Colloidal and Nanosized Catalysts in Organic Synthesis: XX. Continuous Hydrogenation of Imines and Enamines Catalyzed by Nickel Nanoparticles

Article abstract of DOI:10.1134/S1070363218100018

Nickel nanoparticles on the BAU-A active carbon or NaX zeolite catalyze hydrogenation of imines and enamines in a flow reactor in a gas phase or in a gas–liquid–solid catalyst system. The process occurs at atmospheric pressure of hydrogen and gives secondary or tertiary amines in a high yield.

Full text of DOI:10.1134/S1070363218100018

ISSN 1070-3632, Russian Journal of General Chemistry, 2018, Vol. 88, No. 10, pp. 2035–2038. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © Yu.V. Popov, V.M. Mokhov, S.E. Latyshova, D.N. Nebykov, A.O. Panov, T.M. Davydova, 2018, published in Zhurnal Obshchei
Khimii, 2018, Vol. 88, No. 10, pp. 1585–1589.
Colloidal and Nanosized Catalysts in Organic Synthesis:
XX.1 Continuous Hydrogenation of Imines and Enamines
Catalyzed by Nickel Nanoparticles
Yu. V. Popova*, V. M. Mokhova, S. E. Latyshovaa, D. N. Nebykova,
A. O. Panova, and T. M. Davydovaa
a Volgograd State Technical University, pr. Lenina 28, Volgograd, 400131 Russia
*e-mail: tons@vstu.ru
Received May 8, 2018
Abstract—Nickel nanoparticles on the BAU-Aactive carbon or NaX zeolite catalyze hydrogenation of imines and
enamines in a flow reactor in a gas phase or in a gas–liquid–solid catalyst system. The process occurs at
atmospheric pressure of hydrogen and gives secondary or tertiary amines in a high yield.
Keywords: nickel nanoparticles, hydrogenation, enamines, imines, zeolite
DOI: 10.1134/S1070363218100018
Synthesis of secondary and tertiary amines via
reductive amination of carbonyl compounds has been
widely described [2]. The first stage of the process is
the condensation of an aldehyde or ketone with a
primary amine affording an imine or with a secondary
amine yielding an enamine. Such reactions are often
performed separately on available catalysts. However,
hydrogenation of imines and enamines with hydrogen
occurs under harsh conditions, and in the laboratory
conditions it proceeds easily using complex metal
hydrides as a reductant [3, 4]. For example,
hydrogenation of enamines with hydrogen in the
presence of rhodium metal complex catalysts occurs in
methanol at atmospheric pressure within 10 h [5].
Reductive amination of aldehydes and ketones with
secondary amines and hydrogen using the same
complexes occurs at a pressure up to 50 atm [6].
Hydrogenation of enamines on palladium and platinum
catalysts at atmospheric pressure has been studied as
well [7, 8]. It has been shown that colloidal nickel
particles catalyze hydrogenation of enamines with
hydrogen under sufficiently mild conditions [9].
temperature of 100°С or on copper chromite at 68 at
and 150°С [11], using iridium complexes at 1–3 at
[12, 13], with metal platinum at 2 at [14], or at 1–
100 at in the presence of various rhodium, iridium,
ruthenium, and titanium complexes [15].
Titanocene catalysts have revealed activity in the
hydrogenation of a range of acyclic as well as cyclic
imines. High selectivity (95–99%) have been achieved
for a series of cyclic imines in the presence of 1 mol %
of Ti at 10–15 at. However, acyclic imine substrates
have been obtained as mixtures of isomers, leading to
the decrease in selectivity (53–78%) when using the
same catalyst [16].
Reduction of imines in the presence of metal
nanoparticles or complexes using isopropanol or
nascent hydrogen as the reducing agent [17–19] as
well as via bubbling hydrogen through a solution of
imine under catalysis with nickel nanoparticles [20]
has been described.
This study aimed to investigate the continuous hydro-
genation of imines and enamines in plug-flow reactor
reactor with gaseous hydrogen under mild conditions
using nickel nanoparticles immobilized on a solid
support as the catalyst. The reaction was performed in
a stream of 12–120-fold excess of hydrogen under
atmospheric pressure and a temperature of 140–180°C
in the presence of the catalyst. The BAU-A active
Several approaches to catalytic hydrogenation of
imines are known. For example, the С=N bond of
aldimines and ketimines can be reduced with hydrogen
on the Raney nickel at a pressure of 7 at and a
1
For communication XIX, see [1].
2035

2036
POPOV et al.
R4
Scheme 1.
Scheme 2.
