Efficient Tertiary Amine/Weak Acid Bifunctional Mesoporous Silica Catalysts for Michael Addition Reactions

Article abstract of DOI:10.1002/cctc.201200551

We describe the development and application of efficient bifunctional acid-base mesoporous silica catalysts, denoted hereafter as Ext-SBA-15-NMe₂, comprising tertiary amine and silanol (weak acid) groups for the Michael addition reaction. The c

Full text of DOI:10.1002/cctc.201200551

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DOI: 10.1002/cctc.201200551
Efficient Tertiary Amine/Weak Acid Bifunctional
Mesoporous Silica Catalysts for Michael Addition Reactions
Sayantani Das,[a] Anandarup Goswami,[a, b] Nagarajan Murali,[a] and Tewodros Asefa*[a, b, c]
We describe the development and application of efficient bi-
functional acid–base mesoporous silica catalysts, denoted
hereafter as Ext-SBA-15-NMe2, comprising tertiary amine and si-
lanol (weak acid) groups for the Michael addition reaction. The
catalysts were synthesized by grafting tertiary amine contain-
ing organosilane into the mesoporous channel pores of SBA-
15 mesoporous silica in polar protic solvent (isopropyl alcohol)
or nonpolar solvent (toluene). The resulting materials, Ext-SBA-
15-NMe2-IPA or Ext-SBA-15-NMe2-Tol, respectively, were used as
catalysts for the Michael addition reactions between trans-b-ni-
trostyrene(s) and active methylene compounds, such as malo-
nonitrile, acetylacetone, and diethylmalonate, at different tem-
peratures. A variable-temperature NMR-array technique was
used to monitor the reactant conversion and the reaction
progress. Among the active methylene compounds tested
with trans-b-nitrostyrene, malononitrile gave the highest con-
version of 90% in 0.3 h at 08C if catalyzed by the catalyst Ext-
SBA-15-NMe2-IPA, the silanol groups of which were not passi-
vated by trimethysilyl (ÀSiMe3) groups. Compared with Ext-
SBA-15-NMe2-Tol, the Ext-SBA-15-NMe2-IPA material (whose the
amine groups were grafted in polar-protic solvents), in particu-
lar, demonstrated an effective cooperative bifunctional cataly-
sis because of its optimized proportions of tertiary amine
groups and a significant number of surface silanol groups that
were judiciously left behind on the material with this opti-
mized synthetic strategy. The catalytic activity of this material
was found to be significantly higher than that of the corre-
sponding material that was grafted in toluene and that con-
tained less optimized proportions of these two catalytic
groups (i.e., amine and silanol groups). The bifunctional cata-
lysts also showed good recyclability upon washing with ace-
tone, which offered an effective way of regenerating the cata-
lyst free from any undesired product/substrate bound on the
surface of the catalysts.
Introduction
The Michael reaction, which involves 1,4-addition of a nucleo-
phile (preferably a C-nucleophile) to an a,b-unsaturated com-
pound, offers a simple yet effective way of making CÀC bonds,
which makes it one of the most widely used reactions in both
industry and academia. The reaction, as first reported by
Arthur Michael in 1887,[1] involves primarily the attack of a C-
nucleophile, generated from compounds with active methyl-
ene groups, on the b position of unsaturated carbonyl com-
pounds, giving the corresponding enol/enolate, followed by
subsequent protonation and tautomerization of the enol to its
keto form, yielding the so-called 1,4-addition product. Thus,
the customary steps in the reaction include the deprotonation
of the parent active methylene compounds with basic re-
agents and the protonation of enolate with either the conju-
gate acid of that same base or the solvent. The ease of the re-
action and the involvement of a base and the corresponding
conjugate acid to catalyze the reaction have made the devel-
opment of efficient acid or base catalysts, which can catalyze
the reactions reversibly and effectively, the subject of intense
research in catalysis. The concept has been applied to this re-
action (in both stereoselective and non-stereoselective
manner) by using metal-based Lewis acid catalysts[2] and orga-
nocatalysts.[3] Compared with organometallic catalytic systems,
organocatalysts offer advantages because most of them are
easy to handle, are stable under many different synthetic con-
ditions, and have highly tunable synthetic skeletons.[4]
[a] S. Das, Dr. A. Goswami, Dr. N. Murali, Prof. T. Asefa
Department of Chemistry and Chemical Biology
Rutgers, The State University of New Jersey
610 Taylor Road, Piscataway, New Jersey 08854 (USA)
Fax: (+1)848-445-2970
In this particular context of Michael addition reaction, the
most common organocatalytic systems are derived from orga-
noamines because they have been found to be basic enough
to take up the protons from the active methylene groups and
generate carbanions, and their conjugate acids (ammonium
ions) are proven to be capable of delivering the protons back
to the enolate at the end of the catalytic cycle. Alternating
modes of activation, that is, either activation of carbonyl using
acid catalysts or use of highly reactive organometallic reagents
to form the nucleophile,[5] have also been used to make some
of the most important synthetic skeletons.[6] However, until re-
E-mail: tasefa@rci.rutgers.edu
[b] Dr. A. Goswami, Prof. T. Asefa
Department of Chemical and Biochemical Engineering
Rutgers, The State University of New Jersey
98 Brett Road, Piscataway, New Jersey 08854 (USA)
[c] Prof. T. Asefa
Institute for Advanced Materials, Devices and Nanotechnology (IAMDN)
Rutgers, The State University of New Jersey
607 Taylor Road, Piscataway, New Jersey 08854 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201200551.
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cently, the possibility of combining more than one catalytic
system for the Michael reaction remained relatively unexplored
owing to poor compatibility between a base and an acid cata-
lytic group within the same system. This compatibility issue
has lately been circumvented by the use of urea-based hydro-
gen-bonding homogeneous catalytic systems. The pioneering
works by Rawal,[7] Takemoto,[8] Jacobsen,[9] Macmillan,[10] and
others[11] toward the development of these types of bifunction-
al catalysts (as well as their asymmetric variants) illustrated the
importance of such a simple, but novel, idea of dual activation
in this type of reaction. In addition to urea, silanols (SiÀOH), in
both homogeneous and heterogeneous forms, are attracting
more attention as catalysts or co-catalytic species for a number
of reactions. The recent work by Franz[12] and Mattson[13]
showed that silanols could show paramount catalytic activities
in reactions such as Michael, aldol, and Diels–Alder reactions.
