Selectivity Studies Towards the Synthesis of Novel Biaryl Ureas by (Hetero)Nanocatalysis: Size Control and Support Effects

Article abstract of DOI:10.1002/cctc.201500889

A series of novel biaryl ureas containing different structural patterns have been prepared in good yields in the presence of palladium nanoparticles (PdNPs) with narrow particle size distribution. In particular, α, β-, and γ-hydroxypropylated cyclodextrins were used as reductants/stabilizers under different Pd-to-cyclodextrin ratios to tune the particle size and exploit the surface/cavity effects in the Suzuki-Miyaura reaction. Catalyst recovery in this process was pursued as well. Owing to its excellent performance, ceria was explored as a support for these PdNPs. The heterogeneous catalyst was characterized and investigated in the reaction of N-(4-iodo-aryl),N′-alkyl urea and phenylboronic acid. Our preliminary results revealed a high activity of the catalyst at 0.5 mol % Pd⁰ during three consecutive cycles. It is suggested that the specific interface between Pd⁰ and the high-surface-area ceria has a role in the catalytic performance.

Full text of DOI:10.1002/cctc.201500889

DOI: 10.1002/cctc.201500889
Full Papers
Selectivity Studies Towards the Synthesis of Novel Biaryl
Ureas by (Hetero)Nanocatalysis: Size Control and Support
Effects
Jaqueline D. Senra,[a] Gil M. Viana,[b] Luiz Fernando B. Malta,*[c] Alessandro B. C. Simas,[d] and
Lucia C. S. Aguiar[c]
A series of novel biaryl ureas containing different structural
patterns have been prepared in good yields in the presence of
palladium nanoparticles (PdNPs) with narrow particle size dis-
tribution. In particular, a, b-, and g-hydroxypropylated cyclo-
dextrins were used as reductants/stabilizers under different Pd-
to-cyclodextrin ratios to tune the particle size and exploit the
surface/cavity effects in the Suzuki–Miyaura reaction. Catalyst
recovery in this process was pursued as well. Owing to its ex-
cellent performance, ceria was explored as a support for these
PdNPs. The heterogeneous catalyst was characterized and in-
vestigated in the reaction of N-(4-iodo-aryl),N’-alkyl urea and
phenylboronic acid. Our preliminary results revealed a high ac-
tivity of the catalyst at 0.5 mol% Pd0 during three consecutive
cycles. It is suggested that the specific interface between Pd0
and the high-surface-area ceria has a role in the catalytic per-
formance.
Introduction
The ureido scaffold has a ubiquitous presence in several funda-
mental biological processes.[1] For instance, the urea-type back-
bone is related to HIV-1 protease inhibition,[2] antitumor,[3] anti-
trypanossomal,[4] and antituberculosis activities.[5] This structur-
al unit and some related systems (e.g., thioureas, glycoluril-
based ones) feature different properties, such as polarity, rigidi-
ty, and hydrogen-bonding capacity, which play a central role in
molecular recognition, making these systems well suited to
folding and self-assembly.[6] At the interface between biology,
organic synthesis, and materials chemistry, urea units have also
inspired the design of protein-like structures[7] (e.g., foldamers),
as well as the fabrication of nanostructures with potential ap-
plications in bionanotechnology.[8] In addition, nature has in-
corporated ureas into a multitude of biologically active natural
products,[9] thus further solidifying the widespread importance
of this functional group.