R4
H
R2
R2
N
R2
H2 (1 atm), 140−200οC
H2 (1 atm), 140−180οС
N
R2
R1
R1
N
N
R1
R1
Ni0
R3
4а−4f
Ni0
R3
3а−3f
R3
1a, 1b
R3
2a, 2b
R1 = Ph, R2 = H, R3 = Et (а), R1 = t-Bu, R2 = H, R3 = i-Pr
(b), R1 = Bu, R2 = H, R3 = Pr (c), R1 = 1-C6H13, R2 = H,
R3 = Pr (d), R1 = 4-EtPh, R2 = H, R3 = Pr (e), R1 = 2-EtPh,
R2 = H, R3 = Pr (f).
R1 = R2 = CH3, R3 = H, R4 = CH2, (1а, 2а), R1 = CH3, R2 =
R3 = H, R4 = CH2O (1b, 2b).
carbon (Ni0/Сact) or NaX zeolite (Ni0/NaX) served as
the catalyst support. The catalyst was obtained via
impregnation of the carrier with aqueous solution of
nickel(II) chloride during 5–6 h followed by filtering
off and treatment with aqueous solution of sodium
borohydride at 20–25°С during 10–12 min; the size of
nickel particles formed at the carrier surface was 70–
120 nm in the case of carbon [21] and 25–60 nm in the
case of zeolite. The moist catalyst was charged in the
reactor and dried in the hydrogen stream directly
before the reaction. The laboratory-scale reactor was a
metallic pipe of the inner diameter 9 mm and heating
zone height 20 mm placed in an electric oven. The
catalyst layer surrounded by the inert filler (quartz
packing) occupied the middle part of the reactor.
tions was 76%. Hydrogenation of enamine 1b with the
Ni0/NaX catalyst at 160°С and 8-fold excess of hyd-
rogen led to the formation of the product 2b in 90% yield.
Aldimines and ketimines are further intermediates
in the reductive amination of carbonyl compounds.
Hydrogenation of imines was performed in a plug-flow
reactor at 140–180°С with 12–20-fold excess of
hydrogen at its atmospheric pressure in the presence of
both above-mentioned catalysts. Imines 3a–f served as
the substrates; their hydrogenation afforded the
corresponding secondary amines 4a–f in a yield up to
98% (Scheme 2).
It was found that the Ni0/Сact catalyst was efficient
in the reactions of the formation of N-propylaniline or
p-ethyl-N-butylaniline (yield 98 and 75%, respectively,
at 180°С).
Scanning electron microscopy study of the surface
of the catalyst prepared via nickel ions reduction on
the zeolite (Ni0/NaX) revealed the presence of 23–
60 nm nickel nanoparticles. The composition of the
surface part of the catalyst was determined by means
of energy-dispersive X-ray analysis: the content of
nickel in the regions of the nickel nanoparticles location
was 27–35%, being 19% over entire support surface.
The effect of the substituent location in an aromatic
ring was studied using p- and o-ethyl-N-butylanilines.
It was found that ethyl group in the ortho-position
significantly reduced the yield of the target product
(secondary amine) in the hydrogenation reaction: the
yield of p-ethyl-N-butylaniline 4e was 75%, that of o-
ethyl-N-butylaniline 4f being 7%, other conditions
being the same.
N-(2-Methylprop-1-enyl)pyrrolidine 1a and N-(prop-
1-enyl)morpholine 1b were used as the hydrogenation
substrates. The reaction was performed at the enamines
1a, b loading 3.6 mL/h and the hydrogen loading 2 L/h
at 140–160°С, the catalyst content being 0.5 g (Ni0/Сact)
or 2 g (Ni0/NaX). The composition of the resulting
mixture was determined by means of chromato–mass
spectrometry.
Hydrogenation of enamine 1a using the Ni0/Сact as
the catalyst resulted in the almost complete conversion
of the substrate (98%) and the yield of the cor-
responding amine 2a 98% (Scheme 1). Hence, the
Ni0/Сact catalyst revealed high activity in the preparation
of cyclic tertiary amines. When using the Ni0/NaX
catalyst, the yield of amine 2a under the same condi-
Synthesis of N-butylhexylamine was performed at
180°С using the Ni/NaX catalyst. The conversion of
the starting imine was almost complete (99.8%), but
the formation of the side products decreased the yield
of the target product 4d to 25%. The side products
included tributylamine (30%) and dibutylhexylamine
(15%).
Hydrogenation of isobutyl-tert-butylimine 3b was
performed at 140°С using the Ni/Cact catalyst. The
yield of the target secondary amine was 68% at the
imine conversion 87%. Using the Ni0/NaX catalyst, the
conversion of the sterically hindered 3b could not be
improved; the yield of amine 4b was 42% at 200°С.
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COLLOIDAL AND NANOSIZED CATALYSTS IN ORGANIC SYNTHESIS: XX.
