The role of the support properties in the catalytic performance of an anchored copper(ii) aza-bis(oxazoline) in mesoporous silicas and their carbon replicas

Article abstract of DOI:10.1039/c2cy20638b

A copper(ii) chiral aza-bis(oxazoline) catalyst (CuazaBox) was anchored onto ordered mesoporous silicas and their carbon replicas. The materials were characterized by elemental analysis (C, N, H, S), ICP-AES, FTIR, XPS, thermogravimetry and isotherms of N₂ adsorption at -196 °C. The materials were tested as heterogeneous catalysts in the reaction of cyclopropanation of styrene to check the effect of porous material type on the catalytic parameters, as well as on their reutilization. Generally, the composites were more active and enantioselective in the cyclopropanation of styrene than the corresponding homogeneous phase reaction run under similar experimental conditions. The materials pH_pzc proved to be an important factor not only in the CuazaBox anchoring yields, but also in their catalytic performance. Less acidic surfaces (SPSi and CMK-3) yielded heterogeneous catalysts with higher styrene conversion and enantioselectivity. The materials could also be recycled with comparable enantioselectivities or generally a slight decrease in the enantioselectivity.

Full text of DOI:10.1039/c2cy20638b

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The role of the support properties in the catalytic
performance of an anchored copper(II) aza-bis(oxazoline)
in mesoporous silicas and their carbon replicas†
Cite this: Catal. Sci. Technol., 2013,
, 659
3
a
a
b
b
Ana Rosa Silva,* Vanessa Guimar a˜ es, Ana Paula Carvalho and Jo a˜ o Pires
A copper(II) chiral aza-bis(oxazoline) catalyst (CuazaBox) was anchored onto ordered mesoporous silicas
and their carbon replicas. The materials were characterized by elemental analysis (C, N, H, S), ICP-AES,
FTIR, XPS, thermogravimetry and isotherms of N
2
adsorption at À196 1C. The materials were tested as
heterogeneous catalysts in the reaction of cyclopropanation of styrene to check the effect of porous
material type on the catalytic parameters, as well as on their reutilization. Generally, the composites
were more active and enantioselective in the cyclopropanation of styrene than the corresponding
homogeneous phase reaction run under similar experimental conditions. The materials pHpzc proved to
be an important factor not only in the CuazaBox anchoring yields, but also in their catalytic
performance. Less acidic surfaces (SPSi and CMK-3) yielded heterogeneous catalysts with higher styrene
conversion and enantioselectivity. The materials could also be recycled with comparable enantio-
selectivities or generally a slight decrease in the enantioselectivity.
Received 12th September 2012,
Accepted 22nd October 2012
DOI: 10.1039/c2cy20638b
www.rsc.org/catalysis
1. Introduction
The synthesis of chiral cyclopropanes remains a considerable
challenge, especially due to the fact that cyclopropane rings are
often found in a variety of natural products and biologically
1
,2
active compounds. Therefore, it is crucial to develop useful
methods for the synthesis of cyclopropanes. The catalytic
asymmetric cyclopropanation of alkenes with diazoacetates
Scheme 1 Commercial bis(oxazoline) (1 and 2) and aza-bis(oxazoline) (3) chiral
has been intensively studied over the last few years by copper ligands.
complexes with chiral nitrogen ligands such as bis(oxazoline)
2
3,4
(
1 and 2, Scheme 1) and aza-bis(oxazoline) (3, Scheme 1).
These homogeneous catalysts have also demonstrated excellent product contamination due to incomplete separation of the
results in several other asymmetric catalytic reactions, including catalyst and problems with expensive catalyst recovery are
5
6
7
8
10
asymmetric aziridination, Diels–Alder, Henry reactions, etc.
frequent. Heterogeneous catalysts have obvious advantages over
Chiral aza-bis(oxazoline) ligands (3) are obtained in several steps homogeneous catalysts with respect to easy separation of the
3
,8,9
from derivatives from chiral amino alcohols.
There are different advantages associated with the choice efficient recycling, improved handling and process control, mini-
catalyst from the reaction mixture at the end of the reaction,
10–13
between homogeneous and heterogeneous catalysts. Homo- mization of metal leaching and low cost.
However, this type of
geneous catalysts exhibit high activity and selectivity in several catalysts often present limited activity and selectivity, and hence it
organic reactions. Nevertheless, there is the possibility of is important to find additional value information to overcome
their disadvantages and convince the organic synthesis commu-
a
11
Departamento de Qu ´ı mica, CICECO, Universidade de Aveiro,
Campus Universit a´ rio de Santiago, 3810-193 Aveiro, Portugal.
E-mail: ana.rosa.silva@ua.pt; Fax: +351 234 401 470; Tel: +351 234 370 604
CQB, Departamento de Qu ´ı mica e Bioqu ´ı mica, Faculdade de Ci ˆe ncias,
Universidade de Lisboa, 1749-016 Lisboa, Portugal
nity and industry of the advantages of heterogeneous catalysts.
In recent years, the interest in ordered mesoporous materials
has been growing due to their advantageous properties over
traditional porous materials such as high dispersion of catalytic
b
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20638b sites, resulting in a well-defined surface and uniform pore size
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Scheme 2 Anchoring procedure for the copper(II) aza-bis(oxazoline) complex onto the mesoporous materials (MM).
1
4,15
distribution, and large specific surface area.
Hence, these copper(II) trifluoromethanosulfonate (copper triflate, [Cu(CF SO ) ]
3
3 2
materials are recognized as being an excellent option for or [Cu(Otf) ], 98%), (S)-tert-leucinol (98%), sodium cyanide
2
1
5
support of homogeneous catalysts.
(Z97.0%), bromine (reagent grade), ammonia solution 2.0 M
Ryoo et al. reported the first synthesis of ordered meso- in methanol, diethyl carbonate (Z98.0%), sodium ethoxide
porous carbon (OMC) CMK-3 via a nanocasting route using (Z95%), triethyloxonium tetrafluoroborate solution 1.0 M in
1
6
SBA-15 mesoporous silica as the template in 1999. This dichloromethane, p-toluenesulfonic acid monohydrate (Z98.5%),
method consisted of the use of a regular rigid template (SBA-15), sodium hydroxide (p.a.), sodium bicarbonate (p.a.), sodium
filled with a precursor, and posterior polymerization, carboni- sulphate (Z99.0%), sand (50–70 mesh particle size), triethyl-
zation and removal of the template. On the other hand, Ting amine (Et N, Z99%), 3-bromopropyltrimethoxysilane (BrPS,
3
et al. described a method that allows the one pot synthesis of Z97.0%), deuterated chloroform (99.8 atom% D, 0.03% (v/v)
mesoporous materials with different types of porous regular- TMS), dry toluene (99.8%), dry tetrahydrofuran (THF, Z99.9%),
1
7
ity. In this case the silica material (SPSi) and its replicate methanol (p.a.), styrene (Z99%), n-undecane (Z99%), ethyl-
1
5,17
(
SPC) are synthesized at the same time.
Carbonization and diazoacetate (EDA, r15% dichloromethane), phenylhydrazine
removal of the silica template with HF yields mesoporous (97%) and potassium bromide (FT-IR grade, Z99%) were
carbon, whereas heating in air leads to mesoporous silica.
1
5,17
purchased from Aldrich and used as received. Sodium hydride
We already reported the asymmetric cyclopropanation of (55–65%) was from Fluka. Ammonium chloride (p.a.), ethanol
1
8
styrene by copper(II) bis(oxazoline) encapsulated onto zeolites
and anchored onto mesoporous silicas
(p.a.) and HCl (37%) were from Panreac. H SO (95–97%) from
2 4
1
9,20
20
and carbons. The Jos ´e Manuel Gomes dos Santos Lda. Ethyl acetate (>99.9%),
best results were obtained using the anchored copper complex n-hexane (95%) and dichloromethane (DCM, Z99.9%) were
with ligand 2 (2Cu catalyst, Scheme 1) onto mesoporous silica, from Romil. Silica gel 60 (63–200 mm) and TLC silica gel 60 F254
but generally low to moderate enantioselectivities were were from Merck.
