Synthesis, immobilization and catalytic activity of a copper(II) complex with a chiral bis(oxazoline)

Article abstract of DOI:10.3390/molecules190811988

A chiral bis(oxazoline) bearing CH₂OH groups was synthesized from a commercial bis(oxazoline) and characterized by ¹H- and ¹³C-NMR, high resolution ESI-mass spectrometry and FTIR. The corresponding copper(II) complex was immobilized onto the surface of a mesoporous carbonaceous material (Starbon 700) in which the double bonds had been activated via conventional bromination. The materials were characterized by elemental analysis, ICP-OES, XPS, thermogravimetry and nitrogen adsorption at 77 K. The new copper(II) bis(oxazoline) was tested both in the homogeneous phase and once immobilized onto a carbonaceous support for the kinetic resolution of hydrobenzoin. Both were active, enantioselective and selective in the mono-benzoylation of hydrobenzoin, but better enantioselectivities were obtained in the homogeneous phase. The heterogeneous catalyst could be separated from the reaction media at the end of the reaction and reused in another catalytic cycle, but with loss of product yield and enantioselectivity.

Full text of DOI:10.3390/molecules190811988

Molecules 2014, 19, 11988-11998; doi:10.3390/molecules190811988
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis, Immobilization and Catalytic Activity of a Copper(II)
Complex with a Chiral Bis(oxazoline)
Liliana Carneiro 1, Ana R. Silva 1,*, Peter S. Shuttleworth 2, Vitaly Budarin 3 and James H. Clark 3
1
Department of Chemistry, CICECO, University of Aveiro, Aveiro 3810-193, Portugal
2
Departamento de Física de Polímeros, Elastámeros y Applicaciones Energéticas,
Instituto de Ciencia e Tecnología de Polímeros, CSIC, Madrid 28006, Spain
3
Green Chemistry Centre of Excellence, Department of Chemistry, University of York,
York YO10 5DD, UK
* Author to whom correspondence should be addressed; E-Mail: ana.rosa.silva@ua.pt;
Tel.: +351-234-370-604; Fax: +351-234-401-470.
Received: 13 July 2014; in revised form: 30 July 2014 / Accepted: 30 July 2014 /
Published: 11 August 2014
Abstract: A chiral bis(oxazoline) bearing CH2OH groups was synthesized from a commercial
bis(oxazoline) and characterized by 1H- and 13C-NMR, high resolution ESI-mass spectrometry
and FTIR. The corresponding copper(II) complex was immobilized onto the surface of a
mesoporous carbonaceous material (Starbon® 700) in which the double bonds had been
activated via conventional bromination. The materials were characterized by elemental
analysis, ICP-OES, XPS, thermogravimetry and nitrogen adsorption at 77 K. The new
copper(II) bis(oxazoline) was tested both in the homogeneous phase and once immobilized
onto a carbonaceous support for the kinetic resolution of hydrobenzoin. Both were active,
enantioselective and selective in the mono-benzoylation of hydrobenzoin, but better
enantioselectivities were obtained in the homogeneous phase. The heterogeneous catalyst
could be separated from the reaction media at the end of the reaction and reused in another
catalytic cycle, but with loss of product yield and enantioselectivity.
Keywords: copper(II); bis(oxazoline); homogeneous catalysis; heterogeneous catalysis;
asymmetric catalysis

