Access to α-Amino Acid Esters through Palladium-Catalyzed Oxidative Amination of Vinyl Ethers with Hydrogen Peroxide as the Oxidant and Oxygen Source

Article abstract of DOI:10.1002/anie.201709285

A novel and convenient palladium catalytic system for the synthesis of α-amino acid esters from simple starting materials is reported. Hydrogen peroxide not only acts as the green oxidant, but also as the oxygen source. This strategy for the conversion of amines and vinyl ethers into highly functionalized and structurally diverse α-amino acid esters is characterized by the simplicity of the experimental procedure, mild reaction conditions, high atom economy, scalability, and practicability.

Full text of DOI:10.1002/anie.201709285

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Angewandte
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Accepted Article
Title: Access to α-Amino Acid Esters via Palladium-Catalyzed
Oxidative Amination of Vinyl Ethers Using Hydrogen Peroxide as
Oxidant and Oxygen Source
Authors: Huanfeng Jiang, Lu Ouyang, Jianxiao Li, Jia Zheng, Jiuzhong
Huang, Chaorong Qi, and Wanqing Wu
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709285
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10.1002/anie.201709285
Angewandte Chemie International Edition
DOI: 10.1002/anie.
Catch Phrase
201((will be filled in by the editorial staff))
Access to α-Amino Acid Esters via Palladium-Catalyzed Oxidative Amination of
Vinyl Ethers Using Hydrogen Peroxide as Oxidant and Oxygen Source **
Lu Ouyang, Jianxiao Li, Jia Zheng, Jiuzhong Huang, Chaorong Qi, Wanqing Wu* and Huanfeng
Jiang*
Abstract: A novel and convenient palladium catalytic system to
achieve α-amino acid esters with simple starting materials has been
reported. Hydrogen peroxide not merely acts as the green oxidant,
but also the oxygen source. This new strategy salient merits
attention the simplicity of experimental procedure, mild reaction
conditions, high atom economy, scalability and practicability, which
can convert the amines and vinyl ethers to highly functionalized and
structurally diverse α-amino acid esters.
cycle.[9] Also, the alkyl-PdII can be oxidized to alkyl-PdIV to form C-
O, C-N, C-C, C-X bonds (Scheme 1a).[10]
α
-Amino acid, the backbone of proteins, which is one of the most
important amino acids in every organism.[1] The biologically
significant α-amino acid and the derivatives have been extensively
used as medical intermediates, artificial sweetener, food additives,
cosmetic additives, mineral flotation additives, germicides,[2] etc.
Despite its great importance, methods for the direct synthesis of α-
amino acid esters, the most important α-amino acid derivatives, are
relatively rare.[3] Therefore, a simple and efficient procedure for the
synthesis of α-amino acid esters with abundant and accessible
starting materials is still highly demanded.
As we know, alkenes can be activated via coordination with
transition metal catalysts, which have been focused continuous
attention due to the high efficiency in building up molecular
complexity.[4] As a Lewis-acidic transition metal, the palladium
catalyst effectively facilitates the addition of nucleophiles to alkenes
for the difunctionalization.[5] In general, amines (especially for
aromatic amines and aliphatic amines) can coordinate with the
palladium salts more strongly than alkenes, which will reduce the
electrophilicity of palladium to form the stable bis(amine)-Pd
intermediate, thus affects the reactivity of the catalyst.[6] Therefore,
examples about the nucleopalladation with simple aromatic amines
are quite few.[7] To overcome this problem, chemists found that
nonbasic nitrogen nucleophiles,[8] such as carbomates, carboxamides,
and sulfonamides, can be activated by PdII to form alkyl-PdII via the
nucleopalladation process. The key intermediate of alkyl-PdII would
go through the direct β-H elimination and complete the catalytic
Scheme 1. Process of aminopalladation and our design for the synthesis of α-
amino acid esters
Based on our continuous interests in palladium-catalyzed
oxidation reactions with green oxidants,[11] we envisioned that the
aromatic amines could serve as the nucleophiles to attack the
alkenes directly to produce the alkyl-PdII species under mild
conditions, followed by the oxidation to form the alkyl-PdIV(OH)
intermediate in the presence of oxidant such as H2O2(Scheme 1b).[12]
Herein, we present a novel palladium-catalyzed intermolecular
aminopalladation triggered oxidative amination of electron-rich
olefins, wherein hydrogen peroxide is functioned as the sole oxidant
and oxygen source, which is undoubtedly eco-friendly and
inexpensive.[13] This protocol relies on simple and readily available
materials to deliver useful α-amino acid esters under mild reaction
conditions.
