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A. A. Kelkar et al. / Tetrahedron Letters 43 (2002) 7143–7146
(70–82%) of triphenylamine (Table 2). Activity varies
with aryl halides in the order I>Br>Cl. The highest
yield of triarylamine was obtained with p-iodoanisole
as the reactant (81%; Table 2, entry 4). A few substi-
tuted amines were also tested and the catalyst was
found to be tolerant of substituents present on the ring
except for a nitro group. Higher yields of triarylamine
were obtained with the electron donating p-methoxy
group as a substituent for both aryl halide and aniline
as the reactants (Table 2, entries 4, 5 and 7), but the
reaction did not proceed with o- and p-nitroaniline as
reactants.
role of KOt-Bu as a base to develop ligand-free cata-
lysts for selective and efficient synthesis of triarylamines
at lower temperature (135°C). The role of chelating
bidentate ligands in improving activity and selectivity
to triarylamines is also presented.
A number of bases were screened with CuI as a cata-
lyst. With KOt-Bu as a base, 72% yield of triphenyl-
amine was obtained (Table 1, entry 1). This is the first
report on the catalytic synthesis of triarylamines with a
Cu catalyst in the absence of promoting ligands under
relatively mild temperature conditions (135°C). Cata-
lytic turnover numbers in a range of 35–40 were
obtained. In order to check the catalytic effect of CuI,
a non-catalytic reaction was carried out using iodobenz-
ene and aniline as reactants (Table 1, entry 2). In this
reaction, 51% conversion of aniline with a poor yield of
triphenylamine (7%) was obtained with many by-prod-
ucts. Comparison of the results (Table 1, entries 1 and
2) clearly show that CuI enhances the conversion as
well as selectivity to triphenylamine. With other bases
like NaOt-Bu, and DBU (Table 1, entries 3 and 4), 50
and 21% conversion of aniline was obtained with very
poor yields of triphenylamine. However, in these cases
the yield of diphenylamine was higher or comparable to
triphenylamine, even though iodobenzene was used in
large excess. With all other bases studied (NaOMe,
Cs2CO3, KOH, NaHCO3) trace amounts of triphenyl-
amine formation were observed. KOt-Bu as a base
gave the best results for triarylamine synthesis and
further work has been carried out with KOt-Bu as the
base. A few aryl halides were screened as substrates and
it was observed that aryl iodides gave higher yields
A variety of N- or P-containing ligands were examined
using iodobenzene and aniline as model substrates and
CuI as a catalyst: with chelating bidentate ligands,
higher activity was observed (Table 3). With PPh3 as a
ligand (1 equiv. to Cu, Table 3, entry 1) the reaction did
not proceed as observed by Gujadhar et al.12 Gujadhar
et al. observed that the copper complexes
Cu(PPh3)3Br12 and Cu(PPh3)(1,10-phenanthroline)Br13
were good amination catalysts. It is possible that PPh3
(1 equiv. to Cu) is able to bind at only one site and not
able to form an active catalyst. Reaction carried out
with a PPh3:Cu ratio of 3 did result in the formation of
1 59% in 7 h. This indicates that a ligand:Cu ratio of 2
or more is required for catalytic activity with PPh3 as a
ligand. With pyridine or quinoline as a ligand (1 equiv.
to Cu) 62 and 38% yields of 1 were obtained, respec-
tively (Table 3, entries 3 and 4). However, with chelat-
ing ligands such as 1,3-bis(diphenylphosphinopropane)
(DPPP), 2,2%-bipyridine and 8-hydroxyquinoline (1
equiv. to Cu), higher yields of 1 were obtained (Table 3,
compare entries 1, 2 with 10, entry 3 with 5 and entry
4 with 7). In a previous report7 using CuCl as a catalyst
and KOH as a base, rate enhancement was not
obtained with 2,2%-bipyridine or 8-hydroxyquinoline as
ligands. From the results obtained, it is clear that
chelating ligands give improved amination activity with
a higher yield of triphenylamine. A series of chelating
diphosphines were tested and the best results (89%
yield) were obtained with DPPP as a ligand. Activity as
well as yield of 1 increased with increase in carbon
chain length initially (C1ꢀC3) and decreased with fur-
ther increase in carbon chain length between the phos-
phines. It is likely that the ligands (DPPT and DPPH)
behave as monodentate ligands leading to lower activ-
ity. A similar trend was observed for alkyldiamine
ligands (Table 3, entries 13–15). Substituted phenan-
throline derivatives were tested as ligands which
revealed that best results (yield of 91% in 2 h) were
obtained using 1,10-phenanthroline as a ligand. To
compare these results with those of the well defined Cu
complexes, reactions were carried out using
Cu(PPh3)3Br12 and Cu(PPh3)(1,10-phenanthroline)Br13
as catalysts. The yield of 1 was substantially lower (44
and 54%, respectively) in these cases. The results
obtained clearly show a significant improvement in the
yield of 1 with the use of chelating ligands (1 equiv. to
Cu) and KOt-Bu as base. Best results were obtained
using 1,10-phenanthroline (91% 1 in 2 h) and 2,2%-
bipyridine (96% in 3.5 h) as ligands. The higher activity
obtained using chelating ligands (1 equiv. to Cu) may
Table 2. Copper-catalyzed amination: screening of aryl
halides and aryl amines
Entry
R
R%
Conv. (%)
Yield (%)*
1
2
1
H
H
H
H
H
H
H
89
81
57
99
99
96
96
95
70
45
28
81
82
73
79
71
7
12
12
8
12
15
9
2c
3§
4
p-OCH3
H
H
p-OCH3
p-OCH3
5
6
7
8
p-OCH3
o-CH3
p-OCH3
o-CH3
14
Reaction conditions: aryl amine: 4.0 mmol; aryl halide: 12.0 mmol;
CuI: 0.08 mmol; KOt-Bu: 12.0 mmol; solvent: toluene: 20 ml; temper-
ature: 135°C; agitation: 900 rpm; reaction time: 14 h.
c Reaction with bromobenzene.
§ Reaction with chlorobenzene.
* Isolated yields.