The group of homogeneous catalysts these two research
groups reported have well-designed silanols with both sterical-
ly and electronically controlled substituents around the silicon
atoms so that the silanols avoid potential self-condensation
(the possible self-condensation of silanols is one of the main
reasons for the difficulty in the synthesis of homogeneous sila-
nol-based catalytic systems). This in turn enabled silanols to ac-
tivate electrophiles without compromising their catalytic activi-
ty. The latest study by Franz even demonstrated that by care-
fully choosing the substituents on the silanol moiety, these
types of catalysts can also be used as bifunctional catalysts for
enantioselective aldol reactions.[14] Intrigued by these results
and our recent success in the field of heterogeneous cataly-
sis,[15] we investigated the possibility of using amine-functional-
ized mesoporous silica materials, which inherently contain sila-
nol groups or can be judiciously made to contain more opti-
mized density of silanol (weak acid) groups, as acid–base bi-
functional organocatalysts for the Michael addition reaction.
Since the discovery in early 1990s, significant progress has
been made in the field of the synthesis of various mesoporous
silica materials and their potential applications as support ma-
terials for heterogeneous catalysts.[16] This development can
generally be attributed to the unique chemical and conducive
physical properties of these types of materials as catalyst sup-
ports, among many other things.
trollable nanometer pores, which allow controlled molecular
diffusion of reactants. Moreover, besides their catalytically
active groups, further functional groups can easily be intro-
duced into these materials by either grafting or co-condensa-
tion synthetic methods. This in turn can result in surface modi-
fication and controllable physical and chemical properties, and
thus efficient and tunable catalytic activities.[17]
Among many types of organofunctionalized mesoporous
materials, amino-functionalized hybrid mesoporous organosili-
ca materials are well known for their catalytic activity in various
carbon–carbon bond-forming reactions. Studies have shown
that the presence of significant numbers of residual surface si-
lanol groups can be beneficial for the catalytic performance of
these materials, especially if an appropriate density of catalytic
functional groups, such as organoamines, is present in their
channel pores.[18] This has encouraged us to develop a similar
type of catalytic system that utilizes both silanol (as a hydro-
gen-bonding cocatalyst) and tertiary amine (as a base catalyst)
as a mode of dual activation toward the Michael reaction. We
herein report the synthesis of bifunctional mesoporous cata-
lysts containing optimized density of tertiary amine and silanol
groups and the use of the resulting materials as effective bi-
functional catalysts for the Michael addition reactions between
various nitroalkenes and active methylene compounds.
Results and Discussion
Synthesis of tertiary amine-functionalized SBA-15 meso-
porous silica materials or bifunctional catalysts
The synthesis of the bifunctional catalyst Ext-SBA-15-NMe2-Tol
(1) or Ext-SBA-15-NMe2-IPA (1’) containing tertiary organoa-
mine and silanol groups was achieved by stirring Ext-SBA-15
with an excess amount of 3-(N,N-dimethylaminopropyl)trime-
thoxysilane (DATS) in toluene or isopropyl alcohol, respectively,
at 808C for 6 h. The corresponding control samples, labeled as
Cap-SBA-15-NMe2-Tol (or 2) and Cap-SBA-15-NMe2-IPA (or 2’),
respectively, were prepared by capping the external silanol
(weak acid) groups of Ext-SBA-15-NMe2-Tol or Ext-SBA-15-
NMe2-IPA with methyl groups by using hexamethyldisilazane
In addition to the common ad-
vantages of heterogeneous cata-
lysts compared to their homoge-
neous counterparts (e.g., less
waste generation, easy work-up
methods, and recyclability), the
resulting mesoporous silica-sup-
ported heterogeneous catalytic
materials offer high surface
areas, which provide large con-
tact areas between reactants
and catalytic sites and thus high
catalytic activity. In addition,
these catalysts can serve as
shape-/size-selective catalysts as
a result of their uniform yet con- Scheme 1. Synthesis of Ext-SBA-15-NMe2-Tol (1) and Cap-SBA-15-NMe2-Tol (2).
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(HMDS), as depicted in Scheme 1. The latter samples, Cap-SBA-
15-NMe2-Tol (2) and Cap-SBA-15-NMe2-IPA (or 2’), respectively,
were synthesized so that its accessible silanol groups would be
protected by trimethylsilyl (ÀSiMe3) groups and the resulting
catalysts would have no accessible external silanol (active
weak acid) groups, which could aid in hydrogen bonding
during the Michael reaction. Thus, by using these materials (2
and 2’) as reference, the effect of silanol groups as acid co-cat-
alysts in the bifunctional catalysts (1 and 1’) for the Michael re-
action was investigated.
grafted organoamine (and trimethylsilyl) groups in the pores
of these materials.
