Hydrogenation and: N-Alkylation of anilines and imines via transfer hydrogenation with homogeneous nickel compounds

Article abstract of DOI:10.1039/c9dt04111g

The nickel-catalyzed N-Alkylation of a variety of arylamines via transfer hydrogenation in the absence of pressurized hydrogen and basic or acidic additives was achieved in a tandem reaction. This process was further extended to the CN bond reduction and N-Alkylation of a variety of imines with ethanol, the latter acting as a hydrogen and acetaldehyde source, which allowed for the reduction and subsequent condensation to yield the corresponding N-Alkylated products.

Full text of DOI:10.1039/c9dt04111g

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PAPER
Hydrogenation and N-alkylation of anilines and
imines via transfer hydrogenation with
homogeneous nickel compounds†
Cite this: Dalton Trans., 2019, 48,
17579
G. Eliad Benitez-Medina
and Juventino J. García
*
The nickel-catalyzed N-alkylation of a variety of arylamines via transfer hydrogenation in the absence of
pressurized hydrogen and basic or acidic additives was achieved in a tandem reaction. This process was
further extended to the CvN bond reduction and N-alkylation of a variety of imines with ethanol, the
latter acting as a hydrogen and acetaldehyde source, which allowed for the reduction and subsequent
condensation to yield the corresponding N-alkylated products.
Received 21st October 2019,
Accepted 7th November 2019
DOI: 10.1039/c9dt04111g
rsc.li/dalton
2
5
involves a TH process), with moderate to good yields. Also,
Kirchner and co-workers reported the alkylation of aryl-
Introduction
Metal-catalyzed CvN bond reduction and tandem alkylation amines with primary alcohols in the presence of a base
1
,2
(
through a transfer hydrogenation (TH) process) has been (t-BuOK) using Co(II)-catalysts that were stabilized by PCP
1
3
primarily studied with compounds containing precious metals pincer ligands.
3
4–7
8
7,9,10
such as Pd, Ru,
Rh, and Ir,
but is much less devel-
Our group has also been interested in the reduction of
imines with the first-row transition-metal compounds in low
1
1–14
oped using cheap, first-row transition-metal compounds.
1
5,16
Preparation of secondary and tertiary amines
is of rele- oxidation states, particularly with the use of Ni(0), pressurized
vance because they are valuable building blocks in the fine hydrogen and methanol as a solvent, where no evidence of
1
5,16
17,18
19
chemical industry
pharmaceuticals,
and for fungicides,
additives,
the involvement of the solvent in the N-alkylation of fluoro-
2
0,21
22
24
and agrochemicals. These secondary imines has been found. The amino reduction reactions of
and tertiary amines are usually synthesized using conventional imines and anilines along with N-alkylation reactions have
organic methodologies such as amine alkylation through been reported with a few homogeneous and heterogeneous
2
6–31
nucleophilic substitution with toxic reagents that include aryl- nickel catalysts.
Ni-RANEY® catalyzed reductive
or alkyl-halides. The use of precious metals, besides their high N-alkylation of amines and anilines was reported by Rice
2
8
27
cost and limited availability, is associated with high et al. and Ainsworth using alcohols as sources of hydro-
1
,2,23
toxicity,
for which it is necessary to replace them with less gen. Recently, Nandan and co-workers have re-visited the
2
toxic or cheap reagents. Thus, the exploration of alternatives reaction with Ni-RANEY® in xylene under reflux conditions
like the first-row transition-metal catalysts for TH reactions is resulting in the hydrogen autotransfer of alcohols.
3
0
attractive due to their availability in the Earth’s crust, and so Regarding the homogeneous process with nickel, Banerjee
far, however, few reports on the N-alkylation of primary et al. reported the N-alkylation of anilines with benzyl
2
9
amines with alcohols catalyzed by Fe, Co and Ni have been alcohol using NiBr as the catalyst precursor and t-BuOK
2
1
1–14,24
published.
as the additive. Barta and co-workers reported a similar
Regarding the relevant reports on the use of Earth-abun- reaction using [Ni(COD)
2
6
2
] and KOH, and these studies
dant metals, Gandon and Bour et al. have recently documen- highlight the need to use bases such as t-BuOK and KOH
ted the synthesis of unsymmetrical tertiary amines using an in the TH process to obtain good conversions and
iron-catalyzed system by a process of ethylation of imines selectivities.
with ethanol through a borrowing hydrogen process (which
We disclose herein our results dealing with the N-alkylation
of anilines and the CvN bond reduction and N-alkylation of
2
imines, with nickel catalysts, without basic additives and H .
