Carbon nanotube-gold nanohybrid catalyzed N-formylation of amines by using aqueous formaldehyde

Article abstract of DOI:10.1002/cctc.201402225

The N-formylation of a variety of primary and secondary amines by using aqueous formaldehyde at room temperature in open air affords the corresponding formamides in excellent yield under the catalytic influence of a gold-carbon nanotube nanohybrid. The reaction is also marked by excellent chemoselectivity, low catalyst loading, and recyclability of the catalyst.

Full text of DOI:10.1002/cctc.201402225

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DOI: 10.1002/cctc.201402225
Carbon Nanotube–Gold Nanohybrid Catalyzed N-
Formylation of Amines by using Aqueous Formaldehyde
Nimesh Shah,[a] Edmond Gravel,[b] Dhanaji V. Jawale,[b] Eric Doris,*[b] and
Irishi N. N. Namboothiri*[a]
The N-formylation of a variety of primary and secondary
amines by using aqueous formaldehyde at room temperature
in open air affords the corresponding formamides in excellent
yield under the catalytic influence of a gold–carbon nanotube
nanohybrid. The reaction is also marked by excellent chemose-
lectivity, low catalyst loading, and recyclability of the catalyst.
we disclosed the superior catalytic activity of carbon nanotube
(CNT)-supported nanogold in the oxidation of silanes,[16] alco-
hols,[17] and phenols[18] and the reductive amination of alde-
hydes.[19] CNTs were chosen as a support owing to their low
cost, high surface area, tunable topography, inertness, and
their extraordinary stability and ability to stabilize transient
higher oxidation states of the supported metal.[20]
The AuCNT nanohybrids were prepared by introducing
a two-layer assembly around a CNT and then adding pre-
formed AuNPs.[16] The polyanionic inner layer is derived from
a micelle consisting of a photopolymerizable polyacetylenic hy-
drophobic tail and a polar carboxylate head group (DANTA am-
phiphile). The polycationic quaternary ammonium (PDADMAC)
outer layer stabilizes the AuNPs on the surface (Figure 1).
Formamides are valuable intermediates in organic synthesis
and medicinal chemistry.[1,2] For example, the synthesis of the
quinolone antibiotic norfloxacine was achieved via a substitut-
ed formanilide as the key intermediate.[2] The role of forma-
mides as intermediates in the synthesis of other N-heterocy-
cles[3] and as catalysts in the allylation and hydrosilylation of
carbonyl compounds[4] has been well documented. Forma-
mides have been extensively employed in Vilsmeier formyla-
tion[5,6] and as key amino protecting groups in peptide synthe-
sis.[6,7] The formylation of various amines has been reported
with reagents such as formic acid/amide,[1,8] acetic formic anhy-
dride,[9] formate,[10] cyanide,[11] and chloral.[12] However, the
wide applicability of these methods has been curtailed by one
or more shortcomings such as undesirable side reactions, the
toxicity of the reagents, the requirement for the pre-prepara-
tion of the formylating agents, elevated temperatures, and an-
hydrous conditions.
Recently, the direct N-formylation of amines by using
MeOH[13] or formalin[14] as a formyl source in the presence of
AuNPs (gold nanoparticles) was reported. Although these are
elegant approaches, the requirement for a higher temperature
(60–1008C) along with an oxygen atmosphere in the former
case and a higher catalyst loading combined with a lack of re-
cyclability in the latter case prompted us to further investigate
the N-formylation of various amines.
From the state of being a catalytically inert or ineffective
transition metal, gold has emerged in recent years as an attrac-
tive catalyst for a variety of organic transformations, especially
after downsizing into nanometer-sized particles.[15] Recently,
Figure 1. Overview of the AuCNT structure. DANTA=diacetylene nitrilotria-
cetate amphiphile, PDADMAC=poly(diallyldimethylammonium chloride),
AuNP=gold nanoparticle, CNT=multiwalled carbon nanotube.
[a] Dr. N. Shah, Prof. Dr. I. N. N. Namboothiri
Department of Chemistry, Indian Institute of Technology Bombay
Mumbai 400 076 (India)
Herein, we report for the first time the highly selective direct
N-formylation of amines with aqueous HCHO as the formyl
source under the catalytic influence of a AuCNT nanohybrid.
Our recent results in the aerobic oxidation of alcohols by
AuCNTs[17] inspired us to attempt the N-formylation of N-meth-
ylaniline (1a) through the in situ generation of formaldehyde
from methanol (Table 1). However, no reaction was observed
even after 24 h of stirring 1a in MeOH/H2O (1:1) in the pres-
E-mail: irishi@chem.iitb.ac.in
[b] Dr. E. Gravel, Dr. D. V. Jawale, Dr. E. Doris
CEA, iBiTecS, Service de Chimie Bioorganique et de Marquage
91191 Gif-sur-Yvette (France)
E-mail: eric.doris@cea.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402225.
