Copper-Catalyzed Formylation of Amines by using Methanol as the C1 Source

Article abstract of DOI:10.1002/cssc.201903266

Cu/TEMPO catalyst systems are known for the selective transformation of alcohols to aldehydes, as well as for the simultaneous coupling of alcohols and amines to imines under oxidative conditions. In this study, such a Cu/TEMPO catalyst system is found to catalyze the N-formylation of a variety of amines by initial oxidative activation of methanol as the carbonyl source via formaldehyde and formation of N,O-hemiacetals and oxidation of the latter under very mild conditions. A vast range of amines, including aromatic and aliphatic, primary and secondary, and linear and cyclic amines are formylated under these conditions with good to excellent yields. Moreover, paraformaldehyde can be used instead of methanol for the N-formylation.

Full text of DOI:10.1002/cssc.201903266

Accepted Article
Title: Copper-catalysed formylation of amines using methanol as the
C1 source
Authors: Manuel Carmona Pichardo, Ghazal Tavakoli, Jessica E.
Armstrong, Tobias Wilczek, Bradley Thomas, and Martin H.
G. Prechtl
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Copper-catalysed formylation of amines using methanol as
the C1 source
Manuel Carmona Pichardo,§[a] Ghazal Tavakoli,§[a] Jessica. E. Armstrong,#[a, b] Tobias
Wilczek,#[a] Bradley E. Thomas,[a] Martin H. G. Prechtl*[a,c]
Abstract: Cu/TEMPO as catalytic system is known for the selective
transformation of alcohol to aldehyde as well as the simultaneous
coupling of alcohol and amines to imines under oxidative conditions.
Herein we demonstrate for the first time that such a Cu/TEMPO
catalyst system is able to catalyse the N-formylation of a variety of
amines by initial oxidative activation of methanol as the carbonyl
source via formaldehyde and formation of N,O-halfacetals and
oxidation of the latter ones at very mild conditions. A vast range of
amines, from aromatic and aliphatic primary and secondary, as well
as linear and cyclic amines have been formylated under these
conditions with good to excellent yields. Additionally, we
demonstrate here that paraformaldehyde can be used instead of
methanol for the N-formylation.
and green chemistry principles. The pioneering work of Glorius
et al. in 2013 was the first example of formylation of amines via
methanol activation.[16] However, the number of reports on the
application of methanol as the carbonyl precursor is very rare
using molecular complexes[13, 16-18] or heterogeneous catalysts[19-
21]
(Scheme 1). Therefore, finding mild and feasible reaction
conditions for activation and aerobic oxidation of methanol could
be the key for the problem of N-formylation of amines and it still
remains a challenge. Yet there is one problem that should be
considered in this regard: amines, per se, can be oxidised to
their higher oxidation state products under most of the oxidative
conditions. Therefore, the reaction conditions in this case should
be selected carefully such that methanol can be oxidised in the
presence of amine selectively.
N-formylated amines are a class of highly applicable compounds
with different applications in organic synthesis ranged from
starting materials to solvents. The importance of N-formamides
relies on the fact that they can be employed as intermediates for
the synthesis of a large number of pharmaceuticals, fungicides,
nitriles, N-containing heterocycles and isocyanates in addition to
their widespread use in organic synthesis and peptide
synthesis.[1-6] Formylation of amines is also one of the methods
to protect the amine functionality in organic transformations.[7]
This family of chemicals can be synthesised via different
methods. Most of the traditional methods use excessive
amounts of formylating agents such as formic acid,[8] formate,[9]
chloral,[9] CO2,[10-12] acetic formic anhydride,[9] carbodiimide[8] and
formaldehyde[9] which often result in the production of huge
amounts of toxic wastes and poor atom economy.[13] These
methods also suffering from other drawbacks such as low yields
and the use of expensive, instable, air- and moisture- sensitive
and toxic reagents.[14-15] Direct coupling of methanol with amines
in the presence of transition metals as catalyst under oxidative
conditions is on the other hand, in accord with the sustainability
Scheme 1. Previously reported catalytic reactions for N-formylation of amines
using methanol compared to this work. (*ICy.HCl = dicyclohexylimidazolium
chloride)
[a]
M. Sc. M. Carmona Pichardo, Dr. G. Tavakoli, B. Sc. J. E.
Armstrong, B. Sc. Tobias Wilczek, B. Sc. B. E. Thomas, Priv.-Doz.