The materials, both before and after postgrafting with orga-
nosilanes, were also characterized by powder XRD (Figure S1)
and TEM (Figure 2). The XRD pattern of all the samples
showed a sharp peak corresponding to the (100) peak and
very small peaks corresponding to the (110) and (200) peaks
of hexagonally ordered mesostructured materials. The Bragg
peaks remained intact after postgrafting of the materials with
organoamine groups, which indicated that the mesostructures
were stable after the postgrafting synthetic step. By indexing
the (100) peak, the materials’ unit cell sizes were found to be
11–12 nm. These and other physical properties of the materials
are listed in Table S1. The TEM images of the samples con-
firmed the presence of ordered mesostructures in the materials
(Figure 2).
The TGA traces of the organoamine- (and trimethylsilyl-)
functionalized mesoporous silica materials (catalysts) (Figure 3)
demonstrated a slight weight loss below 1008C for all the sam-
Figure 1. Nitrogen adsorption/desorption isotherm and pore size distribu-
&
&
tion of Ext-SBA-15 ( ), Ext-SBA-15-NMe2-Tol ( ), and Cap-SBA-15-NMe2-Tol
&
( ).
The N2 gas adsorption/desorption measurements were per-
formed to characterize the mesoporous structure, surface area,
and pore diameter of the catalysts and their parent materials.
The N2 gas adsorption measurements showed type IV iso-
therms, which were characteristic of mesoporous materials
(Figure 1). The N2 gas adsorption measurements also revealed
that the BJH pore size distribution of the materials was mono-
disperse. The BET surface area and average pore diameter of
the parent material (SBA-15) were 427 m2 gÀ1 and 11.8 nm, re-
spectively, whereas the surface area and pore diameter of Ext-
SBA-15-NMe2-Tol (1) were 235 m2 gÀ1 and 8.4 nm, respectively,
and those of Cap-SBA-15-NMe2-Tol (2) were 225 m2 gÀ1 and
8.4 nm, respectively. The decrease in surface area and pore di-
ameter in the organo-functionalized materials compared with
the parent material was expected because of the presence of
Figure 3. Thermogravimetric traces of Ext-SBA-15, Ext-SBA-15-NMe2-Tol, and
Cap-SBA-15-NMe2-Tol.
ples because of the loss of water adsorbed on the materials.
The weight loss in Ext-SBA-15 (extracted mesoporous silica) in
between 100 and 6008C was attributed mainly to residual tem-
plating agent and partly to water formed by condensation
from the surface silanol groups of the material. The weight
losses in the range between 100 and 6008C from the catalysts
were predominantly due to the loss of the organoamine (or À
SiMe3 trimethylsilyl) groups. The Ext-SBA-15-NMe2-Tol and Cap-
SBA-15-NMe2-Tol samples showed
a similar weight loss
(ꢀ21%).
The catalysts Ext-SBA-15-NMe2-Tol, Cap-SBA-15-NMe2-Tol,
and Cap-SBA-15-NMe2-IPA were characterized by 13C cross-po-
larization magic angle spinning (CP-MAS), 29Si CP-MAS, and
29Si MAS (29Si–1H-decoupled) solid-state NMR spectroscopy (Fig-
ure S2). In the case of 13C CP-MAS NMR spectrum for Ext-SBA-
15-NMe2-Tol, the peaks at 10.2, 20.1, 43.2, and 61.6 ppm were
designated as C4, C3, C1 (NMe2), and C2 of alkylamine groups
(Figure 4). The peaks marked with asterisks (*) correspond to
the peaks from residual surfactants.[19] A similar pattern (as well
as a similar chemical shift) was observed in the case of Cap-
SBA-15-NMe2-Tol. However, the peaks corresponding to methyl
groups that resulted from the capping of ÀSiÀOH groups did
Figure 2. TEM images of Ext-SBA-15-NMe2-Tol.
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action between trans-b-nitrostyr-
ene and active methylene com-
pounds at various temperatures
(Scheme 2). Cap-SBA-15-NMe2-
Tol, which does not have active
residual silanol (weak acid)
groups in it, was used as the ref-
erence material to establish the
combined effect of both the
amine catalyst and the surface
silanol groups of Ext-SBA-15-
NMe2-Tol on the reaction. Al-
though the effect was less prom-
inent at higher temperatures,
the reaction was found to be
faster with Ext-SBA-15-NMe2-Tol
than with Cap-SBA-15-NMe2-Tol.
This was presumably because
the surface silanol groups in the
former aided in the hydrogen
Figure 4. 13C CP-MAS spectra and their peak assignments for Ext-SBA-15-NMe2-IPA and Cap-SBA-15-NMe2-IPA.
not show up in the spectra (at least under the same experi-
mental conditions). Contrarily, in the case of Cap-SBA-15-NMe2-
IPA, the peak at 0.82 ppm indicating the presence of ÀOÀSiMe3
in the material was seen.[20] Based on these observations, it
seems likely that because of the higher degree of grafting in
toluene,[15] the number of free ÀSiÀOH groups reduces, which
results in less number of free ÀSiÀOH groups for HMDS cap-
ping; however, in the case of grafting in isopropanol, the rela-
Scheme 2. The Michael addition reaction between nitrostyrene (or p-substi-
tuted nitrostyrenes) and various active methylene compounds.
tive number of ÀSiÀOH sites remains significant owing to less
grafting in the previous step.[15b] As a result, the latter material
reacts significantly upon treatment with HMDS and gives more
ÀOÀSiMe3 groups that show up in its 13C CP-MAS spectrum.
