An Efficient Protocol for Formylation of Amines Using Carbon Dioxide and PMHS under Transition-Metal-Free Conditions

Article abstract of DOI:10.1055/s-0035-1561571

A highly efficient, green and simple base catalytic system was investigated for the formylation of amines using CO₂ and PMHS [poly(methylhydrosiloxane)] under mild reaction conditions. This reaction proceeds smoothly without additives and furnishes the corresponding N-formylated products from both the 1° and the 2° aliphatic as well as aromatic amines in good to excellent yields.

Full text of DOI:10.1055/s-0035-1561571

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
D. B. Nale, B. M. Bhanage
Letter
Synlett
An Efficient Protocol for Formylation of Amines Using Carbon
Dioxide and PMHS under Transition-Metal-Free Conditions
Deepak B. Nale
R1
R1
NH
R2
K2CO3
Bhalchandra M. Bhanage*
N
R2
CHO
CO2
+
PMHS, 100 °C,
THF, 12 h
Department of Chemistry, Institute of Chemical Technology,
N. Parekh Marg, Matunga, Mumbai 400019, India
bm.bhanage@gmail.com
26 examples
100% conv.
1° or 2° aliphatic, aromatic
up to 99% yield
bm.bhanage@ictmumbai.edu.in
Received: 28.11.2015
Accepted after revision: 17.01.2016
Published online: 18.02.2016
hydrogen (H2) have a similar reduction potential as the Si–H
bond is kinetically more reactive (due to its polarity and
lower bond dissociation energy, i.e. 92 kcal mol–1 in SiH4
and 104 kcal mol–1 in H2).8
DOI: 10.1055/s-0035-1561571; Art ID: st-2015-d0921-l
Abstract A highly efficient, green and simple base catalytic system
was investigated for the formylation of amines using CO2 and PMHS
[poly(methylhydrosiloxane)] under mild reaction conditions. This reac-
tion proceeds smoothly without additives and furnishes the corre-
sponding N-formylated products from both the 1° and the 2° aliphatic
as well as aromatic amines in good to excellent yields.
Formamides have widespread applications as solvents,
raw materials and versatile intermediates in various syn-
thetic processes, especially as precursors to isocyanides and
formamidines.9 The formamide moiety can impart high bi-
ological activity in various pharmaceuticals.10 The formyla-
tion of dimethylamine using CO2 to form N,N-dimethylfor-
mamide (DMF) has gained considerable industrial attention
using homogeneous or heterogeneous catalysis.2a,c,d,11–14
Conventionally, formamides are synthesized using carbon
monoxide as the carbonyl source under forcing conditions
with sodium methoxide as a catalyst.15 Baiker et al.16 and
Beller et al.17 have reported that non-precious metal com-
plexes demonstrate higher activities in the hydrogenation
of CO2 for the production of DMF. The reducing characteris-
tics of hydrosilanes have been used to convert CO2 to car-
boxylic acids, formamides and methylamines.13b–e,13i,j,18,19
However, most of the existing methodologies have some
disadvantages such as requirement for toxic reagents, such
as phosgene or CO, expensive metal phosphine complexes
and often require forcing reaction conditions. Hence, there
is still a need to develop a transition-metal-free protocol for
the synthesis of formamides from aliphatic as well as aro-
matic amines using CO2 as a C1 source. As a result, we here-
in report a transition-metal-free, base-catalyzed route for
the efficient synthesis of formamides from CO2 and amines
using PMHS under mild reaction conditions (Scheme 1).
Table 1 demonstrates the effect of reaction parameters
for the formylation of morpholine 1a using CO2 and PMHS
as a model reaction. Initially, the reaction was carried out in
the absence of catalyst, but the desired product was not ob-
tained (Table 1, entry 1). Then, a series of experiments was
Keywords amine, carbon dioxide, formamides, formylation, potassi-
um carbonate
Carbon dioxide is a ubiquitous C1-building block in syn-
thetic chemistry.1 A green route for the forrmylation of
amines employs CO2 along with poly(methylhydrosiloxane)
(PMHS) and the development of this technology has gener-
ated long-term interest from both academic and industrial
viewpoints.2 However, due to the inherent thermodynamic
and kinetic stability of CO2, its activation is challenging, es-
pecially under mild conditions. It has been reported that al-
kali carbonates3 and hydrosilanes can activate CO2 under
mild conditions in the presence of amines to form the cor-
responding carbamic acids.4 However, the scope of CO2
chemical transformations in industrial applications is re-
stricted to areas such as the synthesis of urea, cyclic and
polymeric carbonates, formic acid, methanol and salicylic
acid.5 An improvement in such synthetic procedures would
be to avoid the use of transition-metal catalysts. Transition-
metal-free reactions have gained considerable attention in
organic synthesis, especially in hydrosilylation reactions.6
PMHS is an abundant waste product of the silicone in-
dustry; is nontoxic, biodegradable, moisture-stable, and in-
expensive. PMHS has been used as an environmentally be-
nign reductant for CO2 and serves as an efficient energy
source as well.5h,7 In fact, hydrosilanes (Si-H) and molecular
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

B
D. B. Nale, B. M. Bhanage
Letter
Synlett
Reaction occurred under solvent-free conditions, but
provided a very low yield of the desired product (Table 1,
entry 6), whereas 1,4-dioxane and THF were found to afford
good to excellent (76–99%) yields of 2a (Table 1, entries 4
and 8). The effect of CO2 pressure on the yield of the desired
product was examined at 100 °C for 12 hours. The yield of
2a increased to quantitative at 3 MPa (Table 1, entries 4, 9
and 10).
