Electrochemical Oxidative C?H/N?H Coupling between γ-Lactams and Anilines

Article abstract of DOI:10.1002/chem.201602454

A mild method for the direct C?H/N?H coupling between γ-lactams and anilines through electrochemical oxidation has been developed. The protocol proceeded smoothly without metal catalysts at room temperature to afford γ-substituted γ-lactams in good yields. It has been revealed that the quasi-divided cell which provided high current density on the anode was crucial for this reaction.

Full text of DOI:10.1002/chem.201602454

DOI: 10.1002/chem.201602454
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&
Synthetic Methods
aE nl e dc t Ar on ci lhi ne em sical Oxidative CÀH/NÀH Coupling between g-Lactams
[a]
Ming Gong and Jing-Mei Huang*
Abstract: A mild method for the direct CÀH/NÀH coupling
between g-lactams and anilines through electrochemical oxi-
dation has been developed. The protocol proceeded
smoothly without metal catalysts at room temperature to
afford g-substituted g-lactams in good yields. It has been re-
vealed that the quasi-divided cell which provided high cur-
rent density on the anode was crucial for this reaction.
Introduction
stages and the nucleophiles are added into the pot after the
power has been switched off. In addition, the laminar-flow
method has been shown to enable amide oxidations in the
Carbon–nitrogen bond formation is important in synthetic or-
ganic chemistry, due to the high prevalence of the nitrogen-
containing compounds in natural products, pharmaceutical-,
[10]
presence of electron-rich nucleophiles such as allylsilanes.
However, as far as operation is concerned, the direct oxidation
of an amide or carbamate with nucleophile in one-pot is
highly desirable. To the best of our knowledge, the direct trap
of the iminium cation of an amide by using anilines as the nu-
cleophile has not been reported in electrochemistry owing to
[
1]
and materials science. Nucleophilic displacement of a leaving
group and reductive amination of carbonyl compounds or
[
2]
imine alkylation are classical transformations. The newly de-
veloped amination methods include the transition-metal-cata-
lyzed Buchwald–Hartwig CÀN coupling, hydroamination, and
[11]
the relatively low oxidation potential of anilines. In continua-
tion of our interest in the application of electrochemical meth-
[
3]
diamination of olefins, and allylic amination. CÀH functionali-
zation is an efficient and highly atom-economic method to
construct complicated structures and great progress has been
[12]
ods to organic synthesis, we report a general method for
the CÀN bond formation through one-step electrochemical CÀ
[
4]
made in this field in recent decades. Recently, alkylation of
H/NÀH coupling between g-lactams and anilines in a quasi-di-
3
[13]
amines to construct CÀN bonds through oxidative sp CÀH
vided cell
yields.
to afford the corresponding products in good
bond activations has been explored by many groups. However,
most of the nitrogen containing substrates were limited to
[
5]
amides, sulfonamides, azoles, and anilines. Alkylation by the
direct use of anilines, a group of substrates susceptible to oxi-
Results and Discussion
3
dization, through oxidative sp CÀH bond activation remains
Initially, we selected 4-bromoaniline (1a) and N-methyl pyrroli-
done (NMP, 2a) as the test substrates. These two compounds
were treated in a solution of NH ClO (0.24m) in 5 mL of THF
[
6]
challenging and typically the substrate scope is limited.
Electrochemical oxidation provides an alternative and envi-
4
4
ronmentally benign method to achieve the CÀH functionaliza-
in a one-compartment cell with a Pt wire anode and a Pt foil
cathode under a constant current (20 mA) at room tempera-
ture for 2.5 h. Product 3a was obtained in a yield of 56%
(Table 1, entry 1). A number of common organic solvents
(Table 1, entries 2, 3, 4, and 5) were examined and we found
[
7]
tion. It has been revealed that an amide or carbamate is oxi-
dized on the anode to generate an iminium cation, which is
[
8]
trapped in situ by a nucleophile. However, only compounds
with high oxidation potentials, such as methanol and cyanide
ions, have been employed as nucleophiles. Introduction of an
electroauxiliary, for example, a silyl group in the a-position to
a nitrogen atom significantly decreases the oxidation potential
that 81% yield of product was obtained when CH CN was em-
3
ployed as solvent (Table 1, entry 2). Studies on the effect of
current density revealed that an increase or decrease of the
current led to the decrease of the product yield (Table 1, en-
tries 6 and 7). Hence, 20 mA was the optimal current for this
reaction. The electrolytes, such as NH ClO , NH BF , Bu NClO ,
[
7b]
of an amide or carbamate.
