H. Liu et al. / Journal of Catalysis 329 (2015) 262–268
263
Scheme 1. (a) Hydrogen autotransfer process for alcohols as electrophiles (top) and (b) hydrogen autotransfer process for amines as electrophiles (below).
hydrogenation conditions in moderate to high yield (20–99%).
Besides ruthenium based homogeneous catalysts, Os (CO)12 [19]
is another transition-metal complex which was able to catalyze
the alkylation of amines with other amines. However, only a low
yield (<30%) to the secondary amine was obtained at 150 °C under
autotransfer process of amine alkylation begins with the oxida-
3
tion/dehydrogenation of an alkylamine to the corresponding imine
(iminium). This is followed by the addition of a second nucleophilic
amine to the in situ formed imine, generating an aminal intermedi-
ate, elimination of the initial dealkylated amine, and formation of a
new imine (iminium), with a final hydrogenation of this imine [17].
The thermodynamics of the whole process are in some cases highly
favorable [28], particularly when ammonia is one of the products
of the reactions (Scheme 1b). In this reaction, the hydrogen donor
for the transfer hydrogenation is the primary amine. Hence, no
additional hydrogen or hydrogen transfer reagent is required. The
advantage of this method compared to most reductive aminations
is the high atom efficiency and the fact that no high-pressure
equipment is required [29]. In this paper, we describe our results
with various copper and silver catalysts. Copper catalysts were
found to be particularly active for the self coupling of amines to
secondary amines. To the best of our knowledge, this is the first
reported copper based heterogeneous catalyst for this type of reac-
tion using the borrowing hydrogen scheme.
7
atm nitrogen pressure. The Ir-based catalyst, (IrCl
2
Cp⁄(NHC)),
recently reported by Peris’ group [20], was found to be active in
the N-alkylation of anilines with aliphatic amines. However, homo-
geneous systems have disadvantages including the difficulties in
the recovery and reuse of the expensive catalysts and the require-
ment for co-catalysts such as bases and stabilizing ligands.
Compared to homogeneous catalysts, much less work has been
done with heterogeneous catalysts for the alkylation of amines
using the amine as an electrophile. Historically, nickel was the first
metal catalyst used to perform the N-alkylation of amines with
other amines as the source of electrophiles. Initially, heteroge-
neous catalysts were used in other processes such as the selective
nitrile hydrogenation where the N-alkylation of amines by other
amines was observed as an unwanted side reaction. However, later
this reaction gained the attention of the scientific community.
During nitrile hydrogenation to obtain primary amines, symmetri-
cal secondary amines were formed in low yields (2–47%). This
reaction was performed at 20 atm of hydrogen and 110–130 °C in
2
. Experimental
2
.1. Synthesis of Cu/Al O catalysts
2 3
tetralin or decalin [21]. A copper-chromite catalyst (CuCr
2 4
O –
BaCr 93 mol%) was reported for the synthesis of the symmetri-
2 4
O
In a typical synthesis for a 5 wt% Cu/Al
ate monohydrate (312 mg/1.56 mmol), aluminum tri-sec-butoxide
9.54 ml/37.2 mmol) and 2-butanol (2 ml) were added together.
The resulting gel was stirred at 100 °C for 3 h to give a light blue
suspension, following the dropwise addition of deionized water
2 3
O sample, copper acet-
cal secondary amine dilaurylamine (didodecylamine) by hydro-
genation of lauramide at 220 °C under 340–365 atm of hydrogen
pressure, and didodecylamine was obtained in moderate yield
(
(
52%) [22]. Processes involving amines as the electrophiles were
also reported over Pd [23–25] and Pt [26,27] based catalysts.
When benzyl amine was treated with palladium black (5 mol%)
at 80 °C, 41% of dibenzylamine was formed after 12 h, together
with 41% of the corresponding imine [23]. Shimizu’s group [27]
reported that alumina-supported Pt nanoclusters with average size
of 0.8 nm were an effective catalyst for mono-N-alkylation of ami-
(
4.5 ml). After stirring for another hour at 100 °C, the suspension
was cooled to room temperature and filtered. The solid catalyst
was washed with acetone, dried at 100 °C and calcined for 1 h at
6
00 °C (10 °C/min ramp). Prior to use, the catalyst was activated
in flowing H at 300 °C for 60 min. The metal content was deter-
2
mined by ICP-AES.
Several other 5 wt% Cu samples were prepared for comparison
by means of a conventional wet impregnation method. Various
2
nes with various amines. (iPr) NH was identified to be a good
source of electrophiles to convert various amines to N-isopropyl
derivatives and give high yields (70–96%). In a subsequent publica-
tion, the same authors showed that alumina-supported Pd with an
average Pd cluster size of 1.8 nm was a highly effective catalyst for
N-alkylation of benzylamines with cyclic secondary amines [24].
Under the conditions of the reaction, the self cross coupling pro-
duct, dibenzylamine, was also formed as an important
by-product with a yield of 4–26%. Coordinatively unsaturated Pd
atoms were found to be necessary for good activity in this sterically
demanding reaction.
supports were used, including commercially available
2 3
c-Al O
(
Merck). These samples were prepared by slurrying 0.95 g of the
support with 15 ml of an aqueous solution of copper acetate mono-
hydrate (0.157 g). The suspension was stirred for 4 h at room tem-
perature before increasing the temperature to 90 °C to evaporate
the water. The slightly wet residue was left to dry overnight at
1
H
00 °C before calcination for 1 h at 600 °C and activation in flowing
at 300 °C for 60 min.
2
We have described a procedure for the synthesis of secondary
amines starting from the corresponding primary amines or
2.2. Procedure for self-coupling of amine
2 3
nitroarenes using Ag/Al O catalysts [9]. Using this method, we
successfully achieved the N-alkylation of amines with different
alcohols as the electrophile. Here, we present a new methodology
for the synthesis of secondary amines following a similar scheme.
In analogy to the amination of alcohols, the reaction occurs
The catalytic self-coupling of benzyl amine was carried out
under 1 atm He at 150 °C in a 22 ml glass liner inside an autoclave.
Typically, 2 mmol of amine was dissolved in 5 ml p-xylene and
0.150 g catalyst was added. The autoclave with the reaction mix-
ture was purged with He three times and then filled with 1 atm
through
a
hydrogen-borrowing mechanism. The hydrogen