Sustainable, green protocols for heterogenized organocatalysts: N-Phenylthiazolium salts heterogenized on organic-inorganic hybrid mesoporous supports

Article abstract of DOI:10.1016/j.molcata.2010.08.022

A heterogenized organocatalyst was synthesized by the covalent anchoring of N-phenyl-thiazolium salts over mesoporous organosilica (phenylene-silica, ethane-silica) and mesoporous silica (MCM-41) supports. Powder X-ray diffraction patterns as well as nitrogen physisorption analysis confirmed the retention of mesoporous structure after the grafting reactions. Solid-state NMR analysis (¹³C CP-MAS NMR, ²⁹Si MAS NMR) certified the integrity of organocatalyst residing inside the pore channels of the mesoporous supports. Catalytic evaluation in the benzoin condensation reaction as well as in the cross-coupling of aldehydes with acyl imines indicated that the organocatalyst heterogenized on mesoporous phenylene organosilica exhibited higher catalytic activity, stability and reusability than the analogous mesoporous silica support. The better activity and stability of the phenylene-silica support was attributed to the enhanced hydrophobic properties arising from the frame wall organic groups.

Full text of DOI:10.1016/j.molcata.2010.08.022

Journal of Molecular Catalysis A: Chemical 332 (2010) 65–69
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journal homepage: www.elsevier.com/locate/molcata
Sustainable, green protocols for heterogenized organocatalysts:
N-Phenylthiazolium salts heterogenized on organic–inorganic hybrid
mesoporous supports
Sankaranarayanapillai Shylesha, Zhou Zhoub, Qingguo Mengc, Alex Wagenera,
Andreas Seifertd, Stefan Ernsta, Werner R. Thiela,∗
a Fachbereich Chemie, TU Kaiserslautern, Erwin-Schrödinger-Str. Geb. 54, 67663 Kaiserslautern, Germany
b Erlangen Catalysis Resource Center, Friedrich-Alexander-Universitat Erlangen-Nurnberg, EgerlandStr. 3, 91058 Erlangen, Germany
c University of South Dakota, Chemistry Department, 414E Clark Street, Vermillion, SD, USA
d Institut für Chemie, TU Chemnitz, Straße der Nationen 62, 09111 Chemnitz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2010
Received in revised form 19 August 2010
Accepted 23 August 2010
Available online 8 September 2010
A heterogenized organocatalyst was synthesized by the covalent anchoring of N-phenyl-thiazolium
salts over mesoporous organosilica (phenylene–silica, ethane–silica) and mesoporous silica (MCM-41)
supports. Powder X-ray diffraction patterns as well as nitrogen physisorption analysis confirmed the
retention of mesoporous structure after the grafting reactions. Solid-state NMR analysis (13C CP-MAS
NMR, 29Si MAS NMR) certified the integrity of organocatalyst residing inside the pore channels of the
mesoporous supports. Catalytic evaluation in the benzoin condensation reaction as well as in the cross-
coupling of aldehydes with acyl imines indicated that the organocatalyst heterogenized on mesoporous
phenylene organosilica exhibited higher catalytic activity, stability and reusability than the analogous
mesoporous silica support. The better activity and stability of the phenylene–silica support was attributed
to the enhanced hydrophobic properties arising from the frame wall organic groups.
Keywords:
Organosilica
Mesoporous materials
Thiazolium salts
Organocatalysis
Heterogenization
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
organocatalysis can be made more greener by using heterogenized
systems.
Metal free methods for the synthesis of organic molecules
termed under organocatalysis have recently attracted widespread
attention [1]. A wide variety of reactions has been investi-
gated in this area of research, among them C–C couplings using
azole type catalysts are widely explored [2]. Interestingly, the
mechanisms behind these reactions often resemble those of
biological processes and hence a better mechanistic understand-
ing of organocatalytic reactions can alternatively improve our
understanding of the basic biocatalytic transformations [2]. How-
ever, in contrast to transition metal based catalytic reactions,
organocatalytic reactions are mainly homogeneous in nature [3].
Thus although organocatalytic transformations can be considered
as green processes, catalyst separation and reuse of the active
components still have to be worked out in detail. A simple solu-
tion relates to the heterogenization of organocatalysts on high
surface area solid supports [4]. Since the unique textural fea-
tures of (meso)porous materials allow high catalyst loadings and
maintain a uniform dispersion of active sites, the protocols of
The thiazolium motif usually found in thiamine (Vitamin B1) is
involved as a co-enzyme in a variety of enzymatically catalyzed
reactions, such as C–C couplings in the fatty acid and carbohy-
drate metabolism [5]. In organocatalytic reactions, thiazolium salts
turned out to catalyze a number of nucleophilic acylation reac-
tions such as the benzoin condensation and the Stetter reaction
by generating an acyl anion equivalent from an aldehyde [6].
