120
C for 12 h (Scheme 24). The amination reaction proceeded smoothly and good
to excellent yields of the products were recorded in all cases.
Cu-PAR is a very versatile catalyst and offers excellent stability, activity, and
reusability for multiple reaction cycles.
Wang et al. in 2011 [89] developed a polystyrene immobilized phenanthroline–Cu(I)
catalyst and applied it successfully for the synthesis of 2-aminobenzothiazoles from
2-halobenzeneamines and phenyl isothiocyanates (Scheme 25).
The optimum reaction conditions were found to be 2.5 mol% copper loading in the
catalyst with DABCO as base and toluene as the solvent at 60
C for 8 h. The efficiency
of the catalytic system was then tested by using 2-chloroaniline, 2-iodoaniline, and
various substituted 2-bromoanilines possessing electron-withdrawing as well as
electron-donating groups along with phenyl isocyanate. Even substituted phenyl
isocyanates bearing methoxy, chloro, methyl, nitro, and trifluoromethyl groups were
found to react appreciably with 2-bromoaniline as well as 2, 4-dibromoaniline. In all
cases good to excellent yields (69–95%) of the corresponding 2-aminobenzothiazoles
were obtained. Even a less active substrate, 2-bromo-5-trifluoromethylaniline, reacted
surprisingly well with phenyl isothiocyanate to give a decent yield (71%) of the
corresponding product. The catalytic system was even found to tolerate sterically
hindered substrate like 2,4-dibromo-6-fluoroaniline to give an excellent yield (84%)
of the corresponding 2-aminobenzothiazole product.
Recyclability studies of the catalyst using 2-iodoaniline and phenyl isothiocyanate as the model substrates demonstrated that the catalyst can be separated from
the reaction mixture simply by filtration, washed dried, and reused for at least ten
reaction cycles with almost no leaching of copper from the support (<0.20 ppm).
I
H
N
N
H
N
N
Cu-PAR
DMSO, K 2 CO 3
100 °C, 14 h
+
Scheme 22 Cu-PAR-catalyzed N-arylation of imidazole with iodobenzene
B(OH) 2
N
N
H
+
Cu-PAR
methanol
40 °C, 12 h
N
N
Scheme 23 Cu-PAR-catalyzed N-arylation of imidazole with phenyl-boronic acid
B(OH) 2
+
NH 2
NH
Cu-PAR
DMSO, 120 °C
12 h
Scheme 24 Cu-PAR-catalyzed amination of aniline with phenylboronic acid
168
M.L. Kantam et al.
C for 12 h (Scheme 24). The amination reaction proceeded smoothly and good
to excellent yields of the products were recorded in all cases.
Cu-PAR is a very versatile catalyst and offers excellent stability, activity, and
reusability for multiple reaction cycles.
Wang et al. in 2011 [89] developed a polystyrene immobilized phenanthroline–Cu(I)
catalyst and applied it successfully for the synthesis of 2-aminobenzothiazoles from
2-halobenzeneamines and phenyl isothiocyanates (Scheme 25).
The optimum reaction conditions were found to be 2.5 mol% copper loading in the
catalyst with DABCO as base and toluene as the solvent at 60
C for 8 h. The efficiency
of the catalytic system was then tested by using 2-chloroaniline, 2-iodoaniline, and
various substituted 2-bromoanilines possessing electron-withdrawing as well as
electron-donating groups along with phenyl isocyanate. Even substituted phenyl
isocyanates bearing methoxy, chloro, methyl, nitro, and trifluoromethyl groups were
found to react appreciably with 2-bromoaniline as well as 2, 4-dibromoaniline. In all
cases good to excellent yields (69–95%) of the corresponding 2-aminobenzothiazoles
were obtained. Even a less active substrate, 2-bromo-5-trifluoromethylaniline, reacted
surprisingly well with phenyl isothiocyanate to give a decent yield (71%) of the
corresponding product. The catalytic system was even found to tolerate sterically
hindered substrate like 2,4-dibromo-6-fluoroaniline to give an excellent yield (84%)
of the corresponding 2-aminobenzothiazole product.
Recyclability studies of the catalyst using 2-iodoaniline and phenyl isothiocyanate as the model substrates demonstrated that the catalyst can be separated from
the reaction mixture simply by filtration, washed dried, and reused for at least ten
reaction cycles with almost no leaching of copper from the support (<0.20 ppm).
I
H
N
N
H
N
N
Cu-PAR
DMSO, K 2 CO 3
100 °C, 14 h
+
Scheme 22 Cu-PAR-catalyzed N-arylation of imidazole with iodobenzene
B(OH) 2
N
N
H
+
Cu-PAR
methanol
40 °C, 12 h
N
N
Scheme 23 Cu-PAR-catalyzed N-arylation of imidazole with phenyl-boronic acid
B(OH) 2
+
NH 2
NH
Cu-PAR
DMSO, 120 °C
12 h
Scheme 24 Cu-PAR-catalyzed amination of aniline with phenylboronic acid
168
M.L. Kantam et al.