Cs 2 CO 3 . Other imidazoles such as benzimidazole, indole, pyrazole, and pyrrole
were successfully coupled with iodobenzene to give the corresponding N-arylated
products in good yields employing 10 mol% of Cu 2 O. The catalyst was reused for
four times with consistent activity for the coupling of iodobenzene with imidazole
at 120
C. In the first cycle 90% yield was obtained while in the fourth cycle 88%
yield was obtained.
8 Copper-Exchanged Fluorapatite
Choudary et al. have chosen [19] a weakly amphoteric apatite as support, since
various kinds of cations and anions can be readily introduced into their framework
due to their large ion exchange ability. Such exchanged apatites are already in use
for several organic transformations [20]. A schematic representation of copperexchanged fluorapatite (1) by incorporating the basic species F
À in apatite in situ by
co-precipitation and subsequent exchange with Cu (II) is shown (Scheme 6).
N-Arylation of imidazoles and other heterocycles with chloroarenes and
fluoroarenes to afford good to excellent yields of aryl heterocycles by using
CuFAP catalyst is described.
The method is general and amenable to the N-arylation of imidazole with a
wide range of chloro- and fluoroarenes using catalyst 1 (Table 5). As illustrated
in Table 5, chloroarenes (electron withdrawing), 2-chloropyridine, and
2-chloropyrimidine provided excellent yields in shorter reaction times than chlorobenzene and chloroarenes (electron donating) (entries 1–11). Cyano, nitro, and
trifluoromethyl groups are well tolerated (entries 1, 3, and 4). The N-arylation
results of deactivated chloroarenes using K 2 CO 3 as a base are quite impressive
over the unreactive system using nanocopper and Cs 2 CO 3 base [21]. Another
significant feature is that under similar conditions, the N-arylation of imidazole
with chlorobenzene afforded moderate yields, which could be further improved by
the addition of KO
t
Bu (entry 9). N-Arylation of heterocycles developed here using
the less expensive chloroarenes is more attractive than the methods using the
expensive bromo- and iodoarenes in terms of economic feasibility. The catalyst is
recycled four times with a slight decrease in activity (entry 1). Interestingly,
fluoroarenes composed of several o- or p-electron-withdrawing (EW) groups
(entries 12–15) are also coupled with imidazole to afford the corresponding
N-arylated products in excellent yields. Faster reactivity over the chloroarenes
(entries 1, 5, 13 and 14) and selective coupling involving C–F activation only in
chlorofluoroarene (entry 15) are reported.
As shown in Table 6, benzimidazole, pyrrole, pyrazole, and piperidine are also
coupled with 1-chloro-4-nitrobenzene and 1-fluoro-4-nitrobenzene to give the
corresponding N-arylated products in excellent yields.
To understand the mechanism of the N-arylation of imidazole, a series of
experiments were conducted. The reaction of catalyst (1) with imidazole gives a
deep blue Cu-imidazole complex (3), which is considered to be the first step of the
128
M.L. Kantam et al.
were successfully coupled with iodobenzene to give the corresponding N-arylated
products in good yields employing 10 mol% of Cu 2 O. The catalyst was reused for
four times with consistent activity for the coupling of iodobenzene with imidazole
at 120
C. In the first cycle 90% yield was obtained while in the fourth cycle 88%
yield was obtained.
8 Copper-Exchanged Fluorapatite
Choudary et al. have chosen [19] a weakly amphoteric apatite as support, since
various kinds of cations and anions can be readily introduced into their framework
due to their large ion exchange ability. Such exchanged apatites are already in use
for several organic transformations [20]. A schematic representation of copperexchanged fluorapatite (1) by incorporating the basic species F
À in apatite in situ by
co-precipitation and subsequent exchange with Cu (II) is shown (Scheme 6).
N-Arylation of imidazoles and other heterocycles with chloroarenes and
fluoroarenes to afford good to excellent yields of aryl heterocycles by using
CuFAP catalyst is described.
The method is general and amenable to the N-arylation of imidazole with a
wide range of chloro- and fluoroarenes using catalyst 1 (Table 5). As illustrated
in Table 5, chloroarenes (electron withdrawing), 2-chloropyridine, and
2-chloropyrimidine provided excellent yields in shorter reaction times than chlorobenzene and chloroarenes (electron donating) (entries 1–11). Cyano, nitro, and
trifluoromethyl groups are well tolerated (entries 1, 3, and 4). The N-arylation
results of deactivated chloroarenes using K 2 CO 3 as a base are quite impressive
over the unreactive system using nanocopper and Cs 2 CO 3 base [21]. Another
significant feature is that under similar conditions, the N-arylation of imidazole
with chlorobenzene afforded moderate yields, which could be further improved by
the addition of KO
t
Bu (entry 9). N-Arylation of heterocycles developed here using
the less expensive chloroarenes is more attractive than the methods using the
expensive bromo- and iodoarenes in terms of economic feasibility. The catalyst is
recycled four times with a slight decrease in activity (entry 1). Interestingly,
fluoroarenes composed of several o- or p-electron-withdrawing (EW) groups
(entries 12–15) are also coupled with imidazole to afford the corresponding
N-arylated products in excellent yields. Faster reactivity over the chloroarenes
(entries 1, 5, 13 and 14) and selective coupling involving C–F activation only in
chlorofluoroarene (entry 15) are reported.
As shown in Table 6, benzimidazole, pyrrole, pyrazole, and piperidine are also
coupled with 1-chloro-4-nitrobenzene and 1-fluoro-4-nitrobenzene to give the
corresponding N-arylated products in excellent yields.
To understand the mechanism of the N-arylation of imidazole, a series of
experiments were conducted. The reaction of catalyst (1) with imidazole gives a
deep blue Cu-imidazole complex (3), which is considered to be the first step of the
128
M.L. Kantam et al.