Solid sodium stannate as a high-efficiency superbase catalyst for anti-Markovnikov hydroamination and hydroalkoxylation of electron-deficient olefins under mild conditions

Article abstract of DOI:10.1016/j.catcom.2010.12.030

A solid superbase with H- above 26.5 has been obtained through thermal treatment of sodium stannate hydrate. It was found to be an efficient catalyst for anti-Markovnikov hydroamination and hydroalkoxylation of electron-deficient olefins under mild conditions.

Full text of DOI:10.1016/j.catcom.2010.12.030

Catalysis Communications 12 (2011) 712–716
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Solid sodium stannate as a high-efficiency superbase catalyst for anti-Markovnikov
hydroamination and hydroalkoxylation of electron-deficient olefins under
mild conditions
a,b
Shuguo Zhang a, Yudan Wei a, Shuangfeng Yin a, , Chak-tong Au
⁎
a
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, Hunan, China
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A solid superbase with H_ above 26.5 has been obtained through thermal treatment of sodium stannate
hydrate. It was found to be an efficient catalyst for anti-Markovnikov hydroamination and hydroalkoxylation
of electron-deficient olefins under mild conditions.
Received 16 November 2010
Received in revised form 16 December 2010
Accepted 17 December 2010
Available online 29 December 2010
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Solid superbase
Sodium stannate
Anti-Markovnikov
Electron-deficient olefins
Acrylonitrile
1. Introduction
electron-deficient olefins is common in the synthesis of drug inter-
mediates and organic compounds of industrial interest [12–17]. The
According to the definition of Tanabe et al. [1], solid superbases are
materials that possess basic sites with strength (H_) higher than 26
(H_ corresponding to the pKa value of the indicator). Materials of this
kind have attracted much attention because they catalyze a number of
organic reactions under mild and environment-benign conditions[2–8].
In the past decade, solid superbases such as Eu2O3/Al2O3 [2], KNO3/ZrO2
[3], KNO3/γ-Al2O3 [4], K/MgO [5,6], Na/NaOH/γ-Al2O3 [7], NaN3/γ-Al2O3
[8], CaO and SrO [9] have been reported. They were used as catalysts for
olefin isomerization [3–5], side-chain alkylation [6], transesterification
[1,7,8], Michael addition [10], and Aldolcondensation [11]. Traditionally,
the preparation of solid superbases can be difficult and expensive. For
example, CaO and SrO have to be evacuated at 850–900 °C for the
generation of superbasic sites [9]. On the other hand, solid superbases K/
MgO and Na/NaOH/γ-Al2O3 that are highly active for alkene isomeri-
zation are tedious and troublesome to prepare [5–7]. Moreover, storage
is a big problem for most of the known solid superbases because they are
sensitive to CO2 and moisture [5–7]. It is hence desirable to design and
develop solid superbases that are simple to prepare and easy to store.
The formation of carbon–nitrogen and carbon–oxygen bonds
through anti-Markovnikov hydroamination and hydroalkoxylation of
general reaction step is shown in Scheme 1. With the addition of N–H or
O–H bondto carbon–carbon double bond in a controlled manner, amines
andethers can be generated in a convenientand atom-economic manner
[18]. Furthermore, through hydrolysis and reduction of nitriles, one can
produce carboxylic acids and amines, respectively. In general, the
reactions proceed in the presence of soluble base catalysts such as alkali
hydroxide [19], alkali alkoxide [20], tetraalkylammonium hydroxide
[20], and triphenylphosphine [21]. The catalysts, however, have to be
neutralized before the extraction of products, leading to waste
generation, catalyst loss, and yield decline. To avoid catalyst loss, the
use of heterogeneous base catalysts such as alkaline earth oxides and
hydroxides, alumina-supported KF and KOH [22–24], Mg(Li)–Al hydro-
talcites [25–29], and anion exchange resins [30] has been explored. It has
been pointed out that for good efficiency over these solid base catalysts,
high temperature [28], prolonged time [27] and excess nucleophile to
electron-deficient olefin ratio [23,24] were required.