2037
Nevertheless, the Ni0/NaX catalyst afforded hydro-
genation of sterically non-hindered imines with high
yields. For example, hydrogenation of butylidene-
butylimine 4c at 200°С gave dibutylamine in 95% yield.
a Watson Marlow 120S peristaltic pump. The reactor
temperature was monitored using
thermocouple.
a
TKhA
N-Isobutylpyrrolidine (2a). a. Catalyst Ni0/Сact
mass 0.5 g, temperature 140°С, hydrogen feeding rate
4 L gcat–1 h–1, enamine 1a feeding rate 3.6 mL gcat–1 h–1.
The enamine conversion 98%, selectivity 100%, yield of
the target product 98%. Mass spectrum, m/e (Irel, %):
129.0 (5) [M + 2]+, 128.0 (60) [M + 1]+, 126.1 (17)
[M – 1]+, 125.2 (3.5), 84.2 (100), 83.2 (17.5), 82.2 (7).
In summary, the elaborated method allowed hydro-
genation of azomethines and enamines at atmospheric
pressure of hydrogen with the target products yield up
to 98% avoiding the use of expensive and difficultly
available catalysts. Nanodisperse metals and their
oxides are known to catalyze certain condensation
reactions [22, 23], therefore it seems promising to
investigate the possibility of single-stage reductive
amination of carbonyl compounds with primary or
secondary amines using a heterogeneous catalyst based
on nickel nanoparticles, stable against deactivation
with the formed water. This will aid in development of
a technological and efficient approach to the synthesis
of a series of secondary and tertiary amines.
b. Catalyst Ni0/NaX mass 2 g, temperature 180°С,
–1
hydrogen feeding rate 1 L gcat h–1, enamine 1a
feeding rate 3.6 mL gcat–1 h–1. The enamine conversion
76%, selectivity 100%, yield of the target product 76%.
N-Propylmorpholine (2b). Catalyst Ni0/NaX mass
2 g, temperature 160°С, hydrogen feeding rate 1 L gcat–1 h–1,
–1
enamine 1b feeding rate 0.9 m L gcat h–1. The
enamine conversion 90%, selectivity 100%, yield of the
target product 90%. Mass spectrum, m/e (Irel, %): 128.8
(7) [M + 2]+, 128.0 (23) [M + 1]+, 127.1 (2) [M]+, 99.9
(100), 84.0 (3), 72.1 (6), 70.0 (18), 56.0 (8), 42.0 (6.5).
EXPERIMENTAL
Chromato–mass spectroscopy analysis was performed
using a Saturn 2100 T/GC3900 instrument (EI, 70 eV).
Scanning electron microscopy studies were performed
using a FEI Versa 3D DualBeam instrument.
Elemental analysis of the catalyst was performed by
means of energy-dispersive X-ray spectroscopy.
N-1-Propylaniline (4a). Catalyst Ni0/Сact mass 0.5 g,
temperature 180°С, hydrogen feeding rate 4 L gcat–1 h–1,
–1
imine 3a feeding rate 3.6 mL gcat h–1. The imine
conversion 99.8%, selectivity with respect to the amine
98%, yield of the target product 98%. Mass spectrum, m/e
(Irel, %): 136.9 (7) [M + 2]+, 136.0 (59) [M + 1]+, 135.0
(50) [M]+, 107.0 (9), 106.0 (100), 79.0 (11), 77.0 (12),
51.0 (7).
Catalysts preparation was carried out via impreg-
nation of a solid support (active carbon or 1–1.2 mm
fraction of the NaX zeolite) with aqueous solution of
nickel(II) chloride hexahydrate during 5–6 h (0.2 g of
the nickel salt per 0.5 g of the carbon or 0.5 g of the
nickel salt per 2 g of the zeolite), filtration and
washing of the modified carrier, and its treatment with
aqueous solution of sodium borohydride at 20–25°С
during 10 min. The so prepared moist catalyst was
loaded in the reactor and dried in a hydrogen stream at
120–140°С just prior to the reaction.
Isobutyl-tert-butylamine (4b). a. Catalyst Ni0/Сact
mass 0.5 g, temperature 140°С, hydrogen feeding rate
–1
–1
4 L gcat h–1, imine 3b feeding rate 3.6 mL gcat h–1.
The imine conversion 87%, selectivity with respect to the
amine 77%, yield of the target product 68%. Mass spec-
trum, m/e (Irel, %): 130.8 (5) [M + 2]+, 129.9 (45) [M +
1]+, 127.8 (1) [M]+, 115.0 (10.4), 114.0 (100), 85.8
(6.5), 74.0 (1), 58.0 (37).
