Nanocrystalline MgO catalysts for the Henry reaction of benzaldehyde and nitromethane

Article abstract of DOI:10.1016/j.molcata.2011.03.018

Nanocrystalline MgO particles of about 3 nm were examined as catalysts for the Henry reaction between benzaldehyde and nitromethane at 273 K. The surface area of nanocrystalline MgO was greater than 600 m² g^-1 except after thermal tr

Full text of DOI:10.1016/j.molcata.2011.03.018

Journal of Molecular Catalysis A: Chemical 341 (2011) 22–27
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Nanocrystalline MgO catalysts for the Henry reaction of benzaldehyde and
nitromethane
Yuanzhou Xi, Robert J. Davis∗
Department of Chemical Engineering, University of Virginia, 102 Engineers’ Way, PO Box 400741, Charlottesville, VA 22904-4741, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanocrystalline MgO particles of about 3 nm were examined as catalysts for the Henry reaction between
benzaldehyde and nitromethane at 273 K. The surface area of nanocrystalline MgO was greater than
Received 4 February 2011
Received in revised form 22 March 2011
Accepted 25 March 2011
Available online 6 April 2011
600 m2 g−1 except after thermal treatment in N2 at 823 K, which reduced the surface area to 359 m2 g−1
.
The areal rate of the Henry reaction over nanocrystalline MgO at 273 K was 0.015 0.002 ␮mol m−2 s−1
and was independent of pretreatment temperature over the range of 523–823 K. Addition of (S)-BINOL
to nanocrystalline MgO has been reported previously to favor the production of one enantiomer of the
Henry reaction. In the current work, negligible enantiomeric excess was observed on (S)-BINOL-modified
nanocrystalline MgO catalysts, regardless of the source of nanocrystalline MgO, the MgO activation
temperature, the reaction temperature, the sequence of reactant addition or the loading of (S)-BINOL.
Adsorption of l-proline onto nanocrystalline MgO catalyst induced a small enantiomeric excess in the
Henry reaction products, but the catalytic activity of the catalyst was low.
Keywords:
Magnesia
Henry reaction
Benzaldehyde
Nitromethane
Chiral catalysis
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
MgO materials depend significantly on their preparation methods
[9–18], the influences of pretreatment conditions and reaction con-
The addition of a nitroalkane to a carbonyl compound, namely
the Henry (or nitroaldol) reaction, is an important C–C bond forma-
tion reaction that produces ␤-nitroalcohols with up to two adjacent
chiral carbons. Reduction of the nitro group in ␤-nitroalcohols
can lead to the formation of aminoalcohols, which occur widely
as natural and synthetic products as well as chemical intermedi-
ates [1–7]. Developing efficient catalysts that form products with
high enantiomeric excess (ee) is of great interest in synthetic
organic chemistry and the pharmaceutical industry. Homogeneous
catalysts, such as a rare earth BINOL complex [1,2], a dinuclear
zinc complex with a chiral semi-azacrown ligand [5,8], a copper
acetate–bis(oxazoline) complex [6] and a bifunctional amine-
thiourea organocatalyst [3,4], are reported to be effective for chiral
Henry reactions. Heterogeneous catalysts for the Henry reaction,
whether chiral or achiral, are much less studied. One of the potential
advantages of a heterogeneous catalyst is that it can be easily recov-
ered from the reaction medium and hopefully recycled. Choudary
et al. reported that nanocrystalline MgO modified with the chiral
ligand (S)-BINOL is an effective and reusable catalyst for the Henry
reaction with comparable ee to homogeneously-catalyzed systems
[9]. However, the nature of the active sites on the modified MgO
surfaces is unknown at this time. Since the catalytic properties of
ditions on the activity and selectivity of the Henry reaction need to
be explored.
In this work, the effects of catalyst source, catalyst activation
temperature, reaction temperature, ligand loading and chemical
addition sequence were explored for the Henry reaction of ben-
zaldehyde and nitromethane over MgO and chirally-modified MgO
catalysts.
