Photoredox-catalyzed decarboxylative cross-coupling of α-amino acids with nitrones

Article abstract of DOI:10.1021/acs.orglett.0c04101

A decarboxylative cross-coupling reaction of α-amino acids with nitrones via visible-light-induced photoredox catalysis has been established for easy access to β-amino hydroxylamines and vicinal diamines with structural diversity, which is featured with simple operation, mild conditions, readily available α-amino acids, and a broad scope of nitrone substrates. The application of this protocol can furnish efficient synthetic strategies for some valuable vicinal diamine-containing molecules.

Full text of DOI:10.1021/acs.orglett.0c04101

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Letter
Photoredox-Catalyzed Decarboxylative Cross-Coupling of α‑Amino
Acids with Nitrones
Heng-Hui Li,§ Jia-Qi Li,§ Xiao Zheng,* and Pei-Qiang Huang*
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ABSTRACT: A decarboxylative cross-coupling reaction of α-amino acids with
nitrones via visible-light-induced photoredox catalysis has been established for
easy access to β-amino hydroxylamines and vicinal diamines with structural
diversity, which is featured with simple operation, mild conditions, readily
available α-amino acids, and a broad scope of nitrone substrates. The application
of this protocol can furnish efficient synthetic strategies for some valuable vicinal
diamine-containing molecules.
Scheme 1. Synthesis of Vicinal Diamines and Analogues by
Visible-Light Photoredox Catalysis
icinal diamine is an essential structural unit embodied in
many important organic compounds. A wide variety of
V
alkaloids, pharmaceuticals, ligands, and organocatalysts armed
with a vicinal diamine moiety show eminent biological and
catalytic activities.1 Owing to these merits, incessant efforts
have been devoted to the efficient synthesis of vicinal diamines
and analogues, and many powerful methods have been
established to date.2,3 Notably, on the basis of the α-aminoalkyl
radicals, the radical coupling of imines4 is a flexible and
straightforward approach that directly combines two C−N
moieties to construct vicinal diamines. Recently, this elegant
strategy has evolved in the realm of visible-light photoredox
catalysis5 with several variants in racemic6 and enantioselec-
tive7 manners under environmentally friendly conditions.8
However, most of these methods rely on the reaction of N-aryl
α-aminomethylene radicals I and aromatic aldimines II and
thus suffer from a narrow substrate scope, rendering the vicinal
diamine products structurally limited (Scheme 1).
The applications of nitrones in radical reactions have drawn
considerable attention in our lab.9 Recently, several photo-
catalytic reactions utilizing nitrones as the radical acceptor
have been reported,10 including a synergistic photocatalytic
and Lewis-acid-catalytic enantioselective reductive cross-
coupling reaction of nitrones with aromatic aldehydes10a and
a cross-coupling reaction of nitrones with aromatic tertiary
amines via visible-light photoredox catalysis.10d On the other
hand, α-amino acids are known as abundant, inexpensive, and
naturally occurring synthetic materials and serve as precursors
of α-aminoalkyl radicals in several photocatalysis reactions via
an oxidative decarboxylation process.11 We thus envisioned
that by trapping α-aminoalkyl radicals generated from α-amino
acids with nitrones, an efficient protocol for preparing vicinal
diamines with structural diversity could be achieved (Scheme
1). Herein we present the decarboxylative cross-coupling
reaction of α-amino acids with nitrones via visible-light-
induced photoredox catalysis.
Received: December 11, 2020
Published: January 12, 2021
https://dx.doi.org/10.1021/acs.orglett.0c04101
© 2021 American Chemical Society
Org. Lett. 2021, 23, 876−880
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Our investigation into this photoredox-catalyzed protocol
began by studying the model reaction of nitrone 1a with N-
phenylglycine 2a. After various photocatalysts, bases, solvents,
and light sources were screened (Table S1), the optimized
reaction conditions were established as follows: By using 7 W
blue LEDs as the light source, the cross-coupling was carried
out in the presence of Ir[dF(CF3)ppy]2(dtbbpy)(PF6) (1.0
mol%) and Li2CO3 (20 mol%) in DMF at room temperature
for 24 h to give the desired β-amino hydroxylamine 3a-1 in
83% yield. Control experiments showed that no product was
formed in the absence of a photocatalyst, light source, or argon
protection, which back up a photoredox catalytic oxidation
mechanism. Furthermore, β-amino hydroxylamine 3a-1 was
obtained in only 30% yield without a base additive. This
indicates that the deprotonation of α-amino acid significantly
facilitates the reaction (Table S2).
increase the concentration of α-aminoalkyl radicals, and thus a
modified condition (Protocol 1) was adopted to give cross-
coupling product 3j-1 in 85% yield. Compared with acyclic
nitrones, cyclic nitrones exhibited higher reactivity under the
general reaction conditions, and the desired products were
formed in excellent yields (3q−s).