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1. Introduction
Bis(oxazoline) ligands are chiral, privileged ligands that when coordinated to for example copper
act as very efficient and enantioselective homogeneous catalysts in several organic transformations,
such as cyclopropanation of alkenes, aziridination of alkenes, Diels-Alder reactions, etc. [1]. A decade
ago they were also found to be homogeneous catalysts in the kinetic resolution of 1,2-diols [2]. Despite
their efficiency, selectivity and enantioselectivity, the work-up of the homogeneous phase reaction is
cumbersome, since homogeneous catalysts are not easily separated from the products. Although some
chiral bis(oxazoline) ligands are commercially available, they are expensive, hindering their industrial
applicability [2].
On the other hand, heterogeneous catalysts can be easily separated from the reaction media by
simple filtration and then the products can be isolated, often without metal contamination. The recovered
heterogeneous catalyst can also be used in further catalytic cycles improving the cost of the process.
Therefore, the heterogenization/immobilization of homogeneous catalysts using solid porous supports
has been a popular strategy to combine the advantages of both homogeneous and heterogeneous
catalysts [3–6]. Several immobilization strategies have been used; however, covalent attachment of the
homogeneous catalysts to the surface of the material increases the resistance of the catalyst to
leaching [3–6]. Due to their availability and low cost, organic polymers have been widely explored as
supports for homogeneous catalysts and in particular for copper bis(oxazoline) ligands [3–6].
Nevertheless, their major drawback has been the lack of porosity and stability upon reuse, especially
when using chlorinated solvents [3,4]. With the discovery of MCM-41 and later other materials, such
as, SBA-15, ordered mesoporous silicas have become popular supports for the immobilization of
homogeneous catalysts [3–12]. Some success has been achieved using this type of material as
support [3–10], especially using mesocelullar silica foams in the case of the copper
bis(oxazolines) [11,12]. Nevertheless the immobilization strategies are limited to organosilane
reagents, which in itself leads to some issues regarding the remaining free silanols requiring extra
silylanation [11,12] or the use of other less acidic mesoporous silicas [8–10].
Porous carbonaceous materials are not as explored, despite the wide use of activated carbons as
supports in several commercially available heterogeneous catalysts as surfaces rich in oxygen surface
groups [13] that allow the design of a wider range of immobilization strategies [14–17]. The structure
of activated carbons is not ordered and is mainly microporous [13]. In 1999, Ryoo et. al., described the
synthesis of an ordered mesoporous carbon prepared via nanocasting using MCM-48 as a hard
template [18]. This material, as well as other ordered mesoporous carbons prepared in a one-pot
synthesis together with the silica precursor, were used as supports for the immobilization of
aza-bis(oxazoline) and commercial bis(oxazoline) via organosilane reagents [8–10]. These
heterogeneous catalysts were active and enantioselective in several organic transformations, but with
inferior performance compared to the corresponding ordered mesoporous silicas supports [8–10].
Starbons® are mesoporous carbonaceous materials prepared by the controlled carbonization of
expanded starch [19] and other polysaccharides. When carbonized at high temperatures they present a
more graphitized surface than the materials carbonized at lower temperatures [19]. These types of
materials have never been explored as supports for asymmetric homogeneous catalysts. Herein we
report on the immobilization of a copper(II) complex with a commercial bis(oxazoline), functionalized

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with hydroxyl groups that allowed the covalent attachment to the brominated surface of a Starbon®
carbonized to 700 °C (Scheme 1). The material was subsequently applied as a heterogeneous catalyst
in the kinetic resolution of a 1,2-diol.
Scheme 1. Modification and immobilization strategy of the CudiolPhBox complex (S3)
onto the brominated (S2) Starbon ® 700 (S1).
2. Results and Discussion
The copper(II) complex with the PhBox bis(oxazoline) was anchored onto the surface of the porous
carbonaceous material, Starbon® 700, in a three step procedure according to Scheme 1. Initially, to be
able to anchor the ligand onto the Starbon® 700 carbonaceous material, the central carbon of the
PhBox ligand bridge was functionalized with CH2OH groups (Scheme 2), by adapting procedures
described in the literature [20]. To the best of our knowledge this is a new organic molecule and thus it
1
was conveniently and completely characterized by H- and 13C-NMR, high resolution ESI-mass
spectrometry and FTIR (see Experimental). This molecule was immobilized onto the surface of the
carbonaceous material Starbon® 700 (S1), in which the double bonds had been brominated in order to
make them more reactive with the hydroxyl groups [17,21].
Scheme 2. Functionalization of the PhBox ligand with CH2OH groups (diolPhBox).
2.1. Characterization of the Materials
Table 1 shows the elemental analysis of the starting (S1) and chemically modified materials (S2 and S3).
As expected, the Starbon® 700 (S1), obtained by the controlled carbonization of starch at 700 °C, has a
high content of carbon (91%) like other conventional porous carbonaceous materials, such as activated
carbon [13]. The XPS analysis of this material only shows the presence at the surface of the material of
carbon in high atomic percentage and oxygen (Table 2). The original Starbon® 700 material possesses
both high pore volume and a Brunauer, Emmett and Teller (BET) surface area of 508 m2/g, with
significant micropore surface impact (Table 3).