Dr. M.H. G. Prechtl
Department of Chemistry, University of Cologne,
Greinstr. 6, 50939 Köln (Germany)
Due to its ubiquitous nature and its availability, methanol has
been always one of the most interesting chemicals since many
years ago. Methanol, as a renewable C1 resource has been
considered as the top five commonly used chemicals in the past
E-mail: martin.prechtl@uni-koeln.de
Homepage: www.h2.bio
years used for preparation of a vast range of products in our
[b]
[c]
B. Sc. J. E. Armstrong
22]
daily life.[16,
Alcohol oxidation, among the other oxidation
Department of Chemistry, Yale University,
225 Prospect St, New Haven, CT, 06511-8499 USA
Prof. Dr. M. H. G. Prechtl
Department of Science and Environment, Roskilde University
Universitetsvej 1, 4000 Roskilde (Denmark)
E-mail: prechtl@ruc.dk
Supporting information for this article is given via a link at the end of
the document.
§: M.C.P. and G.T. contributed equally. #: J.E.A. and T.W.
contributed equally.
reactions, is one of the most applicable and well-known
transformations in the context of organic synthesis and many
different approaches have been reported so far for the selective
and efficient oxidation of alcohols to aldehydes, ketones and
carboxylic acids.[23-24] However, activation of methanol is more
complicated due to the higher energy barrier and lower
reactivity.[25] Actually, most of the previously reported procedures
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10.1002/cssc.201903266
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for converting methanol to its higher oxidation state products
suffer from many drawbacks such as harsh reaction conditions,
high energy consumption, high toxicity, poor atom economy and
Following our research about the activation on C1 molecules for
energy conversion processes[49-50] and synthetic applications,[51]
we report here on a very mild method for the formylation of
amines with methanol or paraformaldehyde at low temperature
with a copper/TEMPO system as robust, selective and cheap
catalyst. For a convenient and practical protocol we aimed to
develop a method based on TEMPO since it is much cheaper
than ABNO and other nitroxyl radicals. Molecular oxygen has
been used as the green oxidant for this reaction. Different N-
formylated amines have been synthesised from a vast range of
amines from primary to secondary, aromatic to aliphatic and
cyclic to linear ones. Additionally, we identified some complex
species which gives us some insights into the mechanism and
reaction pathway.
the necessity of using expensive catalytic systems which limit
25-28]
the applications of these systems.[16,
Furthermore, the
presence of rare transition metals (TM) such as Au, Pd and Ru
in these reactions leads to competing oxidations of other
functional groups present in the reaction mixture including
heterocycles and other groups containing nitrogen, oxygen, and
sulfur.[29-31] The later can be overcome by using more abundant
first-row TM systems which show broader functional group
tolerance.[32-34] Therefore, efforts are still continuing to develop
more feasible and sustainable alternatives for the activation of
alcohols and in particular methanol under mild conditions.
Application of nitroxyl radicals such as TEMPO (2,2,6,6-
tetramethylpiperidine-N-oxyl) as redox-active co-catalyst in
combination with a TM based catalyst has been proved to work
for the oxidation of different types of alcohols. However, most
interestingly these systems have not been further investigated
for methanol oxidation in recent years,[35-36] since it has been
reported previously that when methanol is used as solvent
methyl formate is formed in a side reaction in presence of
copper and nitroxyl radicals.[37] In fact, the one-electron oxidation
of TEMPO affords an oxoammonium species which suffers a
two-electron reduction to yield a hydroxylamine during the
alcohol oxidation; while the function of the auxiliary oxidant is to
regenerate the oxoammonium species. In this regard, one of the
most successful catalytic systems is the combination of a
copper(I) or (II) salt with nitroxyl compounds like TEMPO as co-
As starting point we used conditions reported for the oxidation of
aliphatic alcohols to aldehydes,[44] and optimised them for
methanol oxidation. Notably, we observed that with
a
copper/TEMPO catalyst system methanol oxidation in absence
of amines does not result in the formation of formaldehyde nor
formic acid, but in the formation of methyl formate (Scheme 2).