It is noted that solid superbases are rarely reported as catalysts [3–8].
Herein, we report the generation of a solid superbase through short-time
calcination of sodium stannate hydrate under nitrogen at a relatively low
⁎
Corresponding author. College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, Hunan, China. Tel.: +86 731 88821310; fax: +86 731
88821310.
Scheme 1. Anti-Markovnikov hydroamination and hydroalkoxylation of monosubsti-
tuted olefins.
E-mail address: sfyin73@yahoo.com.cn (S. Yin).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.12.030

S. Zhang et al. / Catalysis Communications 12 (2011) 712–716
713
temperature. The as-prepared material shows high catalytic efficiency
towards the anti-Markovnikov hydroamination and hydroalkoxylation of
electron-deficient olefins, giving 100% selectivity to the product. This is
the first report on anti-Markovnikov hydroamination and hydroalkoxyla-
tion of electron-deficient olefins catalyzed by a solid superbase. In terms
of catalytic activity, the solid superbase catalyst is superior to any of the
heterogeneous base catalysts reported in the literatures [22–29].
2. Experimental
2.1. Chemicals and reagents
The reagents and chemicals (analytic grade unless stated other-
wise) were obtained from Kemiou Chemical Reagent Company
(Tianjin, China) and Tianjin Damao Chemical Reagent Co., Ltd. They
were subject to dehydration using 3 Å molecular sieves at room
temperature. Sodium stannate (chemically pure) was purchased from
the Beijing Hongxing Chemical Plant.
Fig. 1. TG-DSC curves of sodium stannate hydrate.
of 350 to 930 °C is not significant. The results suggest the complete
removal of water from Na2SnO3·3H2O at 350 °C. It is believed that the
calcination of materials has a role to play in material activation for the
generation of superbasic sites [5–9]. To find out the optimized
conditions for calcination, we calcined samples of sodium stannate
hydrate at 300, 350, 400, 500 and 600 °C for a period of 1–4 h. The
superbasic sites of each sample were measured by means of Hammett
indicator–Benzoic acid titration [9], and the catalytic activity of the
samples was evaluated in the anti-Markovnikov hydroalkoxylation of
acrylonitrile with phenol under equal conditions.
2.2. Catalyst preparation
To prepare the catalyst, sodium stannate hydrate was calcined in a
tube furnace at 300–600 °C for 1–4 h under a N2 flow (99.99% purity,
Changsha Gas Co., China). The basicity of the as-prepared materials was
measured by the Hammett indicator method (see Supplementary data).
2.3. Catalyst evaluation
3.2. Effects of calcination temperature and time
Tests for catalytic activity were performed in a glass reactor equipped
with a magnetic stirrer. Electron-deficient olefin (6 mmol), nucleophile
(6 mmol) and catalyst (0.035 g) were added and the mixture was stirred
at room temperature for an appropriate period of time. In the case of using
phenol and acrylonitrile (molar ratio of 1:4) as reactants, the reaction was
carried out in a 50 ml one-necked glass flask at 80 °C fixed with a water-
cooled condenser. After the reaction, the catalyst was separated from the
mixture by centrifugation, and the reaction mixture was analyzed using a
gas chromatograph (Agilent Technologies 7820) equipped with a FID and
a HP-5 capillary column (30 m×0.32 mm×0.25 μm). The conversion of
olefins was determined by using biphenyl as the internal standard. All the
products were analyzed over an Agilent 6890-5973 MSD GC-MS
equipment, and confirmed by H1 NMR. The NMR spectra were recorded
at 25 °C over an INOVA-400 M (Varian) instrument calibrated using
tetramethylsilane (TMS) as the internal standard.