2
0
obtained with low recyclability.
2
.2. Preparation of SBA-15
It has been reported that in ligands 3 (Scheme 1), the presence
of a nitrogen atom instead of a carbon atom at the center leads to
a stronger interaction between the chiral ligand and the copper
than in the case of bis(oxazoline) (Box, Scheme 1) and hence
2
3,24
SBA-15 synthesis was based on the literature.
About 4.0 g of
the co-polymer (EO)20(PO)70(EO)20 was dissolved in a concen-
trated HCl solution. Then tetraethyl orthosilicate (TEOS) was
added to this solution with constant stirring. The obtained
solution was stirred at 35 1C for 24 hours and subsequently
heated at 100 1C for 24 hours, under static conditions. After-
wards, the solution was filtered and the filtrate was set aside for
4
higher stability of the copper complex formed. Therefore better
enantioselectivities in the asymmetric cyclopropanation of
alkenes and stability upon reuse have been observed for these
immobilized homogeneous catalysts onto siliceous mesocellular
3,4,21,22
silica foams and organic polymers.
At the same time, more
drying in air. The dried up filtrate was calcinated at 550 1C for
5
À1
flexibility after immobilization is obtained with these catalysts
h with a ramp of 1 1C min
.
3
than with bis(oxazoline), 1 and 2 type ligands (Scheme 1).
Therefore, herein, we report the anchoring of copper(II) 2.3. Preparation of CMK-3
aza-bis(oxazoline) (3) onto several unexplored mesoporous silicas
24
The preparation of CMK-3 was based on previous works.
(
SBA-15, SPSi, HMS) and carbon materials (CMK-3, SPC), in order
2 4
Briefly, solid sucrose was dissolved in a concentrated H SO
solution to which the SBA-15 sample was added under mixing.
The as obtained paste was heated in an oven at 100 1C for
to evaluate the effect of the type of structure and properties of the
materials on the activity, enantioselectivity and recyclability of the
heterogeneous catalysts (Scheme 2).
6
hours and subsequently at 160 1C for 6 hours. The resulted
black powder was then mixed again with sucrose solution
in H SO ) and reheated with a similar heating program. The
(
2
4
2
. Experimental section
obtained black powder was further carbonized, at 875 1C for
2
.1. Materials
Tetraethyl orthosilicate (TEOS, 98%), (EO)20(PO)70(EO)20
sucrose (Z99.5%), HF (Z48%), 1-dodecylamine (98%), by repeated washing with distilled water (until neutral pH).
À1
1
hour with a ramp of 10 1C min , under vacuum. The silica
,
template was then removed by treatment with HF acid followed
6
60 Catal. Sci. Technol., 2013, 3, 659--672
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The so obtained carbon (CMK-3) was then dried at 95 1C and
stored in a dry place.
Synthesis of oxazolidinone. Secondly, oxazolidinone was
synthesized; 8.53 mmol of sodium ethoxide was added to a
solution of 8.53 mmol of (S)-tert-leucinol and 9.33 mmol of
diethyl carbonate in ethanol (12 mL) and the mixture was
2.4. Preparation of SPSi and SPC in one step
In this methodology a silica–carbon composite material is refluxed for 15 hours. The solvent was evaporated under
firstly formed. If heated, the composite will produce a silica reduced pressure and then the residue was dissolved in DCM
material (SPSi) or, if carbonized, will produce the SPC carbon. (25 mL). The solution was washed with a saturated solution of
The preparation of the porous silica SPSi and the corres- ammonium chloride (13 mL) and then the aqueous phase was
1
5
ponding carbon replica SPC was described in detail elsewhere.
extracted 2 times with DCM (13 mL). The combined organic
SO and the solvent
In short, the co-polymer (EO)20(PO)70(EO)20 was dissolved in extracts were dried with anhydrous Na
2
4
water and a 2 M HCl solution was added under stirring. To the evaporated under reduced pressure. Finally, the white solid was
stirred mixture concentrated H SO , sucrose and tetraethyl- dried under vacuum for 2 days. The substance was used for the
2
4
orthosilicate were added. The solution was kept at 50 1C, and next step without further purification. Yield: 1.10 g (90%);
1
stirred for 24 hours, then it was aged hydrothermally in an
3
H NMR (300 MHz, CDCl ), d/ppm: 5.78 (s, 1H), 4.35–4.41
autoclave at 100 1C for a further 24 hours. The obtained (t, J = 9.0 Hz, 1H), 4.18–4.23 (ddd, J = 12.6, 5.8, 1.3 Hz, 1H),
composite was dried in an oven at 100 1C and then in a furnace 3.57–3.62 (ddd, J = 8.9, 5.8, 1.1 Hz, 1H), 0.91 (s, 9H); MS (ESI),
+
+
À1
at 160 1C for 6 hours. When heated in air under static condi- m/z: 144.10 [M + H] , 166.08 [M + Na] ; FTIR, n/cm : 3300 s,
tions at 550 1C for 6 hours, the synthesized composite gener- 2970 s, 1745 vs, 1720 vs, 1481 m, 1402 m, 1369 m, 1238 s, 1101
ated the silica sample (SPSi). The corresponding carbon sample m, 1053 m, 984 m, 926 m.
(SPC) was formed via carbonization of the composite under a
Synthesis of ethoxyoxazoline. Thirdly, ethoxyoxazoline was
dry nitrogen flow at 900 1C for 12 hours, and then the silica synthesized; 10.1 mmol of a triethyloxonium tetrafluoroborate
being removed with a 40% HF. The solid was finally filtered, solution in dichloromethane (10.1 mL, 1.0 M) was added
washed with water and ethanol, and finally dried in a oven at 70 1C. dropwise for 30 minutes to 7.71 mmol of oxazolidinone in
anhydrous DCM (16 mL) at 0 1C, in an inert atmosphere. The
2.5. Preparation of HMS
solution was stirred for 30 hours at room temperature, in an
25
HMS was made according to the literature. A solution of 10.3 mL inert atmosphere, and then it was added to a cold saturated
of 1-dodecylamine in 88.1 mL of ethanol, and 88.5 mL of solution of sodium hydrogenocarbonate (32 mL). The organic
deionized water, was stirred for a few minutes. Then 37 mL phase was separated and the aqueous phase extracted 3 times
of tetraethyl orthosilicate (TEOS) was added. The mixture was with DCM (8 mL). The organic phases were dried with anhy-
kept under stirring at room temperature for 24 hours. The drous sodium sulphate and then an oil was obtained after
white product was then filtered and washed with deionized water solvent evaporation under reduced pressure. The substance was
until the pH is neutral and then with ethanol. The final product used for the next step without further purification. Yield: 1.22 g
À1
1
was calcinated at 600 1C for 6 hours with a ramp of 1 1C min
.
(92%); H NMR (300 MHz, CDCl
3
), d/ppm: 4.15–4.30 (m, 4H),
3
.73–3.78 (m, 1H), 1.32–1.37 (t, J = 7.1 Hz, 3H), 0.88 (s, 9H); MS
2.6. Preparation and characterization of aza-bis(oxazoline) (3)
+
À1
(ESI), m/z: 172.13 [M + H] ; FTIR, n/cm : 3566 m, 3386 m, 3249
The synthesis of aza-bis(oxazoline) (3, Scheme 1) was performed m, 2966 s, 1732 vs, 1670 vs, 1537 s, 1479 m, 1406 m, 1371 m,
3
,9
in four steps, by adapting reported procedures.
