One-pot organocatalytic asymmetric synthesis of 3-nitro-1,2- dihydroquinolines by a dual-activation protocol

Article abstract of DOI:10.1002/asia.200900298

Generally, amine-catalyzed enantioselective transformations rely on chiral enamine or unsaturated iminium intermediates. Herein, we report a protocol involving dual activation by an aromatic iminium and hydrogenbonding. An enantioselective aza-Michael-Hen

Full text of DOI:10.1002/asia.200900298

FULL PAPERS
DOI: 10.1002/asia.200900298
One-Pot Organocatalytic Asymmetric Synthesis of 3-Nitro-1,2-
dihydroquinolines by a Dual-Activation Protocol
Yi-Feng Wang,[a] Wei Zhang,[a] Shu-Ping Luo,[a] Bai-Lin Li,[a, b] Ai-Bao Xia,[a]
Ai-Guo Zhong,[b] and Dan-Qian Xu*[a]
Abstract: Generally, amine-catalyzed
enantioselective transformations rely
on chiral enamine or unsaturated imi-
nium intermediates. Herein, we report
a protocol involving dual activation by
an aromatic iminium and hydrogen-
bonding. An enantioselective aza-Mi-
chael–Henry domino reaction of 2-ami-
nobenzaldehydes with nitroolefins has
been developed through this protocol
using primary amine thiourea catalysts
to provide a variety of 3-nitro-1,2-dihy-
droquinolines in moderate yields and
with up to 90% ee. The mechanism for
the catalytic enantioselective reaction
was confirmed by ESI mass spectro-
metric detection of the reaction inter-
mediates. The products formed are
substructures found in skeletons of im-
portant biological and pharmaceutical
molecules.
Keywords: asymmetric synthesis ·
domino reactions · organocatalysis ·
reaction mechanisms
Introduction
line by relying on an organocatalytic model of iminium–en-
amine activation of a,b-unsaturated aldehydes to proceed
an enantioselective Michael-aldol reaction with 2-aminoben-
zaldehydes. The counterpart of 3-formyl-1,2-dihydroquino-
line, 3-nitro-1,2-dihydroquinoline, which could be prepared
through the domino Michael–Henry reaction, was studied
by Yaoꢁs group.[4] Nevertheless, its asymmetric form has not
been researched. Given the importance of these valuable
1,2-dihydroquinolines, as well as the lack of efficient meth-
ods for the preparation of these important synthetic inter-
mediates, the development of a new catalytic asymmetric
synthesis of these compounds appeared to be of great im-
portance. Herein, we report a one-pot asymmetric organoca-
talytic procedure for the synthesis of chiral 3-nitro-1,2-dihy-
droquinolines starting from 2-aminobenzaldehydes and ni-
troolefins.
Quinoline derivatives are widely distributed in nature. In
particular, 1,2-dihydroquinoline is a common motif found in
a variety of natural products and is used as a versatile inter-
mediate in organic and natural product synthesis.[1] Conse-
quently, the synthesis of the 1,2-dihydroquinoline core and
its derivatives, especially by asymmetric catalysis, has been
an attractive goal for synthetic organic chemists.[2] Surpris-
ingly, only three examples of their asymmetric catalytic syn-
thesis have been described (by the research groups of Shiba-
saki, Cꢀrdova, and Wang).[3] Shibasaki and co-workers de-
veloped a catalytic asymmetric Reissert reaction for the
chiral Reissert compounds, while Cꢀrdova and Wangꢁs ap-
proaches led to the synthesis of 3-formyl-1,2-dihydroquino-
The concept of activating and binding both reacting part-
ners for a catalytic transformation has been widely reported
for the design of new organocatalytic transformations.[5] In
this case, the nitroolefin as the electrophile could be activat-
ed by hydrogen-bonding as well as transition-state organiza-
tion with hydrogen-bond donor catalysts, such as thioureas.[6]
We sought another means of activating the reaction with 2-
aminobenzaldehyde. On the basis of our previous success in
aromatic iminium activation for the organocatalytic asym-
metric synthesis of 3-nitro-2H-chromenes,[7] we envisaged
that bifunctional primary amine thiourea catalysts might fa-
cilitate this asymmetric transformation through the domino
aza-Michael–Henry[8] reaction. A proposed catalytic cycle is
[a] Y.-F. Wang, W. Zhang, Dr. S.-P. Luo, B.-L. Li, A.-B. Xia,
Dr. D.-Q. Xu
State Key Laboratory Breeding Base of Green Chemistry-Synthesis
Technology
Zhejiang University of Technology
Hangzhou 310014 (China)
Fax : (+86)0571-88320066
E-mail: chrc@zjut.edu.cn
[b] B.-L. Li, Dr. A.-G. Zhong
Department of Pharmaceutical and Chemical Engineering
Taizhou College
Linhai Zhejiang 317000 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900298.
