Oxidative Cyclization Synthesis of Tetrahydroquinolines and Reductive Hydrogenation of Maleimides under Redox-Neutral Conditions

Article abstract of DOI:10.1021/acs.orglett.8b00977

A redox-neutral reaction without using any external oxidant and reductant in one pot is described. By combining a Ru(bpy)₃²⁺ photocatalyst and cobaloxime catalyst, a number of tertiary anilines can be oxidized by Ru(bpy)₃<

Full text of DOI:10.1021/acs.orglett.8b00977

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Oxidative Cyclization Synthesis of Tetrahydroquinolines and
Reductive Hydrogenation of Maleimides under Redox-Neutral
Conditions
Xiu-Long Yang, Jia-Dong Guo, Tao Lei, Bin Chen, Chen-Ho Tung, and Li-Zhu Wu*
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, the
Chinese Academy of Sciences, Beijing 100190, P. R. China
*
S Supporting Information
ABSTRACT: A redox-neutral reaction without using any external
oxidant and reductant in one pot is described. By combining a
Ru(bpy)32 photocatalyst and cobaloxime catalyst, a number of tertiary
+
2
+
anilines can be oxidized by Ru(bpy)3 to realize oxidative cyclization of
tetrahydroquinolines, and the electron and proton eliminated from the
substrate anilines are captured by a cobaloxime catalyst to achieve
hydrogen transfer in situ to maleimides, in good to excellent yields, under
redox-neutral conditions.
he development of a mild, general, and efficient approach
conjunction with hydrogenation under redox-neutral con-
T
to build complex molecular structures is always an
ditions. This protocol utilizes commercially available substrates,
avoids the use of any external oxidant and reductant in the
reaction vessel, and produces the desired products in good to
excellent yields (34 examples, up to 98%) under redox-neutral
conditions.
1
important concern of synthetic chemistry. Photoredox
catalysis, taking advantage of visible light as an energy input,
has recently become a sought-after organic transformation.
Although most organic molecules are transparent to visible
light, a range of transition metal complexes and organic dyes
have been exploited as efficient photocatalysts for the
Our investigations commenced with the reaction of N,N-
dimethylaniline (1a, 0.2 mmol) with N-phenylmaleimide (2a,
2
synthetically valuable reactions. Thanks to the ability of
0
.4 mmol) in CH CN (4 mL). In the presence of 1 mol %
3
photocatalysts to absorb visible light, numerous organic
substrates, including unreactive C−H bonds, have been
activated to undergo either oxidative or reductive reactions,
respectively, with the aid of an external oxidant and reductant.
Notwithstanding these advances, it is rather challenging to
design a more atom- and step-economic reaction that avoids
the use of an external oxidant and reductant for a redox-neutral
reaction.
Ru(bpy) Cl and 10 mol % Co(dmgH) pyCl under an argon
3
2
2
atmosphere, the reaction mixture of 1a and 2a was irradiated by
W blue LEDs at room temperature. The desired annulation
product 3aa and hydrogenation product 4a were obtained in
8% and 91% yields (Table 1, entry 1). However, only a 30%
3
8
yield of 3aa was obtained under air, along with the
decomposition of most 1a into N-methylaniline (Table 1,
6
+
−
In this contribution, we report an oxidative cyclization of
tertiary anilines with maleimides to synthesize tetrahydroquino-
lines, and simultaneously reductive hydrogenation of mal-
entry 11). It was found that neither Acr −MesClO (9-
4
mesityl-10-methylacridinium perchlorate) nor Ir(ppy) was able
3
to improve the transformation further (Table 1, entries 2 and
3
eimides in one pot. Tetrahydroquinolines are significant
3
). As reducing the amount of 2a in the reaction mixture
structures existing in a number of natural products and
decreased the yields of 3aa and 4a (Table 1, entry 6), and 5
4
synthetic drugs. With the aid of external oxidants, the
mol % instead of 10 mol % of Co(dmgH) pyCl was not enough
2
oxidative cycloaddition synthesis of tetrahydroquinolines has
been achieved. Our design here is to realize the oxidative
cyclization without any external oxidant under visible light
irradiation, and the electron and proton eliminated from the
substrate tertiary anilines are captured by a cobaloxime catalyst
to achieve hydrogen transfer in situ to maleimides. In contrast
to those reports of oxidative cyclization to form tetrahydro-
for the redox-neutral reaction (Table 1, entry 7 and Figure S2
in the Supporting Information), it is reasonable to consider that
2
.0 equiv of 2a and the ratio of catalyst to substrate were
essential for generation of 3aa and 4a in good yields. A set of
control experiments suggested the necessity of Ru(bpy) Cl ,
3
2
5
quinolines in the presence of an oxidant, this is the first
Received: March 27, 2018
example of oxidative synthesis of tetrahydroquinolines in
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00977
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of the Reaction Conditions
intermediate information upon irradiation. Prior to irradiation,
the absorption spectrum of Ru(bpy) Cl and Co(dmgH) pyCl
3
2
2
in degassed CH CN was simply the sum of the individual
3
components. After irradiation, the system showed absorption
bands at 440−500 nm and 550−650 nm (Figure 1b), which
II
I
was consistent with the formation of Co and Co intermediate
7
b
species. Evidently, the large luminescent quenching extent and
yield (%)
II
I
reduced Co and Co intermediate formation indicated the
preferred oxidative quenching pathway, i.e., photoinduced
electron transfer from Ru(bpy) Cl to Co(dmgH) pyCl.