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Table 1. Optimization of the reaction conditions for the AuCNT-catalyzed
N-formylation of N-methylaniline (1a).[a]
[b]
Entry
Formylating
agent
Base
(equiv.)
Solvent
Yield
[%]
1[c]
2[e]
3
4
5
6
7
8
9[f]
10[g]
11[h]
12
MeOH
HCHO
HCHO
HCHO
HCHO
HCHO
HCHO
HCHO
HCHO
HCHO
HCHO
HCO2H
NaOH (3)
NaOH (3)
NaOH (3)
LiOH (3)
NaOH (3)
NaOH (2)
NaOH (1)
none
NaOH (3)
NaOH (3)
NaOH (3)
NaOH (3)
H2O
NR[d]
87
94
76
87
83
80
NR[d]
NR[d]
NR[d]
NR[d]
NR[d]
toluene/H2O
toluene/H2O
toluene/H2O
CH2Cl2/H2O
toluene/H2O
toluene/H2O
toluene/H2O
toluene/H2O
toluene/H2O
toluene/H2O
toluene/H2O
Scheme 1. Plausible mechanism for the N-formylation of amine 1a with
HCHO.
dary amine (Table 2, entry 1), representative cyclic secondary
amines such as piperidine (1b, a dialkylamine), and aryl alkyla-
mines 1c and 1d were also subjected to N-formylation to
afford corresponding formamides 2b–2d in excellent yields
(89–94%; Table 2, entries 2–4). Diarylamine 1e did not undergo
formylation even after 24 h, which was attributable to deacti-
vation of the amino group by the two aryl groups (Table 2,
entry 5).
[a] Conditions (unless otherwise stated): Methylaniline (1a, 0.1 mmol), for-
mylating agent (2 equiv.), AuCNT (aqueous suspension, 0.34 mol%), base
(1–3 equiv.), toluene/H2O or CH2Cl2/H2O (1:1, 1 mL), RT, open flask (air),
24 h. [b] Yield of purified isolated product. [c] AuCNT (0.17 mol%).
[d] NR=no reaction. [e] AuCNT (0.17 mol%), 30 h. [f] No catalyst. [g] Only
CNT was used. [h] CNT with the bilayer assembly, but without AuNP, was
used.
Having demonstrated the efficacy of our experimental con-
ditions for the formylation of various secondary amines, the
formylation of primary aromatic amines was investigated
(Table 3). In general, amines with electron-withdrawing and
electron-donating groups on the aromatic ring were amenable
to the reaction over a period of 22–30 h at room temperature.
However, the formylation of parent aniline 3a and that of sub-
strates 3b and 3c with electron-donating groups in the para
position proceeded in marginally higher yields to deliver prod-
ucts 4a–c (92–94%; Table 3, entries 1–3).
ence of AuCNT (0.17 mol%) and NaOH (3 equiv.) at room tem-
perature (Table 1, entry 1). This suggested that direct N-formy-
lation of amines through the in situ oxidation of MeOH was
not feasible under our experimental conditions. Subsequently,
N-formylation of 1a was performed by using 38% aqueous
HCHO as the formyl source under aerobic conditions under
the catalytic influence of AuCNTs (0.17 mol%), which afforded
expected product 2a in 87% yield after 30 h (Table 1, entry 2).
The turnover number (TON) and turnover frequency (TOF) cal-
culated for this reaction were 512 and 17 hꢀ1, respectively (see
the Experimental Section for details). Increasing the catalyst
loading to 0.34 mol% further improved the yield to 94% and
shortened the reaction time to 24 h (Table 1, entry 3). Chang-
ing the base from NaOH to LiOH (Table 1, entry 4), the solvent
from toluene/H2O to CH2Cl2/H2O (Table 1, entry 5), or reducing
the quantity of base (Table 1, entries 6 and 7) did not have any
positive effect. The reaction did not take place in the absence
of either the base (Table 1, entry 8) or the catalyst (Table 1,
entry 9) or if CNT/CNT with a bilayer assembly was used in the
absence of the AuNPs (Table 1, entries 10 and 11). There was
also no reaction if formaldehyde was replaced by formic acid
under otherwise identical conditions (Table 1, entry 12). This
observation shed light onto the plausible mechanism of formy-
lation in that oxidation of formaldehyde to formic acid fol-
lowed by amidation was ruled out (Scheme 1, path B). Instead,
formation of hemiaminal I followed by its oxidation (catalyzed
by AuCNT) to formamide 2a appeared to be the favored path-
way (Scheme 1, path A).[14]
Table 2. AuCNT-catalyzed N-formylation of secondary amines 1.[a]
Entry
1
Substrate
1
Product
2
Yield[b]
[%]
1a
2a
94
2
3
4
1b
1c
1d
2b
2c
2d
90
89
94
5
1e
2e
NR[c]
[a] Conditions: Amine 1 (0.1 mmol), formaldehyde (2 equiv.), AuCNT (aq.
suspension, 0.34 mol%), NaOH (3 equiv.), toluene/H2O (1:1, 1 mL), RT,
open flask (air), 24 h. [b] Yield of purified isolated product. [c] NR=no re-
action.
