Direct synthesis of 2-oxo-acetamidines from methyl ketones, aromatic amines and DMF: Via copper-catalyzed C(sp3)-H amidination

Article abstract of DOI:10.1039/c9ra00616h

A convenient method for the synthesis of 2-oxo-acetamidines from methyl ketones using aromatic amines and DMF as nitrogen sources is reported via copper-catalyzed C(sp³)-H amidination. Various methyl ketones react readily with aromatic amines a

Full text of DOI:10.1039/c9ra00616h

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Direct synthesis of 2-oxo-acetamidines from
methyl ketones, aromatic amines and DMF via
Cite this: RSC Adv., 2019, 9, 7203
copper-catalyzed C(sp3)–H amidination†
*
Wei He, Jiang Xiao, Yao Wu, Yongjia Guo, Qiang Liu
Dianke Xie,
*
and Cancheng Guo
A convenient method for the synthesis of 2-oxo-acetamidines from methyl ketones using aromatic amines
and DMF as nitrogen sources is reported via copper-catalyzed C(sp3)–H amidination. Various methyl
ketones react readily with aromatic amines and DMF, producing 2-oxo-acetamidines in yields of 47 to
92%. This protocol features the simultaneous formation of C–N and C]N bonds using DMF and
aromatic amines as two different nitrogen sources. It thus provides an efficient approach to construct
acyclic amidines via three C(sp3)–H bond amidination. Based on the preliminary experiments, a plausible
mechanism of this transformation is disclosed.
Received 24th January 2019
Accepted 25th February 2019
DOI: 10.1039/c9ra00616h
rsc.li/rsc-advances
decarbonylation of the amide to semicyclic amidines (Scheme 1,
eqn (d)).11 Although signicant progress has been made in this
Introduction
eld, direct and efficient methods for the synthesis of acyclic
amidines from amide are still highly desired.
Amidines are important structural motifs in natural products,
bioactive molecules and functional materials.1 They have
profound applications in diverse areas such as medicinal
chemistry, synthetic intermediates, catalyst design, material
science, supramolecular chemistry, and coordination chem-
istry.2 Therefore, the direct and convenient construction of
amidines is an important and popular research eld, particu-
larly for the synthesis of 2-oxo-acetamidines due to the wide
applications of their carbonyl group as a versatile intermediate
in the synthesis of a broad range of acetamidine derivatives.3
Recently, transition metal and metal-free catalyzed C–N bond
formation has arisen as an excellent synthetic method to build
complex structures because it reduces prefunctionalization
while improving atom economy and energy efficiency.4 These
protocols used nitriles,5 isonitriles,6 organic azides,7 and
amines8 as nitrogenous sources, which are considered impor-
tant pathways to construct cyclic and acyclic amidines.
As our continuing interest in carbon–nitrogen coupling
elds,12 and inspired by the above-mentioned studies, we herein
report a copper-catalyzed adimination of methyl ketones,
aromatic amines and DMF to acyclic amidines, 2-oxo-
acetamidines (Scheme 1, eqn (e)). Even though the reactions
of methyl ketones with amines or amides are known to achieve
the synthesis of a-ketoamides by using metal and metal-free
approaches,13 the synthesis of acyclic amidines using amides
and aromatic amines as two different nitrogen sources has not
been described previously. This approach allows direct amidi-
nation of three C(sp3)–H bonds and affords C–N bond and C]N
bond simultaneously. This protocol provides
a versatile
approach to 2-oxo-acetamidines with good functional group
Amides are key building blocks of proteins and are broadly
found in natural and manufactured organic molecules.9
Meanwhile, amides are also oen used as precursor molecules
to synthesize amidines.10 The general method of synthesizing
amidines by amides is mainly divided into the following two
strategies: (1) direct amidination of the amide and amine
scaffolds without decarbonylation of amide, providing cyclic
amidines as the nal product (eqn (a)–(c) in Scheme 1);10 (2)
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry
and Chemical Engineering, Hunan University, Changsha, 410082, P. R. China.