Our effort to characterize this further by using 29Si CP-MAS and
29Si–1H-decoupling experiments offered satisfactory informa-
tion regarding the Q(s) and T(s) states of the material; howev-
er, the spectra failed to show the peaks for the M state corre-
sponding to the silicon atoms in ÀOSiMe3 in the Cap-SBA-15-
NMe2 sample under the same experimental conditions because
of low signal-to-noise ratio as well as relatively lower number
of possible ÀSiMe3 capping sites in it.[21] A comparative study
of Ext-SBA-15-NMe2 and Cap-SBA-15-NMe2 (obtained from the
same batch of SBA-15) revealed that the Ext-SBA-15-NMe2
sample had a higher percentage of T(s) states (Figure S2). How-
ever, the apparent discrepancy in the percentage of T states
between the two Cap-SBA-15-NMe2 samples could presumably
stem from the different batches of the SBA-15 precursor used
during the experiments in addition to their low signal-to-noise
ratios. The amount of amine groups present in both catalysts
was obtained by using CHN elemental analysis (Table S2).
Figure 5. Proposed mechanism of dual activation involving hydrogen bond-
ing by acid–base bifunctional mesoporous catalysts in the Michael addition
reaction.
bonding of the substrate, which was absent in the latter cata-
lyst. In other words, the SiÀOH groups in the former activated
the nitro group of nitrostyrene, which makes it more electro-
philic, and thus a facile addition of the nucleophile occurred
(Figure 5).
The extent of dual activation by acid and base groups as
well as how concerted such a process can be still remain to be
areas of active research, although various models have been
proposed in the case of some homogeneous versions. In most
of the organocatalytic asymmetric Michael reactions, the acti-
vation of nucleophiles (in this case, active methylene com-
pounds) by an amine-based catalytic system is well precedent
and can certainly be extended to our case. However, the other
Catalytic activity of the catalysts in the Michael addition
reaction
First, the catalytic activities of Ext-SBA-15-NMe2-Tol and Cap-
SBA-15-NMe2-Tol were investigated in the Michael addition re-
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parallel activation mode, that is, the activation of hydrogen
bonding, demands further discussion. Of the possible activa-
tion modes of an electrophile by an organocatalyst, two types
of activation are commonly documented. In one case, an elec-
trophile becomes activated by the formation of an iminium
ion, which was generated through the reaction between the
electrophile (in this case, Michael acceptor) and the catalyst. In
the second case, hydrogen bonding between the catalyst and
the Michael acceptor makes the electrophilicity of the acceptor
molecule to increase. Although the first activation mode is his-
torically important and offers some great advantages in the
context of proline-catalyzed reactions, it is not applicable in
the case of tertiary amine-catalyzed reactions. Contrarily, a rela-
tively newer approach, that is, activation through hydrogen
bonding, shows better resemblance with our catalytic system.
On the basis of the X-ray structures of their alkaloid-based thi-
ourea catalysts along with physical organic studies, Takemoto[8]
and Jacobsen[9] proposed the concept of dual activation
through two hydrogen bonds between the urea moiety and
the electrophile. However, more recent theoretical study by
Pꢁpai and Soꢂs[22] showed an alternative approach, which uses
the hydrogen bonding with amine as a plausible activation
mode. The widely reported double hydrogen bond activation
by urea or thiourea has also been challenged by Lu,[23] who
demonstrated that a single hydrogen bond activation of the
urea NÀH with the electrophile can also catalyze the reaction.
In addition, the effects of additive, solvent, and temperature
have been studied. However, till the studies by Franz[12,14] and
Mattson,[13] silanol as a potential hydrogen-bonding source in
homogeneous catalysis remained unexplored because of its
propensity toward condensation, which leads to poor stability
under normal reaction conditions. In this context, Franz’s X-ray
crystallographic study of the silanol-activated carbonyl groups
along with the theoretical study of the pKa of silanols are quite
encouraging and the idea of similar hydrogen-bonding activa-
tion can easily be extended in case of our heterogeneous ver-
sion, which thereby allows us to draw one of the possible
mechanisms for the heterogeneous Michael reaction. In this
case, in spite of the sharp advances in heterogeneous silica-
supported catalysis, the investigation of the effect of the sur-
face silanol group as a co-catalyst at molecular levels still
poses significant challenges mainly due to the difficulty in
comprehending the bonding pattern of these catalytic groups
on the surface and the nanoscale pore channels of the materi-
als. Furthermore, compared to homogeneous catalysis, obtain-
ing crystal structures of intermediates in heterogeneous cataly-
sis is difficult. Under these circumstances, the reaction mecha-
nisms are often hypothesized on the basis of information from
their homogeneous counterparts. As indicated previously, the
detailed mechanistic investigation is beyond the scope of this
current article; however, the catalytic results suggest that al-
though activation of the nucleophile with basic groups is re-
quired for the reaction, a further rate acceleration can be ach-
ieved by the activation of the electrophile through hydrogen
bonding using the silanol groups of the mesoporous silica sup-
port material. This hydrogen-bonding activation is not opera-
tive in the case of Cap-SBA-15-NMe2-Tol, because this material
does not have many free or accessible SiÀOH groups owing to
its treatment with HMDS. The results of these experiments
clearly showed that Ext-SBA-15-NMe2-Tol, which was not treat-
ed with HMDS, could serve as an effective hydrogen-bond
donor catalyst for the conjugate addition of various active
methylene compounds to trans-b-nitrostyrene or substituted
trans-b-nitrostyrenes.
The variable-temperature 1H NMR-array technique experi-
ment, which we used here, allowed us to study the catalytic re-
actions at different temperatures in real time and to obtain ki-
netic data. The results of the kinetic studies using the variable-
temperature 1H NMR-array technique matched closely with
those of our bench-top experiments and eliminated the risk of
potential rate difference between NMR and bench-top experi-
mental conditions due to external influences (e.g., stirring), be-
cause of the smaller amount of the catalyst used in the former
experiment. In addition, based on our current results, the re-
sults from this technique seem to be more reliable than those
of (our) GC-based experimental catalytic studies for the bench-
top reactions. Despite its apparent usefulness, such an experi-
ment for monitoring heterogeneous catalysis is, however,
rarely performed, and our work here is one of the first exam-
ples that demonstrates the use of such method for probing
the catalytic activities of organoamine-functionalized mesopo-
rous heterogeneous catalysts.[24] The catalytic results are pre-
sented in Tables 1–3.
