reaction of imidazole with iodobenzene after the first cycle and 3.5% after the
fourth cycle.
16 Polyaniline-Cu Catalyst
Polyaniline (PANI) is one of the most widely studied conducting polymers
for electronic and optical applications. It is also receiving considerable attention in
modern organic synthesis as a support and a promoter for metal-catalyzed organic
transformations due to its easy preparative protocol from inexpensive starting material,
high environmental stability, easy acid–base doping–dedoping, and redox properties
[62]. Recently, Choudary et al. have reported polyaniline-supported Os-, Sc-, In-, and
Pd-catalyzed organic transformations [63]. Later, we reported [64] the preparation,
characterization, and catalytic properties of polyaniline-supported cuprous iodide
(Scheme 13) in the N-arylation of N(H)-heterocycles and benzylamines with aryl
halides and arylboronic acids.
PANI-Cu was prepared by stirring CuI and polyaniline in acetonitrile for 24 h
followed by filtration. The prepared PANI-Cu was fully characterized using FTIR,
XPS, ICP-AES, SEM, and EDAX. The most important bands in the FTIR spectrum
of PANI are located at 1,584, 1,494, 1,376, 1,308, 1,163, and 830 cm
À1 . They are
attributed to the stretching vibrations of quinoid (υ C¼N + υ C¼C ), benzenoid (υ C¼C )
units of the polymer, deformations of the C–N bond, stretching vibrations of
the C–N bond, in-plane deformations of CH bonds present in the aromatic rings
of the undoped polymer, and the out-of-plane deformations of CH bonds in the
1,4-substituted aromatic ring, respectively [65]. Upon incorporation of CuI into
PANI, no appreciable shifts in the quinoid or benzenoid ring bands positions have
been observed. XPS analysis of PANI-Cu showed a Cu 2p 3/2 line at 932.4 eV and
Cu 2p 1/2 line at 952 eV, which confirmed the oxidation state of copper in PANI-Cu
to be +I [22]. The PANI catalyst was analyzed by XPS for the N 1s, and it showed
three types of N, namely, -N (398 eV, proportion 45%), -NH
À (399.4 eV, proportion 45%), and -N
+ (402 eV, proportion 10%). Similarly, PANI-Cu showed the
presence of -N (397.7 eV, proportion 13%), -NH
À (400 eV, proportion 65%), and
-N
+ (401.9 eV, proportion 22%). The decrease of imine nitrogen indicates that more
of the copper was bound with the PANI via imine nitrogen than the amine nitrogen.
To explore the scope and limitations of the current catalytic protocol for
N-arylation, several haloarenes and nitrogen-containing heterocycles were allowed
to react under the optimized conditions, and the results are summarized in Table 24.
It was observed that iodoarenes with electron-withdrawing groups (Table 24,
entries 4 and 5) reacted at a faster rate than iodoarenes with electron-donating
groups (Table 24, entries 2 and 3). Sterically hindered 2-iodotoluene took a longer
time to afford a good yield (Table 24, entry 6). Benzylamine with electron-donating
moieties was found to be more active than the simple benzylamine (Table 24,
entries 11 and 12). When dibenzylamine was employed, no product formation was
observed (Table 24, entry 13). This is may be due to steric hindrance of the
Recent Developments in Recyclable Copper Catalyst Systems for C–N Bond. . .
147
fourth cycle.
16 Polyaniline-Cu Catalyst
Polyaniline (PANI) is one of the most widely studied conducting polymers
for electronic and optical applications. It is also receiving considerable attention in
modern organic synthesis as a support and a promoter for metal-catalyzed organic
transformations due to its easy preparative protocol from inexpensive starting material,
high environmental stability, easy acid–base doping–dedoping, and redox properties
[62]. Recently, Choudary et al. have reported polyaniline-supported Os-, Sc-, In-, and
Pd-catalyzed organic transformations [63]. Later, we reported [64] the preparation,
characterization, and catalytic properties of polyaniline-supported cuprous iodide
(Scheme 13) in the N-arylation of N(H)-heterocycles and benzylamines with aryl
halides and arylboronic acids.
PANI-Cu was prepared by stirring CuI and polyaniline in acetonitrile for 24 h
followed by filtration. The prepared PANI-Cu was fully characterized using FTIR,
XPS, ICP-AES, SEM, and EDAX. The most important bands in the FTIR spectrum
of PANI are located at 1,584, 1,494, 1,376, 1,308, 1,163, and 830 cm
À1 . They are
attributed to the stretching vibrations of quinoid (υ C¼N + υ C¼C ), benzenoid (υ C¼C )
units of the polymer, deformations of the C–N bond, stretching vibrations of
the C–N bond, in-plane deformations of CH bonds present in the aromatic rings
of the undoped polymer, and the out-of-plane deformations of CH bonds in the
1,4-substituted aromatic ring, respectively [65]. Upon incorporation of CuI into
PANI, no appreciable shifts in the quinoid or benzenoid ring bands positions have
been observed. XPS analysis of PANI-Cu showed a Cu 2p 3/2 line at 932.4 eV and
Cu 2p 1/2 line at 952 eV, which confirmed the oxidation state of copper in PANI-Cu
to be +I [22]. The PANI catalyst was analyzed by XPS for the N 1s, and it showed
three types of N, namely, -N (398 eV, proportion 45%), -NH
À (399.4 eV, proportion 45%), and -N
+ (402 eV, proportion 10%). Similarly, PANI-Cu showed the
presence of -N (397.7 eV, proportion 13%), -NH
À (400 eV, proportion 65%), and
-N
+ (401.9 eV, proportion 22%). The decrease of imine nitrogen indicates that more
of the copper was bound with the PANI via imine nitrogen than the amine nitrogen.
To explore the scope and limitations of the current catalytic protocol for
N-arylation, several haloarenes and nitrogen-containing heterocycles were allowed
to react under the optimized conditions, and the results are summarized in Table 24.
It was observed that iodoarenes with electron-withdrawing groups (Table 24,
entries 4 and 5) reacted at a faster rate than iodoarenes with electron-donating
groups (Table 24, entries 2 and 3). Sterically hindered 2-iodotoluene took a longer
time to afford a good yield (Table 24, entry 6). Benzylamine with electron-donating
moieties was found to be more active than the simple benzylamine (Table 24,
entries 11 and 12). When dibenzylamine was employed, no product formation was
observed (Table 24, entry 13). This is may be due to steric hindrance of the
Recent Developments in Recyclable Copper Catalyst Systems for C–N Bond. . .
147