric reactions of 4-
methylacetophenone were compared (see the Supporting In-
formation, Figure S7). It was found that catalyst 3 showed ini-
tial activity higher than that of Cp*RhTsDPEN; the initial turn-
[
10]
tems. In general, excellent conversions and high enantiose-
lectivities were obtained for all of the aromatic ketones tested.
Taking acetophenone as an example, over 99% conversion was
achieved and an ee value of 96% was obtained for (S)-1-
phenyl-1-ethanol. This conversion was higher than that of the
[8a]
homogeneous counterpart of 3 in the absence of Bu NBr,
4
and the ee value was comparable to that of its homogeneous
over frequency (TOF) values were 480.0 and 362.5 mol
[
10a]
À1 À1
counterpart.
Of particular note was that the asymmetric re-
mol
h
for 3 and Cp*RhTsDPEN, respectively.
action could be performed at a much high ratio of substrate/
catalyst without markedly affecting its ee value, as exemplified
by the asymmetric transfer hydrogenation of acetophenone at
a substrate-to-catalyst molar ratio of 500 (entry 1 versus value
in brackets).
An important design feature of magnetic catalysts is their
easy separation by application of an appropriate magnetic
field. Ideally, the recovered catalyst should retain its catalytic
activity and enantioselectivity after multiple cycles. In this case,
upon completion of the reaction, magnetic catalyst 3 was re-
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
covered quantitatively by using a small external magnet (Fig-
ure 4b). As shown in Figure 5, the recycling of magnetic cata-
lyst 3 in ten consecutive reactions still afforded (S)-1-phenyl-1-
ethanol with 97% conversion and 96% ee (see the Supporting
Information, Table S1 and Figure S9).
3.0 mL). The combined Et
2
O washings were washed with brine
twice and dried with Na SO . After evaporation of Et O, the residue
2
4
2
was purified by silica gel flash column chromatography to afford
the desired product. The conversion and ee values were deter-
mined by chiral GC using a Supelco b-Dex 120 chiral column (30 m
ꢁ
0.25 mm (i.d.), 0.25 mm film).
Acknowledgements
We are grateful to the Shanghai Sciences and Technologies De-
velopment
3ZR1458700), SRFDP (20133127120006), CSIRT (IRT1269) and
the Shanghai Municipal Education Commission (12ZZ135,
4YZ074, SK201329) for financial support.
Fund
(10JC1412300,
12nm0500500
and
1
1
Keywords: asymmetric transfer catalysts
· heterogeneous
catalysis · magnetic nanoparticles · magnetic separation ·
mesoporous materials
Figure 5. Reusability of catalyst 3 using acetophenone as the substrate.
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Communication
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