From a medicinal chemistry perspective, representative
structure–activity relationships (SAR) studies have pointed to
the fact that hydrophobic substitution on the CÀN bonds of
ureas can improve the robustness of interactions as a result of
the stabilization of some conformations, giving rise to collec-
tively stronger non-covalent forces.[10] N-Aryl ureas are by far
the most investigated as they provide a means to p-stacking
interactions along with hydrophobic ones depending on their
backbones. For example, a number of aryl ureas display signifi-
cant potential to inhibit vascular endothelial growth factor re-
ceptor-2, which is related to angiogenesis suppression.[11]
Moreover, some recent SAR studies suggested that the intro-
duction of the biphenyl moiety could improve the antitumor[12]
as well as anti-obesity[13] potencies of urea derivatives. Other
N-biaryl urea systems are of high interest in the context of
host–guest chemistry, especially in the dimeric form.[6b]
N-Biaryl ureas are most commonly prepared by palladium-
catalyzed carbon–carbon bond-forming cross-coupling reac-
tions[12,14] or by metal-catalyzed N-arylations,[14b–c] which gener-
ally provide high reaction yields. To minimize the relatively
high scavenging ability of palladium,[15a,b] most of these proce-
dures rely on prior N-protection along with the use of homo-
geneous catalysts based on bulky electron-rich commercial li-
gands (especially phosphine) to allow in situ generation of the
active species. Despite the high activity given by this ap-
proach, these palladium-based catalysts suffer from high costs
and difficulties of recovery, which also limit the possibilities for
multistep reactions. The use of cleaner and inherently safer al-
ternatives is promising owing to the attractive emphasis on
sustainable chemistry. In this respect, ligand-free approaches,
especially by using metal nanoparticles, are more cost-effective
and may enable recycling of the catalyst system more effec-
tively compared with the traditional methodologies.[16] In reac-
[a] Prof. J. D. Senra
Chemistry Institute
Universidade do Estdo do Rio de Janeiro
Rio de Janeiro, RJ 20550-900 (Brazil)
[b] Dr. G. M. Viana
Faculty of Pharmacy
Universidade Federal do Rio de Janeiro
Rio de Janeiro, RJ, 21941-614 (Brazil)
[c] Prof. L. F. B. Malta, Prof. L. C. S. Aguiar
Chemistry Institute
Universidade Federal do Rio de Janeiro
Rio de Janeiro, RJ, 21941-903 (Brazil)
E-mail: lfbmalta@iq.ufrj.br
[d] Prof. A. B. C. Simas
Walter Mors Institute of Research on Natural Products
Universidade Federal do Rio de Janeiro
Rio de Janeiro, RJ, 21941-614 (Brazil)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500889.
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tions of sensitive compounds, these metal catalysts
can also be tailored for optimal performance in view
of the effects of catalyst nanoparticle shape and size
on activity. Owing to environmental concerns, differ-
ent approaches have been used for the synthesis of
metal nanoparticles, including their phytosynthesis as
well as the use of naturally derived reducing/capping
agents.[17] Cyclodextrins (CDs), glyconanocavities ob-
tained by biotechnological routes, often play a key
role in these processes,[18] especially in the scope of
aqueous-phase nanocatalysis.[18c–g]
Table 1. Survey of the reaction conditions.[a]
Entry
Catalyst/CD type
Conditions
Yield
[%][e]
(Pd loading [mol%])
1
2
3
4
5
6
7
8
PdNPs[b]/a-HPCD (0.5)
PdNPs[c]/b-HPCD (0.5)
PdNPs[d]/g-HPCD (0.5)
PdNPs[b]/a-HPCD (0.5)
PdNPs[c]/b-HPCD (0.5)
PdNPs[c]/b-HPCD (2)
PdNPs[c]/b-HPCD (2)
PdNPs[c]/b-HPCD (2)
PdNPs[c]/b-HPCD (2)
PdNPs[c]/b-HPCD (2)
PdNPs[c]/b-HPCD (2)
PdNPs[c]/b-HPCD (2)
Na2PdCl4 (2)
H2O, K2CO3, RT
H2O, K2CO3, RT
H2O, K2CO3, RT
H2O, K2CO3, 95–1008C
H2O, K2CO3, 95–1008C
H2O, K2CO3, 95–1008C
H2O, Cs2CO3, 95–1008C
H2O, K3PO4, 95–1008C
H2O/EtOH (2:1), K2CO3, 95–1008C
H2O/EtOH (1:1), K2CO3, 95–1008C
H2O/CH3CN (1:1), K2CO3, 95–1008C
H2O/THF (1:1), K2CO3, 95–1008C
H2O/EtOH (2:1), K2CO3, 95–1008C
H2O/EtOH (1:1), K2CO3, 95–1008C
30
35
<10
48
52
55[f]
41[f]
45[f]
60
73
36
Recently, we reported an efficient and green reac-
tion protocol involving the synthesis of N-benzyl
thioureas from papaya seeds.[19a] Extending the work,
we engaged in the preparation of a novel set of N-
benzyl and N-aryl ureas by the treatment of the re-
spective thioureas with synthetically useful potassium
dichloroiodate.[19b,c] In this case, by tuning the reac-
tion conditions, sequential oxidation–iodination ena-
bled access to 4-iodo aryl ureas in moderate to high
yields. Based on the potential of N-biaryl ureas as
promising lead compounds for medicinal chemistry
and supramolecular chemistry, we herein report the
introduction of CD-protected palladium nanoparticles
(PdNPs) as catalysts in Suzuki–Miyaura reactions with
9
10
11
12
13
14
44
25
33
Na2PdCl4 (2)
[a] Reaction conditions: N-4-iodophenyl-N’-butyl urea (0.5 mmol), phenylboronic acid
(0.6 mmol), base (1.0 mmol), solvent (5 mL), 24 h. [b] Size dispersion range=13–
30 nm. [c] Size dispersion range=20–46 nm. [d] Size dispersion range=31–60 nm.
