Betaine Catalysis for Hierarchical Reduction of CO2 with Amines and Hydrosilane To Form Formamides, Aminals, and Methylamines

Article abstract of DOI:10.1002/anie.201702734

An efficient, sustainable organocatalyst, glycine betaine, was developed for the reductive functionalization of CO₂ with amines and diphenylsilane. Methylamines and formamides were obtained in high yield by tuning the CO₂ pressure and reaction temperature. Based on identification of the key intermediate, that is, the aminal, an alternative mechanism for methylation involving the C⁰ silyl acetal and aminal is proposed. Furthermore, reducing the CO₂ amount afforded aminals with high yield and selectivity. Therefore, betaine catalysis affords products with a diversified energy content that is, formamides, aminals and methylamines, by hierarchical two-, four- and six-electron reduction, respectively, of CO₂ coupled with C?N bond formation.

Full text of DOI:10.1002/anie.201702734

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Accepted Article
Title: Betaine Catalysis for Hierarchical Reduction of CO2 with Amine
and Hydrosilane to Formamide, Aminal and Methylamine
Authors: Xiao-Fang Liu, Xiao-Ya Li, Chang Qiao, Hong-Chen Fu, and
Liang-Nian He
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201702734
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10.1002/anie.201702734
Angewandte Chemie International Edition
COMMUNICATION
Betaine Catalysis for Hierarchical Reduction of CO2 with Amine
and Hydrosilane to Formamide, Aminal and Methylamine
Xiao-Fang Liua, Xiao-Ya Lia, Chang Qiaoa, Hong-Chen Fua, Liang-Nian He*a,b
Abstract: An efficient, sustainable organocatalyst i.e. glycine
betaine was developed for reductive functionalization of CO2 with
amines and diphenylsilane. Methylamines and formamides could
respectively be obtained with high yield via tuning CO2 pressure and
reaction temperature. Betaine catalysis was efficient for the
formation of formamide at 10 bar CO2 and 50 oC. Exclusively
yielding methylamines from various amines with atmospheric
pressure of CO2 was attained at 70 oC. Based on identification of the
key intermediate i.e. aminal, an alternative mechanism for
methylation involving the C0 silyl acetal and aminal was proposed.
Furthermore, reducing CO2 amount to approx. 1 equiv. afforded
aminal with high yield and selectivity. Therefore, betaine catalysis in
this study afforded the products with diversified energy content i.e.
formamide, aminal and methylamine respectively through
hierarchical 2-, 4- and 6-electron reduction of CO2 coupled with C-N
bond formation for the first time.
realized the reduction of CO2 to formaldehyde.[18] Furthermore,
few examples for combining reduction of CO2 into the C0
function (i.e. methylene) with formation of new C−N, C−O or
C−C bonds have ever been uncovered.[17,19,20] For instance,
Cantat et al. for the first time described the four-electron
reduction of CO2 with amines and hydrosilane to access aminals
in 2015.[17] Other reports for CO2 reduction to C0 oxidation state
coupled with formation of new bonds also include
hydroformylation of olefins with CO2 and carbonylative coupling
with CO2 via CO pathway.[21]” Hitherto, the hierarchical reduction
of CO2 with amine and hydrosilane to access formamides,
aminal and methylamines remains unknown.[22]
In recent years, CO2 has been already employed as a
sustainable C1 building block in organic synthesis.[1] Among
these, reductive functionalization of CO2 with amines and
reductant, which combines both reduction of CO2 and C-N bond
construction and is illustrated using the so called "diagonal
approach” by Cantat et al [2], to produce versatile chemicals and
energy-storage materials i.e. formamides, aminals and
methylamines that are usually from petroleum feedstocks[3]
would be appealing and promising as shown in Scheme 1.[4] In
this context, metal-based catalytic systems including Ru,[5] Fe,[6]
Cu,[7] Zn,[8] Ni[9] and Cs[10] have been developed. On the other
hand, a number of organocatalysts, such as TBD (1,5,7-
Scheme 1. Reductive functionalization of CO2 with amine to afford formamide,
aminal and methylamine
To resolve hierarchical reduction of CO2, the selection of
well-balanced catalyst that could subtly control the kinetics of
CO2 reduction is crucial.[17] Betaine is known as an inner salt
with onium cation and with negatively charged moiety e.g. a
carboxylate anion, which could interact with silicon atom of
hydrosilane, thereby generating the penta- or hex-coordinated
silicon species and thus resulting in enhanced hydride donating
ability.[21] With reasonable interaction towards hydrosilane,
betaine catalysis may balance the reactivity of CO2 reduction
and allow the C+II, C0 and C-II species to be stabilized selectively,
whereby reaching hierarchical reduction of CO2.