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Table 1. Elemental analysis for the starting Starbon® 700 (S1), brominated (S2) and with
the CudiolPhBox complex anchored (S3).
Sample
S1
%C a
90.90
80.05
84.02
%N a
0.41
0.32
0.36
%H a
1.11
1.24
1.12
%S a
%Cu b
S2
S3
0.46
0.53
Notes: a Obtained by elemental analysis; b Obtained by ICP-OES.
Table 2. XPS analysis for the starting Starbon® 700 (S1), brominated (S2) and with the
CudiolPhBox complex anchored (S3). a
Sample
S1
%C
%O
12.35
13.78
9.98
%Br
%N
%F
%S
%Cu
87.65
85.62
88.39
S2
0.60
0.32
S3
0.25
0.81
0.19
0.05
Notes: a Atomic percentage.
Table 3. Textural properties for the S1, S2 and S3 samples.
Sample
S1
ABET (m2/g)a Amicro (m2/g) a Aexternal (m2/g) a VBJHads (cm3/g)
DBJHads (nm) b
10.7
508
266
203
296
86
212
179
165
0.654
0.571
0.508
S2
11.1
S3
38
12.1
Notes: a Obtained by t-plot method; b Average pore diameter.
The surface of Starbon® 700 that is rich in double bonds was then activated with bromine, better
leaving group when reacted with hydroxyl groups (Scheme 1) [17,21]. Elemental analysis shows that
there is a drastic decrease in the S2 carbon content (more than 10%), showing that bromine was
introduced at the surface of the S1 material. This decrease in carbon content agrees well with the first
weight loss observed during the thermogravimetric experiments with S2, verifying the presence of
bromine (Scheme 1). The derivative of this thermogravimetric curve shows a peak centered around
215 °C (Figure 1). This is corroborated by the XPS analysis showing the presence of bromine
corresponding to 463 µmol/g (Table 2). The binding energy of the Br 3p3/2 and Br 3d5/2 peaks are
184.2 (Figure 2a) and 70.8 eV (Figure 2b), respectively, which are consistent with reported values for
Br atoms covalently bonded to carbon atoms [21,22]. Nevertheless, the presence of a smaller peak due
to bromide anions can be detected in the XPS spectra (Figure 2) at 182.5 and 68.5 eV, respectively,
due to Br 3p3/2 and Br 3d5/2 peaks, probably due to some hydrolysis of the C-Br bonds [22]. Thus it
may be concluded that the bromination of the Starbon® 700 double bonds was achieved, similar to
other carbon materials [17,21,22]. After bromination the sample pore volume was also reduced in 10%
demonstrating success in bromination. It is interesting to note that the surface area decreases mainly in
the micropore area region (Table 3), showing that the bromine molecule is able to diffuse through the
porous structure of the Starbon® 700 and react even at the surface of the smaller micropores, which are
in larger amount when compared to the mesopores.

Molecules 2014, 19
Figure 1. Derivative of thermogravimetric curves for samples S1 and S2.
11992
Figure 2. High resolution XPS spectra for the S2 and S3 samples in the regions: (a) Br 3p
and (b) Br 3d.
184.2 eV
70.8 eV
(a)
(b)
S2
S3
S2
S3
205
200
195
190
185
180
175
170
80
78
76
74
72
70
68
66
64
binding energy (eV)
binding energy (eV)
The S3 material elemental analysis shows an increase in the carbon content relative to S2, due to the
introduction of the bis(oxazoline) ligand rich in carbon (Scheme 1) and the presence of sulfur from the
triflate anion coordinated to the copper. From this sulfur content the quantity of copper can be
calculated corresponding to 72 µmol/g, which matches well with the quantity of copper directly
determined by ICP-OES of 84 µmol/g (Table 1). Although the parent samples S1 and S2 also present
some nitrogen, there is also a slight increase in the nitrogen content of the S3 sample, corresponding to
12 µmol/g of the bis(oxazoline) ligand [7–9]. Since elemental analysis provides an overall composition
of the samples, it may be concluded that S3 contains a large excess of Cu(II) triflate over
bis(oxazoline). When compared to S2, a decrease in the bromine content can be seen by XPS (Table 2,
Figure 2). A weight loss associated with the peak centered at 215 °C in the derivative of the TG curve
can be seen. This all indicates that the reaction between the hydroxyl groups from the CudiolPhBox
complex and the reactive bromines introduced at the surface of the Starbon® 700 (Scheme 1) took