2. Experimental methods
2.1. Catalyst preparation and characterization
Nanocrystalline MgO was purchased from Strem Chemicals
(product name: magnesium oxide nanopowder (high surface
area)), designated as nano-MgO-1, and from NanoScale Materials
Inc. (product name: NanoActive Magnesium Oxide Plus), desig-
nated as nano-MgO-2. The detailed synthesis method used to
prepare these two MgO materials can be found elsewhere [15]. The
thermal activation of the MgO catalysts was performed by heating
0.5 g of as-received nano-MgO-1 or nano-MgO-2 in 100 cm3 min−1
flowing N2 (GT&S, 99.999%) at 5 K min−1 and holding at vari-
ous temperatures for 1 h. The modification of MgO by (S)-BINOL
(Sigma–Aldrich, 99%) was conducted in the same way as reported
by Choudary et al. [9], i.e. (S)-BINOL was simply mixed with the
thermally-pretreated MgO catalyst in the solvent THF. In addition,
l-proline (Sigma–Aldrich, 99%) modified MgO was also used as a
∗
Corresponding author. Tel.: +1 434 924 6284; fax: +1 434 982 2658.
E-mail addresses: yx8f@virginia.edu (Y. Xi), rjd4f@virginia.edu (R.J. Davis).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.03.018

Y. Xi, R.J. Davis / Journal of Molecular Catalysis A: Chemical 341 (2011) 22–27
23
0.3
0.2
0.1
catalyst for the Henry reaction. Nano-MgO-2 (0.5 g), which was
thermally-treated to 723 K, was magnetically-stirred in a solution
of 1.428 g l-proline and 45 mL methanol (Sigma–Aldrich, anhy-
drous, 99.8%) at room temperature for 1 h. The solid catalyst was
then separated by centrifugation, washed with 45 mL THF, and used
directly in the Henry reaction. Prior to characterizing the material,
l-proline modified nano-MgO-2, it was treated in 100 cm3 min−1
flowing N2 for 24 h at room temperature.
a
(R)-1-phenyl-2-nitroethanol
(S)-1-phenyl-2-nitroethanol
0.0
0.9
At 24 h
b
The physisorption of N2 was performed on a Micromeritics
ASAP 2020 at 77 K. The BET (Brunauer–Emmett–Teller) and BJH
(Barrett–Joyner–Hallender) methods were used to calculate the
surface area and pore size distribution of the samples, respectively.
The XRD (X-ray diffraction) patterns were measured on a Scin-
benzaldehyde conversion: 60.7%
ee: 85.5%
0.6
0.3
0.0
˚
tag XDS 2000 diffractometer using Cu K␣ radiation (ꢀ = 1.54 A).
18
20
22
24
The samples were scanned in the continuous mode from 4◦ to
72◦ at a scan rate of 2◦ min−1. The IR (infra-red) spectra were
recorded in the diffuse reflectance mode on a Bruker Vertex 70
spectrometer equipped with a Harrick Praying Mantis diffuse
reflectance accessory using an MCT detector. The spectra were
averaged from 32 scans in the range of 400–4000 cm−1 at the res-
olution of 4 cm−1. The magnesium and nitrogen contents of the
nano-MgO-2 loaded with l-proline were determined by ICP anal-
ysis and combustion analysis, respectively (Galbraith Laboratories,
Knoxville, TN).
Time / min
Fig. 1. HPLC chromatograms of (a) racemic Henry reaction products of 0.0102 mL
benzaldehyde and 1 mL nitromethane catalyzed by 1 drop of tetramethylguani-
dine [19] at 273 K for 40 min; (b) enantioselective Henry reaction products at
24 h; reaction conditions: 0.267 g Cu(OAc)2·H2O and 0.0401 mL 2,2-Bis((4S)-(−)-
4-isopropyloxazoline)propane as ligand were dissolved in 6.822 mL methanol and
stirred for 1 h at room temperature then 0.101 mL benzaldehyde and 2.923 mL
nitromethane were added and reacted for 24 h [6].