Similar observations have been reported as side reaction while
methanol
has
been
used
as
solvent
in
copper/phenanthroline/nitroxyl radical promoted oxidation of
aldehydes.[37] The methyl formate formation is a result of the
oxidation of the corresponding halfacetal formed from
formaldehyde and methanol, similar to the reported formation of
lactones from diols.[48] Interestingly, in contrast to previous
reports we found a substrate specific selectivity for the methanol
oxidation which differs from the oxidation of other aliphatic
alcohols, but is similar to the oxidation of diols.[48] In vast
contrast, we found that the presence of amines results in the
formation of N-formylated amines. The formation of the
formamide is favoured over the methyl formate since the N,O-
halfacetal formation is faster than the O,O-halfacetal formation.
To this end, we tested different copper precursors in the
oxidation of methanol using TEMPO as co-catalyst, air or O2 as
the oxidant and NMI and bipyridine (bpy) as ligands at 25°C
(Table S1, SI, Scheme 2).
catalyst which can oxidise alcohols under aerobic conditions to
32, 38-43]
aldehydes or ketones,[29,
but the oxidation of methanol
has not been reported yet with TEMPO in synthetic protocols.
(bpy)CuI/NMI (NMI = N-methylimidazole) as catalytic system has
been reported as robust system for the oxidation of a vast range
of primary alcohols.[44] Considering the compatibility of these
conditions with diverse functional groups on the substrates and
using copper as a cheap and accessible first-row TM as well as
using easily available and inexpensive ligands for activating
copper, we took the challenge to investigate the possibility of
formylating amines with methanol under these catalytic
conditions. And since Cu/TEMPO-catalysed methanol oxidation
has not been reported and the alcohol oxidation in presence of
amines usually results in the formation of coupled secondary
imines,[35, 45] likewise amines undergo homo-coupling to imines
in absence of alcohol,[35] this approach seems very unusual on
the first glance. Notably, it has been demonstrated that a
Cu/ABNO (ABNO = 9-azabicyclo-[3.3.1]nonane N-oxyl) catalyst
system is able to convert aliphatic and benzylic alcohols with
amines to the corresponding amides in vast contrast to
Cu/TEMPO systems owing to different sterics of ABNO and
TEMPO during the N,O halfacetal oxidation.[46-47] However, this
argument is not fully true for the conversion of diols to the
corresponding lactones, where also TEMPO has been
successfully used as co-catalysts for the lactone synthesis.[48]
Therefore, there is also substrate specific selectivity.
Noteworthy, neither the Cu/ABNO nor the Cu/TEMPO catalysts
have been demonstrated to be active to convert methanol in
presence of amines to synthesise formamides.
Scheme 2. Methanol oxidation at ambient temperature with Cu/TEMPO.
After the initial optimisation of the quantitative methanol
oxidation to methyl formate and the N-formylation of piperidine
at room temperature and 60°C (for a description see SI: catalyst
screening, solvents, temperature, time and oxidation agents: air,
O2, peroxides), we found for the N-formylation of piperidine the
conditions depicted in Scheme 3.
Scheme 3. N-formylation of piperidine with methanol (Table S2: condition
screening)
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Having the optimised reaction conditions in hand, different types
of amines were tested in the oxidative coupling N-formylation
reaction with methanol (Table 1). Additionally we performed the
reactions with paraformaldehyde (pFA) as C1-source since this
has not been succeeded yet for Cu/TEMPO (or ABNO)
catalysed amidation and formamide synthesis and it gives
further insights about the reaction (Table 1; values given in
brackets were obtained with pFA as formylation reagent).
Noteworthy, N-formylation with pFA has only been reported
using an iridium catalyst using a threefold excess of pFA.[52] The
yields are slightly better with pFA and trends are similar
depending on the substrates independent, if methanol or pFA is
used for the formylation.
