Electrochemical oxidative [4 + 2] annulation of tertiary anilines and alkenes for the synthesis of tetrahydroquinolines

Article abstract of DOI:10.1039/c8gc02463d

Heterocyclic compounds, especially nitrogen heterocycles, are one of the most important classes of compounds in the pharmaceutical and agrochemical industries. The oxidative [4 + 2] annulation reaction provides a powerful tool for the rapid construction of six-membered heterocycles. Herein, we report a metal- and external oxidant-free method for the uniform synthesis of tetrahydroquinolines through the electrochemical oxidative [4 + 2] annulation of tertiary anilines and alkenes. In this strategy, one partner loses only two hydrogen atoms while another partner reduces one degree of unsaturation, accompanied by the generation of hydrogen. Under the conditions of an undivided cell and room temperature, a series of tetrahydroquinoline derivatives were prepared with good yields.

Full text of DOI:10.1039/c8gc02463d

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Electrochemical oxidative [4 + 2] annulation of
tertiary anilines and alkenes for the synthesis of
tetrahydroquinolines†‡
Cite this: Green Chem., 2018, 20,
870
4
Received 6th August 2018,
Accepted 2nd October 2018
Pengfei Huang,§ Pan Wang,§ Shengchun Wang, Shan Tang
and Aiwen Lei
*
DOI: 10.1039/c8gc02463d
rsc.li/greenchem
Heterocyclic compounds, especially nitrogen heterocycles, are one tion is the [4 + 2] annulation between aryl compounds and
of the most important classes of compounds in the pharmaceutical alkenes or alkynes. In 2007, Satoh and Miura’s group devel-
and agrochemical industries. The oxidative [4 + 2] annulation oped an efficient, waste-free coupling of benzoic acids with
reaction provides a powerful tool for the rapid construction of six- unsaturated compounds via regioselective C–H bond cleavage
5
membered heterocycles. Herein, we report a metal- and external using a Rh/Cu catalyst under air. Since then, the transition-
2
oxidant-free method for the uniform synthesis of tetrahydroquino- metal catalyzed activation of the C(sp )–H bond through the
lines through the electrochemical oxidative [4 + 2] annulation of directing groups to construct a six-membered cyclic compound
6
7
tertiary anilines and alkenes. In this strategy, one partner loses only has been widely reported. Except benzoic acid, benzamides,
8
9
two hydrogen atoms while another partner reduces one degree of simple N,N-dimethylaniline, and salicylaldehyde all can
unsaturation, accompanied by the generation of hydrogen. Under undergo [4 + 2] cyclization with alkenes or alkynes. In
the conditions of an undivided cell and room temperature, a series addition, some dehydrogenative [4 + 2] annulation reactions
3
10
of tetrahydroquinoline derivatives were prepared with good yields.
through double C(sp )–H activation have been developed. In
addition to alkenes or alkynes, trifluoromethyl ketones can
also furnish [4 + 2] annulation reactions. In most of the
Nitrogen-containing heterocyclic compounds are one of the
most important structural skeletons of functional molecules,
which are widely found in natural products, biologically
11
above-mentioned reactions, transition-metal catalysts or
chemical oxidants are usually required. Therefore, it is more
environmentally friendly and effective to develop the [4 + 2]
annulation reaction without an external oxidant or metal for
building six-membered compounds.
1
active structures, and pharmaceutical-related compounds.
Therefore, there is continuing interest in the development of
novel synthetic methods for the construction of nitrogen-
2
containing heterocycles. [4 + 2] annulation reactions are the
Electrochemical anodic oxidation has attracted widespread
interest because it can avoid the use of costly and toxic oxi-
most common synthetic protocols for the construction of valu-
3
able six-membered heterocyclic compounds. Among them,
12
dants. We proposed a new idea of oxidation [4 + 2] annula-
the Diels–Alder reaction is one of the most important methods
tion reaction along with hydrogen evolution. In this strategy,
one partner loses only two hydrogen atoms while another
partner reduces one degree of unsaturation, accompanied by
the generation of hydrogen (Scheme 1a). Here, we present an
efficient electrochemical oxidative [4 + 2] annulation between
4
for constructing a six-membered ring, which requires the use
of a conjugated diene or heterodiene as a starting material,
and thus sometimes has limitations. Therefore, the develop-
ment of a new dehydrogenative [4 + 2] annulation reaction for
the construction of six-membered complex molecular struc-
tures still has great practical value.
Recently, dehydrogenative [4 + 2] annulation reactions for
the construction of six-membered complex molecular com-
pounds have been developed. The most common type of reac-
Institute for Advanced Studies (IAS), College of Chemistry and Molecular Sciences,
Wuhan University, Wuhan 430072Hubei, P. R. China. E-mail: aiwenlei@whu.edu.cn
†
‡
Dedicated to Professor Xiyan Lu on the occasion of his 90th birthday.
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc02463d
These authors contributed equally.
Scheme 1 Oxidative [4 + 2] annulation for the synthesis of six-mem-
bered complex molecular compounds.
§
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tertiary anilines and alkenes in a simple undivided cell. It Table 2 Electrochemical oxidative [4 + 2] annulation of alkenes and
a
N,N-dimethylaniline
enabled the selective synthesis of tetrahydroquinolines under
external oxidant- and catalyst-free conditions (Scheme 1b).
N-(1-Phenylvinyl)acetamide (1a) and N,N-dimethylaniline
(
2a) were chosen as model substrates to examine the reaction
n
4 4 3
conditions. Utilizing Bu NBF as the electrolyte and CH CN/
AcOH as co-solvents, N-(1-methyl-4-phenyl-1,2,3,4-tetrahydro-
quinolin-4-yl)acetamide 3aa could be obtained in 72% yield
under 3 mA constant current for 6 h in an undivided cell
(Table 1, entry 1). Acetic acid was found to be crucial since no
desired product could be obtained when CH CN was used
3
solely. In addition, no product was formed when acetic acid
was replaced by organic or inorganic strong acids (Table 1,
entries 2–5) while the yield was only slightly reduced when
replacing the acetic acid with a weak acid (Table 1, entry 6).
Using a cheaper nickel plate or copper plate to replace the
platinum plate cathode led to a slight decrease in reaction
yields (Table 1, entries 7 and 8). Furthermore, using an in-
expensive graphite rod anode (∅ 6 mm) showed similar reactiv-
ity with the platinum plate anode (Table 1, entry 9). Both a
decrease and an increase in current lead to lower reaction
yields (Table 1, entries 10 and 11). Moreover, this reaction also
gave a moderate yield under atmospheric conditions (Table 1,
entry 12). No reaction took place without the electric current
under an air atmosphere or nitrogen atmosphere (Table 1,
entries 13 and 14).
After obtaining the optimized reaction conditions, we exam-
ined the applicability of a series of alkenes in this electro-
chemical reaction (Table 2). Enamines with a methyl group or
a phenyl group at the para position of the benzene ring could
obtain desired products in good yields (3ab–3ac). When the
a
Standard conditions: Pt plate (15 mm × 15 mm × 0.3 mm) anode, Pt
plate (15 mm × 15 mm × 0.3 mm) cathode, constant current = 3 mA, 1
n
a
(0.25 mmol), 2a (1.25 mmol), Bu
4 4 3
NBF (0.20 mmol), CH CN/AcOH
Table 1 Conditions for optimization
(
4.0 mL/1.0 mL), room temperature, 6 h, and an undivided cell (2.7 F).
b
c
Constant current = 5 mA and 3 h (2.3 F). Constant current = 3 mA, 1
0.50 mmol), 2a (1.0 mmol), and 12 h (2.7 F).
(
methyl group is at the meta position of the benzene ring, the
yield was decreased due to the steric hindrance effect (3ad).
Enamines with a halogen group such as fluoro, chloro, and
bromo at the para position of the benzene ring could give the
corresponding tetrahydroquinolines in good yield (3ae–3ag). It
is noteworthy that the electron-withdrawing trifluoromethyl
group resulted in a good reaction yield while the electron-
donating methoxyl group furnished the desired product with a
decreased reaction yield (3ah–3ai). In addition to phenyl
enamines, 2-naphthyl enamine and heterocyclic enamines,
such as pyridyl enamine and thienyl enamine, were also well
compatible (3aj–3al). The N-propionyl enamine could give the
desired product (3am) in good yield. The cyclic enamine was
also suitable and gave the corresponding tetracyclic product
b
Entry
Variation from the standard conditions
None
Yield
1
2
3
4
5
6
7
8
9
72%
n.d.
n.d.
n.d.
n.d.
65%
59%
58%
65%
18%
56%
42%
n.d.
n.d.
5 mL CH
TFA instead of AcOH
PO instead of AcOH
HCO H instead of AcOH
BuCO H instead of AcOH
3
CN
H
3
4
2
n
2
Pt(+)|Cu(−) instead of Pt (+)|Pt(−)
Pt(+)|Ni(−) instead of Pt (+)|Pt(−)
C(+)|Pt(−) instead of Pt (+)|Pt(−)
1 mA instead of 6 mA, 18 h
5 mA instead of 6 mA, 3.6 h
Under air
1
1
1
1
1
0
1
2
3
4
No electricity, under air
No electricity
a
Standard conditions: Pt plate (15 mm × 15 mm × 0.3 mm) anode, Pt (3an) in good yield. Moreover, aliphatic enamines were also
plate (15 mm × 15 mm × 0.3 mm) cathode, constant current = 3 mA, 1a
NBF (0.20 mmol), CH CN/AcOH
4 4 3
4.0 mL/1.0 mL), room temperature, 6 h, and an undivided cell (2.7 F).
compatible with the reaction and lead to moderate yields
3ao–3ap). Except for enamines, dihydrofuran was also able to
n
(0.25 mmol), 2a (1.25 mmol), Bu
(
(
b
Isolated yield, n.d. = not detected.
obtain the desired product in good yield (3aq). What’s more,
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maleimide derivatives were also well suited for this electro-
chemical oxidative [4 + 2] annulation reaction (3ar–3at).
Next, efforts were made to investigate the substrate scope of
tertiary anilines (Table 3). N,N-Dimethylanilines with methyl
and tert-butyl groups at the para position of the benzene ring
gave the desired products (3ba–3ca) in moderate to good
yields. N,N-Dimethylaniline with the electron-donating
methoxy group at the para position of the benzene ring could
also get a moderate yield (3da). N,N-Dimethylaniline with the
electron-withdrawing trifluoromethyl group on the aromatic
ring showed good reaction efficiency in this transformation
Scheme 2 Gram scale synthesis.
because N-methylaniline can break the C–N bond at the
anode.
When increasing the current, this electrochemical oxidative
(
3ea). Besides, N,N-dimethylanilines with halogen groups such
[
4 + 2] annulation reaction was able to amplify to 6 mmol,
providing the desired product 3aa in 56% yield (0.95 g)
Scheme 2). This shows that such an electrochemical strategy
as fluoro, chloro, bromo and even iodo, on the aromatic ring
were also tolerated and afforded the corresponding products
(
(
3fa–3ia) in moderate to good yields. Importantly, it was found
has potential for application.
that the reaction preferably takes place on the N-methyl group.
When one of the methyl groups of N,N-dimethylaniline was
replaced by ethyl, isopropyl, or even cyclohexyl, which has a
relatively large steric hindrance, the desired products (3ja–3la)
were obtained as the only products in good yields.
What surprised us is N-methyldiphenylamine was also toler-
ated and afforded the corresponding tetrahydroquinoline
Then the mechanism of the reaction was investigated. No
desired product was observed when the reaction was per-
formed by the addition of triethyl phosphite under the stan-
dard reaction conditions and the phospholipid product N,N-di-
methylaniline 4 could be detected, which revealed the involve-
ment of a tertiary α-amino carbon radical in the transform-
ation (Scheme 3a). In addition, no desired product or phos-
pholipid product was produced without acetic acid
(
3ma) in moderate yield. However, when we used N-methyl-p-
toluidine as the substrate, 3ba was obtained. It is possible
(
Scheme 3b). Furthermore, the mechanism of this transform-
ation was also proven by the electron paramagnetic resonance
EPR) study. Based on the fitting and corrected results (g =
.0038, A = 14.1 G, A = 7 G, A = 7 G, A = 7 G, A = 7 G and
AH = 7 G), a N,N-dimethylaniline radical cation was detected
Fig. 1 and for details see the ESI, Fig. S2†). By contrast, this
(
Table 3 Electrochemical oxidative [4 + 2] annulation of enamines 1a
2
N
H
H
H
H
a
and tertiary aniline
(
EPR signal was hardly observed in the absence of acetic acid.
We proposed that the radical cation can be stabilized by acetic
acid.
Based on the observations mentioned above and literature
1
3
reports, a plausible mechanism of the reaction between 1a
and 2a is proposed in Scheme 4. First, 2a is oxidized at the
anode to generate a radical cation which can be stabilized by
acetic acid. Next, the radical cation resonates and then depro-
tonates to get the tertiary α-amino carbon radical. The radical
can then react with 1a via a radical addition process. The
ensuing radical species could engage in an intramolecular
cyclization reaction, and the subsequent anodic oxidation will
then deliver the desired product 3aa. Concomitant cathodic
reduction of acetic acid leads to the formation of hydrogen
gas.
a
Standard conditions: Pt plate (15 mm × 15 mm × 0.3 mm) anode, Pt
plate (15 mm × 15 mm × 0.3 mm) cathode, constant current = 3 mA, 1a
n
(
0.25 mmol), 2 (1.25 mmol), Bu
4
NBF
4
3
(0.20 mmol), CH CN/AcOH
(
4.0 mL/1.0 mL), room temperature, 6 h, and an undivided cell (2.7 F).
Constant current = 5 mA and 3 h (2.3 F).
b
Scheme 3 Mechanistic studies.
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Province Natural Science Foundation of China (2017CFA010).
The Program of Introducing Talents of Discipline to
Universities of China (111 Program) is also appreciated.
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In conclusion, we have developed a metal- and external
oxidant-free method for the uniform synthesis of tetrahydro-
quinolines through the electrochemical oxidative [4 + 2] annu-
lation of tertiary anilines and alkenes. This reaction is compa-
tible with various functional groups and occurs at room temp-
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addition of acetic acid is the key to achieve the reaction due to
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