The scope of this formylation was investigated with various
secondary and primary amines under the above-optimized
conditions (Tables 2 and 3). Besides 1a, an open-chain secon-
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this indicates a high degree of chemoselectivity (Table 3,
entry 8). However, primary alkyl amines were unreactive under
our experimental conditions (Table 3, entry 9).
Table 3. AuCNT-catalyzed N-formylation of primary amines 3.[a]
The inertness of diarylamine 1e prompted us to further in-
vestigate the chemoselectivity in the N-formylation of primary
and secondary amines. Thus N-formylation of diamine 5 led to
the formation of benzimidazole 6 in 88% yield (Scheme 2).
Entry
1
Substrate
3
Product
4
Time
[h]
Yield[b]
[%]
3a
4a
26
24
92
92
2
3
3b
3c
4b
4c
22
94
4
5
3d
3e
4d
4e
27
30
83
80
Scheme 2. Chemoselectivity in the N-formylation of amines.
6
3 f
4 f
24
NR[c]
This is ascribable to a cascade reaction involving chemoselec-
tive N-formylation of the primary amino group to form inter-
mediate II by hydroxymethylation of 5 and the AuCNT-cata-
lyzed oxidation of I, followed by intramolecular cyclization of II
to III and dehydration to give benzimidazole 6. Upon perform-
ing this reaction in the absence of AuCNT, there was no con-
version at all; this confirmed that benzimidazole formation
proceeded through AuCNT-catalyzed N-formylation and not
through an uncatalyzed imine/iminium intermediate. Yet an-
other chemoselective formylation was performed by using p-
aminodiphenylamine (7) as the substrate (Scheme 2). As ex-
pected, the primary amino group underwent selective formyla-
tion to provide formamide 8 in excellent yield (92%).
To compare the catalytic efficiency of AuCNT with that of
colloidal AuNPs and a gold salt, the formylation of 1a to 2a
was performed with identical loading (0.34 mol%) of different
catalysts (Table 4). Only trace amounts of 2a were observed in
the presence of colloidal AuNPs (Table 4, entry 2), and there
was no product formation at all in the presence of gold salt
HAuCl4 even after a reaction time of 3 days (Table 4, entry 3),
which thus confirmed the superior catalytic activity of AuCNT
(Table 4, entry 1).
7
8
9
3g
3h
3i
4g
4h
4i
28
25
24
89
86
NR[c]
[a] Conditions: Amine 1 (0.1 mmol), formaldehyde (2 equiv.), AuCNT (aq
suspension, 0.34 mol%), NaOH (3 equiv.), toluene/H2O (1:1, 1 mL), RT,
open flask (air), 24 h. [b] Yield of purified isolated product. [c] NR=no re-
action.
Anilines substituted in the meta position reacted well, re-
gardless of the electronic nature of the substituent; for exam-
ple, 3d and 3e provided products 4d and 4e in 83 and 80%
yield, respectively (Table 3, entries 4 and 5), whereas no reac-
tion was observed in the case of p-nitroaniline (3 f) even after
24 h, presumably owing to strong deactivation of the amino
group (Table 3, entry 6). Anilines possessing various substitu-
ents in the ortho position, for example, anthranilic acid (3g)
and o-aminophenol (3h), also underwent facile formylation
under our experimental conditions to furnish products 4g and
4h in 89 and 86% yield, respectively (Table 3, entries 7 and 8).
There was no competitive O-formylation in the latter case, and
Besides the requirement for only a very low catalyst loading
(0.34 mol%), the recyclability of AuCNT is remarkable (Table 5).
The catalyst was easily recovered by simple centrifugation and
was reused for N-formylation without any significant loss of ac-
tivity up to four cycles with only a marginal drop in the yield
from 94 to 92% (Table 5, entries 1–5).
The key role of the AuCNT nanohybrid as the active catalytic
species and the heterogeneous nature of the catalyst were
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starting material (24–30 h, as monitored by TLC). The aqueous
layer was then extracted with EtOAc (3ꢁ10 mL). The combined or-
ganic layer was dried (anhydrous Na2SO4) and concentrated under
vacuum. The crude residue was directly subjected to column chro-
matography to give pure product 2, 4, 6, or 8.