3. Results and discussion
The Henry reaction of benzaldehyde and nitromethane was
conducted in a 100 mL 3-necked round-bottomed flask that was
magnetically-stirred at 800 rpm under N2 atmosphere. Gener-
ally, 0.5 g nano-MgO-1 or nano-MgO-2 catalyst was thermally
pretreated and transferred to the reactor under N2 atmosphere
for the Henry reaction with 0.408 mL (4 mmol) of benzaldehyde
(Sigma–Aldrich, 99.5%) and 1.074 mL (20 mmol) of nitromethane
(Sigma–Aldrich, 99.0%) as reactants and 20 mL tetrahydrofuran
(THF, Sigma–Aldrich, anhydrous, 99.9%) as solvent at 273 K unless
otherwise indicated. High-performance liquid chromatography
(HPLC, e2695 Separations Module equipped with a Waters 2489
UV/Visible Detector at 215 nm) was used to quantify the reactants
and products. A CHIRALPAK IB column (4.6 mm I.D., 250 mm L;
Chiral Technologies Inc.) operating at 298 K with 1.0 mL min−1 n-
hexane/2-propanol (95/5, v/v)as mobilephasewas used to separate
the reactants and products. The samples from the reaction were fil-
tered (0.2 ␮m) and diluted in mobile phase (sample/mobile phase,
1/10, v/v) before analysis. Since the reaction products (R)-1-phenyl-
2-nitroethanol and (S)-1-phenyl-2-nitroethanol are not available
commercially, tetramethylguanidine (Acros, 99%) was used as a
homogeneous catalyst to react benzaldehyde at various concen-
trations with nitromethane to produce racemic products [19]. A
representative HPLC chromatogram of racemic products is dis-
played in Fig. 1a. The racemic products were easily resolved and
the calibration curves of (R)-1-phenyl-2-nitroethanol and (S)-1-
phenyl-2-nitroethanol were based on a mass balance between the
converted benzaldehyde and the formation of racemic product. The
identification of (R)-1-phenyl-2-nitroethanol versus (S)-1-phenyl-
2-nitroethanol was based on an known Henry reaction catalyzed by
a copper acetate–bis(oxazoline) complex [6]. In that experiment,
0.267 g Cu(OAc)2·H2O (Sigma–Aldrich, 99.99%) and 0.0401 mL 2,2-
bis((4S)-(−)-4-isopropyloxazoline)propane (Sigma–Aldrich, 96%)
were first dissolved in 6.822 mL methanol and stirred for
1 h at room temperature before adding 0.101 mL benzalde-
hyde and 2.923 mL nitromethane and stirring for an additional
24 h. A representative HPLC chromatogram of the enantiose-
lective products of the Henry reaction is shown in Fig. 1b.
By comparison to previously reported results [6], the HPLC
peaks appearing at 18.8 and 22.2 min are attributed to (R)-1-
phenyl-2-nitroethanol and (S)-1-phenyl-2-nitroethanol, respec-
tively. The enantiomeric excess is reported here based on
(S)-1-phenyl-2-nitroethanol.
3.1. Catalyst characterization
X-ray diffraction patterns of the nano MgO materials are pre-
sented in Fig. 2. The as-received nano-MgO-1 and nano-MgO-2
samples exhibit two broad peaks at 42.6◦ and 61.6◦, which cor-
respond to the (2 0 0) and (2 2 0) reflections of cubic MgO. The
crystallite sizes of the MgO samples were estimated from a line
broadening analysis of the XRD patterns in Fig. 2 and the results
summarized in Table 1 indicate both samples were 3 nm in crys-
tallite size. Although the crystal structure of the l-proline modified
nano-MgO-2 (Fig. 1c) was still cubic MgO, the size of the modified
sample increased slightly to 3.4 and 3.6 nm along the (2 0 0) and
(2 2 0) directions.
The N2 adsorption–desorption isotherms of nano-MgO-1 and
nano-MgO-2 are provided in Fig. 3a. Both of the samples exhibited
type II adsorption isotherms with a small hysteresis loop on the
desorption branch. The nano-MgO-1 and nano-MgO-2 had high BET
surface areas of 619 m2 g−1 and 775 m2 g−1, respectively (Table 1),
which is consistent with previously reported results [9,15]. The
pore size distributions of nano-MgO-1 and nano-MgO-2 (Fig. 3b)
revealed pores in nano-MgO-1 in the range of 2–6 nm with a max-
imum at ∼2.7 nm, whereas nano-MgO-2 had a slightly narrower
(200)
(220)
(c)
(b)
(a)
0
10
20
30
40
50
60
70
80
2θ / degrees
Fig. 2. XRD patterns of (a) nano-MgO-1; (b) nano-MgO-2; and (c) l-proline modified
nano-MgO-2.

24
Y. Xi, R.J. Davis / Journal of Molecular Catalysis A: Chemical 341 (2011) 22–27
Table 1
Physical properties of nano MgO catalysts.