Recently, we reported the first protocol for the heterogenization of
a N-phenylthiazolium organocatalyst on mesoporous MCM-41 [7].
Therein we noted that hydrophilic support is not really preferable
for recycling of this catalyst, since its activity decreases drasti-
cally already after the first run. However, we could prove that by
removing residual silanol groups and thereby alternating the sur-
face properties from hydrophilic to more hydrophobic, the catalytic
activity and the stability of the heterogenized catalyst systems can
be improved. These results prompted us to investigate the appli-
cation of hydrophobic periodic mesoporous organosilicas (PMOs)
as supports for thiazolium catalyzed reactions. Recently, it is well
accepted that hydrophobic surfaces can improve the catalytic activ-
ity and stability of certain active sites in inclusion chemistry [8,9].
PMOs are organic–inorganic hybrid materials having organic
moieties as bridging motifs in the frame wall positions. They are
∗
Corresponding author. Tel.: +49 631 2052752; fax: +49 631 2054647.
E-mail address: thiel@chemie.uni-kl.de (W.R. Thiel).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.08.022

66
S. Shylesh et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 65–69
the desorption branches of nitrogen isotherms using the BJH model.
Cl
N
Solid-state 13C and 29Si CP-MAS NMR spectra were recorded at
100.6 and 79.49 MHz, respectively, using a Bruker AVANCE 400
spectrometer.
+
Si(OMe)3
S
CH3CN
80°C, 24 h
2.3. Catalytic reactions
S
2.3.1. Benzoin condensation
Benzaldehyde was distilled in vacuum and triethylamine was
distilled under ambient pressure before use. The heterogenized
organocatalyst was suspended in the solvent under an atmosphere
of nitrogen and benzaldehyde was added followed by the drop-
wise addition of Et3N. The reactions were performed in refluxing
solvents under stirring. After the reaction, the mixture was filtered
under an atmosphere of N2 and the yield of benzoin was deter-
mined by GC–MS analysis.
N
Si(OMe)3
Cl-
1
Scheme 1. Synthesis of the thiazolium precursor 1.
obtained by a template directed condensation of bifunctional pre-
cursors of the type (RO)3Si–X–Si(OR)3 and have a homogeneous
distribution of organic fragments and silica moieties in the frame-
work [10]. Due to these organic moieties, they possess hydrophobic
surfaces which consequently improve their hydrothermal stabil-
ity compared to mesoporous materials derived from neat silica
and their applicability in various catalytic transformations. For
this purpose, hydrophobic PMOs possessing phenylene (–C6H4–)
and ethylene (–CH2CH2–) bridges were prepared by a templating
method using cetyltrimethylammonium bromide as the struc-
ture directing agent [8c]. An outstanding feature of the phenylene
derivative is its molecular organization of the pore walls wherein
hydrophobic arene and hydrophilic silica layers are alternatively
2.3.2. Cross-coupling reaction for the synthesis of a-amido
ketones
N-[(4-Ethylphenyl sulfonyl)(phenyl)methyl]cyclohexanecar-
boxamide was prepared according to a literature procedure [11].
The reactions were carried out in a 25 mL flask under nitrogen
atmosphere. Typically, 0.55 g (1.5 mmol) of the carboxamide and
300 mg of the heterogenized catalyst were set under vacuum at
35 ◦C for 1 h. Then 15 ml of the appropriate solvent was added fol-
lowed by the addition of 0.17 g (1.6 mmol) 4-pyridinecarbaldehyde.
An appropriate amount of distilled triethylamine was added in
one portion via a syringe to initiate the reaction. After the reaction,
the catalyst was filtered off and the product was isolated by flash
chromatography over silica (70% EtOAc and 30% hexane).
˚
arranged with a periodicity of 7.6 A [10]. The thiazolium precursor
1 was prepared by the synthetic procedure shown in Scheme 1.