Herein, we would like to report glycine betaine (GB, 1-
carboxy-N,N,N-trimethylmethanaminium inner salt) as an
excellent and sustainable organocatalyst for reductive
functionalization of CO2 with various amines and diphenylsilane.
Selectively yielding methylamines and formamides, respectively,
could be achieved by switching CO2 pressure. Particularly, the
reductive reaction could terminate selectively at the
formaldehyde level i.e. aminal through subtly regulating CO2
amount. As a whole, this protocol represents the first example
for selective 2-, 4- and 6-electron reduction of CO2 with amine
and hydrosilane to products with various energy content i.e.
formamide, aminal and methylamine, respectively.
triazabicyclo[4.4.0]dec-5-ene),[2]
carbenes),[11]
ILs (ionic
NHCs
(N-heterocyclic
as well as
and TBAF
liquids)[12]
proazaphosphatrane superbases,[13] B(C6F5)3
[14]
(tetrabutylammonium fluoride)[15] have also shown high activity
to afford formamides and/or methylamines.[16] Notably, the
carbon oxidation state in formyl of formamides and methyl of
methylamines is +II and –II respectively, and the formation of C0
species (i.e. formaldehyde, aminal) from CO2 remains
a
challenge. This may be because reduction of the C0 species to
C-II species (methanol, methylamine) is more rapid than
reduction of formate derivatives (i.e. formic acid, formamide) to
C0 species, which renders the C0 species elusive to be trapped
and isolated.[17] “As a consequence, only a few reports have
[a]
[b]
Xiao-Fang Liu, Xiao-Ya Li, Chang Qiao, Hong-Chen Fu, Prof.
Dr, Liang-Nian He
State Key Laboratory and Institute of Elemento-organic
Chemistry, College of Chemistry
Nankai University, Tianjin 300071 (China)
E-mail: heln@nankai.edu.cn
Prof. Dr. L.-N. He
Collaborative Innovation Center of Chemical Science and
Engineering
Nankai University, Tianjin 300071 (P.R. China)
For our initial investigation, the reductive functionalization of
CO2 with N-methylaniline (1a) in the presence of diphenylsilane
as reductant was chosen as a benchmark reaction (Table 1). No
product was detected without any catalyst (entry 1, Table 1), and
betaine hydrochloride GB•HCl bearing proton at the oxygen
atom of carboxylate showed no catalytic activity neither (entry 2).
Excitingly, quantitative yield of methylation product i.e. N,N-
dimethylaniline 1c was obtained by employing GB as the
catalyst (entry 3). Those results hint that free oxygen-
nucleophilic species play a vital role in promoting the reaction. In
Supporting information for this article is given via a link at the
end of the document.
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10.1002/anie.201702734
Angewandte Chemie International Edition
COMMUNICATION
Table 1. Catalyst screening and condition optimization for reductive
functionalization of CO2 with N-methylaniline1a and hydrosilane
other words, because carboxylate is presumably able to activate
CO2 molecule as well as diphenylsilane via interaction of oxygen
of betaine and silicon of hydrosilane to form the hypervalent
silicon species, thus improving reducibility of hydrosilane.[23]
Further control experiments showed that carboxylate salts with
ammonium cation work smoothly and the cation (e.g. ammonium
group) tunes the catalytic activity via adjusting the nucleophilicity
of carboxylate anion (Table S1, SI). Without CO2, the reaction
did not occur at all (entry 4), suggesting CO2 is indispensable for
this methylation. Furthermore, halving the catalyst loading did
not influence on 1c yield (entry 5); while decreasing the amount
of diphenylsilane, 1c yield declined to some extent (entry 6).
Interestingly, a considerable amount of N-methylformanilide
1b coupled with 1c was formed at relatively low temperature e.g.
50 oC (entry 7). The similar trend that low temperature favors for
the formylation was also observed in the TBAF and Cs2CO3
catalysis.[10a, 15] However, further decreasing temperature to 30
oC just gave a trace amount of 1b (entry 8). On the other hand,
increasing CO2 pressure to 10 bar, quantitative yield of 1b could
be achieved (entry 9). This is because high CO2 pressure
represents high CO2 concentration and large excess CO2 would
consume diphenylsilane, depressing further reduction and
causing the reaction to terminate at N-formylation step. Even
halving the amount of diphenylsilane, a 92% yield of 1b could
still be attained (entry 10). Therefore, selective reductive
functionalization of CO2 with amine to formamide and
methylamine respectively was realized through switching the
temperature and CO2 pressure. The effects of different
hydrosilanes were examined, demonstrating the priority of
diphenylsilane for this reaction (Table S2).