Sample
BET (m2 g−1
)
Pore volume (cm3 g−1
)
D(2 0 0)d,e (nm)
D(2 2 0)d,f (nm)
Nano-MgO-1a
619
635
359
775
457
0.47
0.57
0.59
0.52
0.50
3.1
–
–
3.3
3.4
3.0
–
–
3.0
3.6
Nano-MgO-1 (723 K)b
Nano-MgO-1 (823 K)b
Nano-MgO-2a
l-Proline modified nano-MgO-2c
a
Catalysts were degassed at 523 K for 2 h before analysis.
b
c
Catalysts were thermal treated at 723 K or 823 K for 1 h under 100 cm3 min−1 N2 flowing and then degassed at 523 K for 2 h before analysis.
Catalyst were degassed at room temperature for 2 h before analysis.
d
The full width at half maximum (FWHM) of the XRD diffraction peaks was calculated by fitting the XRD patterns to pseudo-Voigt functions. Crystallite size was estimated
from Debye–Scherrer equation, using silicon as standard to correct the line broadening due to instrument.
e
Crystallite size along (2 0 0) direction.
Crystallite size along (2 2 0) direction.
f
and ∼1440 cm−1 are thus assigned to the asymmetric and sym-
pore size distribution in the range of 2–5 nm with a maximum
at ∼2.5 nm. Although thermally-treating nano-MgO-1 at 723 K for
1 h did not significantly alter its surface area, treatment at 823 K
for 1 h decreased the surface area to 359 m2 g−1, as summarized
in Table 1.
metric stretches of carboxylate anions [22,23]. The C–H stretches
that appeared in the range of 2800–3000 cm−1 are also attributed
to the adsorbed l-proline moiety on nano-MgO-2.
Modifying nano-MgO-2 with l-proline decreased its sur-
face area to 457 m2 g−1 compared to the untreated precursor
(775 m2 g−1). The decrease in surface area is attributed to a sub-
stantial loading of l-proline on the nano-MgO-2 surface, which
was determined to be 23.9 wt% from elemental analysis and
corresponds to a surface coverage of 4.44 ␮mol m−2. Infrared
spectroscopy was also used to characterize nano-MgO-2 and its
l-proline modified analogue. As shown in Fig. 4, the as-received
nano-MgO-2 exhibited features associated with adsorbed CO2
[13,20] and H2O molecules, presumably because of its high basicity.
The absorption bands at ∼1400 and ∼1470 cm−1 can be ascribed to
the various carbonate species, whereas the band at ∼1630 cm−1 is
attributed to the bending mode of adsorbed H2O molecules. The
hydroxyl stretching region in the range of 2700–3800 cm−1 is typi-
cal on MgO materials [15,21]. New bands can be observed in Fig. 4b
that are assigned to the adsorbed l-proline. The bands at ∼1600
3.2. Henry reaction
Since IR spectroscopy revealed the presence of CO2 and H2O
on the surface of MgO, the effect of thermal pretreatment on the
rate of the Henry reaction was explored. The nano-MgO-1 cata-
lyst was thermally activated in N2 at temperatures ranging from
523 to 823 K and transferred to the reactor without exposure to
air. The initial reaction rate and the conversion of benzaldehyde
at 4 h as a function of pretreatment temperature are presented in
Fig. 5. The initial rate of benzaldehyde reaction increased from 0.55
to 0.64 mmol g−1 min−1 by increasing the thermal pretreatment
temperature from 523 to 723 K; however, the rate decreased to
0.27 mmol g−1 min−1 after a pretreatment at 823 K. The benzalde-
hyde conversion after 4 h was about 92% for the lower pretreatment
temperatures but decreased to 68% at the catalyst pretreatment
temperature of 823 K. The lower reactivity and lower benzalde-
hyde conversion observed after treating nano-MgO-1 at 823 K is
explained by its lower surface area (Table 1). Montero et al. also
reported that MgO nano particles prepared solvothermally had
surface area of 580 m2 g−1, which decreased to 140 m2 g−1 after
calcination at 873 K [24]. The reaction rates normalized by sur-
face area of nano-MgO-1 pretreated at 523, 723 and 823 K were
quite similar (0.015, 0.017 and 0.013 ␮mol m−2 s−1, respectively).