13
14
79 (90)
81 (84)
15
16
14 (16)
12 (12)
Table 1. N-formylation of different amines using methanol or
paraformaldehyde (pFA) as the carbonyl source.a
17
18
n. q.
86 (90)
Yield
Entry
1
Amine
Product
(%)b,c
[a] Conditions: Acetonitrile (5 ml), CuI (0.053 mmol), bpy (0.053 mmol), NMI
(0.11 mmol), amine (1.1 mmol), methanol (1.1 mmol), TEMPO (0.107 mmol),
O2 (1.5 bar), 60 °C, 20 h. [b] 1H-NMR yields are reported. n. q. = not
quantified. [c] Values given in brackets used pFA instead of methanol as
formylating agent; 1.1 mmol refers to the monomeric unit of pFA [CH2O]n.
96 (97)
2
92 (95)
A broad variety of amines can be formylated with good to
excellent yields. Aliphatic secondary amines such as piperidine,
morpholine, tetrahydroquinoline and tetrahydroisoquinoline due
to their smaller steric hindrance compared to the linear ones
result in better yields (Table 1, entries 1-4). However pyrrolidine
(Table 1, entry 5) gave a lower yield (MeOH: 59%; pFA: 57%)
which might be related to different sterics and the smaller ring
size resulting in more stable copper amine complexes with
pyrrolidine as ligand.
3
4
85 (84)
86 (93)
5
6
7
59 (57)
61 (74)
40 (50)
Comparing the results of N-formylation of linear amines reveals
that secondary amines are better substrates than the primary
ones. In the reaction with N,N-dibutylamine, N,N-
dibutylformamide was obtained with 61% yield using MeOH (or
pFA: 74%); while its corresponding primary amine, 1-octylamine,
forms N-octylformamide only with 40% yield using MeOH (or
pFA: 50%) (Table 1, entries 6-7). Primary phenyl alkyl amines
were also tested (Table 1, 8-11). And the yields decrease with
the increase of the chain length between the phenyl ring and the
amino group. Therefore, the best result in this series was
obtained using benzylamine with up to 86% using MeOH (or
8
86 (94)
9
74 (77)
50 (61)
pFA:
94%)
yield
followed
by
phenethylamine,
3-
10
phenylpropylamine and 4-phenylbutylamine, respectively. This
trend can be attributed to a back side effect from the phenyl ring
where it can donate its electron density partially to the amino
11
12
45 (54)
84 (91)
group and make the nitrogen
a better nucleophile. The
possibility of involvement of some π-interactions between the
phenyl ring and the catalyst in the transition states can also be
considered as a reason for better yields of aromatic primary
amines compared to aliphatic ones. The presence of such
interactions has been proved in similar catalytic systems which
can help the substrate to find the active catalytic species in the
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medium easier.[53] In addition, we tested substituted benzylic
amines (Table 1, entries 12-14) with very good yields using
MeOH or pFA for the formylation: The N-monomethylated
benzylamine (Table 1, entry 12 gave 84%/91% yield,
phenylethylamine gave 79%/90% (Table 1, entry 13) and
naphthylethylamine gave 81%/84% (Table 1, entry 14).
to the peroxo-bridged binuclear species C which then oxidises
one molecule of TEMPOH to TEMPO radical via H-atom transfer
delivering the peroxo complex D (SI-Fig. 21-28).
2-Picolylamine resulted in the formation of only 14% (16% with
pFA) of its corresponding formylated product again due to
electron withdrawing nature of the pyridine ring diminishing the
nucleophilicity of the amino group, likewise 2-picolylamine can
act as chelating ligand which blocks the required labile
coordinating site of a copper species (Table 1, entry 15). As
mentioned above, tetrahydroquinoline was formylated with the
very good yield of 85% selectively. As expected, increasing the
steric hindrance in the vicinity of the nitrogen can affect the
formylation performance. This effect can be seen by comparison
of the reaction of N,N-dibutylamine and N,N-diisopropylamine
(Table 1, entries 6 and 16). The latter one has a lower yield as
the isopropyl groups prevent the substrates to react easily with
each other. In the case of aniline, (E)-1,2-diphenyldiazene was
formed as the main product (Table 1, entry 17), since aniline is
known to be sensitive under oxidative conditions.[54] In vast
contrast, the formylation of 8-aminoquinoline (Table 1; entry 18)
was successful yielding 86% of the corresponding formamide
(respectively 90% with pFA), and it does not undergo
homocoupling to form the corresponding diazene.
