Reusable shuttles for exchangeable functional cargos: Reversibly assembled, magnetically powered organocatalysts for asymmetric aldol reactions

Article abstract of DOI:10.1016/j.tet.2019.130592

A supramolecular approach has been followed to support adamantyl substituted proline organocatalysts onto the surface of magnetite nanoparticles decorated with a β-cyclodextrin motif. The resulting magnetic nanoparticles (ca. ～10 nm diameter) were used as modular, magnetically recyclable catalysts in the asymmetric aldol reaction of aromatic aldehydes with cyclic ketones in water. The catalytic assemblies can be easily dismantled in organic media, and the recovered nanoparticles (magnetically powered chemical shuttles) re-complexed with another suitably substituted catalytic unit (replaceable functional cargo).

Full text of DOI:10.1016/j.tet.2019.130592

Tetrahedron 75 (2019) 130592
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Reusable shuttles for exchangeable functional cargos: Reversibly
assembled, magnetically powered organocatalysts for asymmetric
aldol reactions
,
,
, b
Carolina Mendoza a 1, Augustin de la Croix a, Paola Riente a 2, Lluís Llorens a
,
Javier de Mendoza a , Miquel A. Pericas
a Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Avgda. Països Catalans, 16, E-43007 Tarragona, Spain
,
**
a, *
ꢀ
b
ꢀ
ꢀ
ꢁ
Departament de Química Inorganica i Organica, Universitat de Barcelona, c/ Martí i Franques 1-11, E-08028 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A supramolecular approach has been followed to support adamantyl substituted proline organocatalysts
Received 21 June 2019
Received in revised form
26 August 2019
Accepted 2 September 2019
Available online 10 September 2019
onto the surface of magnetite nanoparticles decorated with a b-cyclodextrin motif. The resulting mag-
netic nanoparticles (ca. ~10 nm diameter) were used as modular, magnetically recyclable catalysts in the
asymmetric aldol reaction of aromatic aldehydes with cyclic ketones in water. The catalytic assemblies
can be easily dismantled in organic media, and the recovered nanoparticles (magnetically powered
chemical shuttles) re-complexed with another suitably substituted catalytic unit (replaceable functional
cargo).
This paper is warmly dedicated to Professor
Stephen G. Davies, to honor his long stand-
ing commitment as an author and an editor
in favor of asymmetric synthesis and
asymmetric catalysis.
© 2019 Elsevier Ltd. All rights reserved.
Keywords:
Aldol reaction
Cyclodextrins
Nanoparticles
Organocatalysis
Recyclable catalyst
Supramolecular catalyst
1. Introduction
catalytic process, arising from another part of the molecule.
Immobilization of asymmetric catalysts on non-soluble sup-
Shuttles have become popular in space exploration because they
are (at least partially) reusable, thus significantly reducing the high
cost of each mission. Similarly, chemical shuttles are desirable in a
variety of areas, such as catalysis and drug delivery. In particular,
shuttles are attractive when translocation, induced by an external
physical or chemical stimulus, combines with function, such as a
ports constitutes an attractive alternative to homogeneous catal-
ysis, as it allows easy recovery and recycling [1]. Thus, a wide
variety of support types have been used, such as soluble and
insoluble polymers [2], silica [3], dendrimers [4], as well as mag-
netic nanoparticles (MNPs) [5]. Among these supports, nano-
particles can present the additional advantages of increased
catalytic activity, due to the accumulation of active centres on their
surface, and to enhanced enantiocontrol, due to the close similarity
with purely homogeneous processes[5b,6].
* Corresponding author.
Presumably, due to reasons such as the avoidance of catalyst
leaching, minimization of product contamination and the possi-
bility of recovery and reuse, catalyst immobilization by covalent
tethering has become prevalent, although non-covalent attach-
ment modes, such as encapsulation, adsorption or electrostatic
interactions have also been explored [7]. In any case, complex
** Corresponding author.
E-mail addresses: jmendoza@iciq.es (J. de Mendoza), mapericas@iciq.es
ꢀ
(M.A. Pericas).
1
ꢁ
Current address: Química Inorganica, Departamento Estrella Campos, Facultad
de Química, Universidad de la República, Montevideo, Uruguay.
2
Current address: Micro Flow Chemistry and Synthetic Methodology, Depart-
ment of Chemical Engineering and Chemistry, Eindhoven University of Technology.
https://doi.org/10.1016/j.tet.2019.130592
0040-4020/© 2019 Elsevier Ltd. All rights reserved.

2
C. Mendoza et al. / Tetrahedron 75 (2019) 130592
catalytic systems assembled by covalent immobilization suffer from
important vulnerabilities arising from the non-applicability of the
shuttling principle. Thus, any failure in operation leads to the loss of
the vehicle and its cargo.
Nowadays, the most versatile and readily available chemical
shuttles are magnetite nanoparticles, displaying controllable
mobility under the stimulus of an external magnetic field, due to
their superparamagnetism [8]. In addition, their outer surface, rich
in hydroxyl groups, can be readily functionalised by grafting with a
inclusion into CDs are mainly van der Waals forces, hydrophobic
interactions, electronic effects and steric factors [12b,17].
