Recoverable organorhodium-functionalized polyhedral oligomeric silsesquioxane: A bifunctional heterogeneous catalyst for asymmetric transfer hydrogenation of aromatic ketones in aqueous medium

Article abstract of DOI:10.1039/c2cc31927f

A bifuctional heterogeneous chiral rhodium catalyst exhibited excellent catalytic activity and enantioselectivity in asymmetric transfer hydrogenation of aromatic ketones and their analogues in aqueous medium, which could be recovered easily and used repetitively without affecting obviously its enantioselectivity. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc31927f

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COMMUNICATION
Recoverable organorhodium-functionalized polyhedral oligomeric
silsesquioxane: a bifunctional heterogeneous catalyst for asymmetric
transfer hydrogenation of aromatic ketones in aqueous mediumw
Shuang Tang, Ronghua Jin, Huaisheng Zhang, Hui Yao, Jinglan Zhuang,
Guohua Liu* and Hexing Li*
Received 16th March 2012, Accepted 3rd May 2012
DOI: 10.1039/c2cc31927f
A bifuctional heterogeneous chiral rhodium catalyst exhibited
excellent catalytic activity and enantioselectivity in asymmetric
transfer hydrogenation of aromatic ketones and their analogues
in aqueous medium, which could be recovered easily and used
repetitively without affecting obviously its enantioselectivity.
site-directing vinyl groups in octavinylsilsesquioxane and
report a bifunctionalized polyhedral oligomeric silsesquioxane
for the first time, in which a quaternary ammonium salt
functionality as a phase transfer catalyst and a chiral
Cp*RhTsDPEN functionality [(Cp* = pentamethyl cyclo-
pentadiene and TsDPEN = 4-(methylphenylsulfonyl)-1,2-
diphenylethylenediamine)] as a chiral catalyst7 are combined
successfully together within POSS. This bifunctional hetero-
geneous catalyst not only controls the dispersibility of
bifunctionality, but also promotes significant catalytic effici-
ency, showing an obvious superiority in asymmetric transfer
hydrogenation of aromatic ketones and their analogues.
Meanwhile, an investigation of its reusability is also discussed
in detail.
Developments of silica-supported heterogeneous catalysts for
asymmetric catalysis have led to significant achievements.1
In particular, various self-assemblies of chiral-functionalized
inorganosilanes or organosilanes have dominated the prepara-
tion of heterogeneous catalysts.2 However, intrinsic limits,
such as uncontrollable hydrolysis of the silane resource and
complicated compatibility of functionalities, often make chiral
functionalities anchor randomly within frameworks, resulting
in difficulty to form precisely site-directed frameworks. Thus,
searching for well-defined silica-based building blocks to control
the arrangement of chiral functionalities within materials is still
a scientific and technological challenge. Octavinylsilsesquioxane
(CH2CH)8Si8O12, as a well-defined molecular building block,
shows some salient features. Octavinylsilsesquioxane, as one of
the cubic silsesquioxanes family,3 is readily prepared in large
quantity, and with vinyl groups at each vertex is easily function-
alized, in which various hydrosilylations, alkyl, epoxy, isocyanate
and acrylate groups have been introduced.4 Furthermore, self-
assemblies of these functional silsesquioxane units are convenient
to construct various polyhedral oligomeric silsesquioxane (POSS)
materials.5 In particular, potentially site-directed multifunc-
tionality of octavinylsilsesquioxane within POSS is beneficial
for forming frameworks bearing desired chiral functional groups.
However, practical self-assembly of chiral-fuctionalized silses-
quioxane units for asymmetric catalysis is still rare.
The Cp*RhTsDPEN-functionalized polyhedral oligomeric
silsesquioxane, Cp*RhTsDPEN-POSS (3), was prepared
according to the procedure illustrated in Scheme 1. First, the
octavinylsilsesquioxane-derived 1 with benzyl chloride groups
at each vertex was synthesized readily via Grubbs coupling
method.4a The condensation of 1 with (R,R)-2 (Fig. S1, ESIw)
We are interested in mesoporous silica-supported heterogeneous
Catalysts.6 In this contribution, we utilize the potentially
The Key Laboratory of Resource Chemistry of Ministry of Education,
Shanghai Normal University, Shanghai, China.
