Synthesis of silyl formates, formamides, and aldehydesviasolvent-free organocatalytic hydrosilylation of CO2

Article abstract of DOI:10.1039/d0cc01371d

Carbon dioxide (CO₂) was used as a C1 source to prepare silyl formates, formamides, and aldehydes. Tetrabutylammonium acetate (TBAA) catalyzed the solvent-freeN-formylation of amines with CO₂and hydrosilane to give formamides including Weinreb formamide, Me(MeO)NCHO, which was successively converted into aldehydes by one-pot reactions with Grignard reagents.

Full text of DOI:10.1039/d0cc01371d

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DOI: 10.1039/D0CC01371D
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Synthesis of silyl formates, formamides, and aldehydes via
solvent-free organocatalytic hydrosilylation of CO2 †
Takumi Murata,a Mahoko Hiyoshi,a Manussada Ratanasak,b Jun-ya Hasegawa*,b and Tadashi
Received 00th January 20xx,
Accepted 00th January 20xx
Ema*,a,‡
DOI: 10.1039/x0xx00000x
Carbon dioxide (CO2) was used as a C1 source to prepare silyl bond formation in solvent, for example, using an organometallic
formates, formamides, and aldehydes. Tetrabutylammonium reagent such as a Grignard reagent (RMgX). Here we found
acetate (TBAA) catalyzed the solvent-free N-formylation of amines tetrabutylammonium acetate (TBAA) as a catalyst for both the
with CO2 and hydrosilane to give formamides including Weinreb hydrosilylation of CO2 and the N-formylation of amines in the
formamide, Me(MeO)NCHO, which was successively converted into presence of hydrosilane under solvent-free conditions, the
aldehydes by one-pot reactions with Grignard reagents.
latter of which gave various formamides including Weinreb
formamide, Me(MeO)NCHO. Weinreb formamide was
successively converted into aldehydes by one-pot reactions
with Grignard reagents. This is a convenient and reliable
method for one-pot aldehyde synthesis with CO2.11
Carbon dioxide (CO2) is an inexpensive and renewable chemical
feedstock that has attracted much attention of chemists.
Despite the kinetic and thermodynamic stability of CO2, efficient
and useful reactions have been developed.1 Among various
methods for CO2 fixations, CO2 reduction is becoming a hot
research area,2 and catalytic CO2 reduction with hydrosilanes
gives value-added substances.3–5 Hydrosilylation of CO2 can
proceed stepwise to generate silyl formates (HCO2SiR3),
bis(silyl)acetals (CH2(OSiR3)2), methoxysilanes (CH3OSiR3), and
methane, depending on reaction conditions.3 A variety of metal
catalysts and organocatalysts have been developed.3–5
Furthermore, CO2 and hydrosilanes can be combined with
amines to effect N-functionalization such as N-formylation and
N-methylation.6,7 Even selective N-formylation/N-methylation
of amines has been achieved simply by changing reaction
parameters such as temperature and CO2 pressure.7 Polar
aprotic solvents such as CH3CN, DMF, and DMSO are often used.
Solvent-free reactions have great potential in sustainable
organic synthesis.8 As part of our ongoing efforts in solvent-free
catalysis,7i,9,10 we decided to search for a catalyst suitable for
the solvent-free hydrosilylation of CO2. The outline of our
strategy is shown in Scheme 1, which involves the C–H bond
formation without solvent and the subsequent one-pot C–C
hydrosilane
HNR1R2
(i) RMgX
solvent
H
NR1R2
H
C(R)
C
O
C
O
CO2
catalyst
no solvent
(ii) H3O+
Scheme 1 Solvent-free N-formylation of amines and one-pot aldehyde synthesis.
a
Table 1 Solvent-free hydrosilylation of CO2
O
hydrosilane
CO2
(1 atm)
TBAA (5 mol%)
no solvent
H
OSiR3
Entry
Hydrosilane
PhMe2SiH
Ph2MeSiH
Ph3SiH
Temp. (°C)
Conv. (%)b
Yield (%)c
60
60
60
60
30
1
2
3
4
5
98
97
76
70
78
10
48
100
100
99
PhSiH3
PhSiH3
a Reaction conditions: CO2 (1 atm), hydrosilane (2.0 mmol), TBAA (5 mol%), 8 h. b
Conversion of silane. c Total yield of silyl formate and formic acid. Determined by
1H NMR using mesitylene as an internal standard.