2. Experimental
3. Results and discussion
2.1. Synthetic procedures
3.1. Structural characterization
2.1.1. Synthesis of 4,5-dimethyl-
Nucleophilic substitution at (p-chloromethyl)phenyltri-
methoxysilane using 4,5-dimethyl thiazole produces the ionic
precusor 1 in quantitative yields. The mesoporous supports were
developed according to literature protocols [7,8c]. Treatment of
phenylene (PMO-Ph) and ethylene bridged PMO (PMO-Et) as well
as silylated mesoporous MCM-41 (Sil-MCM) and MCM-41 (MCM)
with compound 1 in dry toluene gives the desired heterogenized
catalysts PMO-Ph-OC, PMO-Et-OC, Sil-MCM-OC and MCM-OC
(Scheme 2).
3-(4-trimethoxysilylphenylmethyl) thiazolium chloride (1)
1.13 g (10 mmol) of 4,5-dimethylthiazole and 2.47 g (10 mmol)
of (p-chloromethyl) phenyltrimethoxysilane were dissolved in
5 mL of dry CH3CN and the mixture was refluxed for 24 h under
inert conditions. All volatiles were removed in vacuum, leaving 1
as an yellow powder in quantitative yield.
2.1.2. Synthesis of the heterogenized catalysts
1.0 g of the desired mesoporous support was degassed at 150 ◦C
for 2 h and then suspended in 100 mL of dry toluene. To this suspen-
sion, 1 mmol of 1 was added and the mixture was heated to reflux
for 24 h. The solid was filtered, washed with ethanol and subse-
quently extracted with ethanol in a Soxhlet extractor for additional
12 h. The solid materials were then dried under vacuum.
The success of these syntheses can be demonstrated by means
of solid-state NMR spectroscopy. The 13C CP-MAS NMR spectra
S
N
Si(OMe)3
2.1.3. Synthesis of silylated supported catalyst (Sil-MCM-OC)
1.0 g of organocatalyst heterogenized MCM-41 support (MCM-
41-OC) suspended in 30 mL of toluene was treated with 1.0 g
(9.2 mmol) of Me3SiCl at room temperature under continuous stir-
ring for 24 h. The solid was then filtered, washed with ethanol and
dichloromethane and dried in vacuum.
Cl-
1
support
toluene, 110°C
S
O
2.2. Characterization
N
Cl-
Si
O
O
Powder X-ray diffraction patterns were obtained on a Siemens
D5005 diffractometer with Cu K␣ radiation (30 kV, 30 mA). Nitro-
gen adsorption desorption isotherms were measured at 77 K on
a Sorptomatic 1990 analyzer after evacuation of the samples at
413 K overnight. The specific surface areas were calculated using
the BET method and pore size distributions were evaluated from
PMO-Ph-OC, PMO-Et-OC,
Sil-MCM-OC, or MCM-OC
Scheme 2. Synthesis of the mesoporous supported organocatalysts.

S. Shylesh et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 65–69
67
100
(A)
(B)
d
c
110
200
b
a
d
c
b
a
200
150
100
50
0
-50
ppm
2
4
6
8
10 10
20
30
40
50
Fig. 1. Solid-state 13C C MAS NMR of (a) MCM-OC, (b) Sil-MCM-OC, (c) PMO-Et-OC
and (d) PMO-Ph-OC.
2 theta / degree
Fig. 3. (A) Small angle XRD patterns of (a) MCM-OC, (b) Sil-MCM-OC, (c) PMO-Et-
OC, (d) PMO-Ph-OC and (B) wide angle XRD patterns of PMO-Ph-OC showing the
crystalline nature of the pore walls.
of PMO-Ph-OC (PMO-Et-OC) show sharp peaks at ı ca. 133.7
(5.3) ppm, corresponding to the –Si–C6H4–Si– (–Si–CH2CH2–Si–)
groups in the framework walls (Fig. 1). All heterogenized samples
additionally exhibit resonances in their 13C CP-MAS NMR spectra
similar to those of the thiazolium precursor 1, but with decreased
intensities for the methoxy groups which are cleaved off during the
condensation process. In PMO-Ph-OC, the shoulder peak centering
at ı ca. 127 ppm can be assigned to the aromatic carbon atoms of
the benzyl group of the thiazolium salt, demonstrating the success-
ful support modification. Resonances at ı 58, 49, and 12 ppm give
additional evidence for the presence of thiazolium sites covalently
bound to the mesoporous solid supports. Remnant ethanol from
the extracting process produces resonances at ı 60 and 16 ppm
and the –OSiMe3 groups on the surface of Sil-MCM-OC give a very
sharp resonance at −0.56 ppm confirming the surface silylation of
the mesoporous solid.
signals between −110 and −90 ppm (Q sites) typical for the silica
frame work and weak resonances between −75 and −65 ppm (T
sites) originating from the heterogenized thiazolium precursor 1.
