A NHC-silyliumylidene cation for catalytic N?formylation of amines using carbon dioxide

Article abstract of DOI:10.1021/acscatal.0c03795

This study describes the use of a silicon(II) complex, namely, the NHC-silyliumylidene cation complex [(I_Me)₂SiH]I (1, I_Me =:C{N(Me)C(Me)}₂), to catalyze the chemoselective N-formylation of primary and secondary amines using CO₂ and PhSiH₃ under mild conditions to afford the corresponding formamides as a sole product (average reaction time: 4.5 h; primary amines, average yield: 95%, average TOF: 8 h^?1; secondary amines, average yield: 98%, average TOF: 17 h^?1). The activity of 1 and product yields outperform the currently available non-transition-metal catalysts used for this catalysis. Mechanistic studies show that the silicon(II) center in complex 1 catalyzes the C?N bond formation via a different pathway in comparison with non-transition-metal catalysts. It sequentially activates CO₂, PhSiH₃, and amines, which proceeds via a dihydrogen elimination mechanism, to form formamides, siloxanes, and dihydrogen gas.

Full text of DOI:10.1021/acscatal.0c03795

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Research Article
A NHC-Silyliumylidene Cation for Catalytic N‑Formylation of Amines
Using Carbon Dioxide
Bi-Xiang Leong, Yeow-Chuan Teo, Cloe
∥
∥
́
Condamines, Ming-Chung Yang, Ming-Der Su,*
and Cheuk-Wai So*
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*
sı Supporting Information
ABSTRACT: This study describes the use of a silicon(II) complex, namely, the NHC-silyliumylidene cation complex [(I ) SiH]I
Me 2
(
1, IMe = :C{N(Me)C(Me)} ), to catalyze the chemoselective N-formylation of primary and secondary amines using CO and
2
2
PhSiH under mild conditions to afford the corresponding formamides as a sole product (average reaction time: 4.5 h; primary
3
−
1
−1
amines, average yield: 95%, average TOF: 8 h ; secondary amines, average yield: 98%, average TOF: 17 h ). The activity of 1 and
product yields outperform the currently available non-transition-metal catalysts used for this catalysis. Mechanistic studies show that
the silicon(II) center in complex 1 catalyzes the C−N bond formation via a different pathway in comparison with non-transition-
metal catalysts. It sequentially activates CO , PhSiH , and amines, which proceeds via a dihydrogen elimination mechanism, to form
2
3
formamides, siloxanes, and dihydrogen gas.
KEYWORDS: silylene, carbon dioxide, amines, silane, N-formylation
INTRODUCTION
aminals with the selectivity dependent upon reaction temper-
■
ature, CO pressure, and type of solvent and
2
Catalytic conversion of carbon dioxide (CO ) into value-added
chemicals and fuels has attracted much attention due to the
increasing amount of anthropogenic CO in the atmosphere
2
9,10,15,16,20,24
amines.
In addition, organocatalytic C−N cycliza-
tion reactions using CO were feasible in the synthesis of
2
2
26
27
1
quinazoline-2,4-diones and oxazolidinones. Recently, a
different catalytic platform was developed by employing
main-group element compounds as catalysts. These main
group elements are considered as sustainable alternatives to
precious transition-metal species due to their high abundance
and relatively low toxicity. A handful of examples were
and its corresponding climatic problems. Considering that C−
N bonds are commonly found in agrochemicals and
2
pharmaceuticals, investigation of catalytic C−N bond
formation using CO2 as C1 building block becomes
indispensable. Consequently, a large number of precious
transition-metal catalysts have been developed to overcome
the high thermodynamic stability of CO for such catalyses.
However, harsh conditions such as high pressure of CO and
long reaction time were required. Organocatalysis then
emerged and was established as an alternative strategy, where
2
8
29
31
3
reported, whereby carbodicarbene, proazaphosphatrane,
1
2
30
,3,2-diazaphospholene, and NHC-germaacylium cations
2
undergo different mechanisms to mediate catalytic N-
amination using CO . These results prompted us to search
2
4
−6
7−10
for other main-group element compounds that could promote
N-heterocyclic carbenes (NHCs), superbases,
and ionic
3
c1112131415161718
more efficient catalytic C−N bond formation using CO via
2
liquids,
were used as catalysts to promote a
new reaction pathways.
range of C−N bond forming reactions between amines and
1
9
CO . Mechanistic studies show that organocatalysts activate
2
Received: August 31, 2020
Revised: November 18, 2020
amines [R NH] (R = substituent) to form a [catalyst−
2
H][R N] complex, which consequently reacts with CO to
2
2
form a carbamate salt in the form of [catalyst−H][R NCOO]
2
9
,20−25
(Figure 1).
