Sequential Photoredox Catalysis for Cascade Aerobic Decarboxylative Povarov and Oxidative Dehydrogenation Reactions of N-Aryl α-Amino Acids

Article abstract of DOI:10.1002/adsc.201800135

A visible-light-driven sequential photoredox catalysis to allow N-aryl α-amino acids to experience efficient cascade aerobic decarboxylative Povarov and oxidative dehydrogenation (ODH) reactions is described. With a dicyanopyrazine-derived chromophore (DP

Full text of DOI:10.1002/adsc.201800135

Title: Sequential Photoredox Catalysis for Cascade Aerobic
Decarboxylative Povarov and Oxidative Dehydrogenation
Reactions of N-Aryl α-Amino Acids
Authors: Tianju Shao, Yanli Yin, Richmond Lee, Xiaowei Zhao, Guobi
Chai, and Zhiyong Jiang
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800135
Link to VoR: http://dx.doi.org/10.1002/adsc.201800135

10.1002/adsc.201800135
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Sequential Photoredox Catalysis for Cascade Aerobic
Decarboxylative Povarov and Oxidative Dehydrogenation
Reactions of N-Aryl α-Amino Acids
Tianju Shao,a Yanli Yin,a,b Richmond Lee,c Xiaowei Zhao,a Guobi Chai,d and Zhiyong
Jianga*
a
Key Laboratory of Natural Medicine and Immuno-Engineering, Henan University, Kaifeng, Henan, P. R. China
475004
E-mail: chmjzy@henu.edu.cn
College of Bioengineering, Henan University of Technology, Zhengzhou 450001, China
Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372
Key Laboratory of Tobacco Flavor Basic Research of CNTC, Zhengzhou Tobacco Research Institute of CNTC,
b
c
d
Zhengzhou 450001, China
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))
avoiding tedious multistep preparations of the starting
Abstract. A visible-light-driven sequential photoredox
catalysis to allow N-aryl α-amino acids to experience materials and additional functionalization to
introduce substituents at target locations.[6-7] Of note,
efficient cascade aerobic decarboxylative Povarov and
oxidative dehydrogenation (ODH) reactions is described.
With a dicyanopyrazine-derived chromophore (DPZ) as a
photoredox catalyst in both transformations, two series of
valuable azaarenes, i.e., 4-amino tetrahydroquinolines
(THQs) and quinolines, were obtained in satisfactory yields
featuring diverse 2- and 2,3-substituent patterns. To enable
the ODH reaction of 4-amino THQs, a cooperative
catalysis with N-hydroxyphthalimide was developed.
Additionally, an unprecedented synthesis of chiral N-
amino-2-methyl THQs with high enantioselectivities was
realized.
sequential catalysis (or relay catalysis) as a
biomimetic strategy offers a unique platform for
organic synthesis.[8] The tandem use of catalytic
reactions with minimum workup presents remarkable
advantages, such as the use of simple and readily
available materials to build complex molecules,
reduced consumption of energy and time and lowered
waste production and yield losses associated with the
isolation and purification of intermediates through a
multistep process. Accordingly, the development of a
novel Povarov reaction with readily accessible,
abundant and stable starting substrates and a relay
catalysis platform to experience an efficient Povarov
reaction and the sequential ODH transformation
remains highly desirable, which would provide a
robust approach to furnish these two series of
important azaarenes concurrently with a wide variety
of substituent patterns. Besides exploring competent
feedstocks for the Povarov reaction, the
underdeveloped catalytic ODH reaction of 4-amino
THQs[7] and the potential incompatibility of the
second catalytic system with residual materials from
the preceding step in sequential catalysis constitute
another two key challenges in this task.