Although the surface area of nano-MgO-1 decreased significantly
after thermal pretreatment at 823 K, the observed rate per exposed
surface area was relatively constant over the range of pretreat-
ment temperatures used here. From the results presented in Fig. 5,
the optimum catalyst activation temperature was 723 K for the
nano-MgO-1 catalyst because of the high surface area of the
400
a
300
200
100
nano-MgO-1
nano-MgO-2
0
0.0
0.2
0.4
0.6
0.8
1.0
P/P
0
3
2
1
0
b
nano-MgO-1
nano-MgO-2
1
10 100
200
Pore width / nm
Fig. 3. N2 adsorption (black symbols) and desorption (white symbols) isotherm
(a) and BJH pore size distribution from N2 adsorption branches (b) of nano MgO
catalysts.
Fig. 4. Diffuse reflectance infrared Fourier transformed spectra (DRIFTS) of (a) nano-
MgO-2 and (b) l-proline modified nano-MgO-2.

Y. Xi, R.J. Davis / Journal of Molecular Catalysis A: Chemical 341 (2011) 22–27
25
100
80
60
40
20
0
is about 16% of the initial activity of nano-MgO-1 without added
(S)-BINOL (Fig. 4). The lower initial activity of (S)-BINOL modified
nano-MgO-1 is likely due to the adsorption of the ligand, (S)-BINOL,
which has a pKa value of 10.28 [25], onto the basic surface of MgO.
After 4 h of reaction, benzaldehyde reached 68% conversion in the
presence of (S)-BINOL, but the ee of the products was negligible at
−1.4%. The low ee observed in entry 1 of Table 2 was not consistent
with the prior work of Choudary et al. [9], however, there were
some differences in the reaction conditions. Therefore, in entry
2 of Table 2, identical ratios of catalyst, (S)-BINOL and reactants
were used and the same catalyst activation temperature of 523 K
was used. Again a negligible ee of −0.8% was observed. Entry 3
of Table 2 presents our best attempt to match the source of the
MgO, the ratios of the catalyst, (S)-BINOL and reactants, and the
activation temperature to those used by Choudary et al. Again, neg-
ligible ee was observed after 4 h or 7 h of reaction. The reaction
profile of entry 3 of Table 2 is presented in Fig. 6b. In an attempt to
explore the influence of other variables in the reaction, entries 4–7
in Table 2 illustrate the effects of the order of addition of reactants,
the amount of (S)-BINOL added to the reactor, and the reaction tem-
perature. As expected, altering the sequence of addition of reagents
did not affect the rate or the ee. Moreover, addition of up to an order
of magnitude more (S)-BINOL did not improve the ee. Although
the initial rate at 195 K (entry 7) was two orders of magnitude
lower than that at 273 K (entry 3 in Table 2), the observed ee was
still negligible, even after 12 h of reaction (6.5% conversion). The
reaction profile associated with entry 7 in Table 2 is presented in
Fig. 6c. Interestingly, Choudary et al. reported that 100% benzalde-
hyde conversion and 90% ee were obtained after 12 h of reaction
under similar conditions.
0.8
0.6
0.4
0.2
0.0
500
600
700
800
Activation temperature / K
Fig. 5. Effect of activation temperature on nano-MgO-1 catalyst reactivity. Reaction
condition: benzaldehyde (8.0 mmol, 0.815 mL); nitromethane (40 mmol, 2.147 mL);
THF (40 mL); 0.5 g nano-MgO-1 activated at 523, 623, 723 and 823 K for 1 h
(5 K min−1) under 100 cm3 min−1 N2 flowing; reaction temperature 273 K.
sample. A representative reaction profile is presented in Fig. 6a. It
is important to note that only the racemic products, (R)-1-phenyl-
2-nitromethanol and (S)-1-phenyl-2-nitroethanol, were observed
during the course of the reaction.