Scheme 4. Reaction pathways for the aerobic Cu/TEMPO-catalysed oxidation
of methanol in absence and presence of amines resulting in the formation of
methyl formate or formamides.
It has been demonstrated that the hydrogen atom from TEMPO-
H
can be abstracted by dimeric peroxo-bridged Cu2O2
complexes.[44] However, the exact mechanism of this
transformation has been vague so far and two different
pathways have been proposed for that: It has been considered
the involvement of oxoammonium species in presence of
copper/TEMPO catalytic system[40] and it has been also
suggested that a two-electron alcohol oxidation system in which
a Cu(II) and an oxy radical each acts as a one-electron
oxidant.[44] Shedding a light on this challenging step, our ESI MS
results showed that both the species TEMPO+ and TEMPOH are
detectable. This observation is in agreement with previous work
where it has been proven via EPR spectroscopy and cyclic
voltammetry that although Cu(II) is capable of oxidizing TEMPO
to TEMPO+, the rate of TEMPO+-mediated alcohol oxidation is
much less than the catalytic reaction with TEMPOH.[40, 44, 56-57]
Later on, the Cu(II) peroxo complex D in the presence of water
is transformed into the Cu(II) hydroxo complex E (SI-Fig. 25-26).
The released H2O2 molecule is not stable under the reaction
conditions and disproportionate immediately in presence of the
catalyst.[44] In continue, a ligand exchange occurs between
methoxide and hydroxide resulting in the formation of the Cu(II)
methoxide complex F and releasing a water molecule. The
following interaction between a TEMPO radical and the Cu(II)
complex F results in the abstraction of an H-atom from F via the
transition state G delivering the Cu(I) complex H. It is noteworthy
that although it has been proposed previously that breaking the
C-H bond to form an aldehyde in this step should probably occur
as a result of the involvement of both TEMPO and Cu(II) center
at the same time in a two electron/ one proton process, there
has been no evidence for the formation of such species in the
literature so far.[44] Our ESI MS analysis indicates, however, the
possible formation of G (SI-Fig. 29) in the reaction mixture. The
necessity of the cooperation of Cu(II) centre and TEMPO at the
same time in the C-H bond breaking step shows that this step is
not just a simple H-atom abstraction as suggested previously.[44]
Probably, coordination of methanol to the Cu center makes the
C-H bond weaker such that TEMPO can abstract the hydrogen
atom easier. Afterwards, the coordinated formaldehyde reacts in
a nucleophilic attack by the amine forming the Cu(II) complex
Considering the synthetic feasibility and utility of the presented
procedure for the formylation of different amines as well as
employing inexpensive catalytic system and abundant
formylating agents making this protocol more interesting, we
decided to perform some more experiments which give us better
insights into the reaction mechanism. The mechanism of
oxidation of alcohols - other than methanol - in the presence of
NMI/Cu/TEMPO
catalytic
system
has
been already
investigated.[44, 46-48] However, there are still some uncertainties
about the mechanism of the alcohol oxidation which need further
examinations. In addition, apparently methanol was not
successfully activated in the previous reports as it has higher
energy barriers and therefore, has lower reactivity compared to
other alcohols. And, since this is the first report on
copper/TEMPO catalysed N-formylation, there are no
experimental mechanistic details available in literature.
To this purpose, reaction mixtures of the N-formylation of the
benchmark substrate piperidine has been analysed by ESI-MS
for the identification of complex species potentially involved in
the reaction network (Scheme 4) and the proposed catalytic
cycle (Scheme 5).