Keywords: Photoredox catalysis; Sequential photoredox
catalysis; Amino acids; Quinolines; Tetrahydroquinolines
Both 4-amino tetrahydroquinolines (THQs)[1] and
quinolines[2] are extensively present in natural and
synthetic compounds with interesting physical and
pharmacological properties. The Povarov reaction is
the commonly accepted strategy to furnish 4-amino
THQs.[1,3-5] The direct use of enamines and imines as
the feedstocks represents the most popular method,
but the substrate scope using this method is limited,
as enamines are usually restricted to those with
terminal olefins, while imines, especially those
bearing alkyl substituents, are typically difficult to
isolate owing to their instability.[3,4] Although a few
complementary methods have been developed, a
general method to render 4-amino THQs with 2- and
2,3-substituents is still deficient.[1,5] On the other hand,
the catalytic oxidative dehydrogenation (ODH) of
THQs is an expedient approach to access quinolines,
α-Amino acids are
a
large category of
environmentally benign and nonfossil carbon sources
and have therefore been pursued by chemists as
starting substrates.[9] In 2013, Tan and co-workers
reported
fluorescein-catalyzed
aminoalkylation
reactions of N-aryl glycine with nucleophiles under
visible-light irradiation.[10] This elegant work
demonstrates the feasibility of N-aryl glycines to
generate imines via tandem SET oxidative
decarboxylation and SET oxidation. We were
1
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10.1002/adsc.201800135
Advanced Synthesis & Catalysis
particularly intrigued by the lower yield in the
Mannich-type chemistry using N-phenyl alanine as
opposed to glycine. This anomalous result suggests
that a possible side-reaction might be responsible for
the decreased yield. As methyl substituted imines are
prone to tautomerization, we surmised that [4+2]
cycloaddition between an enamine and imine (i.e. the
Povarov reaction) could be occurring. If a general
phenomenon, this reaction would be an ideal method
to approach 4-amino THQs with a broad substrate
scope, as no purification of the unstable alkyl imines
would be necessary, and a broad diversity of α-amino
acids and N-aryls are available. As an extension of
our research interest[11] in photoredox catalysis,[12] we
were thus intrigued to verify this hypothesis, that is
decarboxylative Povarov reaction, and develop an
unprecedented photoredox ODH reaction of 4-amino
THQs to quinolines. To provide a highly efficient and
sustainable synthetic approach, we also anticipated to
(LEDs) as the energy source, at ambient temperature
and under air were determined as optimal, affording
2a in 84% yield with 3:1 d.r. (trans:cis) within 12
hours (entry 1, Table 1). Other photoredox catalysts,
such as [Ru(bpy)3]Cl2 and fluorescein, were also
examined (entries 2−3), but the yields were not
improved. The use of a 23 W compact fluorescent
light (CFL) or sunlight instead of the blue LED as the
energy source gave decreased yield (entries 4−5).
Without DPZ, 2a was obtained in a much lower yield,
affirming the indispensability of the photoredox
catalyst to the reactivity (23% yield, entry 6). No
reaction was observed in control experiments
performed in the absence of air or visible light
(entries 7−8, respectively), revealing the need for
both air and visible light in the reaction.
Table 2.Synthesis of 4-amino THQs.[a]
develop
a
visible-light-driven
photocatalytic
sequence[13] involving a cascade mechanism of these
two transformations in a one-pot. Our preliminary
results are described in this communication.
Table 1.Variation of the reaction parameters.[a]
Deviation from
standard conditions
None
[Ru(bpy)3]Cl2 instead
of DPZ
Yield of
2a [%][b]
84
Entry
Trans:cis[c]
3:1
1
2
52
34
51
32
1:1
Fluorescein instead of
DPZ
3
4
5
1:2
23 W CFL instead of 2
x 1W blue LED
Sunlight instead of 2 x
1W blue LED
No DPZ
2.7:1
3:1
6
7
8
23
0
0
1:2.6
N.A.
N.A.
Argon instead of air
No light
[a] Reaction conditions: 1a (0.1 mmol), DPZ (0.4 mol%), 5
Å MS (25 mg), CHCl3 (1.0 mL), 25 oC. [b] Yield of
isolated product. [c] The ratio was determined by 1H NMR
spectroscopy of the crude product. N.A. = not available.
We initiated our study by selecting N-phenyl
alanine 1a as the model substrate, and our developed
dicyanopyrazine-derived
instead of fluorescein, due to their comparable redox
potentials[14] as
photoredox catalyst. The
chromophore
(DPZ),
a
[a] Reaction conditions: 1 (0.2 mmol), DPZ (0.4 mol%), 4
preliminary study furnished the desired product 2a in
23% yield (see Table S1 in the Supporting
Information [SI]), which prompted us to evaluate the
reaction parameters further. The conditions involving
0.4 mol% of DPZ as a catalyst, 25 mg of 4 Å
molecular sieves (MS) as an additive, CHCl3 as a
solvent (0.1 M), two 1 W blue light-emitting diodes
Å MS (50 mg), CHCl3 (2.0 mL), 25 °C. Yield of isolated
product. The ratio of trans:cis (d.r.) was determined by H
NMR spectroscopy of the crude products. The ratio of p-
:o- (r.r.) was determined by the analysis of corresponding
ODH products 5 as shown in Table 3. [b] 1a:3 = 1.5:1,
LiH2PO4 (1.0 equiv), CHCl3 (2.0 mL), 25 °C, 10 h.