CDs have also been attached to nanoparticles, for the adsorption
of contaminants [18], drug delivery [19] and catalysis [20]. Kaifer
and co-workers reported the preparation of water-soluble CD-
modified gold nanospheres forming host-guest complexes with
ferrocenemethanol and 1-adamantanol [21]. Magnetic FecorePtshell
nanoparticles have also been modified with g-CD (eight glucose
subunits). The resulting water soluble nanoparticles have been
variety of
u
-substituted 1,1,1-trialkoxysilanes. A variety of func-
employed as efficient catalysts for the aqueous hydrogenation of
tional units have been incorporated on magnetic nanoparticles by
this approach, most frequently using click chemistry (Fig. 1a) [9].
The vulnerability of the covalent approach mentioned above
could be in principle overcome through the integration of high-
affinity complexing units onto the magnetic nanoparticles sur-
face. In this way, magnetically-powered chemical shuttles would be
constructed, and functional cargos for different applications could
allyl alcohol [22] and, quite recently, MNPs decorated with b-CD
were employed to recover an adamantyl-containing catalyst from
the reaction medium [23]. However, a magnetically-powered cat-
alytic shuttle with a catalytic cargo based on the b-CD-adamantyl
interaction has not yet been reported in the literature.
In this work we describe a supramolecular approach to catalysis
with immobilised systems that benefits from both covalent teth-
ering and non-covalent attachment strategies as key elements of
the shuttle design. This is based on the introduction of a supra-
molecular motif (namely, a cyclodextrin-adamantane contact) as a
reversible linker between the magnetic nanoparticles and the
catalyst molecule. This could allow the easy recovery of a given
catalyst from the surface of the nanoparticle for the sequential use
of different catalysts with the same shuttle (Fig. 2) or even
exploring the operation of tandem catalytic effects with the
simultaneous presence of different catalysts on the shuttle [24]. As
a proof of concept for our design, we selected the proline catalysed
aldol reaction between aromatic aldehydes and cyclohexanone in
water. An adamantyl-decorated proline catalyst encapsulated into a
be
decomplexation-recomplexation
instance, the -cyclodextrin-adamantyl supramolecular interaction
sequentially
introduced
by
simple
complexation-
sequences
involving, for
b
(Fig. 1b). In addition to the increased versatility of this approach,
any possible deactivation of the functional unit could be easily
remediated by a solvent-induced disassembly of the non-covalent
construct that would leave the valuable magnetically-powered
shuttle ready for reuse through simple complexation with fresh
catalytic units.
Although crown ethers and polyamines have been previously
immobilised onto magnetic nanoparticles [10], we decided to focus
on receptors with high affinities for organic molecules. This is the
case for
b
-cyclodextrin (a naturally occurring cyclic array of seven
b-CD has been reported by Liu and co-workers [16b], yielding the
D
-glucopyranosyl subunits linked by
a
-1,4-glycosidic bonds), which
corresponding hydroxyketones with high diastereo- and
enantioselectivities.
strongly binds adamantyl derivatives in water (Kass ¼ 105-106 Mꢀ1
)
[11,12]. Due to their low price, water solubility, lack of toxicity and
highly hydrophobic internal cavities, cyclodextrins (CDs) have been
widely exploited in the food, cosmetics and pharmaceutical in-
dustries [13]. When dissolved in an aqueous phase, the inner space
of CDs provides a hydrophobic micro-environment. Thanks to this
property, CDs have been widely employed as enzyme mimics [14]
and in catalysis, either covalently attached to chiral catalysts to act
on an included substrate [15], or by complexing the catalyst inside
the cavity to act on external substrates [16]. Driving forces for
In our case, to attach the b-CD to the magnetic nanoparticles, we
decided to prepare functional nanoparticles 1 (Fig. 3) incorporating
a 1,2,3-triazole subunit in the linker, which can be easily formed
through a copper-catalysed alkyne-azide cycloaddition (CuAAC)
reaction [25], the best known example of click chemistry [26].
In recent years, we and other groups [27] have studied and
established the validity of this approach for the immobilization of
catalytic species without negative effects on either the catalytic
activity or stereoselectivity. As part of the ongoing work in our
group on the preparation of functional ε-cobalt [28] and Fe3O4 [9d-
f,29a-b] magnetic nanoparticles (MNPs) as easily recoverable and
reusable catalysts, we decided to use azido-functionalised
Fig. 1. A single-purpose, magnetically powered catalyst (a) and a multipurpose,
b-
cyclodextrin-adamantyl reversibly assembled analogue designed to act as a chemical
shuttle for a functional cargo (b).
Fig. 2. A multi-purpose, reversibly assembled magnetically powered catalyst for
sequential use.

C. Mendoza et al. / Tetrahedron 75 (2019) 130592
3
sodium ascorbate, using a 1:1 water/t-butyl alcohol mixture as
solvent. The reaction was monitored by IR spectroscopy (Fig. 5), and
once the azide band of the functional nanoparticles (around
2097 cmꢀ1) had disappeared, the particles 1 were separated by
magnetic decantation. The incorporation of the b-CD moiety was
indicated by the presence of bands in the region of 900e1200 cmꢀ1
associated with the CeO bonds in the IR spectrum of 1. The evo-
lution of the shape and size of the functional nanoparticles, from 4
to 1, was followed by TEM micrography (Fig. 4aec) Nanoparticles 1
were obtained as a mixture of small aggregates and discrete par-
ticles with a diameter of 10.3 2.9 nm (Fig. 4c), without significant
change in the aspect or size with respect to precursors 4 and 6. The
degree of functionalization of MNPs
elemental analysis (%N) and found to have
approximately 0.21 mmol/g.
1
was determined by
-CD loadings of
b
To test the shuttling principle in catalysis, proline derivatives 2
and 3 (Fig. 3, see ESI for synthetic details) were first submitted to
complexation with
b-CD for comparative purposes (Scheme 2).