Synthesis of aminoxazoline. First, aminoxazoline was
1263 s, 1057 s (broad), 937 m.
Synthesis of aza-bis(oxazoline) (3). Finally, aza-bis(oxazoline)
synthesized; 9.38 mmol of sodium cyanide was added in small was synthesized; 0.61 mmol of p-toluenesulfonic acid were
portions to a solution of 9.38 mmol of bromine in methanol added to 3.80 mmol of ethoxyoxazoline and 6.05 mmol of
(12 mL) at 0 1C. Then 8.53 mmol of (S)-tert-leucinol in methanol aminoxazoline in 40 mL of dry toluene and refluxed for
(2 mL) was added and the mixture was stirred at room tem- 24 hours, in an inert atmosphere. The solvent was evaporated
perature for one hour. After treatment with ammonia solution under reduced pressure and the crude product was purified by
in methanol (31 mL, 2.0 M), the solvent was evaporated under chromatography on silica gel using ethyl acetate/n-hexane 9 : 1
reduced pressure. The residue was dissolved in an aqueous as the eluent to yield an oil. Colorless crystals could be obtained
1
solution of NaOH (36 mL, 20% w/w) and was then extracted by recrystallization from acetone. Yield: 0.69 g (68%); H NMR
5
times with 15 mL of ethyl acetate. The combined organic (300 MHz, CDCl
3
), d/ppm: 4.27–4.33 (t, J = 9.2 Hz, 2H),
phases were dried with anhydrous Na SO , the solvent was 4.12–4.17 (dd, J = 6.7and 8.9 Hz, 2H), 3.78–3.84 (dd, J = 6.7
2
4
+
evaporated under reduced pressure, and the remaining (S)-tert- and 9.4 Hz, 2H), 0.90 (s, 18H); MS (ESI), m/z: 268.20 [M + H] ;
leucinol was removed under reduced pressure at 60 1C for FTIR, n/cm : 3438 m, 2958 m, 1637 s, 1585 s, 1387 m, 1092 m.
À1
2
days. The substance was used for the next step without
1
2.7. Preparation and characterization of the homogeneous
catalyst (3Cu)
further purification. White solid; yield: 0.86 g (71%); H NMR
(300 MHz, CDCl ), d/ppm: 4.19–4.25 (t, J = 8.8 Hz, 1 H), 4.05–4.10
3
(
1
3
dd, J = 7.1 and 8.2 Hz, 1 H), 3.72–3.77 (dd, J = 7.1 and 9.2 Hz, The copper(II) complex with ligand 3 was prepared by dissolu-
+
À1
H), 0.87 (s, 9 H); MS (ESI), m/z: 143.12 [M + H] ; FTIR, n/cm
:
2
tion in DCM of equimolar quantities of [Cu(OTf) ] (0.012 mmol)
425 s, 2954 s, 1699 vs, 1680 vs, 1649 m, 1406 m, 1011 m.
and azaBox (0.012 mmol) and stirring for 4 hours at room
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temperature. The solution was filtered and upon evaporation of mesoporous silicas at 100 1C for 6 hours, using a resolution of
À1
À1
the solvent a green oil was obtained. FTIR, n/cm : 2970 m, 4 cm and 256 scans.
1
728 m, 1626 s, 1261 vs, 1174 s, 1036 s, 648 s.
X-ray photoelectron spectroscopy was performed at Centro
de Materiais da Universidade do Porto (Portugal), using a
VG Scientific ESCALAB 200A spectrometer using non-
monochromatized Mg Ka radiation (1253.6 eV). All the materials
were compressed into pellets prior to the XPS studies. In order
to correct possible deviations caused by electric charge of the
samples, the C 1s line at 285.0 eV was taken as the internal
standard.
2.8. Anchoring of the CuazaBox (3Cu) onto the mesoporous
materials
2
1
This method was adapted from a reported procedure and has
three main steps: (i) functionalization of ligand 3, (ii) coordina-
tion of copper(II) onto ligand 3 and (iii) anchoring of 3Cu onto
the surface of the material (Scheme 2). Typically, 0.112 mmol of
The pH measurements for determination of the pHpzc (pH at
which the material has a net zero surface charge) were made
with a SympHony SP70P VWR pH meter. The assays were made
by reverse mass titration following the method proposed by
3
were reacted with 0.124 mmol of sodium hydride (1.1 equiv.)
in 2.5 mL of dry THF and the mixture was stirred at 60 1C for
hours. Then, 0.112 mmol of 3-bromopropyltrimethoxysilane
were added to the clear solution and further stirred overnight at
6
2
6
Noh and Schwarz. Thermogravimetry was performed under
6
1
0 1C. The solvent was evaporated under reduced pressure and
5 mL of DCM were added, followed by 0.112 mmol of
À1
air flux with a ramp of 5 1C min in a TG-DSC apparatus,
model 111 from Setaram.
[Cu(OTf)
2
]. After stirring for 24 hours at room temperature,
Nitrogen adsorption isotherms at À196 1C were measured in
an automatic apparatus (Asap 2010; Micromeritics). Before the
adsorption experiments the samples were outgassed under
vacuum for 2.5 h at 150 1C.
0
.50 g of mesoporous material was added to the green solution
and the mixture was stirred for 5 days at room temperature. The
resulting solid was filtered off, washed with DCM and then
further stirred with DCM overnight, at room temperature, to
remove untethered species. Finally, after filtration the solid was
dried under vacuum at 60 1C for 2 days, yielding green solids in
the case of the mesoporous silicas and obviously black in the
case of mesoporous carbon.
2.10. Catalysis experiments
The catalytic activity of the silica based materials, in the
cyclopropanation of styrene, was tested at room temperature
in batch reactors at atmospheric pressure and with constant
stirring. In the experiments, typically, 2.4 mmol of styrene,
3Cu@SBA-15: elemental analysis (%) N 0.51, C 3.12, H 0.90,
À1
S 0.94; ICP-AES Cu 1.0%; loading of 3 121 mmol g and loading
of Cu 157 mmol g
0
1
.65 mmol of n-undecane, 70 mg of heterogeneous catalyst in
0.0 mL of dichloromethane and 2 mL of phenylhydrazine were
À1
.
3Cu@CMK-3: elemental analysis (%) N 0.50, C 80.71, H 1.12,
used. Finally, 2.75 mmol of ethyldiazoacetate (EDA) was added
to the reaction mixture over 2 hours using a syringe pump.
When the addition of ethyldiazoacetate was completed,
aliquots (0.05 mL) were periodically withdrawn from the reac-
tion mixture with a hypodermic syringe, filtered through PTFE
À1
S 0.94; ICP-AES Cu 0.9%; loading of 3 119 mmol g and loading
of Cu 142 mmol g
À1
.
3Cu@SPSi: elemental analysis (%) N 0.64, C 3.92, H 0.70,
À1
S 0.89; ICP-AES Cu 1.0%; loading of 3 151 mmol g and loading
of Cu 157 mmol g
À1
.
0.2 mm syringe filters, and analyzed by GC-FID by the internal
3
Cu@SPC: elemental analysis (%) N 0.40, C 79.95, H 0.74;
standard method. At the end of the reactions the hetero-
geneous catalyst was recovered by filtration, washed extensively
with DCM, dried under vacuum and reused in another cycle
using the same experimental procedure.
Control experiments were also performed using this experi-
mental procedure in a homogeneous phase with 3Cu or equi-
À1
ICP-AES Cu 0.57%; loading of 3 94 mmol g and loading of Cu
9
À1
0 mmol g
.
3Cu@HMS: elemental analysis (%) N 0.35, C 2.61, H 1.13,
À1
S 0.78; ICP-AES Cu 0.88%; loading of 3 83 mmol g and loading
of Cu: 138 mmol g
À1
.