1
2
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10.1002/adsc.201800135
Advanced Synthesis & Catalysis
To identify the substrate scope, various N-aryl-α-
an iminium intermediate is likely not effective for 4-
amino THQs under these conditions. Inspired by the
report that a phthalimide-N-oxyl (PINO) radical is
more active than a peroxyl radical in hydrogen
abstraction,[16] we performed the cooperative
alkyl α-amino acids 1 were subjected to the aerobic
decarboxylative Povarov reaction under the
established reaction conditions (Table 2). All the
reactions were found complete within 5 to 15 hours,
providing [4+2] annulation adducts 2a-t in 52−82%
yields. Both electron-deficient and electron-donating
substituents located at different positions of the
aromatic unit were well tolerated. Moreover, 2,3-
disubstituted 4-amino THQs 2m-t were successfully
obtained by modulating the α-alkyl groups of the α-
amino acids. Since the generated imines from N-aryl-
α-aryl α-amino acids cannot tautomerize to form
enamines, we anticipated that these species would
react with enamines derived from α-alkyl α-amino
acids. This reaction might feature competitive
chemoselectivity for higher-reactivity alkyl imines,
leading to the easier generation of adducts 2. The
desired products 4a and 4b were obtained in 69% and
62% yields, respectively, by evaluating two
representative transformations of 1a with N-aryl-α-
aryl α-amino acids 3a-b under slightly modified
reaction conditions.
photoredox catalysis
hydroxyphthalimide (NHPI, E = 0.78 vs saturated
with DPZ and
N-
red
1/2
calomel electrode [SCE] in CH3CN) as a catalyst.[16b]
We were pleased to find that 5a was obtained in 81%
isolated yield after 3 hours using 0.4 mol% DPZ and
10 mol% Cl4-NHPI 6 in CH3CN and DMA mixed
solvent (v/v = 4:1) under irradiation by a 3 W blue
LED (see Table S2 in SI). The direct transformation
of α-amino acids 1 to quinolines 5 through the
sequential catalytic strategy was subsequently
examined (see Table S3 in SI). A series of 2-alkyl
and 2-alkyl-3-alkyl/aryl-substituted quinolines 5a-t
were furnished in 46−68% yields (Table 3). Notably,
0.4 mol% DPZ was still necessary in the second
process likely due to the oxidation of DPZ by the
generated H2O2 in the reaction system, leading to a
loss of efficacy after the first step.
Table 4. Synthesis of 2-aryl-substituted quinolines.[a]
Table 3. Synthesis of 2-alkyl-substituted quinolines.[a]
[a] 0.2 mmol scale. Yield of isolated product.
The synthesis of quinolines 7 featuring an aryl
group at the 2-position was then carried out. Given
the yield of THQs 4 was moderate (e.g., 4a-b, Table
2), which should inevitably result in a poor yield of 7,
the reaction conditions for the first decarboxylative
Povarov transformation had to be further improved.
We found that the reactions between α-alkyl amino
acids 1 and α-aryl amino acids 3 were complete
within 4 to 10 hours using 0.4 mol% DPZ with 5.0
mol% Na2S2O3 and 1.0 equiv. of LiH2PO4 as
additives in methanol as a solvent; both 4-amino
THQs and 4-methoxy THQs were observed as the
mixed products (see Table S4 in SI). While the
chemoselectivity was not satisfactory, both THQs
could be smoothly transferred to the desired
quinolines 7 in 52−65% yields over two steps when
performing the subsequent cooperative catalysis
[a] 0.2 mmol scale. Yield of isolated product. The ratio of
p-:o- was determined by H NMR spectroscopy of the
crude product.
1
We next sought to develop an aerobic ODH of 4-
amino THQs to quinolines via visible-light
photoredox catalysis. The method reported by
Balaraman for the transformation of THQs, but not
the 4-amino variants, to quinolones[6i] was first
attempted; the reaction of 2a was performed with
Rose Bengal as catalyst in N,N-dimethylacetamide
o
(DMA) at 25 C and under irradiation by a 1 or 3 W
blue LED, but only trace product 5a was obtained.[15]
This performance suggests that the hydrogen-atom
•−
transfer (HAT) of N-α-C−H enabled by O2 to form
3
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10.1002/adsc.201800135
Advanced Synthesis & Catalysis
without purification (Table 4). Notably, the
substituent patterns of both 5 and 7 are extremely
broad, as N-aryls and α-substituents of α-amino acids
can be flexibly changed (Tables 3 and 4). The
satisfactory yields of quinolines 5 and 7 also provide
robust evidence that the second catalysis is
compatible with the first catalytic system.