Under the reaction conditions, the catalyst with 34.3 mmol of
catalytic groups (calculated based on ꢀ3 wt% or
ꢀ2.14 mmolgÀ1 of the amine-functionalized material obtained
by elemental analysis) was used for the reaction between 0.2m
nitrostyrene and 0.25m active methylene compounds. The re-
action between nitrostyrene and malononitrile was completed
within 0.3 h at 08C in the presence of the catalyst Ext-SBA-15-
NMe2 (Figure 6), whereas the control experiment without the
catalyst showed 80% conversion in 1.5 h at room temperature
(Figure S3). The corresponding reactions involving acetylace-
tone and diethylmalonate as active methylene partners oc-
curred only at a much higher temperature (508C), which yield-
Table 1. Catalytic activity of the supported catalyst used in the Michael
addition reaction.[a]
Entry[b]
Catalyst[c,d]
X[e]
T
[8C]
t
[h]
Conversion[f]
[%]
TOF
[hÀ1
]
[g]
1
2
3
4
5
6
1
2
1
2
1
2[h]
CN
CN
COMe
COMe
CO2Et
CO2Et
0
0
50
50
50
50
0.3
0.3
3
3
12
–
90
80
85
88
62
–
17142
15238
1619
1676
295
–
[a] Refer to Scheme 2 for the reaction scheme and substituents in the re-
actants; [b] In each case, 0.2m trans-b-nitrostyrene in CDCl3 was used;
[c] 16 mg of solid catalyst was used; [d] 1=Ext-SBA-15-NMe2-Tol and 2=
Cap-SBA-15-NMe2-Tol (see also Scheme 1); [e] 0.25m in CDCl3; [f] Based
on the disappearance of olefinic 1H signal in 1H NMR; [g] Obtained by
using the number of catalytic (amine) groups in moles in the catalyst,
which were determined by elemental analysis; [h] Reaction stalled after
4 h with 40% conversion.
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Table 2. Comparison of catalytic activity of the supported catalyst used
in the Michael addition reaction.[a]
Entry[b]
Catalyst[c,d]
X[e]
T
[8C]
t
[h]
Conversion[f]
[%]
1
2
3
4
5
Ext-SBA-15
DATS
Ext-SBA-15+TEA
1
2
COMe
COMe
COMe
COMe
COMe
50
50
50
50
50
2
2
2
3
3
<2
98
90
85
88
[a] Refer to Scheme 2 for the reaction scheme and substituents in the re-
actants; [b] In each case, 0.2m trans-b-nitrostyrene in CDCl3 was used;
[c] 16 mg of solid catalyst was used; [d] 1=Ext-SBA-15-NMe2-Tol and 2=
Cap-SBA-15-NMe2-Tol (see also Scheme 1), the millimoles of triethylamine
(TEA) and 3-(N,N-Dimethylaminopropyl)trimethoxysilane (DATS) were cal-
culated on the basis of the nitrogen content of the catalyst 1; [e] 0.25m
in CDCl3; [f] Based on the disappearance of olefinic 1H signal in 1H NMR.
Figure 7. Percent conversion of nitrostyrene versus reaction time of Michael
addition reaction between active methylene compounds and nitrostyrene
&
^
catalyzed with Ext-SBA-15-NMe2-Tol. Malonitrile ( ), acetylacetone ( ), dieth-
~
ylmalonate ( ).
Table 3. Substrate scope of the supported catalyst used in the Michael
addition reaction.[a]
greater propensity to form the carbanion (also the nucleo-
phile). Consequently, the reaction between malononitrile and
nitrostyrene showed faster reactivity, followed by acetyl-
acetone and finally diethylmalonate (Table 1).
Entry[b]
Catalyst[c,d]
Y[e]
T
[8C]
t
[h]
Conversion
[%][f]
1
2
3
1’
1’
1’
H
Cl
OMe
50
50
50
3
3
3
95
75
30
We further investigated the reaction in more detail with the
catalysts Ext-SBA-15-NMe2-IPA and Cap-SBA-15-NMe2-IPA, the
tertiary amine catalytic groups of which were grafted in
a more polar solvent, isopropyl alcohol. This choice of solvent
was, however, not completely arbitrary. It was previously
shown by our group that grafting aminosilanes on mesopo-
rous silicas in polar solvents results in lesser amount of amine
groups, which leaves behind more accessible silanol groups on
the material compared with the one obtained using toluene—
the more commonly used solvent for organosilane graft-
ing.[15,26] In the presence of Ext-SBA-15-NMe2-IPA, the Michael
addition between trans-b-nitrostyrene and malononitrile at
08C showed faster reactivity than its counterpart, which in-
volved the catalyst Ext-SBA-15-NMe2-Tol (Figure 8a). This di-
minishing catalytic efficiency in the case of Ext-SBA-15-NMe2-
Tol can be attributed to the fewer silanol (weak acid) sites this
material has for dual activation; however, the possibility of ag-
gregation of the catalytic groups or formation of densely
populated organoamine groups within the mesopores cannot
be ruled out.[27] On comparing Ext-SBA-15-NMe2-IPA with Cap-
SBA-15-NMe2-IPA, the latter showed lower catalytic activity, as
expected, because of the lesser number of silanol groups pres-
ent in it to activate nitrostyrene (Figure 8b). These experiments
signify the importance of silanol groups, or conversely spatially
isolated amine groups, for dual catalysis. This study was ex-
tended to other substrate and catalyst combinations. For in-
stance, a comparative study of the catalytic activity of Ext-SBA-
15-NMe2-IPA and Ext-SBA-15-NMe2-Tol was also performed for
the substrate acetylacetone at 508C, and a similar trend was
observed. The reaction was again faster with Ext-SBA-15-NMe2-
IPA than with Ext-SBA-15-NMe2-Tol (Figure S4). Finally, the sub-
strate scope was studied with the catalyst Ext-SBA-15-NMe2-
IPA, which also showed the same trend as Ext-SBA-15-NMe2-Tol
(Figure S5).