To this end, equimolar amounts of 2 or 3 and
b
-CD were dis-
solved in 95:5 water/methanol. The reaction mixture was stirred at
room temperature for 2e48 h, and the solvent was then removed
under reduced pressure to afford the corresponding complex 8 or 9.
Both 1H NMR spectroscopy and MS provided evidence of the for-
mation of the host-guest complexes (see ESI for NMR spectra). In
the case of 8, TOF-MS (positive mode) shows a peak at 1438.3 amu
that was assigned to the cation [MþNa]þ, where M corresponds to
Fig. 3.
b-CD-functionalised MNPs (1) and partner L-hydroxyproline derivatives modi-
fied with adamantyl residues (2 and 3).
magnetite nanoparticles as platforms for the grafting of a mono-
alkynylated b-CD subunit through a “click” cycloaddition reaction.
Two organocatalytic guests 2 and 3 were explored, differing mainly
in the nature and length of the linker between the proline catalytic
unit and the adamantyl anchor. While 2 was designed to behave as
a loose guest due to the short distance between the hydrophobic
moiety and the catalytic site, we considered that 3, thanks to the
longer linker connecting these two units, would lead to more
tightly bound complexes with 1. According to this design, 2 would
serve as a proof of principle for the recoverability and recyclability
of 1 (the chemical shuttle) should its catalytic cargo be lost by
leaching.
the mass of the inclusion complex between
b-CD and guest 2
(C57H93N4O37S). On the other hand, the TOF-MS spectrum (negative
mode) of complex 9 shows a peak at m/z ¼ 1507.3 amu, assigned to
the anion [M ꢀ H]- where M is the mass of the inclusion complex
between b-CD and guest 3.
Following the same procedure, proline derivatives 2 and 3 were
immobilised onto the surface of the cyclodextrin-modified
magnetite nanoparticles 6 to provide the shuttle þ catalyst nano-
particle assemblies 10 and 11 (Fig. 6).
TEM images (Fig. 4d) showed that the size of the nanoparticles
was maintained (9.1 2.2 nm for nanoparticles 10 and 9.7 2.8 nm
for 11).
2. Results and discussion
The appearance of an additional band at 2989 cmꢀ1 in the IR
spectrum of the assemblies (Fig. 7) supports the presence of the
proline catalytic unit on the surface of the nanoparticles. Ther-
mogravimetric analysis (Fig. 8), is also strongly indicative of the
presence of the proline derivative on the surface of the nano-
particles. Thus, 25% of the initial weight is lost between 300 and
800 ꢁC from nanoparticles 11, whereas a much lower weight loss is
2.1. Synthesis and characterization of the catalytic materials 8e9
and 10e11
MNPs of Fe3O4 (4) were prepared following the co-precipitation
method, in the presence of oleic acid as a surfactant (Scheme 1)
[30]. Transmission electron microscopy (TEM) images revealed the
formation of 8.4 2.6 nm nanoparticles (Fig. 4a). Next, the 3-
azidopropyl moiety was grafted onto the surface of the MNPs by
reaction with 3-azidopropyltrimethoxysilane (5) in toluene [31],
leading to 9.8 2.9 nm nanoparticles 6 (Fig. 4b) with a function-
alization of approximately 0.30 mmol N3/g (as determined by ni-
trogen elemental analysis).
recorded for
b-CD-functionalised nanoparticles 1 over the same
temperature range.
2.2. Catalytic behaviour of 2e3 and 8e9 in the asymmetric aldol
reaction in water
Integration of the
carried out using a CuAAC reaction [25a-b] promoted by CuSO4/
b-CD derivative 7 [32] onto the MNPs was
We first evaluated prolines 2e3 as catalysts in the model
asymmetric aldol reaction of benzaldehyde (12a) and cyclohexa-
none (13) in water, using 10 mol% of the catalyst (Table 1, entries 1
and 4). In both cases, moderate yields of the aldol product were
obtained after 24 h, with good to excellent enantioselectivities. In
terms of diastereo- and enantioselectivity, the best results were
recorded for catalyst 3 (>99% ee). The scope of these catalysts for
different aldehydes and ketones was examined under the same
conditions (see ESI) leading to results that are comparable with
those recorded with analogous 4-hydroxyproline derivatives [33].
The b-CD complexes 8 and 9 were also tested for the same reaction
using conditions analogous to the previous experiments (10 mol%
of catalyst, 60 eq. of water and rt). Excellent stereoselectivities were
observed for both catalysts (entries 6 and 7), although in the case of
Scheme 1. Preparation of Fe3O4 MNPs 1 functionalised with b-CD.

4
C. Mendoza et al. / Tetrahedron 75 (2019) 130592
Fig. 4. TEM image and size distribution of a) MNPs 4, b) azide-functionalised MNPs 6, c) b-CD-functionalised MNPs 1 and d) MNPs 11.
8 a decrease in activity was observed with respect to the uncom-
plexed catalyst (compare entries 1 and 6). When catalyst 9 was
allowed to react for 48 h, a nearly complete conversion was ach-
ieved (compare entries 10 and 11). Overall, the results are very good
both in terms of enantio- and diastereoselectivity. For catalyst 8 the
enantioselectivity was improved for both 12a and p-tri-
fluoromethylbenzaldehyde (12b). In the case of 9 the results were
very similar to those obtained for the uncomplexed proline 3, both
for 12a and 12b.