The carbamate anion subsequently acts as a
nucleophile to reduce CO in the presence of silane or borane
2
derivatives, affording formamides, N-methylamines, and/or
©
XXXX American Chemical Society
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1
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Table 1. Optimization of N-Formylation of Amines with
a
CO and PhSiH3
2
H− source
(equiv)
yield
(%)
TOF
−1
entry
X
T (°C) t (h)
TON
(h )
1
2
3
4
5
6
7
8
9
10
10
10
10
10
10
10
10
1
None
HBpin (3)
60
60
60
60
60
24
40
90
60
60
60
60
96
1
96
1
1
3
1
1
1
1
0
0.0
6.8
3.0
4.3
9.1
9.5
7.5
7.1
8.4
0.0
6.8
68
30
43
91
95
75
71
84
54
90
88
Ph SiH (3)
0.03
4.3
9.1
3.2
7.5
7.1
8.4
10.8
18.0
17.6
3
Ph SiH (3)
2
2
PhSiH (3)
PhSiH (3)
PhSiH (3)
PhSiH (3)
PhSiH (3)
PhSiH (1)
PhSiH (2)
PhSiH
3
3
3
3
3
1
1
0
1
5
5
5
10.8
18.0
17.6
3
b
1
1
3
19a
Figure 1. Organocatalytic C−N bond formation using CO2.
12
3
(3)
a
Reaction conditions: amine 4b (0.10 mmol), required amount of
silane, C D (0.5 mL), catalytic amount of complex 1 and CO .
Catalyst loading is relative to amine. The yield was obtained by H
Among main-group elements, silylene compounds could be
a good option due to their unique properties: the presence of a
lone pair of electrons and a vacant p orbital on the silicon(II)
atom of a silylene. These electronic properties enable them to
possess both electrophilic and nucleophilic characters, leading
to Lewis ambiphilicity. In this context, they can display
reactivity similar to transition-metal complexes in the area of
6
6
2
1
NMR using the CH protons of cyclohexane as an internal standard.
2
TON and TOF are assigned to the number of hydride transferred. All
b
the catalytic trials were repeated in triplicate. Optimized conditions.
Among the hydride reagents, PhSiH showed the best activity
3
32
small molecules and enthalpically strong bond activation,
thus showing possible application in catalyzing organic
in terms of TOF (entry 5). The catalytic loading of 1 (entries
5
, 9, and 12), molar ratio of PhSiH (entries 10−12), and
3
3
3
reactions. To prove this concept, various silylene compounds
reaction temperature (entries 5−8) were then modulated,
where complex 1 showed the best performance in catalytic N-
formylation under 5 mol % catalytic loading, reaction
32n−q
have been shown to separately undergo CO activation
2
and ammonia, amine, and hydrazine N−H bond activatio-
3
2e−m
n.
However, there is still no example capable of
temperature of 60 °C, and two equivalents of PhSiH (entry
3
stoichiometrically or catalytically activating both CO and a
2
11).
N−H bond simultaneously, resulting in C−N bond formation.
In addition, other coupling reactions between two small
molecules mediated by a silylene compound are currently
unknown. This aroused our interest to investigate the catalytic
capability of a silicon(II) compound in C−N bond formation.
Recently, we reported the synthesis of a NHC-silyliumylidene
The catalytic reaction of primary amines with CO and
2
PhSiH was then investigated (Table 2, Table S3) using the
3
optimized conditions (Table 1, entry 11). First, isopropyl-
amine 2a did not undergo catalytic N-formylation under any
conditions. Second, complex 1 (5 mol %) was found to be
capable of catalyzing N-formylation of less sterically hindered
aliphatic amines, namely, 2-methoxyethan-1-amine 2b and n-
butylamine 2c to form the corresponding N-formamides (yield
34
cation complex [(I ) SiH]I (1, Figure 1), which contains a
Me 2
base-stabilized silicon(II) cation. It showed transition-metal-
−
1
like reactivity in functionalizing the ortho-C−H bond of
for 3b and 3c: >99%; TOF = 20.0 h ). The olefinic
functionality in prop-2-yn-1-amine 2d remained intact during
the catalysis, showing the possibility of chemoselective N-
formylation. Third, >99% yield was achieved for the N-
34
fluorobenzene, leading to C−C bond formation. Henceforth,
complex 1 is anticipated to be an excellent candidate to
activate both CO and amines simultaneously for C−N bond
2
−
1
formation. Herein, we report the NHC-silyliumylidene cation
complex-catalyzed N-formylation of amines and CO2 to
selectively afford formamides (Figure 1).
formylation of aromatic aniline 2e (TOF = 10.0 h ). Its
derivatives with electron-donating, electron-withdrawing, and
sterically hindered substituents were well tolerated (2f−2t).
Fourth, benzylamine 2u underwent catalytic N-formylation to
RESULTS AND DISCUSSION
Optimization was first performed using diisopropylamine 4b
Table 1, see Supporting Information, Table S1). The NHC-
−1
■
(
form 3u with >99% conversion (TOF: 6.7 h ). However,
when electron-donating and electron-withdrawing groups were
introduced at the ortho and para positions of 2u, complex 1
−
1
silyliumylidene cation complex 1 (10 mol %) was unable to
was less active in these reactions (TOF = 4.1−6.1 h ). Fifth,
diformylated products were not observed in most reactions.