Facultad de Química, Universidad Nacional Autónoma de México,
México City 04510, Mexico. E-mail: juvent@unam.mx
These experiments used P-donor ancillary ligands and several
alcohols, such as ethanol, methanol and 2-propanol, which
acted as solvents and reagents in the reaction.
†
Electronic supplementary information (ESI) available: Selected IR and NMR
spectra and GC-MS determination. See DOI: 10.1039/C9DT04111G
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gen and aldehyde), at 150 °C and 18 h of reaction time as the
optimal conditions.
Results and discussion
Optimization of reaction conditions
Using the optimal reaction conditions, a variety of substi-
Initially, we assessed a variety of alcohols as solvents and tuted anilines were assessed in the N-alkylation reaction.
reagents, with the [Ni(COD) ]/dippe system, COD = 1,5-cyclo- Toluidine substrates were employed (entries 2–4, Table 2) which
2
octadiene, and using aniline as the substrate. We considered the resulted in 100% conversion and good selectivity towards the
3
2
conditions used in previous work of our group; the catalytic N-monoalkylated products (2b, 2c and 2d). Entry 3 was,
system was composed of 2 mol% [Ni(COD) ] and 3 mol% however, the exception. In entry 3 the selectivity was found to
2
dippe.
At 120 °C and with 2 mol% loading of [Ni(COD)
be distributed between N-ethylaniline 2c and N,N-diethylaniline
], the con- 3c (85% and 15% respectively). Using m-fluoroaniline, good
2
version was low (27%), but completely selective for conversion and selectivity were obtained and the presence of a
N-ethylaniline 2a (Table 1, entry 1). At 130 °C the conversion minute amount of the corresponding enamine, which led to the
increased to 50%, with 2a as the only product (Table 1, entry formation of the N-dialkylated product 2ee, was observed by
2
), and at the same temperature of reaction, but using 4 mol% GC-MS (Table 2, entry 5). The use of chlorinated anilines
[
Ni(COD) ], changes in conversion and selectivity were not sig- (Table 2, entries 6–8), o-chloroaniline and p-chloroaniline
2
nificant (Table 1, entry 3). A full conversion was obtained by (Table 2, entries 6 and 8), resulted in low to fair conversions of
maintaining the catalytic loading at 2 mol%, but increasing these substrates (30% and 55%, respectively), likely due to
the reaction temperature to 150 °C (24 h) (Table 1, entry 4). the deactivating character of their substituents. This is in con-
Under these conditions, the selectivity of N-mono-ethylated trast to m-chloroaniline, which achieved 74% conversion
aniline 2a and N,N-di-ethylaniline 3a was 73% and 27%, (Table 2, entry 7). The electron withdrawing groups in the
respectively.
ortho-position of the phenyl ring were found to affect the
A reduction in the reaction time from 24 h to 18 h resulted reduction process; these electron withdrawing groups were less
3
3
in 100% conversion and selectivity for the N-monoalkylated effective than those in the meta- and para- positions. In
product 2a (Table 1, entry 6) (under the same reaction con- addition, when using methoxyanilines, good conversions and
ditions). Importantly, the conversion was lower when 2-propa- selectivities were obtained and were comparable to the conver-
nol and methanol were used as solvents (Table 1, entries 7 and sions obtained with o-, m-, and p-toluidine, due to their similar
8
), and this may be due to the low stability of the corres- behavior to electron-donating substituents (Table 2, entries
ponding intermediates (imines or enamines) involved when 9–11).
using these solvents.
