Carbonylation of C?N Bonds in Tertiary Amines Catalyzed by Low-Valent Iron Catalysts

Article abstract of DOI:10.1002/anie.201903740

The first iron catalysts able to promote the formal insertion of CO into the C?N bond of amines are reported. Using low-valent iron complexes, including K₂[Fe(CO)₄], amides are formed from aromatic and aliphatic amines, in the presence of an iodoalkane promoter. Inorganic Lewis acids, such as AlCl₃ and Nd(OTf)₃, have a positive influence on the catalytic activity of the iron salts, enabling the carbonylation at a low pressure of CO (6 to 8 bars).

Full text of DOI:10.1002/anie.201903740

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Accepted Article
Title: Carbonylation of C–N Bonds in Tertiary Amines Catalyzed by
Low Valent Iron Catalysts
Authors: Tawfiq Nasr Allah, Solène Savourey, Jean-Claude Berthet,
Emmanuel Nicolas, and Thibault Cantat
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903740
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10.1002/anie.201903740
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Carbonylation of C–N Bonds in Tertiary Amines Catalyzed by Low
Valent Iron Catalysts
Tawfiq Nasr Allah, Solène Savourey, Jean-Claude Berthet, Emmanuel Nicolas, and Thibault Cantat*[a]
Abstract: The first iron catalysts able to promote the formal insertion
of CO into the C–N bond of amines are reported. Using low valent iron
complexes, including K2[Fe(CO)4], amides are formed from aromatic
and aliphatic amines, in the presence of an iodoalkane promoter.
Inorganic Lewis acids, such as AlCl3 and Nd(OTf)3, have a positive
influence on the catalytic activity of the iron salts, enabling the
carbonylation at a low pressure of CO (6 to 8 bars).
role of a metal nucleophile, the authors reported in 2013 the use
of Na[Co(CO)4] as an efficient catalyst for the carbonylation of
trimethylamine at >180 °C and 30 bars of CO, with CH3I as a
promotor.[11] Following a different approach, Hu, Huang et al. used
I2 to induce charge-transfer complexes and weaken the C–N bond
in benzylamines and promote their carbonylation with nickel(II)
catalysts.[12] To realize the potential of the carbonylation of amines,
novel catalysts are needed that enable insertion of CO in C–N
bonds under a low pressure of CO, for a variety of substrates. To
achieve this goal, iron complexes are ideal candidates. Indeed,
low valent iron complexes are capable of coupling nucleophiles
and electrophiles with CO in stoichiometric as well as catalytic
reactions;[13] and we report herein the first iron catalysts able to
carbonylate aliphatic amines to tertiary amides.
Carbon monoxide is a convenient carbon building block and its
importance in the chemical industry is illustrated in major
processes, such as the Fisher-Tropsch synthesis of hydrocarbons
from syngas, the production of acetic acid at the megaton scale
and the conversion of olefins to alcohols and aldehydes (Aldox
and Oxo processes).[1] Because CO can be produced from the
gasification of waste biomass or the electroreduction of CO2, it
could be regarded as a renewable carbon source in the future.[2]
The exploration of carbonylation reactions hence remains a very
active field of research.[3] In fact, the carbene character of the CO
molecule offers a unique reactivity and the possibility to insert CO
in covalent sigma bonds to form carbonyl derivatives with an ideal
atom efficiency. The success of this approach is clearly
highlighted in the Monsanto process for the synthesis of acetic
acid by CO insertion in the O–CH3 bond of methanol,[4] or in the
production of phosgene from CO and Cl2.[5] By extension, the
carbonylation of a C–N bond in an amine would afford an
attractive route to amides. Although amides are efficiently
synthesized through metal-catalyzed amino-carbonylation
reactions of aryl-halides,[6] these procedures also generate by-
products in stoichiometric quantities and this drawback could be
advantageously circumvented with catalysts able to promote the
insertion of CO in existing C–N bonds. This strategy is hampered
by the difficulty to activate amines, as the heterolytic cleavage of
a C–N bond is ca. 20 kcal.mol-1 more endothermic than a C–O
bond.[7] Because of this limitation, very few carbonylation
reactions of amines have been developed. Strained amines, such
as aziridines, have been successfully carbonylated, in a reaction
that mimics the carbonylation of epoxides to β-lactones.[8] Only a
handful of catalysts are known to carbonylate linear amines.[9]
Utilizing palladium(II) salts, Li and co-authors successfully
prepared N,N’-dimethylacetamide by carbonylation of a mixture of
trimethylamine and tetramethylammonium iodide at 200 °C with
30 bars of CO.[10] Reasoning that the palladium catalyst plays the
Scheme 1. Mechanistic proposal for the carbonylation of amines to amides
using nucleophilic low valent metal complexes.