Only the N-formylation of 2k afforded mono- and diformy-
lated products in a 3:2 ratio. In these catalyses, the activity of
complex 1 in terms of TOF and product yield are better than
catalyze the reaction of 4b with CO in the absence of a
2
hydride source at 60 °C. Subsequently, three equivalents of
hydride reagent (HBpin, Ph SiH, Ph SiH , PhSiH ) was added
3
2
2
3
into the reaction mixture to afford the N-formamide 5b,
demonstrating that complex 1 can mediate N-formylation
regardless of the type of hydride source used (entries 2−5).
the N-heterocyclic carbene (NHC), IPr (:C{N(Ar)CH} , Ar =
2
2,6-iPr C H . Examples of IPr, 2e: yield = 35%, TOF = 0.3
2
6
3
1
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a
−1
−1
−1 15
Table 2. Scope of Primary Amine Substrates
h ; 2m: 43%, 4.5 h ; 2u: 81%, 4.5 h ), and lecithin (2c:
8
−
1
−1 18
9%, 1.7 h ; 2m: no reaction; 2u: 84%, 1.7 h ). Lastly, the
formation of N-formamides in the catalyses was further
confirmed by X-ray crystallography of 3m, 3p, and 3w (Figures
S71−S73). Moreover, it is noteworthy that the above-
6
8,9
16,17
mentioned NHC, superbases,
and ionic liquids
mediated both catalytic N-formylation and N-methylation of
amines and CO . However, complex 1 can only selectively
2
mediate N-formylation regardless of reaction conditions. In
other words, the N-formamides formed in the catalyses
(
Tables 2 and 3) did not undergo further reduction to form
N-methylamines.
a
Table 3. Scope of Secondary Amine Substrates
a
Reaction conditions: primary amine (0.10 mmol), 2 equiv of PhSiH3
(
0.20 mmol), catalyst 1 (5 mol %), C D (0.50 mL), CO (∼50.0
6
6
2
mg). Catalyst loading is relative to amine. Yields were determined by
1
H NMR spectroscopy using cyclohexane as an internal standard, on
the basis of the consumption of the amine. The identity of the
product was confirmed as RHNC(O)H. Isolated yields were
reported in parentheses. All the catalytic trials were repeated in
b
c
triplicate. Reaction performed at 40 °C. Mono- and diformylations
were observed in a 3:2 ratio.
a
Reaction conditions: secondary amine (0.10 mmol), 2 equiv of
−
1
−1
−1
PhSiH (0.20 mmol), catalyst 1 (5 mol %), C D (0.50 mL), CO
h ; 2m: 47%, 0.4 h ; 2n: 86%, 0.7 h ; 2q: 0%; 2u: 85%, 0.7
3
6
6
2
−
1
5
(∼50.0 mg). Catalyst loading is relative to amine. Yields were
h ). When the Ar substituent is replaced by Mes (2,4,6-
Me C H ), IMes (:C{N(Mes)CH} ) can only mediate the
1
determined by H NMR spectroscopy using cyclohexane as an
3
6
2
2
internal standard on the basis of the consumption of the amine. The
reaction of amines with CO and Ph SiH to solely form N-
methylamines. Another NHC, namely, thiazolium carbene
1 2
2
2
2
identity of the product was confirmed as R R NC(O)H. Isolated
4
yields were reported in parentheses. All the catalytic trials were
b
c
(
:C{N(Bn)CHC(H)S}, Bn = CH Ph), was capable of
2
repeated in triplicate. Reaction performed at 40 °C. Reaction
d
catalyzing both N-formylation and N-methylation of amines
performed at 90 °C. Reaction was also performed in a 1 g scale,
leading to full conversion, under the same reaction conditions.
with polymethylhydrosiloxane and CO depending on the
2
reaction temperature, but its activity (example of thiazolium
−
1
6
carbene: 2e: yield = 53%, TOF = 0.4 h ) is slower than 1.
Sixth, the activity of 1 is better than superbases such as 1,5,7-
triazabicyclo[4.4.0]dec-5-ene, TBD (examples of TBD, 2e:
The catalytic reaction of secondary amines with CO and
2
PhSiH was further investigated (Table 3, Table S4). First, 5
3
−
1
7
yield = 24%, TOF = 0.2 h ) and N-butyl-N′,N′,N″,N″-
mol % 1 was able to catalyze the N-formylation of aliphatic
secondary amines, namely, diethylamine (4a) and diisopropyl-
amine (4b), to afford the corresponding N-formamides in high
yield. Selective N-formylation was observed for the product 5c,
where the olefinic functionality in N-methylprop-2-yn-1-amine
4c remained intact during the catalysis. In particular, the
activity of 1 toward sterically hindered 4f is much higher than
tetramethylguanidine, BTMG (examples of BTMG, 2e: yield =
−
1
9
9
5%, TOF = 4.3 h ; 2u: mixture of products). In particular,
complex 1 exhibits better activity in comparison with ionic
liquids, for example, 1-butyl-3-methylimidazolium chloride
[
=
BMIm]Cl (examples of [BMIm]Cl, 2e: yield = 76%, TOF
0.15 h ; 2u: 60%, 0.12 h ), glycine betaine (2c: 85%, 4.5
−
1
−1 11
1
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−
1
8
that of the superbase DBU (yield = 54%, TOF = 0.1 h ) and
Scheme 1. DFT Calculations for Catalytic N-Formylation of
−
1
11
ionic liquids [BMIm]Cl (69%, 0.14 h ) and glycine betaine
77%, 0.8 h ). Second, less activity was found in aromatic
N-methylaniline 4e (95%, 6.3 h ), but the TOF of 1 is still
higher than those of NHC, IPr (>99%, 0.8 h ), superbase,
TBD (40%, 0.16 h ), and ionic liquid, [BMIm]Cl (95%, 0.19
h ). Third, excellent yields (85 to >99%) and high activity
TOF: 20.0 h ) were found in the 1-catalyzed N-formylation
of cyclic amines such as piperidine 4g, morpholine 4h, indoline
k, and 1,2,3,4-tetrahydroquinoline 4m. >99% yield and high
activity (TOF: 6.7−10 h ) were also achieved for the N-
formylation of N-heterocycles, namely, dimethylpyrazole 4n,
imidazole 4p, and benzimidazole 4q. The catalytic activity
Amines with CO and PhSiH3
2
−
1
15
(
−1
−
1
5
−
1
7
−
1
11
−
1
(
4
−
1
5
outperformed that of NHC catalysts found in the literature.