The silylated Sil-MCM-OC sample shows an additional resonance
at 12 ppm for the −SiOMe3 groups.
Powder XRD patterns of the samples with heterogenized thia-
zolium sites exhibit characteristic peaks at low angles related to the
hexagonal mesoporous structure (Fig. 3). Additional peaks appear
for PMO-Ph-OC in the range of 10◦ < 2ꢀ < 50◦ which can be assigned
˚
to a periodicity with a spacing of 7.6 A in the frame walls [12].
N2 adsorption/desorption experiments produced type IV
isotherms typical for mesoporous materials having a uniform pore
structure (Fig. 4). Compared to the pristine mesoporous supports,
the modified materials showed a decrease in surface area, pore
size, and pore volume due to the presence of bulky organic groups
inside the pore channels. The measured BET surface areas and BJH
pore sizes of all thiazolium heterogenized mesoporous solids are
collected in Table 1. Furthermore, the catalyst loading of all meso-
porous materials was found to be 0.34–0.48 mmol g−1 (calculated
from the elemental analysis), corresponding to ∼10 wt% of organic
moieties in the materials (Table 1).
The 29Si MAS NMR spectrum of PMO-Ph-OC displays three sig-
nals (Fig. 2), which can be assigned to T2 and T3 silicon centres
bound to phenylene moieties (−81.2 and −71.4 ppm) and to the
linker unit of the thiazolium motif (−71.4 and −63.3 ppm) [12].
PMO-Et-OC shows two peaks: one at −58 (T2: C(OH)Si(OSi)2) and
the another at −67 ppm (T3: CSi(OSi)3). There are no resonances
typical for Qn species (Qn = Si(OSi)n (OH)4−n, n = 2–4), which means
that no silicon–carbon bond cleavage occurred during the synthesis
of the supports and the subsequent high temperature grafting pro-
cess. As expected, the 29Si MAS NMR spectrum of MCM-OC shows
Thermogravimetric data were recorded in air for the surfactant
free phenylene–silica PMO-Ph and thiazolium modified PMO-Ph-
OC material (Fig. 5). The TG–DTG curve of neat PMO-Ph exhibits a
T2
T3
a
800
+200
d
c
600
400
200
0
+100
b
c
Q3
Q4
d
b
a
0.0
0.2
0.4
0.6
P/Po
0.8
1.0
50
0
-50
-100
-150
ppm
Fig. 2. Solid-state 29Si MAS NMR of (a) MCM-OC, (b) Sil-MCM-OC, (c) PMO-Et-OC
and (d) PMO-Ph-OC.
Fig. 4. . Nitrogen adsorption–desorption isotherms of (a) PMO-Ph-OC, (b) PMO-Et-
OC, (c) MCM-OC and (d) Sil-MCM-OC.

68
S. Shylesh et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 65–69
Table 1
Textural and compositional details of the mesoporous solids functionalized with the thiazolium organocatalyst.
a
Sample
SBET (m2 g−1
)
Vpore (cm3 g−1
)
Dpore (nm)
TOG (mmol g−1
)
Wt. loss (%)b
PMO-Ph-OC
PMO-Et-OC
Sil-MCM-OC
MCM-OC
882
735
766
897
0.87
0.81
0.64
0.71
2.57
2.51
2.52
2.69
0.34 (0.33)
0.39
0.34
9.9
–
10.7
9.4
0.48 (0.18)
a
Total amount of organic moieties, determined by elemental analysis.
b
Weight loss between 200 and 500 ◦C (from TG data). Values in parenthesis show the results of the second run.
small weight loss below 100 ◦C with an endothermic peak, corre-
sponding to the desorption of physisorbed water. No weight loss for
the decomposition of remnant surfactant in the range of 100–300 ◦C
was noted, confirming the complete removal of the surfactant by
solvent extraction. A main weight loss occurs from 500 to 650 ◦C
with a strong exothermic peak corresponding to the combustion
and release of organic fragments from the pore walls [13]. For PMO-
Ph-OC, a weight loss with an exothermic peak can be observed
from 200 to 350 ◦C, which can be assigned to the partial decom-
position of the thiazolium salt since this feature is absent in the
decomposition curve of PMO-Ph. These results also confirm the
successful introduction of the organocatalyst into the mesoporous
organosilica support.