We screened several potential catalysts (5 mol%) for the
solvent-free hydrosilylation of CO2 (1 atm, balloon) with
PhMe2SiH at 60 °C (Table S1 in ESI). As
a result,
tetrabutylammonium fluoride (TBAF) and TBAA successfully
produced silyl formate (HCO2SiMe2Ph) in 52% and 76% yields,
respectively. Control experiments without CO2 or catalyst
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demonstrated that both of them were essential for the reaction. PhSiH3, the remaining H atoms on the Si atom can also be used
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In contrast, other TBA halides and inorganic salts such as Cs2CO3 for the reduction of CO2 (not shown).6e This catalytic cycle was
DOI: 10.1039/D0CC01371D
and CsF, some of which have been reported to work as catalysts supported by DFT calculations, where PhSiH3 was employed as
in polar aprotic solvents,5a showed no catalytic activity under a model hydrosilane (Fig. 1 and Fig. S5 in ESI).12 All the
–
the present reaction conditions. These results are partly due to components (Bu4N+HCO2 , PhSiH3, and CO2) are included in
either insolubility in liquid hydrosilane or poor nucleophilicity. reactant complex R. The formate anion interacts weakly with
We employed TBAA for further investigations to confirm that the Si atom of PhSiH3 to form a metastable complex I1, and a
TBAA was effective for several hydrosilanes (Table 1). PhMe2SiH pentacoordinate silicate species I2 generates via transition
and Ph2MeSiH exhibited good reactivity at 60 °C (entries 1 and state TS1 with a low energy barrier (∆G‡ = 7.6 kcal/mol). The
2). Ph3SiH, a solid at room temperature, melted at 60 °C, and pentacoordinate silicate species then makes a contact with CO2
the reaction proceeded to give the corresponding silyl formate to give a weak complex I3. The subsequent hydride-transfer
(entry 3). In sharp contrast, the use of PhSiH3 at 60 °C afforded reaction reaches transition state TS2 to afford intermediate I4
the product in only 10% despite 100% conversion of PhSiH3 where the negatively charged O atom of HCO2 interacts with
(entry 4). Lowering temperature to 30 °C improved the yield to the Si atom of the silyl formate. The dissociation of the adduct
,
–
48% (entry 5). The yields were modest probably because of a gives silyl formate
P
with the regeneration of the catalyst. The
side reaction generating H2 (ESI).
desorption energy from I4 to is calculated to be ∆E = 3.4
P
kcal/mol (∆G° = 0.5 kcal/mol). The whole energy profile clearly
indicates that this reaction is exothermic and that the entropy
term becomes negatively large in the later stage of the reaction
as the number and symmetry of the molecules decrease. The
highest-energy transition state TS2 is only 11.5 kcal/mol higher
–
–
R
R
CO2
Bu4N+ AcO Si
AcO Si
H
O
H
C
R
R
R
R
O
Bu4N+
R3SiH
AcOSiR3
than R, which indicates that this catalytic cycle is likely to occur
HCO2SiR3
Bu4N+AcO–
(TBAA)
Bu4N+HCO2
–
at ambient temperature. The anionic species in the key
transition state, TS2, is stabilized by the electrostatic field of the
TBA cation and the hydrogen bonds of the formyl O atom of the
pentacoordinate silicate species with the positively charged H
atoms of the TBA cation (2.14 Å and 2.31 Å).13
(TBA formate)
–
R
R3SiH
O
HCO2 Si
H
–
R
C
R
R
Bu4N+ HCO2 Si
H
O
Bu4N+
Table 2 Solvent-free N-formylation of 1aa
CO2
R
R
CO2 (1 atm)
CHO
hydrosilane
H
N
Scheme 2 A plausible reaction mechanism.
N
Ph
Ph
TBAA
no solvent
(a)15.0
(b)
1a
2a
11.5
10.0
5.0
7.9
7.6
Entry
Hydrosilane
Temp. (°C)
TBAA (mol%)
Yield (%)b
9.6
6.4
TS2
7.5
4.3
6.8
-0.33(H)
1.71
+1.20(Si)
-0.81(O)
-0.59(O)
TS1
1
2
3
4
5
6
7
8
9c
PhMe2SiH
Ph2MeSiH
Ph3SiH
60
60
60
30
30
30
30
30
30
5
5
42
64
72
56
82
90
83
74
82
I2
4.9
+1.01(C)
-0.62(O)
+0.13(H)
0.7
I3
0.0
2.55
0.0
2.31
-0.75(O)
I1
5
0.0
2.53
2.14
R
–5.0
–5.5
Ph2SiH2
PhSiH3
PhSiH3
5
+0.30(H)
-5.0
-10.0
-15.0
5
3
–11.3
P
PhSiH3
2
TS2
–14.7
PhSiH3
1
I4
PMHS
2.5
Fig. 1 (a) Energy profile for the hydrosilylation of CO2 with PhSiH3 and TBA formate.
