Table 1. Reaction condition optimization with a Ag/Mo hybrid material
as the catalyst.[a]
ized catalysts, heterogeneous catalysts have advantages for
product and catalyst isolation, catalyst reuse, and operation-
al handling and some nice results were obtained recently.[13]
Nevertheless, till today, the development of heterogeneous
catalyst systems was severely limited due to its poor general-
ity or its specific catalytic activity for specific reactions, es-
pecially in fine-chemical synthesis. For example, the newly
reported iron oxide supported nanoruthenium could be an
active catalyst in sulfonamide alkylation with alcohols.[14]
However, this catalyst has no activity in the alkylation of
carboxamides, amines, and ketones. Also, there is no possi-
bility to use aliphatic alcohols as starting materials. There-
fore, one of the major tasks for catalyst researchers is to de-
velop more active and general heterogeneous catalysts for
the N-alkylation reactions with alcohol without the addition
of organic ligand.
According to the mechanistic investigation, the key steps
for the reaction are the activation or dehydrogenation of al-
cohol to form the carbonyl compound and the hydrogena-
tion of the imine intermediate to yield the product (Sche-
me 1).[5a,9b,15] Up-to-now, one of the most active catalysts,
that is, hydrotalcite-supported silver, for the dehydrogena-
tion of alcohols has been reported.[16] Meantime, molybde-
num complex was found to be active for the nuclephilic ad-
dition or reduction of imines.[17] Possibly a new and simple
catalyst could be designed for the N-alkylation of amines
with alcohol if we can combine silver and molybdenum to-
gether through a suitable method. Here, we report our new
findings on the selective growth of Ag/Mo hybrid materials,
that is, Ag6Mo10O33 and its catalytic activity in the coupling
reactions of amines, carboxamides, sulfonamides, and ke-
tones with alcohols (Scheme 1).
Entry
Cat.
Base
Conv. [%][b]
Sel. [%][c]
1
2
3
4
5
6
7
8
Ag2O
MoO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
NaHCO3
KOtBu
KOH
41
13
36
82
32
48
72
44
0
0
Ag2MoO4
Ag-Mo-22
Ag-Mo-5
Ag-Mo-15
Ag-Mo-20
Ag-Mo-25
Ag-Mo-22
Ag-Mo-22
Ag-Mo-22
Ag-Mo-22
Ag-Mo-22
Ag-Mo-22
24
82
35
50
76
51
85
0
9
85
17
24
10
11
12
13
14
0
ꢀ95 (ꢀ95)[d]
ꢀ95 (ꢀ 95)[d]
88
89
92
80
Cs2CO3
[a] Reaction conditions: aniline (2 mmol, 186 mg), benzyl alcohol
(10 mmol, 1080 mg), catalyst (40 mg), base (20 mol%), 12 h, 1408C (en-
tries 1–4), 1608C (entries 5–14). Conversion of aniline was determined by
GC-FID. [b] Conversion of aniline. [c] Selectivity to N-benzyl aniline de-
termined by GC-FID. The major byproduct derived from aniline is N-
benzylideneaniline. [d] The catalyst was recovered and reused for the
second time.
for the N-alkylation of amines. The Ag/Mo hybrid materials
were prepared through a hydrothermal method with slight
modification.[18] All the materials were characterized by
BET, XPS, XRD, SEM, and TEM. BET measurements
showed that the BET surface area was 1.5–4.5 m2 g À1for all
the samples (see Table S1 in the Supporting Information).
XPS analysis confirmed the presence of Ag+ and Mo6+ spe-
cies (Figure 1).
The SEM and TEM pictures showed that the sample was
closed packed and confirmed the formation of the
Ag6Mo10O33 crystal for sample Ag-Mo-22 (Figure 2 and Fig-
ure S1 in the Supporting Information). XRD analysis gave
more interesting results (Figure 3). Normally, Ag2Mo2O7
crystals could be obtained with a pH value of about 0.90–
1.50 after the addition of nitric acid (samples Ag-Mo-5, -15,
and -20). After finely tuning the acidity, Ag6Mo10O33 crystal,
that is, Ag-Mo-22, could be selectively synthesized by the
addition of nitric acid (22 mL, 2m), although the pH value
of the reaction mixture with the addition of 20–25 mL nitric
acid could not be measured accurately by a pH meter. The
crystal structure of Ag6Mo10O33 was different from former
reports,[18a] which was denoted as Ag-Mo-R and prepared in
solution with pH 2 and the ratio of Ag to Mo was 1:1 (see
Figure S2 in the Supporting Information).
Scheme 1. Amine, carboxamide, sulfonamide, and ketone alkylation with
alcohol (M=Ag/Mo).
Results and Discussion
By applying these Ag/Mo hybrid materials in the reaction
of aniline alkylation with benzyl alcohol, nice results were
achieved (Table 1, entries 5–14). The conversion of aniline
and the selectivity of N-benzyl aniline were all above 90%
with Ag-Mo-22 as the catalyst and no deactivation occurred
when it was recovered and reused with KOtBu as additive
base. For comparison, the Ag-Mo-R prepared by the report-
ed procedure was also tested under the same reaction condi-
tions (not shown in Table 1). The conversion was 52% with
56% selectivity, which suggests that Ag-Mo-R was also a
Initially, the catalytic activity of commercially available
Ag2O, MoO3, and Ag2MoO4 were tested (Table 1). Clearly,
only N-benzylideneaniline formed in the presence of Ag2O
or MoO3 (Table 1, entries 1 and 2). As we imagined, the cat-
alytic activity was efficiently improved with Ag2MoO4 as the
catalyst. Although the conversion was not high, 24% N-
benzyl aniline was observed (entry 3).
Inspired by these interesting results, we tried to find a
suitable method to prepare Ag/Mo hybrid material catalysts
1022
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 1021 – 1028