We also explored the scope of α-amino acids. As shown in
Scheme 3, decarboxylative cross-coupling of N-arylglycines
Scheme 3. Photoredox Decarboxylative Cross-Coupling of
a b
,
N-Substituted Glycines with Nitrones
After identifying the optimized reaction conditions, we
examined the scope of nitrones and found that the reaction
tolerates a wide array of nitrone substrates. As for acyclic
nitrones, various C-substituents (1a−l) and N-substituents
(1m−p) on the nitrones were examined using N-phenylglycine
2a as the reaction partner, and the desired β-amino
hydroxylamines were obtained in moderate to good yields
(Scheme 2). When electron-rich nitrone 1j was used as an
electrophilic radical acceptor, more Li2CO3 with water was
needed to promote the deprotonation of α-amino acids and
Scheme 2. Photoredox Decarboxylative Cross-Coupling of
a b
,
N-Phenylglycine with Nitrones
a
General method: Nitrone (0.30 mmol), α-amino acid (0.90 mmol),
Ir[dF(CF3)ppy]2(dtbbpy)(PF6) (1.0 mol%), Li2CO3 (20 mol%),
b
DMF (1.5 mL), 7 W blue LEDs, room temperature, 24 h. Isolated
c
yield. Protocol 2: Nitrone (0.40 mmol), α-amino acid (0.40 mmol),
Ir[dF(CF3)ppy]2(dtbbpy)(PF6) (1.0 mol%), CsF (100 mol%), DMF
d
e
(2.0 mL), 32 W blue LEDs, room temperature, 36 h. Protocol 1. dr
1
values were determined by H NMR analysis of crude products.
with nitrone 1a gave the desired β-amino hydroxylamines in
moderate to good yields. As for para-substituted N-
arylglycines, electron-donating methoxy gave a lower yield
compared with electron-withdrawing halides and cyano
substituents (3a-5 vs 3a-2−4). Ortho-, meta-, or para-Cl-
substituted N-arylglycines gave almost identical and highest
yields (3a-3, 3a-6, and 3a-8). N-Arylglycine 2i with a
polysubstituted aryl group gave a moderate yield of 65%
(3a-9). For C-aryl nitrone 1g, the reaction outcome was
irrelevant to N-arylglycines, as products 3g-2−4 were obtained
in nearly identical yields. Furthermore, the reaction of N-bis-
substituted glycines proceeded smoothly under the general
conditions to provide desired cross-coupling products in
moderate yields (3a-10−a-12). It is worth mentioning that
the decarboxylation of N-Boc glycine proceeded under harsher
conditions (Protocol 2) to give the desired product 3a-13 in
61% yield. In addition, the reaction could also be applied to α-
branched N-phenyl α-amino acids in moderate to good yields
when the less sterically hindered nitrone 1j was used as the
a
General method: Nitrone (0.30 mmol), α-amino acid (0.90 mmol),
Ir[dF(CF3)ppy]2(dtbbpy)(PF6) (1.0 mol%), Li2CO3 (20 mol%),
DMF (1.5 mL), 7 W blue LEDs, room temperature, 24 h. Isolated
yield. Protocol 1: Nitrone (0.30 mmol), α-amino acid (0.90 mmol),
Ir[dF(CF3)ppy]2(dtbbpy)(PF6) (1.0 mol%) and Li2CO3 (100 mol%),
DMF (1.2 mL) and H2O (0.30 mL), 7 W blue LEDs, room
temperature, 24 h.
b
c
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reaction partner under the modified conditions (Protocol 1,
3j-2−4). Notably, halide, nitrile, and indolinyl groups were
well tolerated in this reaction. The decarboxylative cross-
coupling of indoline-2-carboxylic acid 2r with nitrone 1r
provided the desired product in good yield with a low
diastereoselectivity (3r-2).