Table 4. Comparison of various Au sources in the N-formylation of N-
methylaniline (1a).[a]
Entry
Catalyst
Loading
[mol%]
Time
[h]
Yield[b]
[%]
Recycling experiment
1
2
3
AuCNT
AuNP colloid
HAuCl4
0.34
0.34
0.34
24
72
72
94
trace
NR[c]
A solution of N-methylaniline (1a; 10.7 mg, 0.1 mmol) in a mixture
of toluene/water (1:1, 1 mL) was prepared and aqueous HCHO
(25 mL, 2 equiv.), NaOH (12 mg, 0.3 mmol, 3 equiv.), and aqueous
AuCNT (100 mL, from 3.4 mm suspension in H2O, 0.34 mol%) were
added. The mixture was stirred until the complete consumption of
the starting material (24 h, as monitored by TLC) at room tempera-
ture. The catalyst was then recovered by simple centrifugation and
reused without further purification.
[a] Conditions: Amine 1a (0.1 mmol), formaldehyde (2 equiv.), catalyst
(0.34 mol%), NaOH (3 equiv.), toluene/H2O (1:1, 1 mL), RT, open flask (air).
[b] Yield of purified isolated product. [c] NR=no reaction.
Table 5. Recycling experiments of the AuCNT catalyst.[a]
TON and TOF experiments
A solution of N-methylaniline (1a; 10.7 mg, 0.1 mmol) in a mixture
of toluene/water (1 ml, 1:1) was prepared and aqueous HCHO
(25 mL, 2 equiv.), NaOH (12 mg, 0.3 mmol, 3 equiv.), and aqueous
AuCNT (50 mL, from 3.4 mm suspension in H2O, 0.17 mol%) were
added. The mixture was stirred for 30 h at room temperature, and
then the catalyst was removed by centrifugation. The mixture was
concentrated under vacuum. The crude residue was directly sub-
jected to silica gel column chromatography to afford pure 2a
(87% yield). The TON and TOF were calculated by using Equa-
tions (1) and (2):
Entry
AuCNT
Time
[h]
Yield[b]
[%]
1
2
3
4
5
fresh
24
24
24
24
24
94
93
93
92
92
recycle 1
recycle 2
recycle 3
recycle 4
[a] Conditions: Amine 1a (0.1 mmol), formaldehyde (2 equiv.), AuCNT (aq
suspension, 0.34 mol%), NaOH (3 equiv.), toluene/H2O (1:1, 1 mL), RT,
open flask (air), 24 h. [b] Yield of purified isolated product.
TON for amine¼ product ðmmolÞ = catalyst ðmmolÞ
¼ 0:087 = 0:00017
¼ 512
ð1Þ
ð2Þ
confirmed by performing the N-formylation of 1a by using
AuCNT (0.34 mol%) and removing the catalyst after 12 h by
centrifugation. At that stage, approximately 44% conversion
was observed and there was no further progress upon stirring
the catalyst-free reaction mixture overnight at room tempera-
ture.
TOF for amine¼ TON = time
¼ 512 = 30
¼ 17 hꢀ1
The product was isolated in 94% yield by increasing the catalyst
loading to 0.34 mol%, which also reduced the reaction time to
24 h.
In conclusion, we developed a simple protocol for the N-for-
mylation of primary and secondary amines with aqueous
HCHO as the formyl source by using a AuCNT nanohybrid cata-
lyst. The reported conditions are effective in open air, without
an O2 atmosphere, and require no heating. Excellent chemose-
lectivity was observed in that a primary amino group could be
formylated in the presence of phenolic OH and diarylamino
groups. The AuCNT nanohybrid compares favorably to other
supported gold nanoparticle based catalytic systems in terms
of catalyst loading, selectivity, recyclability, and mild reaction
conditions.
Acknowledgements
Support from the Indo-French Centre for the Promotion of Ad-
vanced Research (IFCPAR)/Centre Franco-Indien pour la Promo-
tion de la Recherche Avancꢀe (CEFIPRA) is gratefully acknowl-
edged (Project no. 4705-1). The Service de Chimie Bioorganique
et de Marquage belongs to the Laboratory of Excellence in Re-
search on Medication and Innovative Therapeutics (ANR-10-
LABX-0033-LERMIT).
Experimental Section
Keywords: gold · heterogeneous catalysis · nanoparticles ·
nanotubes · recyclability
General procedure for the N-formylation of amines
A solution of amine 1, 3, 5, or 7 (0.1 mmol) in a mixture of tolu-
ene/water (1:1, 1 mL) was prepared and 38% aqueous HCHO
(25 mL, 2 equiv.), NaOH (12 mg, 0.3 mmol, 3 equiv.), and aqueous
AuCNT (100 mL from 3.4 mm suspension in H2O, 0.34 mol%) were
added. The mixture was stirred until complete consumption of the
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