The generated Ru(bpy)3 [E1/2 Ru(bpy)3 /Ru(bpy)
+1.29 V vs SCE, E1/2
oxidize 1a to form radical cation 1a , which eliminated a
proton to form radical 1a for the following photocatalytic
reaction.
Kinetic isotopic effects (KIE) ascertained the dissociation of
protons from the oxidized 1a . When all-deuterated D -1a
was subjected to the reaction, D -4a was obtained in 75% yield
with 56% D along with D -3aa in 78% yield (Scheme 1a). As a
entry
photocatalyst
3aa
4a
7
,8
1
2
3
Ru(bpy) Cl
88
77
83
78
trace
60
80
0
91
65
75
75
0
3
2
3 2 2
+
−
3+
ox
3+
2+
3
Acr −MesClO
fac-Ir(ppy)3
4
=
ox
+•
2a
1a /1a = +0.79 V vs SCE] is able to
c
+
•
4
Ru(bpy) Cl
3
2
2
2
2
2
d
•
5
e
Ru(bpy) Cl
3
,
f
6
7
8
9
1
1
1
Ru(bpy) Cl
37
61
0
3
g
Ru(bpy) Cl
3
h
Ru(bpy) Cl
+•
3
1
1
−
trace
0
0
2
i
0
1
2
Ru(bpy) Cl
0
3
2
2
2
9
j
Ru(bpy) Cl
30
80
−
3
k
Ru(bpy) Cl
81
3
Scheme 1. Kinetic Study and Radical Capture Experiments
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), photocatalyst (1
mol %), and Co(dmgH) pyCl (10 mol %) were added in CH CN (4
2
3
mL) under an argon atmosphere and irradiation of 3 W blue LEDs for
b
c
2
4 h at rt. Yield of isolated products based on 0.2 mmol of 1a. 2 mL
d e f
of CH CN. 4 mL of EtOH. 0.2 mmol of 2a was used. 25% H was
3
2
detected by gas chromatography, using pure methane as an internal
g
h
i
standard. 5 mol % Co(dmgH) pyCl. In the dark. Without
2
j
k
Co(dmgH) pyCl In open air. In 1.0 mmol scale.
2
Co(dmgH) pyCl, and visible light for this transformation
2
(
Table 1, entries 8−10).
To understand the primary process of the photocatalytic
reaction, the luminescent spectra of Ru(bpy) Cl2 in the
3
presence of either 1a and Co(dmgH) pyCl were examined.
2
As shown in Figure 1a, both 1a and Co(dmgH) pyCl quenched
2
−
5
Figure 1. (a) Luminescence spectra of Ru(bpy) Cl (4 × 10 M)
3
2
−4
−3
with Co(dmgH) pyCl (4 × 10 M) or 1a (4 × 10 M) in degassed
2
CH CN with excitation at 450 nm. (b) UV−vis absorption spectra of
3
−5
−4
system containing 4 × 10 M Ru(bpy) Cl and 4 × 10
M
3
2
Co(dmgH) pyCl irradiated under blue LEDs for 0−3 min in degassed
2
result of proton exchange with a trace amount of water in
CH CN.
3
CH CN or Ru(bpy) Cl ·6H O, the ratio of D in 4a also
3
3
2
2
decreased in the hydrogen transfer process by Co-
(dmgH) pyCl. When CD CN was used instead of CH CN as
the luminescence of Ru(bpy) Cl , but the quenching extent by
3
2
2
3
3
Co(dmgH) pyCl was much larger than that of 1a (see Figures
the solvent, 3aa and 4a decreased from 88% and 91% to 76%
and 85% yields, respectively (Scheme 1b). All of the
observations suggested that hydrogen transfer from tertiary
anilines to maleimides occurred. The (k /k ) intra- and
2
S5 and S6 in the Supporting Information for details).
ox
3+
Considering the redox potential [E
Ru(bpy)3 /*Ru-
1
/2
+
red
II
2+
+
(
+
bpy)32 = −0.81 V vs SCE, E
*Ru(bpy)3 /Ru(bpy)3
=
1/2
H
D
red
III
ox
1/2
0.77 V vs SCE, E
Co /Co = −0.67 V vs SCE, E
intermolecular isotopic values were determined to be 2.1 and
2.2, respectively (Scheme 1c and 1d). The KIE values were
much lower for the (k /k ) intra- and intermolecular isotropic
1/2
+
•
2a,7
1
a /1a = +0.79 V vs SCE]
(Figure S3), the quenching
behavior could be attributed to photoinduced electron transfer
H
D
5b
from Ru(bpy) Cl to Co(dmgH) pyCl. The UV−vis absorp-
effects (6.4 and 4.0) determined under air conditions,
suggesting that an electron/proton transfer instead of a
3
2
2
9
tion spectrum was further monitored to provide the
B
DOI: 10.1021/acs.orglett.8b00977
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
,
a b
hydrogen atom transfer (HAT) process should occur
predominantly for C−H bond cleavage. An obvious secondary
kinetic isotope effect on the benzene ring (the intermolecular
k /k = 1.5) indicated that the C−C bond formation occurs
Scheme 3. Scope of Tertiary Anilines
H
D
before C−H bond cleavage (Scheme 1e). Moreover, a series of
radical inhibition experiments were conducted to confirm the
radical formation for this reaction. With the addition of radical
inhibitor 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) or 2,6-
di-tert-butyl-4-methylphenol (BHT), the cyclization reaction
was gradually suppressed (Scheme 1f and 1g). When 2 equiv of
2-benzylidenemalononitrile 2x, a well-known radical scavenger
of α-aminoalkyl radical, were added under the standard
conditions, 5ax rather than the cyclization product 3ax was
obtained in only 63% yield (Scheme 1h). Therefore, the
10
photogenerated α-aminoalkyl radicals are significant and
responsible for the addition/cyclization process.
Based on the aforementioned mechanistic studies, we
proposed a plausible mechanism that is shown in Scheme 2.
Scheme 2. Proposed Mechanism
a
Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol), 1 mol %
Ru(bpy) Cl , and 10 mol % Co(dmgH) pyCl were added in CH CN
3
2
2
3
(
4 mL) under an argon atmosphere and irradiation of 3 W blue LEDs
b
c
for 24 h at rt. Yield of isolated products based on 0.2 mmol 1. The
1
d
configuration of the diastereomer was determined by H NMR. 3 mol
of Ru(bpy) Cl and 5 mol % Co(dmgH) pyCl were used.
%
3
2
2
2a resulted in the formation of a mixture of regioisomers 3ga
and 3ga′ in a ratio of 1.8:1 and 46% combined yield. When N-
ethyl-N-methylaniline 1i was used as the substrate, the reaction
took place mainly at the N-methyl carbon with the less
sterically hindered position (3ia and 3ia′). Though the same
selectivity was observed using N-isopropyl-N-methylaniline
(3ja), N-phenyl or benzyl substituted tertiary anilines hardly
gave the desired products 3ka and 3la. Significantly, the use of
N-aryltetrahydroisoquinolines (THIQ) as the partners afforded
the desired heterocyclic scaffolds (3na−3pa) as diaster-
eoisomers in 30%−78% yields. Mifepristone (1q), a repre-
sentative pharmaceutical containing N,N-dimethylaniline, en-
abled the formation of 3qa as a complex mixture of
diastereomers in a combined 62% yield.
2
+
Upon visible light irradiation of Ru(bpy)
by blue LEDs,
3
Ru(bpy)3 2 in its excited state is oxidatively quenched by
and Co via single-
electron transfer (SET). The formed Ru(bpy)
with 1a to regenerate Ru(bpy) and produce radical cation
a , which is deprotonated to yield α-aminoalkyl radical 1a .
