Well-Defined Phosphine-Free Iron-Catalyzed N-Ethylation and N-Methylation of Amines with Ethanol and Methanol

Article abstract of DOI:10.1021/acs.orglett.8b02080

An iron(0) complex bearing a cyclopentadienone ligand catalyzed N-methylation and N-ethylation of aryl and aliphatic amines with methanol or ethanol in mild and basic conditions through a hydrogen autotransfer borrowing process is reported. A broad range of aromatic and aliphatic amines underwent mono- or dimethylation in high yields. DFT calculations suggest molecular hydrogen acts not only as a reducing agent but also as an additive to displace thermodynamic equilibria.

Full text of DOI:10.1021/acs.orglett.8b02080

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Well-Defined Phosphine-Free Iron-Catalyzed N‑Ethylation and
N‑Methylation of Amines with Ethanol and Methanol
Alexis Lator,† Sylvain Gaillard,† Albert Poater,*,‡ and Jean-Luc Renaud*,†
†
́
́
Normandie Universite, LCMT, ENSICAEN, UNICAEN, CNRS, 6 boulevard du Marechal Juin, 14000 Caen, France
‡
̀
̀
Departament de Química, Institut de Química Computacional i Catalisi (IQCC), Universitat de Girona, c/Ma Aurelia Capmany
69, 17003 Girona, Catalonia, Spain
S
* Supporting Information
ABSTRACT: An iron(0) complex bearing a cyclopentadienone
ligand catalyzed N-methylation and N-ethylation of aryl and
aliphatic amines with methanol or ethanol in mild and basic
conditions through a hydrogen autotransfer borrowing process is
reported. A broad range of aromatic and aliphatic amines
underwent mono- or dimethylation in high yields. DFT
calculations suggest molecular hydrogen acts not only as a
reducing agent but also as an additive to displace thermodynamic
equilibria.
methane, dimethyl sulfate, or diazomethane, palladium-form-
aldehyde reductive amination, or reduction of carbamates with
a stoichiometric amount of hydride reagents.1b However, the
low selectivity (due to the formation of overalkylated
products), the employment of toxic and hazardous chemicals,
and the simultaneous production of waste are not in agreement
with an environmentally friendly process. In recent years,
several efficient strategies have been developed for the creation
of nitrogen−carbon bonds and greener strategies for N-
methylation. Transition metal-catalyzed small-molecules acti-
vation such as carbon dioxide,2 carbonate,3 or formic acid4 are
among these recent developments. A hydrogen autotransfer or
borrowing hydrogen strategy for methylation is also a powerful
strategy.5 Advantages of this approach is the use of easy-to-
handle methanol, as a source of methylating agent, and the
formation of water as a unique byproduct. Based on a
simplified mechanism, this one-pot sequence involved a
methanol oxidation step, followed by a condensation of the
amine and reduction of the iminium intermediate. Following
such a strategy, methanol could be considered as a convenient
and sustainable alkylating agent for amine methylation.
However, dehydrogenation of methanol is more energetically
demanding compared to other alcohols (ΔH = +84 kJ.mol‑1 for
methanol vs ΔH = +68 kJ.mol‑1 for ethanol). So the use of
methanol as a methylating agent is still a challenging reaction,
and few examples have been reported so far.6−9
lkylation of amines, notably N-methylation, is a vibrant
field of investigation for pharmaceuticals and industrials,
A
due to their high bioactivity and importance in nature
(Adrenaline) and medicine (antidepressant, anti-Parkinsonian,
painkiller) (Figure 1). Industrial synthesis of N-methylamines
mainly involved formaldehyde via Eschweiller−Clarke type
reactions.1a On the other hand, academic researchers prefer a
traditional stoichiometric methylating agent such as iodo-
Efficient catalysts for N-methylation of amines with
methanol are often based on precious and noble metal-based
complexes such as iridium6 and ruthenium.7 However, due to
Figure 1. Bioactive molecules bearing a N-methyl or N-ethyl amine
moiety.