[a] Refer to Scheme 2b for the reaction scheme and substituents in the
reactants; [b] In each case, 0.25m acetylacetone in CDCl3 was used;
[c] 16 mg of solid catalyst was used; [d] 1’=Ext-SBA-15-NMe2-IPA;
[e] 0.2m in CDCl3; [f] Based on disappearance of olefinic 1H signal in
1H NMR.
Figure 6. Percent conversion of nitrostyrene versus reaction time in the Mi-
chael addition reaction between malononitrile and nitrostyrene catalyzed
&
^
with Ext-SBA-15-NMe2-Tol ( ) and Cap-SBA-15-NMe2-Tol ( ).
ed the respective Michael addition products (Figure 7). The
trend in reactivity corresponds closely to the pKa values of the
respective substrates and can be explained by the ease of for-
mation of the carbanions from the substrates. The relative
acidity of active methylene compounds in DMSO follows the
trend: malononitrile (pKa 11.1)>acetylacetone (pKa 13.3)>di-
ethylmalonate (pKa 16.4).[25] Hence, in the presence of the
amine catalyst (base), the compound with lower pKa showed
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sults indicate that dual activation by the grafted aminosilane
and surface silanol groups is still important for better perfor-
mance of the heterogeneous catalyst.
With the most active catalyst Ext-SBA-15-NMe2-IPA (1’), we
then investigated the substrate scope with different para-sub-
stituted nitrostyrenes (Table 3, entry 1). As discussed earlier, the
yield of the reaction between nitrostyrene and acetylacetone
was very good (95% conversion; see Figure S4) at 508C in 3 h.
However, upon using substituted nitrostyrenes with the sub-
stituents at the para-position made the substrate demonstrate
different reactivity with acetylacetone because of the change
in its electronic properties. For instance, under the same reac-
tion condition, 4-chloro-substituted nitrostyrene reacted at
a slower rate compared with its unsubstituted counterpart,
which yielded 70% conversion at 508C in 3 h (Table 3, entry 2).
Although the electron-withdrawing halogen group is expected
to increase the electrophilicity of the a carbon of nitrostyrene,
the resonance effect (+R) of chlorine seems to be overriding
the inductive effect (-I), which ultimately results in the dimin-
ished electrophilicity of the a carbon of nitrostyrene. This
effect becomes more prominent if more electron-donating À
OMe group was substituted at the para-position of nitrostyr-
ene because this substrate gave only 30% conversion under
the same reaction conditions.
Compared to the previous studies of heterogeneous cata-
lysts for Michael addition reactions, the catalyst reported here
is found to be more active. In previous studies using silica-
based amino-functionalized catalysts, high temperatures and
long reaction times were used to obtain comparable or much
lower yields. For example, the Michael addition reaction be-
tween nitroalkane and a,b-unsaturated ketone in the presence
of an MCM-41-based amine catalyst (with a loading of 50 mg)
yielded 96% product at 908C in 20 h.[28] In another study,
using a high reaction temperature of 1408C and reaction time
of 8 h, a conversion of 92% was achieved in an intermolecular
Michael addition reaction between benzaldehyde and 2’-hy-
droxyacetonphenone catalyzed by an amine-functionalized
SBA-type catalyst.[29] The Michael addition of nitroalkane to
a,b-unsaturated ketone, catalyzed by using the N,N-dimethyl-
3-aminopropyl-derivatized hexagonal mesoporous silica cata-
lyst, gave a yield of 70% in 3 h under reflux conditions.[30] In
our case, all the reactions were performed either at 08C or at
508C to achieve excellent conversion. For example, with the
most reactive substrate malononitrile, Ext-SBA-15-NMe2 (only
16 mg) catalyzed the reaction to completion at 08C in 0.3 h.
These comparative results are listed in Table S2.
Figure 8. a) Kinetic plot of the Michael addition reaction between nitrostyr-
^
ene and malononitrile at 08C catalyzed by Ext-SBA-15-NMe2-IPA ( ) and Ext-
&
SBA-15-NMe2-Tol ( ). b) Kinetic plot of the Michael addition reaction be-
tween nitrostyrene and malononitrile at 08C catalyzed by Ext-SBA-15-NMe2-
&
^
IPA ( ) and Cap-SBA-15-NMe2-IPA ( ).