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Chem. Asian J. 2009, 4, 1834 – 1838

Figure 1. The screened organocatalysts.
Table 1. Organocatalytic enantioselective domino aza-Michael–Henry re-
action.[a]
Entry
Cat.
Acid
Solvent
Yield [%][b]
ee [%][c]
1[d]
2[d]
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Ia
Ib
Ia
Ia
Ia
Ia
Ia
Ia
Ia
Ia
Ia
Ia
Ia
Ia
Ia
II
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
4-NO2C6H4CO2H
4-CH3C6H4CO2H
4-FC6H4CO2H
TFA
iPrOH
iPrOH
iPrOH
MeOH
EtOH
CH2Cl2
Et2O
THF
PhCH3
DMF
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
56
51
54
60
52
12
14
22
30
<5
37
41
42
<5
60
37
70
24
51
21
66
5
Scheme 1. Proposed catalytic cycle of the asymmetric synthesis of 3-
nitro-1,2-dihydroquinolines.
77
29
72
60
78
76
72
n.d.
72
73
70
n.d.
75
12
85
52
86
70
shown in Scheme 1. The preferred orientation of aromatic
iminium A, resulting from the condensation of 2-aminoben-
zaldehyde and the catalyst, allows the nitroolefin to form
hydrogen bonds, which bring it into proximity for the forma-
tion of a nitrogen–carbon bond (transition state B). The imi-
nium promotes the Henry reaction with the aromatic alde-
hyde,[9] and then elimination of the formed Mannich base
(intermediate C) affords the enantioselective products and
concurrently regenerates the catalyst.
AcOH
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
IIIa
IIIb
IIIc
IIId
Results and Discussion
[a] Unless otherwise specified, all reactions were carried out with 2-ami-
nobenzaldehyde (0.5 mmol), b-nitrostyrene (0.5 mmol), catalyst
(0.1 mmol, 20 mol%), the specified co-catalyst of an acid (0.1 mmol,
20 mol%), and 4 ꢃ-M.S. (0.2 g) in iPrOH (1.0 mL) at room temperature
for 48 h. DMF=N,N-dimethylformamide; TFA=trifluoroacetic acid.
[b] Yield of isolated product. [c] Determined by chiral HPLC analysis
(Daicel Chiralpak OD-H, hexane/iPrOH=85:15). n.d.: not determined.
[d] In the absence of 4 ꢃ-M.S.