R1
NH
R2
R1
O
H
K2CO3
N
R2
+
O C O
PMHS, THF,
100 °C, 12 h
1
2
Scheme 1 Scope of formamide derivatives from amines, CO2 and
PMHS
performed to examine the reaction parameters such as na-
ture of the catalyst, CO2 pressure, temperature, reaction
time, effect of PMHS loading and solvent to optimize the
synthesis of the desired product 2a from morpholine.
We investigated the effect of temperature on the yield of
2a using K2CO3 as catalyst at 3 MPa CO2 pressure ranging
from 80 °C to 120 °C with a reaction time of 12 hours (Table
1, entries 4, 11 and 12). It was observed that, at 80 °C, the
yield of 2a was quite low (Table 1, entry 12). On increasing
the reaction temperature from 100 °C to 120 °C the desired
product was obtained in an excellent yield (99%) within 12
hours (Table 1, entry 11).
We studied the effect of reaction time, ranging from 8
hours to 16 hours (Table 1, entries 13 and 14). The maxi-
mum yield (99%) was obtained within 12 hours at 3 MPa
pressure.
Finally, we focused on the amount of the PMHS (Table 1,
entries 4, 15 and 16) and it was found that 1 mmol of mor-
pholine and 4 mmol of PMHS provided the highest yield of
the desired product (Table 1, entry 4). Accordingly, the opti-
mized reaction conditions were: amine (1 mmol), K2CO3 as
catalyst (10.0 mol%), THF as solvent (5.0 mL), PMHS (4.0
mmol), CO2 pressure of 3 MPa, temperature of 100 °C and a
reaction time of 12 hours.20
To check the generality of these reaction conditions, we
examined the optimized protocol for N-formylation of a
range of primary and secondary aliphatic and aromatic
amines 1a–z.
The N-formylation of aliphatic secondary amines 1b–f,
provided good yields (75–92%) of the corresponding desired
products 2b–f (Table 2, entries 2–6). It was observed that
the bulkier aliphatic secondary amines provided moderate
yields of the desired products, maybe due to steric hin-
drance (Table 2, entry 5). Substrate 1g furnished selectively
the N-formylation product 2g in 54% yield without reduc-
tion of the double bond (Table 2, entry 7). Cyclic secondary
amines gave the corresponding N-formylation products in a
good to excellent yields (Table 2, entries 8, 9, 11–14). Unfor-
tunately, the reaction did not proceed in the case of 1j (Ta-
ble 2, entry 10), presumably because of the sterically hin-
dered and deactivating COOH group on the adjacent carbon
atom of the amine. Furthermore, a second formylation of 1n
did not occur at the hindered site, with only monoformyla-
tion product 1n being isolated in 85% yield (Table 2, entry
14).
Table 1 Study of Reaction Parameters for Formylation of Morpholine
Using CO2 and PMHSa
H
cat.
O
NH
+
O
O
C O
N
reductant, solvent,
temperature, time
O
1a
2a
Entry Morpholine/PMHS Catalyst
(mmol:mmol)
CO2
(MPa) (°C)
Temp
Time Yield of 2a
(h)
(%)b
1
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:4
1:6
1:2
–
3
3
3
3
–
3
3
3
2
4
3
3
3
3
3
3
100
12
NR
99c
85
99c
NR
42
76
39
68
99
99
65
99
69
98
50
2
Cs2CO3
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
100
100
100
100
100
100
100
100
100
120
80
12
12
12
24
12
12
12
12
12
12
12
16
8
3
4
5
6d
7e
8f
9
10
11
12
13
14
15
16
100
100
100
100
12
12
a Reaction conditions: morpholine (1 mmol), catalyst (10.0 mol%), THF (5.0
mL), 100 °C, 12 h.
b Based on GC/GC–MS analysis. The yield was quantified by the external
standard method using morpholine-1-carbaldehyde.
c Isolated yield after column chromatography purification. NR: no reaction.
d The reaction was performed neat.
e Solvent used was 1,4-dioxane.
f Solvent used was glycerol.