To compare the catalytic activity of the bifunctional catalysts
with their corresponding main individual components, we
studied the catalytic activity of these different materials in the
Michael addition reaction between trans-b-nitrostyrene and
acetylacetone. Acetylacetone was chosen as an active methyl-
ene compound because of its slower reactivity, which also
allows us to compare the conversion at a higher temperature
with reasonable accuracy compared to malononitrile (which
reacts at a faster rate even at a lower temperature). Ext-SBA-15
did not show appreciable conversion at 508C in 3 h (Table 2,
entry 1), which suggests that the activation of nitrostyrene by
the surface silanol group is not enough to drive the reaction
forward. The homogeneous DATS reacted at a faster rate com-
pared with either of the grafted catalysts (Table 2, entry 2). Our
attempt to use a physical mixture of SBA-15 and DATS as the
catalyst showed unexpected lower activity, presumably be-
cause at a higher temperature, grafting of aminosilane on SBA-
15 competes with the actual catalysis method. To avoid this
undesired reaction, Ext-SBA-15 was mixed with TEA (Table 2,
entry 3) and the mixture showed catalytic activity comparable
to that of its corresponding homogeneous organosilane cata-
lyst (Table 2, entry 2). Although the homogeneous version is
still slightly faster than the heterogeneous catalyst, these re-
Recyclability studies using such a small amount of catalyst
(16 mg) were challenging under NMR kinetic conditions mainly
because of poor recovery of the catalyst at the end of the reac-
tion after each cycle. To overcome these difficulties, a different
approach was adopted. For the initial recyclability studies, the
reaction between nitrostyrene and acetylacetone using the cat-
alyst Ext-SBA-15-NMe2-Tol at 508C for 2 h was performed on
a larger scale (5 times than what was used in the catalytic
studies above) on bench-top in parallel to the usual catalytic
reaction under NMR condition. The catalyst for the bench-top
reaction was isolated by centrifugation. At this moment, we
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faced difficulty in removing the pale yellow color of the solid,
which seemed to be coming from either the intermediate or
final products adsorbed on the surface of the catalyst. At this
stage, washing with CDCl3, chloroform, dichloromethane, di-
chloromethane+TEA (to remove the acid species generated
during the reaction), ether, and water did not help much. The
situation improved a little when we used alcohols, as they
could remove most of the yellowish color from the catalyst.
After several attempts with different solvents as well as their
different combinations, acetone worked best, and after thor-
ough washing with acetone (at least 5 times), the catalyst was
dried and used for the next cycle. From this recovered catalyst,
16 mg was used for NMR kinetic studies, in which the reaction
was performed under the original catalytic conditions and the
rest of the catalyst was utilized for the same reaction but
under bench-top reaction conditions. The same washing pro-
cess was repeated for the subsequent cycles.
catalytic surface has to be addressed very carefully (e.g., for
our primary and secondary amine catalysts,[15,26] acetone
cannot be used as a solvent to wash the catalyst because it
will react with amine groups and form imine/iminium species,
which could be difficult to eliminate). We believe that further
systematic investigation is possible to completely remove any
undesired species bound on the catalysts effectively without
decreasing the catalytic activity, as shown here.
Conclusions
In conclusion, we have described the synthesis of efficient
mesoporous nanocatalysts for the Michael addition reaction by
postgrafting of organoamine groups onto the surfaces of mes-
oporous silica. The synthesis of these nanocatalysts was ach-
ieved by reacting aminosilanes with mesoporous silica in
either toluene or isopropyl alcohol. The catalysts with tertiary
amine and enough silanol (weak acid) groups were found to
demonstrate cooperative catalytic properties and thereby
more efficient catalytic activities than the corresponding mate-
rials containing no active silanol groups. Furthermore, the cata-
lysts were investigated with different active methylene com-
pounds as nucleophiles to study the scope of the catalytic re-
action and the versatility of the catalysts. The reactions were
observed to be the fastest with malononitrile. In addition to
their high catalytic activity, the catalysts reported here have
many advantages, such as their ease of separation from reac-
tion mixtures and greater shelf life as well good recyclability
with insignificant loss of catalytic activity.
The reaction between nitrostyrene and acetylacetone at
508C showed 70% conversion to the desired Michael product
in 2 h. After second, third, and fourth cycles of reactions, the
reaction with the recycled catalyst gave 62, 60, and 68% con-
version of nitrostyrene, respectively (Figure 9). The TEM image
Experimental Section
Materials and reagents
Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethyl-
ene glycol) copolymer (Pluronic 123, with an average molecular
mass of ꢀ5800) was obtained from BASF. Tetraethylorthosilicate,
hexamethyldisilazane (HMDS), malononitrile, acetylacetone, diethyl-
malonate, trans-b-nitrostyrene, triethylamine (TEA), dichloro-
methane, ethylacetate, hexane, acetone, isopropyl alcohol, and tol-
uene were purchased from Sigma–Aldrich. Hydrochloric acid
(36.5%) was obtained from Fischer Scientific. 3-(N,N-Dimethyl-
aminopropyl)trimethoxysilane (DATS) was received from Gelest, Inc.
Deuterated chloroform (CDCl3) was obtained from Cambridge Iso-
tope Laboratories.
Figure 9. Recyclability results of the Michael addition reaction between ni-
trostyrene and acetylacetone at 508C catalyzed by Ext-SBA-15-NMe2-Tol (1).
Conversions of nitrostyrene were reported after 2 h based on the H NMR in-
1
tegration of each array of experiment.
of the catalyst after the third cycle showed that the mesostruc-
ture of the recycled catalyst remained intact (Figure S6). The
relatively lower catalytic activity with the recycled catalysts
could stem from (1) the presence of catalyst-bound reactants/
products (which was difficult even after several acetone wash)
and/or (2) the incomplete removal of acid species generated
during the reaction, which thereby decreased the active cata-
lytic sites as well as the amount of the total catalyst. Although
our recyclability studies did not reproduce the yield and con-
version the way we wanted (possibly owing to the reasons
stated above), our improved washing process with acetone
can offer additional advantages for other heterogeneous cata-
lysts, which often suffer from the complete lack of recyclability.
However, during the washing process, the compatibility be-
tween acetone (the solvent) and the functional groups on the
Instrumentation
Analytical TLC was performed on EM Reagent 0.25 mm silica gel 60
F254 plates, which were obtained from VWR. Visualization was ac-
complished with UV light and freshly prepared KMnO4 stain.