Thereby the scope of nucleo-
philes is expanded. Another strategy developed by Yoshida et
[
9]
al, described as a “cation-pool” method, involves two discreet
4
4
4
4
4
4
and LiClO were examined and it was found that NH ClO was
4
4
4
[
a] M. Gong, Prof. Dr. J.-M. Huang
the most efficient for this electrochemical reaction (Table 1, en-
tries 2, 8, 9 and 10). A lower yield of the product was obtained
with the addition of an acid or a base (Table 1, entries 11 and
Key Laboratory of Functional Molecular Engineering of Guangdong Prov-
ince, School of Chemistry and Chemical Engineering
South China University of Technology
Guangzhou, Guangdong 510640 (P. R. China)
E-mail: chehjm@scut.edu.cn
1
2). When the reaction was performed under an N atmos-
2
phere, the yield dropped to 66% (Table 1, entry 13). On the
Chem. Eur. J. 2016, 22, 1 – 5
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[
a]
[a]
Table 1. Optimization of the reaction conditions.
Table 2. Substrate scope for synthesis of 3a–t.
Entry
Solvent
mL]
Current
[mA]
Electrolyte
Yield
[%]
[
b]
[
1
2
3
4
5
6
7
8
9
THF
CH CN
DMSO
CH NO
NMP
20
20
20
20
20
15
25
20
20
20
20
20
20
20
20
20
NH
NH
NH
NH
NH
NH
NH
NH
4
4
4
4
4
4
4
4
ClO
ClO
ClO
ClO
ClO
ClO
ClO
4
4
4
4
4
4
4
56
81
18
18
60
69
70
34
64
54
22
63
66
63
70
0
3
3
2
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CH
3
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
BF
NClO
LiClO
4
(Bu)
4
4
1
0
4
[
c]
1
1
NH
NH
NH
NH
NH
NH
4
4
4
4
4
4
ClO
ClO
ClO
ClO
ClO
ClO
4
4
4
4
4
4
[
d]
e]
f]
1
1
1
1
1
2
3
4
5
6
[
[
[
[
g]
h]
[
a] Reaction conditions: 1a (0.25 mmol) and 2a (0.5 mL, 5.2 mmol) in sol-
vent (5 mL) with 0.24m electrolyte was electrolyzed at constant current
20 mA) in an undivided cell at RT, 2.5 h, open air. Anode: Pt (wire, diame-
(
2
ter: 0.5 mm, height: 1.1 cm). Cathode: Pt foil (1ꢁ1.5 cm ). [b] Yields of iso-
lated products. [c] CH COOH (0.43 mmol). [d] (Et) N (0.43 mmol). [e] Reac-
tion performed under an N atmosphere (balloon). [f] Reaction performed
under an O atmosphere (balloon). [g] Reaction performed under an air
3
3
2
2
atmosphere (balloon). [h] No current.
other hand, the yield also decreased in the presence of O2
(
Table 1, entry 14). When the reaction was carried out under
a balloon filled with air, the product was collected in a yield of
0% (Table 1, entry 15). Finally, it was found that no reaction
7
occurred with starting materials recovered when the reaction
was performed in the absence of current (Table 1, entry 16).
With the optimized reaction conditions in hand, we then ex-
plored the substrate scope. As shown in Table 2, a wide range
of anilines could afford the corresponding products in moder-
ate to good yields. Anilines containing electron-withdrawing
groups, such as CF , COCH , COOEt, and OCF , underwent the
[
(
a] Reaction conditions: 1a (0.25 mmol), 2a (0.5 mL, 5.2 mmol), CH
3
CN
5 mL), 0.24m NH ClO as electrolyte, 2.5 h, RT. Anode: Pt wire (diame-
4
4
2
ter=0.5 mm, height=1.1 cm). Cathode: Pt foil (1ꢁ1.5 cm ), constant cur-
rent (20 mA), undivided cell, open air. Yield of isolated products.
[
b] CH
and 10.5 h. [c] After 3 h. [d] After 5 h 40 min. [e] After 3 h. [f] After 1.5 h.
g] After 5 h.
3 3 3
CN:CH COOEt=3:2, (Et) N=0.43 mmol, constant current (10 mA)
3
3
3
[
reaction to give good yields (3c–f). A series of halogen-con-
taining anilines were tested. The results showed that halogen
could be well tolerated in this reaction (3a, 3g, and 3h), pro-
viding a great opportunity for the further synthetic manipula-
tions. Additionally, anilines containing electron-donating
groups such as Me, Et, and OMe, afforded the corresponding
products in slightly lower yields (3i–k).The reaction of hetero-
cyclic aromatic primary amines, such as 6-bromobenzo[d]thia-
zol-2-amine also gave the respective product in good yield
a good partner with pyrrolidin-2-one to give the correspond-
ing product in a yield of 69% (3t). Notably, iodine substituted
aniline could also give the desired products in 49% yield (3p).
Cyclic voltammetry (CV) experiments (Figure 1, for more de-
tails see the Supporting Information) revealed the oxidation
+
peak of NMP at a potential of 1.90 V versus Ag/Ag and the
+
(
3l). Next, to verify the utility of the methodology for pyrrolidi-
oxidation peak of 4-MeC H NH at 0.77 V versus Ag/Ag .