Fig. 2 depicts the effects of calcination temperature and time on
activity along with the amount of superbasic sites of the obtained
2.4. Catalyst characterization
TG-DSC analysis was performed on a NETZSCH-STA-449C instrument
(Operation condition: N2, 8 K/min heating rate). The CO2 temperature-
programmed desorption (CO2-TPD) was performed to determine the
basicity of sodium stannate. The experiments were conducted on a
Micromeritics 2920 apparatus using a thermal conductivity detector
(TCD). The powder X-ray diffraction (XRD) experiment was conducted on
a Rigaku Automatic Diffractometer (Rigaku D-MAX) using monochro-
matized Cu Kα radiation (λ=0.15406 nm).
3. Results and discussion
3.1. TG-DSC
The TG-DSC curves of a sample of sodium stannate hydrate
obtained under the protection of nitrogen are shown in Fig. 1. There
are two endothermic peaks in the DSC profile corresponding to two
stages of weight loss. The weak endothermic peak from 53 to 93 °C is
attributed to water desorption, while the sharp one at 292 °C to the
disassembling of water from Na2SnO3·3H2O. The TG curve shows that
the weight loss at 350 °C is around 20%, whereas that within the range
Fig. 2. (A) Effect of calcination temperature on the activity and amount of superbasic sites
of sodium stannate calcined for 2 h. (B) Effect of calcination time on the activity and
amount of superbasic sites of sodium stannate calcined at 350 °C. Reaction conditions:
acrylonitrile, 0.04 mol; phenol, 0.01 mol; catalyst, 0.100 g; at 80 °C for 120 min.

714
S. Zhang et al. / Catalysis Communications 12 (2011) 712–716
catalysts. Despite not far apart in superbase strength (all in the range
of 26.5≤H_b33 as determined by using 4-chloroaniline and triphe-
nylmethane as indicators, see Supplementary data), the calcined
samples are different in the amount of superbasic sites as well as in
catalytic activity. One can see from Fig. 2(A) that at a calcination time
of 2 h, the effect of calcination temperature on superbasicity parallels
to that on catalytic activity. It is noted that the sample calcined at
350 °C shows the highest superbasicity and acrylonitrile conversion.
On the other hand, it is observed from Fig. 2 (B) that at a calcination
temperature of 350 °C, the amount of superbasic sites and catalytic
activity first increases, reaching a maximum at a calcination time of
2 h, and then decreases. The trend is similar to that observed by Guo et
al. over solid base catalysts [31]. It is rational to infer that the ample
superbasic sites thus generated are responsible for the high catalytic
activity. Hence, the optimum calcination temperature and time for the
generation of a superbase sodium stannate catalyst are 350 °C and 2 h.
The catalyst prepared under such a condition was chosen for further
investigation.
Fig. 4. XRD patterns of sodium stannate before and after calcination at 200, 350, and
500 °C for 2 h. Diffraction peaks belonging to the Na2SnO3·3H2O phase are marked (●);
diffraction peaks corresponding to anhydrous Na2SnO3 are marked (■).
catalyst apt to catalyze the anti-Markovnikov hydroamination and
hydroalkoxylation of monosubstituted olefins.
3.3. CO2-TPD
3.5. Catalytic activity
The results of CO2-TPD over sodium stannate before and after
calcination are depicted in Fig. 3. The uncalcined sample has poor ability
for CO2 adsorption, showing only low-intensity peaks at around 130 and
580 °C. Over the calcined one, however, intense desorption peaks are
observed at ca. 300, 530, and 700 °C. The desorption of CO2 at 700 °C is a
clear indication of superbasicity [4]. Obviously, the CO2-TPD results are
in good agreement with the results obtained by the Hammett indicator
method (Fig. 2). It is apparent that the thermal treatment has resulted in
the generation of superbasic sites on sodium stannate.
As listed in Table 1, the conversion of olefins under the adopted
reaction conditions is 100% except for the cases of tertiary butanol,
tert-butylamine, aniline and benzylamine. The anti-Markovnikov
mono-addition products (viz. β-cyanoethyl ethers) are formed with
100% selectivity. When tertiary butanol, tert-butylamine, aniline and
Table 1
Catalytic activities of calcined sodium stannate towards the anti-Markovnikov
hydroamination and hydroalkoxylation of electron-deficient olefins with various
amines and alcohols.a
3.4. XRD
Entry Olefin
Nucleophile Time
(min)
Productb
Conv.
