920
Chemistry Letters Vol.38, No.9 (2009)
Heterogeneously Catalyzed One-pot Synthesis of Aldimines from Primary Alcohols
and Amines by Supported Ruthenium Hydroxides
Jung Won Kim,1 Jinling He,1 Kazuya Yamaguchi,1;2 and Noritaka MizunoÃ1;2
1Department of Applied Chemistry, School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
2Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST),
4-1-8 Honcho, Kawaguchi 332-0012
(Received July 3, 2009; CL-090628; E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp)
The one-pot synthesis of aldimines from primary alcohols
Table 1. The one-pot synthesis of 3a by various catalystsa
and amines via sequential reactions of alcohol oxidation and de-
hydrative condensation could efficiently be promoted by sup-
ported ruthenium hydroxide catalysts, in particular Ru(OH)x/
TiO2. The observed catalysis was truly heterogeneous and the re-
trieved catalyst could be reused without an appreciable loss of
high catalytic performance.
NH
2
catalyst
OH
N
+
+ 1/2O2
+ 2H2O
1a
2a
3a
Entry
Catalyst
Yield of 3a/%
1
2
Ru(OH)x/TiO2
Ru(OH)x/Al2O3
Ru(OH)x/Al2O3
RuClx/TiO2
Ru/C
Ru/HAP
Ru(OH)x
anhydrous RuO2
95
44
92
<1
15
2
<1
<1
<1
2
<1
<1
<1
<1
<1
<1
<1
3b
4
Sequential catalysis that can mediate one-pot multistep syn-
thesis has attracted much attention in both research and chemical
industries.1–3 In comparison with stepwise syntheses, one-pot syn-
thesis can avoid the isolation of (unstable) intermediates and can
reduce the production of chemical waste.1–3 Although many tran-
sition-metal-based catalytic one-pot synthetic procedures have
been developed, these systems have disadvantages of the recovery
and reuse of (expensive) catalysts and/or the need for co-catalysts
such as bases and stabilizing ligands.1 Recently, focus has shifted
from homogeneous to heterogeneous catalysis and several heter-
ogeneously catalyzed efficient one-pot procedures have been re-
ported.2,3 Also, we have developed various heterogeneously cat-
alyzed efficient functional group transformations including one-
pot multistep syntheses by metal hydroxide catalysts.3
5
6
7
8
.
9
RuCl3 nH2O
10
11
12
13
14
15c
16c
17
RuCl2(PPh3)3
RuCl2(bpy)2
[RuCl2(p-cymene)]2
Ru(acac)3
Ru3(CO)12
TiO2
Al2O3
None
Imines (aldimines and ketimines) are an important class of
chemicals that have widely been used as synthons for the organic
synthesis, pharmaceuticals, and agricultural chemicals.4 Very re-
cently, one-pot procedures with heterogeneous catalysts such as
manganese octahedral molecular sieves (K-OMS-2)5a and gold
nanoparticles supported on hydroxyapatite (Au/HAP)5b for the
synthesis of aldimines directly from primary alcohols and
amines have been reported. However, the turnover frequencies
(TOFs) are 1.3–83 hÀ1 and still low.5 In this paper, we report
the efficient one-pot syntheses of aldimines from primary alco-
hols and amines by supported ruthenium hydroxide catalysts.
First, the catalytic activities for the reaction of benzyl alco-
hol (1a) with aniline (2a) to produce N-benzylideneaniline (3a)
were compared among various catalysts (Table 1). Supported
ruthenium hydroxide catalysts such as Ru(OH)x/TiO2 and
Ru(OH)x/Al2O3 (see Supporting Information6 for preparation
and characterization) showed high catalytic activities for the
synthesis of 3a. In these cases, the corresponding secondary
amine was not formed. No reaction proceeded in the absence
of the catalysts, or in the presence of TiO2 or Al2O3. In the pres-
aReaction conditions: Catalyst (Ru: 2 mol %), 1a (0.5 mmol),
2a (0.75 mmol), toluene (2 mL), O2 (1 atm), 100 ꢀC, 60 min.
Yields (based on 1a) were determined by GC analyses.
b180 min. c50 mg.
neous catalysts such as Ru/C, Ru/HAP,7 anhydrous RuO2,
and Ru(OH)x. The reaction hardly proceeded in the presence
of ruthenium chloride species supported on TiO2 prepared with-
out base pretreatment (RuClx/TiO2, see Supporting Informa-
tion6 for preparation). Therefore, the generation of the active
ruthenium hydroxide species on supports is very important to ob-
tain high catalytic performance.
The Ru(OH)x/TiO2-catalyzed reaction of 1a with 2a was
carried out under the conditions described in Table 1 and the cat-
alyst was removed from the reaction mixture by hot filtration at
ca. 40% conversion of 1a. After removal of the catalyst, the re-
action was again carried out with the filtrate under the same con-
ditions. In this case, no further reaction proceeded (Figure S1).6
In addition, it was confirmed by ICP-AES analysis that no ruthe-
nium was detected in the filtrate (below detection limit of 7 ppb).
These facts can rule out any contribution to the observed catal-
ysis from ruthenium species that leached into the reaction solu-
tion and the observed catalysis is intrinsically heterogeneous.8
In the presence of the most active Ru(OH)x/TiO2 catalyst,
various combinations of substrates (seven alcohols and five
.
ence of the catalyst precursor RuCl3 nH2O, the desired aldimine
3a was not formed. Also, ruthenium complexes RuCl2(PPh3)3,
RuCl2(bpy)2, [RuCl2(p-cymene)]2, Ru(acac)3, and Ru3(CO)12
were not effective. The catalytic activities of supported rutheni-
um hydroxides were much higher than those of other heteroge-
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