2.3. Testing the shuttling principle with 10e11. Recovery and reuse
of the MNPþ -CD shuttle
b
For the catalytic study, functional nanoparticles 10 and 11 were
suspended in water (60 eq.) and tested in the asymmetric aldol
reaction of p-trifluoromethylbenzaldehyde (12b) with 13 (Table 1,
entries 12e14). With nanoparticles 10, the aldol product was ob-
tained in moderate yield (32%) after 24 h (entry 12) and, even after
72 h, complete conversion was not achieved (entry 13). Although
this clearly indicates a significant decrease in the activity of the
catalyst upon complexation, in terms of diastereo- and enantiose-
lectivity the results are comparable with those obtained with the

C. Mendoza et al. / Tetrahedron 75 (2019) 130592
5
Fig. 7. Comparison of the IR spectra of proline derivative 3, MNPs 1 and MNPs 11.
Fig. 5. IR spectra of azide-functionalised MNPs 6 and b-CD-functionalised MNPs 1.
Scheme 2. Preparation of inclusion complexes of
3.
b-CD with L-proline derivatives 2 and
Fig. 8. TGA curves showing the weight loss of nanoparticles 11 and
300e800 ꢁC temperature range.
1 over the
decomplexation takes place upon repeated use or is provoked, for
instance, by exposure of the complex to an organic medium, the
most expensive part of the construct (the magnetic nanoparticles)
can still be recovered by magnetic decantation and recomplexed
(with either the same catalytic unit or with a different one) for
further use.
Fig. 6. Fully assembled shuttle þ catalyst nanoparticles 10 and 11.
To demonstrate the recyclability and reconfigurability of mag-
netic shuttle 1, freshly prepared 10 was first used as the catalyst in
four consecutive runs of the aldol reaction between p-tri-
fluoromethylbenzaldehyde and cyclohexanone (Table 2, entries
1e4). Indeed, while the enantioselectivity of the reaction remained
constant during the recycling, the yield progressively decreased.
This unsatisfactory result is due to the poor solubility of the aldol
product 14b in water, which requires the use of water/alcohol
mixtures to separate the aldol product from the catalytic nano-
particles. While 10 is stable in water, the work-up is responsible for
substantial leaching of the catalytic unit 2 and for the associated
catalyst deactivation. Gratifyingly, the recovery of the unloaded
shuttle 1 after the fourth reaction cycle was essentially quantitative.
The recovered nanoparticles 1 were washed with THF to secure the
removal of residual 2, dried under vacuum, and submitted to re-
complexation with 3. Recycling of catalyst 11 assembled in this
manner proved to be much more rewarding (entries 5e7). As could
be anticipated from the design of the catalytic unit 3, involving a
longer linker/spacer that improves the supramolecular interaction
free catalyst 2 and with the molecular complex 8. For catalyst 11
(entry 14) the reaction of p-trifluoromethylbenzaldehyde was
achieved with excellent yield (90%) after 72 h reaction, but both
diastereo- and enantioselectivities were eroded with respect to the
reference, free catalyst 3 or its molecular complex 9.
As we have already mentioned, one of the main advantages
associated with supporting a catalyst onto MNPs is the possibility of
recovery by simple magnetic decantation and reuse. Through the
assembling process, the magnetic properties of the supported
nanoparticles are transmitted to the chiral catalyst they contain,
becoming magnetically powered, and can be moved through a liquid
phase or held within it by application of an external magnetic field.
From the sustainability point of view, a catalyst loaded onto such a
chemical shuttle can be separated from the reaction media and
subsequently recovered and reused by magnetic decantation
involving the simple application of inexpensive neodymium mag-
nets to the walls of the reaction vessel. In addition, if

6
C. Mendoza et al. / Tetrahedron 75 (2019) 130592
Table 1
proline derivatives bearing adamantyl residues. The resulting
nanoparticles were used as magnetically recoverable catalysts in
the asymmetric aldol reaction of aromatic aldehydes with cyclo-
hexanone in water, displaying high diastereo- and enantiose-
lectivities. We have also shown that the reversibly-assembled
catalysts can be easily disassembled in organic media, and that the
recovered functional nanoparticles (magnetically powered chemical
shuttles) can be re-complexed with a different catalytic guest. This
paves the way for the development of multipurpose catalytic kits
assembled through non-covalent, yet strong hydrophobic in-
teractions between magnetic nanoparticles decorated with suitable
hosts (acting as chemical shuttles) and catalytic units equipped with
complementary guests (as replaceable functional cargos).
Asymmetric aldol reaction of 12a-b and 13 using proline derivatives 2 and 3,a
complexes 8 and 9,b and functional nanoparticles 10 and 11c as catalysts.