Scheme 1. Enantioselective synthesis of chiral 4-amino-2-
methyl THQs.
The synthesis of N-amino THQs and quinolines
with our catalysis system allowed abundant
substituent diversity attributed to the use of α-amino
acids as feedstocks. This feedstock greatly enriches
the diversity of enamines and enables the generated
imines, especially unstable alkyl imines, to
participate directly in the transformations. Therefore,
we anticipated the development of a dual-catalysis[17]
enantioselective
manifold
for
the
aerobic
Figure 1. Plausible mechanism.
decarboxylative Povarov reaction of N-aryl alanines,
thus leading to valuable chiral N-amino-2-methyl
substituted THQs.[1a-d] In recent years, many catalytic
asymmetric Povarov reactions have been established
to access chiral N-amino THQs featuring diverse 2-
substituents,[18,19] but no examples of the synthesis of
2-methyl-substituted entities have been reported,
likely due to the inconvenient manipulation of
acetaldehyde[18] and the instability of acetaldehyde-
derived imines.[19] After examining the reaction
conditions (see Table S5 in SI), the desired chiral N-
amino-2-methyl-substituted THQs ent-trans-2a−c
and ent-trans-2f were obtained after 30 hours in
58−73% yields with 71−92% ee and 3.6:1 to 5.2:1
d.r. by reacting N-aryl alanines 1 using 1.0 mol%
DPZ and 10 mol% chiral SPINOL-phosphoric acid[20]
8a as cooperative catalysts, 1.0 equiv. of tetra-n-
octylammonium bromide (TOAB) as an additive in
Figure 1 shows the proposed mechanism for the
sequential catalysis platform involving aerobic
decarboxylative Povarov and ODH reactions. A
Stern-Volmer experiment showed that N-aryl-α-alkyl
α-amino acid 1 as a reductive quencher can be
oxidized by DPZ* (see Figure S1 in SI). The
generated radical cation
8
will undergo
decarboxylation to form radical species 9, which is
•
subsequently oxidized by HO2 to afford imine 10.[10]
The [4+2] annulation between imine 10 and its
tautomer, i.e., enamine 11, will furnish 4-amino THQ
2 as one of the desired products and the key
intermediate for the next catalytic system. Since
NHPI 6 is crucial to the ODH reaction and can be
conveniently oxidized by DPZ* due to their matched
redox potentials,[14,16] PINO 12 would be first
generated from 6 in the second photoredox catalytic
cycle. Hydrogen abstraction of 2 by PINO 12 can
o
CH2Cl2 at −40 C and irradiation with a 1 W blue
LED (Scheme 1). Of note, the absolute configurations
of both ent-trans-2a and ent-cis-2a were assigned
based on the structures of the corresponding
derivatives 17 and 18 determined as solved by single
crystal X-ray diffraction.[21] It was found that the
diastereoselectivity was formed at the 4-position. As
shown in Table 1 (entries 3 and 5), the cis-isomer was
the major product when the reaction was performed
with fluorescein or without a catalyst. The results
indicate that a distinct imine isomer is generated after
the first Mannich reaction when with the DPZ
catalyst.
produce radical species 13. After further oxidation by
• [6i]
HO2 ,
imine 14 is produced from 13 and
tautomerizes to enamine 15. The dissociation of
arylamine, which can be detected by thin layer
chromatography (TLC) and gas chromatography-
mass spectrometry (GC-MS) analyses, furnishes
quinoline 5.
In conclusion, we developed a sequential catalysis
platform to achieve a one-pot cascade aerobic
4
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10.1002/adsc.201800135
Advanced Synthesis & Catalysis
solvent was evaporated in vacuo and the product was
purified from a short of silica gel (basified with Et3N) with
hexane/ethyl acetate (80/1-20/1) or hexane/DCM (5/1-2/1)
to present the desired products 5.
reaction sequence including decarboxylative Povarov
and ODH reactions. With DPZ as a photoredox
organocatalyst and using N-aryl α-amino acids as the
naturally abundant and inexpensive feedstocks, two
series of valuable azaarenes, N-amino THQs and
quinolines, were obtained in satisfactory yields
featuring diverse 2- and 2,3-substituents, which
indicates the good compatibility of the second
catalysis with residual materials from the first
catalysis and the versatility of this catalytic system. In
addition to the advisable choice of α-amino acids as
competent starting substrates, another crucial point in
this method is the development of a cooperative DPZ
and NHPI catalysis with the ODH transformation of
4-amino THQs to quinolines. The capability of in situ
generating labile alkyl imines to experience the
Povarov reaction also inspired an enantioselective
manifold, facilitating the first asymmetric synthesis
of important chiral N-amino-2-methyl THQs from N-
aryl alanines.