Preliminary investigation to the Pd-catalyzed oxidative amination
was conducted with N-methylaniline (1a), ethyl vinyl ether (2a), and
the green oxidant H2O2. Pleasingly, 61% consumption of 3a was
found when 1a was treated with 5 mol % of Pd(OAc)2, 4 equivalents
[]
L. Ouyang, J. Li, J. Zheng, J. Huang, C. Qi, Prof. Dr. W. Wu, Prof. Dr.
H. Jiang
o
of 35% aqueous H2O2 at 70 C under air in 1,4-dioxane (Table 1,
Key Laboratory of Functional Molecular Engineering of Guangdong
Province, School of Chemistry and Chemical Engineering, South
China University of Technology, 381 Wushan Road, Guangzhou,
510640, (China)
entry 1). Subsequently, the exploration of different solvents led to
the discovery that DMF was the best choice for this process and the
desired product 3aa could be obtained in 83% yield (Table 1, entry
2). However, solvents such as THF, MeCN, DCE, were not
compatible for this reaction (Table 1, entries 5-7). Screening of
other Pd salts, such as Pd(PPh3)2Cl2, Pd(MeCN)2Cl2, Pd(PhCN)2Cl2,
Pd(dba)2 and Pd(PPh3)4, revealed that Pd(OAc)2 was the optimal
(Table 1, entries 8-13). Notably, no desired product was detected
when the reaction was performed without metal catalyst (Table 1,
Fax: (+86)20-87112906
E-mail: cewuwq@scut.edu.cn; jianghf@scut.edu.cn
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
1
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10.1002/anie.201709285
Angewandte Chemie International Edition
as L-menhol, (-)-borneol (2n, 2o), could participate in the present
transformation to generate the corresponding α-amino acid esters
3an and 3ao, respectively.
entry 14). Compared to the aqueous H2O2, other strong oxidants
including TBHP (70% aq.), DTBP and K2S2O8 were inefficient to
provide the desired product (Table 1, entries 15-17).
Table 1. Optimization of the reaction conditions [a]
entry
Pd catalyst
oxidant
solvent
yield (%)
1
2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
35% aq. H2O2
70% aq. TBHP
DTBP
1,4-dioxane
DMF
DMA
EtOH
THF
61
83 (76)
74
3
4
57
5
trace
n.d.
trace
trace
n.d.
12
6
DCE
7
MeCN
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
8
9
Pd(PPh3)2Cl2
Pd(MeCN)2Cl2
Pd(PhCN)2Cl2
Pd(dba)2
10
11
12
13
14
15
16
17
57
Scheme 2. The scope of the alkenes for the synthesis of α-amino acid esters.
Reaction conditions: N-methylaniline (1a) as the limiting reagent (0.25 mmol),
Pd(OAc)2 (5 mol %), H2O2 (35% aq., 4 equiv), ethyl vinyl ether (2a, 2 equiv),
DMF (0.25 M substrate); under air. Yields of isolated products are shown.
50
Pd(PPh3)4
--
69
n.r.
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
n.d.
n.d.
n.d.
K2S2O8
[a] Reaction conditions: N-methylaniline (1a) as the limiting reagent (0.25 mmol),
Pd catalyst (5 mol %), oxidant (4 equiv), ethyl vinyl ether (2a, 2 equiv), solvent
(0.25 M substrate); under air. Yield was determined by GC with dodecane as
internal standard based on 1a. Isolated yield is in the parentheses. n.r. = no
reaction; n.d. = not determined.