(%)
Fig. 4 shows the XRD patterns of sodium stannate hydrate before
and after calcination at 200, 350, and 500 °C for 2 h. The XRD pattern
of the sample calcined at 200 °C is almost identical to that of the
uncalcined sample, showing a sharp peak at 2θ of 18.7° and four small
peaks at 35.7, 38.0, 49.2 and 58.6° that are attributable to Na2SnO3·3-
H2O (JCPDS 01-1115 card). The results are in accordance with the
results of TG-DSC investigation (Fig. 1). After calcination at 350 °C, the
peaks of Na2SnO3·3H2O weaken, and peaks at 2θ of 16.2°, 37.4° and
39.3° appear. Further calcination at 500 °C does not result in any
change of the XRD pattern. Based on the TG-DSC data, the new pattern
is related to anhydrous Na2SnO3. With due consideration given to the
results of superbasic sites and catalytic performance (Fig. 2), one can
deduce that the removal of water from Na2SnO3·3H2O during
calcination at 350 °C would result in the creation of superbasic sites.
Despite the formation mechanism and nature of the active centres are
unclear, one can see that the anhydrous Na2SnO3 is a superbasic
1
CH2=CHCN
MeOH
40
100
2
3
CH2=CHCN
CH2=CHCN
EtOH
PrOH
40
60
100
100
4
5
6
7
8
CH2=CHCN
CH2=CHCN
CH2=CHCN
CH2=CHCN
CH2=CHCN
i-PrOH
BuOH
60
120
240
120
180
100
100
57
t-BuOH
PhCH2OH
100
100
9
10c
CH2=CHCN
CH2=CHCN
120
120
100
86
PhOH
Et2NH
11
12
CH2=CHCN
CH2=CHCN
120
30
100
100
n-PrNH2
t-BuNH2
13
CH2=CHCN
240
51
14
15
16
CH2=CHCN
CH2=CHCN
CH2=CHCN
BuNH2
60
240
240
100
0
PhNH2
PhCH2NH2
47
17
CH2=CHCOOCH3 MeOH
120
120
100
100
18
CH3COOCH=CH3
MeOH
a
Reaction conditions: 6 mmol nucleophile, 6 mmol electron-deficient olefin, and
0.035 g catalyst, RT.
b
The selectivity to the target products was 100%.
PhOH: 0.01 mol, acrylonitrile: 0.04 mol, reaction conducted at 80 °C.
c
Fig. 3. CO2-TPD profiles of sodium stannate (a) before and (b) after calcination.

S. Zhang et al. / Catalysis Communications 12 (2011) 712–716
715
benzylamine are used as substrates, acrylonitrile conversion is 57%,
51%, 0% and 47% respectively after a reaction period of 240 min
(entries 6, 13, 15 and 16). For tertiary butanol and tert-butylamine,
the decline of activity could be ascribed to steric hindrance that
weakens nucleophilic ability. It should be pointed out that there is
steric hindrance in the case of cyclohexanol but in a much lower
degree. That is why cyclohexanol conversion can still be 100%, a value
similar to that reported by Jiang et al. [32]. At this moment, we cannot
give a rational explanation for the poor reactivity of benzylamine over
the superbase catalyst, and investigation is being undertaken on this
aspect.