[e] Isolated yield (3a). [f] 8–13% biphenyl (4).
unprotected N-iodoaryl ureas. To exploit the influence of differ-
ent particle sizes in the performance/reaction selectivity, differ-
ent nanocatalysts were investigated to prepare novel the N-
biaryl-N’-alkyl ureas. Bearing in mind the advantages for cata-
lyst reusability, the PdNPs were heterogenized[20a] by using
a rare-earth metal oxide support (CeO2), and were evaluated
for reusability in three consecutive catalytic cycles.
propyl-CD relative to the Pd precursor (Table 1, entries 1,2)
generated the desired N-biaryl urea (3a) in low yields (30–
35%) at 258C. With these conditions, it was noticed that the
conversion of the substrate into the corresponding biaryl was
similar regardless of whether the a- or b-form was used. Inter-
estingly, the g-form failed to provide significant starting materi-
al conversion in pure water (Table 1, entry 3). Application of re-
fluxing water with PdNPs containing the a- or b-forms resulted
in moderate yields of 3a (Table 1, entries 4 and 5). The cou-
pling reaction with 2 mol% PdNPs provided only a slight im-
provement in the performance, whereas the use of different
bases was not rewarding and facilitated the formation of unde-
sired byproducts (Table 1, entries 6–8). The effect of water on
the cross-coupling was also studied and the results showed
that substitution by a water/ethanol mixture improved the
yield of 3a (Table 1, entries 9 and 10). However, the use of less
polar aprotic solvents did not provide good yields (Table 1, en-
tries 11 and 12), suggesting a solvent effect.[22] Notably, control
experiments showed that Na2PdCl4 alone afforded the expect-
ed product in low yields, even in a water/ethanol mixture
(Table 1, entries 13 and 14). Nevertheless, to prove the impor-
tance of the NPs stabilization, the assay with 4-fold Pd loading
(entry 14) was performed and shown to be still less active than
the one with PdNPs, even with the NPs in water (entry 2). The
particle size distribution with the 10-fold excess of CD is rela-
tively broad;[20b] this result suggests that the poor activation/
selectivity of PdNPs towards the unprotected aryl urea may be
a result of strong interactions from the functional domains
with the metal surface.
Results and Discussion
Our initial experiments were guided by previous investigations
concerning the effect of the CD/Pd ratio on the size of palladi-
um nanoparticles stabilized by hydroxypropylated cyclodex-
trins.[20b] We demonstrated that the CD concentration influen-
ces the formation of regular assemblies, which may direct
PdNPs growth towards uniform particles. Therefore, we were
committed to identifying the correct ratio to achieve the
utmost efficiency and economy in the implementation of this
method. In addition, it was not clear whether we would en-
counter different catalytic activity for the a-, b-, and g-forms of
hydroxypropylated CDs given the fact that their internal cavity
sizes are different and the number of a-d-glucopyranoside
units has been demonstrated to exhibit distinct NPs size con-
trol.[18] Thus, we judiciously screened all platforms, assuming
that their internal environments could influence the reaction
outcome.
Firstly, our studies were focused on the coupling of phenyl-
boronic acid with N-butyl-N’-(4-iodo)phenyl (1a) in water as
the reductant/stabilizing agent is hydrophilic. Three separate
initial sets of conditions were used based on the CD type.
From Table 1 it can be seen that the use of 0.5 mol% PdNPs
prepared with a lower excess (10-fold) of a- or b-2-hydroxy-
Recent elegant studies[18c,22] provide evidence that the sol-
vent and base are decisive for the in situ formation of some
Pd-based active species. In our case, however, we focused on
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using a higher CD/Pd molar ratio as this condition allowed us
previously to improve the size control of the PdNPs.[20b]
As the ability to catalyze the formation of biaryl ureas in
moderate yields was similar with Pd0 dispersions made from
either a/b-CD, both of them were examined to explore the
impact of catalyst nanoparticle size on the reactivity of the
urea substrate.