Potential energies and free energies (303 K) are shown in red and black, respectively. (b)
Structure of TS2 with distances (Å) in blue or red and NBO charges in black.
a Reaction conditions: 1a (0.50 mmol), CO2 (1 atm), hydrosilane (2.0 mmol), TBAA
(loading % based on silane), 12 h. b Determined by 1H NMR using mesitylene as an
internal standard. c PMHS (400 μL, Si–H: 6 mmol).
A plausible reaction mechanism is shown in Scheme 2. The
nucleophilic attack of –OAc of TBAA on the Si atom of a
We next examined the catalytic activity of TBAA for the
solvent-free N-formylation of amines with CO2 and hydrosilane.
A mixture of N-methylaniline, hydrosilane, and TBAA was stirred
under CO2 (1 atm) for 12 h (Table 2). The use of PhSiH3 at 30 °C
gave the best result (entries 1–5). When the amount of TBAA
was reduced to 1 mol%, the yield decreased to some degree
(entries 6–8). When polymethylhydrosiloxane (PMHS), an
inexpensive polymer containing the Si–H bond, was used as a
reductant, the reaction took place efficiently at 30 °C (entry 9).
hydrosilane gives
a
pentacoordinate silicate species.
Subsequent hydride transfer to CO2 gives silyl acetate (AcOSiR3)
–
and TBA formate (Bu4N+HCO2 ), the latter of which is a
–
catalytically active species (ESI).5d This HCO2 acts as a
nucleophile to generate another pentacoordinate silicate
species. Subsequent hydride transfer to CO2 gives silyl formate
(HCO2SiR3), regenerating TBA formate. TBA formate may react
with AcOSiR3 to give TBAA and HCO2SiR3, and in the case of
2 | J. Name., 2013, 00, 1-3
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Substrate scope was investigated with PhSiH3 or PMHS (Table 3). formation (Scheme 1).16 To realize this concept, we paid
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DOI: 10.1039/D0CC01371D
N-Methylanilines 1a–e with the electron-withdrawing or attention to Weinreb amides, which enable one to synthesize
electron-donating groups were successfully converted into N- carbonyl compounds even with an excess amount of
methylformanilides 2a in good yields. Other N-substituted organometallic We found that N,O-
reagent.17,18
anilines 1f were also tolerated. More bulky amine 1i was a dimethylhydroxylamine (1k) could be successfully converted
poor substrate, while N-benzylmethylamine (1j) exhibited good into N-methoxy-N-methylformamide 2k under slightly
–
e
–
h
(
)
reactivity.
modified conditions with PhSiH3 (Table 3, entry 11). This is the
first synthesis of Weinreb formamide 2k from CO2.
Subsequently, the reaction mixture was cooled in an ice bath,
and a suspension of a Grignard reagent (RMgBr) in THF, which
was prepared from either RBr or RH, was added under N2. After
stirring at 0 °C for 2 h, the reaction was quenched. To our delight,
the C–C(sp3, sp2, or sp) bond was formed to give various
Table 3 Scope of solvent-free N-formylation of aminesa
CO2 (1 atm)
hydrosilane
CHO
N
H
N
R1
R2
R1
R2
TBAA
no solvent
1
2
aldehydes
propynals 3g
3
such as (hetero)aromatic aldehydes 3a
h, and aliphatic aldehyde 3i in good to high yields
–f,
Yield (%)b
Entry
1
R1
R2
–
PhSiH3
PMHS
(Scheme 4). H and 13C NMR spectra of 3c' synthesized with
13CO2 showed a doublet at 9.91 ppm (JCH = 176 Hz) and an
intense singlet at 192.1 ppm (ESI), respectively, which clearly
demonstrates that the carbon atom of the formyl group
originated from carbon dioxide. In contrast, one-pot reactions
of Grignard reagents with HCO2SiMe2Ph instead of
Me(MeO)NCHO resulted in no formation of aldehydes.