2
molar quantities of [Cu(OTf) ] plus 3 in order to compare with
the heterogeneous ones (Table 3).
2.9. Physical and chemical methods
Elemental analysis (C, N, H and S) was performed in duplicate
The reaction mixture was analyzed by GC-FID (using the
at Servicio de An ´a lisis Instrumental, CACTI Vigo, Universidade internal standard method) on a Varian 450 GC gas chromato-
de Vigo, Spain, or at Department of Chemistry of the University graph equipped with a fused silica Varian Chrompack capillary
of Aveiro, Portugal. The copper ICP-AES was performed at column CP-Sil 8 CB Low Bleed/MS (15 m  0.25 mm id; 0.15 mm
‘
‘Laborat o´ rio Central de An ´a lises’’ of the University of Aveiro, film thickness), using helium as a carrier gas. The enantiomeric
Portugal.
excesses (%ee) of cyclopropanes were determined in the same
FTIR spectra of the materials were obtained as KBr pellets chromatograph but using a fused silica Varian Chrompack
(
2.5–3 mg of sample diluted with 200 mg of KBr or for oils a capillary column CP-Chiralsil-Dex CB (25 m  0.15 mm i.d.
À1
drop on the surface of a KBr pellet), in the range of 400–4000 cm
,
 0.25 mm film thickness). Conditions used: 60 1C (3 min),
À1 À1
with a FT Mattson 7000 galaxy series spectrophotometer or by 5 1C min , 170 1C (2 min), 20 1C min , 200 1C (5 min); injector
ATR (mesoporous silicas) with a Bruker Tensor 27 spectro- temperature, 200 1C; detector temperature, 230 1C. The several
photometer; all spectra were collected at room temperature, chromatographic peaks were identified against commercially
after drying the pellets in an oven at 75 1C overnight or the available samples and/or by GC-MS (Finnigan Trace).
6
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3
. Results and discussion
centered at À91.5, À100.9 and À110.1 ppm (Table 1), in
1
9
2
3
agreement with the literature. The amount of Q and Q Si
sites due to the silanols [Si–(OSi) –(OH) ] and [Si–(OSi) –OH],
3.1. Materials characterization
2
2
3
Parent mesoporous materials. In this work three different respectively, is significantly lower than the silica framework Q4
mesoporous silicas that have never been used as supports for sites, [Si–(OSi) ] (Table 1). Hence all mesoporous silicas possess
4
2
3
aza-bis(oxazoline) ligands were synthesized: SBA-15, an ordered the essential Q and Q silanols for their surface post-function-
mesoporous silica well-known and used in heterogeneous alization in the order HMS > SBA-15 > SPSi. This order is the
1
0,12
catalysis;
described by Ting et al.
HMS) prepared by using a simple procedure reported by Tanev silanols.
SPSi, prepared by a one pot methodology same for the materials surface area and thus the higher the
1
5,17
and a hexagonal mesoporous silica materials surface area the higher the amount of available
(
2
5
and Pinnavaia. The analysis of the isotherms of adsorption of
nitrogen at À196 1C (Fig. 1) shows that SBA-15 and SPSi possess correlated with the acidity, which might influence the course
nm mesopores, whereas the HMS possesses a smaller meso- of asymmetric organic transformations and/or the stability of
pore of about 2.5 nm (Table 1). The BET areas follow the order the immobilized homogeneous catalysts. The pHpzc values for
HMS > SBA-15 > SPSi (Table 1). The Si MAS and CPMAS NMR SBA-15 and SPSi were 3.5 and 5, respectively, which is in
of these materials (Fig. 2) clearly show three different Si types: accordance with the amount of silanols on the surface. But
Nevertheless, the amount of surface silanols might be
7
2
9
2
3
4
Q , Q and Q . Deconvoluting the first spectra using three for the HMS sample the pHpzc value was 4.1, higher than
Gaussian peaks, a perfect fit is obtained (Fig. S1 of ESI†), that for SBA-15 which possesses lower amounts of silanols.
Fig. 1 (a) and (b) Nitrogen adsorption–desorption isotherms, at À196 1C, and mesopore size distributions for the indicated samples (open points adsorption; closed
points desorption). (c) and (d) Mesopore size distributions from the BdB method.
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29
a
Table 1 Deconvolution of the Si MAS NMR of the mesoporous silicas, pHpzc and textural properties of all the materials
2
3
4
Q
Q
Q
Textural properties
b
c
2
À1
d
3
À1
e
%
Area
ppm (width)
% Area
ppm (width)
% Area
ppm (width)
pHpzc
A
BET /m g
V
total /cm g
w /nm
HMS
SBA-15
SPSi
CMK-3
SPC
4.5
3.5
2.5
À91.9 (4.2)
À92.0 (4.8)
À90.7 (4.6)
29.4
21.5
18.2
À101.0 (6.4)
À100.9 (6.3)
À100.7 (6.1)
66.1
75.0
79.3
À110.0 (8.4)
À110.1 (8.6)
À110.2 (9.1)
4.1
3.5
5.0
4.7
3.3
994
756
571
1396
1140
0.72
0.99
0.84
1.26
1.23
2.4; 2.7
7.1
6.6; 7.4
4.0; 7.7
3.7–5.8
a
b
c
d
Deconvoluted spectra in Fig. S1 of ESI. pH at which the material has a net zero surface charge. Specific surface area. Total porous volume.
e
Maxima in the widths (w) of the mesopore size distributions for the various samples.
of 4.0 and 7.7 nm for CMK-3 and in the range of 3.7–5.8 for SPC.
The pHpzc values are 4.7 and 3.3, for CMK-3 and SPC, respec-
tively. Hence lower surface acidity was obtained for CMK-3
when compared to its parent SBA-15, whereas the SPC possesses
higher surface acidity than SPSi. Again by using two different
synthesis methodologies two mesoporous carbon materials were
obtained with different surface and textural properties.
Composition modified materials. 3-Bromopropyltrimethoxy-
silane functionalized copper(II) aza-bis(oxazoline) (CuazaBox,
3Cu) was anchored according to Scheme 2 onto the two ordered
mesoporous silicas, SBA-15 and SPSi, their carbon replicas,
CMK-3 and SPC, respectively, and a smaller pore mesoporous
silica (HMS). The alkoxy groups of the functionalized 3Cu
complex react with the silanol groups at the surface of the
mesoporous silicas and the surface phenol groups of the
carbon materials, likewise our previous work on the immobili-
2
7,28
zation of homogeneous catalysts onto activated carbons.
As
described in the experimental section, at the end of the
anchoring procedure the materials were extensively washed
and refluxed in order to remove the eventual physisorbed
catalyst. Thus simple physisorption onto the carbon surface
can be ruled out, besides p–p interactions since the azaBox
ligand does not present a p system, like the previously explored
salen ligands. This is also supported by the fact that the
3Cu@SPC heterogeneous catalyst did not deactivate upon reuse
and this heterogeneous catalyst and 3Cu@CMK-3 also work
with lower homogeneous catalyst densities than in the case of
the mesoporous silicas (see Section 3.2).
The elemental analysis of all the materials, compiled in
Table 2, shows the presence of nitrogen which indicates that
the anchoring of the chiral ligand 3 onto the materials was
successful. Taking into consideration that a 3 molecule
contains three atoms of nitrogen (Scheme 2), the amount of
anchored 3 can be calculated, which is also compiled in
29
29
Fig. 2 Solid state NMR of the mesoporous silicas: (a) Si MAS and (b) Si CPMAS.