(Scheme 1). As expected, 4-substituted aryl substrates with
electron-donating groups demonstrated good reactivity
toward 1a, resulting in satisfactory yields. Increasing the steric
bulk on the boronic acid counterpart did not have any influ-
ence on the outcome of the cross-coupling reaction (products
3a2–3a4, 3b2, 3b4). The use of electron-poor aryl boronic
acids completely failed to provide any observable product
under these conditions. Notably, the addition of an excess of
base (4 equiv.) restored the catalytic cycle, thus providing the
corresponding products in moderate to good yields (products
3a3 and 3a5). This result supports the critical involvement of
base in the second step (transmetalation) and is evidence in
favor of the organoboron compound activation. Switching
from 4-substituted aryl boronic acid to a heteroaro-
matic core, the catalytic reactivity led to similar yields
(products 3a6–3b5).
The strategy was therefore adjusted by using PdNPs with
narrow size distributions synthesized under a 50-fold excess of
CD.[20b] Gratifyingly, the catalyst system composed of 2-hydrox-
ypropyl-b-cyclodextrin (b-HCPD)-stabilized PdNPs improved
the yield of 3a to 98% under the same conditions as described
(Table 2, entry 1). More satisfyingly, similar yields were obtained
Table 2. Studies with PdNPs with narrow size distributions.[a]
With these reaction conditions in hand and recog-
nizing the utility of ureas 3a7, 3a8, and 3b6 for
domino Suzuki/Heck reactions, we envisioned a con-
secutive
CÀC
bond-forming
transformation
(Scheme 1). Treatment of urea 1a with the vinyl bor-
onate counterpart followed by the addition of 4-io-
dotoluene led to the corresponding N-styrylbenzyl
ureas 5a and 5b in moderate to good yields with
the b-HPCD-stabilized PdNPs as the catalyst
(Scheme 2). However, when we attempted to employ
the styrylboronic acid, the expected product 6a was
only observed in a low yield (<10%). In fact, the
1H NMR spectra indicated recovery of the precursor,
3a7. With regard to the stereoselectivity, a coupling
constant of 13 Hz was observed for 5a and 5b, sug-
gesting the formation of the more favorable E isomer
(Scheme 1).
Entry
Catalyst/CD type
R
Conditions
Yield [%][d]
Product
1
2
3
4
5
6
7
PdNPs[b]/b-HPCD
PdNPs[b]/b-HPCD
PdNPs[c]/a-HPCD
PdNPs[c]/a-HPCD
PdNPs[b]/b-HPCD
PdNPs[b]/b-HPCD
PdNPs[b]/b-HPCD
À(CH2)3CH3
À(CH2)3CH3
À(CH2)3CH3
À(CH2)3CH3
ÀCH2Ph
95–1008C, 24 h
95–1008C, 6 h
95–1008C, 24 h
95–1008C, 6 h
95–1008C, 6 h
95–1008C, 6 h
508C, 24 h
98
95
90
81
87
3a
3a
3a
3a
3b
3c
3c
[c]
ÀCH(CH3)2
ÀCH(CH3)2
–
75
[a] Reaction conditions: N-4-iodophenyl-N’-butyl urea (0.5 mmol), phenylboronic acid
(0.6 mmol), K2CO3 (1.0 mmol), H2O/EtOH (1:1, 5 mL), PdNPs (2 mol%). [b] Size disper-
sion range=10–15 nm. [c] Size dispersion range=2–5 nm. [d] Isolated yield.
within 6 h (Table 2, entry 2). Notably, reaction with the a form
proceeded with comparable yields (Table 2, entries 3 and 4).
Substitution of the butyl by a benzyl group also allowed us
the access to the biaryl urea in high yield within 6 h (Table 2,
entry 5, 87%). In contrast, the presence of an isopropyl group
(Table 2, entry 6) led to sub-products from the cleavage of the
lateral chain at high temperatures (~1008C). However, explora-
tion of milder reaction conditions enabled the synthesis of the
corresponding N-biphenyl-N’-isopropyl urea (Table 2, entry 7,
75%).
We propose that the reactivity differences between the CD
forms are due to the inherent formation of inclusion com-
plexes, which can affect the kinetics of the oxidative addition/
transmetalation processes, according to the NP model.[20b] In
this case, the cavity with the strongest complexation capaci-
ty—the b form—seems to dictate the relative reactivity instead
of surface effects related to particle size distributions. When
considering a short chain such as isopropyl, it is conceivable
that the weaker association with the cavity can promote its
fast cleavage.
Scheme 1. Substrate scope. [a] N-4-iodophenyl-N’-butyl urea or N-4-iodo-
phenyl-N’-benzyl urea (0.5 mmol), arylboronic acid (0.6 mmol), K2CO3
(1.0 mmol), H2O/EtOH (1:1, 5 mL), PdNPs (b-HPCD, CD/Pd=50, 2 mol%Pd),
24 h.
