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H.-Y. Kuo et al.
PAPER
ium complexes as the catalyst for comparison (Table 1), cohols such as 1-octanol and ethyl alcohol can also be
given that iridium complexes are frequently used as prec- employed as the alkylating agent. Thus, reaction of m- and
atalysts for C–N bond formation. Based on the 100% con- p-benzenediamine with 1-octanol yielded N,N′-dioctyl-m-
version and product analysis, all of the tested iridium benzenediamine and -p-benzenediamine in 85% and 90%
complexes have excellent activity for the oxidation of yield, respectively (Table 2, entries 7 and 11). In contrast,
benzyl alcohol into benzaldehyde; however, we found that the nitro-substituted benzyl alcohol is not suitable as the
the catalytic systems based on mononuclear iridium com- alkylating agent, as the reaction gave a mixture of uniden-
plexes associated with various ligands gave benzimid- tified products (Table 2, entry 12). Presumably, partial re-
azole 3 as the major product, not dialkylation product 2, duction of the nitro group resulted in complication of the
even though the reactions were carried out under hydro- reaction.
gen atmosphere (Table 1). From this screening, it clearly
shows that complex 1 is the most promising catalyst for
the N,N′-dialkylation of benzenediamines with alcohols.
In summary, we have described an efficient diiridium cat-
alytic process for the dialkylation of benzenediamines
with various alcohols. This catalytic system offers several
Presumably, the cooperative effect between the two metal
advantages for the preparation of such benzenediamines,
centers in complex 1 plays an important role in facilitating
such as low loading of catalyst, no required organic sol-
the reduction of imine, an effect which is absent in a
vent, high selectivity, and high yields, which make it a
mononuclear system.
useful and attractive methodology for organic synthesis.
In this study, we have established that the dimetallic cata-
Table 1 N,N′-Dialkylation of o-Benzenediamine Catalyzed by
lyst affects the catalytic selectivity dramatically, presum-
ably due to a synergistic effect. Detailed mechanistic
studies and further applications are currently under inves-
tigation.
Various Iridium Complexesa
Entry
Iridium catalyst
Conv.b (%) Yieldc (%)
3
2
1H and 13C NMR spectra were recorded in CDCl3 on a Bruker AM-
300 or Avance 400 spectrometer. Chemical shifts are given in parts
per million relative to TMS. HRMS data were recorded by using
ESI-TOF (Waters Micromass LCT Premier XE) and FABMS
(JEOL JMS-700) techniques. Chemicals and solvents were of ana-
lytical grade and were used as received, unless otherwise stated.
Complex 1 was prepared according to the method previously report-
ed.8
1
2
3
4
5
6
complex 1
100
100
100
98
–
90
–
Ir(IBn)(CO)2Cld
[Ir(cod)2Cl]2
78
80
50
70
38
–
[Ir(cod)2Cl]2/Ph3P
[Ir(cod)2Cl]2/dpppe
47
20
50
100
IrCl3/Me2N(CH2)2NMe2 97
Iridium-Catalyzed N,N′-Dialkylation of Benzenediamines;
General Procedure
a Reaction conditions: diamine (0.5 mmol), alcohol (3 mmol), iridium
complex (3 μmol), CsOH·H2O (0.15 mmol), molecular sieves (0.3 g),
120 °C, 48 h, under H2 atmosphere.
A mixture of the benzenediamine (54 mg, 0.5 mmol), an alcohol (3
mmol), iridium complex 1 (3 mg, 3 μmol), CsOH·H2O (25.2 mg,
0.15 mmol), and 4 Å molecular sieves (0.3 g) in a reaction tube was
flushed with nitrogen gas. The reaction mixture was heated at
120 °C for 48 h under hydrogen atmosphere. After the reaction
completion, H2O (5 mL) and EtOAc (3 × 5 mL) were added. The or-
ganic extract was separated, dried (MgSO4), and concentrated. The
desired product was purified by chromatography (CH2Cl2–EtOAc,
30:1 to 10:1). The structures of the products were confirmed from
their spectroscopic (1H and 13C NMR, and MS) data, which were
similar to those reported in the literature. Spectroscopic data of new
compounds are summarized below.
b Based on the consumption of diamine.
c NMR yields.
d IBn = 1,3-dibenzylimidazol-2-ylidene.
e dppp = Ph2P(CH2)3PPh2.
With this promising result, the scope and generality of the
N,N′-dialkylation reaction was investigated (Table 2). As
expected, all of the reactions proceeded smoothly and
some functional groups were tolerated, including methyl,
methoxy, bromo, and trifluoromethyl groups, but not the
nitro group. Thus, reaction of o-benzenediamine with var-
ious benzyl alcohols afforded the corresponding N,N′-
dibenzyl-o-benzenediamines in good to excellent yields
(Table 2, entries 1–4). The yield decreased slightly with
the use of the ortho-substituted benzyl alcohol presum-
ably due to steric hindrance (Table 2, entry 5).
N,N′-Bis(p-bromobenzyl)-o-benzenediamine10
Yield: 0.168 g (75%); yellow solid; mp 120–122 °C.
1H NMR (400 MHz, CDCl3): δ = 4.28 (s, 4 H, -ArCH2N-), 6.66–
6.68 (m, 2 H), 6.81–6.83 (m, 2 H), 7.26 (d, J = 6.4 Hz, 4 H), 7.47
(d, J = 6.4 Hz, 4 H).
13C NMR (100 MHz, CDCl3): δ = 138.2, 136.6, 131.5, 129.2, 120.8,
119.6, 112.2, 48.2.
HRMS–FAB: m/z [M]+ calcd for C20H18N2Br2: 443.9837; found:
443.9830.
Next, we explored the isomeric benzenediamines. Dial-
kylations of m- and p-benzenediamine with various alco-
hols, including aliphatic alcohols, proceeded smoothly to
yield the corresponding amines in good yields (Table 2,
entries 6–11). It should be noted that these reactions can
be carried out under nitrogen atmosphere, i.e. the hydro-
gen atmosphere is not required. Furthermore, aliphatic al-
N,N′-Bis[p-(trifluoromethyl)benzyl]-o-benzenediamine
Yield: 0.149 g (70%); yellow solid; mp 118–120 °C.
1H NMR (400 MHz, CDCl3): δ = 4.41 (s, 4 H, -ArCH2N-), 6.64–
6.67 (m, 2 H), 6.78–6.80 (m, 2 H), 7.49 (d, J = 8.0 Hz, 4 H), 7.59
(d, J = 8.0 Hz, 4 H).
Synthesis 2013, 45, 189–192
© Georg Thieme Verlag Stuttgart · New York