Notably, complex 1 shows better selectivity in comparison with
other main-group element compounds, where 1,3,2-diazaphos-
pholene unselectively catalyzes N-formylation of secondary
amines leading to a mixture of N-formylated and N-methylated
3
0
products. It is noteworthy that complex 1 is capable of
showing selective N-formylation of both primary and
secondary amines.
In order to shed some light into the practical utility of our
catalytic protocol using complex 1, a gram-scale reaction was
carried out. Under the standard reaction conditions, dibenzyl-
amine 4f was fully converted to 5f (yield: >99%, TOF: 20.0
h ) as the sole product. In other words, both selectivity and
yield were preserved upon upscaling of our catalytic protocol.
Upon completing the above catalytic N-formylation of
amines, a weak singlet at δ ∼4.5 ppm was observed (Figures
S1, S6, S15, S24, S33, S38, S43, S48, S57, S60, and S63), which
inserts into the C−O bond via a low kinetic barrier (TS02,
⧧
ΔG = 20.3 kcal/mol), resulting in the formation of Int03
rel
−
1
and regeneration of 1 (ΔG = −15.7 kcal/mol). Siloxane
rel
(
PhH Si) O is then eliminated from Int03 via a kinetic barrier
2 2
of 15.9 kcal/mol (TS03: 0.2 kcal/mol) to form 3u (ΔG
=
rel
−
38.7 kcal/mol). The DFT calculations are consistent with the
catalytic conditions, whereby two equivalents of PhSiH are
3
sufficient for the completion of catalysis (Table 1). In addition,
the mechanism is different from that of the abovementioned
organocatalytic N-formylation of amines and CO2 using
NHCs, superbases, and ionic liquids as catalysts, where they
is attributable to H gas. To clarify these phenomena, N-
2
methylaniline 4e was treated with PhSiH in C D at room
3
6
6
temperature or 60 °C, whereby no reaction occurred. This
demonstrates that the N−H proton of 4e and Si−H hydride of
activate the amine (R NH) to form [catalyst−H][R N] in the
PhSiH did not react to form H2 and the silylamine
2
2
3
9,20−25
first step of catalyses.
[
Ph(Me)NSiH Ph] in C D in the absence of complex 1.
2 6 6
To further support the proposed mechanism for the
dehydrogenation step occurring between TS01 and Int02, a
deuterium-labeling study was performed using d -piperidine
This result is consistent with the recent findings, where a
catalyst is essential for the dehydrocoupling of silanes and
35
1
1
amines to form silylamines and H2. Moreover, there was no
reaction between complexes 1 and 4e, which demonstrates that
the Si−H bond in complex 1 did not react with the N−H
proton of 4e in C D to form H and the amidosilyliumylidene
4
g-D (D, 98%, Scheme 2). First, the catalysis was carried out
Scheme 2. Deuterium-Labeling Study Using d -Piperidine
11
6
6
2
(
D, 98%) (All Catalytic Trials Were Repeated at Least in
cation [(I ) SiN(Me)Ph]I. In this context, complex 1
Me
2
Triplicate)
behaves differently from other organocatalysts, which activate
amine (R NH) to form [catalyst−H][R N] (Figure 1, R =
2
2
1
9a
substituent). Furthermore, complex 1 does not react with
PhSiH at 60 °C in C D , which illustrates that they are not
3
6
6
involved in the first step of catalysis. Therefore, the first step of
catalysis should involve the interaction of complex 1 with
3
3a
CO2.
In addition, H can only be formed when 1 was
2
reacted with N-methylaniline, CO , and PhSiH together. Such
2
3
a hypothesis is evidenced by the following DFT calculations.