Collman’s reagent Na2[Fe(CO)4] and its congener Na[CpFe(CO)2]
are strong iron-based nucleophiles, able to react with alkylhalides
to generate carbonyl groups by CO migratory insertion.[14] In fact,
the [CpFe(CO)2]- anion is about 70∙106 times more nucleophilic
than the [Co(CO)4]– anion.[15] We hence reasoned that low valent
iron carbonyl complexes could advantageously replace
Na[Co(CO)4] in a catalytic sequence where the metal nucleophile
is alkylated with an iodoalkane promotor to generate an alkyl
complex that is carbonylated and further trapped by the amine
nucleophile (Scheme 1).[16] The carbonylation of N,N-
dimethylaniline (1a) was first undertaken using 6 mol% Fe(CO)5,
with 55 bars CO and 0.8 equiv. CH3I as a promoter. After 15 h at
200 °C in acetonitrile, the selective carbonylation of the N–CH3
linkage was observed and N-methylacetanilide 1b was formed in
40 % yield. This reaction represents the first example of the use
of an iron catalyst in the carbonylation of amines. Fe3(CO)12 and
[CpFe(CO)2]2 exhibit a similar catalytic activity (Entries 2-5 in
Table 1), and a prolonged reaction time of 60 h was necessary to
accomplish a quantitative carbonylation of the amine starting
material to 1b (Table 1, Entries 1 and 5). Interestingly, the
[a]
Dr. T. Nasr Allah, Dr. S. Savourey, Dr. E. Nicolas, Dr. J.-C. Berthet,
Dr. T. Cantat
NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay 91191
Gif-sur-Yvette cedex, France
Fax: (+33) 1.6908.6640
E–mail: thibault.cantat@cea.fr
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201903740
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potassium salt of Collman’s reagent, K2[Fe(CO)4], was more
active and it afforded 1b in >99 % yield after 15 h at 200 °C. It is
notable that the carbonylation reaction is highly chemoselective
and only proceeds through the activation of the N–C(sp3) bond,
leaving the stronger N–C(sp2) linkage of the aniline backbone
untouched. In addition, the resulting amide is unreactive under the
applied reaction conditions so that only a single N–CH3 linkage in
1a is carbonylated.
from the previous work of Collman, Shriver and others which
demonstrated, via stoiechiometric reactions, that the migratory
insertion of CO into the iron-alkyl bond of a low valent iron
carbonyl complex is highly sensitive to the pressure of CO and is
accelerated by Lewis acids able to coordinate a ligated CO
18]
molecule.[17b,
Interestingly, this strategy enables the
quantitative carbonylation of 1a to amide 1b using only 8 bars CO
at 200 °C (Entry 10 in Table 1). The need for a rather large
quantity of the Lewis acid (60 mol%) likely results from the
progressive poisoning of this co-catalyst by the amide product. As
the reaction proceeds, a strong donor is produced, in the form of
amide 1b, which can coordinate to the Lewis acid. In fact, the
reaction was completely inhibited when N-methyl-2-pyrrolidinone
was used as a solvent. This cyclic amide is indeed known to
solvate Lewis acids and form solvent separated species.[19]
Table 1. Screening of iron catalysts for the carbonylation of N,N-
dimethylaniline 1a.
Table 2. Screening of Lewis acids for the carbonylation of N,N-dimethylaniline
1a.
