Table 20 Characterization of silica-supported copper catalysts
Entry
Catalyst
Stretching frequency of
C¼N (cm
À1
) before
complexation
Change in C¼N bands
(cm
À1
) after
complexation
Loading of
Cu (mmol/g)
1
Cat1
–
–
0.38
2
Cat2
1,640
1,631
0.43
3
Cat3
1,641
1,338
0.41
4
Cat4
1,645
1,631
0.41
5
Cat5
1,643
Almost no shift
0.39
Table 21 Screening of different copper catalysts for N-arylation of imidazole with iodobenzene
a
I
N
H
N
N
N
+
Catalyst (5 mol%)
toluene,
Cs 2 CO 3 , 8h, N 2
Entry
Catalyst
Yield (%)
Leaching of Cu (%)
b
1
Cat1
23
25
2
Cat2
92
2.5
3
Cat3
69
7
4
Cat4
31
9
5
Cat5
91
6.5
6
Cu(OAc) 2
3
–
a
Reaction conditions: iodobenzene (1 mmol), imidazole (1.2 mmol), Cs 2 CO 3 (2 mmol), toluene
(3 mL), catalyst (5 mol%), 100
C, N 2 atmosphere. GC yields
b
Determined by ICP-AES and expressed as percentage of the total copper charged
NH 2
N
S
Cu
AcO
OAc
Cu(OAc) 2
N
N
N
SiO 2
SiO 2
SiO 2
SiO 2
SiO 2
Cu(OAc) 2
Cu
AcO
AcO
Cu
AcO
OAc
N
N
Cat1
Cat2
Cat3
Cat4
Cat5
N
O
Scheme 12 Structure of silica tethered copper catalysts
Recent Developments in Recyclable Copper Catalyst Systems for C–N Bond. . .
143