A broad range of anilines and electron-rich alkenes were
investigated to afford the α-amino acid ester compounds under the
optimized conditions. As illustrated in Scheme 2, the oxidative
amination of alkenes proceeded well over a wide range of alkenes
with moderate to excellent yields. Substrates with different alkoxy
groups reacted smoothly, either the alkoxy groups come in chain
(3aa-3ad) or comes in cyclic (3ae). The reaction was also amenable
to halogen-substituted vinyl ethers (3af, 3ag), which might change
their properties and transfer to different functional groups. It is
worth noting that 1-adamantanol vinyl ether which is a rigid
molecule performed well to access the α-amino acid ester molecule
(3ah). Furthermore, when benzyl vinyl ether (3ai), 2-phenyl ethyl
vinyl ethers (3aj, 3ak) and 2-napthyl ethyl vinyl ether (3al) were
subjected to the reaction system, the reactions proceeded with good
efficiency under the conditions used. In order to further improve the
versatility of this reaction, different kinds of electron-rich olefins
were also examined. Gratifyingly, under the standard reaction
conditions, 1-vinyl-2-pyrrolidinone (2m) could also react with N-
methylaniline (1a) smoothly to give the desired product 3am in 65%
yield.
Scheme 3. Late-stage modification of α-amino acid esters
The scope of the Pd-catalyzed oxidative amination of alkenes
leading to α-amino acid esters was further investigated to a variety
of substituted amines (Scheme 4). Intriguingly, halogen atoms (–F, –
Cl, –Br) on the aromatic ring could be applied to this reaction (3ba-
3da). The methyl substituent at different positions of N-
methytoluidine (para-, meta-, and ortho-positions) showed steric
hindrance effect, and the corresponding products 3ea, 3fa, 3ga were
obtained in decreasing yields. It should be noted that the cyclic
amines were also tolerated in this process, which gave the
corresponding products 3ha and 3ia in moderate yields. Moreover,
the palladium catalyst was also compatible with primary aromatic
amine, and the desired product 3ja was formed in 60% yield with 8
equivalents of oxidants. Notably, the substrates possessing halogen
group could undergo the oxidative amination process, leading to the
corresponding α-amino acid esters (3ka-3ma, 3pa) in moderate
yields. The absolute configuration of 3la was unambiguously
characterized by X-ray analysis (see the Supporting Information for
details). Additionally, the reaction was amenable to the electron-
donating group (isopropyl, methyl, phenoxyl and methoxyl) of
amines for further transformations (3na, 3oa, 3qa, 3ra). Different
N-alkyl substituted amines served well for this transformation (3sa,
3ta). However, the reaction was sensitive to the functional groups
In view of the great importance of α-amino acid esters in
bioactive natural products and pharmaceutical compounds, it is
essential to develop late-stage modification of α-amino acid esters
(Scheme 3). To our delight, the α-amino acid esterification process
performed well to access different complex products under the
standard conditions. For instance, natural alcohol derivatives, such
2
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10.1002/anie.201709285
Angewandte Chemie International Edition
reaction, anhydrous DMF was used as the solvent, and the reaction
occurred smoothly to afford the desired product in 74% yield,
indicating that the oxygen should not come from H2O in DMF
(Scheme 6B). Critical evidence was obtained in the isotope labeling
experiment by adding ten equivalents of H218O to the anhydrous
DMF, which afforded product 3aa in 70% yield without 18O
incorporation product (3aa-18O) (Scheme 6C). Moreover, by using
H218O2 (2.0% aq.) as the oxidant, the N-methylaniline could be
converted to 3aa-18O in 64% yield instead of 3aa (Scheme 6D).
Oxygen coming from dioxygen was also ruled out (see the
Supporting Information for details). Based on the above control
experiments, one might draw a conclusion that the oxygen of α-
amino acid esters comes from the H2O2.
such as allyl and nitrile, affording the corresponding products 3ua
and 3va in lower yields.
Besides broad functional group tolerance, potential applications
of this oxidative amination reaction were tested. A scale-up
experiment (1a, 5 mmol) was smoothly conducted under the
standard conditions with 61% yield of 3aa obtained (Scheme 5A).
Further chemical transformations of the ester functionality in the
product molecules make the method more useful and attractive.