Scheme 6. Plausible Mechanism for Photoredox
Decarboxylative Cross-Coupling of α-Amino Acids with
Nitrones
We next investigated the stereoselective decarboxylative
cross-coupling of N-arylglycines with chiral nitrone 4 derived
from D-tartaric acid.9b As shown in Scheme 4, the electron-
Scheme 4. Photoredox Decarboxylative Cross-Coupling of
a b c
, ,
N-Aryl Glycines with Nitrone 4
an antihistamine drug (Scheme 7).15 Under the modified
reaction conditions (Protocol I), the decarboxylative cross-
Scheme 7. Synthesis of Mepyramine
a
b
c
1
General method. Isolated yield. dr values were determined by H
NMR analysis of crude products.
deficient N-arylglycines gave the highest yields. Moderate to
good diastereoselectivities were obtained, and N-arylglycines
with para-fluoro and nitrile groups gave the higher
diastereoselectivity (5b and 5d). The relative stereochemistry
of 5b was established on the basis of nuclear Overhauser effect
spectroscopy (NOESY) experiment to be 2,3-anti. In addition,
through the cleavage of N−O12 and O−Bn13 bonds, β-amino
hydroxylamine 5a was easy converted to vicinal diamine 6,
which is a LAB derivative with glycosidase inhibitory
properties (Scheme 5).14
coupling of N,N-dimethylglycine 2l with nitrone 1j gave β-
amino hydroxylamine 3j-5 in 82% yield. After the zinc-
mediated reduction12 of 3j-5, vicinal diamine 13 was obtained
in 95% yield. Finally, the Pd-catalyzed N-arylation16 of 13 with
2-Cl-pyridine gave mepyramine (14) in 84% yield.
Scheme 5. Synthesis of LAB Derivative 6
In summary, we have developed a visible-light photoredox
Ir-catalyzed decarboxylative cross-coupling of α-amino acids
with nitrones for easy access to β-amino hydroxylamines and
vicinal diamines with structural diversity. This method affords
the advantages of simple operation, mild conditions, readily
available α-amino acids, and a broad scope of nitrone
substrates. By using this method, a step-economy synthesis
of a glycosidase inhibitory LAB derivative and a concise
synthetic strategy for the antihistamine drug mepyramine have
been established. The extension of the photoredox-catalyzed
enantioselective decarboxylative cross-coupling of α-amino
acids with nitrones is in progress in our laboratory and will be
reported in due course.
By combining the results from the aforementioned studies
and the control experiments shown in Supporting Information,
a reaction mechanism was proposed, as shown in Scheme 6.
The deprotonation of α-amino acid 2 generates α-amino acid
anion 10, which is oxidized by the visible-light-exited Ir(III)
complex to form α-aminoalkyl radical 11 with the release of
carbon dioxide. The α-aminoalkyl radical 11 is captured by
nitrone 1 to form radical 12, which is reduced by Ir(II)
complex 9 and is protonated by α-amino acid 2 to furnish β-
amino hydroxylamine 3. In this free-radical chain mechanism,
the base is just an initiator of the reaction. This rationalizes the
fact that a substoichiometric amount of base is sufficient for
completion of the reaction.
ASSOCIATED CONTENT
■
sı
* Supporting Information
With the aim of demonstrating the utility of this photoredox
catalytic protocol, we undertook the synthesis of mepyramine,
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04101.
878
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transition metal/photoredox catalysis. Sci. China: Chem. 2020, 63,
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Experimental procedures, characterization, and spectral
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AUTHOR INFORMATION
Corresponding Authors
■
Chem. Res. 2014, 47, 3665. (c) Muniz, K. Promoting Intermolecular
̃
Pei-Qiang Huang − Department of Chemistry, Fujian
Provincial Key Laboratory of Chemical Biology, College of
Chemistry and Chemical Engineering, Xiamen University,
Xiamen, Fujian 361005, China; orcid.org/0000-0003-
3230-0457; Email: pqhuang@xmu.edu.cn
Xiao Zheng − Department of Chemistry, Fujian Provincial Key
Laboratory of Chemical Biology, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen, Fujian
361005, China; School of Pharmaceutical Sciences, Xiamen
University, Xiamen, Fujian 361102, China; orcid.org/
0000-0001-7634-0042; Email: zxiao@xmu.edu.cn
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Heng-Hui Li − Department of Chemistry, Fujian Provincial
Key Laboratory of Chemical Biology, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen,
Fujian 361005, China
Jia-Qi Li − Department of Chemistry, Fujian Provincial Key
Laboratory of Chemical Biology, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen, Fujian
361005, China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04101
Author Contributions
§H.-H.L. and J.-Q.L. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
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The authors are grateful for financial support from the
National Key R&D Program of China (grant no.
2017YFA0207302), the NSF of China (21672175,
91856110, 21931010, and 22071204), and the Program for
Changjiang Scholars and Innovative Research Team in
University (PCSIRT) of Ministry of Education, China.
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