To assess the viability of this domino process, the investiga-
tion was started by treating 2-aminobenzaldehyde with ni-
trostyrene in the presence of primary amine thiourea Ia de-
rived from diaminocyclohexane (Figure 1). As shown in
Table 1, the acid PhCO2H was added to facilitate the cata-
lytic cycle and iminium intermediate formation. To our de-
light, by performing the reaction in iPrOH, the desired
product was isolated in 56% yield with moderate enantiose-
lectivity; the tertiary amine thiourea Ib[5f] afforded only
almost racemic products (Table 1, entries 1 and 2). As H2O
is generated in the formation of active iminium intermediate
A, which might influence the expected hydrogen-bonding
interaction between the catalyst and nitrostyrene, molecular
sieves (M.S., 4 ꢃ) were added to remove these trace
amounts of water. In this way, the ee value was raised to
77% (Table 1, entry 3). The reaction medium of this catalyt-
ic system may need to be balanced between polar and non-
polar, because a relatively nonpolar solvent favors the hy-
drogen-bonding catalysis whereas the aromatic iminium may
be more stable in polar solvents. Whereas in some solvents,
Abstract in Chinese:
Chem. Asian J. 2009, 4, 1834 – 1838
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1835

D.-Q. Xu et al.
FULL PAPERS
Table 2. Organoatalyst IIIa promoted domino aza-Michael–Henry reac-
such as MeOH, EtOH, and iPrOH, the reaction proceeded
smoothly, and the stereocontrol of the reaction course in-
creased with decreased polarity of solvents (Table 1, en-
tries 3–5), the reaction in less polar solvents, such as CH2Cl2,
Et2O, THF, and PhCH3, generally proceeded with relatively
high enantioselectivity but low yields (Table 1, entries 6–9).
As expected, little or no reaction was observed in hydrogen-
bond accepting solvents such as DMF (Table 1, entry 10).
Variation of the acid additive failed to improve the effi-
ciency of the reaction (Table 1, entries 11–15), but care
should be taken in the choice of the acid additive for this re-
action. The pKa value of the acid should not be greatly
larger than the that of the conjugate acid of 2-aminobenzal-
dehyde; otherwise the protonated amino group could pro-
hibit the aza-Michael reaction (the pKa values of anilineꢁs
conjugate acid, PhCO2H, and AcOH are 4.6, 4.2, and 4.8, re-
spectively, while that of TFA is 0.5).
tions.[a]
Entry
X
R
Product Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
Ph
3a
3b
3c
3e
70
65
45
58
37
65
53
63
47
44
39
49
64
67
85
84
83
88
90
81
83
75
88
70
77
64
55
52
4-MeC6H4
2-MeOC6H4
4-MeOC6H4
3,4-(MeO)2C6H3 3 f
4-ClC6H4
4-BrC6H4
3g
3h
naphthalen-2-yl 3j
Ph
9
5-Br
4,5-(MeO)2 Ph
4,5-(MeO)2 4-MeOC6H4
4,5-(MeO)2 4-ClC6H4
H
H
3k
3l
3m
3n
3o
3p
10
11
12
13
14
The diphenylethylenediamine-derived catalyst II[10] was
tested for this reaction under the optimized conditions, but
the result was disappointing (Table 1, entry 16). The efficien-
cy of the reaction could be enhanced by modifying the struc-
ture of the catalyst Ia with a substituted chiral functional
group, the bifunctional thiourea (S,S,S)-IIIa[5d,e] afforded the
products in good yield (70%) with high enantioselectivity
(85% ee). (S,S,R)-IIIb, a stereoisomer of (S,S,S)-IIIa, provid-
ed the same enantiomer of the product. However, lower
yield and enantioselectivity were afforded by catalyst
(S,S,R)-IIIb (Table 1, entry 17 vs entry 18). Further enhanc-
ing the chiral inducement by introducing a naphthyl group
into the catalyst structure did not lead to significant im-
provement relative to the catalyst (S,S,S)-IIIa (Table 1, en-
tries 19 and 20). Therefore, (S,S,S)-IIIa should be a suitable
organocatalyst for the asymmetric transformation.
n-propyl
i-propyl
[a] Unless otherwise specified, all reactions were carried out with 2-ami-
nobenzaldehydes (0.5 mmol), b-nitroolefins (0.5 mmol), catalyst IIIa
(0.1 mmol, 20 mol%), benzoic acid (0.1 mmol, 20 mol%) and 4 ꢃ-M.S.-
AHCTUNGERTG(NNUN 0.2 g) in iPrOH (1.0 mL) at room temperature for 48 h. [b] Yield of iso-
lated product. [c] Determined by chiral HPLC analysis (Daicel Chiralpak
OD-H/AD-H).