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place. Furthermore, by XPS nitrogen can also be detected confirming the presence of the
bis(oxazoline) ligand, as well as copper, fluorine and sulfur from the copper(II) triflate (Table 2).
However, in contrast to the elemental and ICP-OES analysis, from the values in Table 3 a higher
amount of bis(oxazoline) (98 µmol/g) compared to copper (39 µmol/g) can be calculated. This
indicates that there is more chiral ligand at the surface of material than copper. The binding energy for
the Cu 3p3/2 peak is 933.2 eV and is typical of Cu(II) complexes with bis(oxazoline)-type ligands [23,24]
A further decrease in pore volume and surface area can be observed after modification of the surface
with the CudiolPhBox complex, also confirming the presence of the complex at the surface of the
Starbon® 700 (Table 3).
2.2. Catalytic Experiments
The copper(II) complexes with bis(oxazoline) ligands act as efficient homogeneous catalysts in several
asymmetric organic transformations [1]. In particular, it has been reported that copper(II) complexes
with ligand PhBox act as efficient homogeneous catalysts in the kinetic resolution of 1,2-diols [2].
Hence, it was decided to test the S3 material as a heterogeneous catalyst in the asymmetric
benzoylation of hydrobenzoin (Scheme 3). The results are compiled in Table 4 together with the
homogeneous phase reactions with the reference PhBox and diolPhBox ligands.
Scheme 3. Kinetic resolution of 1,2-diphenylethane-1,2-diol (3) with the S3 material.
Ph
O
N
O
O
OTf
Cu
N
OTf
Ph
O
Ph
Ph
Ph
Ph
HO
OOCPh
HO
OH
PhCOCl (0.5 eq)
DIPEA (1.0 eq)
0°C, CH2Cl2
2
(± 1)
Table 4. Kinetic resolution of 1,2-diphenylethane-1,2-diol by the S3 material and CuPhBox,
CudiolPhBox in homogeneous phase (Scheme 1). a
mol b (%)
Catalyst
Cycle
%yield c
%ee d
S e TON f
Cu
0.7
1.0
1.1
0.7
0.7
diolPhBox
[Cu(OTf)2]
[Cu(OTf)2] + PhBox
[Cu(OTf)2] + diolPhBox
S3
1st
1st
1st
1st
2nd
33
46
47
38
20
0
50
1.0
1.1
0.1
0.1
84
84
38
9
27
26
3
46
43
55
30
1
a
Notes: Reactions performed for 24 h at 0 °C using 0.48 mmol (R,R)-3, 0.48 mmol (S,S)-3, 1.00 mmol
DIPEA, 1.0% mol based on Cu and 0.50 mmol of benzoyl chloride in 5.0 mL of CH2Cl2; b % of copper and
diolPhBox ligand in the catalyst in relation to 1 (see Table 1 and Figure 2); for the recycling experiments
c
d
corrected for the loss of heterogeneous catalyst weight; Isolated yield of 4 (Scheme 3); Enantiomeric
e
excess of 4, determined by HPLC; Selectivity (S) = ln[1 − yield (1 + ee)]/ln[1 − yield (1 − ee)];
f TON = moles of isolated 4 /moles of Cu.