6
4
2
nones, selected reactions were conducted using pyrrolidin-2-
one. Pyrrolidin-2-ones could react with anilines bearing elec-
tron-deficient groups or electron-rich groups on the aromatic
rings, gave corresponding products in moderate yields (3n–r).
Sterically hindered substrates, for example, biphenyl-2-amine
gave 5-([1,10-biphenyl]-2-ylamino)pyrrolidin-2-one in 43% yield
Hence, the effects of the anode were investigated [Eq. (1),
2
Scheme 1]. When a Pt foil (1ꢁ1.5 cm ) anode instead of the Pt
wire was employed, only a trace amount of desired product
was obtained with all the anilines consumed. After reduction
of the length of the Pt wire by one half, the yield decreased to
43% with 37% of anilines recovered. These results demonstrat-
ed that this reaction was very sensitive to current density.
(
3s). Heterocyclic aromatic primary amine also worked as
&
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Scheme 2. Reaction pathway.
+
1
a to give the target product 3a. Reduction of NH4 to re-
lease hydrogen gas occurred on the cathode.
Conclusions
In conclusion, we have developed a mild and metal-free proce-
dure for electrochemical CÀH amination of N-methyl pyrroli-
dones and pyrrolidones in a quasi-divided cell. The commer-
cially available anilines have been used directly as a nitrogen
source in a one-step protocol. The present method provides
an easy to operate route for the synthesis of nitrogen-contain-
ing products in good yields with a wide substrate scope. Fur-
ther investigations into the mechanistic details and applica-
tions of this method are underway in our laboratory.
Figure 1. Cyclic voltammogram of 0.07m NH
4
ClO
4 3
solution in CH CN at RT:
a) none, b) ferrocene (0.002m), c) 4-MeC NH
6
H
4
2
(0.009m), d) NMP (0.004m).
The voltammogram was obtained with Pt wire as auxiliary electrode and Ag
wire as pseudo-reference electrode. The scan rate was 0.1 Vs on a platinum
disk electrode (d=2 mm).
À1
Experimental Section
In a round-bottomed flask (5 mL), 4-bromoaniline (0.25 mmol) and
N-methylpyrrolidone (NMP, 0.5 mL,5.2 mmol) were dissolved in
5
mL CH CN with NH ClO (0.24m) as an electrolyte. The reaction
3 4 4
flask was equipped with Pt wire as anode and Pt foil as cathode
2
(
(
(
1.5 cm ). The solution was electrolyzed at a constant current
20 mA) for 2.5 h under open air at ambient temperature
À1
7.5 Fmol ). After electrolysis, it was quenched by water and ex-
tracted with ethyl acetate (3ꢁ15 mL). The combined organic ex-
tracts were washed with brine (5 mL), dried over MgSO , filtered
and concentrated under vacuum. The crude product was purified
by column chromatography on silica gel with ethyl acetate.
4
Scheme 1. Studies on the effects of the anode.
When a certain length of Pt wire (diameter=0.5 mm, height=
Acknowledgements
1.1 cm) was used as an anode (a cell design referred to as
quasi-divided cell), the high current density on the anode
might have favored the oxidation of NMP instead of anilines as
the concentration of anilines was much lower than that of
NMP. In addition, it is proposed that in the current reaction the
acidic electrolyte, ammonium perchlorate, might have assisted
to reduce the concentration of aniline in the double layer of
anode by protonation of aniline.
We are grateful to the National Natural Science Foundation of
China (Grant Nos. 21172080 and 21372089) for financial sup-
port.
Keywords: CÀH/NÀH coupling · electrochemistry · metal free ·
g-lactams · anilines
Control reactions employing the “cation pool method” were
carried out [Eq. (2), Scheme 1]. The electrochemical oxidation
of NMP was initially conducted under constant current condi-
tions (20 mA) at room temperature for 2.5 h. The current was
then stopped followed by the addition of 4-bromoaniline
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Published online on && &&, 0000
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Chem. Eur. J. 2016, 22, 1 – 5
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Full Paper
FULL PAPER
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Synthetic Methods
M. Gong, J.-M. Huang*
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Electrochemical Oxidative CÀH/NÀH
Coupling between g-Lactams and
Anilines
One-pot lullaby: An easy to operate
method for the electrochemical oxida-
tive coupling of N-methyl pyrrolidone
with anilines to synthesize N-aryl-g-
amino-g-lactams was developed. Ani-
lines were employed directly together
with N-methyl-2-pyrrolidone in one-pot
and no metal was needed. The reaction
was carried out under open air at ambi-
ent temperature to afford the corre-
sponding products in moderate to
good yields with a wide range of func-
tional groups tolerated.
Chem. Eur. J. 2016, 22, 1 – 5
www.chemeurj.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