The reaction mechanism was studied by DFT calculations
using the standard reaction conditions, where HD (triplet, 4.43
(M06-2X/def2-TZVP, Scheme 1, Scheme S1). It was found
ppm) and formyl −C(O)H (7.76 ppm) signals were
1
that complex 1 is capable of mediating the catalysis, whereby
observed in the H NMR after an hour (Figure S66). The
the Si lone pair of electrons interact with CO first to form
formation of HD proves the mechanism in Scheme 1, which
proceeds via a dihydrogen elimination mechanism. Second, 4g
and 4g-D in a 1:1 ratio were subjected to catalytic N-
formylation using the standard reaction conditions (Scheme
2
33a
Int01 (ΔG = −3.1 kcal/mol).
The O center in Int01
attacks PhSiH , and subsequently, the Si−H bond reacts with
rel
3
the N−H bond of 2u to form H . The N center of 2u
2
simultaneously attacks the CO bond via a low kinetic barrier
3). Full conversion was observed where H and HD were
2
⧧
of 13.2 kcal/mol (TS01: ΔG
= 10.1 kcal/mol) to form
formed, as well as 5g-D and 5g were afforded in a 1:1 ratio
rel
Int02 (ΔG = −0.7 kcal/mol). The O center in Int02 then
attacks the second PhSiH , where the Si−H bond of PhSiH
(Figure S67). Third, 4g or 4g-D did not react with PhSiH in
rel
3
the absence of complex 1 at any temperature, refuting the
3
3
1
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Scheme 3. Scrambling Reaction with d -Piperidine (D,
Scheme 5. DFT Calculations for Catalytic Hydrosilyation of
CO2
11
98%) and Piperidine in a 1:1 Ratio (All Catalytic Trials
Were Repeated at Least in Triplicate)
In particular, the silylformates [PhSiH {OC(O)H}3−x]
x
formed further reacted with PhSiH to form siloxanes in the
3
1
-catalyzed hydrosilylation of CO (Scheme 4). This result
2
possibility of H formation from the coupling reaction between
indicates that the 1-catalyzed N-formylation (Tables 2 and 3)
should not proceed via silylformates; otherwise, low yields of
formamides would occur. Even if the 1-catalyzed N-
formylation proceeds via silylformates, which then react with
2
PhSiH and amine. Fourth, no reaction was observed in the
3
reaction of PhSiH and piperidine (4g-D and/or 4g) in the
3
presence of catalyst 1 at any reaction temperature (Figure
S68). This illustrates that complex 1 cannot attack PhSiH and
amine, silanol PhSiH (OH)3−x formed could hydrolyze
x
3
the subsequent amine to form H2 and silylamide [Ph-
complex 1 and then impede its activity. However, complex 1
shows high activity toward the N-formylation of amines with
(
H) SiNR ]. In other words, the mechanism via Int01 and
2 2
TS01 should be feasible.
CO and PhSiH , signifying the implausible mechanism via a
2 3
It is noteworthy that complex 1 (10 mol %) was able to
catalyze the hydrosilylation of CO with PhSiH in C D to
silylformate intermediate.
To support our hypothesis, a stepwise mechanistic study was
2
3
6
6
form a mixture of silylformates [PhSiH {OC(O)H}3−x] and
implemented, where 1-catalyzed hydrosilylation of CO
performed as the first step followed by the removal of CO
2
was
and
x
siloxanes (Scheme 4, x = 1−2; conversion: >99%; TOF = 19.4
2
addition of amine 2f or 2m (Table 4). The results showed that
Scheme 4. 1-Catalyzed Hydrosilylation of CO (All
Catalytic Trials Were Repeated at Least in Triplicate)
2
Table 4. N-Formylation of Amines via Silylformates
Resulting in Low Yields of 3
a
h ), which was confirmed by 1H and C{ H} NMR
−
1
13
1
spectroscopy (Figures S69a,b and S70) and HRMS. Hydro-
entry T (°C) t (h) consumption of silane (%) product yield (%)
silylation of CO mediated by a low-oxidation state silicon
2
1
2
3
60
40
90
3
19
2
15
24
23
3f
3m
11
21
compound was not reported yet. These results brought up a
question of whether the mechanism of 1-catalyzed N-
control reaction
formylation should proceed via silylformates [PhSiH {OC-
x
a
Reaction conditions: PhSiH (0.20 mmol), C D (0.5 mL), catalytic
3
6
6
(
O)H}3−x], which then react with amines to form N-
amount of complex 1 and CO (∼50.0 mg). After the hydrosilylation
2
formamides and siloxanes.
reaction, CO was removed with its crude mixture filtered into
2
To answer this question, the mechanism of the 1-catalyzed
another J Young NMR tube loaded with amine (0.10 mmol). Catalyst
hydrosilylation of CO was elucidated by DFT calculations
1
2
loading is relative to amine. The yield was obtained by H NMR using
(
Scheme 5, Scheme S2), whereby the Si lone pair electrons
the CH protons of cyclohexane as an internal standard. All the
2
33a
interact with CO to form Int01 (ΔG = −3.1 kcal/mol).
catalytic trials were repeated in triplicate.