b-CD
Entry
Catalyst
R
Time [h]
Yield [%]d
anti:syne
eeanti [%]f
1
2
3
4
5
6
7
8
2
2
2
3
3
8
9
8
8
9
9
10
10
11
11
H
H
CF3
H
CF3
H
24
48
24
24
24
24
24
24
48
24
48
24
72
72
72
26
35
77
41
81
13
54
61
75
70
98
32
49
90
52
6:1
6:1
85
81
93
>99
97
94
99
96
96
96
99
92
91
87
78
10:1
18:1
30:1
4.5:1
14:1
12:1
10:1
32:1
33:1
9:1
4. Experimental section
4.1. General information
H
CF3
CF3
CF3
CF3
CF3
CF3
CF3
H
Unless otherwise stated, all commercial compounds were used
as received without any further purification. Ultra pure water was
obtained from an SG Water Ultra clear basic system, that provides
9
10
11
12g
13
14
15
water with a conductivity at 25 ꢁC of 0.055
mS. NMR spectra were
9:1
13:1
4:1
recorded on a Bruker Avance 400 Ultrashield NMR spectrometer at
room temperature unless otherwise stated. For 1H NMR all chem-
ical shifts are reported in ppm relative to the proton resonance
resulting from incomplete deuteration of the corresponding NMR
solvent: CD3OD (3.31 ppm), CDCl3 (7.27 ppm) and CD2Cl2
(5.32 ppm). In the case of 13C NMR spectra all chemical shifts are
reported in ppm relative to the carbon resonance of the corre-
sponding deuterated NMR solvent: CD3OD (49.15 ppm), CD2Cl2
(54 ppm) and CDCl3 (77.23 ppm). IR spectra were recorded on a
Bruker Tensor 27 FTIR spectrometer fitted with an ATR cell. IR
spectra of nanoparticles were recorded on a Thermo Nicolet 5700
FTIR spectrometer, using KBr pellets. Reactions under microwave
irradiation were performed in a CEM Discover microwave synthesis
apparatus using 10 mL vessels with septa for reactions performed
at elevated temperatures and pressures. HPLC analyses were per-
formed with Agilent Technologies HP-1100 or HP-1200 apparatus,
equipped with UV detector or diode array detectors and MS de-
a
2 or 3 (0.04 mmol), 12a-b (0.4 mmol), 13 (2.0 mmol), water (8 mmol.
8 or 9 (0.032 mmol), 12a-b (0.3 mmol), 13 (1.5 mmol), water (18 mmol).
10 or 11 (0.016e0.020 mmol), 12a-b (0.2 mmol), 13 (1.0 mmol), water
b
c
(12 mmol).
d
Isolated yield of the combined diastereoisomers.
e
f
Determined by 1H NMR spectroscopy.
Determined by chiral HPLC.
g
12b (0.06 mmol), 13 (0.3 mmol), water (3.6 mmol).
Table 2
Recycling and reconfiguration of magnetic shuttle 1, from catalyst 10 to 11 in the
aldol reaction of 12b with 13.a
Catalyst
Cycle
Time [h]
Yield [%]b
anti:sync
eeanti [%]d
10
1
2
3
4
5
6
7
24
24
24
24
72
72
72
32
17
Traces
Traces
89
9:1
9:1
6:1
6:1
14:1
10:1
5:1
92
93
91
92
89
87
85
tector respectively. Chiralcel OD-H 5
alpak AD-H
(0.46 cm ꢂ 25 cm), Chiralpak AS-H
(0.46 cm ꢂ 25 cm), Chiralpak IC 5 m (0.46 cm ꢂ 25 cm), Chiralpak
IA 5
m (0.46 cm ꢂ 25 cm)
m
m (0.46 cm ꢂ 25 cm), Chir-
5
mm
5 mm
11
m
57
35
mm (0.46 cm ꢂ 25 cm), Chiralpak IB 5
m
chiral columns fitted with guard columns were used to separate
enantiomers. For the determination of the experimental conditions
for the analysis, pure racemic samples were prepared and used as
standards. Elemental analyses were performed at Servei de
a
Reaction conditions for catalyst 10: aldehyde (0.2 mmol), 13 (1.0 mmol), water
(12 mmol), catalyst (8e10 mol%), room temperature, 24 h; reaction conditions for
catalyst 11: aldehyde (0.2 mmol), ketone (1.0 mmol), water (12 mmol), catalyst
(8 mol%), room temperature, 72 h.
b
ꢀ
Microanalisi de l'Institut d'Investigacions Químiques i Ambientals
Isolated yield of the combined diastereoisomers.
c
Determined by 1H NMR spectroscopy.
de Barcelona (IIQAB) or at Centro de Microanalisis Elemental,
ꢁ
d
Determined by chiral HPLC.
ꢁ
Departamento de Química Farmaceutica, Facultad de Farmacia de la
Universidad Complutense de Madrid. Transmission electron mi-
croscopy (TEM) images were obtained with a JEOL JEM-1011
transmission electron microscope equipped with a lanthanum
hexaboride filament operated at an acceleration voltage of 100 kV.
Thermogravimetric analysis were performed in a Mettler Toledo
TGA/SDTA 851e thermo balance, working in a range of tempera-
tures from room temperature to 1000 ꢁC, with a precision of
0.25%. TGA results in the product description were recorded under
nitrogen stream (80 mL/min) and with a heating rate of 10 ꢁC/min.
between the b-CD and the adamantyl subunits, catalyst 11 behaves
as a more stable entity under the employed reaction and work-up
conditions, allowing multiple recycling without significant deteri-
oration of the catalytic performance. In this manner, a proof of the
shuttling principle has been established, although further work
would be required to make this approach competitive with
covalent-based catalyst immobilization.
Alumina crucibles of 100 mL of capacity were used.
3. Conclusion
4.2. Preparation of magnetite nanoparticles coated with oleic acid
In summary, we have shown that superparamagnetic magnetite
(4)
nanoparticles functionalised with
b-cyclodextrin via click chemis-
try are suitable hosts for the non-covalent immobilization of
L-
These nanoparticles were prepared by slight modifications of a

C. Mendoza et al. / Tetrahedron 75 (2019) 130592
7
previously reported procedure [30]. Iron(II) chloride tetrahydrate
4.5. CuAAC reaction of alkyne (7) with azido-functionalised
(99%) (3.01 g, 15 mmol) and iron(III) chloride hexahydrate (97%)
(8.36 g, 30.0 mmol) were dissolved in degassed ultrapure water
(140 mL) in a 500 mL round-bottomed flask and heated to 80 ꢁC,
shaking vigorously with a shaker. Oleic acid (98%) (2.1 mmol, 0.60 g,
0.68 mL) dissolved in degassed acetone (15 mL) was added to the
reaction mixture, followed by 30% aqueous NH3 solution (18.2 mL).