Thus, to expand the repertoire of aryl ureas suitable for bio-
logical studies, we set out to test the generality of this catalytic
condition by employing a variety of boronic acids and esters
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antitumor compounds[23,24]—it was interesting to improve their
synthetic conditions in terms of catalysis (mostly, turnover
number and reusability). Considering the requirements envis-
aged for scale-up approaches, some issues might be solved by
using heterogenized catalysts. From this point of view, small-
sized palladium supported on carbonaceous[25] or inorganic
oxides[26] has attracted considerable attention as some of these
catalysts are able to promote selective transformations
through collaborative effects.[26a] Previous reports indicate that
the dispersion of colloidal metal species onto some supports,
in the presence of cyclodextrins as protective agents, can im-
prove the metallic dispersion and induce new selectivities.[25]
At the same time, pure crystalline cerium oxide (ceria) contains
sites that are able to perform several useful transformations.[26]
Surprisingly, only a handful of studies have focused on the
use of pure ceria as a catalyst[27a–b] or as a support for CÀC
bond-forming reactions.[27c–d] An alternative approach is the
preparation of CeO2 with an alkaline-earth dopant (e.g,. Ca-
doped ceria) as this reduces costs through use of much lower
amounts of the expensive rare-earth metal. Based on the cata-
lytic perspectives along with our interest in ceria-based materi-
als,[28] we investigated Ca-doped ceria as a support for PdNPs.
As a-HPCDs demonstrated similar catalytic behavior and were
more able to stabilize the PdNPs after work-up compared with
b-HPCDs, we decided to use them for the design of heterogen-
ized PdNPs on CeO2.
Scheme 2. Preliminary studies for PdNP-catalyzed one-pot cross-couplings.
In an effort to fulfill the demand for an ecologically and eco-
nomically advantageous approach for the preparation of novel
ureas by nanocatalysis, we initially evaluate the reusability of
the PdNP-containing aqueous phase by exploring the reaction
between urea 1a and phenylboronic acid. Thus, after work-up
from the first cycle, the aqueous phase revealed partial activity
(Scheme 3), indicating that the PdNPs retain some activity de-
The synthesis of the supported catalyst was carried out in
two stages: a) the preparation of doped ceria by means of
a co-precipitation method and b) wet impregnation with the
aqueous Pd/a-HPCD dispersion. The preparation route is de-
scribed in the Supporting Information. Through this method,
we performed the co-precipitation at pH 14 by using NaOH
1.0m with CAN (cerium(IV) ammonium nitrate) and calcium ni-
trate as the precursors (in a proper stoichiometry) in an at-
Scheme 3. Catalytic tests with recycled aqueous phases.
spite the slight agglomeration that was apparent on visual in-
spection. Under the same catalytic conditions, the NP disper-
sions containing a-HPCDs showed superior potential for recy-
cling (Scheme 3). In fact, the PdNPs mean size distribution
measured from AFM images indicates that the a-HPCD disper-
sion is more stable after work-up. However, it is often unclear
if the PdNPs are just a reservoir of monometallic active species
or are the real catalysts. By considering the first hypothesis,
leaching to the organic phase could be a possible mechanism.
Therefore, a tentative experiment involving adding iodinated
aryl urea 1a to the reaction medium containing the PdNPs
was performed, as aryl halides (iodides/bromides) can induce
the leaching process under appropriate conditions.[21] Analysis
of X-ray fluorescence spectroscopy results indicated no signifi-
cant Pd content was leached from the aqueous phase. On the
other hand, the notable decrease in the activity of the aqueous
phase with the simultaneous increase in particle agglomera-
tion can be directly explained through Ostwald ripening. This
fact is also in accordance with the findings recently reported
by our group concerning the partial uncapping of CDs from
metal surfaces induced by cavity complexation.[22]
tempt to get material with a nominal composition of
Ce0.90Ca0.10O1.90 (see the Supporting Information). X-ray fluores-
cence indicated that the doped ceria has the composition
Ce0.88Ca0.12O1.88. Elemental analysis (CHN) detected 1.84%
carbon in the doped ceria. We believe that this could be due
to the presence of carbonate on the ceria particles surface.
