DBU-Catalyzed Selective N-Methylation and N-Formylation of Amines with CO2 and Polymethylhydrosiloxane

Article abstract of DOI:10.1002/adsc.201800140

We describe herein an efficient organocatalytic system for the selective N-methylation and N-formylation of amines with carbon dioxide (CO₂) as a sustainable C1 feedstock and polymethylhydrosiloxane (PMHS) as a cost-effectvie reducing reagent. High-yielding N-methylation products are obtained with low catalyst loading (1%) of DBU. Selective N-formylation of amines is achieved using the same catalytic system at a lower reaction temperature. (Figure presented.).

Full text of DOI:10.1002/adsc.201800140

Title: DBU-Catalyzed Selective N-Methylation and N-Formylation of
Amines with CO2 and Polymethylhydrosiloxane
Authors: Gang Li, Jie Chen, Daoyong Zhu, Ye Chen, and Ji-Bao Xia
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800140
Link to VoR: http://dx.doi.org/10.1002/adsc.201800140

10.1002/adsc.201800140
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
DBU-Catalyzed Selective N-Methylation and N-Formylation of
Amines with CO2 and Polymethylhydrosiloxane
Gang Li,a,b Jie Chen,a Dao-Yong Zhu,a Ye Chenb,* and Ji-Bao Xiaa,*
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou
Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou, 730000, P. R. China.
Phone: 86 13656204826; Fax: 86 521 81880906; E-mail: jibaoxia@licp.cas.cn
School Chemistry of Chemical Engineering, Guizhou University, Guiyang, 550025, P. R. China.
b
Phone: 86 13985131235; Fax: 86 851 83625867; E-mail: ychen19@gzu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. We describe herein an efficient organocatalytic
system for the selective N-methylation and N-formylation of
amines with carbon dioxide (CO2) as a sustainable C1
feedstock and polymethylhydrosiloxane (PMHS) as a cost-
effectvie reducing reagent. High-yielding N-methylation
products are obtained with low catalyst loading (1%) of
DBU. Selective N-formylation of amines is achieved using
the same catalytic system at a lower reaction temperature.
Keywords: carbon dioxide; catalysis; reduction;
methylation; formylation
Besides, N-methylation of amines producing
methylamines using CO2 and silanes have also been
achieved with organocatalysts[11] or transition-metal-
catalysts[12]. However, an expensive catalyst and/or a
high catalyst loading were usually necessary for the
reported catalytic systems, especially for the catalytic
N-methylation of amines, which prevented the
application of those methods. On the other hand,
selective N-methylation and N-formylation of amines
with one catalytic system is rare. Cantat’s iron-
tetraphosphine-system[13] and Dyson’s thiazolium
carbene system[14] were efficient for the reductive N-
formylation of amines with CO2, but showed low
efficiency or limited substrate scopes for the
reductive N-methylation of amines. Notably, Fu’s and
Lin’s recent cesium carbonate system[15] and He’s and
Han’s independent glycine betaine system[16]
displayed remarkable results for both N-formylation
and N-formylation of amines with CO2 and
phenylsilanes. Although phenylsilanes were widely
used in the previous reductive fixation of CO2 onto
Introduction
The incorporation of carbon dioxide into organic
molecules is highly desirable since CO2 is an
abundant, inexpensive, nontoxic, and sustainable
carbon feedstock.[1] The past decade has witnessed a
rapid improvement in the homogeneous catalytic
reduction of CO2 into energy-rich small molecules
such as formic acid, carbon monoxide, methanol, and
methane.[2] In the meanwhile, reductive fixation of
CO2 onto amines by means of forming a new C–N
bond to yield formamides or methylamines has
received growing attention.[3] Both of the products are
fine chemicals widely used as solvents or
intermediates for syntheses in the area of
agrochemicals, medicines and material sciences.[4]
Hydrogen has been used in the reductive fixation
of CO2 onto amines to afford formamides or
methylamines since 1970.[5] However, expensive
metal-based catalysts and harsh reaction conditions
are usually needed in these reactions, such as high
amines,
they
are
more
expensive
than
reaction temperature and high pressure of CO2 and H2. polymethylhydrosiloxane (PMHS), an abundant
In contrast, chemists have found that the catalytic
reduction of CO2 could be achieved with easily
handled hydrosilanes[6] or boranes[7] under milder
conditions. Following the initial report in 2012 by
Cantat and co-workers of the organic base-catalyzed
reductive N-formylation of amines using CO2 and
phenylsilane,[8] a variety of metal-free[9] and metal-
based[10] catalytic systems have been developed for
the synthesis of formamides with CO2 and silanes.
chemical waste with low cost, nontoxicity, and high
moisture stability from silicone chemistry.[17] It will
be more attractive using PMHS as reductant in the
reductive fixation of CO2.