Considering the above results and the previous studies of
2
4,32–36
In addition, a mercury drop test was performed (Table 1, our group and others,
we present a mechanistic propo-
entry 9), which resulted in 94% conversion, thus not causing a sal in Scheme 1 that is initiated by the oxidative addition
significant drop in conversion. Therefore, we conclude that of EtOH to [Ni(COD)(dippe)] (formed in situ) to produce
this catalytic system is likely to be homogeneous and we con- “[Ni(dippe)(H)
sider the use of a catalytic loading of 2 mol% [Ni(COD) ], respective enamine 1-A from aniline. This enamine is then co-
mol% dippe, ethanol as the solvent (and the source of hydro- ordinated to A to form the imine-complex B, and then the
2
]” A and acetaldehyde, which then form the
2
3
N-monoalkylated product 2 is obtained through the insertion
of the enamine into the nickel hydride to yield C, followed by
the reductive elimination of the product and the oxidative
addition of EtOH to re-form complex A.
a
Table 1 Optimization of N-alkylation reaction
A similar methodology was used for closely related imines
that were synthesized in situ from p-tolualdehyde and p-tolui-
dine. The best reaction conditions for this reaction were found
to be 24 h and 150 °C (Table 3, entry 4), with a selectivity of
94% for product 5d.
Considering the reaction conditions determined for entry 4,
1
Entry T (°C) t (h) R -OH
% Conversion Selectivity 2a : 3a
other alcohols were assessed (Table 3, entries 6 and 7). We
determined that the reaction in methanol resulted in a conver-
sion of 83% with a selectivity of 81% for amine 5d and with
-propanol the conversion was modest (78%), with good
selectivity for 5d (97%).
1
2
3
4
5
6
7
8
9
120
130
130
150
150
150
150
150
150
24
24
24
24
20
18
18
18
18
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
2-Propanol
EtOH
27
50
40
100
100
100
13
100 : 0
100 : 0
100 : 0
73 : 27
92 : 8
100 : 0
100 : 0
100 : 0
100 : 0
b
2
Because our interest was in obtaining the N-alkylated
product (7d), we decided to use a higher catalytic loading of
4
94
c
5 mol% [Ni(COD) ] and 7 mol% dippe and a 48 h reaction time.
2
Under these conditions, the conversion was 99% and there was
low selectivity between amine 5d, enamine 6d, and N-ethylamine
7d (64%, 1%, and 35% respectively) (Table 3, entry 8).
a
General conditions: 1 mmol of aniline, [Ni(COD)
2
] (2 mol%), dippe
b
(
(
3 mol%) and dry ethanol (10 mL). [Ni(COD) ] (4 mol%), dippe
6 mol%). Mercury drop test.
2
c
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a
Table 2 (Contd.)
Table 2 Substrate scope for anilines
Entry
Substrate
% Conversion
100
% Selectivity
Entry
1
Substrate
% Conversion
100
% Selectivity
10
2
3
4
100%
100
11
100
a
General conditions: 1 mmol of aniline, [Ni(COD)
3 mol%) and dry ethanol (10 mL).
2
] (2 mol%), dippe
(
100
An increase in temperature and reaction time improved the
selectivity towards the production of 7d (Table 3, entries 9 to
2). The addition of molecular sieves at 170 °C resulted in
better selectivity of 7d over 5d (89% and 11%, respectively)
Table 3, entry 12). Finally, the use of dcype (1,2-bis(dicyclo-
1
5
100
(
hexylphosphino)ethane) as the ancillary ligand (instead of
dippe) resulted in 100% conversion and 97% selectivity
towards 7d (Table 3, entry 13).
With these optimal reaction conditions in hand, we
assessed the reactivity of closely related substrates that are
depicted in Scheme 2. At 170 °C the reaction resulted in very
good conversion and selectivity towards the N-alkylated
product with ethanol (Scheme 2, b–d). It is noteworthy that the
use of o-toluidine resulted in a very low yield, and this is prob-
ably due to steric effects.