To gain structural information, X-ray diffraction analysis was
also used. The diffraction pattern is shown in Figure S1 (in the
Supporting Information). No reliable data refinement from the
indexing of the diffraction pattern was possible owing to the
high signal-to-noise ratio; hence, no cell parameter was calcu-
lated. However, by using the Scherrer formula (see details in
Supporting Information) we obtained an estimated value of
2.4 nm for the crystal size, indicating that it was nanoceria.
Additional confirmation of the doped nanoceria structure
was acquired by using Raman spectroscopy (Figure S2, Sup-
porting Information). For comparison, we also analyzed a non-
doped ceria produced by using the same method. The F2g
band at ~450 cmÀ1, assigned as the “breathing” vibration
mode of oxygen atoms around cerium in the CeO8 units, pres-
ents an asymmetric characteristic, which is indicative of nano-
materials.[29] Additionally, the second-order Raman mode at
~600 cmÀ1, indicative of oxygen vacancies (Vo), is also ob-
On the basis of the potential biological profile of these
biaryl ureas—by virtue of their structural resemblance to some
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served[30,31] for both samples. For the non-doped ceria, this
leads to the conclusion that Ce3+ is present. For the doped
ceria, it is apparent from the lower signal-to-noise ratio that
the intensity of the Raman scattering is decreased. These find-
ings definitively confirm the existence of non-stoichiometric
nanoceria[29,32] and Ca-doping.[33]
The synthesis of the supported PdNPs involved the initial
preparation of aqueous Pd0 dispersions followed by stirring in
the presence of doped ceria for 24 h at room temperature.
After centrifugation, the impregnated material was washed
with ethanol and finally dried at 708C for further characteriza-
tions.
We considered that all palladium content was adsorbed as,
after centrifugation, the aqueous dispersion became clear and
colorless, indicating the presence of less than 1% Pd (w/w).
The presence of the organic moieties in the material was dem-
onstrated by means of FTIR spectroscopy (Figure S3, Support-
ing Information). In this case, the interval 1250–1000 cmÀ1 indi-
cated the presence of physically adsorbed cyclodextrin, the
bands of which can be assigned as nCÀOÀC (1154 cmÀ1), nCÀC/CÀO
(1076 cmÀ1), and nCÀO (1033 cmÀ1). Additional characterization
by SEM energy-dispersive X-ray spectroscopy (EDS) showed
that calcium and palladium are homogeneously distributed
over the ceria surface (Figure 1). Finally, the atomic force mi-
croscopy (AFM) phase image demonstrates the presence of
well-oriented CD domains forming the solid network (Figure 2,
dark contrast). However, the noble-metal NP phase is not clear
along the CeO2 surface. This could be, in part, because of the
Figure 2. AFM image of PdNPs/CeO2 (non-contact image, phase).
possible contaminant layer (e.g., water), which can interfere
with oscillation.
With the heterogeneous system in hand, we proceeded to
test the catalytic efficiency starting with 0.5 mol% Pd0 (w/w)
by using urea 1a as the substrate (Table 3, entry 2). We were
delighted to find that this catalytic system provided compound
3a in a markedly high yield (97%). In this case, the calculated
turnover number (TON) was 194. Working with an even lower
Pd loading resulted in a moderate conversion to the desired
cross-coupling product (Table 3, entry 1).
Encouraged by these results, we next investigated whether
this catalyst had recycling potential. If so, it could shed a light
on whether the supported PdNPs act as a reservoir or as the
Figure 1. (a) SEM micrography of the supported catalyst. (b–e) EDS spectra showing Ce (most prominent), Ca, and Pd sites.
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abled the preparation of a series of N-biaryl-N’-alkyl ureas,
novel candidate compounds with potential uses in medicinal
chemistry. Considering the advantages of catalytic reusability
along with our interest in future scalable reactions, preliminary
catalytic tests involving supported PdNPs on CeO2 as a hetero-
geneous catalyst provided a reliable synthesis of N-biaryl-N’-
butyl urea over three consecutive catalytic cycles. The biologi-
cal potential of these novel biaryl scaffolds as well as the role
of ceria and its possible surface effects in the cross-coupling
reaction are currently under investigation in our group.
Table 3. PdNPs/CeO2-promoted Suzuki–Miyaura cross-coupling between
urea 1a and phenylboronic acid.[a]
Entry
Pd loading [%]
Yield 3a [%]
1
2
3
0.1
0.5
–
62[b]
97[b]
<1[c]
[a] N-4-iodophenyl-N’-butyl urea (0.5 mmol), phenylboronic acid
(0.6 mmol), K2CO3 (1.0 mmol), H2O/EtOH (1:1, 5 mL), PdNPs/CeO2, 908C,
24 h. [b] Isolated yield. [c] Estimated by H NMR.