Received: July 3, 2018
© XXXX American Chemical Society
A
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Letter
economic pressure and engagement in the development of
more sustainable processes, the replacement of these noble
metals by Earth-abundant ones becomes a challenging and
attractive topic. In 2016, Beller and Sortais reported the first
non-noble metal catalyst, involving a PNP pincer type
manganese complex, for monomethylation of anilines in the
presence of a (sub-) stoichiometric amount of a base at 100−
120 °C (Figure 2).8 Liu reported a combination of a cobalt(II)
In our ongoing research on iron-catalyzed reduction of
polarized CC and CX bonds,14,15 we recently presented
several electron-rich (cyclopentadienone) iron tricarbonyl
complexes which are highly active in alkylation of both
aliphatic and aromatic ketones (Fe1, Figure 2).16 Based on in
silico predictions and experimental works, we highlighted the
role of the base both in the dehydrogenation step (decreasing
the barrier of activation) and in the reduction step.15 We
report herein that our complex Fe1 is also an active complex
for the methylation and ethylation of amine derivatives in basic
conditions.
We highlighted recently that the dehydrogenation of ethanol
and isopropanol in basic medium in the presence of Fe1 was
more facile than the hydrogen cleavage.15 Preliminary studies
with ethanol as an alkylating reagent in the presence of cesium
carbonate (10 mol %) and complex Fe1 (2 mol %) at 90 °C
confirmed this result. N-Ethyl anisidine 3a was obtained in
40% conversion. Changing the base to cesium hydroxide and
increasing the temperature to 110 °C led to a full conversion
and a 97% isolated yield (Scheme 1). In these conditions, no
Scheme 1. N-Ethylation of Anilines and Some Aliphatic
a
Amines
Figure 2. Previous works in hydrogen autotransfer methylation using
methanol.
a
salt and a tetraphosphine ligand for the monomethylation of
anilines and secondary aliphatic amines, and dimethylation of
primary aliphatic amines in the presence of a catalytic amount
of K3PO4 at 140 °C (Figure 2).9 However, even if these
complexes paved the way to more sustainable processes, they
are based on expensive phosphorus ligands.
Alkylation of amines and ketones via the borrowing
hydrogen strategy has also been demonstrated in the presence
of the (cyclopentadienone) iron tricarbonyl complexes Fe2
and Fe3 (Figure 2).10−12 However, methylation has been
scarcely investigated, due to the higher enthalpy of
dehydrogenation for methanol compared to higher alcohols.13
General conditions: amine (0.5 mmol), Fe1 (2 mol %), CsOH (10
b
mol %), EtOH (1 mL) for 16 h. Yield was based on isolated product.
preactivation of the complex (under UV-A irradiation or with
Me3NO to remove a CO ligand) was required and the active
species was generated via a Hieber-type reaction.17 Functional
groups such as nitrile, acetal, or ether were tolerated in this
reaction affording compounds 3a-e in good yields (73−99%,
Scheme 1). Benzylamine type derivatives also underwent
ethylation in excellent yields (4a-b, f-g, 73−98%) as
highlighted in Scheme 1. Secondary aliphatic amines were
B
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ethylated with ethanol in good to excellent yield, while primary
amines were dialkylated, as already observed with noble metals
and with the in-situ generated cobalt complex.8
Scheme 2. N-Methylation of Various Anilines Bearing
Different Functional Group
a
Having demonstrated the feasibility of the ethylation of
amines, we wondered whether methanol, the less reactive
alcohol for dehydrogenation, could be a C1-alkylating reagent.
