DOI: 10.1002/cctc.201501346
Communications
Pd/C as an Efficient and Reusable Catalyst for the
[
a]
Xiang Liu, Philippe Hermange,* Jaime Ruiz, and Didier Astruc*
An efficient Pd/C-catalyzed synthesis of secondary amines in-
alcohols, which allows easy access to this “borrowing-hydro-
gen” strategy.
volving CÀN bond formation was achieved from amines and
alcohols with H O as the only byproduct. This common, green,
The reaction conditions were optimized by using aniline
(1a) and benzyl alcohol (2a) as model substrates. As shown in
Table 1, the reaction was first performed with 1a (0.55 mmol),
2
commercial, and cheap catalyst was recycled five times in this
sustainable, simple, and economic procedure.
2
a (0.5 mmol), and HCOONa (0.5 mmol) in the presence of
5
mol% Pd/C (5%w/w) in refluxing toluene (2 mL) under an at-
[
11]
Secondary amines are among the most valuable functional
mosphere of N2 for 24 h. Interestingly, desired product 3a
was obtained in 34% yield (Table 1, entry 1). Examining the
effect of the amount of sodium formate, the yield of 3a was
gradually improved to 88% by increasing the stoichiometry to
3 equivalents (Table 1, entries 2 and 3). A higher loading result-
ed in a reduced yield (Table 1, entry 4), and the absence of
base totally shut down the reaction (Table 1, entry 5). Then, the
reaction was performed with catalyst loadings of 2 to 10 mol%
in Pd, and it was finally found that 5 mol% Pd was the opti-
mum loading (Table 1, entries 6–8). The reaction was per-
formed with various bases, including KOH, K CO , NaHCO ,
[1]
compounds in natural products such as alkaloids. They are
also extensively used for the synthesis of lubricants, pharma-
ceuticals, surfactants, and agricultural chemicals. In addition,
they are found in many functional materials, including poly-
[2]
mers and biologically active drugs. Numerous methods have
been developed for the synthesis of secondary amines, but the
nucleophilic substitution of alkyl halides with primary amines
[
3]
often remains the most commonly employed method. How-
ever, targeting more environmentally friendly procedures, the
N-alkylation of primary amines with alcohols has received par-
2
3
3
[2,4]
ticular attention during the last decades. This so-called “bor-
rowing-hydrogen” or “transfer-hydrogenation” strategy has
been reported by using a variety of transition-metal complexes
K PO , and CH COONa, but no increase in the yield of product
3 4 3
3a was observed (Table 1, entries 9–13). However, the good
yields that are generally obtained (relative to those obtained
with other bases) show that potential in situ H2 generation
II
as catalysts. They most frequently used ones are Ru - and
I
Ir -based species, but gold, silver, copper, nickel, palladium,
[
5]
osmium, and rhodium catalysts have also been explored.
Very recently, a significant step forward was reported with mo-
lecular iron catalysts instead of noble transition-metal cata-
[a]
Table 1. Optimization of the reaction conditions.
[
6]
lysts. However, these methods often require pre-prepared lig-
ands, strong bases, long reaction times, and high tempera-
tures, and in all cases, the catalysts are not recycled. Therefore,
the development of general and efficient catalytic methodolo-
gies with recoverable catalysts for the synthesis of secondary
amines is a paramount goal in modern organic synthesis and
[
b]
Entry
Pd/C [mol%]
Base (equiv.)
Solvent
Yield [%]
1
2
3
4
5
6
7
8
9
5
5
5
5
5
10
7
2
5
5
5
5
5
5
5
5
5
5
HCO Na (1)
2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
p-xylene
DMSO
34
86
88
75
0
[
7]
medicinal chemistry. Along this line, heterogeneous catalysts
HCO
2
Na (2)
[
8]
[9]
HCO Na (3)
2
have been reported, including Pd/Fe O , Pd/MgO, and Pd
3
4
HCO
none
HCO Na (3)
2
Na (4)
[
10]
nanoclusters/TiO2.
However, none of them is commercial;
therefore, our focus here is the search for a commercial and
cheap catalyst for this reaction of great importance in chemis-
try and materials science. Herein, we report that simple Pd/C is
an efficient and recyclable catalyst for the synthesis of second-
ary amines by CÀN bond formation from primary amines and
64
78
37
79
62
43
54
78
88
34
56
62
51
2
HCO
HCO
2
Na (3)
Na (3)
2
KOH (3)
CO (3)
NaHCO (3)
10
K
2
3
11
3
1
1
1
15
16
17
18
2
3
4
K
3 4
PO (3)
NaOAc (3)
[c]
[c]
[c]
[c]
[d]
HCO
HCO
HCO
HCO
HCO
2
Na (3)
2
Na (3)
2
Na (3)
2
Na (3)
2
Na (3)
[
a] X. Liu, Dr. P. Hermange, Dr. J. Ruiz, Prof. D. Astruc
ISM, UniversitØ de Bordeaux
DMF
DMA
toluene
3
51 Cours de la Liberation, 33405 Talence Cedex (France)
E-mail: d.astruc@ism.u-bordeaux1.fr
[
a] Reaction conditions: 1a (0.55 mmol), 2a (0.50 mmol), Pd/C (5% w/w),
base (1.50 mmol), toluene (2 mL), N , reflux, 24 h. [b] Yield of isolated
2
product. [c] 1208C. [d] 908C.
ChemCatChem 2016, 8, 1043 – 1045
1043
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim