Selective: N-formylation/N-methylation of amines and N-formylation of amides and carbamates with carbon dioxide and hydrosilanes: Promotion of the basic counter anions of the zinc catalyst

Article abstract of DOI:10.1039/d0gc02890h

A catalyst composed of commercially available Zn(OAc)2 and 1,10-phenanthroline (phen) was effective in the N-formylation/N-methylation of amines using CO2 as the C1 source in the presence of hydrosilanes. An equimolar reaction of N-methylaniline with PhSiH3 under a CO2 atmosphere yielded the N-formylation product in 92% yield at 25 °C. Scale-up of the reaction using 10 mmol substrate was also successful in affording the desired product in 83% yield (1.1 g). This catalyst exhibits a high thermal stability and a turnover number (TON) of 385000 at 150 °C. In addition, the reaction of N-methylaniline in the presence of excess Ph2SiH2 produced N,N-dimethylaniline. Furthermore, our catalytic protocol was developed for the N-formylation of amides and carbamates, which have smaller pKa values and lower reactivities than the corresponding amines. The present Zn(OAc)2/phen catalyst was found to show versatility in the conversion of CO2 and amines into several functionalized organic chemicals under mild conditions. We propose that the basic counter anion (i.e., the acetate) of the catalyst activates both the Si-H and N-H bonds.

Full text of DOI:10.1039/d0gc02890h

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ARTICLE
Selective N-Formylation/N-Methylation of Amines and N-
Formylation of Amides and Carbamates with Carbon Dioxide and
Hydrosilanes: Promotion of the Basic Counter Anion of the Zinc
Catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Qiao Zhang,a Xiao-Tao Lin,ab Norihisa Fukaya,a Tadahiro Fujitani,aKazuhiko Sato,a and Jun-Chul
Choi*ab
A catalyst composed of commercially available Zn(OAc)2 and 1,10-phenanthroline (phen) was effective in the N-
formylation/N-methylation of amines using CO2 as the C1 source in the presence of hydrosilanes. An equimolar reaction of
N-methylaniline with PhSiH3 under a CO2 atomosphere yielded the N-formylation product in 92% yield at 25 °C. Scale-up of
the reaction using 10 mmol substrate was also successful to afford the desired product in 83% yield (1.1 g). This catalyst
exhibits a high thermal stability and a turnover number (TON) of 385,000 at 150 °C. In addition, the reaction of N-
methylaniline in the presence of excess Ph2SiH2 produced N,N-dimethylaniline. Furthermore, our catalytic protocol was
developed for the N-formylation of amides and carbamates, which have smaller pKa values and lower reactivities than the
corresponding amines. The present Zn(OAc)2/phen catalyst was found to show versatility in the conversion of CO2 and
amines into several functionalized organic chemicals under mild conditions. We propose that the basic counter anion (i.e.,
the acetate) of the catalyst activates both the Si−H and N−H bonds.
horizontal utilization and vertical reduction as a series of
“diagonal reactions,” allowed the preparation of a
number of valuable chemicals containing newly created
Introduction
Carbon dioxide (CO2) is one of the major greenhouse
gases, and so increasing attention has been paid to the
application of CO2 as a C1 building block for the
preparation of valuable chemicals.1–7 More specifically,
a range of catalysts have been developed to transform
the thermodynamically stable CO2 into a variety of
organic compounds. For example, the functionalization
of CO2 with amines to give ureas and carbamates
maintains a +4 oxidation state on the carbon atom, and
is known as “horizontal utilization.”8 In addition, the
conversion of CO2 into formic acid, formaldehyde,
methanol, and methane requires reduction of the
carbon atom and the elimination of one or two oxygen
atoms. Such transformations are known as “vertical
reduction.”8 Cantat et al. reported a combination of
C−E bonds (E = N, O, S, etc.).
The N-formylation of amines using CO2 in the
presence of reductants through 2e− reduction processes
yields formamides, which is classified as a diagonal
reaction of CO2. Formamides are widely used as solvents
and medicine components, in addition to being common
in agricultural chemistry. For example, N,N-
dimethylformamide (DMF) is a common polar solvent in
organic synthesis. The traditional synthesis of DMF
involves the use of the toxic carbon monoxide (CO) and
dimethylamine. In contrast, the use of H2 as the
reductant in the presence of CO2 and amines provides a
convenient alternative, but requires the use of noble
metal catalysts under relatively harsh conditions (e.g.,
high temperatures).9, 10 The addition of hydrosilanes to
such reactions renders the N-formylation of amines by
CO2 relatively smooth, and the N-methylation of amines
by CO2 requires three equivalents of the desired
hydrosilane to promote the 6e− reduction reaction to
a. National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba
Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. Email:
junchul.choi@aist.go.jp; Tel: (+81) 029-861-9283.
b. Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1
Tennodai, Tsukuba, Ibaraki 305-8573, Japan.
Electronic Supplementary Information (ESI) available: Screening of reaction
conditions and NMR spectra for the products. See DOI: 10.1039/x0xx00000x
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yield the desired methylated amine. The methylation of the reaction selectivity are also examined. As indicated
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DOI: 10.1039/D0GC02890H
amines is of particular importance in terms of the in Scheme 1, the reactions of CO2 with primary amines
regulation of biological functions and also drug in the presence of hydrosilanes are explored, in addition
discovery.11−16 Indeed, several reviews have recently to the N-formylations of amides and carbamates.
been published regarding the N-formylation and N-
methylation of amines with CO2.17−23 The catalysis of this
Selective N-formylation / N-methylation of amines
R
process in the presence of hydrosilanes can be classified
N
CHO
into four groups: (i) metal catalysis;24−31 (ii) organo-
R
N-formylation
32−38
catalysis;8,
(iii) heterogeneous and solid
R
R
cat. = Zn(OAc)2 / phen
phen =
or
R
+ CO2 + R3SiH
H
N
catalysts;39−49 and (iv) solvent-promoted catalyst-free
conditions.50 N-methylation reactions can also be
classified using the same system, i.e., metal
catalysis,51−58 organo-catalysis,59−61 heterogeneous
N
CH3
N
N
R
selectvity tuned by temperature
and the ratio of CO2 / hydrosilane
N-methylation
67
catalysis,62−65 and catalyst-free conditions.66,
N-formylation of amides
Importantly, mechanistic and computational studies
O
O
cat. = Zn(OAc)2 / phen
+ CO2 + R3SiH
R
R
R
N
R
N
supported the results of the above experimental
H
CHO
68−72
investigations.36,
Upon examination of these
R = H, primary amide
R = H, secondary amide (first reported herein)
previous reports, three issues become apparent. Firstly,
different catalysts are required for the N-formylation
and N-methylation reactions, and the application of a
single catalyst for both processes has been limited.70,
73−85 Selective 2e−, 4e−, and 6e− CO2 reduction processes
of amines to give formamides, aminals, and N-
methylated amines are also rare.86−88 Secondly, the
discovery of suitable non-noble metal catalysts and
readily available efficient ligands remains a challenge.
Thirdly, the N-formylation of amides with CO2 and
hydrosilanes is more challenging than the corresponding
N-formylation of carbamates
(first reported herein)
O
O
cat. = Zn(OAc)2 / phen
+ CO2 + R3SiH
R
R
RO
N
RO
N
H
CHO
Scheme 1
A summary of this work: selective N-formylation / N-methylation of
amines and N-formylation of amides and carbamates with CO2 and hydrosilanes.
Results and Discussion
N-methylaniline was chosen as the substrate for the
reaction using amines. Although the first report into the standard reaction outlined in Table 1. Initially, it was
N-formylation of primary amides with CO2 and PhSiH3 found that the reaction of N-methylaniline with 5 bar
was published in 2018,89 the use of secondary amides as CO2 and 1 equiv. PhSiH3 in a 10 mL autoclave did not
substrates remains unexplored.
occur in the presence of the Zn(OAc)2 catalyst (2 mol%).
However, the addition of the phen ligand ([Zn]/[phen] =
1:3) resulted in successful N-formylation after 4 h to give
N-methylformanilide (1a) in 92% yield, as confirmed by
1H NMR spectroscopy (Table 1, entry 2). Other catalysts,
such as ZnCl2, ZnBr2, ZnI2, ZnSO4, and Zn(OTf)2 (OTf =
trifluoromethanesulfonate), were significantly less
effective in this reaction (Table 1, entries 3–7), while the
reaction catalyzed by zinc pivalate Zn(OPiv)2 in the
presence of the phen ligand yielded 1a in 83% yield
(Table 1, entry 8). The above results indicate that N-
formylation using the Zn catalyst requires both a basic
counter anion (e.g., acetate) and an N-donor ligand.
We recently reported the transformation of CO2 to
carbamate,90−95 in addition to a step-by-step CO2
hydrosilylation to give methane via silyl formate,
bis(silyl)acetal, and methoxysilane through the use of
zinc catalysts.96 Thus, we herein describe the use of
commercially available substances as the components of
a
Zn(OAc)2 catalyst in addition to the 1,10-
phenanthroline (phen) ligand for the selective N-
formylation and N-methylation of amines with CO2 in
the presence of hydrosilanes. The effects of the
temperature, CO2 pressure, and kind of hydrosilane on
2 | J. Name., 2012, 00, 1-3
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Recently, we reported the excellent activity of the Substituted 1,10-phenanthrolines (L1, L2, and L3) were
View Article Online
DOI: 10.1039/D0GC02890H
Zn(OAc)2/phen catalyst in the CO2 hydrosilylation also investigated as potential ligands (Table 1, entries
reaction, in addition to
moderate activities for 9−11). In the case of L1, a methyl-substituted 1,10-
ZnCl2/phen and ZnSO4/phen catalysts, and low activities phenanthroline, we found a slightly lower yield of 1a
for ZnBr2/phen and ZnI2/phen catalysts.96 Similarly, (90%) due to the electron donating effect of the methyl
Dyson and co-workers reported a model reaction using group. In contrast, for L2 and L3, the yields of 1a were
tetrabutylammonium as the catalyst (5 mol%, 1 h), and 28 and 22%, respectively. This was attributed to the
they also observed the significant influence played by electron-withdrawing nitro group in L2 and the two
the counter anion. More specifically, the catalyst methyl groups at sterically hindered positions in L3
bearing the OAc counter anion gave 1a in 46% yield, lowering the reactivities of these legands. In addition,
while negligible activities were observed for catalysts tetra-N-donor
ligand
bispicen
(N,N’-bis(2-
bearing Cl, Br, and OTf anions (i.e., <1% yields).36
pyridylmethyl)-1,2-ethanediamine) and hexa-N(O)-
donor ligand debpn (N,N’-bis(2-pyridylmethyl)-
CHO
H
N
2 mol% ZnX2
6 mol% ligand
98
bis(ethylacetate)-1,2-ethanediamine)97,
were also
N
+ CO2 + hydrosilane
5 bar 1 mmol
MeCN, 25 oC, 4 h
found to be reactive, but their reactivities were slightly
lower than when the phen ligand was employed (Table
1, entries 12−13). Furthermore, it was found that the use
of Ph2SiH2 as the reductant reduced the yield of 1a to
29%, and no reaction was observed when Ph3SiH, Et3SiH,
and (EtO)3SiH were employed (Table 1, entries 14−17).
1a
1 mmol
NO2
N
N
N
N
N
N
N
N
L2
phen
L1
L3
COOEt
EtOOC
N
N
NH HN
bispicen
Table 2 Determination of TONs for the synthesis of 1a by N-formylation of N-
methylaniline with CO2 and PhSiH3 catalyzed by Zn(OAc)2 / phen.a
N
N
N
N
Yield b
TON
debpn
Entry
catalyst
(mmol)
2 × 10−2
2 × 10−3
2 × 10−3
2 × 10−3
2 × 10−3
2 × 10−4
2 × 10−4
2 × 10−4
2 × 10−4
2 × 10−5
2 × 10−5
2 × 10−5
2 × 10−5
2 × 10−6
2 × 10−6
2 × 10−6
2 × 10−6
T
(°C)
25
25
50
75
25
50
75
100
50
75
100
125
75
100
125
150
100
time
(h)
4
4
4
4
72
4
4
4
72
4
4
4
72
4
4
4
72
Table 1 Catalyst screening for N-formylation of N-methylaniline with CO2 and
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
92%
30%
82%
87%
87%
13%
37%
71%
92%
9%
60%
91%
90%
29%
51%
77%
70%
46
150
410
435
435
hydrosilane.a
Entry
1
2
3
4
5
6
7
8
catalyst
Zn(OAc)2
Zn(OAc)2/phen
ZnCl2/phen
ZnBr2/phen
ZnI2/phen
hydrosilane
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
Ph2SiH2
Ph3SiH
Et3SiH
yield b
trace
92%
650
11%
1850
3550
4600
4500
30000
45500
45000
145000
255000
385000
350000
trace
trace
38%
trace
83%
90%
28%
22%
78%
69%
29%
trace
trace
trace
ZnSO4/phen
Zn(OTf)2/phen
Zn(OPiv)2/phen
Zn(OAc)2/L1c
Zn(OAc)2/L2d
Zn(OAc)2/L3e
Zn(OAc)2/bispicen
Zn(OAc)2/debpn
Zn(OAc)2/phen
Zn(OAc)2/phen
Zn(OAc)2/phen
Zn(OAc)2/phen
9
10
11
12
13
14
15
16
17
a
Reaction conditions: 1 (1 mmol), CO2 (5 bar), PhSiH3 (1 mmol), MeCN (3
mL). The reduced amount catalyst was diluted from a concentrated solution.
b
The yield was determined by 1H NMR in CDCl3 using mesitylene as the
(EtO)3SiH
internal standard.
a Reaction conditions: N-methylaniline (1 mmol), CO2 (5 bar), PhSiH3 (1 mmol),
b
Zn salt (0.02 mmol), phen (0.06 mmol), MeCN (3 mL).
Reaction
Our results also indicated that catalyst exhibits a high
thermo-stability and high catalytic efficiency. More
specifically, in Table 2, the TONs based on reduction of
the catalyst loading from 2 × 10−2 to 2 × 10−6 mmol can
temperature: 25 °C. Reaction time: 4 h. The yield was determined by 1H NMR
c
in CDCl3 using mesitylene as the internal standard. L1: 5-dimethyl-1,10-
phenanthroline. L2: 5-nitro-1,10-phenanthroline. e L3: 2,9-dimethyl-1,10-
d
phenanthroline.
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also be observed. Although this reduction slowed the
The N-formylation of N-methylaniline with CO and
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DOI: 10.1039/D0GC02890H
2
reaction, this was compensated for by either increasing PhSiH3 was also found to be successful for the gram-
the temperature or extending the reaction time from 4 scale synthesis. In addition, the bubbling of CO2 into a
to 72 h. For example, in Table 2 entry 2, the use of 2 × three-necked flask was also possible to avoid the use of
10−3 mmol catalyst gave a 30% yield of 1a after 4 h at 25 an autoclave. Figure 1 outlines the N-formylation
°C, while increased reaction temperatures of 50 and 75 reaction of 10 mmol N-methylaniline (1.07 g) with
°C promoted the yield of 1a to 82 and 87%, respectively. bubbled CO2 (1 atm) and PhSiH3 (10 mmol) in MeCN (30
In addition, a 87% yield of 1a was obtained when the mL) catalyzed by 2 mol% Zn(OAc)2/6 mol% phen. After 8
reaction time was extended to 72 h. When 2 × 10−6 mmol h, an 83% yield of 1a was obtained.
catalyst was employed, a TON of 385,000 was reached
Our protocol can therefore be applied to the N-
within 4 h at 150 °C. Our results therefore indicate that
formylation of a range of amines, as summarized in
the Zn(OAc)2/phen catalyst tolerates high temperatures
Scheme 2. The isolated yields for the corresponding
and efficiently enhances the N-formylation of amine
formamides reached up to 96%. It was also found that
with CO2 in the presence of PhSiH3.
substrates bearing halide (2a) and unsaturated bond
CHO
(3a) components could tolerate our reaction conditions.
H
N
Zn(OAc)2/phen
MeCN, 25 o
N
+ CO2 + PhSiH3
2 mol% Zn(OAc)2
6 mol% phen
C
R
R
R
R
+ CO2 + PhSiH3
H
N
N
CHO
10 mmol
bubbling
3 mL MeCN, 25 oC, 4 h
10 mmol
isolated yield
1 mmol 5 bar 1 mmol
CHO
CHO
N
CHO
N
Cl
N
1a
, 92%
3a
, 80%
2a
, 74%
Ph
Ph
CHO
CHO
CHO
N
CHO
N
MeO
N
6a
, 96%
4a
5a
, 73%
, 85%
CHO
CHO
N
N
N
9a
(A)
, 94%
8a
7a
, 80%
, 95%
CHO
N
N
CHO
N
CHO
10a
, 18%(4 h)
66%(24 h)
12a
, 87%
11a
, 73%
N
CHO
13a
, 62% (4 h)
94% (8 h)
Scheme 2
N-formylation of various amines with CO2 and PhSiH3 catalyzed by
Zn(OAc)2/phen.
Two different scenarios can be considered for the
(B)
activation of primary amines with CO2 and PhSiH3,
namely mono-N-formylation and double-N-formylation.
Although mono-N-formylation has been documented
Figure 1 Gram scale N-formylation of N-methylaniline with bubbled CO2 and
PhSiH3. (A) the yield was a function of time. (B) equipment setup.
4 | J. Name., 2012, 00, 1-3
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for the activation of primary amines, the selective Under these conditions, the mono-N-formylated
View Article Online
DOI: 10.1039/D0GC02890H
double-N-formylation of primary amines has been product (1b) was obtained in 48% yield. Increasing the
reported only by Li and co-workers,84 wherein they used amount of PhSiH3 to 2 equiv. has a 74% yield over 4 h. In
a Mn2(CO)10 complex bearing a bulky P-donor ligand as the case of an aliphatic amine, i.e., benzylamine, a
the catalyst.84 We therefore investigated the selective similar result was obtained, with 1 equiv. PhSiH3 giving a
mono- and double-N-formylation of primary amines 24% yield of the mono-N-formylated product (2b), and 2
with CO2 and PhSiH3 catalyzed by Zn(OAc)2/phen in equiv. promoting the yield to 97%. Thus, an excess of
detail, as described below. The initial reaction PhSiH3 was therefore considered to be key in the mono-
conditions for the mono-N-formylation involved the use N-formylation of primary amines. The substrate scope of
of 1 mmol aniline, 1 equiv. PhSiH3, and 5 bar CO2 at 25 the mono-N-formylation reaction was then examined as
°C over 4 h in the presence of the Zn(OAc)2/phen catalyst summarized in Scheme 4. As indicated, that the yields of
(Scheme 3).
the mono-N-formylation products were time
dependent, and for some substrates, longer time
reactions led to the formation of double-N-formylation
products (see the Supporting Information).
mono-N-formylation of aromatic primary amine
H
N
2 mol% Zn(OAc)2
6 mol% phen
NH2
CHO
+ CO2 + PhSiH3
MeCN, 25 oC, 4 h
To increase the yields of double-N-formylation
products, the reaction was carried out using 4 equiv.
PhSiH3 (Scheme 5) to give 1c6c in 7391% yields.
1b
1 mmol 5 bar 1 equiv.
2 equiv.
48%
74%
mono-N-formylation of aliphatic primary amine
2 mol% Zn(OAc)2
CHO
6 mol% phen
2 mol% Zn(OAc)2
RNH2
+ CO2 + PhSiH3
RN
CHO
3 mL MeCN, 25 oC, 24 h
6 mol% phen
NH2
CHO
N
+ CO2 + PhSiH3
MeCN, 25 oC, 4 h
H
0.5 mmol 5 bar 2 mmol
Isolated yield
2b
CHO
CHO
1 mmol
5 bar 1 equiv.
2 equiv.
24%
97%
CHO
N
N
N
CHO
CHO
CHO
Cl
Scheme 3
The influence of the amount of PhSiH3 for mono-N-formylation of
3c
, 73%
CHO
1c
4c
, 90%
2c
5c
, 85%
aromatic and aliphatic primary amines. Yields were isolated yields.
CHO
N
CHO
N
MeO
N
CHO
CHO
CHO
2 mol% Zn(OAc)2
6 mol% phen
MeO
H
RNH2
+ CO2 + PhSiH3
RN
, 83%
, 89%
6c
, 91%
3 mL MeCN, 25 oC, 4 h
CHO
1 mmol 5 bar 2 mmol
Isolated yield
Scheme 5
Double-N-formylation of primary amines.
H
CHO
N
CHO
The N-methylation of amines with CO2 as the C1
synthon in the presence of hydrosilanes was first
reported by Cantat51 and Beller52 independently in 2013,
and the first selective N-formylation and N-methylation
of amines with CO2 in the presence of hydrosilane was
documented in 2014.26 The N-methylation of amines
using CO2 as the C1 building block requires a 6e−
reduction process, and so at least 3 equivalents of Si−H
are required for 1 equivalent CO2. Furthermore, a high
N
H
CHO
N
H
Cl
2b
, 97%
1b
4b
, 74%
3b
, 68% (3 h)
H
H
H
N
MeO
N
N
CHO
CHO
CHO
MeO
6b
, 64% (2 h)
5b
, 65%
, 58% (1 h)
Scheme 4
Mono-N-formylation of primary amines.
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(A)
CHO
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temperature is required for this process. Initially, we
employed 3 equiv. PhSiH3 for the N-methylation of 1
mmol N-methylaniline in the presence of 3 bar CO2 at
120 °C. After allowing the reaction to proceed for 24 h,
the yield of N,N-dimethylaniline (1d) was 34%, while in
the presence of 4 equiv. Ph2SiH2, an increased yield of
0.02 mmol Zn(OAc)2
0.06 mmol phen
H
N
N
DOI: 10.1039/D0GC02890H
PhSiH3 + CO2
+
3 mL MeCN, 25oC
1a
1 mmol 5 bar 1 mmol
42% (1 h)
88% (2 h)
H
N
CHO
(B)
0.02 mmol Zn(OAc)2
0.06 mmol phen
N
1 mmol
92% was achieved. Scheme
6 summarizes the
silyl formate
PhSiH3 + CO2
1 mmol 5 bar
3 mL MeCN, 25oC, 4 h
10 min
Zn(OAc)2/phen-catalyzed N-methylation of amines using
CO2 and Ph2SiH2, whereby the methylated amines were
obtained in isolated yields of 6399%.
1a
84%
Scheme 7
Reaction determining step for N-formylation with CO2 and PhSiH3.
Yields were isolated yields.
2 mol% Zn(OAc)2
6 mol% phen
The success of the Zn(OAc)2/phen-catalyzed N-
formylation of amines in the presence of CO2 and PhSiH3
phen motivated us to expand the substrate scope from
amines to amides. It should be noted here that amides
have lower pKa values than amines, and are less reactive
to nucleophilies. Mandal and co-workers first reported
the N-formylation of primary amides in the presence of
CO2 and PhSiH3 catalyzed by bulky N-heterocyclic
carbenes.89 However, the N-formylation of secondary
amides was not considered in their study. Thus, we
investigated the N-formylation of both primary and
secondary amides catalyzed by Zn(OAc)2/phen. More
specifically, Scheme 8 summarizes the N-formylation
reaction of primary amides (0.5 mmol) using 5 bar CO2
and 1 mmol PhSiH3 to give the corresponding N-
formylated products in yields of 5586%.
R
R
R
R
+ CO2 + Ph2SiH2
H
N
N
CH3
MeCN, 120 oC, 24 h
1 mmol 5 bar 4 mmol
Isolated yield
CH3
N
CH3
CH3
N
MeO
Me
N
3d
1d
4d
, 88%
, 92%
2d
, 92%
CH3
CH3
CH3
Ph
Ph
N
Cl
N
N
CH3
, 63%
6d
, 77%
5d
, 99%
N
, 84%
N
N
N
CH3
CH3
7d
, 80%
8d
9d
, 79%
CH3
, 65%
10d
0.02 mmol Zn(OAc)2
0.06 mmol phen
Scheme 6
N-methylation of various amines.
O
O
+ CO2 + PhSiH3
CHO
3 mL MeCN, 25 oC, 24 h
R
N
R
NH2
Scheme 7 shows the reaction steps involved in the N-
formylation and CO2 hydrosilylation processes. More
specifically, for the N-formylation of N-methylaniline in
the presence of CO2 and PhSiH3 (Scheme 7A), a 42% yield
of 1a was obtained after 1 h, while an 88% yield was
obtained after 2 h. In addition, as shown in Scheme 7B,
we attempted to generate silyl formate from PhSiH3 and
CO2. Subsequently, this silyl formate was expected to
react with N-methylaniline to yield 1a. We found that
the latter reaction gave 1a in an 84% yield within 10 min.
These results confirmed that the reaction between CO2
and PhSiH3 to yield silyl formate was significantly slower
than the subsequent reaction between silyl formate and
the amine during the N-formylation stage.
H
0.5 mmol 5 bar 1 mmol
O
Isolated yield
O
O
CHO
CHO
CHO
N
H
N
H
N
H
1e
, 74%
3e
2e
, 74%
O
, 75%
O
O
CHO
CHO
CHO
N
H
N
N
H
H
OEt
4e
, 71%
5e
, 55%
6e
, 86%
Scheme 8
N-formylation of primary amides.
To the best of our knowledge, the N-formylation of
secondary amides in the presence of CO2 and
hydrosilanes was previously unreported prior to our
study. Thus, our initial investigation began using 1 mmol
6 | J. Name., 2012, 00, 1-3
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ARTICLE
acetanilide as the substrate with 5 bar CO2 and 1 mmol
After the successful N-formylation of primary and
View Article Online
PhSiH3 catalyzed by Zn(OAc)2/phen at 25 °C in MeCN secondary amides, we movedDOoI:u10r.103i9n/Dte0rGeCs0t2890toH
(Table 3, entry 1). However, after 24 h, only 15% yield of carbamates, since the resulting products can be used as
the desired product was obtained, and even upon mosquito repellents. Thus, we investigated the N-
increasing the temperature to 80 °C, the yield reached formylation of carbamates in the presence of CO2 and
only 27% (Table 3, entry 2). However, toluene, a non- PhSiH3 using a similar synthetic method to that
polar solvent, was found to increase the yield to 76 and employed for the N-formylation of secondary amides.
87% at 80 and 100 °C, respectively (Table 3, entries 4−5). Again, it was found that temperature played a role in the
Thus, the N-formylation of secondary amides requires a reaction, whereby product 2g was obtained in 48% yield
higher temperature than that needed for the reaction of at 100 °C, and in 63% yield at 120 °C. Overall, yields up
primary amides (>80 °C), and a non-polar solvent is also to 84% were achieved, as shown in Scheme 10.
essential. As shown in Scheme 9, our protocol was
applicable to a range of secondary amides, giving the
corresponding products in isolated yields of 8199%.
0.02 mmol Zn(OAc)2
0.06 mmol phen
O
O
R
R
+ CO2 + PhSiH3
5 bar 1 mmol
RO
N
RO
N
3 mL toluene, 100 oC, 24 h
H
CHO
1 mmol
Isolated yield
Table 3 Reaction condition screening for the N-formylation of acetanilide (a secondary
amide) with CO2 and PhSiH3 catalyzed by Zn(OAc)2/phen.a
CHO
CHO
CHO
0.02 mmol Zn(OAc)2
0.06 mmol phen
N
OMe
N
OEt
N
OMe
O
O
+ CO2 + PhSiH3
5 bar 1 mmol
O
3 mL solvent, T (oC), 24 h
O
O
N
N
H
3g
, 79%
CHO
1f
1g
2g
, 48%
, 73%
1 mmol
63% (120 oC)
CHO
CHO
N
entry
temperature (°C)
solvent
MeCN
MeCN
toluene
toluene
toluene
dioxane
THF
yield b
15%
27%
trace
76%
87%
82%
58%
41%
CHO
N
N
OMe
O
OMe
1
2
3
4
5
6
7
8
25
80
25
O
O
O
Cl
MeO
, 77% (120 oC)
6g
5g
, 74%
80
4g
, 84%
100
100
100
100
Scheme 10 N-formylation of carbamates.
EtOAc
Previously, we reported the Zn(OAc)2/phen-
catalyzed CO2 hydrosilylation,96 whereby we proposed a
six-membered intermediate composed of Zn(II), an OAc−
ligand, and the H−Si group (Scheme 11A). As a Lewis
acid, Zn(II) bonds with the H−Si bond through
electrophilic activation, while the carbonyl oxygen atom
present in OAc− exhibits a Lewis base character and so
interacts with the Si.99 The second role of Zn(OAc)2 was
activation of the N−H bond of the amine (Scheme 11B).
From the results of our previous research, selection of
the basic counter anion (e.g., acetate or carbonate) was
the key for transformation of the amines to carbamates
using CO2 and silicate esters. This is due to the fact that
these counter anions promote proton abstraction from
a
Reaction conditions: acetanilide (1 mmol), CO2 (5 bar), PhSiH3 (1 mmol),
b
solvent (3 mL), Zn(OAc)2 (0.02 mmol), phen (0.06 mmol), 24 h. The yield
was determined by 1H NMR in CDCl3 using mesitylene as the internal
standard.
0.02 mmol Zn(OAc)2
0.06 mmol phen
O
O
R
+ CO2 + PhSiH3
5 bar 1 mmol
R
R
N
3 mL toluene, 100 oC, 24 h
R
N
H
CHO
1 mmol
Isolated yield
O
O
O
N
N
N
CHO
CHO
CHO
1f
2f
, 91%
, 87%
3f
, 87%
91, 93
the N−H bonds through hydrogen bonding.90,
OEt
Although our previously reported ZnCl2/phen-catalyzed
CO2 hydrosilylation to silyl formate proceeded in a high
efficiency,96 the same catalyst was inactive in the N-
formylation of N-methylaniline in the presence of CO2
and PhSiH3 (Table 1 entry 3). These results indicate that
ZnCl2/phen can activate the Si-H bond in a similar
O
O
12
N
N
CHO
4f
CHO
, 81%
5f
, 99%
Scheme 9
N-formylation of secondary amides.
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J. Name., 2013, 00, 1-3 | 7
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ARTICLE
Journal Name
Table 4 Catalyst effect for the synthesis of 1f by N-formylation of acetanilide with
manner to Zn(OAc)2/phen (Scheme 11A), but that it is
inefficient in this context of N−H activation (Scheme
11B). A suitable catalyst for the N-formylation reaction
must therefore activate both the Si−H and N−H bonds.