The resultant α-aminoalkyl radical intermediate 1a undergoes
an intermolecular radical addition/cyclization to form the
radical intermediates A and B. As a result, the desired
tetrahydroquinoline 3aa is generated through a rearomatization
process. The released proton (H ) and electron (e), on the
other hand, are immediately captured by Co(dmgH) pyCl to
form the reduced species Co and Co as well as Co −H
species, which achieves hydrogen transfer in situ to maleimides
+
*
3
3
+
II
Co(dmgH) pyCl to afford Ru(bpy)
2
3
3
+
associates
2
3
+
+
•
•
1
•
Having successfully achieved the reaction with various
tertiary anilines, we shifted our attention to the scope of
electron-deficient olefins (Scheme 4 and 5). Remarkably, N-aryl
maleimides substituted with electron-donating or -withdrawing
groups gave tetrahydroquinolines 3ab−3ak in good yields. Due
to the atropisomeric ortho-substituted acrylamide, N-(o-tolyl)
maleimide and N-(2-bromophenyl) maleimide were suitable for
the reaction, giving rise to 3aj and 3ak as a mixture of
diastereomers in combined 96% and 68% yields, respectively.
Furthermore, reactions of the N-aliphatic (methyl, n-propyl,
tert-butyl, cyclopentyl, cyclohexyl, and benzyl) maleimides also
proceeded smoothly to afford their corresponding tetrahy-
droquinolines in excellent yield (3al−3ar). However, 1a failed
to react with the other olefins (2s−2w) (Table S3) and
+
2
II
I
III
and regeneration of Co(dmgH) pyCl for the next catalytic
2
cycle.
The generality of the redox-neutral reaction was well
established with tertiary anilines substituted with electron-
withdrawing, electron-donating, and bulky alkyl groups. As
summarized in Scheme 3, N,N-dimethylanilines incorporating
4
-Me, 4-OMe, 4-F, and 4-Cl on the phenyl ring reacted with 2a
to afford their corresponding tetrahydroquinolines 3aa−3ha in
3
0−89% yields. However, the N,N-dimethylanilines bearing a
strong electron-withdrawing group 4-CN (1f) with higher
oxidation potential (Figure S4) did not produce any of the
desired product. Irradiation of N,N-3-trimethylaniline (1g) and
decomposed partly into H and N-methylaniline due to its
2
unfavored cyclization under the reaction conditions. Interest-
ingly, the redox-neutral reaction, i.e., an oxidative cyclization
C
DOI: 10.1021/acs.orglett.8b00977
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
,
a b
Scheme 4. Scope of Electron-Deficient Olefins
simultaneously, the electron and proton eliminated from
tertiary anilines are captured by a cobaloxime catalyst to
achieve hydrogen transfer in situ to the maleimides. It is
anticipated that exploitation of photoredox/cobalt catalytic
systems would represent a more step- and atom-economic
approach for organic transformation.
11
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00977.
Experimental procedures, methods and product charac-
terizations (PDF)
AUTHOR INFORMATION
Corresponding Author
■
a
*
E-mail: lzwu@mail.ipc.ac.cn.
Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), 1 mol %
Ru(bpy) Cl , and 10 mol % Co(dmgH) pyCl were added in CH CN
3
2
2
3
ORCID
(
4 mL) under an argon atmosphere and irradiation of 3 W blue LEDs
b
Bin Chen: 0000-0003-0437-1442
Chen-Ho Tung: 0000-0001-9999-9755
Li-Zhu Wu: 0000-0002-5561-9922
for 24 h at rt. Yield of isolated products based on 0.2 mmol of 1a.
c
1
The configuration of diastereomer was determined by H NMR.
d
9
5% of 2 was recovered.
Notes
,
a b
Scheme 5. Scope of Di-N-substituted Maleimides
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for financial support from the Ministry of
Science and Technology of China (2014CB239402 and
■
2
017YFA0206903), the National Science Foundation of
China (91427303 and 21390404), the Strategic Priority
Research Program of the Chinese Academy of Sciences
(
XDB17000000), and the Key Research Program of Frontier
Science of the Chinese Academy of Sciences (QYZDY-SSW-
JSC029).
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Org. Lett. XXXX, XXX, XXX−XXX