As can be seen from Table 19, other nitrogen-containing heterocycles like
pyrrole, pyrazole, indole, and piperidine gave the corresponding N-arylated products
with 1-chloro-4-nitrobenzene and 1-fluoro-4-nitrobenzene in excellent yields.
Lower reaction rates but comparable yields were observed with resin supported 1
compared to its homogeneous counterpart. The resin supported 1 can be recovered
by simple filtration and reused for three cycles with consistent activity.
14 Silica Immobilized Copper Complexes
Silica modified with different functionalities such as -NH 2 , -SH, diamines, and
amino acids is reported to be a good support for various metals like Pd, Cu, Sc, Ru,
Pt, and V for different organic transformations [52]. Recently, Likhar et al. [52]
reported the N-arylation of N(H)-heterocycles and benzylamines with aryl halides
and arylboronic acids using silica immobilized copper complexes (Scheme 12).
The FTIR spectrum of chemically modified silica (imine) shows a peak due to
the C¼N bond around 1,640 cm
À1 , which on complexation with copper shifts to a
lower value. The lowering in frequencies of the C¼N peak is indicative of the
formation of the metal–ligand bond. The difference in the values of the C¼N
stretching band before and after complexation for all the catalysts is shown in
Table 20.
XPS analysis of the catalysts shows that in catalysts cat1-cat5, two intense peaks
appear at 933–935 eV and 952–954 eV. These peaks are attributable to the Cu 2p 3/2
and Cu 2p 1/2 levels of Cu
0-δ+ species, which may be due to the formation of the
mono/bi/multidentate copper complexes with nitrogen ligands [53, 54]. A
simplified catalyst structure is shown in Scheme 12. The copper content of the
catalysts were estimated by using ICP-AES, and the results are shown in Table 20.
To find the best catalyst for the N-arylation reaction, all the catalysts (5 mol%
Cu) were screened for the reaction of iodobenzene with imidazole using Cs 2 CO 3 at
100
C under nitrogen atmosphere in toluene, and the results are summarized in
Table 21. From Table 21, it can be seen that the catalysts derived from pyridine-2carboxaldehyde (Cat2) and 2-bipyridyl ketone (Cat5) were equally good; Cat3
showed moderate activities whereas Cat1 and Cat4 gave poor yields. Interestingly,
when neat Cu(OAc) 2 , was used there was almost no reaction (Table 21, entry 6).
To explore the scope and limitations of the current catalyst (Cat2), several
haloarenes were used for the arylation of the imidazole under the optimized
conditions, and the results are summarized in Table 22. It was observed that
iodoarenes with an electron-withdrawing group (entries 3 and 4) reacted at a faster
rate than iodoarenes with an electron-donating group (entry 2). Decreasing the
amount of catalyst from 5 mol% to 2 mol% gave good yields only after 24 h
(entry1). Interestingly, heterogeneous Cat2 afforded a comparable yield in shorter
duration when compared with the Buchwalds CuOTf-1,10-phenanthroline system
(entry 3) [55]. To expand the scope of the catalyst, a variety of other nitrogencontaining heterocycles such as benzimidazole, pyrrole, and pyrazole were successfully coupled with iodoarenes to give the corresponding N-arylated products in
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M.L. Kantam et al.
pyrrole, pyrazole, indole, and piperidine gave the corresponding N-arylated products
with 1-chloro-4-nitrobenzene and 1-fluoro-4-nitrobenzene in excellent yields.
Lower reaction rates but comparable yields were observed with resin supported 1
compared to its homogeneous counterpart. The resin supported 1 can be recovered
by simple filtration and reused for three cycles with consistent activity.
14 Silica Immobilized Copper Complexes
Silica modified with different functionalities such as -NH 2 , -SH, diamines, and
amino acids is reported to be a good support for various metals like Pd, Cu, Sc, Ru,
Pt, and V for different organic transformations [52]. Recently, Likhar et al. [52]
reported the N-arylation of N(H)-heterocycles and benzylamines with aryl halides
and arylboronic acids using silica immobilized copper complexes (Scheme 12).
The FTIR spectrum of chemically modified silica (imine) shows a peak due to
the C¼N bond around 1,640 cm
À1 , which on complexation with copper shifts to a
lower value. The lowering in frequencies of the C¼N peak is indicative of the
formation of the metal–ligand bond. The difference in the values of the C¼N
stretching band before and after complexation for all the catalysts is shown in
Table 20.
XPS analysis of the catalysts shows that in catalysts cat1-cat5, two intense peaks
appear at 933–935 eV and 952–954 eV. These peaks are attributable to the Cu 2p 3/2
and Cu 2p 1/2 levels of Cu
0-δ+ species, which may be due to the formation of the
mono/bi/multidentate copper complexes with nitrogen ligands [53, 54]. A
simplified catalyst structure is shown in Scheme 12. The copper content of the
catalysts were estimated by using ICP-AES, and the results are shown in Table 20.
To find the best catalyst for the N-arylation reaction, all the catalysts (5 mol%
Cu) were screened for the reaction of iodobenzene with imidazole using Cs 2 CO 3 at
100
C under nitrogen atmosphere in toluene, and the results are summarized in
Table 21. From Table 21, it can be seen that the catalysts derived from pyridine-2carboxaldehyde (Cat2) and 2-bipyridyl ketone (Cat5) were equally good; Cat3
showed moderate activities whereas Cat1 and Cat4 gave poor yields. Interestingly,
when neat Cu(OAc) 2 , was used there was almost no reaction (Table 21, entry 6).
To explore the scope and limitations of the current catalyst (Cat2), several
haloarenes were used for the arylation of the imidazole under the optimized
conditions, and the results are summarized in Table 22. It was observed that
iodoarenes with an electron-withdrawing group (entries 3 and 4) reacted at a faster
rate than iodoarenes with an electron-donating group (entry 2). Decreasing the
amount of catalyst from 5 mol% to 2 mol% gave good yields only after 24 h
(entry1). Interestingly, heterogeneous Cat2 afforded a comparable yield in shorter
duration when compared with the Buchwalds CuOTf-1,10-phenanthroline system
(entry 3) [55]. To expand the scope of the catalyst, a variety of other nitrogencontaining heterocycles such as benzimidazole, pyrrole, and pyrazole were successfully coupled with iodoarenes to give the corresponding N-arylated products in
142
M.L. Kantam et al.