Hence there is no linear correlation between the amount of Table 2. Hence, the order of the chiral ligand loading is
silanols of mesoporous silicas and their pHpzc. However this SPSi > SBA-15 E CMK3 > SPC > HMS. Although SBA-15 and
result shows that materials with similar pore size, SBA-15 and its carbon replica possess similar amounts of anchored 3, SPSi
SPSi, prepared using the same structuring agent, but by differ- has a higher amount of 3 than its carbon replica (SPC). The
ent procedures, yield different surface acidities.
highest amount of ligand loading found for SPSi is most
These last materials, SBA-15 and SPSi, were also used as probably related to the nature of its surface since, as indicated
templates for the preparation of two carbon mesoporous mate- above, the pHpzc values showed that the SPSi material has the
rials, CMK-3 and SPC, but using different methodologies. Both least acidic surface. In fact, considering on one hand the silicas
materials possess higher areas than the parent mesoporous and, on the other hand, the replicas, ligand loadings are in
silicas, due to the presence of micropores also, with mesopores general highest for the least acidic materials.
6
64 Catal. Sci. Technol., 2013, 3, 659--672
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Table 2 Chemical analysis and textural properties of the modified materials
À1
À2
Elemental analysis
mmol g
Textural properties
Density/mmol m
ICP
a
b
c
f
2
À1
g
3
À1
Sample
%N %C
%H %S
%Cu
3
OTf
Cu Cu/3 1/2 Â OTf/3
A
BET /m g
Vtotal /cm g
Cu
3
3
3
3
3
3
Cu@SBA-15 0.51
Cu@CMK-3 0.50 80.71 1.12 0.94 0.9
3.12 0.90 0.94 1.0
121 293
119 292
151 2e 78
94
157 1.3
142 1.2
157 1.0
90 1.0
1.2
1.2
0.9
438
717
339
772
584
0.68
0.94
0.60
0.85
0.44
0.21
0.10
0.28
0.08
0.14
0.16
0.09
0.26
0.08
0.08
Cu@SPSi
Cu@SPC
Cu@HMS
0.64
3.92 0.70 d0.89 1.0
0.57
2.61 1.13 0.78 0.88
0.40 79.95 0.74
0.35
83 242
138 1.7
1.5
a
b
c
Calculated from the EA nitrogen content. Calculated from the EA sulfur content. Calculated from the ICP copper content by using the
À6
d
e
À1
f
g
formula: %Cu  10 /(100  63.456). Below the detection limit: 0.30%. Below the detection limit: 94 mmol g
. Specific surface area. Total
porous volume.
From the elemental analysis the presence of sulphur which mesoporous silicas are typical of the Si–O bonds of the frame-
is from the triflate (OTf) counter-anion of copper(II) can also be work, whereas a rich oxygen surface chemistry can be deducted
3
2
observed, showing that copper(II) coordination took place. for the SPC material. This last conclusion is supported by the
Taking into consideration that each triflate contains one lowest pHpzc value observed for the SPC mesoporous carbon
sulphur atom, the amounts of triflate anion can be calculated (Table 1). Although the signal to noise of the Cu 2p spectra of
(Table 2). Thus, the order of the triflate anion content is 3Cu@SPC is low, the higher energy and peak width observed
SBA-15 E CMK-3 > SPSi > HMS. Hence the order of loading for Cu 2p3/2 of 3Cu@SPC suggests that there must be copper(II)
of triflate is different from that of ligand 3 suggesting that the in different environments, when compared to 3Cu@SBA-15 and
amount of copper will also be different from that of 3 and 3Cu@HMS. In the C 1s high resolution spectra besides the
generally slightly higher as can be seen by the 1/2 Â OTf/3 ratio, bands due to the carbon backbone of oxazoline and the propyl
also compiled in Table 2.
linker at lower energy, for the 3Cu@SBA-15 which contains a
To confirm this, the copper content of the materials was also higher surface amount of copper, a band at 292 eV due to the
obtained by ICP-AES in order to determine the amount of CF carbon of the triflate anion can be clearly observed
3
copper coordinated to ligand 3. As can be seen in Table 2, the (Table S2, ESI†). This anion also gives rise to an intense peak
amounts of copper are higher than those of 3 from the Cu/3 in the F 1s region, with a band at 688 eV (Table S2, ESI†),
ratio, for 3Cu@SBA-15, 3Cu@CMK-3 and 3Cu@HMS, which are whereas sulphonate sulfur can sometimes be detected at 168 eV
similar to the 1/2 Â OTf/3 ratio, indicating that the copper (Table S2, ESI†).
content can be roughly determined through its counter-anion.
FTIR. The FTIR spectra of the mesoporous silica materials,
However the order of copper loading onto the materials is SBA-15, SPSi and HMS, are dominated by the Si–O asymmetric
slightly different from that of the OTf anion: SBA-15 = SPSi > and symmetric stretching vibrations, respectively, at 1050 and
À1
À1
CMK-3 > HMS > SPC. The ordered mesoporous silicas (SBA-15 800 cm (Fig. 3). Moreover by ATR a low intensity peak at
and SPSi) possess more copper than their corresponding 3746 cm can be distinguished in the spectra of these materi-
carbon replicas (CMK-3 and SPC).
als, which is due to the stretching of the O–H groups from the
3
3
XPS. For some of the heterogeneous catalysts prepared, for isolated silanols (SiO–H). Although the samples were dried
which enough sample was available, XPS was also performed: for 6 hours at 100 1C under vacuum, O–H stretching and
À1
3
Cu@SBA-15, 3Cu@HMS and 3Cu@SPC. Besides of a large bending vibrations around 3450 and 1640 cm , respectively,
amount of silica and oxygen, and carbon in the case of the from adsorbed water can still be observed. The carbon meso-
3
Cu@SPC material, copper, sulphur and fluorine were detected porous materials, CMK-3 and SPC, show very low intensity
À1
at the surface of these materials indicating the presence of broad bands also around 3500 and 1639 cm which may be
copper(II) triflate, as well as nitrogen showing the presence of due to the O–H stretching and bending vibrations, respectively,
À1
the chiral ligand 3 (Table S1, ESI†).
of adsorbed water, as well as a broad band at 1100 cm
The N 1s peaks of the materials are centered between 399.6 probably due to the Si–O stretching vibration of the remaining
and 400.8 eV and are large indicating the presence of the chiral silica templates.
2
9,30
ligand 3 with imine nitrogen
as well as tertiary amine
In the materials containing the anchored 3Cu, represented
(Table S2, ESI†). The Cu 2p3/2 peaks are centered between in Fig. 3, besides the characteristic bands of the materials very
9
32.8 and 934.2 eV, which are typical of copper(II) complexes low intensity bands at 2976, between 1560 and 1360 and about
2
9–31
À1
in a mixed N,O coordination sphere.
However for the SPC 640 cm due, respectively, to the C–H stretching vibrations of
materials the Cu 2p3/2 peak is larger and higher in energy than the 3 tert-butyl groups, vibrations characteristic of ligand 3 and
for the other two analyzed silica materials. The O 1s spectra of SQO vibration of the sulfonate group of the triflate counter
3Cu@SBA-15 and 3Cu@HMS can be deconvoluted into a single anion of the copper(II) can be further seen (Fig. 3f and
peak centered at 532.8 eV, whereas the one for 3Cu@SPC is Scheme 2). The characteristic CQN stretching vibration of
larger and can be deconvoluted into 3 peaks at 531.8, 533.7 and ligand 3 (Fig. 3f and Scheme 2) can be clearly seen in the
À1
5
36.8 eV (Table S2, ESI†). Thus the O 1s XPS spectra of the ATR spectra at 1619 cm , but some extent of superimposition
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Fig. 3 FTIR spectra for the mesoporous materials (MM) and 3Cu@MM materials, before and after catalysis: (a) SBA-15, (b) CMK-3, (c) SPSi, (d) SPC and (e) HMS,
(
f) azaBox (3), CuazaBox (3Cu) and CuazaBoxPS (3CuPS) and (g) EDA, styrene cyclopropane, diethyl fumarate and diethyl maleate.