Procedure for preparing 7: 56 μL of DPZ (0.4 mol%, 1.0
mg of DPZ in 200 μL of toluene) were syringed into a 10
mL reaction bottle which was equipped with a stir bar, and
then solvent was removed in vacuo. Subsequently, 3 (0.2
mmol), LiH2PO4 (0.2 mmol, 1.0 equiv.), Na2S2O3 (0.01
mmol, 0.05 equiv.), MeOH (2 mL) were added
successively, and then equipped with a rubber septum and
an air balloon. The reaction mixture was stirred and
o
irradiated by a 3 W blue LED (λ = 450−455 nm) at 25 C
(the temperature was maintained in an incubator) from a
2.0 cm distance. TLC monitored until full consumption of
3, then 1a (0.3 mmol) were added within 2 hours. After 5-
10 h, solvent was evaporated in vacuo, followed by the
addition of DPZ (56 μL, 1.0 mg of DPZ in 200 μL of
MeCN), Cl4-NHPI (0.02 mmol, 0.1 equiv.), MeCN : DMA
= 4:1 (2 mL), the reaction mixture was stirred and
o
irradiated by a 3W blue LED (λ = 450−455 nm) at 25 C
(the temperature was maintained in an incubator) from 2.0
cm distance. After 3-5 h, evaporated the solvent in vacuo,
dissolved with DCM (0.5 mL) and purified from a short of
silica gel (basified with Et3N) with hexane/ethyl acetate
(80/1-20/1) or hexane/DCM (5/1-2/1) to provide the
desired product 7.
Acknowledgements
Experimental Section
Grants from NSFC (21402239, 21672052), Innovation Scientists
and Technicians Troop Construction Projects of Henan Province
and Henan Province (14IRTSTHN006, 162300410002) are
gratefully acknowledged.
General experimental procedure for the synthesis of 4-
amino THQs
General Procedure for preparing 2: 56 μL (0.0008 mmol,
0.4 mol%) of DPZ (1.0 mg of DPZ in 200 μL of toluene)
was syringed into a 10 mL reaction bottle which equipped
with a stir bar, and then solvent was removed in vacuo.
Subsequently, 1 (0.2 mmol), 4 Å MS (50 mg), CHCl3 (2
mL) were added successively, and then equipped with a
rubber septum and an air balloon. The reaction mixture
was stirred under irradiation by two 1 W blue LEDs (λ =
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General Procedure for preparing 4a-4b: 56 μL (0.0008
mmol, 0.4 mol%) of DPZ (1.0 mg of DPZ in 200 μL of
toluene) was syringed into a 10 mL reaction bottle which
equipped with a stir bar, and then solvent was removed in
vacuo. Subsequently, 3 (0.2 mmol), LiH2PO4 (0.2 mmol,
1.0 equiv.), CHCl3 (2 mL) were added successively, and
then equipped with a rubber septum and an air balloon.
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o
blue LEDs (λ = 450−455 nm) at 25 C (the temperature
was maintained in an incubator) from a 2.0 cm distance.
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were added within 2 hours. After 10 hours, solvent was
evaporated in vacuo, and the residue was purified by
column chromatography on silica gel with hexane/ethyl
acetate (80/1 to 40/1 ratio) to give 4.
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substituted quinolines.
Procedure for preparing 5: The step 1 was same as the
above mentioned on the synthesis of 2. After complete of
the step 1, CHCl3 was removed in vacuo, followed by the
addition of DPZ (56 μL, 1.0 mg of DPZ in 200 μL of
MeCN), Cl4-NHPI (0.02 mmol, 0.1 equiv.), MeCN : DMA
= 4:1 (2 mL), the reaction mixture was stirred and
irradiated by a 3 W blue LED with a distance of 1.5 cm at
25 oC. With a full conversion of 2 under TLC analysis, the
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5
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10.1002/adsc.201800135
Advanced Synthesis & Catalysis
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1/2
Et(S* /S•−) = +0.78 V vs SCE in CH3CN, E = −1.27 V
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[15] We also evaluated the reaction conditions for ODH
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