After this addition, further amounts of oleic acid were added
(5 ꢂ 1.0 mL) over a 5e10 min period. The black reaction mixture
was hold for 30 min at 80 ꢁC and then slowly cooled to room
temperature. A 1:1 v/v mixture of methanol and acetone (100 mL)
was then added to support the precipitation. After allowing the
nanoparticles to settle overnight with the help of a magnet, the
supernatant was separated using a cannula and the particles were
magnetite nanoparticles (6). Preparation of functional nanoparticles
(1)
Functional nanoparticles 6 (0.1 g, f ¼ 0.283 mmol/g, 9.8 2.9 nm
diameter) were dispersed in 8 mL of a 1:1 mixture of ultrapure
water and tert-butyl alcohol in a 100 mL round bottomed flask
using ultrasonication for 20 min. Alkyne 7 (0.066 g, 0.057 mmol)
was added to the mixture, followed by the l-sodium ascorbate 99%
(7.85 mg, 0.040 mmol) and copper(II) sulfate pentahydrate
(0.919 mg, 3.68 mmol) as a solid. The reaction mixture was stirred at
70 ꢁC for 72 h. Then the reaction was stopped and water was added
to the reaction mixture. The particles were magnetically separated
and washed as follows: water (5 mL), 20% v/v NH4OH in water
(2 mL), water (5 mL) and methanol (25 mL). The particles were
dried in vacuo at 40 ꢁC. The nanoparticles (60 mg) were recovered
as a brown solid. According to the %N determined by elemental
analysis the functionalization of the nanoparticles was
f ¼ 0.123 mmol/g. Particle size: (n ¼ 120, nm): 10.28 (s ¼ 2.90);
washed with
a 1:1 v/v mixture of methanol and acetone
(5 ꢂ 100 mL). The nanoparticles were dried, first with an argon
stream and then under vacuum. In this manner, 3.8 g of nano-
particles were isolated as a dark brown solid. Particle size (n ¼ 70,
nm): 8.40 (s ¼ 2.56); FT-IR (KBr):
n 3453, 3005, 2956, 2923, 2852,
1632, 1426, 1409, 1261, 1229, 1100, 1056, 802, 580 cmꢀ1
.
Found: C, 4.35; H, 0.93; N, 0.69; FT-IR (KBr): n 3385, 2956, 2922,
2852, 1637, 1399, 1151, 1079, 1029, 581 cmꢀ1. TGA (30e1000 ꢁC,
10 ꢁC/min, under N2; for a 2.7370 mg sample, % weight loss):
13.3668 (left limit: 99.28 ꢁC, right limit: 488.27 ꢁC), 3.9734 (left
limit: 488.27 ꢁC, right limit: 847.22 ꢁC).
4.3. Preparation of 3-azidopropyltrimethoxysilane (5)
This compound was prepared by slight modifications of a pre-
viously reported procedure [31a]. Sodium azide 95.5% (4.13 g,
60.7 mmol) was suspended in a mixture of anhydrous acetonitrile
(80 mL, molecular sieves 4 Å) and 2 mL DMF, under argon in a
Schlenk flask. 3-iodopropyltrimethoxysilane 95% (5 mL, 24.3 mmol)
was added with a syringe under stirring. The resulting white sus-
pension was then stirred overnight at reflux. The reaction mixture
was allowed to cool down to room temperature, the solvent was
evaporated under reduced pressure, dry pentane (45 mL) was
added and the reaction mixture was stirred for 15 min. The pentane
layer was transferred to another Schlenk flask using a cannula fitted
with filter paper, a second addition of dry pentane was done and
the process was repeated. The solvent was evaporated under
reduced pressure to yield 5 (4.24 g, 20.7 mmol, 85% yield) as clear
yellow liquid. All the spectroscopic data matched with those re-
4.6. Preparation of inclusion complexes of b-cyclodextrin with
proline derivatives 2 and 3
4.6.1. Inclusion complex of b-cyclodextrin and compound 2.
synthesis of complex 8
b-Cyclodextrin (0.206 g, 0.178 mmol) was placed in a 500 mL
round-bottomed flask and was dissolved in water (230 mL) to give a
colourless solution. To this solution, 2 (0.050 g, 0.178 mmol) dis-
solved in methanol (12 mL) was added and the mixture was stirred
at room temperature for two days. After this time, the solvents
were evaporated under reduced pressure and the solid residue was
dried in vacuo for 5 h to render the title product 8 (0.219 g, 87%
yield) as a white solid. When the complexation time was shortened
ported in the literature [6]. 1H NMR (400 MHz, CDCl3):
d
3.58 (s, 9H
to 2 h, yield was 76%. 1H NMR (400 MHz, D2O):
d 5.11 (d,
CH3), 3.27 (t, 3J(HeH) ¼ 6.9 Hz, 2H), 1.77e1.67 (m, 2H),
3J(H1,H2) ¼ 3.4 Hz, 7H, H1
b-CD), 4.17 (m, 1H), 3.92 (m, 28H, H3 b-
0.75e0.66 ppm (m, 2H); 13C NMR (100 MHz, CDCl3)
50.7 (CH3), 22.7 (CH2), 6.5 ppm (CH2); FT-IR (ATR):
d
53.9 (CH2),
CD
þ
H5
b-CD
þ
H6
b
-CD), 3.71 (dd, 3J(H2eH3) ¼ 9.8,
n
2942, 2841,
3J(H1eH2) ¼ 3.4 Hz, 7H, H2
b
-CD), 3.63 (t, 3J(HeH) ¼ 9.3 Hz, 7H, H4
2094 (N3), 1457, 1413, 1344, 1275, 1241, 1189, 1079, 883, 816,
b-CD), 3.56 (m, 2H), 3.32e3.20 (m, 1H), 2.80 (m, 1H), 2.29 (m, 3H,
628 cmꢀ1
.