À1
of the O–H bending vibration at 1637 cm
of the water confirming that grafting between this group and the propylsilane
33
adsorbed on the surface of the materials can also be observed. functionalized 3Cu took place according to Scheme 2.
Furthermore the low intensity band at 3746 cm , due to the
À1
It is worth mentioning that copper(II) coordination to the
stretching of the O–H groups from the isolated silanols, disappeared functionalized ligand 3 can be detected as shown in Fig. 3f by
6
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Scheme 3 Cyclopropanation of styrene with ethyldiazoacetate.
À1
the shift of the CQN stretching band from 1641 cm in ligand samples where a relative inversion on the heights of the
À1
3
to 1620 cm in the functionalized ligand 3, before addition maxima near 4 and near 7.7–10 nm seems to occur after
of the materials to the reaction mixture (see Section 2.4 and functionalization with the 3Cu complex. CMK-3 also presents
Scheme 3). the highest decrease in the porous volume.
Thermogravimetry with differential scanning calorimetry.
The TG-DSC results obtained in an oxidizing atmosphere are
3.2. Catalysis experiments
usually more informative than those obtained in an inert The copper(II) complexes with aza-bis(oxazoline) ligands are
atmosphere but, to avoid the decomposition of the matrix, in efficient homogeneous catalysts in the asymmetric cyclo-
3
,4,21,22
the case of carbon replicas an inert atmosphere was used. By propanation of alkenes.
Therefore, the nanostructured
comparing the curves (Fig. 4), for the same material, before and materials with anchored 3Cu were tested as heterogeneous
after functionalization, the weight loss up to 250 1C, and the catalysts in the asymmetric cyclopropanation of styrene with
corresponding endothermic broad DSC peak, is ascribed ethyldiazoacetate at room temperature (Scheme 3) and the
mainly to water loss. The transformations above 250 1C can results are compiled in Table 3.
be identified with the decomposition of the 3Cu complex. For
Catalytic activity, diastereoselectivity and enantioselectivity.
the carbon replicas the progressive fall of the weight curve, and From Table 3 (please also see Scheme 3 for the nomenclature of
the lack of clear transformation in the DSC curve, makes the reaction products) it can be seen that all the materials are active
interpretation difficult. For the silicas, the pattern is much in the asymmetric cyclopropanation of styrene with high trans-4
similar for both, SBA-15 and HMS with a weight loss between diastereoselectivity (70–72%) and moderate to high enantio-
250 1C and 415 1C associated to an exothermic transformation, selectivities. The order of the values for this last parameter is
both compatible with the decomposition of the complex by SPSi (72% cis-4 and 80% trans-4) > CMK-3 (70% cis-4 and 79%
thermal oxidation. In the case of the SPSi sample the DSC trans-4) > SPC (63% cis-4 and 75% trans-4) > SBA-15 (62% cis-4
exothermic transformation extends until higher temperatures and 72% trans-4) c HMS (36% cis-4 and 47% trans-4). Hence
(450 1C), than for the other silicas, and presents a more well the ordered mesoporous silica SPSi gives rise to the most
defined peak. We attribute this enhanced definition in the DSC enantioselective heterogeneous catalyst. Moreover, as can be
peak to a more uniform distribution of the complex in the seen in Table 3, the enantioselectivities of the heterogeneous
matrix, in relation to the other silicas, which is also in line with catalysts are much higher than the corresponding homo-
the least acidity of the SPSi sample, as mentioned above in the geneous phase reactions run under the same experimental
discussion of the pHpzc values. In fact, being a less acidic conditions and using the same amount of Cu (0.46 or 0.26% mol,
material, a more homogeneous reactivity of the surface silanol Table 3), with the only exception of 3Cu@HMS. Therefore
groups could be expected.
positive effects on the enantioselection of the 3Cu catalyst
Nitrogen adsorption at À196 8C. Fig. 1 displays the nitrogen can be observed upon immobilization for all the materials
adsorption–desorption isotherms at À196 1C, of the materials used, probably due to the introduction of the propyl groups
before and after functionalization with 3Cu, and the respective on the acidic 3 nitrogen (Scheme 2). In fact, higher enantio-
mesopore size distributions estimated from the Broekhoff–de selectivities were reported for methylated 3 and thus propyla-
Boer method, in a version simplified with the Frenkel–Halsey– tion of 3 should also increase its enantioselectivity as observed
3
4
3
Hill equation (BdB–FHH). The general pattern of reduction in herein. Taking into consideration only the silica mesoporous
the porous volume and specific surface areas (Tables 1 and 2) materials, it is curious to note that the SBA-15 material
after functionalization, already mentioned in the literature for gives rise to a less enantioselective heterogeneous catalyst than
1
8,19
other systems, is noticed.
In the case of the silica materials, using SPSi as a support, despite the similarity of the textural
most probably because the mesopore size distributions are properties of both materials. The least enantioselective hetero-
much more uniform in relation to the carbon replicas (Fig. 1c geneous catalyst was nevertheless when the HMS material
and d), the shape of the distribution is, essentially, kept. For was used as the support, indicating that a smaller pore size is
the carbon replicas, the main changes occur for the CMK-3 not beneficial to the course of the asymmetric cyclopropanation
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Fig. 4 Thermogravimetric curves for the various samples (a) SBA-15, (b) CMK-3, (c) SPSi, (d) SPC, (e) HMS.
of styrene. Nonetheless, this last material has lower amounts
The catalyst densities for both Cu and azaBox were also
of copper and 3 than the other mesoporous silicas, besides calculated and can be found in Table 3. However the catalyst
the highest Cu/3 ratio, which can also partially justify the density and type of material could not explain the differences
lowered performance compared with the other materials. observed for the enantioselectivity of the several heterogeneous
For the other heterogeneous catalysts with a slightly higher catalysts. The catalytic performance in the first cycle of
À2
content of copper than ligand it does not seem to be a 3Cu@CMK-3 with 0.1 mmol m is comparable to the best
À2
À2
very important factor, as 3Cu@CMK-3 presents comparable catalyst 3Cu@SPSi with the highest density of 0.3 mmol m
enantioselectivity as 3Cu@SPSi, with the same amount of Comparing 3Cu@CMK-3 and 3Cu@SPC, both with 0.1 mmol m
.
copper and ligand.
of active phase, the first is found to be a better heterogeneous
6
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a
Table 3 Cyclopropanation of styrene with azaBox ligand immobilized onto various porous supports and in a homogeneous phase
b
e
mol%
Cu
%ee (4)
c
d
f
Catalyst
t/h
Run
3
%C
4 trans/cis
cis
trans
TON
3
3
3
3
+ [Cu(OTf)
+ [Cu(OTf)
+ [Cu(OTf)
Cu@SBA-15
Cu@CMK-3
Cu@SPSi
2
2
2
]
]
]
24
24
3
1st
1st
1st
1st
2nd
1st
2nd
1st
2nd
1st
2nd
1st
0.27
0.46
1.0
0.26
0.48
1.0
19
42
56
22
15
47
39
53
26
37
41
51
10
66/34
73/27
73/27
70/30
65/35
72/28
65/35
71/29
70/30
70/30
67/33
70/30
64/36
41
63
76
62
41
70
38
72
64
63
52
36
40
44
73
84
72
51
79
45
80
72
75
59
47
49
70
90
56
47
43
126
104
118
65
130
158
125
28
24
0.46
0.36
0.37
0.37
0.45
0.40
0.26
0.26
0.40
0.38
0.35
0.28
0.32
0.31
0.43
0.38
0.27
0.27
0.24
0.23
24
3
3
3
3
24
24
24
24
Cu@SPC
24
24
Cu@HMS
24
24
2nd
a
Reactions performed at room temperature using 2.40 mmol styrene, 0.65 mmol n-undecane (internal standard), 70 mg of heterogeneous catalyst,
b
2
mL phenylhydrazine and 2.75 mmol of EDA in 10.0 mL of CH
2
Cl
2
.