E-mail: fqiangliu@qq.com; ccguo@hnu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra00616h
Scheme 1 Methods for the synthesis of amidines.
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Table 1 Optimization of the reaction conditionsa
Entry
[Cu] (mol%)
Base
Solvent
Oxidant (equiv.)
T (ꢀC)
Yieldb %
1
2
3
CuCl2 (20)
CuCl2 (20)
CuCl2 (20)
CuCl2 (20)
CuCl2 (20)
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOONa
CH3COONa
CH3ONa
Cs2CO3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DTBP(4)
TBHP(4)
BPO(4)
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
80
75
15
0
0
85
0
4
AIBN(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
DTBP(4)
5c
6
7d
8e
9
CuCl2 (20)
CuCl2 (20)
CuCl (20)
CuBr (20)
CuI (20)
Cu(OAc)2 (20)
Cu(acac)2 (20)
CuCl2 (20)
CuCl2 (20)
CuCl2 (20)
CuCl2 (20)
CuCl2 (20)
CuCl2 (20)
CuCl2 (20)
CuCl2 (20)
CuCl2 (20)
36
0
56
23
19
50
47
60
55
45
Trace
36
50
71
55
74
10
11
12
13
14
15
16
17
18
19
20
21f
22g
PhCOOK
PhCOOK
PhCOOK
PhCOOK
PhCOOK
100
140
120
120
a
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), catalyst (0.1 mmol), and base (0.5 mmol), oxidant, additive and solvent (2 mL) under O2
b
c
d
e
f
g
atmosphere at 120 ꢀC for 36 h. Isolated yields. Phen (0.1 mmol). Air. N2 atmosphere. 12 h. 24 h. DTBP ¼ di-tert-butyl peroxide, TBHP
¼ tert-butyl hydroperoxide, BPO ¼ benzoyl peroxide, AIBN ¼ 2,20-azobis(2-methylpropionitrile), phen ¼ 1,10-phenanthroline monohydrate,
DMSO ¼ dimethyl sulfoxide, DMF ¼ N,N-dimethylformamide, THF ¼ tetrahydrofuran.
tolerance. Notably, less reactive aliphatic methyl ketone could atmosphere, 1a and 2a could not react to form 4aa (entry 8;
also be converted into the corresponding amidine derivatives.
Table 1). This fact implied that oxygen was essential for this
reaction. Reaction temperature and reaction time were also
scanned to improve the yield, and 120 C and 36 h were deter-
ꢀ
Results and discussion
mined as optimum for the oxidative amidination reaction
(entries 18–22; Table 1). Aer screening on different parame-
ters, the highest yield of 4aa (85%) was achieved when the
reaction was carried out with CuCl2 (0.1 mmol), phen (0.1
mmol), PhCOOK (0.5 mmol) and DTBP (2 mmol) at 120 ꢀC
under the atmosphere of oxygen in DMF (entry 9; Table 1).
With the optimized reaction conditions in hand, the scope
with respect to the methyl ketones was rstly evaluated (Table 2).
A wide variety of methyl ketones bearing electron-donating and
electron-withdrawing functional groups gave the corresponding
products in moderate to excellent yields (4aa–4ra). The position
of the substituents on the aryl ring had a minor effect on the
efficiency of this transformation. For example, not only methyl
ketones 1d and 1i possessing para substituents but also
substrates (1c,1e) and 1b, bearing substituents in meta and ortho
position, afforded the corresponding products in good yields. In
addition, our newly developed protocol tolerated a variety of
functionalities, including halogens (4ea, 4fa, 4ga and 4ha),
methoxy (4ka), ethoxy (4la), ester groups (4ma), t-butyl (4na).