Entry
Cat. (mol%)
CO (bar)
GC Yield (%)
1
2
Fe(CO)5 (6)
Fe3(CO)12 (6)
Fe3(CO)12 (2)
[CpFe(CO)2]2 (6)
[CpFe(CO)2]2 (3)
K2[Fe(CO)4] (6)
Fe(CO)5 (6)
55
55
55
55
55
55
8
40 (>99)b
72
49
3
Entrya
Lewis acid (mol%)
Temperature
GC Yield (%)
4
45
5
40 (>99)b
>99
42
1
2
-
200
200
200
200
200
200
200
180
160
200
58
17
6
Ca(OTf)2 (60)
Sc(OTf)3 (60)
AlCl3 (60)
7
3
>99
78b
>99
77
8
Fe3(CO)12 (6)
[CpFe(CO)2]2 (6)
K2[Fe(CO)4] (6)
8
17
4
9
8
57
5
Nd(OTf)3 (60)
Nd(OTf)3 (30)
Nd(OTf)3 (6)
Nd(OTf)3 (60)
Nd(OTf)3 (60)
Nd(OTf)3 (60)
10
8
58
6
7
19
[a] Reaction conditions : Dimethylaniline (0.98 mmol), cat (2-6 mol%),
MeI (0.8 eq.), 15h, MeCN [b] after 60h of reaction
8
35
Capitalizing on this reactivity, the carbonylation of 1a was
attempted at a low pressure of 8 bar CO. Although a decrease in
catalytic activity could be noted for all the catalysts, 1b was
formed in 58 % yield after 15 h in the presence of 6 mol%
K2[Fe(CO)4] and 0.8 equiv. CH3I (Entries 7-10 in Table 1). In order
to overcome the lower carbonylation efficiency under low CO
pressure, we investigated the influence of Lewis acids on the
reaction. Indeed, the latter are well known promotors in
carbonylation steps, having a positive impact both on the kinetics
and the thermodynamics of CO migratory insertion reactions.[17]
Several Lewis acids have been tested for the carbonylation of 1a,
(Table 2) including calcium, scandium and neodymium triflate as
well as aluminium trichloride. While Ca(OTf)2 has a deleterious
influence on the reaction, 60 mol% AlCl3 somewhat improved the
production of 1b. More satisfyingly, Sc(OTf)3 and Nd(OTf)3 enable
the carbonylation of 1a in a quantitative yield after 15 h at 200 °C,
whereas its conversion is limited to 58 % in the absence of the
Lewis acid additive. The positive influence of a Lewis acid on the
catalytic activity of the low valent iron system can be rationalized
9
1
10b
>99
[a] Reaction conditions : K2Fe(CO)4 (6 mol%), dimethylaniline (0.98 mmol), MeI
(0.8 eq.), CO (8bar) 15h, 6 mL MeCN, 200°C [b] Average of two runs
The carbonylation of 1a was found to depend on the reaction
temperature and the yield of 1b dropped to 35 % and 1 %, when
the temperature is lowered to 180 and 160 °C, respectively
(Table 2). This finding is consistent with the proposal from Li et al.
that the catalytic cycle involves an equilibrium between the free
amine and its quarternary salt [PhNMe3]I.[10-11] This equilibrium
must be reversed at high temperature to increase the
concentration of the free and reactive amine and iodomethane.
Altogether, these observations and data support the mechanistic
proposal depicted in Scheme 2: Alkylation of Collman’s reagent
with iodomethane (involved in a deleterious equilibrium with the
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amine reagent) affords the anionic alkyl complex [Fe(CH3)(CO)4]-
I. The subsequent CO migratory insertion is facilitated by the
coordination of the Lewis acid co-catalyst to yield an acylferrate
intermediate II. Nucleophilic addition of the free amine on the acyl
ligand forms, after elimination, a quaternarized amide product III
that subsequently decomposes to the amide product and
regenerates iodomethane. Importantly, this mechanism
circumvents the problematic activation of the strong C–N bond,
by generating an electrophilic acyl moiety, able to react with the
amine reagent, so that the C–C bond between the alkyl residue
and CO is formed prior to the C–N bond cleavage.
trimethylammonium cation [ArNMe3]I, which provides smaller
quantities of the reactive promotor MeI at 200 °C, via the
equilibrium depicted in Scheme 2. The presence of a methyl
group in the ortho position of N,N-dimethylaniline does not
hamper the catalysis and 5a was converted to amide 5b in >99 %
yield. Beyond N,N-dimethylaniline derivatives, diphenyl-N-methyl
amine 7a was found to undergo a carbonylation of the N-CH3
linkage and, under the same reaction conditions utilized for 1a,
amide 7b was formed in 56 % yield. Similarly, the trialkylamine N-
methyl morpholine 9a gave 9b in 28% yield after 15 h. The same
trend is observed for 1,2,3,4-N-methyl tetraisoquinoline 8a which
provided amide 8b in 22 and 54 % yield after 15 and 60 h,
respectively. The lower activity likely stems from the use of
aliphatic amines that strongly bind MeI.
Table 3. Screening of tertiary amines.