Hydrolysis of 3aa could afford N-methyl-N-phenyl glycine (4aa)
directly and efficiently. Compounds with glycine skeleton can be
observed in many pharmaceutically active drugs, and showed broad
biological activities. And the reduction of 3aa gave α-amino alcohol
5aa in excellent yields (Scheme 5B). Similarly, amino alcohols are
important compounds and have found wide applications in many
synthetic molecules and natural products. In addition, 6aa could be
easily obtained from the α-alkylation of α-amino acid ester in 62%
yield with the aid of DTBP (Scheme 5C)
Scheme 6. Mechanistic investigations
Scheme 4. The scope of amine for the α-amino acid esterification. Reaction
conditions: aniline (1a) as the limiting reagent (0.25 mmol), Pd(OAc)2 (5 mol %),
H2O2 (35% aq., 4 equiv), ethyl vinyl ether (2a, 2 equiv), DMF (0.25 M substrate);
Scheme 7. Proposed mechanism
a
under air. Yields of isolated products are shown. 8 equiv of H2O2 (35% aq.)
was used.
On the basis of the above results, a reasonable mechanism for Pd-
catalyzed intermolecular oxidative amination is illustrated in
Scheme 7. First, 2a can be activated via the coordination with the
Pd(II) catalyst, suggesting that the aminopalladation process should
be involved to give the alkylpalladium int-I.[14] Subsequently, the
H2O2 oxidizes the alkylpalladium int-I to form the alkyl PdIV(OH)
(int-II), which might go through the direct reductive elimination to
generate the int-III and then would be oxidized to the final product
3aa.[12b, 15] Alternatively, H2O2 may also act as a nucleophile to
attack the int-II to produce int-IV,[16] which undergoes the
dehydration process to afford 3aa.
In conclusion, we have developed a novel and convenient
protocol for the synthesis of functionalized α-amino acid esters via
palladium-catalyzed oxidative amination of vinyl ethers. 18O-
Labeling experiments demonstrated that H2O2 was not only served
as a green and inexpensive oxidant, but also the source of oxygen.
This method may open up
a
new viewpoint on the
Scheme 5. Synthetic applications
difunctionalization (installing of both amino and ester groups) of
alkenes. Moreover, the simple and abundant starting materials
without prefunctionalization make this method particularly practical,
neutral and mild. Ongoing studies are focused on exploring the
applications of the reaction and the detailed mechanism of the
reaction.
To gain more insight into the C-O bond formation steps, several
control experiments were conducted (Scheme 6). The reaction of N-
methylaniline (1a) and ethyl vinyl ether (2a) using urea-H2O2 as the
oxidant proceeded well with DMF as the solvent, giving the product
in 71% yield (Scheme 6A). In order to estimate whether the water in
the solvent of DMF could serve as a nucleophile involved in the
3
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10.1002/anie.201709285
Angewandte Chemie International Edition
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Acknowledgements
The authors thank the National Program on Key Research Project
(2016YFA0602900), the National Natural Science Foundation of
China (21420102003 and 21672072), and the Fundamental Research
Funds for the Central Universities (2015ZY001 and 2017ZD062) for
financial support.
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The authors declare no conflict of interest.
Keywords: palladium-catalyzed ·amination oxidation α-amino acid
ester · hydrogen peroxide
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4
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Angewandte Chemie International Edition
Entry for the Table of Contents (Please choose one layout)
Catch Phrase
Lu Ouyang, Jianxiao Li, Jia Zheng,
Jiuzhong Huang, Chaorong Qi, Wanqing
Wu,* and Huanfeng Jiang*
__________ Page – Page
Access to α-Amino Acid Esters via
Palladium-Catalyzed
Oxidative
A novel and convenient palladium catalytic system to achieve α-amino acid esters
with simple starting materials has been reported. Hydrogen peroxide not merely
acts as the green oxidant, but also the oxygen source. This new strategy salient
merits attention the simplicity of experimental procedure, mild reaction conditions,
high atom economy, scalability and practicability, which can convert the amines and
vinyl ethers to highly functionalized and structurally diverse α-amino acid esters.
Amination of Vinyl Ethers Using
Hydrogen Peroxide as Oxidant and
Oxygen Source
5
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