6
7
30
74
The use of other alcohols was also assessed with 2-propanol
and methanol. With these solvents the N-alkylated products
were not observed, and only the production of the corres-
ponding amine (5d) was observed (Scheme 3). For methanol, it
is possible that the corresponding imine formed (6) may be
8
9
55
3
7
unstable; in addition, aliphatic alcohols are more difficult to
3
8
activate than aromatic or unsaturated alcohols. Also, among
methanol and ethanol, methanol has a relatively high dehydro-
−
1
genation energy (ΔH = +84 kJ mol ) with respect to ethanol
100
−1 39
(
ΔH = +68 kJ mol ). However, because the reduction of
imine was observed, the low selectivity towards the
4
N-alkylated product would be related to the instability of the
iminic intermediate 6, rather than to the dehydrogenation of
the alcohol. Regarding 2-propanol, the produced ketone after
TH does not form the corresponding imine, and likely a Lewis
acid is required to promote the condensation of the aniline
4
0
and the ketone.
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Scheme 1 Mechanistic proposal for the N-alkylation TH-process.
a
Table 3 The reduction and N-alkylation of imines
1
Entry
T/°C
t (h)
R -OH
% Conv.
5d : 6d : 7d : 1d : 2d : 3d
1
2
3
4
5
6
7
100
130
150
150
150
150
150
150
160
160
160
170
170
48
48
48
24
14
24
24
48
24
48
48
48
48
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
2-Propanol
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
0
46
93
98
7
83
78
99
100
100
100
100
100
—
91 : 0 : 2 : 0 : 7 : 0
94 : 1 : 0 : 0 : 5 : 0
94 : 2 : 1 : 0 : 3 : 0
97 : 0 : 3 : 0 : 0 : 0
81 : 2 : 0 : 2 : 1 : 14
97 : 0 : 0 : 3 : 0 : 0
64 : 1 : 35 : 0 : 0 : 0
56 : 0 : 42.8 : 0.2 : 1
60 : 6 : 34 : 0 : 0 : 0
12 : 0 : 87 : 0 : 0 : 1
11 : 0 : 89 : 0 : 0 : 0
3 : 0 : 97 : 0 : 0 : 0
b
8
9
b
b
1
1
1
1
0
1
2
3
b,c
b,c
c,d
a
c
b
General conditions: 1 mmol of imine, [Ni(COD)
Molecular sieves (4 Å) were employed as the desiccant. 5 mol% [Ni(COD)
2
] (2 mol%), dippe (3 mol%) and dry ethanol (10 mL). 5 mol% [Ni(COD) ], 7 mol% dippe.
2
d
2
], 7 mol% dcype.
Because the reduction of imines and further alkylation would ology that has been previously used in our group and adapted it
2
4,32,33
involve the coordination of the –CvN– moiety to the nickel to the current case. This is presented in Scheme 4.
center, monitoring this reaction by NMR was used to identify
3
1
1
The corresponding P{ H}-NMR spectrum of the reac-
some of the key intermediates. Thus, we used a similar method- tion mixture confirmed the presence of both compounds I
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Scheme 2 Substrate scope for reduction and N-alkylation.
Scheme 3 Scope of alcohols in the reduction of 4.
2
Scheme 4 Formation of [(dippe)Ni(η -C,N)-PhHC = NPh] (II).
and II (Fig. 1), with doublets at 65.15 ppm and 71.55 ppm, singlets were observed at 51.59 ppm and 70.14 ppm and
with a JP–P of 61 Hz, assigned to complex II, in which were assigned to free dippe and [Ni(dippe)(COD)],
2
4,33
the imine is coordinated side-on.
In addition, two respectively.
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3
1
1
8
Fig. 1 The P{ H}-NMR spectrum of a mixture of I and II in THF-d .
1
The H-NMR spectrum (Fig. 2) shows a signature signal at aniline alkylation mechanism presented in Scheme 1 (vide
.97 ppm (dd, JP–H = 7.4 Hz and 2.9 Hz) assigned to the imine supra).
4
moiety coordinated to the metal which is characteristic of the
Here, imine 4 is coordinated to an active species A to
coupling with non-equivalent phosphorus and an up-field yield amine 5 (the right side of the cycle), followed by the for-
3
3
4
1,42
shift of the signal.