A range of 2-aminobenzaldehyde/nitroolefin combina-
tions were surveyed to determine the scope and limitations
of the methodology. As shown in Table 2, a variety of b-aryl-
nitroolefins underwent the domino reaction in moderate
yields with high selectivity (up to 90% ee), regardless of
their electronic properties (Table 2, entries 1–8). Moreover,
substituted 2-aminobenzaldehydes were explored and found
also to be tolerated in this domino reaction (Table 2, en-
tries 9–12). Notably, the domino reaction of aliphatic nitroo-
lefins proceeded smoothly to afford the desired products in
good yields with moderate enantioselectivities (Table 2, en-
tries 13 and 14).
The absolute stereochemistry of the 3-nitro-1,2-dihydro-
quinoline was established by comparison of the electronic
circular dichroism (ECD) spectra of the product with calcu-
lated time-dependent DFT results; this is now a widely ac-
cepted technique for the determination of absolute configu-
rations.[11] As shown in Figure 2, the experimental ECD
spectrum matched the theoretical data for the S configura-
tion, which implies that the primary amine group of the aro-
matic iminium intermediate B should favor attack of the re-
face of the nitroolefin, as shown in Scheme 1.
Figure 2. The theoretical ECD spectrum (dot and dash traces) for the
product 3a simulated by the TD-DFT/6-311+G*//DFT/6-311+G*
method, compared with the experimental spectrum (solid trace).
which enabled us to identify all critical intermediates in the
reaction mixture (Figure 3). The spectrum of the sample ob-
tained from the reaction mixture after stirring for 3 h re-
vealed the ions at m/z 278.2, 366.3, 381.3, 403.3, 530.2, and
552.2, corresponding to the signals for the catalyst
([IIIa+H]+), the aromatic iminium intermediate
A
([AÀNH2+2H]+, [A+H]+, and [A+Na]+), and the Mannich
base ([C+H]+ and [C+Na]+), respectively. Moreover, a
characteristic signal at m/z 469.1 was observed in the ESI-
MS/MS spectrum of ion [C+H]+ (m/z 530.2; Figure 4). This
signal was consistent with the remaining fragment of the
aza-Michael adduct after the scission of CH3NO2, indicating
that signals at m/z 530.2 in Figure 3 and at m/z 530.3 in
Figure 4 should correspond precisely to both intermediate C
For direct proof of the rationality of the proposed mecha-
nism, we investigated the domino reaction using ESI-MS,[12]
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ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 1834 – 1838

One-Pot Organocatalytic Asymmetric Synthesis
ICR MS, Table 3). Therefore,
the above MS results fully con-
firmed the proposed catalytic
cycle involved in the sequential
process of the axa-Michael–
Henry mechanism as shown in
Scheme 1.
Conclusions
In conclusion, we have devel-
oped the first organocatalytic
enantioselective domino aza-
Michael–Henry reaction of 2-
aminobenzaldehydes and aro-
matic nitroolefins to generate
synthetically versatile 3-nitro-
1,2-dihydroquinolines. Synergis-
tic activation of both reactants
through stereoselective covalent
activation and hydrogen-bond-
ing interactions, which is similar
to catalysis by enzymes, allows
this challenging transformation
to take place under mild reac-
tion conditions. We hope that
the strategy applied in this
study may give some hints in
the development of asymmetric
transformations.
Figure 3. ESI-MS spectrum of the domino reaction for the synthesis of 3-nitro-1,2-dihydroquinoline, 3 h after
the start of the reaction.