Recently, we developed a TBD-catalyzed reductive
tandem C–C and C–N bond-forming reaction with
CO2.[18] During the mechanistic studies, we found that
TBD-catalyzed N-methylation of 2,6-diisopropyl-N-
methylaniline with CO2 and phenylsilane afforded the
1
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10.1002/adsc.201800140
Advanced Synthesis & Catalysis
dioxane, ethanol, dichloromethane, chloroform, and
ethyl
acetate
(Supplementary
information).
Significant solvent effect of CH3CN was observed
and the reason was not clear presently. When the
reaction was performed at 30 °C, the selectivity of the
reaction
was
reversed
and
N-methyl-N-
phenylformamide 3a was obtained as a major product
(entry 12). After extensive screening, 84% yield of 3a
was obtained with 10% DBU at a higher
concentration (entry 15). Control experiments
showed that trace of products were obtained without
catalyst at 100 °C and no reaction occurred without
catalyst at 30 °C (entries 16 and 17).
Table 1. Selective N-Methylation and N-Formylation of 1a
with CO2 and polymethylhydrosiloxane (PMHS) a
Figure 1. DBU-Catalyzed Selective N-Methylation and N-
Formylation of Amines using CO2 and Polymethylhydro-
siloxane (PMHS).
corresponding N-methylated product in excellent
yield in CH3CN at 100 °C (Figure 1a). It is an
interesting and unexpected result since N-formylated
product has been obtained under the similar
conditions with THF as solvent (Figure 1b).[8]
Inspired by this result, we aimed to develop an
efficient catalytic system for the selective N-
methylation and N-formylation with CO2 and cost-
effective PMHS. We reported herein an efficient
DBU-catalyzed selective N-methylation and N-
formylation of amines to produce methylamines or
formamides with CO2 as a sustainable C1 source and
PMHS as a cheap reducing reagent (Figure 1c).
T (oC)
/t (h)
Yield of
Yield of
Entry Catalyst
2a (%)a
3a (%)a
1
TBD
100/12
100/12
100/12
100/12
100/12
100/12
100/12
100/12
100/12
100/48
80/48
9
11
7
2
DBU
DBN
DABCO
DMAP
DIPEA
Et3N
86
22
1
3
8
4
4
5
1
4
6
2
4
7
1
4
8b
9c
10c
11c
12d
13d,e
14d,e
DBU
DBU
DBU
DBU
DBU
DBU
DBU
81
66
88
78
3
8
Results and Discussion
10
11
17
42
59
79
84
4
At the outset of our research, we selected
commercially available N-methylaniline 1a as a
model substrate to test our idea. The initial tiral
afforded N,N-dimethylaniline 2a and N-methyl-N-
phenylformamide 3a as a mixture with TBD as a
catalyst in CH3CN at 100 °C (Table 1, entry 1). To
our delight, DBU was an efficient catalyst giving 2a
as a major product along with small amount of 3a as
a minor product (entry 2). In comparison, lower
efficiency was observed with DBN as catalyst, which
has similar structure with DBU (entry 3). Trace of the
product was obtained with other organic bases as
catalysts, such as DABCO, DMAP, DIPEA, and Et3N
(entries 4-7). These results indicate that the structure
of organic bases have dramatic effect on this reaction.
High efficiency of DBU may be attributed be its
unique structure and its high nucleophilic property.