Entry
Catalyst
T/
P(CO2)
Conv. of
1a/%[b]
0
Yield of
1b/%[b]
0
Yield of
1c/%[b]
0
oC
70
70
70
70
70
70
50
30
50
50
/atm
1
1
-
2
GB• HCl
GB
GB
GB
GB
GB
GB
GB
GB
1
trace
>99
0
trace
trace
0
trace
98 (95)[g]
0
3
1
4[c]
5[d]
6[e]
7
0
1
>99
73
trace
trace
16
96 (91)[g]
72
1
1
80
63
8
1
trace
98
trace
96
trace
trace
trace
9
10
10
10[f]
94
92
[a] Conditions: 1a (0.25 mmol, 27 μL), GB (3.0 mg, 10 mol%), diphenylsilane
(1 mmol, 186 μL), CH3CN (2 mL), 12 h. [b] Determined by GC using 1,3,5-
trimethyoxybenzene as an internal standard. [c] N2 balloon. [d] GB (1.5 mg, 5
mol%). [e] Diphenylsilane (0.75 mmol, 140 μL). [f] Diphenylsilane (0.50 mmol,
93 μL) and reaction time 18 h. [g] Isolated yield.
With the optimal conditions in hand, we evaluated the scope
of this newly-developed GB-catalyzed N-methylation protocol as
shown in Scheme 2. A variety of secondary aromatic amines
were smoothly methylated with good to excellent yields (1c-10c).
Remarkably, excellent functional-group tolerance was observed
for alkynyl (5c), ester (6c), alkenyl (8c) and hydroxyl (10c) group.
In addition, secondary aliphatic amines showed moderate
reactivity for methylation (11c-12c). Primary amines also were
successfully methylated to furnish the corresponding N,N-
dimethylamines with moderate to good yield by doubling the
amount of diphenylsilane and reaction time (13c-18c).
Furthermore, we explored the utility of the formylation of
amines with CO2 using GB as an organocatalyst (Scheme 2).
Secondary aromatic amines with both electron-donating and
electron-withdrawing substituent on the phenyl ring were
suitable for this formylation, providing the corresponding
formamide in 62–96% yields (1b-4b, 8b and 19b). It is worth
mentioning that alkenyl (8b) and carbonyl (19b) groups were
well tolerated in this catalytic system, demonstrating nice
functional group compatibility. Aliphatic amines showed better
activity than aromatic counterparts (11b-12b, 20b vs. 1b-4b),
probably being ascribed to the stronger nucleophilicity.
Additionally, primary amines behaved in a similar fashion to
secondary amines, forming mono-formylated products (13b-17b).
Several control experiments were performed to explore
possible intermediate in N-methylation reaction as depicted in SI.
The N-methylation delivered methylamine 1c coupled with trace
of formamide 1b (Figure S1), suggesting that formamide may
serve as the active intermediate. However, formamide 1b as
substrate was unable to give the methylated product 1c at all
under otherwise identical conditions, which may exclude the
possibility of formamide as the intermediate (eqn. 1, SI). On the
other hand, urea 1d, which has ever been proposed as an
intermediate in the Ru-catalyzed methylation of amine with CO2
and hydrosilane,[5a] could not deliver 1c neither (eqn. 2, SI).
Furthermore, CO2 could be reduced by diphenylsilane to
diphenyldimethoxysilane (methanol level) under the identical
conditions (SI), thus diphenyldimethoxysilane was examined as
possible intermediate. However, methylamine 1c could not be
obtained neither (eqn. 3, SI). These control experiments
illustrate that three pathways for N-methylation involving
formamide, urea and methanol, respectively do not work in this
betaine catalysis.