Consequently, the deprotonated amine nucleophile
attacks silyl formate to yield an N-formylated amine as
the final product (Scheme 11C). Furthermore, Scheme
11D summarizes the diagonal transformation of CO2
with amines. The horizontal transformation describes
the CO2 reduction process in the presence of
hydrosilanes to yield C(II)-, C(0)-, C(−II)-, and C(−IV)-
based products, while the vertical functionalization
processes of silyl formate and methoxysilane with the
amines yields formamides and methylated amines,
respectively.
Ph2Si(OCHO)2.
View Article Online
DOI: 10.1039/D0GC02890H
0.02 mmol Zn
0.06 mmol phen
O
O
O
Ph2Si(OCH)2
silyl formate
+
100 oC, toluene, 24 h
N
H
N
CHO
1f
1 mmol
1 mmol
entry
1
2
3
4c
5
catalyst
-
Zn(OAc)2
Zn(OAc)2
Zn(OAc)2
ZnCl2
ligand
-
-
phen
phen
-
yield b
0%
21%
32%
31%
2%
6
ZnCl2
phen
3%
a Reaction conditions: acetanilide (1 mmol), Ph2Si(OCHO)2 (1 mmol), toluene
(3 mL), Zn (0.02 mmol), phen (0.06 mmol), 24 h. b The yield was determined
C
by 1H NMR in CDCl3 using mesitylene as the internal standard. In the
presence of CO2 (5 bar).
To confirm the proposed mechanism (Scheme 11),
we performed control experiments, as outlined in Table
4. Thus, the reaction of the prepared Ph2Si(OCHO)2
A. The activation of Si-H bond and CO2 insertion
OAc
R3SiOAc
R3SiH
(L)Zn
100
Zn(OAc)2(L)
H
Zn(OAc)(L)
CO2
O
H
R3Si
with acetanilide in toluene resulted in the formation of
N-formyl acetanilide (1f) when catalyzed by
Zn(OAc)2/phen (Table 4, entry 3). In contrast, in the
absence of catalyst (Table 4, entry 1) no product
formation was detected. This implies that the presence
of a catalyst is critical for this process. However, when
the reaction is catalyzed by ZnCl2, remarkably lower
yields of product were obtained (Table 4, entries 5 and
6), indicating that the presence of a carbonyl group (i.e.,
in the AcO− ion) is important for catalytic activation of
the N-H bond (Scheme 11B). On the other hand, for the
N-formylation of acetanilide in the presence of CO2 (5
bar) and Ph2SiH2 using the Zn(OAc)2/phen catalyst, a 58%
yield of 1f was obtained after 24 h (see the Supporting
Information).
O
O
R3SiOAc
L =
N
N
HCOOZn(OAc)(L)
H
OSiR3
Zn(OAc)2(L)
B. The activation of amine derivatives
OAc
(L)Zn
RRN
R
R
N
H
AcOH
R
O
Zn(OAc)2(L)
N
Zn(OAc)(L)
R
H
O
C. The reaction between silyl formate and amine derivatives
O
O
H
R
R
R
R
+
N
Zn(OAc)(L)
N
H
OSiR3
D. Zn(OAc)2 / phen catalyzed CO2 diagonal reactions
Carbon reduction
R3SiH
cat.
R3SiH
cat.
R3SiH
cat.
R3SiH
+II
0
-II
-IV
+IV
HCOOSiR3
CH2(OSiR3)2
CH4
CH3OSiR3
CO2
cat.
with B(C6F5
)
3
R
R
R
R
cat.
or
not
cat
or
not.
N
H
N
H
N
2 R3SiH
R
R
O
H
R
R
N
CH3
Scheme 11 Proposed mechanism for Zn(OAc)2/phen catalyzed N-formylation
and N-methylation of amine derivatives with CO2 and hydrosilanes.
Conclusions
The commercially available Zn(OAc)2/phen (phen =
1,10-phenanthroline) catalyst was found to be efficient
in the selective N-formylation and N-methylation of
amines using CO2 as the C1 source in the presence of
hydrosilanes. As an example, the turnover number for
the model N-formylation reaction in the presence of CO2
and PhSiH3 reached 385,000 at 150 °C. In addition, for
the N-formylation of primary amines, the selective
mono- and double-N-formylation processes were
8 | J. Name., 2012, 00, 1-3
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successfully activated amides for N-formylation
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toluene for the first time. Based on our results, we
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Conflicts of interest
There are no conflicts to declare.
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