3.6. Catalyst recyclability
The recyclability of sodium stannate catalyst was tested in the
hydroalkoxylation of acrylonitrile with methanol. Table 2 shows the
results of five runs under the adopted reaction conditions. After each
run, the catalyst was separated from the product by centrifugation, and
was immediately reused (with the addition of fresh reactants) without
any treatment. It was observed that the conversion of acrylonitrile
declines (from 100%) to 98% in the second run and to ca. 59% in the
fourth and fifth runs. The deactivation of the catalyst is likely due to the
blocking of the basic and superbasic sites by organic entities. With
decline in the availability of active centres, there is a decrease in
acrylonitrile conversion. Once the organic entities are removed through
the adopted calcination process, the spent catalyst recovers in activity as
reflected in the 100% recovery of acrylonitrile conversion (see entries 6
and 7 in Table 2). In other words, through a simple calcination in
nitrogen (or in oxygen, results not shown) atmosphere, the catalyst can
be regenerated and recycled. We also investigated the surface basicity of
the regenerated catalyst and found that it is comparable to that of the
fresh catalyst. Therefore, the superbase catalyst is highly recyclable.
Scheme 2. Plausible mechanism for the hydroalkoxylation of acrylonitrile with alcohols.
alcohol. A plausible mechanism is hence proposed for the hydroalk-
oxylation of acrylonitrile with alcohols as depicted in Scheme 2. The
surface O2− ions of the solid superbase act as active basic sites to
abstract H+ from alcohol, resulting in the generation of alkoxide
anions. Then an alkoxide anion attacks the carbocation of the C=C
bond of acrylonitrile to form a 3-alkoxypropanenitrile anion on the
surface of the catalyst. Finally, the 3-alkoxypropanenitrile anion
abstracts H+ to yield 3-alkoxypropanenitrile. Much work is being
conducted in our laboratory for the clarification of the mechanistic
steps of this catalytic system.
4. Conclusions
A novel solid superbase was obtained simply by thermal treatment
of sodium stannate hydrate. The material possesses ample superbasic
sites. With in situ generation of the catalyst for the target reactions,
storage of the superbasic material is not a problem. The calcined
sodium stannate shows high catalytic activity and selectivity towards
the anti-Markovnikov addition of N–H and O–H bonds across the C=C
bonds of electron-deficient olefins. The catalyst is non-toxic, and
shows good thermal stability and recyclability. Moreover, it was found
that there is a relationship between the amount of superbasic sites
and catalytic activity. It is envisaged that the solid superbase will find
wide applications in the sectors of catalysis and fine chemical
industry. Further investigation is being conducted to clarify the
formation mechanism as well as the nature of the superbasic sites.
3.7. Plausible reaction mechanism
Hattori et al. [23] reported a mechanism on cyanoethylation of
alcohols over metal oxide catalysts, where an H+ is abstracted from the
hydroxyl group of alcohols by a basic site to form surface alkoxide anion
that is believed to be stabilized on the metal cation, then the alkoxide
anion reacts with acrylonitrile to form 3-alkoxypropanenitrile anion,
followed by picking up H+ from the surface to yield 3-alkoxypropane-
nitrile. They also deduced that the abstraction of an H+ was a rapid step
while the formation of 3-alkoxypropanenitrile anion was a slow step.
Choudary et al. [29] also investigated the reaction mechanism on the
cyanoethylation of alcohols with acrylonitrile over Mg–Al–O–But–
Hydrotalcite catalyst. They regarded that the incorporation of a ButO
anion enhances catalyst basicity. With the activation of alcohol, there is
the facile formation of alkoxide intermediate that reacts with
acrylonitrile to generate 3-alkoxypropanenitrile anion [29].
Acknowledgments
The financial supports of the National Natural Science Foundation of
China (Grant No. 20873038), and the Doctoral Program Foundation of
Institutions of Higher Education of China (Grant No. 200805320001) are
gratefully acknowledged. C.T. Au thanks the Hunan University for an
adjunct professorship.
From the XPS results (see Supplementary data of XPS analysis), we
can deduce that there are O2− ions on the superbase catalyst. We
speculate that these ions are beneficial for the abstracting of H+ from
Table 2
Appendix A. Supplementary data
Study of catalyst recyclability.a
Cycle
Conversion (%)
Supplementary data (including superbasicity measurement, XPS,
N2 adsorption/desorption and BET measurement and 1H NMR data) to
this article can be found online at doi:10.1016/j.catcom.2010.12.030.
1
2
3
100
98
81
4
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59
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