Notably, reactive primary amine on the aromatic ring were also
Initially, the reaction of acetophenone 1a, aniline 2a and DMF
catalyzed by copper salts was performed to optimize the reac-
tion conditions. The results were listed as shown in Table 1. To
our delight, under CuCl2 conditions, reaction of 1a, 2a with
DMF afforded mainly oxidative amidination product 4aa in 15–
85% yield (entries 1–8, Table 1). Subsequently, a number of
other oxidants including DTBP, TBHP, BPO, AIBN were
explored. Only the DTBP and TBHP effected the reaction,
among which DTBP proved to be superior oxidant, giving 4aa in
75% yield (entries 1–4; Table 1). Gratifyingly, further improve-
ment of the process was achieved when the reaction conducted
under the ligand of phen (1,10-phenanthroline monohydrate),
affording 4aa in 85% yield (entry 5; Table 1). Copper salts
exhibited unique ability in this transformation, as the reaction
did not occur without copper catalyst (entry 6; Table 1). Aer
a series of copper salts were tested (entries 9–13; Table 1), CuCl2
was demonstrated to be the best choice. Further studies indi-
cated that base can promote this transformation, PhCOOK is
optimal (entries 14–17; Table 1). Under the nitrogen
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Table 2 Scope of methyl ketonesa
Table 3 Scope of aromatic aminesa
a
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), CuCl2 (0.1 mmol),
PhCOOK (0.5 mmol), DTBP (2 mmol), phen (0.1 mmol), N,N-
a
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), CuCl2 (0.1 mmol),
b
dimethylformamide (2 mL), O2 (1 atm), 36 h, 120 ꢀC. Isolated yields.
PhCOOK (0.5 mmol), DTBP (2 mmol), phen (0.1 mmol), N,N-
b
dimethylformamide (2 mL), O2 (1 atm), 36 h, 120 ꢀC. Isolated yields.
GC yields. d N.D. ¼ not detected.
c
c
GC yields.
Then, the scope of amides was explored (Table 4). Unfortu-
nately, this protocol was not general to a wide range of form-
amides including N,N-diethylformamide, N-methylformamide,
tolerated (4oa). Fortunately, when p-nitroaniline were used as
substrates, the desired products were detected (4ja) in moderate
yield. Heteroaryl methyl ketones were also investigated, and the
corresponding products were obtained in moderate to excellent
yields (4pa, 4qa, 4ra). Pyridine methyl ketones failed to partici-
pate in the reaction to obtain the desired product (4sa,4ta).
Moreover, this methodology could be extended to alkyl methyl
ketones as well, although in moderate yield (4ua).
Next, the scope of aromatic amines was explored (Table 3). A
wide range of structurally diverse aromatic amines were suit-
able substrates for this transformation. For example, aryl
amines bearing methyl groups could generate the correspond-
ing 2-oxo-acetamidines 4ab–4ad in excellent yields. Under the
optimal reaction conditions, the meta-halogen substrates (4ae–
4ag) and the para-halogen substrates (4ah–4ak) are both
completely tolerated. Also, functional groups such 4-isopropyl
(4am), 3,5-dimethoxy (4an) were well tolerated under our reac-
tion conditions. Fortunately, when p-nitroaniline were used as
substrates, the desired products were detected (4al) in moderate
yield. Moreover, heteroaryl amines were also investigated, and
the corresponding products were obtained in moderate yields
N-methyl-N-phenylformamide,
morpholine-4-carbaldehyde
with piperidine-1-carbaldehyde, giving no desired products.