Choudary et al. reported that (S)-BINOL modified nano-MgO-
2 with high surface area can induce enantioselectivity for Henry
reaction [9]. In their paper, (S)-BINOL modified nano-MgO-2 cat-
alyzed the Henry reaction of benzaldehyde and nitromethane with
70% ee after 7 h of reaction at 273 K and 90% ee after 12 h reaction at
195 K. Therefore, we tested (S)-BINOL modified nano MgO for Henry
reaction of benzaldehyde and nitromethane and the results are
presented in entry 1 of Table 2. The (S)-BINOL/nano-MgO-1 com-
bination had lower initial activity of 0.12 mmol g−1 min−1, which
Since we were unable to observe any enantiomeric excess by
modifying MgO with (S)-BINOL, we attempted a few control experi-
ments that are summarized in Table 3. Entry 10 of Table 3 confirmed
that the Henry reaction did not proceed at room temperature
8
4
a
benzaldehyde
(R)-1-phenyl-2-nitroethanol
b
benzaldehyde
(R)-1-phenyl-2-nitroethanol
(S)-1-phenyl-2-nitroethanol
(S)-1-phenyl-2-nitroethanol
6
3
4
2
0
2
1
0
0
50
100 150 200 250 300
Time / min
0
50
100
150
200
250 420
Time / min
4
3
2
1
0
4
3
2
1
0
d
benzaldehyde
(R)-1-phenyl-2-nitroethanol
(S)-1-phenyl-2-nitroethanol
c
benzaldehyde
(R)-1-phenyl-2-nitroethanol
(S)-1-phenyl-2-nitroethanol
0
100
200
300
400 720
0
50
100
150
200
250
300
Time / min
Time / min
Fig. 6. Reaction profiles of activated nano MgO catalyst for Henry reaction. (a) Nano-MgO-1 (723 K activated), reaction condition same as in Fig. 5; (b) (S)-BINOL modified
nano-MgO-2 (523 K activated) catalyst, reaction temperature was 273 K; (c) (S)-BINOL modified nano-MgO-2 (523 K activated) catalyst, reaction temperature was 195 K; (d)
l-proline modified nano-MgO-2 (723 K activated) catalyst, reaction temperature was room temperature.

26
Y. Xi, R.J. Davis / Journal of Molecular Catalysis A: Chemical 341 (2011) 22–27
Table 2
Reaction tests of activated nano MgO catalysts for Henry reaction.
Entry
Catalyst
(S)-BINOL (g)
Thermal
treatment, T (K)
Reaction, T (K)
Initial benzaldehyde
reaction rate
Benzaldehyde
conversion at 4 h (%)
ee at 4 h (%)
precursori
(mmol g−1 min−1
)
1a,e
2b,e
3b,e
Nano-MgO-1
Nano-MgO-1
Nano-MgO-2
0.32
0.16
0.16
723
523
523
273
273
273
0.12
0.25
0.23
68
95
−1.4
−0.8
−0.8
−0.5c
−0.6
−2.3
−3.5
−2.2d
3.2
94
95c
94
4b,f
5b,f
6b,g
7b,e
8b,h
9b,h
Nano-MgO-2
Nano-MgO-2
Nano-MgO-2
Nano-MgO-2
Nano-MgO-2
Nano-MgO-2
0.16
0.8
1.6
0.16
–
523
523
523
523
723
723
273
273
273
195
296
195
0.29
0.21
0.21
0.002
0.073
–
91
92
6.5d
82
–
∼0d
20.4d
a
8 mmol benzaldehyde and 40 mmol nitromethane were used as reactants in 40 mL THF as solvent.
4 mmol benzaldehyde and 20 mmol nitromethane were used as reactants in 20 mL THF as solvent.
At 7 h.
At 12 h.
b
c
d
e
Nitromethane and (S)-BINOL were added to THF at reaction temperature and then activated catalyst was added and stirred for 1 h before adding benzaldehyde to initiate
reaction [9].
f
In the mixture of THF, activated catalyst and benzaldehyde at 273 K, (S)-BINOL was added and stirred for 5 min before adding nitromethane to initiate reaction.
Stirring the mixture of THF, (S)-BINOL and activated catalyst at 273 K for 1 h before adding benzaldehyde and nitromethane to initiate the reaction.
l-Proline modified nano-MgO-2 (723 K activated). The activated catalyst was added to THF at reaction temperature and benzaldehyde and nitromethane were then added
g
h
to initiate the reaction.
i
0.5 g catalyst was activated.
without the presence of the MgO catalyst. Since CO2 is a com-
mon poison of base catalysts, we attempted to thermally activate
MgCO3 as a catalyst for the Henry reaction. A thermally activated
(523 K for 1 h under 100 cm3 min−1 N2 flowing) commercial MgCO3
(Sigma–Aldrich, ≥40% MgO) was inactive for the Henry reaction
after 4 h, as shown in Table 3. To exclude the possibility of leached
magnesium cations coordinating with (S)-BINOL and serving as
a homogeneous catalyst, magnesium acetate and (S)-BINOL were
used in the Henry reaction. As summarized in entry 12 of Table 3,
only 2.7% benzaldehyde conversion was observed after 24 h and no
ee was observed.
cessful. As an alternative, l-proline modified nano-MgO-2 was
tested in the reaction. l-Proline has a carboxyl group and an
amino-group associated with the same chiral center, which makes
l-proline a potential catalyst or ligand for asymmetric reactions.
