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M. Nasrollahzadeh et al. / Tetrahedron Letters 55 (2014) 2813–2817
Table 2 (continued)
Entry
Arylboronic acid
Amine
Product
Yielda (%)
92
NH2
NH2
H
N
F3C
18
B(OH)2
F3C
H
N
19
78
B(OH)2
N
N
NH
N
N
20
21
74
73
N
B(OH)2
B(OH)2
NH
N
N
N
a
Yields after work-up.
The yields from four subsequent runs using the same recovered catalyst.
b
a fast and efficient reaction (Table 1, entry 6). No desired product
was observed in the absence of air (under a nitrogen atmosphere;
Table 1, entry 16), which indicated that an oxidative process was
involved in the formation of the product.
In summary, we have successfully synthesized Fe@Pd nano-
wires through a facile procedure. The catalyst exhibited good activ-
ities in ligand-free cross coupling reactions of arylboronic acids
with amines in aqueous medium. Furthermore the Fe@Pd nano-
wires could be easily separated by a magnet, and could be recycled
several times without loss of the catalytic activity. Further investi-
gations on the application of this system on other catalytically syn-
thetic reactions are in progress.
After determining the optimized conditions, we next investi-
gated the scope of the magnetic catalyst for the CAN cross-cou-
pling reaction of a diverse range of arylboronic acids containing
both electron-releasing and electron-withdrawing groups with
various amines (Table 2). This newly developed Fe@Pd-catalyzed
amination protocol was also applied to aliphatic and heterocyclic
amines (Table 2, entries 10–13, 18 and 19–21). In all cases, this
protocol afforded the desired products in good to excellent yields.
As shown in Table 2, reactions of ortho-substituted arylboronic
acids with amines afforded good yields (entries 4–6). An ortho-sub-
stituent on aniline could also promote this reaction (entry 6). 1-
Naphthylboronic acid gave the corresponding adduct in good yield
(entry 14).
Next, we studied the reusability of this heterogeneous Fe@Pd
catalyst in CAN coupling reactions (Table 2). After completion of
the reaction, the catalyst was recovered by the application of an
external magnet, then washed with ethyl acetate, and dried in a
hot air oven at 100 °C for 2 h. The recovered catalyst was reused
under similar conditions for the next run, and the catalytic behav-
ior of the Fe@Pd nanowires was found to be unaltered over five
consecutive cycles (Table 2, entry 3).
Preparation of Fe@Pd nanowires
Fe nanowires were prepared using a system consisting of two
main parts: a high current DC power supply and a reactor including
an anode, cathode, and a micrometer, which moves the anode in
contact with the cathode. In this method, an 8 V DC voltage and
5 A current are applied between two metallic iron electrodes; it
was found that the voltage dropped to 5 V during arcing but the
current remained constant. Both the anode and cathode were made
of Fe, wire-shaped, 2 mm in diameter and of 99.99% purity. Ini-
tially, we brought the two electrodes into contact, leading to a
small contact cross section and thus to a high current density. As
more Fe was ablated from the anode, the plasma expands, pushing
the liquid away, and a gaseous bubble forms. Melted species can
react with the plasma and then condense into the liquid. In order
to extract the dispersed wires, the solution was evaporated at a
pressure of 10À1 Torr and centrifuged several times and then dis-
persed on a glass substrate. Deposition of Pd on the surface of Fe
nanowires was accomplished via a simple drop-drying process by
dropping PdCl2 solution onto Fe nanowire films and drying them
at room temperature. This solution was prepared by ultrasonically
solving 0.02 g of PdCl2 powder (5 N), 99.9 mL DI water and 0.1 mL
HCl. After Pd deposition, samples were washed with DI water sev-
eral times and then dried in air.
A general catalytic cycle for this reaction is presented in
Scheme 2. This reaction allows aryl carbonAnitrogen bond forma-
tion via the oxidative coupling of arylboronic acids with ANH con-
taining compounds in air.
Ar'B(OH)2
Pd(II)
ArHN
ArNH2
transmetallation
B(OH)3
Preparation of diphenylamine; typical procedure
Pd(II)
H2O
To a stirred solution of phenylboronic acid (1.0 mmol), aniline
(1.0 mmol), and K2CO3 (2.0 mmol) in deionized H2O (10 mL) at
room temperature was added an aqueous suspension of Fe@Pd
nanowires (3.0 mol % in 3 mL of H2O). The mixture was stirred at
room temperature for 5 h. After completion of the reaction (as
monitored by TLC), 2 M HCl was added and the catalyst was sepa-
rated by applying an external magnet. The catalyst was washed
with EtOAc. The mixture was extracted with EtOAc (2 Â 20 mL),
dried, and concentrated. The residue was subjected to gel perme-
ation chromatography to afford pure product. Mp 51–53 °C (lit.9a
ArHN
Ar'
Pd
(II)
oxidation
Pd(0)
O2
reductive
elimination
ArNHAr'
Scheme 2. Proposed mechanism for the Pd-catalyzed oxidative amination of
arylboronic acids.