2
rel
Subsequently, the Si−H bond of PhSiH inserts into the C−O
3
⧧
bond in Int01 via TS04 (ΔG = 13.9 kcal/mol) to form the
rel
silylformate [PhH SiOC(O)H] (ΔG = −5.7 kcal/mol) and
2
rel
regenerates complex 1. The reaction occurs with higher
activation barrier (17.0 kcal/mol) in comparison with the
reaction of 1, 2u, CO and PhSiH via TS01 (13.2 kcal/mol).
the conversion of the silylformates [PhSiH {OC(O)H}3−x]
x
were found to be exceedingly low at 40 and 60 °C, which
resulted in the poor yield of 3f (11%) and 3m (21%),
indicative of minimal contribution of silylformates
2
3
This suggests that the 1-catalyzed N-formylation of amines
with CO and PhSiH should not proceed via a silylformate
2
3
intermediate. Our results are in contrast to the DFT
calculations of the mechanism of the 1,3,2-diazaphospholene-
catalyzed N-formylation and N-methylation, which involve the
[PhSiH {OC(O)H}3−x] in 1-catalyzed N-formylation. The
x
low conversion of [PhSiH {OC(O)H}3−x] at 40 and 60 °C is
x
in line with a higher kinetic barrier in DFT calculations
(Scheme 5).
formation of silylformate [PhSiH {OC(O)H} ] intermediate
x
3‑x
36
via σ-P−H bond metathesis mechanism pathway.
1
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Research Article
CONCLUSIONS
was repeated twice. The reaction conditions are indicated in
Tables S3 and S4 in the Supporting Information, and the
reaction was monitored by NMR spectroscopy. The catalytic
studies were repeated at least in triplicate. The yields of
■
The NHC−silyliumylidene cation complex 1 is a low-
oxidation-state group 14 element catalytic system to promote
the metal-free chemoselective N-formylation of amines with
carbon dioxide, which efficiently and selectively delivers N-
formamides. The byproducts of the reactions are dihydrogen
gas and siloxane. The activity of 1 and product yields are better
than that of currently available nonmetal catalysts used for
such catalysis. The outperformance of 1 is due to its transition-
metal-like catalysis, whereby the silicon(II) center sequentially
activates CO , PhSiH , and amines, which proceeds via a
1
products were reported according to the integration of H
1
2
NMR signals of R R NC(O)H at ∼8 ppm with reference to
the CH protons (1.40 ppm) of the internal standard,
2
cyclohexane. Formation of methylated amine was not observed
throughout the catalytic reaction. The resulting formamides
were purified by silica plug or silica gel chromatography using
either ethyl acetate/hexane or tetrahydrofuran as the eluent to
give the corresponding N-formamides as an oil or crystalline
solid. The NMR data of 3a−3x and 5a−5m can be found in
the Supporting Information.
2
3
dihydrogen elimination mechanism, to form formamides,
siloxanes, and dihydrogen gas.
Procedures for the Gram-Scale Catalytic Functional-
ization of CO Using PhSiH and Dibenzylamine 4f.
EXPERIMENTAL PROCEDURE
■
2
3
General Procedures. All manipulations were carried out
under an inert atmosphere of argon gas by standard Schlenk
techniques. Compound 1 was prepared according to the
Catalyst 1 (0.102 g, 0.253 mmol), internal standard cyclo-
hexane (1.00 mL, 5.07 mmol, 20 equiv), phenylsilane (1.25
mL, 10.1 mmol, 40 equiv), dibenzylamine 4f (1.00 g, 0.97 mL,
5.07 mmol, 20 equiv), and C D (25 mL) were loaded into a
33
literature procedure. C D was dried over potassium metal
6
6
6
6
and distilled prior to use. CDCl was dried over CaH and
Schlenk bottle. The Schlenk bottle was immersed in liquid
3
2
distilled prior to use. d -piperidine (D, 98%) was purchased
nitrogen under vacuum to obtain a frozen solution for freeze−
11
from Cambridge Isotope Laboratories, Inc. and dried with 4 Å
pump−thaw cycle, and 1 bar of CO gas was then added. The
2
molecular sieves prior to use. All amine substrates are
freeze−pump−thaw cycle was repeated twice. The reaction
mixture was heated from room temperature to 60 °C. The
reaction was monitored by NMR spectroscopy. The catalytic
studies were repeated at least in triplicate. Full conversion was
1
commercially available and dried using CaH . The H and
2
1
3
1
C{ H} NMR spectra were recorded on a JEOL ECA 400
spectrometer. The NMR spectra were recorded in C D or
6
6
1
CDCl , and the chemical shifts are relative to SiMe for H and
observed after an hour. A 99% yield was reported according to
3
4
1
3
1
C. The following abbreviations are used to describe signal
the integration of H NMR signals of (PhCH
)
2
2
NC(O)H
(
5f) at 8.07 ppm with reference to the CH protons (1.40
multiplicities: s = singlet, d = doublet, t = triplet, q = quartet,
sept = septet, and m = multiplet. Coupling constants J are
given in Hertz (Hz). Electrospray ionization (ESI) mass
spectra were obtained at the Mass Spectrometry Laboratory at
the Division of Chemistry and Biological Chemistry, Nanyang
Technological University.
2
ppm) of the internal standard, cyclohexane. Formation of
methylated amine was not observed throughout the catalytic
reaction. 5f was purified by silica gel chromatography using
ethyl acetate/hexane as the eluent, which was isolated as an oil
(99% yield).