1
1
2
1a
1b
1c
1d
1e
1f
Ph
4-Cl-C6H4
Me
Me
Me
Me
Me
Et
76
64
64
64
91
97
74
58
3
63
76
64
76
89
77
77
82
0
3
4-Br-C6H4
4-OMe-C6H4
4-Me-C6H4
Ph
4
5
6
7
1g
1h
1i
Ph
n-Pr
8
9
Ph
CH2CH=CH2
4-Me-C6H4
Me
CO2 (1 atm)
PhSiH3
(i) RMgBr
THF
CHO
4-Me-C6H4
PhCH2
Me
H
N
N
O
R
CHO
10
11
1j
1k
TBAA (3 mol%)
no solvent
(ii) H3O+
69
95c
72
0
O
3
OMe
1k
2k
a Reaction conditions: 1 (0.50 mmol), CO2 (1 atm), PhSiH3 (2.0 mmol) or PMHS (400
CHO
CHO
CHO
μL, Si–H: 6 mmol), TBAA (5 mol% based on PhSiH3 or 2.5 mol% based on PMHS),
b
c
30 °C, 12 h. Isolated yield. Reaction conditions: 1k (2.0 mmol), CO2 (1 atm),
PhSiH3 (2.0 mmol), TBAA (3 mol%), 0 °C, 1 h then 20 °C, 5 h. Yield determined by
1H NMR using mesitylene as an internal standard.
CHO
3e (57%)
S
MeO
3b: R = H (84%)
3c: R = OMe (86%) 3d (76%)
R
3a (71%)
N
S
Ph
Because various mechanisms have been proposed for N-
formylation with different catalysts,6a,6e,6g,7c,14,15 we conducted
control experiments (Scheme 3). After the TBAA-catalyzed
hydrosilylation of CO2 with PhSiH3 or PhMe2SiH, CO2 was
replaced with Ar, and 1a was then added to allow the reaction.
As a result, 2a was obtained in 45% or 92% yields, which
suggests that 2a was produced via the reactions of 1a with silyl
formates. This difference in productivity between PhSiH3 and
PhMe2SiH can be ascribed to the difference in water sensitivity
between the corresponding silyl formates or intermediates.
CHO
3i (56%)
CHO
R
CHO
3f (62%)
3g: R = H (59%)
3h: R = t-Bu (64%)
isolated yields
based on 1k
13CHO
JCH = 176 Hz
MeO
OMe
3c' (55% with 13CO2) 1H NMR spectrum of 3c'
Scheme 4 One-pot synthesis of aldehydes.
PhSiH3
TBAA (5 mol%)
(a)
CO2
In summary, tetrabutylammonium acetate (TBAA) worked
as a catalyst for the solvent-free hydrosilylation of CO2 (1 atm).
DFT calculations indicated that TBA formate generated in situ is
a catalytically active species. TBAA was also effective for the N-
formylation of amines with CO2 (1 atm) and hydrosilanes to give
various formamides, including Weinreb formamide,
Me(MeO)NCHO. Taking advantage of solvent-free conditions,
Weinreb formamide was successively converted into aldehydes
by one-pot treatment with Grignard reagents. This achievement
is significant because various reactions with different
substrate/reagent/catalyst/solvent can be employed in the
second step of one-pot synthesis (Scheme 1). This strategy is a
new entry to one-pot C–H/C–C bond formation with CO2.11,16,19
O
1a
2a
(45%)
no solvent, 30 °C, 8 h
30 °C, 15 h
under Ar
H
H
OSi
PhMe2SiH
TBAA (5 mol%)
(b)
CO2
O
1a
2a
(92%)
no solvent, 60 °C, 8 h
60 °C, 15 h
under Ar
OSiMe2Ph
Scheme 3 Control experiments.
The conversion of CO2 into the formyl group of aldehydes is
extremely important because of their considerable synthetic
utility and value.11 We envisioned that one-pot aldehyde
synthesis might be achieved by taking advantage of the solvent-
free C–H bond formation because any reagent and solvent
could be added to the reaction mixture to effect the C–C bond
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C. Nakagawa, R. A. Pramudita and Y. Manaka, ACS Sustainable
This work was supported by The Yakumo Foundation for
Environmental Science and by the Cooperative Research
Program of Institute for Catalysis, Hokkaido University (Grant
19A1003). JH thanks MEXT for "Integrated Research Consortium
on Chemical Sciences" and Hokkaido University for
“Photoexcitenix” project. Part of the computations were carried
out at RCCS (Okazaki, Japan).
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Chem. Eng., 2019, 7, 11056–11061.
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DOI: 10.1039/D0CC01371D
Silyl formates, formamides, and aldehydes were synthesized via the solvent-free
hydrosilylation of carbon dioxide using tetrabutylammonium acetate as a catalyst.