For the initial studies, the effect of catalysts such as
Cs2CO3, Na2CO3 and K2CO3 was investigated (Table 1, entries
2–4). We observed that both Cs2CO3 and K2CO3 provided an
excellent yield (99%) of the desired product (Table 1, entries
2 and 4) and so the less expensive K2CO3 was chosen as the
best catalyst for the title reaction.
The indoline substrate 1o underwent efficient conver-
sion to 2o (78%; Table 2, entry 15), whereas 1,2,3,4-tetrahy-
droquinoline provided only trace amounts of product 1p
(Table 2, entry 16). The benzylic secondary amines 1q and
1r proved to react well, providing excellent yields (95% and
91%) of the desired formamides after 12 hours at 100 °C un-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

C
D. B. Nale, B. M. Bhanage
Letter
Synlett
Table 2 (continued)
der the optimized reaction conditions (Table 2, entries 17
and 18). The N-benzyl-2-methylpropan-2-amine (1t), pos-
sessing the N-tert-butyl group, was not converted into the
corresponding product (2t; Table 2, entry 20). The aromatic
secondary amine, N-methylaniline readily provided the de-
sired product in good yield (93%; Table 2, entry 19), but N-
methyl-2-nitroaniline (1u) did not provide the desired
product, presumably due to steric reasons (Table 2, entry
21). Cyclohexylamine and aniline furnished the desired
products in a moderate to good yields (Table 2, entries 22
and 23). However, anilines bearing ortho substituents (1x,
1x′ and 1x′′) did not give the corresponding formamide
products (Table 2, entry 24). Reaction of [1,1′-biphenyl]-4-
amine and tert-butylamine afforded the corresponding de-
sired products 2y and 2z in moderate to good yields (59%
and 70%; Table 2, entries 25 and 26).
Entry
Substrate
Product
Yield (%)b
98
N
CHO
NH
8
2h
1h
N
N
9
92
H
CHO
1i
2i
COOH
COOH
N
N
H
10
NR
CHO
1j
2j
N
NH
N
N CHO
11
12
13
88
84
71
In summary, herein we have reported the first example
of the use of K2CO3 as an efficient catalyst for the formyla-
tion of amines by hydrosilylation of CO2 using PMHS. We
believe this system will add to the methodology for the syn-
thesis of formamides.
1k
2k
Ph
N
NH
Ph
Ph
N
N CHO
1l
2l
N
NH
N
N CHO
Table 2 Synthesis of Formamidesa
Ph
1m
2m
H
O
cat.
NH
N
O
C O
+
reductant, temp,
solvent, time
HN
NH
HN
N
CHO
1° or 2°
14
85
2
1
2n
1n
1o
Entry
1
Substrate
Product
Yield (%)b
99c
15
16
78c
O
N
CHO
O
NH
N
N
H
CHO
2a
2o
1a
N
H
N H
2
3
4
92c
89c
87
trace
N
N
H
1b
1b
CHO
1p
2p
Et
Et
Et
Et
N
H
CHO
N
N
N
H
17
18
95c
91
CHO
2c
1c
2q
1q
N
N
Ph
N
Ph
Ph
N
H
Ph
H
CHO
CHO
1d
2d
1r
2r
N
H
N
NH
5
51
N
CHO
19
20
93c
NR
CHO
1e
2e
2s
1s
nBu
nBu
nBu
N
CHO
N
N
H
N
6
7
75c
54
nBu
N
H
N
2f
1f
CHO
1t
2t
H
CHO
1g
2g
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
D. B. Nale, B. M. Bhanage
Letter
Synlett
Table 2 (continued)
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Entry
Substrate
Product
Yield (%)b
NR
H
N
N
CHO
21
NO2
NH2
NO2
2u
1u
1v
H
N
(3) (a) Li, S.; Jin, H.; Mumford, K. A.; Smith, K.; Stevens, G. J. Energy
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CHO
22
23
82c
85c
2v
H
NH2
N
CHO
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1w
2w
H
NH2
X
N
CHO
X
24
NR
X= F (1x),
Cl (1x'),
Br (1x")
X= F (2x),
Cl (2x'),
Br (2x")
NH
CHO
NH2
25
26
59
2y
1y
NH
CHO
NH2
70c
1z
2z
a Reaction conditions: substrate (1.0 mmol), anhyd K2CO3 (10.0 mol%), THF
(5.0 mL), CO2 (3 MPa), PMHS (4.0 mmol), 100 °C, 12 h. NR: no reaction.
b Based on GC/GC–MS analysis. The yield was quantified by using the exter-
nal standard method using morpholine-1-carbaldehyde.
c Isolated yield after column chromatography purification.