Optimization for the Reductive Functionalization of
Deuterium-Labeling Study of Catalytic N-Formyla-
tion Using d11-Piperidine 4g-D. Catalyst 1 (2.0 mg, 0.005
mmol), internal standard cyclohexane (10.8 μL, 0.10 mmol, 20
CO Using PhSiH and Diisopropylamine 4b. Catalyst 1
2
3
(
0
2.0 mg, 0.005 mmol), internal standard cyclohexane (10.8 μL,
.10 mmol, 20 equiv), phenylsilane (24.6 μL, 0.20 mmol, 40
equiv), diisopropylamine 4b (14.0 μL, 0.10 mmol, 20 equiv),
equiv), phenylsilane (24.6 μL, 0.20 mmol, 40 equiv), d11
piperidine 4g-D (D, 98%, 9.87 μL, 0.10 mmol, 20 equiv), and
(0.5 mL) were loaded into a J Young NMR tube. The J
-
and C D (0.5 mL) were loaded into a J Young NMR tube.
C D
6 6
6
6
The J Young NMR tube was immersed in liquid nitrogen
Young NMR tube was immersed in liquid nitrogen under
vacuum to obtain a frozen solution for freeze−pump−thaw
cycle, and 1 bar of CO gas was then added. The freeze−
2
under vacuum to obtain a frozen solution for freeze−pump−
thaw cycle, and 1 bar of CO gas was then added. The freeze−
2
pump−thaw cycle was repeated twice. The reaction conditions
are indicated in Table S1 in the Supporting Information, and
the reaction was monitored by NMR spectroscopy, which
pump−thaw cycle was repeated twice. The reaction mixture
was heated from room temperature to 60 °C. The reaction was
monitored by NMR spectroscopy. The catalytic trials were
repeated at least in triplicate. The identity of the product was
confirmed the formation of (iPr) NC(O)H 5b. Formation
2
1
1 2
of methylated amine was not observed throughout the catalytic
determined according to the H NMR signals of R R NC(
O)H (5g-D) at 7.76 ppm. A mixture of H (4.47 ppm) and
HD (triplet, 4.43 ppm, JHD = 42.7 Hz) were observed in the
reaction. The yields of products were reported according to the
2
1
1
integration of H NMR signals of (iPr) NC(O)H at 8.17
2
ppm with reference to the −CH protons (1.40 ppm) of the
internal standard, cyclohexane. The catalytic studies were
repeated at least in triplicate.
crude reaction after an hour due to piperidine (2%) and d11-
piperidine (98%), respectively.
2
Catalytic N-Formylation Using Piperidine 4g and d -
11
General Procedures for the Catalytic Functionaliza-
Piperidine 4g-D in a 1:1 Ratio. Catalyst 1 (2.0 mg, 0.005
mmol), internal standard cyclohexane (10.8 μL, 0.10 mmol, 20
equiv), phenylsilane (24.6 μL, 0.20 mmol, 40 equiv),
piperidine 4g (4.94 μL, 0.05 mmol, 10 equiv), d -piperidine
tion of CO Using PhSiH and Amines. Catalyst 1 (2.0 mg,
2
3
0
.005 mmol), internal standard cyclohexane (10.8 μL, 0.10
mmol, 20 equiv), phenylsilane (24.6 μL, 0.20 mmol, 40 equiv),
1
1
required amount of amine (0.10 mmol, 20 equiv), and C D
4g-D (D, 98%, 4.94 μL, 0.05 mmol, 10 equiv), and C D (0.5
6
6
6 6
(
0.5 mL) were loaded into a J Young NMR tube. The J Young
mL) were loaded into a J-Young NMR tube. The J Young
NMR tube was immersed in liquid nitrogen under vacuum to
obtain a frozen solution for freeze−pump−thaw cycle, and 1
NMR tube was immersed in liquid nitrogen under vacuum to
obtain a frozen solution for freeze−pump−thaw cycle, and 1
bar of CO gas was then added. The freeze−pump−thaw cycle
bar of CO gas was then added. The freeze−pump−thaw cycle
2
2
1
4829
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ACS Catal. 2020, 10, 14824−14833

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pubs.acs.org/acscatalysis
Research Article
was repeated twice. The reaction mixture was heated from
room temperature to 60 °C. The reaction was monitored by
NMR spectroscopy. The catalytic studies were repeated at least
in triplicate. The yield was calculated based on consumption of
the amine 4g in addition to the integration of the formoxy
ASSOCIATED CONTENT
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c03795.
