Enantioselective Oxidative Coupling of β-Ketocarbonyls and Anilines by Joint Chiral Primary Amine and Selenium Catalysis

Article abstract of DOI:10.1021/acs.orglett.9b02636

An enantioselective primary amine-catalyzed total N-selective nitroso aldol reaction (N-NA) was achieved through the oxidation of primary aromatic amines to the corresponding nitrosoarenes catalyzed by selenium reagents and 30% H₂O₂. This protocol provides a facile and highly efficient access to α-hydroxyamino carbonyls bearing chiral quaternary centers under exceedingly mild and green reaction conditions with high chemo-and enantiocontrol.

Full text of DOI:10.1021/acs.orglett.9b02636

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Enantioselective Oxidative Coupling of β‑Ketocarbonyls and
Anilines by Joint Chiral Primary Amine and Selenium Catalysis
Wanting Chen,† Yanni Wang,† Xueling Mi,*,† and Sanzhong Luo*,‡
†College of Chemistry, Beijing Normal University, Xinjiekouwai Street 19, Beijing 100875, China
‡Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 100084, China
S
* Supporting Information
ABSTRACT: An enantioselective primary amine-catalyzed
total N-selective nitroso aldol reaction (N-NA) was achieved
through the oxidation of primary aromatic amines to the
corresponding nitrosoarenes catalyzed by selenium reagents
and 30% H2O2. This protocol provides a facile and highly
efficient access to α-hydroxyamino carbonyls bearing chiral
quaternary centers under exceedingly mild and green reaction
conditions with high chemo- and enantiocontrol.
preparations and handling of nitrosoarenes pose serious issues,
hiral aniline groups are key structural units in many
Cbiologically active natural products and therapeutic limiting their synthetic applications. An easily conceivable
reagents.1 The N-nitroso aldol reaction with nitrosoarenes
represents one of the most straightforward approaches to
solution is to in-situ-generate nitroso compounds under
oxidative conditions.7 In this regard, anilines are the most
promising candidates because they are readily available
precursors for nitrosoarenes. However, such a catalytic
asymmetric oxidative N-nitroso aldol reaction of anilines
remains a considerable challenge and has not been reported.8
Recently, we have examined the enantioselective coupling of
acyclic β-keto esters and nitrosobenzene, for which high
stereocontrol has not been achieved so far. In the presence of
our chiral primary amine catalysts, the reactions of the
benchmark nitrosobenzene afforded the regioselective N-
nitroso aldol adduct with high enantioselectivity (Scheme 2,
3a, 64% yield, 98% ee and 3b, 71% yield, 97% ee). However,
the extension to other nitrosoarenes such as those electron-
withdrawing-group-substituted ones led to a serious reduction
in both yield and enantioselectivity, likely as a result of
background reactions and the decomposition of the nitro-
sobenzenes (observed). An in situ oxidative strategy was hence
pursued to address this issue.
access chiral anilines. In 2004, Yamamoto reported the first
example of a catalytic enantioselective N-nitroso aldol reaction
using BINAP-silver complexes.2 Later on, chiral amino-
catalysis3 and H-bonding bifunctional catalysis4,5 were also
reported for similar reactions with aldehydes and β-
ketocarbonyls, respectively. Despite this progress, catalytic
asymmetric N-selective nitroso aldol reactions are still limited
regarding their applicability and scope (Scheme 1). In
particular, nitrosoarenes6 are notoriously unstable and easily
undergo polymerization and decomposition. Hence the
Scheme 1. Asymmetric N-Selective Nitroso Aldol Reaction
of β-Ketocarbonyls
As known, nitrosoarenes could be prepared by the oxidation
of primary amines and hydroxylamines or the reduction of
nitroarenes.9 The selenium-catalyzed oxidation of aryl amines
̈
with H2O2 developed by Backvall appeared to be one of the
most attractive routes to synthesize nitroso compounds.10
However, the oxidation of anilines is highly dependent on the
substitution pattern, and successful examples of unactivated
anilines with electron-withdrawing substitutes are still rare. In
addition, the compatibility of the oxidative conditions with
delicate asymmetric catalysis remains an open question, and no
example has been reported. Intrigued by the intrinsic green
credentials of the Se/H2O2 system, we investigated the chiral
Received: July 26, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02636
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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a
Scheme 2. N-Nitroso Aldol Reaction with Nitrosoarenes
Table 1. Screening and Optimization of the Oxidative
a
Coupling to Aniline
b
c
entry Se cat. (X mol %) Ar = 1l/2b
CHCl3
yield (%)
Ee (%)
1
2
Ph (5 mol %)
PhSeO2H (10 mol %)
SeO2 (20 mol %)
Ph (5 mol %)
Ph (5 mol %)
Ph (2.5 mol %)
Ph (10 mol %)
Ph (5 mol %)
Ph (5 mol %)
3-MeO-C6H4
4-MeO-C6H4
2-F-C6H4
3-F-C6H4
4-Cl-C6H4
3-Cl-C6H4
3,5-(CF3)2-C6H3
none
1:2
1:2
1:2
1:2
1:2
1:2
1:2
2:1
3:1
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
CHCl3
CHCl3
CHCl3
MeCN
MeOH
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
32
21
10
36
10
14
32
62
70
27
39
45
55
53
70
60
99
96
97
76
70
97
97
97
97
94
96
97
96
98
99
97
3
4
5
6
7
8
9
10
11
12
13
14
15
16
a
All reactions were performed at 0 °C in 0.5 mL of CHCl3 with 2
(0.10 mmol), 1 (0.15 mmol), A (20 mol %), 2−4 h. Isolated yields. Ee
was determined by HPLC analysis.