Considering the results outlined herein mation of enamine 6 by the condensation reaction with acet-
2
4,32–36
and in previous closely related reports,
a mechanism for aldehyde formed in situ to produce intermediate D, followed
the tandem hydrogenation and subsequent N-alkylation of by hydrogenation and reductive elimination to afford
imines is presented in Scheme 5, which is similar to the product 7.
1
8
Fig. 2 The H-NMR spectrum of a mixture of I and II in THF-d .
17584 | Dalton Trans., 2019, 48, 17579–17587
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Scheme 5 Mechanistic proposal for the amino-reductive and N-alkylation TH process of imines.
before use. All anilines, aldehydes, molecular sieves and
Ni(COD) ] were purchased from Aldrich and stored in the glove
Conclusions
[
2
In summary, a Ni-catalyzed tandem process achieved the box. Deuterated solvents were purchased from Cambridge
N-alkylation of anilines with electron withdrawing and donat- Isotope Laboratories and stored over 4 Å molecular sieves in a
2
ing groups that were found to show excellent conversion glove box. [(dippe)Ni(η -C, N)-PhHC = NPh] was prepared
33
toward N-ethylaniline. Similarly, the reduction of N-di-p-toly- according to methods described in previous reports, and
methanimine 4 in the absence of pressurized hydrogen was N-di-p-tolymethanimine 3 was prepared using the general pro-
1
2,24,33
achieved in the presence of ethanol, methanol and 2-propanol; cedure described elsewhere.
NMR spectra were recorded
however, the imine N-alkylation reaction was carried out at room temperature on a 300 MHz Bruker AVANCE III spectro-
1
through an extra tandem step (only for ethanol), which meter. H NMR spectra (δ parts per million) were reported rela-
1
3
1
allowed the formation of the enaminic intermediate that is tive to the residual protio-solvent. C{ H} spectra showed the
3
1
1
reduced to form the N-ethyl product 7.
characteristic carbon signal of each solvent. P{ H} NMR
chemical shifts (δ parts per million) were reported relative to
external 85% H PO . Coupling constants (J values) are given in
3 4
Hz. The following abbreviations are used for the NMR data: s =
singlet, d = doublet, t = triplet, m = multiplet and br = broad.
Experimental
The catalytic experiments were carried out using standard GC-MS determination was performed using an Agilent
Schlenk techniques in an inert-gas/vacuum double manifold Technologies G3171A equipped with a column comprised of
or under argon (Praxair, 99.998) in an MBraun UniLab glove 5% phenylmethyl silicone, 30 m × 0.25 mm × 0.25 µm; internal
box (<1 ppm H
dard methods, refluxed under an inert atmosphere with mag- lated product, done in triplicate. H and C{ H} NMR spectra
nesium/iodine, and subsequently distilled and stored over of the reduction products were obtained in CDCl . The cata-
2 2
O and O
). The alcohols were dried using stan- standards were used for every sample recovered from the iso-
1
13
1
3
molecular sieves under an argon atmosphere. All liquid lytic experiments were carried out in 4750-125 and 4700-25 mL
reagents were purchased as reagent grade and were degassed stainless-steel Parr reactors.
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General procedure for N-alkylation of anilines
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(
A solution of [Ni(COD) ] (5 mol%) and dcype (7 mol%) in
2
1
mL of THF was added to the imine (1.0 mmol) in 5 mL of
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(
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(
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stirred for 12 h. Subsequently, it was concentrated in vacuo
8
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Acknowledgements
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We thank the UNAM-DGAPA Postdoctoral Scholarship 15 V. Froidevaux, C. Negrell, S. Caillol, J. P. Pascault and
Program for its support. We are grateful for funding from B. Boutevin, Chem. Rev., 2016, 116, 14181–14224.
DGAPA-PAPIT IN-200119 and CONACYT-A-1-S-7657. Also, we 16 P. Kalck and M. Urrutigoïty, Chem. Rev., 2018, 118, 3833–
thank Dr Alma Arévalo for technical assistance. 3861.
17586 | Dalton Trans., 2019, 48, 17579–17587
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