To gain insight into mechanism of N-methylation, the
dependence of reaction time over catalytic performance was
further studied, and the results were given in Figure S1. A
certain deviation appears between conversion of 1a and total
yield of 1b and 1c from 4 h to 6 h. The formation of silyl formate,
acetal and methoxide (reductive products of CO2 in the presence
of diphenylsilane) as well as formamide 1b, methylamine 1c and
N,N’-dimethyl-N,N’-diphenylmethanediamine 1e (products of
reductive functionalization of CO2 with amine and hydrosilane)
within 4-6 h could be confirmed by GC-MS (SI). Among those
reactive species, the aminal 1e was detected with 31% NMR
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10.1002/anie.201702734
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COMMUNICATION
yield in 4 h and almost vanished in 6 h, implying 1e may be the
intermediate. Indeed, the aminal 1e was successfully isolated in
this study,[24] and further reacted with diphenlysilane under
identical conditions afford a quantitative yield of 1c (Scheme S4).
These results support that the aminal is the feasible
intermediates. In this context, Cantat et al. recently reported that
the aminal was obtained through condensation between amine
and C0 silyl acetal[17] which has been detected in this GB-
catalyzed reaction. Therefore, it can be concluded that this
reaction goes through the C0 silyl acetal and aminal pathway.
Such results stimulated us to explore if the reaction could
terminate at aminal i.e. C0 level. Therefore, further condition
exploration was conducted as shown in Table 2. Temperature
the amine affords the aminal VI. Finally, VI is reduced to the
methylated product VII in the presence of GB and diphenylsilane.
o
screening showed that 50 C is optimal (entries 1-3, Table 2).
Increasing the amount of CO2 to approx. 2 equiv. relative to the
amine led to a sharp drop in the yield of N,N’-dimethyl-N,N’-
diphenylmethanediamine 1e (entry 2 vs 4), which implies the
amount of CO2 is crucial in selectively obtaining aminal. This
may be because inadequate CO2 suppresses further reduction
of aminal to methylamine. Halving dosage of GB did not
influence on the yield of 1e (entry 2 vs 5). As such, employing
phenylsilane as reductant delivered similar results (entry 2 vs 6).
The reductive functionalization of CO2 with amines to aminal
was explored tentatively. Secondary aromatic amines N-methyl-
p-toluidine 3a and 4-chloro-N-methylaniline 4a as well as
heterocyclic compounds 21a worked well to afford
corresponding aminals 3e, 4e, 21e (Scheme 2).
Scheme 2. Reductive functionalization of CO2 with amine to afford formamide,
aminal and methylamine. N-formylation condition: amine (0.25 mmol), GB (3.0
mg, 10 mol%), Ph2SiH2 (93 μL, 0.5 mmol), 50 oC, 10 bar CO2, 18 h; N-
methylation condition: amine (0.25 mmol), GB (1.5 mg, 5 mol%), Ph2SiH2 (186
μL, 1 mmol), and CH3CN (2 mL), 70 oC, 1 bar CO2 for 12 h; Conditions of
aminal formation: amine (1 mmol), GB (12.0 mg, 10 mol%), Ph2SiH2 (558 μL, 3
mmol), 50 oC, CO2 (approx. 1 mmol, closed), 4 h; yields were determined by
1H NMR using 1,3,5-trimethyoxybenzene as an internal standard. *Primary
amine (0.25 mmol), GB (5 mol%), Ph2SiH2 (372 μL, 2 mmol), and CH3CN (2
mL), 70 oC, 1 bar CO2, 24 h and dimethylamines were obtained dominantly.
By raising CO2 pressure and lowering reaction temperature,
the formylated product V can be obtained with high yield and
selectivity. Nucleophilic attack of the amine towards II delivers
the formamide V. This is because excess CO2 exhausts
diphenylsilane to afford the silyl formate II, which is unable to go
through further reduction without reductant. In addition, further
reduction of CO2 to the silyl acetal III may require high energy
input compared with the reduction to the silyl formate II, and
lower temperature thus disfavors for further reduction to III. In
other words, both high CO2 pressure and low temperature
suppress further reduction and render the reduction terminating
at the stage of the silyl formate, which leads to the formation of
formamide V rather than VI.
Thirdly, reducing the amount of CO2 to approx. 1 equiv.
relative to amine delivers selectively VI rather than methylamine
VII. When CO2 (1 equiv. relative to amine) reacts with amine and
diphenylsilane, CO2 is quickly consumed, thus resulting in failure
in conversion of VI to VII due to CO2 deficiency and renders the
reaction terminating at the step of aminal VI. That is, inadequate
CO2 amount restricts further reduction of VI to VII.