Entrya
Substrates
Products
GC Yield (%)
1a, R = H
1b, R = H
>99 (83)c
1
2a, R = Me
3a, R = F
2b, R = Me
3b, R = F
81
95
55
4a, R = CN
4b, R = CN
2
3
>99
>99
5a
6a
5b
6b
Scheme 2. Proposed mechanism for the carbonylation of 1a catalysed by
K2[Fe(CO)4] + Nd(OTf)3, in the presence of MeI.
Motivated by the catalytic activity of the low valent iron catalysts,
we were interested in exploring the scope of reactive amine
substrates in this carbonylation reaction. Nd(OTf)3 was selected
as the Lewis acidic additive, because it exhibits an enhanced
stability and a greater resilience to hydrolysis, compared to AlCl3.
To assess the electronic influence of the amine reagent,
substituted N,N-dimethylaniline derivatives were first tested.
Aniline 4a, bearing–CN electron withdrawing group (EWG) at the
para position, was carbonylated in moderate yield of 55 % after
15 h at 200 °C (Table 3, Entry 1). The lower conversion likely
stems from the decreased nucleophilicity of the nitrogen atom
resulting in a slower transfer of the acyl group from iron to the
amine. In fact, replacing the cyano EWG with the weaker fluoride
EWG enabled the near-quantitative carbonylation of 3a to 3b.
This result is consistent with the high selectivity of this reaction:
only a single N–CH3 group can undergo a carbonylation event
because the resulting amide has a nucleophilicity much lower
than the amine starting material.
5
56
7a
7b
6
7
22 / 54b
8a
9a
8b
9b
28b
[a] Reaction conditions : K2[Fe(CO)4] (6 mol%), amine (0.98mmol),
Nd(OTf)3 60mol%, MeI (0.8 eq.), CO (8bar) 15h, 6 mL MeCN [b] After 60h
of reaction. [c] Isolated yield
Catalytic tests also revealed that the introduction of electron
donating groups (EDGs) somewhat lowers the conversion yield to
the amide product. For instance, the m-Me and p-Me substituted
N,N-dimethylaniline derivatives where converted to amides 6b,
and 2b in 99 and 81 % yield, respectively. This trend follows the
relative electron donating ability of the substituents, as given by
their Hammett sigma constants: -0.07 (m-Me) and -0.17 (p-Me).[20]
This effect likely results from the enhanced stability of the
Scheme 3. Carbonylation of 10a catalysed by K2[Fe(CO)4] + Nd(OTf)3, in the
presence of EtI.
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When the carbonylation of N,N-diethylaniline 10a was carried out
using EtI as a promoter in the presence of 6 mol% K2Fe(CO)4,
60 mol% Nd(OTf)3 and 8 bar CO, the carbonylation of the N-
CH2CH3 functionality afforded amide 10b in a very good 81 %
yield (Scheme 3). This is an especially interesting result as it
demonstrates that the carbonylation is not limited to N–Me
functionalities but is also viable for longer alkyl chains.
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In summary, we have reported the first iron catalysts able to
promote the carbonylation of tertiary amines, by formal insertion
of CO into a C–N bond, using low valent iron complexes, such as
K2[Fe(CO)4]. The transformation requires an iodoalkane promotor
and inorganic Lewis acids (e.g. Nd(OTf)3) acting as co-catalysts,
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Experimental Section
General procedure for the catalytic carbonylation of tertiary amines
in autoclaves: In a glove box the autoclave is charged with the catalyst
(2, 3 or 6 mol%), the amine (1 mmol), MeI (0.8 eq.) and a Lewis acid
(60mol%). The solvent (6 mL) is added and the autoclave is purged 4 times
with 10 bar of CO, then pressurized at the required pressure. The reaction
is then allowed to heat at 200°C for 15h or 60h. At the end of the reaction,
the autoclave is cooled down to RT and the pressure is released slowly.
Finally, the solution is filtrated on a Celite pad before analysis on GC/MS
chromatography.
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Keywords: Amides • Carbon Monoxide • Iron • Catalysis • Lewis
Acids
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Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Tawfiq Nasr Allah, Solène Savourey,
Jean-Claude Berthet, Emmanuel
Nicolas, and Thibault Cantat *
Page No. – Page No.
Title
Low valent iron complexes, such as K2[Fe(CO)4], efficiently catalyze the
carbonylation of C-N bonds in amines, by CO insertion. Inorganic Lewis acids, such
as AlCl3 and Nd(OTf)3, have a positive influence on the catalytic activity and enable
the carbonylation at a low pressure of CO.
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