Hydrogenation (general procedure). The reaction
was performed in a plug-flow reactor at atmo-spheric
pressure at temperature 140–200°С. Under these
conditions the reaction could proceed in the gas phase
as well as in the gas–liquid–solid catalyst system,
depending on the boiling point of the substrate. The
laboratory-scale reactor was a 12Kh18N10T steel pipe
of internal diameter 9 mm and the heating zone height
50 mm placed in an electric oven. The catalyst was
loaded into the reactor, and the stream of hydrogen
was simultaneously applied (the hydrogen feeding rate
was controlled using a GV-7 hydrogen generator);
liquid substrates 1a, 1b, or 3a–3f were supplied using
b. Catalyst Ni0/NaX mass 2 g, temperature 200°С,
–1
hydrogen feeding rate 1 L gcat h–1, imine 3b feeding
rate 3.6 mL gcat–1 h–1. The imine conversion 48%, selec-
tivity with respect to the amine 87%, yield of the target
product 42%.
Di-n-butylamine (4c). Catalyst Ni0/NaX mass 2 g,
temperature 180°С, hydrogen feeding rate 1 L gcat–1 h–1,
–1
imine 3c feeding rate 0.9 mL gcat h–1. The imine
conversion 97%, selectivity with respect to the amine
98%, yield of the target product 95%. Mass spectrum,
m/e (Irel, %): 130.0 (26) [M + 1]+, 128.7 (2) [M]+, 44.1
(100), 85.9 (39), 41.1 (31), 42.0 (17), 57.0 (62), 43.0 (5).
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2038
POPOV et al.
vol. 61, p. 3849. doi 10.1021/jo960057x
N-Butylhexylamine (4d). Catalyst Ni0/NaX mass 2 g,
temperature 180°С, hydrogen feeding rate 1 L gcat–1 h–1,
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–1
imine 3d feeding rate 0.9 mL gcat h–1. The imine
conversion 99.8%, selectivity with respect to the target
amine 25%, yield of the target product 25%. Mass
spectrum, m/e (Irel, %): 158.8 (12) [M + 2]+, 158.0
(100) [M + 1]+, 113.8 (3), 85.8 (3), 44.2 (57), 41.2 (3).
Selectivity with respect to tri-1-butylamine 30%, yield
30%. Mass spectrum, m/e (Irel, %):186.9 (11) [M + 2]+,
186.0 (100) [M + 1]+, 185.0 (3) [M]+, 114.2 (6), 113.2
(5), 44.2 (80), 41.2 (6). Selectivity with respect to di-
butylhexylamine 15%, yield 15%. Mass spectrum, m/e
(Irel, %): 214.8 (11) [M + 2]+, 214.1 (100) [M + 1]+,
115.1 (3), 114.2 (22), 113.2 (4), 44.2 (60), 41.2 (5).
p-Ethyl-N-butylaniline (4e). Catalyst Ni0/Сact mass
0.5 g, temperature 180°С, hydrogen feeding rate
–1
–1
4 L gcat h–1, imine 3e feeding rate 3.6 mL gcat h–1.
The imine conversion 95%, selectivity with respect to
the target amine 77%, yield of the target product 74%.
Mass spectrum, m/e (Irel, %): 178.8 (10) [M + 2]+,
177.8 (100) [M + 1]+, 176.9 (28) [M]+, 176.0 (5) [M –
1]+, 134.1 (21), 106.0 (4). Selectivity with respect to p-
ethyl-N,N-dibutylaniline 23%, yield 22%. Mass spec-
trum, m/e (Irel, %): 234.0 (2) [M + 1]+, 233.0 (15) [M]+,
232.1 (100) [M – 1]+, 175.0 (8), 174.0 (5), 160.0 (5),
129.8 (2).
o-Ethyl-N-butylaniline (4f). Catalyst Ni0/Сact mass
0.5 g, temperature 180°С, hydrogen feeding rate
–1
–1
4 L gcat h–1, imine 3f feeding rate 3.6 mL gcat h–1.
The imine conversion 75%, selectivity with respect to
the target amine 11%, yield of the target product
7%.Mass spectrum, m/e (Irel, %): 178.8 (11) [M + 2]+,
177.8 (100) [M + 1]+, 177.2 (48) [M]+, 176.3 (8) [M –
1]+, 134.0 (29), 105.8 (5). Selectivity with respect to
o-ethyl-N,N-dibutylaniline 47%, yield 47%. Mass
spectrum, m/e (Irel, %): 232.0 (2) [M – 1]+, 231.0 (16),
230.0 (100), 229.3 (68), 214.2 (30), 200.2 (9), 131.9
(9), 106.0 (6).
CONFLICT OF INTERESTS
No conflict of interests was declared by the authors.
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