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The S3 material acts as a heterogeneous catalyst in the kinetic resolution of hydrobenzoin with 38%
isolated yield of the monobenzoylated product, in a maximum of 50% yield, and 38%
enantioselectivity, confirming the presence of the CudiolPhBox at the surface of the carbonaceous
material. Higher catalytic activity was obtained than in all the homogeneous phase reactions. Since the
modified CudiolPhBox complex (Scheme 2) behaves as an efficient, enantioselective and selective
homogeneous catalyst as the original CuPhBox, the significantly lower enantioselectivity obtained in
heterogeneous phase must be due to the low content in the chiral bis(oxazoline) ligand of the S3
material (Table 1). The yield was lower than the homogeneous phase reactions, but slightly higher than
the homogeneous phase reaction run with [Cu(OTf)2]. The CuPhBox has also been immobilized onto
the surface of other mesoporous carbonaceous materials, CMK-3 and SPC, using a different strategy.
The monobenzoylated product yield obtained herein is comparable to the one obtained with CMK-3
and higher than the one with SPC as support, but the enantioselectivity is significantly lower. Again
this may be attributed to the low content of chiral bis(oxazoline) ligand of the S3 material, whereas the
other heterogeneous catalysts present similar quantities of Cu(II) and chiral PhBox essential to obtain
good enantioselectivites. The higher copper content obtained by ICP-OES than nitrogen by elemental
analysis also suggests that there might be Cu(II) triflate that is directly coordinated to the surface of the
Starbon® 700. This would be a non enantioselective way to obtain monobenzoylated product, which
may also be contributing to the lowering of the overall S3 heterogeneous catalyst enantioselectivity.
At the end of the reaction with the S3 material, it was removed by vacuum filtration, washed and
dried. Then it was used in another cycle of the kinetic resolution of hydrobenzoin, but with reduced
monobenzoylated yield and enantioselectivity (Table 4). This may be due to the instability
immobilized active species, the copper(II) complex formed with diolPhBox, as found for other
immobilized copper(II) bis(oxazoline) systems [5,6,8–10].
3. Experimental Section
3.1. General Information
Copper(II) trifluoromethanosulfonate (copper triflate, [Cu(OTf)2], 98%), (S)-(−)-2,2'-isopropylidenebis(4-
phenyl-2-oxazoline) (1, PhBox, 97%), paraformaldehyde (95%), triethylamine (Et3N, ≥99%), dry
tetrahydrofuran (THF, ≥99.9%), bromine (reagent grade%), and potassium bromide (FT-IR grade, ≥99%)
were purchased from Aldrich and used as received. Ethanol (p.a.), methanol (p.a.) were from Riedel de
Häen. 1,4-dioxane was from Fisher Scientific and n-pentane from VWR. Dichloromethane, acetonitrile,
n-hexane, isopropanol and ethyl acetate were HPLC grade and from Romil. Elemental analysis and
copper ICP-OES were performed in duplicate by CACTI Vigo, University of Vigo (Vigo, Spain). Low
resolution ESI-MS was performed at QOPNA of the University of Aveiro (Aveiro, Portugal) and high
resolution ESI-MS was performed by CACTI Vigo, University of Vigo. 1H- and 13C-NMR was performed
using a Bruker Avance 300 instrument. FTIR spectra were collected in the range 400–4000 cm−1 at
room temperature using a resolution of 4 cm−1 and 256 scans as KBr pellets with a FT Mattson 7000
galaxy series spectrophotometer; the samples were previously dried in an oven at 100 °C for 6 h.
Thermogravimetry analyses (TGA) were performed under nitrogen flow with a ramp of 5 °C/min in a
TGA apparatus, model Shimadzu TGA-50. X-ray photoelectron spectroscopy was performed at “Centro de

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Materiais da Universidade do Porto” (CEMUP, Porto, Portugal), in a KRATOS AXIS Ultra HSA -VISION
spectrometer using a nonmonochromatized MgKα radiation (1,253.6 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 internal standard. The elemental
contents of the various samples were calculated from the areas of the relevant bands in the high
resolution XPS spectra, which were also curve fitted using symmetric Gaussian curves, after fitting to
a Shirley background, using XPSpeak version 4.1. Nitrogen adsorption isotherms at −196 °C were
measured in an automatic apparatus (Gemini V 2.00 instrument model 2,380; Micromeritics). Before
the adsorption experiments the samples were outgassed under vacuum overnight at 120 °C to an
ultimate pressure of 1,024 mbar and then cooled to room temperature prior to adsorption.
3.2. Synthesis of the Diolphbox
To PhBox (0.0953 g, 0.311 mmol) and paraformaldehyde (0.0254 g, 0.846 mmol, 2.7 eq),
dichloromethane (2.5 mL) and 1,4-dioxane (0.6 mL) were added. To this mixture a solution of
triethylamine (0.2 mL) in tetrahydrofuran (1.6 mL) was slowly added over a period of 1 h and the
solids dissolved gradually during this period. The resulting solution was stirred for 3 days at room
temperature and then poured into 8 mL of n-pentane. A precipitate appeared immediately which was
isolated by vacuum filtration. The solution was evaporated under vacuum and a white foam was
1
obtained (0.052 g, 46% yield). H-NMR (300 MHz, CDCl3), δ/ppm: 7.38–7.25 (m, 10 H), 5.30–5.24
(dd, 2 H, J = 10.2 Hz, J = 7.9 Hz), 4.74–4.67 (dd, 2 H, J = 10.2 Hz, J = 8.5 Hz), 4.25–4.15 (m, 6 H).
13C-NMR (75 MHz, CDCl3), δ/ppm: 166.6, 141.6, 128.8, 127.8, 126.6, 75.0, 69.1, 52.7, 49.9.
+
ESI-HRMS, m/z: calculated (C21H23N2O4 ) 367.16523, experimental 367.16575. FTIR, ν/cm−1: 2,983 m,
2923 m, 2,853 m (propyl C-H stretching), 1,650 (C=N stretching).
3.3. Bromination of the Starbon® 700
To Starbon® 700 (0.5 g), previously dried at 150 °C under vacuum, tetrahydrofuran (THF, 10 mL) and
bromine (60 μL) were added. The mixture was stirred at room temperature for 24 h and the red color
of the solution faded. The material was isolated by vacuum filtration using 45 μm nylon membranes,
washed with THF and dried in an oven at 100 °C under vacuum.
3.4. Immobilization of the Cu(II) Complex with Diolphbox Onto the Brominated Starbon® 700
To diolPhBox (0.0279 g, 0.076 mmol) and [Cu(OTf)2] (0.0279 g, 0.076 mmol), dichloromethane
(60 mL) was added. A blue solution was obtained, but after some minutes stirring it turned green and
was left stirring for more 2 h. The brominated Starbon® 700 was added to this solution and refluxed for
36 h. The material was filtered and washed under reflux with fresh dichloromethane (20 mL) in order
to remove any physisorbed complex. Finally, the materials were isolated by filtration and dried
under vacuum.