Acknowledgment
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Catalysis; Wiley-VCH: Weinheim, 2007. (b) Yuan, Y.; Thome, I.;
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The author Deepak B. Nale is grateful to the University Grant Com-
mission, India for providing financial support under the UGC-SAP-SRF
program, and the Green Technology Centre of Institute of Chemical
Technology (ICT), Mumbai, India.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1561571.
S
u
p
p
ortioInfgrmoaitn
S
u
p
p
ortiInfogrmoaitn
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D. B. Nale, B. M. Bhanage
Letter
Synlett
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flushing the reactor three times with 10 atm of N2, followed by
the CO2 to the desired pressure, the reactor was heated and
stirred vigorously at 530–600 rpm for 12 h. After completion of
the reaction, the reactor was cooled to r.t. and the pressure was
carefully released. Basic hydrolysis of the reaction mixture was
carried out as previously described.5h The reaction mixture was
then extracted with EtOAc (3 × 10 mL), the combined organic
layers were dried over anhyd Na2SO4, filtered and evaporated in
vacuo. The isolated crude oil product was further purified by
column chromatography on 100–200 mesh size silica gel (elu-
tion with 20:4 → 10:2 petroleum ether–EtOAc) to provide the
corresponding pure compound. The spectroscopic data are con-
sistent with those reported in the literature and in agreement
with the assigned structures.21
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NMR (400 MHz, 30 °C, CDCl3, TMS): δ = 7.92 (s, 1 H), 3.58–3.52
(m, 4 H), 3.44 (t, J = 4.0 Hz, 2 H), 3.28 (t, J = 4.0 Hz, 2 H). 13C NMR
(100 MHz, 30 °C, CDCl3, TMS): δ = 160.81, 67.08, 66.26, 45.67,
40.44. GC–MS (EI): m/z (%) = 115 (100) [M]+.
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Indoline-1-carbaldehyde (2o): brown solid; yield: 78%. 1H
NMR (400 MHz, 30 °C, CDCl3, TMS): δ (mixture of rotamers) =
8.92 (s, 1 H, major), 8.50 (s, 1 H, minor), 7.24–7.14 (m, 3 H),
7.05–7.01 (m, 1 H), 4.05 (t, J = 8.0 Hz, 2 H), 3.14 (t, J = 8.0 Hz, 2
H). 13C NMR (100 MHz, 30 °C, CDCl3, TMS): δ = 157.58, 140.93,
127.57, 126.06, 125.13, 124.28, 109.38, 44.64, 27.17. GC–MS
(EI): m/z (%) = 147 (65) [M]+.
N-Benzyl-N-methylformamide (2q): colorless oil; yield: 95%.
1H NMR (400 MHz, CDCl3, 30 °C, TMS): δ (mixture of rotamers)
= 8.26 (s, 1 H, major), 8.13 (s, 1 H, minor), 7.37–7.17 (m, 5 H),
4.50 (s, 2 H, minor), 4.37 (s, 2 H, major), 2.82 (s, 3 H, minor),
2.76 (s, 3 H, major). 13C NMR (100 MHz, 30 °C, CDCl3, TMS): δ
(mixture of rotamers) = 162.74 (major), 162.57 (minor), 135.96
(minor), 135.69 (major), 128.88 (major), 128.67 (minor), 128.21
(major), 128.08 (minor), 127.62 (minor), 127.36 (major), 53.47
(major), 47.74 (minor), 34.05 (major), 29.44 (minor). GC–MS
(EI): m/z (%) = 149 (100) [M]+.
N-Methyl-N-phenylformamide (2s): brownish oil; yield: 93%.
1H NMR (400 MHz, 30 °C, CDCl3, TMS): δ = 8.39 (s, 1 H), 7.33 (t,
J = 8.0 Hz, 2 H), 7.20 (t, J = 8.0 Hz, 1 H), 7.10 (d, J = 4.0 Hz, 2 H),
3.24 (s, 3 H). 13C NMR (100 MHz, CDCl3, 30 °C, TMS): δ (mixture
of rotamers) = 162.27 (major), 162.17 (minor), 142.08, 129.55,
128.96, 126.32 (major), 126.16 (minor), 123.53, 122.26, 31.95.
GC–MS (EI): m/z (%) = 135 (85) [M]+.
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(20) General Procedure for the Synthesis of Formamide Deriva-
tives: The catalyst (10.0 mol%) was introduced into a 100-mL
autoclave equipped with an overhead stirrer and an automatic
temperature control system5i containing amine (1 mmol),
PMHS (4.0 mmol) and THF (5.0 mL) at r.t. After sealing and
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E