Table 2. Condition exploration for reductive functionalization of CO2 with N-
methylaniline 1a to N,N’-dimethyl-N,N’-diphenylmethanediamine 1e
Entry
T/oC
CO2
Conv. of
1a/%[b]
trace
99
Yield of
1b/%[b]
trace
trace
trace
9
Yield of
1c/%[b]
trace
trace
trace
19
Yield of
1e/%[b]
trace
93
equiv
1
30
50
70
50
50
50
1
1
1
2
1
1
2
3
61
59
4
65
37
5[c]
6[d]
99
trace
trace
trace
4
93 (85)[e]
87
92
[a] Condition: 1a (1 mmol, 108 μL), GB (11.8 mg, 10 mol%), Ph2SiH2 (6 mmol
Si-H), CO2 (approx. 1 mmol, closed), CH3CN (2 mL). [b] Determined by 1H
NMR using 1,3,5-trimethyoxybenzene as an internal standard. [c] GB (5.9 mg,
5 mol%). [d] PhSiH3 (Si-H 6 mmol). [e] Isolated yield.
To sum up, CO2 amount in this betaine catalysis determines
product diversity (formamide, aminal and methylamine,
respectively) as well as hierarchical reduction of CO2 to formic
acid (+II), formaldehyde (0) and methanol (-II) oxidation level.
In summary, we have developed an efficient organocatalytic
process for reductive functionalization of CO2 with amines and
diphenylsilane promoted by glycine betaine. Through tuning the
CO2 amount and reaction temperature, this protocol enables
three levels of reductive products with various energy content i.e.
formamide, methylamine and aminal, which represents
hierarchical reduction of CO2 to formic acid, formaldehyde and
methanol oxidation level, to be accessed respectively. This is
the first time that switchable 2-, 4- and 6-electron reduction of
CO2 coupled with construction of C-N bond was realized under
organocatalysis.
On the basis of the above results and previous reports,[4b]
a
possible reaction pathway for the present GB-catalyzed
reductive functionalization of CO2 with amine was proposed as
depicted in Scheme 3. Employing atmospheric pressure CO2,
methylamine could be acquired exclusively. GB initially interacts
with diphenylsilane through nucleophilic oxygen of GB and
silicon of diphenylsilane, forming the hypervalent silicon species
I bearing active hydride, which is also supported by DFT
calculation.[25] Subsequent hydride transfer from I to CO2
delivers the silyl formate II, which goes through further reduction
to give the C0 silyl acetal III. Then condensation between III and
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10.1002/anie.201702734
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COMMUNICATION
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Scheme 3. Proposed reaction pathway on reductive functionalization of CO2
with amine to selectively afford formamide, methylamine and aminal
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This work was financially supported by National Key Research
and Development Program (2016YFA0602900), National
Natural Science Foundation of China (21472103, 21421001,
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21421062),
Natural
Science
Foundation
of
Tianjin
(16JCZDJC39900).
Keywords: Carbon dioxide fixation, organocatalysis, betaine,
hierarchical reduction, aminal
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[5]
Reviews on reductive functionalization of CO2 with amines a) A. Tlili,; E.
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X. Cui; K. Dong, K. Junge, M. Beller, ACS Catal. 2017, 1077; c) J.
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[22] Selectively yielding formamide and methylamine have sprouted via
reductive functionalization of CO2 with amine and hydrosilane, see Ref.
6, 10a, 15 and 16c.
[23] Selected examples on reduction of carbonyl compound catalyzed by
oxygen-nucleophiles: a) K. Revunova, G. I. Nikonov, Chem. Eur. J.,
2014, 20, 839; b) W. Xie, M. Zhao, C. Cui, Organometallics, 2013, 32,
7440.
a) Y. Li, X. Fang; K. Junge, M. Beller Angew. Chem. Int. Ed. 2013, 52,
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[24] Characterization data and NMR spectra of 1e were given in SI.
[25] DFT study showed that the Si-H bond of hydrosilane is further polarized
once coordinated with GB, resulting in the hydrosilane activation by GB,
see SI.
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10.1002/anie.201702734
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
Hierarchical reduction of CO2
：
Xiao-Fang Liu, Xiao-Ya Li, Chang Qiao,
Hong-Chen Fu, Liang-Nian He*
efficient and sustainable glycine
betaine catalysis was developed for
reductive functionalization of CO2 with
amines and diphenylsilane. Such
organocatalysis afforded the products
with diversified energy content i.e.
formamide, aminal and methylamine
respectively through hierarchical 2-, 4-
and 6-electron reduction of CO2
coupled with C-N bond formation for
the first time.
Page No. – Page No.
Betaine Catalysis for Hierarchical
Reduction of CO2 with Amine and
Hydrosilane to Formamides, Aminal
and Methylamines
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