% of copper and 3 in the catalyst in relation to styrene (see Table 1); for the
c
d
recycling experiments corrected for the loss of heterogeneous catalyst weight. Conversion of styrene determined by CG-FID. trans/cis ratio of 4
e
f
(Scheme 3). cis and trans 4 (Scheme 3) enantiomeric excesses determined by chiral GC-FID. TON = moles of styrene converted/moles of Cu.
catalyst than the second. Nevertheless, in the case of the material seems to be superior as a support than conventional
mesoporous silicas the catalyst density might be an important SBA-15, and their carbon replicas.
À2
factor as 3Cu@SPSi, with 0.3 mmol m of active phase, presents
On the other hand, immobilization of the 3Cu catalyst on
À2
better enantioselectivity than 3Cu@SBA-15 (0.2 mmol m ) and these mesoporous silica materials also leads to better styrene
Cu@HMS (0.1 mmol m ). Hence we were driven to search for conversions and TON than the corresponding homogeneous
À2
3
another support property that could explain the different phase reactions (0.46 or 0.26 mol%, Table 3). Therefore positive
enantioselectivities observed between all the heterogeneous effects on the catalytic activity of the 3Cu catalyst can also be
catalysts. It is difficult to correlate the results from enantios- observed upon immobilization for these silica materials,
electivity with the textural and surface chemistry properties of probably due to the introduction of the propyl groups on the
3
the materials. Nevertheless an attempt can be made for acidic 3 nitrogen (Scheme 2). The order for the styrene con-
instance considering the SBA-15 and the SPSi samples. For versions is SPSi (53%) > HMS (51%) > CMK-3 (47%) > SPC (34%)
these samples the mesopore size distributions are comparable, > SBA-15 (22%). These values do not follow the materials’
although the porous volume of SBA-15 is 18% higher (Table 1). copper contents (Table 2, see Section 3.1.1). The SPSi and
Therefore, the most important parameter in this case seems HMS mesoporous silicas were the ones which convert more
to be the surface chemistry since, as discussed above, the styrene followed by the mesoporous carbons and finally SBA-15.
SPSi surface is less acidic than the one of SBA-15 allowing as The TON of the heterogeneous phase reactions are high and in
well a more uniform distribution of the complex in the SPSi the order: SPC (130) > CMK-3 (126) E HMS (125) > SPSi (118) c
matrix, as suggested earlier by the DSC results (Fig. 4). SBA-15 (47). It can be concluded that the ordered mesoporous
Moreover, the SPSi carbon replica (SPC) yields a least enantio- carbons, SPC and CMK-3, yield more active heterogeneous
selective heterogeneous catalyst, whereas CMK-3 yields a more catalysts than when the corresponding mesoporous silicas are
enantioselective heterogeneous catalyst than SBA-15. Despite used as 3Cu catalyst supports. This is the opposite of that
2
0
the differences in the framework composition, textural observed for the heterogenized 2Cu on the same supports and
and surface chemistry properties the same dependence on could be due to a different effect of the type of matrix support,
surface acidity can be inferred. SPC presents a more acidic i.e. carbon vs. silica on the heterogenized homogeneous
surface than its parent SPSi and lower enantioselectivities catalyst. Since the heterogeneous catalyst with the HMS mate-
1
9,20
were obtained, whereas the lower acidity of the CMK-3 surface rial as a support, bearing a pore size near to 2.5 nm,
has a
yields a more enantioselective heterogeneous catalyst than comparable TON as the ones with the mesoporous carbon
its parent SBA-15 (Tables 1 and 3). It is also clear from the materials, their higher activity can also be due to the pore size
results in this work that small mesopore sizes, in the range of the materials, since they possess a pore size of 4.0 nm,
of those presented by the HMS material, might be disadvanta- whereas their parent ordered mesoporous silicas possess pore
2
0
geous in the present context since this solid presented the sizes of around 7 nm. The rate determining step in the
lowest chiral ligand loading and the lowest enantioselectivity cyclopropanation of styrene with copper bis(oxazoline) ligands
values.
is the formation of a copper(I) carbene complex by coordination
1
0,31
It is noteworthy that in our previous report, upon hetero- of ethyl diazoacetate to the copper centre.
By influence of
genization of 2Cu (Scheme 1) onto mesoporous silicas and their the spatial restrictions the change in the orientation of the
carbon replicas, the SPSi material also led to the heterogeneous reacting alkene with respect to the catalytic center on the
2
0
catalyst with the highest enantioselectivity. Therefore this surface of the material, the activity of the system will change,
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as in the case of enzymes. In smaller pores the trajectory of the enantioselectivity for the other heterogeneous catalysts. Higher
reacting alkene might be shorter and directed by the con- decreases in enantioselectivity can be observed for the hetero-
straints imposed by the pore, yielding therefore an improved geneous catalysts with SBA-15 and CMK-3. With the exception
activity.
Cu immobilized onto siliceous mesocellular foam (MCF) indicate that although the stability of the copper(II) complexes
using the same anchoring procedure as the one reported herein with 3 is higher than those of commercial bis(oxazoline),
of 3Cu@SPC there is loss of activity upon reuse, which may
3
1
8,19
20
gave 75% yield of cyclopropane, 86% ee for the trans and 78% 1Cu
and 2Cu, there is still some degree of instability of
ee for the cis cyclopropane with a diastereoselectivity of 62% the chiral complex or partial deactivation of the immobilized
2
1
trans to 38% of cis. The enantioselectivities are comparable to 3Cu complex. The FTIR spectra of the materials at the end of
our best heterogeneous catalyst, 3Cu@SPSi, but the yields are the recycling experiments (Fig. 3) show a very intense band at
À1
35
higher probably because this reaction was performed in an 1726 cm , which is typical of the carbonyl in the ester group,
inert atmosphere and using an excess of styrene in relation to as can be confirmed from Fig. 3g, representing the spectra of
EDA. The reported diastereoselectivity is however lower. Never- the reagent EDA and products styrene cyclopropane and
theless, by capping the surface silanols of the MCF Ying et al. by-products diethyl fumarate and diethyl maleate. The vibra-
could further increase the cyclopropane yield and enantioselec- tion of the carbonyl band in the recycled heterogeneous
2
1
tivity to 69 and 88% ee for the trans and 83% ee for cis. 3Cu catalysts is closer to that of the styrene cyclopropane and
3
,22,35
has also been immobilized onto a polymeric support
diethyl maleate and diethyl fumarate by-products, formed from
methoxypolyethylene glycol and polystyrene; using the first EDA dimerization. Therefore, as observed by us previously in
polymer in the cyclopropanation of styrene but with methyl the presence of more nitrogen and carbon (EA) on recycled
1
8
diazoacetate gave 69% yield of cyclopropanes, with 71% trans to catalysts, strong adsorption of by-products cannot be ruled
9% cis cyclopropane diastereoselectivity and 91 and 87% out as it would also justify the lowering of the catalytic activity
2
3
36
enantioselectivity, respectively; whereas with polystyrene and of the heterogenized catalysts.
again methyl diazoacetate, 28% yield, 70% trans to 30% cis
cyclopropane and 88% trans enantioselectivity was obtained.