primary amine and selenium combined catalysis for the
oxidative N-nitroso aldol reactions with anilines.11 A diaryl
diselenide catalyst was identified to effectively facilitate the
oxidation of anilines to nitrosoarenes, and its successful
coupling with β-ketocarbonyls was achieved in the presence
of a primary amine catalyst.
N.D.
a
Reactions were performed with 1l (0.10 mmol), 2 (0.20 mmol),
In this study, a primary challenge is the oxidative
compatibility of aminocatalysis under the oxidative selenium
catalyst A (20 mol %), and Se catalyst (5 mol %, entries 9−15), H2O2
(30%) (2.2 equiv) at room temperature in 0.5 mL of solvent, 4−6 h
conditions because the aminocatalyst may be readily oxidized.
b
c
10
total reaction time. Yields were determined by 1H NMR analysis. Ee
was determined by the HPLC analysis on a chiral stationary phase.
N.D. = no desired product.
̈
Inspired by Backvall’s work, a two-stage operation was
utilized to obviate this problem. As such, the oxidation of
aniline was first conducted, and the organic phase after
complete conversion was then treated with aminocatalyst and
carbonyl compounds to initialize enantioselective C−N
coupling. In the first stage, it was found that the simple
removal of an aqueous uplayer was critical for productivity, and
the reasons might be two-fold: to remove excess H2O2 that is
potentially detrimental to the aminocatalyst and to eliminate
the inhibiting effect of water on the enamine catalysis. The
whole process did not require any isolation or purification of
the unstable nitroso intermediates (Table 1).
and 10). The diselenide-bearing 3-chloro substitute turned out
to be the optimal catalyst to give 70% yield with 99% ee (Table
1, entry 14). A more electron-deficient one with a
ditrifluoromethyl group provided a slightly lower yield
(Table 1, entry 15). In control experiments without a selenium
catalyst, no desired product was detected (Table 1, entry 16),
pinpointing the critical role of selenium catalysis in the in situ
generation of nitrosoarene.
An electron-deficient 4-aminobenzoate 1l was chosen as a
model substrate for screening and optimization. Initially, the
readily available diphenyl diselenide was found to give the
desired product 4l in 32% yield with 99% ee (Table 1, entry 1),
showing a better outcome compared with PhSeO2H or
selenium dioxide (Table 1, entry 1 vs 2). Chloroform was
identified as the optimal solvent in terms of both productivity
and enantioselectivity (Table 1, entries 1 vs 3 and 4).
Decreasing the loading of the selenium catalyst led to a
reduced yield (entry 5), but increasing the loading showed no
obvious effect (entry 6). Using excess aniline gave improved
yields (based on the limiting 2b) with maintained
enantioselectivity (entries 7 and 8) along with a large amount
of azoxyarene side products. The results obtained were
encouraging but still far from satisfaction with respect to the
conversion of aniline as well as the formation of side products.
Therefore, we next explored substituted selenium catalysts, and
a series of diaryl diselenides were prepared according to known
methods.12 A general trend was observed, with the electron-
deficient selenium catalyst showing superior productivity to
their electron-rich counterpart (Table 1, entries 11−14 vs 9
With the optimized conditions in hand, we first examined
the scope of β-ketocarbonyl substrates, of which the results are
summarized in Scheme 3. First, using β-keto esters with
different ester moieties (Me, Et, n/iPr, tBu, allyl, Bn) all gave
the desired products in moderate to good yield and with high
enantioselectivities (Scheme 3, 3a−g, 44−55% yield, 96−98%
ee). The substrate scope could be further extended to β-
ketoamides. Both N-aryl (2h−j) and N-aliphatic amides (2k)
worked well under such reaction conditions to afford the
desired α-oxyamination adducts in good yield and with high
enantioselectivities as well (Scheme 3, 3h−k, 53−65% yield,
97−98%ee). Furthermore, six-membered-ring cyclic ketoester
and ketoamides could be transformed into the corresponding
products with high enantioselectivities (Scheme 3, 57% yield,
>99% ee for 3l; 55% yield, 95% ee for 3m). Unfortunately, β-
ketoesters bearing a larger α-alkyl group and 1,3-diketones did
not work well, leading to rather poor yields (not shown).