CH adamantyl), 2.08e1.75 ppm (m, 13H, 6 ꢂ CH2 adamantyl þ CH
proline); 13C NMR (101 MHz, D2O)
176.09 (COOH), 104.92 (CH, C1
-CD), 75.86 (CH, C3
-CD), 74.58 (CH, C2 þ C5
-CD), 63.21 (CH), 62.64 (CH2, C6 -CD), 56.83 (CH2), 48.76 (C),
d
4.4. Azido-functionalised magnetite nanoparticles (6)
b-CD), 84.14 (CH, C4
b
b
b
b
These nanoparticles were prepared by slight modifications of a
previously reported procedure [31c]. In a typical preparation
nanoparticles 4 (0.96 g) were dispersed by sonication (20 min) in
46.67 (CH2), 40.18 (CH), 38.45 (CH2), 38.30 (CH2), 31.71 ppm (CH).
FT-IR (ATR):
n 3279, 2910, 2851, 1624, 1449, 1367, 1329, 1299, 1245,
1152, 1101, 1079, 1022, 998, 936 cmꢀ1; MS (TOF MS ESþ): m/z (%)
about
480 mL
of
degassed
toluene
and
then
3-
1438.3 (12%) [MþNa]þ, 1157.2 (37%) [(
[C15H23NO2S þ Na]þ.
b
-CD)þNa]þ, 304.1 (100%)
azidopropyltrimethoxysilane 5 (1.92 g, 9.33 mmol) was added, fol-
lowed by glacial acetic acid 99.5%, d ¼ 1.05 g/mL (0.192 mL,
3.36 mmol) and ultrapure water (0.269 mL, 14.93 mmol). The re-
action mixture was stirred at room temperature for 3.5 days. The
black particles were allowed to settle overnight with the help of an
external magnet. The supernatant was separated using a cannula,
and the nanoparticles were washed with toluene (3 ꢂ 42 mL) and
methanol (3 ꢂ 26 mL) and dried under vacuum. In this manner,
0.78 g of nanoparticles were recovered as a brown powder. Ac-
cording to the %N determined by elemental analysis the function-
alization of the nanoparticles was f ¼ 0.35 mmol N3/g. Particle size:
(n ¼ 41, nm): 8.83 (s ¼ 3.01); Found: C, 3.61; H, 0.80; N, 1.48; FT-IR
4.6.2. Inclusion complex of b-cyclodextrin and compound 3.
synthesis of complex 9
b
-cyclodextrin (0.085 g, 0.073 mmol) was placed in a 250 mL
round-bottomed flask and was dissolved in water (94 mL) to give a
colourless solution. To this solution, compound (0.0275 g,
3
0.073 mmol) dissolved in methanol (7.1 mL) was added and the
mixture was stirred at room temperature for 2 h. After this time, the
solvents were eliminated using a rotary evaporator and the solid
residue was dried in vacuo for 5 h to render the title product 9
(0.097 g, 88% yield) as a white solid. 1H NMR (400 MHz, D2O):
d 8.17
(KBr):
n
3442, 2955, 2924, 2852, 2098 (N3), 1637, 1420, 1239, 1182,
(s, 1H, H triazole), 5.65e5.48 (m, 1H), 5.10 (d, 3J(H1eH2) ¼ 3.6 Hz,
7H), 4.68 (s, 2H), 4.56 (t, J(HeH) ¼ 8.8 Hz, 1H), 4.03 (m, 2H),
1103, 1022, 585 cmꢀ1
.

8
C. Mendoza et al. / Tetrahedron 75 (2019) 130592
3.96e3.76 (m, 30H), 3.75e3.53 (m, 17H), 3.00e2.62 (m, 2H), 2.15 (s,
3H), 1.79 (m, 6H), 1.64 (s, 6H), 1.47 ppm (t, J(HeH) ¼ 7.4 Hz, 2H). FT-
4.8. Procedures for the asymmetric aldol reactions catalysed by
cyclodextrin complexes
b-
IR (ATR):
n 3259, 2898, 2844, 1628, 1407, 1367, 1329, 1244, 1203,
1152, 1079, 1024, 998, 940, 846 cmꢀ1; MS (TOF MS ESꢀ): m/z (%)
1507.3 (52) [M ꢀ H]- (C62H99N4O38 requires 1507.59), 1133.1 (20)
4.8.1. With monomeric catalysts 8 and 9
The catalyst 8 or 9 (0.032 mmol, 0.1 eq) was added to a sus-
pension of the aldehyde 12 (0.3 mmol) and cyclohexanone 13
[(b
-CD)-H]-, 373.1 (100) [C20H29N4O3]-.