When reducing the catalyst loading of DBU to 1%,
excellent yield of 2a was still obtained albeit with a
long reaction time (88% yield, entry 10). When
decreasing the reaction temperature to 80 °C, the
reaction run smoothly in a slightly reduced yield
(entry 11). Diminished yield or no reaction was
observed using other solvents, such as THF, 1,4-
30/24
30/24
1
30/72
12
14
1
15d,e,f DBU
30/72
16g
17d,e,g
-
-
100/12
30/72
0
0
a)
Reaction conditions: 1a (0.2 mmol), catalyst (5 mol %),
CO2 (1 atm), PMHS (143 µL), CH3CN (2 mL) in a sealed
25 mL screw Schlenk tube, the yield was determined by
b)
GC using n-dodecane as an internal standard. With 2.5
mol % DBU. c) With 1 mol % DBU. d) With 216 µL PMHS.
e)
f
With 0.5 mL CH3CN. With 10 mol % DBU. g Without
catalyst.
With the optimized reaction conditions in hand, we
firstly evaluated the scope of the N-methylation of
amines using CO2 and PMHS. As illustrated in Table
2, secondary monoaromatic amines can be smoothly
2
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10.1002/adsc.201800140
Advanced Synthesis & Catalysis
Table 2. Scope of the DBU-Catalyzed N-Methylation of
heteroaryl-substituted amine (N-methylpyridin-2-
amine, entry 13). Notably, good yields were also
obtained with diarylamine and dibenzylamine as
substrates (entries 14 and 15). When diethylamine
was employed as substrate, N-methylation (2q) and
N-formylation product (3q) were both obtained and
3q was the major product (entry 17). Interestingly,
formamide was formed as the sole product when
cyclic aliphatic amine was used, such as piperidine
and morpholine (entries 17 and 18). However, no
reaction occurred using aniline as the substrate.
Amines with CO2 and PMHS a
Table 3. Scope of the DBU-Catalyzed N-Formylation of
Amines with CO2 and PMHS a
a) Reactions conducted on 0.2 mmol scale. Isolated yields
are provided. b) 5 mol % DBU as catalyst. c) The yield
was determined by GC using n-dodecane as an internal
standard.
methylated affording the corresponding products in
good to excellent yields. Both electron-donating and
electron withdrawing group on the aryl ring of amine
were well tolerated (entries 1-6). In addition, good
yield was obtained when using N-methylaniline with
bulky group on the ortho position of aniline (entry 7).
The N-alkyl substituent on the secondary
monoaromatic amines could be bulky ethyl, iso-
propyl, cyclohexyl, and allyl (entries 8-11). All of
these substrates gave the N-methylated amines in
good yields. Moderate yields were obtained with
cyclic amine (2-methylindoline, entry 12) and
a)
Reactions conducted on 0.4 mmol scale. Isolated yields
are provided. b) The yield was determined by GC using n-
dodecane as an internal standard.
As shown in Table 3, we next evaluated the
scope of the N-formylation of amines using CO2 and
PMHS. Electron-donating group on the aryl ring of
3
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10.1002/adsc.201800140
Advanced Synthesis & Catalysis
Scheme 1. DBU-catalyzed N-methylation of cinacalcet with CO2 and polymethylhydrosiloxane (PMHS)
amine were well tolerated (entries 1, 2, and 5).
However, slow reaction was found with electron
withdrawing group on the para position of aniline
and low yield of 3d and 3e was obtained (entries 3
and 4). Similarly, N-formylation of N,N-
dibenzylamine 1p gave the corresponding formamide
3p in 36% isolated yield (entry 5). In addition, high
yields of formamides were obtained when cyclic
aliphatic amine was used, such as piperidine and
morpholine (entries 8 and 9).
obtained by nucleophilic attack of amines to III. In
another pathway, silyl formate III can be further
reduced to bis(silyl) acetal IV, which is a common
intermediate observed by in-situ NMR in the previous
report on reduction of CO2.[6] Unfortunately, the
detection or isolation of intermediate III and IV in
our system was unsuccessful because of the polymer
property of PMHS. Reaction of amines with IV gives
N-methylated amines 2 as the final product through
possible aminal intermediate.
Methyl group has important biological effect in
bioactive compounds and drug molecules. Methyl-
substituted amines with better activity have been
found in comparing with NH-amines, for example,
morphine vs normorphine.[19] To display the potential
synthetic application of our methodology, we the
DBU-catalyzed N-methylation of drug molecules
with CO2 and PMHS. DBU-catalyzed N-methylation
of cinacalcet 4, a drug molecule used to treat
secondary hyperparathyroidism, afforded the N-
methylated product 5 in good yield under the
standard conditions (Scheme 1).