E-mail: ghliu@shnu.edu.cn, HeXing-Li@shnu.edu.cn;
Tel: +8621 64321819
w Electronic supplementary information (ESI) available: Experimental
procedures and analytical data for obtained chiral alcohols. See DOI:
10.1039/c2cc31927f
Scheme 1 Preparation of catalyst 3.
c
6286 Chem. Commun., 2012, 48, 6286–6288
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Table 1 Asymmetric transfer hydrogenation of aromatic ketonesa
Entry
Ar
Conv.b (%)
Eeb (%)
TOFf
1
2
3
4
5
6
7
8
Ph
Ph
Ph
499 (88)c
93
84
96 (96)c
92d
96e
93
248
232
209
250
250
250
250
245
248
250
248
4-FC6H4
4-ClC6H4
4-BrC6H4
3-BrC6H4
4-MeC6H4
4-OMeC6H4
3-OMeC6H4
2-Naphthyl
99
499
499
499
97
499
499
499
92
92
95
91
94
93
92
Fig. 1 (a) TG/DTA curves of
Cp*RhTsDPEN and 3.
3 and (b) XPS spectra of
9
10
11
under basic conditions afforded the (R,R)-TsDPEN-modified
POSS material. Finally, the heterogeneous catalyst 3 was
successfully obtained via the deprotection of Boc groups
followed by the complexation with [Cp*RhCl2]2 (see ESIw).
Thermal gravimetric (TG) analysis (Fig. 1a) showed that a
new exothermic peak around 850 K with weight loss of 8%
could be assigned to the oxidation of rhodium chloride when
compared to the TG of (R,R)-TsDPEN-modified POSS
(Fig. S2, ESIw), which was consistent with 77.88 mg (0.76 mmol,
7.78%) of Rh loading per gram catalyst as detected by inductively
coupled plasma (ICP) analysis. Elemental analysis disclosed
N and S at 2.78 and 0.76 mmol per gram of catalyst as
calculated from mass% of N and S atoms (N 3.89, S 2.42%),
in which a 3.66 mole ratio of N to S suggested that five
functionalities of each octavinylsilsesquioxane within the POSS
formed the quaternary ammonium units.
a
Reaction conditions: catalyst (10.58 mg, 8.00 mmol of Rh based on
ICP analysis), HCO2Na (0.68 g, 10.0 mmol), ketone (2.0 mmol) and
2.0 mL water, reaction temperature (40 1C), reaction time (1.0 h).
Determined by chiral GC or HPLC analysis (see Fig. S6,m ESIw).
Data were obtained using the homogeneous Cp*RhTsDPEN
b
c
d
catalyst. Data were obtained using (R,R)-TsDPEN-modified POSS
e
plus [Cp*RhCl2]2 as a catalyst. Data were obtained using (R,R)-
f
TsDPEN-modified POSS plus RhTsDPEN as a catalyst. Turnover
frequency (TOF = mole of substrate converted per mole of Rh
complex per hour).
while similar enantioselectivity ascribed to the unchanged
chiral microenvironment was verified by XPS spectra.
To gain better insight into the nature of this heterogeneous
catalysis and to eliminate the effect of non-covalent adsorption
during the catalytic process, two comparable experiments
were carried out using (R,R)-TsDPEN-modified POSS plus
[Cp*RhCl2]2 and (R,R)-TsDPEN-modified POSS plus
Cp*RhTsDPEN as heterogeneous catalysts. It was found that
the former afforded the corresponding alcohol with 93%
conversion and 92% ee value (entry 2), while the latter gave
the alcohol with 84% conversion and 96% ee value (entry 3).
The former suggested that the catalyst synthesized by an in situ
complexation method could result in a catalytic performance.
Lower catalytic activity and enantioselectivity than those of 3
should be due to the fact that a small part of [Cp*RhCl2]2 had
not been coordinated during the catalytic process (and loss of
Rh was detected by ICP analysis in solution). The latter
system indicated that the chiral microenvironment had not
been changed a result of non-covalent adsorption. Lower
conversion than that of 3 suggested that part of catalyst did
not participate in the reaction due to blockage from physical
absorption.6b–d However, when both the above catalysts were
subjected to Soxhlet extraction, the reused catalysts were
essentially inactive. These facts ruled out the role of non-
covalent adsorption, demonstrating that the nature of hetero-
geneous catalysis was derived from the catalyst 3 itself.