(155 mL, 1.5 mmol, 5 eq) in water (324 mL, 18 mmol, 60 eq). The re-
action mixture was stirred for 24e48 h at room temperature. After
that, water (5 mL) was added and the aqueous layer was extracted
with ethyl acetate (3 ꢂ 5 mL). The organic layer was dried with
MgSO4, filtered and concentrated in vacuo. The crude product was
purified by silica gel column chromatography using 9:1 hexane-
ethyl acetate as eluent.
4.7. Preparation of supramolecular complexes of functional
magnetic nanoparticles 1 with proline derivatives 2 and 3
4.7.1. Catalytic magnetic nanoparticles 10
Nanoparticles 1 (f ¼ 0.14 mmol/g, 1.27 g, 0.141 mmol) were sus-
pended in ultrapure water (278 mL) to give a brown suspension
with sonication for 10 min. Proline derivative
2 (0.079 g,
4.8.2. With functional magnetic nanoparticles 10 and 11
0.282 mmol) was dissolved in methanol (14 mL) and was added to
the nanoparticles dispersion. The mixture was stirred at room
temperature for 48 h. After this period, the nanoparticles were
submitted to magnetic decantation, the supernatant was separated
and the particles washed with methanol (4 ꢂ 20 mL) to eliminate
the excess of proline derivative 2. Next, the functional nanoparticles
were dried under vacuum, first in the vacuum line and then over-
night at 40 ꢁC in a vacuum desiccator. In this manner, 1.18 g of
nanoparticles were recovered. According to the %N determined by
elemental analysis the functionalization of the nanoparticles was
f ¼ 0.083 mmol/g. Particle size: (n ¼ 32, nm): 9.08 (s ¼ 2.23). Found:
The nanoparticles supported catalyst 10 or 11 (8e10 mol%) was
suspended in water (216
sion were added the aldehyde 12 (0.2 mmol) and cyclohexanone 13
(104 L, 1.0 mmol, 5 eq). The reaction mixture was stirred at room
mL, 12 mmol, 60 eq). Then to the suspen-
m
temperature for 24e72 h. Upon reaction, water (2 mL) and MeOH
(3 mL) were subsequently added and the mixture was separated by
magnetic decantation of the nanoparticles. The magnetic nano-
particles were subsequently rinsed with water (3 mL) and the
aqueous layer together with the MeOH were extracted with ethyl
acetate (3 ꢂ 5 mL). The organic layer was dried with anhydrous
MgSO4, filtrated and concentrated under reduced pressure. The
crude product was purified by silica gel column chromatography
using 9:1 hexane-ethyl acetate as eluent. The nanoparticles were
then vortexed with additional water, decanted, dried and reused. To
exchange the adsorbed catalyst, the magnetic nanoparticles were
vortexed in THF (3ꢂ 3 mL), magnetically decanted, rinsed with
MeOH, water, and dried.
C, 6.34; H, 1.10; N, 0.58; S, 0.12. FT-IR (KBr):
n 3425, 2956, 2923,
2853, 1734, 1636, 1457, 1080, 1032, 582 cmꢀ1. TGA: (30e1000 ꢁC,
10 ꢁC/min, under N2; for a 4.0470 mg sample, % weight loss):
14.3850 (left limit: 111.05 ꢁC, right limit: 477.63), 6.6399 (left limit:
476.66 ꢁC, right limit: 795.08), 7.2616 (left limit: 795.08 ꢁC, right
limit: 923.61).
4.7.2. Solvent-induced disassembly of functional magnetite
nanoparticles 10
Acknowledgements
Nanoparticles 10 (0.30 g) were suspended in anhydrous THF and
stirred at room temperature for 2 h. After this period, the nano-
particles were submitted to magnetic decantation, the supernatant
was separated and the particles washed with anhydrous THF
(5 mL). The organic phases were combined and evaporated under
vacuum to afford amino acid 2 (7.4 mg, quantitative recovery) as an
oil. The recovered nanoparticles were identical to MNPs 1 by IR.
This work was funded by CERCA Programme-Generalitat de
ꢁ
Catalunya and MINECO-Ministerio de Innovacion, Ciencia y Uni-
versidades, Spain (grant CTQ2015-69136-R and Severo Ochoa
Excellence Accreditation 2014e2018 [SEV-2013-0319]). C.M. thanks
AGAUR-Agency for Management of Universities and Research
Grants, Generalitat de Catalunya for a predoctoral fellowship.
Appendix A. Supplementary data
4.7.3. Preparation of functional nanoparticles 11
Nanoparticles 1 (f ¼ 0.13 mmol/g, 1.02 g, 0.146 mmol) were sus-
pended in ultrapure water (100 mL) to give a brown suspension.
The suspension was sonicated for 10 min, to ensure the dispersion
of the nanoparticles. Proline derivative 3 (0.104 g, 0.277 mmol) was
dissolved in 160 mL water with sonication and heating and was
added to the nanoparticles dispersion. The mixture was stirred at
room temperature for 48 h. After this period the nanoparticles were
magnetically separated and washed with water (5 ꢂ 30 mL) to
eliminate the excess of proline derivative 3. The solid material was
dried under vacuum, first in the vacuum line and then overnight at
40 ꢁC in a vacuum desiccator. In this manner 0.941 g of nano-
particles were recovered as a brown solid. According to the %N
determined by elemental analysis the functionalization of the
nanoparticles was f ¼ 0.094 mmol/g. Particle size: (n ¼ 101, nm):
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2019.130592.
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