In the reductive fixation of CO2 onto N-
methylaniline 1a, N-methylated product 2a was
obtained as a major product at higher temperature,
whereas N-formylation product 3a was obtained as a
major product at lower temperature. This suggested
that formamide may be an intermediate in the N-
methylation of amines. However, control experiment
showed that 3a couldn’t be converted into the
methylated product 2a under the identical conditions,
indicating that N-formylation product wasn’t the
intermediate to N-methylated product (Scheme 2, eq.
1).
Scheme 3. Possible reaction pathways.
Conclusion
In summary, we have developed a selective
organocatalytic N-methylation and N-formylation of
amines for the synthesis of methylamines or
formaides with CO2 as a sustainable C1 source and
polymethylhydrosiloxane (PMHS) as a reductant.
The reaction features high efficiency and good
substrate scope using cheap catalyst (DBU) with low
catalyst loading and cost-effective silane (PMHS).
Further investigation of the reaction mechanism and
expansion of the reductive C–C bond formation
method are ongoing in our lab.
Scheme 2. Control experiment.
Experimental Section
General procedure A for the DBU-catalyzed N-methylation
of amines with CO2 and PMHS: An oven-dried Schlenk
tube (25 mL) was evacuated and back-filled with carbon
dioxide (CO2) for 3 times. Then it was charged with the
substrate 1 (0.20 mmol, 21.4 mg). The solution of DBU
(0.002 mmol, 0.3 mg) in CH3CN (20 µL), anhydrous
acetonitrile (2.0 mL) and PMHS (143 µL) were added
sequentially via syringe under CO2. Then the Schlenk tube
was sealed at atmospheric pressure of CO2 (1 atm) and the
resulting mixture was stirred for 48 h at 100 °C. The
mixture was cooled to room temperature and extracted by
Et2O, then concentrated under vacuum. The residue was
purified by column chromatography to give the pure
desired product 2.
Based on our results and previous mechanistic
studies on the reductive fixation of CO2 onto
amines,[20] the possible reaction pathways have been
proposed in this reductive fixation of CO2 onto
amines (Scheme 3). The carbamic zwitterion
intermediate I is first formed by nucleophilic attack
of DBU towards CO2, which has been confirmed by
previous report.[21] Subsequent intramolecular or
intermolecular hydride transfer through possible
species II may afford silyl formate III and
regenerates DBU. In one pathway, formamides 3 are
4
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10.1002/adsc.201800140
Advanced Synthesis & Catalysis
General procedure B for the DBU-catalyzed N-fomylation
of amines with CO2 and PMHS: An oven-dried Schlenk
tube (25 mL) was evacuated and back-filled with carbon
dioxide (CO2) for 3 times. Then it was charged with the
substrate 1 (0.40 mmol, 43.0 mg). The solution of DBU
(0.04 mmol, 6.0 mg) in CH3CN (200 µL), anhydrous
acetonitrile (0.3 mL) and PMHS (216 µL) were added
sequentially via syringe under CO2. Then the Schlenk tube
was sealed at atmospheric pressure of CO2 (1 atm) and the
resulting mixture was stirred for 72 h at 30 °C. The
mixture was extracted by Et2O, then concentrated under
vacuum. The residue was purified by column
chromatography to give the pure desired product 3.
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We are grateful for the financial support from National Natural
Science Foundation of China (21772208, 21702212, 21602230,
21633013), Natural Science Foundation of Jiangsu Province
(BK20161260, BK20170404), and the Hundred-Talented
Program of the Chinese Academy of Sciences.
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10.1002/adsc.201800140
Advanced Synthesis & Catalysis
FULL PAPER
DBU-Catalyzed Selective N-Methylation and N-
Formylation of Amines with CO2 and
Polymethylhydrosiloxane
Adv. Synth. Catal. 2018, Volume, Page – Page
Gang Li,a,b Jie Chen,a Dao-Yong Zhu,a Ye Chenb,*
and Ji-Bao Xiaa,*
7
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