1H NMR spectra were recorded on Varian VNMRS 400 MHz and
Varian VNMRS 500 MHz instruments, and the spectra were reported
and compared with the literature value (in ppm) by using solvent
as an internal standard (CDCl3 at 7.26 ppm). For kinetic studies at
different temperatures, the “pad”-macro (pre-acquisition delay)
command (built in vnmrj, version 2.1B) was used. The integration
data for kinetic studies at different time points were processed by
using vnmrj and plotted by using Microsoft Excel. Nitrogen gas ad-
sorption–desorption measurements were performed on Micromer-
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itics Tristar 3000 volumetric adsorption analyzer after degassing
the samples at 1608C for 12 h. The powder XRD patterns of the
materials were measured by a Histar diffractometer at 295 K by
using monochromatized CuKa radiation. Thermogravimetric analy-
sis (TGA) or decomposition profiles of mesoporous materials were
acquired with a Q50 thermogravimetric analyzer. The TGA data
were collected under nitrogen atmosphere (60 cm3 minÀ1) in the
temperature range of 25–7008C (heating rate: 108CminÀ1). The
solid-state 13C (100.64 MHz) and 29Si (79.51 MHz) NMR spectra were
acquired by using a Bruker 400 MHz solid-state NMR spectrometer
containing a 3.2 mm bore HXY Bruker probe operating in the HX
mode at 298 K. For 13C CP-MAS NMR experiments, 10.0 kHz spin
rate, 3 s recycle delay, 2 ms contact time, p/2 pulse width of 5 ms
Capping the silanol groups of tertiary amine-functionalized
SBA-15 mesoporous silica
To prepare a control sample containing the tertiary amine catalytic
groups but no active residual silanol groups in it, either Ext-SBA-
15-NMe2-Tol or Ext-SBA-15-NMe2-IPA (200 mg) was suspended in
toluene (11.5 mL) and HMDS (1.5 mL); the solution was then stirred
at RT for 6 h to functionalize the external SiÀOH groups of Ext-
SBA-15-NMe2 with ÀSiMe3 groups. The solid samples were recov-
ered by filtration, washed with toluene and ethanol, and finally air
dried. The resulting samples, the accessible silanol groups of which
were capped with methyl groups, were labeled as Cap-SBA-15-
NMe2-Tol (2) and Cap-SBA-15-NMe2-IPA (2’), respectively.
1
(at 66 W), and at least 16 K scans using the SPINAL-64 H decou-
pling method were used. For the 29Si NMR experiments, 10.0 kHz
Michael addition reaction
spin rate, 10 s recycle delay, 5 ms contact time, p/2 pulse width of
1
5 ms (at 25.5 W), and at least 16 K scans using the SPINAL-64 H de-
The Michael addition reaction was performed in both reaction
flasks and NMR tubes. The latter allowed us to monitor the reac-
tion progress and to reliably quantify the percentage conversion of
the reaction directly in real time at different temperatures by using
NMR spectroscopy. In a typical Michael addition experiment, an
NMR tube was charged with either Ext-SBA-15-NMe2 or Cap-SBA-
15-NMe2 (16 mg) and dried at 808C for 30 min. The NMR spectrom-
eter was then set up at the desired temperature. Freshly prepared
stock solution (in CDCl3) of trans-b-nitrostyrene (1 equiv, 0.2m in
the NMR tube) and the corresponding active methylene com-
pound (1.25 equiv, 0.25m in the NMR tube) in CDCl3 (total volume
of 0.6 mL) were added to the NMR tube, capped (sealed with par-
afilm), and shaken well. For an experiment at 08C, this addition
was performed in an ice bath and transferred quickly to the spec-
trometer. The tube was inserted into the spectrometer, and the re-
action was monitored by following the disappearance of olefinic
coupling method were used. The 13C and 29Si chemical shifts were
referenced by using adamantine and standard silica sample, re-
spectively, as external standards. The spectra were processed in
Bruker’s TopSpin (version 3.1) by using conventional techniques,
and a 50 Hz of line broadening window function was applied to all
the spectra. For future convenience, details of representative “ac-
quisition” parameters have been included in the Supporting
Information.
Synthesis of SBA-15 mesoporous silica material
SBA-15 was synthesized by using the templating agent Pluronic
123 in an acid solution, as reported previously by Zhao et al.[16d]
Typically, a solution of Pluronic 123/2m HCl/tetraethylorthosilicate/
H2O (2:12:4.3:26, mass ratio in grams) was prepared and stirred at
408C for 24 h. The solution was then aged at 658C for another
24 h. The resulting solution was filtered, and the solid material was
washed with a large amount of water, which produced a material
labeled as “as-synthesized mesostructured SBA-15”. The as-synthe-
sized mesostructured SBA-15 (4 g) was then dispersed in ethanol
(400 mL) and diethyl ether (400 mL) and stirred at 508C for 5 h to
remove the surfactant. After filtration and washing with ethanol,
the final solid material was dried in oven, which gave SBA-15 mes-
oporous silica.
1
proton peaks of the starting materials in H NMR.
Acknowledgements
T.A. gratefully acknowledges the financial assistance provided by
the US National Science Foundation (grant numbers CAREER
CHE-1004218 and NSF DMR-0968937), the NSF-American Com-
petitiveness and Innovation Fellowships (NSF-ACIF), and the NSF
Creativity Award. We thank Dr. Thomas J. Emge for powder XRD
analysis.
Keywords: acid–base catalyst · bifunctional nanocatalyst ·
heterogeneous catalysis · mesoporous materials · Michael
addition
Synthesis of tertiary amine-functionalized SBA-15 meso-
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excess DATS (3.68 mmol, 1.5 mL) in anhydrous toluene (Tol;
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Received: August 11, 2012
Revised: October 15, 2012
Published online on January 29, 2013
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ChemCatChem 2013, 5, 910 – 919 919