Indeed, l-proline [26], l-proline-derived organocatalyst [27] and
supported l-proline [28] have been reported to catalyze asymmet-
ric aldol reactions. The acidity of the carboxyl group could lead to
high loading of l-proline onto the basic surface of MgO surface,
while the basicity of the amino-group and the chiral carbon cen-
ter of l-proline might lead to activity and enantioselectivity for the
Henry reaction.
Choudary et al. report that silylation of nano-MgO substantially
reduces its activity and eliminated the ee for the Henry reaction
in the presence of (S)-BINOL. These results appear to suggest that
the hydroxyl groups on the nano MgO contribute significantly to
the catalytic activity for the Henry reaction. Apparently, the activa-
tion temperature of 523 K is not high enough to remove all of the
hydroxyl groups on MgO surface [13,24] and the addition of (S)-
BINOL might actually increase the hydroxyl groups on the catalyst
surface due to the transfer of protons from (S)-BINOL to Lewis base
sites on the catalyst surface. To test the role of hydroxyl groups
and control its inventory, the nano-MgO-2 catalyst was hydrated
by water vapor for 24 h at room temperature and then activated
at 523 K for 1 h before it was tested for Henry reaction. The result
summarized entry 13 of Table 3 indicates that the hydrated nano-
MgO-2 was poorly active and did not exhibit any ee.
The results of the Henry reaction on l-proline modified MgO are
presented in Table 2 and Fig. 6d. At room temperature, the l-proline
modified nano-MgO-2 exhibited moderate reactivity with an ini-
tial benzaldehyde reaction rate of 0.073 mmol g−1 min−1 and little
selectivity to (S)-1-phenyl-2-nitroethanol (ee of 3.2%) at 4 h. The
ee was improved to 20.4% by lowering the temperature to 195 K,
however the reaction rate was so low that the benzaldehyde con-
version after 12 h was nearly undetectable. The surface coverage of
l-proline on nano-MgO-2 was 4.44 ␮mol m−2, which is the same as
the reported CO2 adsorption capacity on a synthesized MgO mate-
rial (4.4 ␮mol m−2) [29]. Evidently, the surface base sites on MgO
were effectively titrated by l-proline. The formation of carboxy-
late anion as observed by the IR absorption bands (Fig. 4) at 1600
and 1440 cm−1 on the l-proline modified nano-MgO-2, suggest that
the proton from the carboxyl group of l-proline has been trans-
ferred to the basic surface of MgO. Therefore, the activity observed
on the l-proline modified nano-MgO-2 is attributed to the amino-
All of our attempts to produce a base catalyst from MgO and
(S)-BINOL to promote the Henry reaction with high ee were unsuc-
Table 3
Control experiment tests for Henry reaction.
Entry
Catalyst
(S)-BINOL (g)
Reaction, T (K)
Benzaldehyde conversion (%)
ee (%)
10a,b
11a,c
12d
No catalyst
MgCO3
0
296
273
296
273
0 (1 h)
0 (4 h)
2.7 (24 h)
2.6 (4 h)
–
–
0.16
0.189
0.16
Mg(CH3COO)2·4H2O
−0.1 (24 h)
13a,e
Hydrated nano-MgO-2
−0.6 (4 h)
a
4 mmol benzaldehyde and 20 mmol nitromethane were used as reactants in 20 mL THF as solvent.
Blank test without catalyst and (S)-BINOL.
523 K treated 0.5 g MgCO3 was used as catalyst.
b
c
d
In the mixture of 0.0429 g magnesium acetate tetrahydrate (Fisher, 99.2%) and 20 mL methanol, 4 mmol benzaldehyde and 20 mmol nitromethane were added and stirred
at room temperature.
e
Nano-MgO-2 was hydrated in water vapor at room temperature for 24 h, then 0.5 g hydrated nano-MgO-2 catalyst was activated at 523 K and used as catalyst.

Y. Xi, R.J. Davis / Journal of Molecular Catalysis A: Chemical 341 (2011) 22–27
27
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Acknowledgments
This work was supported by the U.S. Department of Energy,
Basic Energy Sciences, for financial support through Catalysis
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