Nonetheless, the heterogeneous catalysts using the carbon
materials as supports remain quite active upon reuse, when
2
2,35
The enantioselectivities and diastereoselectivities are compar- compared with the mesoporous silicas, and the SPC retains
able to our best heterogeneous catalyst, 3Cu@SPSi. Hence a most of its enantioselectivity upon reuse.
hydrophobic surface seems to be an important factor in design-
Deactivation upon reuse was also observed for 3Cu immo-
ing new robust materials for being used as supports in asym- bilized onto a Laponite clay and a Nafion silica through
4
22
metric catalysis. On the other hand, 3Cu immobilized onto a electrostatic interactions, as well as polystyrene. It is also
Laponite clay and a Nafion silica through electrostatic inter- commonly observed for commercial bis(oxazoline) catalysts
1
9,20,37
actions showed slightly higher enantioselectivities (76% cis, immobilized onto mesoporous silica materials.
3% trans and 81% cis, 88% trans, respectively), but lower when immobilized onto siliceous mesocellular foam, which
yields (46 and 30%, respectively) than our best heterogeneous possesses ultra-large pores of 25 nm, and methoxypolyethylene
Only
2
1
8
4
3
catalyst, 3Cu@SPSi, under comparable experimental conditions.
glycol that these systems present high enantioselective activity
Also in comparison with our previous reports on the immobi- and recyclability. It is noteworthy, however, that the catalytic
1
8,19
lization of commercial bis(oxazoline) catalysts, 1Cu
2
and studies on these reports were performed using 1–2 mol% of
2
0
21,22
Cu (Scheme 1), better enantioselectivities and styrene con- Cu
versions were generally achieved herein with the 3Cu catalysts homogeneous phase reactions, whereas in our catalytic experi-
in relation to the substrate, double than the necessary in
2
indicating that their copper(II) complexes must be more stable ments the Cu and azaBox contents were between 0.26–0.51 and
than with the former homogeneous catalysts, as described 0.11–0.43 mol%, respectively. Thus deactivation is more diffi-
4
in the literature.
cult to detect than in our case.
It is noteworthy that in the present reaction no by-products
other than diethyl fumarate and diethyl maleate, which are
from the dimerization of EDA, were detected. Hence the styrene
conversions can be taken as the yields of cyclopropanes.
4
. Conclusions
A copper(II) chiral aza-bis(oxazoline) homogeneous catalyst
Recycling. The main aim of the immobilization of homo- (3Cu) could be anchored onto ordered silica mesoporous
geneous catalysts is its easy separation from the reaction materials and their carbon replicas. The materials pHpzc proved
media, at the end of the reaction, and the possibility of further to be an important factor in the 3Cu anchoring yields, as well as
use in more catalytic cycles. Besides that, it is well known that in the uniform distribution of the complex in the SPSi matrix.
single site isolation of homogeneous catalysts onto a porous All the composites prepared were more active and enantio-
material may increase its stability. Hence the composite mate- selective in the cyclopropanation of styrene than in the corres-
rials were reused in more successive catalytic cycles after ponding homogeneous phase reactions, run under the same
filtration from the solution, washing with dichloromethane experimental conditions, suggesting positive immobilization
and drying. The results are also compiled in Table 3. It can effects, with the only exception of 3Cu@HMS that presented
be concluded that they can be reused with comparable lower enantioselectivity. The most enantioselective hetero-
enantioselectivities, for HMS and SPSi, and decrease in the geneous catalyst, with %ee cis and %ee trans of 72 and 80,
6
70 Catal. Sci. Technol., 2013, 3, 659--672
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respectively, was the one using the ordered mesoporous silica
SPSi. Since CMK-3 was the second most enantioselective
catalyst with %ee cis and %ee trans of 70 and 79, surface
chemistry seems to be an important parameter as these two
materials present the highest pHpzc values of the studied
supports. The ordered mesoporous carbons, CMK-3 and SPC,
7 D. A. Evans, D. Seidel, M. Rueping, H. W. Lam, J. T. Shaw and
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silicas, SBA-15 and SPSi, are used as catalyst supports. This
Rev., 2009, 109, 418–514.
could be due to the pore size of the materials as the HMS 11 D. Rechavi and M. Lemaire, Chem. Rev., 2002, 102,
material, with a pore size much smaller than SBA-15 and SPSi,
is also a very active catalyst.
3467–3493.
12 J. M. Fraile, J. I. Garcia, C. I. Herrerias, J. A. Mayoral and
E. Pires, Chem. Soc. Rev., 2009, 38, 695–706.
The heterogeneous catalysts with HMS, SPSi and SPC can be
reused with comparable enantioselectivities, or decrease in the 13 J. M. Fraile, J. I. Garcia and J. A. Mayoral, Chem. Rev., 2009,
enantioselectivity, and with higher activity in the case of the
109, 360–417.
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reuse which could be due to partial deactivation, probably by
coordination of the EDA dimerization by-products.
The results presented herein are comparable to the ones of
2005, 77, 1–45.
3
6
15 J. Pires, S. Borges, A. P. Carvalho and A. R. Silva, Adsorpt. Sci.
Technol., 2010, 28, 717–726.
the same 3Cu homogeneous catalyst immobilized onto meso- 16 R. Ryoo, S. H. Joo and S. Jun, J. Phys. Chem. B, 1999, 103,
2
1
cellular foams, despite different experimental conditions, and
7743–7746.
superior to the same catalysts immobilized by electrostatic 17 C.-C. Ting, H.-Y. Wu, S. Vetrivel, D. Saikia, Y.-C. Pan,
4
interactions into clays.
G. T. K. Fey and H.-M. Kao, Microporous Mesoporous Mater.,
2010, 128, 1–11.
Herein, with this comprehensive study, we were able to show
that when preparing asymmetric heterogeneous catalysts the 18 A. R. Silva, H. Albuquerque, A. Fontes, S. Borges, A. Martins,
acidity of the support surfaces can have a detrimental role on
the enantioselection of the immobilized chiral homogeneous
A. P. Carvalho and J. Pires, Ind. Eng. Chem. Res., 2011, 50,
11495–11501.
catalyst. Therefore the preparation of new robust porous 19 A. R. Silva, H. Albuquerque, S. Borges, R. Siegel, L. Mafra,
materials bearing less acidic surfaces is needed in order to
design superior asymmetric heterogeneous catalysts, without
the need for post-hydrophobization of the surface.
A. P. Carvalho and J. Pires, Microporous Mesoporous Mater.,
2012, 158, 26–38.
20 A. R. Silva, L. Carneiro, A. P. Carvalho and J. Pires, unpublished
results.
2
1 J. Lim, S. N. Riduan, S. S. Lee and J. Y. Ying, Adv. Synth.
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Acknowledgements
This work was funded by Fundaç ˜a o para a Ci ˆe ncia e a 22 H. Werner, C. I. Herrerias, M. Glos, A. Gissibl, J. M. Fraile,
Tecnologia (FCT) through the project PTDC/QUI/64770/2006,
which was co-financed by EU under the programs COMPETE,
I. Perez, J. A. Mayoral and O. Reiser, Adv. Synth. Catal., 2006,
348, 125–132.
QREN and FEDER. FCT is also acknowledged for financing 23 J. Pires, M. Pinto, J. Estella and J. C. Echeverria, J. Colloid
under the Projetos Estrat ´e gicos Pest-C/CTM/LA0011/2011
Interface Sci., 2008, 317, 206–213.
CICECO) and PEst-OE/QUI/UI0612/2011 (CQB/FC/UL). ARS 24 V. K. Saini, M. Andrade, M. L. Pinto, A. P. Carvalho and
(
thanks FCT, Fundo Social Europeu and Programa Operacional
J. Pires, Sep. Purif. Technol., 2010, 75, 366–376.
Potencial Humano for the contract under the program 25 P. T. Tanev and T. J. Pinnavaia, Science, 1995, 267, 865–867.
Ci ˆe ncia 2008.
26 J. S. Noh and J. A. Schwarz, J. Colloid Interface Sci., 1989, 130,
57–164.
7 B. Jarrais, A. R. Silva and C. Freire, Eur. J. Inorg. Chem., 2005,
582–4589.
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