As a model reaction, the methylation of anisidine 1a in
methanol was optimized in the presence of 10 mol % of a base
and 2 mol % of Fe1 (Table 1). Again, cesium hydroxide
a
Table 1. N-Methylation of Anisidine
b
entry
catalyst
base
conv. (%)
c
1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe2
Fe3
Cs2CO3
CsOH
CsOH
CsOH
CsOH
CsOH
CsOH
9
>99
70
63
58
35
0
2
d
3
e
4
f
5
6
7
a
General conditions: anisidine (0.5 mmol), catalyst (2 mol %), base
b
(10 mol %), solvent (1 mL) for 16 h. Conversion was determined by
c
1H-NMR. Catalyst was activated under UV-A (400−315 nm)
d
e
f
irradiation for 30 min. For 8 h. 5 mol % of iron complex. 1 mol %
of iron complex.
provided enhanced conversions compared to cesium carbonate
(entries 1 and 2, Table 1). Shortening the reaction time to 8 h
led to a decrease of the conversion (entries 2 and 3, Table 1).
Finally, deviation of the catalyst loading or the nature of the
complex provided lower conversions (entries 4−7, Table 1).
With the optimized alkylation conditions in hand, we
delineated the scope of the methylation by modifying the
nature of the substituent on the aromatic ring (Scheme 2).
Both electron-donating and electron-withdrawing substituents
were tolerated in this alkylation, and the N-methyl anilines 5
were isolated in moderate to excellent yields (58−99%,
Scheme 2). Alkylation of the halogen-substituted anilines
gave the methylated anilines 5d-g in excellent yields without
any reduction of the carbon−halogen bonds (Scheme 2).
Other functionalities, such as reducible functionalities (nitrile,
ketone), an acetal or a styrene fragment, remained also inert in
these conditions. Heteroaromatic derivatives were also
tolerated, and the methylated amines 5j-k were isolated in
94−99% yields. Noticeably, chemoselective methylation was
obtained with 4-amino-acetophenone and 4-amino-benzophe-
none. The corresponding 4-N-methylamino-acetophenone 5n
and 4-N-methylamino-benzophenone 5l were obtained in 77%
and 96% yields, respectively (Scheme 2). Neither alkylation in
the α-position to the ketone nor reduction of the ketone was
observed.
a
General conditions: aryl amine (0.5 mmol), Fe1 (2 mol %), CsOH
b
(10 mol %), MeOH (1 mL) for 16 h. Yield was based on isolated
product.
reflects the lower reactivity of methanol. In a recent
contribution, Hong reported a ruthenium-catalyzed methyl-
ation of amines with methanol under hydrogen pressure and
demonstrated that hydrogen pressure was necessary to allow
selective dehydration over dehydrogenation of the hemiaminal
intermediate.7g Following this line, methylation of benzylamine
in the presence of Fe1 was achieved (full conversion, 95%
isolated yield, Scheme 3). Deviation of these reaction
conditions (hydrogen pressure, temperature or reaction time)
did not allow methylation of benzylamine (see SI, entries 5−8,
Table S1).
To delineate the scope of this alkylation, various primary
and secondary alkyl amines were evaluated (Scheme 3). The
reactivity of both types of amines is similar, and good to
excellent yields were obtained. As monoalkylation occurs with
aniline, chemoselective methylation of an amino-indole was
obtained (6j: 83% yield). Gratifyingly, (chiral) amino-alcohols
provided the corresponding methylated amines in high yields
(6g: 87% and 6h: 88%, Scheme 3).
To increase the interest of this procedure and demonstrate
its versatility, the scope was extended to aliphatic amines
(Scheme 3). Initial attempts, using the previous reaction
conditions with benzylamine, did not deliver any methylated
product, even by raising the temperature up to 140 °C (see
Supporting Information (SI), entries 1−4, Table S2). Such a
result is in sharp contrast with the ethylation procedure but
To provide a mechanistic framework consistent with our
experiments, DFT calculations were undertaken (Figure 3) and
both amines were considered (aniline as model for aromatic
amines and methylamine for alkyl amines). If the organic steps
C
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are first studied, the condensation between aniline and
formaldehyde generated in situ (vide infra) is a metal free
two-step reaction, which is thermodynamically neutral (2.5 and
0.0 kcal.mol‑1, respectively) but highly kinetically energy
demanding with energy barriers of 29.4 and 39.6 kcal.mol‑1,
respectively (see Figure S66 in the SI). The latter is the rate-
determining step in the overall catalytic cycle, and the solvent
assists both steps. The same process occurs with methylamine.