Then, we turned our attention toward the scope of
substituted anilines in this tandem oxidation−N-nitroso aldol
reaction. To achieve excellent transformation of nitroso
derivatives, it is necessary to avoid peroxidation to the
B
DOI: 10.1021/acs.orglett.9b02636
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Organic Letters
Letter
a
a
Scheme 3. Substrate Scope of β-Ketocarbonyls
Scheme 4. Substrate Scope of Anilines
a
All reactions were performed with aniline 1a (0.10 mmol), 2 (0.20
mmol), A (20 mol %), (3-Cl-C6H4Se)2 (5 mol %), and H2O2 (30%)
(2.2 equiv) at room temperature in 0.5 mL of chloroform for 2−4 h.
Isolated yields. Ee was determined by HPLC analysis.
corresponding nitro derivatives or dimerization to the
respective azo derivatives. It was found that a reaction time
of 2 h was sufficient to ensure the maximization of nitroso
conversion while minimizing overoxidation, and in cases with
electron-deficient anilines, slightly prolonging the reaction time
to 4 h was necessary to ensure conversion. Reactions with
anilines containing electron-deficient substituents formed the
final products via the respective nitroso compounds in
moderate to good isolated yield and with high enantioselectiv-
ities (Scheme 4, 4a−g, 70−84% yield, 94−99%ee). It is
noteworthy that anilines with strong electron-withdrawing
groups on the aromatic ring including 4-keto-, 3-nitro, 4- or 3-
cyano, and 4- or 3-carboxylate substitutes could be well
tolerated to the desired products in moderate to good yield
and with high enantioselectivities (Scheme 4, 4h−m, 49−72%
yield, 98−99%ee). 3-Fluoroaniline could be oxidized rather
quickly to complicate byproducts, leading to inferior results in
low yield but with high enantioselectivity (Scheme 4, 4n, 32%
yield, 99% ee). Disubstituted aniline also worked well (Scheme
4, 4o, 65% yield, 98% ee). Ortho-substituted anilines gave
slightly lower yields probably due to steric effect (Scheme 4,
4p−4r, 43−54% yields, 95−99%ee). For o-methyl formate
aniline, the targeted product was not obtained, but an
intramolecular cyclization product was isolated (Scheme 4,
4s, 53% yield, 97% ee and 4t, 46% yield, >99% ee). These
results clearly reveal that the present method was well
applicable for the oxidation of a series of aromatic primary
amines.
To further demonstrate the synthetic utility of this
methodology, a gram-scale reaction was carried out in the
presence of 10 mol % of chiral primary amine catalyst (Scheme
5). This transformation worked well with a comparable yield
and enantioselectivity.
As known, anilines can be directly oxidized to the
corresponding nitroso derivatives using benzene seleninoper-
oxoic acid PhSe(O)OOH as the real active oxidant, which was
generated in situ by the oxidation of diphenyl diselenide by
H2O2 in this system after further hydrolysis.13 The observed
a
All reactions were performed at room temperature in 0.5 mL of
CHCl3 with 1 (0.10 mmol), 2b (0.20 mmol), A (20 mol %), (3-Cl-
C6H4Se)2 (5 mol %), and H2O2 (30%) (2.2 equiv) for 2−4 h. Isolated
yields. Ee was determined by HPLC analysis.
C
DOI: 10.1021/acs.orglett.9b02636
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Scheme 5. Gram-Scale Synthesis
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02636.
Experimental procedures, screening data, steric param-
eters and model development, computational details,
and NMR spectra (PDF)
chemo- and enantioselectivities could be explained by a
protonated N−H H-bonding network to the nitroso moiety, as
we previously reported14(Figure 1).
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: xlmi@bnu.edu.cn (X.M.).
*E-mail: luosz@tsinghua.edu.cn (S.L.).
ORCID
Xueling Mi: 0000-0002-5264-8269
Sanzhong Luo: 0000-0001-8714-4047
Notes
The authors declare no competing financial interest.
Figure 1. Proposed transition state.
ACKNOWLEDGMENTS
To confirm the N/O selectivity of this methodology, the
reduction of product 3b to the corresponding aniline 5a was
carried out in quantitative yield by using hydrochloric acid and
zinc dust as the reducing agent (Scheme 6, eq 1). Moreover,
■
We thank the Natural Science Foundation of China
(21390400, 21521002, 21572232, and 21672217) and
Tsinghua University for financial support. S.L. is supported
by the National Program of Top-notch Young Professionals.
Scheme 6. Cleavage of N−O Bond
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