the catalysts was examined by isolating it from the reaction mix-
ture (centrifugation, washing with ethanol and dichloromethane
and drying). The catalytic activity of PMO-Ph-OC remains almost
unchanged upon reuse for three consecutive runs, whereas MCM-
OC showed very sharp decrease in activity already after the second
run (Table 2). Ethylene bridged PMO-Et-OC and silylated Sil-
MCM-OC showed almost comparable reactivity and stability. The
pronounced decrease in activity of the MCM-OC probably relates
to a structural degradation of the support by the basic and pro-
tic reaction medium, which results in a leaching of the active
sites and in a collapse of the pore structure. The spent MCM-
OC catalyst showed disordered mesoporous structure (from XRD
patterns and N2 physisorption), a loss of catalytic active sites
(from elemental analysis) and thereby a decrease in catalytic
activity. Such effects will be decreased by increasing the hydropho-
bicity and structurally stability of the supports. The crystalline
pore walls of PMO-Ph-OC may provide a special benefit in this
context.
3.2. Catalytic activity evaluation
The activity of the heterogenized thiazolium catalysts was
investigated for the benzoin condensation (Scheme 3) as well as
for the cross-coupling of aldehydes with acyl imines (Scheme 4),
which are typical test reactions for thiazolium based organocat-
alytic transformations. Optimization of the solvent and the amount
of external base provided that methanol as the solvent and an Et3N
concentration of 0.22 mol/L gave the highest productivity for the
benzoin condensation (Table 2). Under identical conditions, PMO-
Ph-OC showed the best activity among all supported catalysts. The
organocatalyst loading was almost similar for all mesoporous mate-
rials evaluated.
As a further proof of principle, the thiazolium catalyzed cross-
coupling of aldehydes with acyl imines to ␣-amidoketones was
investigated (Scheme 4) [11,14]. This reaction effectively produces
biologically relevant ␣-amido ketones and the presence of Et3N
as the base assists in the conversion of tosylamides to the acyl
imine as well as in the deprotonation of the thiazolium moiety. As
expected, the coupling reaction between 4-pyridine carbaldehyde
O
For practical applications of heterogeneous catalysts the sta-
bility and reusability are important factors. The recyclability of
O
S
O O
H
+
N
N
H
100
60
50
40
30
20
10
0
0
95
90
85
80
75
70
catalyst
-1
-2
-3
-4
-5
H
N
O
O
N
Scheme 4. Cross-coupling of aldehydes with acyl imines to amido ketones.
Table 2
200
400
600
800
Catalytic activity in the benzoin condensation of benzaldehyde.a
Temperature ºC
Entry
Catalyst
Solvent
[BA]:[Et3N]
Yield (%)b
1
2
3
4
5
6
PMO-Ph-OC
PMO-Ph-OC
PMO-Ph-OC
PMO-Et-OC
Sil-MCM-OC
MCM-OC
CH2Cl2
CHCl3
MeOH
MeOH
MeOH
MeOH
3:1
3:1
3:1
3:1
3:1
3:1
55
42
85 (73)
71 (48)
66 (36)
62 (18)
Fig. 5. Thermogravimetric (TG) and differential thermogravimetric (TG–DTG) anal-
yses of pristine phenylene silica PMO-Ph (solid lines) and PMO-Ph-OC (dash lines).
O
O
catalyst
H
2
a
OH
Reaction conditions: benzaldehyde (BA, 0.67 mol/L), Et3N (0.22 mol/L), reflux
temperatures, t: 24 h.
b
Scheme 3. Benzoin condensation starting from benzaldehyde.
Determined by GC–MS, values in parenthesis show the results of the second run.

S. Shylesh et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 65–69
69
Table 3
funding Z.Z. This work was further supported by the joint research
project “NanoKat” at the TU Kaiserslautern.
Organocatalytic addition of 4-pyridine carbaldehyde to N-[(4-methylphenyl
sulfonyl)(phenyl)methyl]cyclohexanecarboxamide.a
Entry
Catalyst
Yield (%)b
Appendix A. Supplementary data
1
2
3
4
PMO-Ph-OC
PMO-Et-OC
Sil-MCM-OC
MCM-OC
91 (86)c
89
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.08.022.
90 (79)d
82 (71)d
a
Reaction conditions: N-[(4-methylphenylsulfonyl)(phenyl)methyl]cyclohexane
carboxamide (1 equiv.), 4-pyridine carbaldehyde (1.1 equiv.), base (Et3N, 5 equiv.),
catalyst (7 mol%), t: 2 h, CH2Cl2 (15 mL), T: 35 ◦C.
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Acknowledgments
We thank the Alexander-von-Humboldt Foundation for
research grant to S.S. and the Deutsche Forschungsgemeinschaft for
a