■
*
Experimental section; selected spectra; X-ray data
collection and structural refinement; and theoretical
studies (PDF)
−
CHO and non-labeled −CH2 functional groups of the
corresponding N-formamide against the −CH integration of
2
the internal standard cyclohexane. The identity of the product
1
X-ray crystallographic data for 3m, 3p, and 3w (CIF)
was determined according to the H NMR signals of
1
2
R R NC(O)H of 5g and 5g-D at 7.76 ppm and
corresponding 5g signals of 3.22−3.25 (m, 2H, CH ), 2.51−
AUTHOR INFORMATION
Corresponding Authors
2
■
2
2
4
.54 (m, 2H, CH ), 1.04−1.13 (m, 4H, CH ), 0.91−0.96 (m,
2
2
H, CH ) in C D . A mixture of H (4.47 ppm) and HD (4.33,
2
6
6
2
Ming-Der Su − Department of Applied Chemistry, National
Chiayi University, Chiayi 60004, Taiwan; Department of
Medicinal and Applied Chemistry, Kaohsiung Medical
University, Kaohsiung 80708, Taiwan; Email: midesu@
mail.ncyu.edu.tw
Cheuk-Wai So − Division of Chemistry and Biological
Chemistry, School of Physical and Mathematical Sciences,
Nanyang Technological University, 637371, Singapore;
orcid.org/0000-0003-4816-9801; Email: CWSo@
ntu.edu.sg
1
.43, 4.54 ppm, J = 42.7 Hz) was observed in the crude
HD
reaction after an hour.
Reaction of PhSiH with d -Piperidine 4g-D. Internal
3
11
standard cyclohexane (10.8 μL, 0.10 mmol), phenylsilane
(
9
24.6 μL, 0.20 mmol, 2.0 equiv), d -piperidine 4g-D (D, 98%,
.87 μL, 0.10 mmol, 1.0 equiv), and C D (0.5 mL) were
6 6
11
loaded into a J Young NMR tube. The reaction was monitored
by NMR spectroscopy at 24, 60, and 90 °C over 24 h. The
catalytic studies were repeated at least in triplicate.
Consumption of PhSiH was not observed based on the
3
integration of the silane signal −SiH against the −CH
Authors
3
2
integration of the internal standard cyclohexane. H and HD
Bi-Xiang Leong − Division of Chemistry and Biological
Chemistry, School of Physical and Mathematical Sciences,
Nanyang Technological University, 637371, Singapore
Yeow-Chuan Teo − Division of Chemistry and Biological
Chemistry, School of Physical and Mathematical Sciences,
Nanyang Technological University, 637371, Singapore
2
signals were absent. In addition, no reaction was observed
when piperidine 4g (9.88 μL, 0.10 mmol, 1 equiv) was used
instead of d -piperidine.
1
1
General Procedure for Stepwise Hydrosilylation and
Subsequent Amine Addition. Catalyst 1 (2.0 mg, 0.005
mmol), internal standard cyclohexane (10.8 μL, 0.10 mmol, 20
equiv), phenylsilane (24.6 μL, 0.20 mmol, 40 equiv), and C D
Cloe Condamines − Division of Chemistry and Biological
́
Chemistry, School of Physical and Mathematical Sciences,
Nanyang Technological University, 637371, Singapore
Ming-Chung Yang − Department of Applied Chemistry,
National Chiayi University, Chiayi 60004, Taiwan
6
6
(
0.5 mL) were loaded into a J Young NMR tube. The J Young
NMR tube was immersed in liquid nitrogen under vacuum to
obtain a frozen solution for freeze−pump−thaw cycle, and 1
bar of CO gas was then added. The freeze−pump−thaw cycle
2
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c03795
was repeated twice. The reaction conditions are indicated in
Table 4, and the reaction was monitored by NMR spectros-
copy. After catalytic hydrosilylation, CO was removed in vacuo
2
Author Contributions
B.-X.L. and Y.-C.T. contributed equally. The manuscript was
written through contributions of all authors.
Notes
∥
and the resulting solution was filtered into another J Young
NMR tube. Subsequently, the required amount of amine (0.10
mmol, 20 equiv) was added. The resultant mixture was then
heated and was monitored by NMR spectroscopy. The
catalytic studies were repeated at least in triplicate.
The authors declare no competing financial interest.
Catalytic Hydrosilylation of Carbon Dioxide Using
ACKNOWLEDGMENTS
■
PhSiH . Catalyst 1 (4.0 mg, 0.01 mmol), internal standard
3
This work is supported by the National Research Foundation
Singapore, NRF-ANR (NRF2018-NRF-ANR026 Si-POP) and
Ministry of Education Singapore, AcRF Tier 2 (MOE2019-T2-
cyclohexane (10.8 μL, 0.10 mmol, 10 equiv), phenylsilane
(
24.6 μL, 0.20 mmol, 20 equiv), and C D (0.5 mL) were
6 6
loaded into a J Young NMR tube. The J Young NMR tube was
immersed in liquid nitrogen under vacuum to obtain a frozen
2
-129) (C.-W. So). M.-C.Y. and M.-D.S. are grateful to the
National Center for High-Performance Computing of Taiwan
for generous amounts of computing time and the Ministry of
Science and Technology of Taiwan for the financial support.
solution for freeze−pump−thaw cycle, and 1 bar of CO gas
2
was then added. The freeze−pump−thaw cycle was repeated
twice. The reaction mixture was heated from room temper-
ature to 90 °C. The reaction was monitored by NMR
spectroscopy, which confirmed the formation of silylformates
and siloxanes as the major products after 1 h. All catalytic trials
were repeated at least in triplicate. The existence of
intermediates and products (silylformates and siloxanes) was
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