However, the rate-determining step is then 7.7 kcal.mol‑1
kinetically less demanding (see Tables S89 to S91 in the SI).
Regarding the iron catalytic cycle, the initial Hieber’s type
activation required two steps. A barrierless addition of the
hydroxide on one CO ligand is followed by a hydride
formation. The latter requested a quite energy-demanding
barrier (34.1 kcal.mol‑1; see also Figure S67a in the SI for the
transition state I−II). Overall, this activation is highly
exergonic ( 29.7 kcal.mol‑1). Despite the relatively high
stability of II, methanol interacts with the FeH···OCs moiety
leading to the release of CsOMe with a kinetic cost of 31.4
kcal.mol‑1 (see Figure S67b in the SI for the transition state
II−III) and the unexpected formation of the intermediate
[Fe(CO)2(cyclopentadienone)(η2-H2)] III. Two possible
pathways have then been examined. Either, a hydrogen release
could liberate the unsaturated intermediate VI with an energy
barrier of 9.5 kcal.mol‑1, or a hydrogen cleavage, assisted by a
methanol molecule, could generate the intermediate IV,
overcoming a transition state with an energy barrier of 19.1
kcal.mol‑1 (see Figure S67c in the SI for the transition state
III−IV). Regarding these pathways, the indirect way is 9.6
kcal.mol‑1 more expensive than the direct hydrogen release. In
other words, hydrogen pressure could drive the equilibrium
(III to VI) to the formation of IV, and consequently to the
Scheme 3. N-Methylation of Various Primary and
Secondary Amines
b
a
General conditions: amine (0.5 mmol), Fe1 (2 mol %), CsOH (10
b
mol %), MeOH (1 mL) for 16 h. Yield was based on isolated
product.
Figure 3. Energy profile for Fe1-catalyzed methylation of amines (Gibbs free energies in solvent in kcal.mol‑1; in red, relative energy for transition
states; R = Ph, Me (R = Ph for the energy values included here)).
D
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reduction of the imines.14,18 Dehydrogenation of methanol
could occur then via barrierless interactions between the
ketone function of VI and the cesium cation in CsOMe. The
release of formaldehyde from the iron center is again
barrierless. Finally, reduction of the imine function is a
concerted mechanism with an energy barrier of 19.7 kcal.mol‑1
for aniline derivatives (see Figure S67d in the SI) and 23.1
kcal.mol‑1 for alkylamine derivatives (see Table S84 in the SI).
All these results highlight both an easier condensation of
alkylamines and a lower reducibility of the corresponding
imines. Finally, the methylated amine coordinated the iron
center but can dissociate easily to regenerate VI.
In conclusion, we have developed a general ethylation and
methylation procedure for a broad range of aliphatic and
aromatic amines using ethanol or methanol as an alkylating
reagent in the presence of a well-defined bifunctional iron
complex. This alkylation process provided alkylated amines in
high yields, in the presence of other reducible functionalities.
Both DFT calculations and experimental work unveil some key
features in this process: the lower reactivity of alkylamines
compared to anilines, the role of the hydrogen pressure and of
the base, and finally the participation of the solvent in the
synthesis of some intermediates.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02080.
Preparation details, optimization conditions, NMR
spectra, computational details, DFT calculations (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: jean-luc.renaud@ensicaen.fr.
*E-mail: albert.poater@udg.edu.
ORCID
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Albert Poater: 0000-0002-8997-2599
Jean-Luc Renaud: 0000-0001-8757-9622
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge financial support from the
■
̀
“Ministere de la Recherche et des Nouvelles Technologies”,
́
Normandie Universite, CNRS, and the LABEX SynOrg (ANR-
11-LABX-0029). A.P. thanks the Spanish MINECO for a
project CTQ2014-59832-JIN, and the EU for a FEDER fund
(UNGI08-4E-003).
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