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3.5. Catalysis Experiments
All the catalytic reactions of the prepared materials were performed in batch reactors at atmospheric
pressure and with constant stirring. The kinetic resolution of 1,2-diphenylethane-1,2-diol (1) was
performed at 0 °C using 0.48 mmol (R,R)-1,2-diphenylethane-1,2-diol, 0.48 mmol (S,S)-1,2-
diphenylethane-1,2-diol, 1.00 mmol DIPEA (170 µL), amount of heterogeneous catalyst containing
0.7% mol Cu and 0.50 mmol of benzoyl chloride (58 µL) in dichloromethane (5.00 mL) [2,9].
The mixture was stirred for 24 h and after filtration of the heterogeneous catalyst the solvent was
evaporated from the filtrate and the monobenzoylated product (2, Scheme 3) isolated by column
chromatography over silica gel using n-hexane/ethyl acetate 3:1 as eluent. The 2 enantiomeric excess
was determined by HPLC at 254 nm using a Chiralcel OD column (250 mm × 4.6 ID, 5 µm) and
n-hexane/isopropanol 9:1 as eluent at 1 mL/min. The retention times of the (R)-2 and (S)-2
enantiomers were identified by comparison with those of a racemic 2. The reaction selectivity (S) was
calculated based on the isolated yields of 2 and respective enantiomeric excess by using the formulae:
ln[1 − yield (1 + ee)]/ln[1 − yield (1 − ee)]. The isolated materials at the end of the reactions were
washed extensively with the appropriate solvent, dried under vacuum and reused in another cycle using
the same experimental procedure. Control experiments were also performed using the same
experimental procedure in homogeneous phase with equimolar quantities of [Cu(OTf)2] plus PhBox or
diolPhBox in order to compare with the heterogeneous ones.
4. Conclusions
A copper(II) complex with a commercial bis(oxazoline) ligand functionalized with CH2OH groups
was successfully reacted with the brominated surface of a mesoporous carbonaceous material.
The material acted as a selective and enantioselective heterogeneous catalyst in the kinetic resolution
of hydrobenzoin, with high catalytic activity. Due to the low chiral bis(oxazoline) ligand content the
enantioselectivity was reduced in comparison to the homogeneous phase reactions. Upon reuse of the
material a further decrease in the product yield and enantioselectivity was observed, probably due to
the instability of the copper(II) complex. Further work is being undertaken in order to improve the
performance of the immobilized homogeneous catalyst onto porous carbonaceous materials in
asymmetric transformations.
Acknowledgments
This work was funded by Fundação para a Ciência e a Tecnologia (FCT) through the project
PTDC/QUI/64770/2006, which was co-financed by EU under the programs COMPETE, QREN and
FEDER, and performed under the COST Action TD1203. ARS thanks FCT, FSE and POPH for the
contract under the program Investigator FCT 2012 (IF/1300/2012), as well as for financing under the
program Pest-C/CTM/LA0011/2013 (CICECO). PS gratefully acknowledges the Ministerio de Ciencia
e Innovacion for the concession of a Juan de la Cierva (JCI-2011-10836) contract.

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Author Contributions
ARS designed research and wrote the paper; LC performed research; PSS synthesized the Starbon®.
ARS and VB analyzed the data. JHC provided supervision and technical advice to the project. All
authors read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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