An important feature of design of the heterogeneous
catalyst 3 was its easy separation via simple filtration and
the reused catalyst could still retain its catalytic activity and
enantioselectivity over multiple cycles. As shown in Fig. 2, the
catalyst 3 was quantitatively recovered via nanofiltration.
It was to be noted that, in twelve consecutive reactions, the
recycled catalyst 3 still afforded 96.2% conversion and 92.8%
ee value (Fig. S7, ESIw).
The incorporation of the chiral Cp*RhTsDPEN function-
ality within POSS could be confirmed by solid-state NMR
spectra. From the 13C CP MAS NMR spectrum (Fig. S4,
ESIw), typical peaks of the TsDPEN moiety (21 ppm for
ArCH2, 70–72 ppm for NCHPh and 126–136 ppm for C6H5)
and of the CpMe5 moiety (94 ppm for C5 and 8 ppm for
CpCH3) could be observed clearly. These observations were
similar to those of homogeneous Cp*RhTsDPEN,8 proving
that the heterogeneous catalyst 3 possessed the same single-site
well-defined active centers as Cp*RhTsDPEN. This behavior
could be further confirmed by XPS spectra (Fig. 1b), in which
the heterogeneous catalyst 3 had the same Rh 3d5/2 electron
binding energy as Cp*RhTsDPEN (309.39 vs. 3309.35 eV).
In addition, the 29Si MAS NMR spectrum (Fig. S5, ESIw)
showed that catalyst 3 possessed an organosilicate framework,
in which the exclusively characteristic T3 signal at À77.5 ppm
corresponding to T3 [C–Si(OSi)3] suggested that the structure
of cubic silsesquioxane was preserved and all Si species were
covalently attached to carbon atoms.9
Table 1 summarizes the catalytic performances of asym-
metric transfer hydrogenation of aromatic ketones10 without
Bu4NBr additive. In general, excellent conversions and high
enantioselectivities were obtained for all tested aromatic
ketones. Taking acetophenone as an example, it was found
that 499% conversion and 96% ee value of (R)-1-phenyl-
1-ethanol were obtained, in which the conversion was obviously
higher than that of the homogeneous Cp*RhTsDPEN while
the ee value was comparable to that of Cp*RhTsDPEN
(entry 1 vs. entry 1 in parentheses). The excellent catalytic
activity could be attributed to the phase transfer function of
the quaternary ammonium salt in a two-phase reaction system
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6286–6288 6287

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We are grateful to China NSF (20673072), Shanghai STDF
(10dj1400103 and 10jc1412300) and Shanghai MEC (12ZZ135
and S30406) for financial supports.
Notes and references
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Entry
Substrate
Product
Conv.b (%)
Eeb (%)
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1
499
92
2
3
4
99
94
99
499
99
92
97
5
499
a
Reaction conditions: catalyst (10.58 mg, 8.00 mmol of Rh based on
ICP analysis), HCO2Na (0.68 g, 10.0 mmol), ketone (2.0 mmol) and
2.0 mL water, reaction temperature (40 1C), reaction time (1.0 h).
b
Determined by chiral HPLC analysis (Fig. S8, ESIw).
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applied to the asymmetric transfer hydrogenation of ketones
and analogues (Table 2). The results showed that all tested
ketones and analogues gave excellent conversions and good
enantioselectivities, with yields obviously higher than those
of Cp*RhTsDPEN without Bu4NBr, and with similar
enantioselectivities.10a
In conclusion, we employed a facile approach to prepare a
bifunctional octavinylsilsesquioxane-based heterogeneous
chiral catalyst, which exhibited excellent catalytic activities
and high enantioselectivities in the asymmetric transfer hydro-
genation of aromatic ketones and analogues. In particular, the
anchoring of the quaternary ammonium salt onto POSS
significantly enhanced the catalytic efficiency in a two-phase
reaction system. More importantly, the heterogeneous catalyst
could be recovered easily and reused repeatedly. The recycled
catalyst (reused 12 times) still showed high catalytic efficiency
without affecting obviously its enantioselectivity, thus showing
good potential in industrial application.
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6288 Chem. Commun., 2012, 48, 6286–6288
This journal is The Royal Society of Chemistry 2012