Experimental Section
General Methods
All regents were purchased from com-
mercial suppliers and used without pu-
rification. The solvents were dried and
distilled prior to use according to the
guidelines of Armarego.[13] All reac-
tions were carried out in oven-dried
glassware. Melting points were deter-
mined on Buchi Melting Point B-545
apparatus. 1H and 13C NMR spectra
were recorded in CDCl3 on a Bruker
AVANCE III NMR spectrometer
(500 MHz). TMS served as internal
standard (d=0 ppm) for 1H NMR
spectroscopy and CDCl3 was used as
internal standard (d=77.0 ppm) for
13C NMR spectroscopy. HPLC experi-
Figure 4. ESI-MS/MS spectrum of ion [C+H]+.
Table 3. High-resolution mass data of detected intermediates.
Species
Mass (measured)
Mass (theoretical)
Formula
Error [ppm]
A
278.1678
366.1994
403.1915
552.2396
278.1685
366.1998
403.1927
552.2404
C15H24N3S
C22H28N3S
C22H28N4NaS
C30H35N5NaO2S
2.5
1.1
3.0
1.4
[AÀNH2+2H]+
ments were carried out using
a
[A+Na]+
JASCO LC-2000 Plus system with a
MD-2010 HPLC diode array detector.
CD spectra were measured on
a
JASCO J-815 CD spectrometer in
0.5 gLÀ1 MeOH. ESI-MS spectra were
a LCQ Deca XP Plus ion-trap mass spectrometer
equipped with an ESI source and controlled by Xcalibur software. The
spray voltage was set to 3000 V for positive ions, the heated capillary
and the intermediate of the Michael adduct. These results
were further confirmed by accurate mass data using Fourier
transform ion cyclotron resonance mass spectrometry (FT-
determined on
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1837

D.-Q. Xu et al.
FULL PAPERS
temperature was at 2708C. For the MS/MS analysis, the collision energy
was 20 eV, the mass spectrometer was operated in the full scan modes,
with the mass width at 1.0 Da. High-resolution mass spectra were ob-
tained on a Bruker APEX III 7.0 Tesla ion cyclotron resonance Fourier
transform mass spectrometer.
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Typical Procedure for the Organocatalytic Asymmetric Aza-Michael–
Henry Reaction
To a solution of catalyst IIIa (0.1 mmol, 20 mol%) and benzoic acid
(0.1 mmol, 20 mol%) in iPrOH (1.0 mL) was added 2-aminobenzalde-
hyde (0.5 mmol), b-nitrostyrene (0.5 mmol), and 4 ꢃ-M.S. (0.2 g) sequen-
tially. The reaction mixture was then stirred at room temperature (about
158C) for 48 h. The crude mixture was purified by flash chromatography
to furnish a dark red solid in 70% yield. M.p.: 108–1098C; 1H NMR
(500 MHz, CDCl3, 258C, TMS): d=7.96 (s, 1H), 7.37–7.39 (m, 2H), 7.27–
7.31 (m, 3H), 7.15–7.18 (m, 2H), 6.70 (t, J=7.5 Hz, 1H), 6.45 (d, J=
7.5 Hz, 1H), 5.98 (s, 1H), 4.69 ppm (s, 1H); 13C NMR (125 MHz, CDCl3,
258C, TMS): d=144.4, 142.1, 141.0, 134.1, 131.3, 131.2, 129.0 (2C), 128.7,
126.2 (2C), 118.6, 114.9, 113.4, 55.4 ppm; The enantiomeric excess was
determined by HPLC with
a Chiralpak OD-H column (hexane :
iPrOH=85:15; flow rate 1.0 mLminÀ1; tR major isomer=19.2 min, tR
minor isomer=13.7 min).
Acknowledgements
This work was supported by the National Natural Science Foundation of
China (No. 20772110) and the Program for Changjiang Scholars and In-
novative Research Team in University of China.
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Received: July 23, 2009
Published online: October 14, 2009
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