Experimental Section
1
General remarks
All reactions were performed under air using conventional reflux
glassware. All chemicals were purchased at the highest commercial
grade and used as received. Analytical thin-layer chromatography
(TLC) was performed by using E. Merck silica gel 60 F254 pre-
coated glass plates (0.25 mm thickness). Flash column chromatog-
raphy was carried out by using E. Merck silica gel BW-300 (200–
400 mesh).
actual catalytically active species. Catalyst reusability was stud-
ied by recycling the material after each run. The yields of the
recycled dispersion in the reaction between N-(4-iodo)phenyl-
N’-butyl urea (1a) and phenylboronic acid in water, after 24 h,
are presented in Figure 3. Gratifyingly, the heterogenized cata-
lyst could be recycled without significant loss in activity over
three consecutive runs.
1H and 13C NMR spectra were recorded on a Varian VNMRSYS-500
or a Bruker DPX-200 spectrometer at 258C. Chemical shift values
are reported in d (ppm) relative to tetramethylsilane with reference
to internal residual solvent [1H NMR: (CH3)2SO (2.5); 13C NMR:
(CD3)2SO (40.7–38.2)]. Coupling constants (J) are reported in hertz
(Hz). Multiplicities of signals are designated as follows: s=singlet;
d=doublet; t=triplet; m=multiplet; br=broad. High-resolution
mass spectra (HRMS) were obtained on a Bruker microTOF II mass
spectrometer using ESI as source type.
X-ray fluorescence (XRF) analysis was performed with a Rigaku
spectrometer model RIX3100, using a Rh tube with a power of
4 kW.
Elemental analysis (CHN) was carried out with a PerkinElmer 2400
CHN apparatus, using a PerkinElmer AD-4 Autobalance.
Figure 3. Catalytic activity of PdNPs/CeO2 in three consecutive runs.
FTIR spectra were acquired with a Nicolet Magna-IR 760 apparatus.
Samples were analyzed in KBr pellets in the 4000–400 cmÀ1 inter-
val. The number of scans was 16, with resolution of 4 cmÀ1
.
A control experiment with purely Ca-doped ceria was also
performed to investigate the influence of the Cex+ (2ꢀxꢀ4)
sites on the catalysis. Interestingly, in the absence of palladium,
the support proved to be inert under the reaction conditions.
However, the possibility of synergic effects between both
metals towards the cross-coupling process is strong, even
though the specific role of cerium oxide remains unclear at
this stage. A mechanistic hypothesis that electron-rich Pd do-
mains may help to reduce the cationic Ce3+/Ce4+ species in
the support (facilitating greater surface interaction with the
Csp2 I sites) might be tentatively put forward.
X-ray diffraction patterns were obtained with a Rigaku diffractome-
ter, model Ultima IV, with a Cu X-ray tube (l-2q goniometer, a Ni
kb filter, 40 kV voltage, and 20 mA current). Samples were analyzed
in the 58<2q<808 interval, with 0.058 steps and 1 second per
step.
Scanning electron microscopy images were acquired with a JEOL
JSM 6460-LV apparatus, operating in the 10–20 kV range coupled
to an X-ray energy-dispersive spectrometer.
Raman spectra were collected on a Horiba Raman Microscope
HR800 with an Olympus microscope and a CCD detector, the excit-
ing radiation being a HeNe 20 mW laser at 632.8 nm
The atomic force microscope used was the Nanowizard model (JPK
Instruments, USA). The equipment operation was performed using
NCR-10 (AIBS) or CSC11 (AIBS) tips manufactured by Nanoworld.
The height and phase contrast measurement distributions for the
nanoparticles were determined by tapping-mode atomic force mi-
croscopy (AFM) under ambient conditions in air. To obtain AFM
images of the dispersed PdNPs, the aqueous dispersions were di-
luted 20-fold in acetone before deposition of three drops (10 mL)
Conclusions
We have demonstrated that unprotected 4-iodo aryl ureas can
be reactive in Suzuki–Miyaura cross-couplings when using
PdNPs stabilized by CDs with controlled size. In addition, con-
secutive Suzuki–Miyaura/Heck reactions were also explored to
test the NPs ability for domino reactions. These procedures en-
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onto glass microscope slides and allowed to dry freely in air. The
analysis of PdNPs/CeO2 was done by deposition of a solid thin
layer on a plate mounting tape contained in a glass slide. Before
analyses, the glass microscope slides were cleaned with nitric acid/
hydrochloric acid (1:3) for 30 min.