One need to consider that the mass of one N-methylimidazole
ligand is equivalent to two acetonitrile ligands, therefore a
complex could carry either one N-methylimidazole ligand or two
acetonitrile ligands yielding in complexes with identical
molecular masses. Accordingly, oxidative addition of oxygen to
the Cu(I) complex A is the first step of the reaction resulting in
the formation of Cu(II) superoxide species B. This assumption
1
was supported by ESI-MS, and by H and 13C NMR analyses
where the results confirm the paramagnetic nature of the
catalyst.[55] Introduction of the second molecule of A to B leads
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10.1002/cssc.201903266
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COMMUNICATION
intermediate I carrying a hemi-aminal ligand. Finally, a second
two electron / one proton H-atom abstraction via the transition
state Cu(II) complex J yields the desired formamide while
releasing TEMPOH. This step reforms the catalytically active
Cu(I) species A. The presence of a signal in the ESI MS spectra
again allows the assumption for the Cu(II) species J (SI-Fig. 30)
in the reaction mixture. Remarkably, we could detect some
copper complexes carrying methanol and/or piperidine and
TEMPO as ligands, but we could not detect complexes carrying
a formate as ligand. This indicates that the formation of methyl
formate only occurs in absence of amine and therefore we also
of 1:1 ratio of methanol to amine also increases the atom
economy of the presented protocol.
Furthermore, in order to spread a light on the details of the
mechanism of this reaction, ESI MS spectra of the reaction
mixtures showed some intermediates which have been not
detected before in related alcohol oxidation processes. For our
future studies this protocol will serve as starting point for further
developments to use methanol for N-formylation based on
copper and nitroxyl radical catalysts other than TEMPO.
can exclude
a secondary reaction pathway towards the
Acknowledgements
formamide formation via methyl formate following
a
transamidation in the reaction network (Scheme 4). In addition,
we could not detect free formic acid in solution which implies
neither the methyl formate nor the formamide is formed from
significant amounts of free formic acid, but rather that the
We gratefully acknowledge financial support provided by the
MIWF-NRW (NRW-returnee award 2009), the Heisenberg-
Program (DFG), the COST Action “C-H Activation in Organic
Synthesis (CHAOS)”, the Ernst-Haage-Prize 2014 of the Max-
Planck-Institute for Chemical Energy Conversion. Moreover we
formaldehyde
reacts
rapidly
to
the
corresponding
halfacetal/halfaminal which are than oxidized readily. The
catalyst activation cycle (Scheme 5) should be identical
independent of the used formylation reagent methanol or
paraformaldehyde and indeed the same species have been
detected by ESI-MS (SI-Fig. 15-28).
acknowledge
the
following
funding
agencies
for
fellowships/scholarships: DAAD (M.C.P. and B.E.T), CONACyT
(M.C.P.), Fulbright Foundation (J.E.A.) and the Alexander-von-
Humboldt Foundation (G.T.).
In summary, methanol and pFA, as a cheap and renewable C1
building blocks, have been used for the N-formylation of amines
using an effective and inexpensive (bpy)CuI(NMI)/TEMPO
catalytic system under the aerobic conditions for the first time.
Direct coupling of methanol with a vast range of amines using
molecular oxygen as a nontoxic, inexpensive and abundant
oxidant makes the whole process more sustainable. Application
Keywords: methanol activation • N-formylation • copper •
oxidative coupling • TEMPO
Scheme 5. Proposed catalytic cycle for the aerobic N-formylation of amines with methanol and the (bpy)Cu/TEMPO/NMI catalytic system
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10.1002/cssc.201903266
ChemSusChem
COMMUNICATION
Entry for the Table of Contents
Communication
Manuel Carmona Pichardo, Ghazal
Tavakoli, Jessica. E. Armstrong, Tobias
Wilczek, Bradley E. Thomas, Martin H.
G. Prechtl*
Page No. – Page No.
Copper-catalysed formylation of
amines using methanol as the C1
source
Smooth oxidative coupling of methanol with amines to form N-formylated amines. A
pathway to formamides with high atom economy at mild conditions by the use of
cheap catalyst precursors and oxygen as abundant and green oxidant
This article is protected by copyright. All rights reserved.