To elucidate the mechanism, some control experiments were
performed (Scheme 2). At rst, the reaction of acetophenone 1a with
aniline 2a was carried out without CuCl2, and the desired product
4aa was not detected (Scheme 2 [eqn (1)]). When the reaction was
done in the absence of O2, 4aa was also not detected (Scheme 2 [eqn
(1)]). These results suggested that both CuCl2 and O2 were necessary
for the reaction. When 2.0 equiv. of BHT (2,6-di-tert-butyl-4-
methylphenol) was added under the standard conditions, the
reaction was inhibited substantially (Scheme 2 [eqn (1)]). The result
suggests that the reaction may involve a radical reaction. Our
reaction may be involved dimethylamine generated in situ from
DMF.14 To verify this pathway, dimethylamine 5 was then used
instead of DMF under the same conditions (Scheme 2 [eqn (2)]), and
only a little amount of 4aa was detected. This result implies that an
aminyl radical, not an amine, was the intermediate in the trans-
formation process. According to the literatures and experimental
results,15 we predicted that acetophenone 1a may be oxidized to
phenylglyoxal intermediate, and then form Schiff base to complete
the transformation process. Therefore, we conducted a control
experiment in which phenylglyoxal monohydrate 6 and imine 7
(4ao,4ap).
Unfortunately,
pyridin-2-amine
and
5-
methylisoxazol-3-amine are not prone to this reaction (4aq,4ar).
Meanwhile, quinolin-8-amine does not react (4as).
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Table 4 Scope of N,N-disubstituted formamidesa
a
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), CuCl2 (0.1 mmol), PhCOOK (0.5 mmol), DTBP (2 mmol), phen (0.1 mmol), DMSO (2 mL), O2 (1 atm),
36 h, 120 ꢀC. b N.D. ¼ not detected. c GC yields.
were used as substrates under standard conditions (Scheme 2 [eqn
(3) and (4)]). The experimental results show that the target product is
obtained in a lower yield, indicating that 6 and 7 may not be the key
intermediates in the reaction. As shown in eqn (5), the reactions
between 2a and 2-(dimethylamino)-1-phenylethan-1-one 8 were
investigated. Under the optimized conditions, the product 4aa
could be furnished in 80% yields (Scheme 2 [eqn (5)]). The results
demonstrated that the reaction may have undergone 8 intermediate
process. Notably, 2-oxo-N,2-diphenylacetamide 9 and 1-phenyl-2-
(phenylamino)ethan-1-one 10 both were not suitable substrates
for the reaction (Scheme 2 [eqn (6) and (7)]). When the reaction was
carried out in the absence of acetophenone 1a, N,N-dimethyl-N0-
phenylformimidamide 11 and 1,2-diphenyldiazene 12 were detec-
ted (Scheme 2 [eqn (8)]). This result indicated the cationic aniline
radicals maybe formed in this reaction.16
On the basis of the above results, a plausible mechanism
for the copper-catalyzed aerobic oxidative coupling is illus-
trated in Scheme 3. In the rst step, the tert-butoxyl radicals
trapped hydrogen from the aryl methyl ketone and DMF
respectively to form radical B13b,17 and aminyl radical A.18 Then,
decarboxylation of A produced aminyl radical C, which reacted
with radical B to generate intermediate D. D was then
Scheme 2 Different control experiments.
Scheme 3 A proposed mechanism for the direct transformation.
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converted to E in the presence of tert-butoxyl radical. Mean-
while, a single-electron oxidation of anilines mediated by
Cu(II) occurred, affording corresponding radical cations F.16
Cationic radical F coupled with radical E to obtain G.
Hydrogen ion of G was removed by base to give the product H.
Finally, H is oxidized under standard conditions to form the
desired product I (Scheme 3).
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Conclusions
In conclusion, the copper-catalyzed oxidative amidination of
methyl ketones with aromatic amines and DMF has been
developed. Acyclic 2-oxo-acetamidines could be obtained with
moderate to good yields. This protocol features with acyclic
amidines formation using DMF and aromatic amines as two
different nitrogen sources. Further investigations on reaction
scope and synthetic application are underway in our laboratory.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (21372068, 21572049), Science
and Technology Program of Hunan Province, China
(2014GK3115) and Science and Technology Program of Chang-
sha, China (K1508004-11).
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