PdNPs/CeO2 synthesis: a-HPCD (1.18 g, 1.0 mmol), Na2PdCl4 aque-
ous solution (4.0 mL of 5 mmolLÀ1, 20 mmolPd), distilled water
(6 mL) and ceria (250 mg) were added in a round-bottom flask. The
mixture was stirred at room temperature for 96 h. After this time,
the mixture was centrifuged twice and the solid phase was dried
at room temperature. Homogenization was carried out in an agate
mortar.
Representative experimental procedure for the synthesis of
3a by PdNPs
Representative experimental procedure for the synthesis of
3a by PdNPs/CeO2
Pd nanoparticles were prepared by using a slightly modified1 ver-
sion of the published procedure.[20b] b-HPCD (0.345 g, 0.25 mmol),
Na2PdCl4 aqueous solution (1.0 mL of 5 mmolLÀ1, 5 mmolPd), and
distilled water (1.5 mL) were added in a round-bottom flask
equipped with a reflux condenser. The mixture was stirred and
heated at 908C (bath temperature) for 5 min. Then, urea 1a
(79.5 mg, 0.25 mmol), phenylboronic acid (30.4 mg, 0.25 mmol), po-
tassium carbonate (69.1 mg, 0.5 mmol), and ethanol (2.5 mL) were
added to the resultant dispersion. After stirring/heating for 6 h, the
mixture was cooled and the solution was filtered through a pad of
Celite and then extracted with EtOAc/CH2Cl2. The organic layer was
washed with brine, dried (Na2SO4), filtered, and concentrated
under reduced pressure. Purification of the crude product by flash
column chromatography (silica gel, 5–15% ethyl acetate/hexane)
gave N-biaryl-N’-butyl urea 3a (63.5 mg, 95%) as a light-yellow
solid.
Urea 1a (79.5 mg, 0.25 mmol), phenylboronic acid (30.4 mg,
0.25 mmol), potassium carbonate (69.1 mg, 0.5 mmol), and the het-
erogeneous catalyst PdNPs/CeO2 (6.6 mg, 1.25 mmolPd) were
added to a solution of H2O/EtOH (1:1, 5.0 mL). The mixture was
stirred and heated at 908C for 24 h. After this time, the reaction
mixture was filtered and the catalyst was washed with water and
dried. The filtrate was subjected to the work-up described above.
Purification of the crude product by flash column chromatography
(silica gel, 5–15% ethyl acetate/hexane) gave N-biaryl-N’-butyl urea
3a (64.8 mg, 97%) as a light-yellow solid.
Acknowledgements
We are grateful to the Instituto de Pesquisas de Produtos Natur-
ais Walter Mors (IPPN, UFRJ) for analytical support and to Prof.
Renata A. Sim¼o (COPPE, UFRJ) for her help with the AFM meas-
urements. We also acknowledge financial support from Fundażo
de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ),
Conselho Nacional de Desenvolvimento Científico e Tecnológico
(CNPq), and Coordenażo de AperfeiÅoamento de Pessoal de
Nível Superior (CAPES).
Representative one-pot experimental procedure for the syn-
thesis of 5b by PdNPs
b-HPCD (0.345 g, 0.25 mmol), Na2PdCl4 aqueous solution (1.0 mL of
5 mmolLÀ1, 5 mmolPd), and distilled water (1.5 mL) were added in
a round-bottom flask equipped with a reflux condenser. The mix-
ture was stirred and heated at 908C for 5 min. Then, urea 1a
(79.5 mg, 0.25 mmol), vinylboronic N-methyliminodiacetic acid
(MIDA) ester (45.7 mg, 0.25 mmol), potassium carbonate (69.1 mg,
0.5 mmol), and ethanol (2.5 mL) were added to the resulting dis-
persion. After stirring/heating for 24 h, 4-iodotoluene (54.5 mg,
0.25 mmol) was added to the mixture and the reaction was main-
tained at the same conditions for an additional 24 h. The solution
was cooled, filtered through a pad of Celite, and extracted with
EtOAc/CH2Cl2. The organic layer was washed with brine, dried
(Na2SO4), filtered, and concentrated under reduced pressure. Purifi-
cation of the crude product by flash column chromatography
(silica gel, 10–15% ethyl acetate/hexane) gave N-[(4-methyl)-styl-
benyl]-N’-benzyl urea 5b (59 mg, 69%) as a light-yellow solid.
Keywords: ceria · cyclodextrins · palladium nanoparticles ·
Suzuki–Miyaura reaction · ureas
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