A novel enantioselective catalytic tandem oxa-Michael-Henry reaction: One-pot organocatalytic asymmetric synthesis of 3-nitro-2H-chromenes

Article abstract of DOI:10.1002/adsc.200800535

An enantioselective oxa-Michael-Henry reaction of substituted salicylaldehydes with nitroolefins that proceeds though an aromatic iminium activation (AIA) has been developed by using a chiral secondary amine organocatalyst and salicylic acid as a co-catalyst. The corresponding 3-nitro-2H-chromenes were obtained in moderate-to-good yields with up to 91% ee under mild conditions. Based on the experimental results and ESI-mass spectrometric detection of the intermediates, a plausible transition state has been proposed to explain the origin of the activation and the asymmetric induction.

Full text of DOI:10.1002/adsc.200800535

FULL PAPERS
DOI: 10.1002/adsc.200800535
A Novel Enantioselective Catalytic Tandem Oxa-Michael–Henry
Reaction: One-Pot Organocatalytic Asymmetric Synthesis of
3-Nitro-2H-chromenes
Dan-Qian Xu,a Yi-Feng Wang,a Shu-Ping Luo,a Shuai Zhang,a Ai-Guo Zhong,b
Hui Chen,a and Zhen-Yuan Xua,*
a
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology,
Hangzhou 310014, Peopleꢀs Republic of China
Fax : (+86)-0571-8832-0066; e-mail: greenchem@zjut.edu.cn
Department of Pharmaceutical and Chemical Engineering Department, Taizhou College, Linhai Zhejiang 317000,
b
Peopleꢀs Republic of China
Received: August 27, 2008; Published online: November 4, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800535.
Abstract: An enantioselective oxa-Michael–Henry the experimental results and ESI-mass spectrometric
reaction of substituted salicylaldehydes with nitroole- detection of the intermediates, a plausible transition
fins that proceeds though an aromatic iminium acti- state has been proposed to explain the origin of the
vation (AIA) has been developed by using a chiral activation and the asymmetric induction.
secondary amine organocatalyst and salicylic acid as
a co-catalyst. The corresponding 3-nitro-2H-chro- Keywords: asymmetric catalysis; Henry reaction; Mi-
menes were obtained in moderate-to-good yields chael reaction; reaction mechanisms; tandem reac-
with up to 91% ee under mild conditions. Based on tions
Introduction
targets.[5] Some representative examples are shown in
Figure 1.
Chromenes, or benzopyrans, are an important class of
Enantioselective organocatalysis, in which an asym-
scaffolds found in numerous naturally occurring and metric reaction proceeds with small oganic molecules
synthetic molecules possessing a broad spectrum of as catalysts, has witnessed tremendous developments
biological activities.[1] Accordingly, interest in devel- due to its efficiency and capacity to extend the scope
oping syntheses of these privileged structures contin- of chiral organic synthesis in recent years, although its
ues to grow,[2] especially concerning condensations be- scope is mainly limited to carbonyl systems.[6] Until
tween salicylaldehyde derivatives and Michael accept- recently, four different methods to activate carbonyl
ors such as a,b-unsaturated aldehydes (ketones) and compounds have been known using amine catalysis:
nitroalkenes which have proven to be a versatile strat-
egy to the benzopyan moiety.[3] Furthermore, for ob-
taining chiral chromenes, an enantioselective tandem
Michael-aldol reaction of salicylaldehyde derivatives
with a,b-unsaturated aldehydes (ketones) catalyzed
by proline derivatives was developed by Wang, Ar-
vidsson, Cꢁrdova and their co-workers, respectively.[4]
Nevertheless, a chiral amine catalytic tandem oxa-Mi-
chael–Henry reaction of salicylaldehyde derivatives
with nitroolefins for chiral 3-nitro-2H-chromene de-
rivatives has not been researched. The importance of
3-nitro-2H-chromenes is recognized due to their ver-
satile modifiability, for example, they could serve as
precursor for flavonols, amines and other biological active compounds derived from 3-nitro-2H-chromenes.
Figure 1. Some representative examples of biologically
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A Novel Enantioselective Catalytic Tandem Oxa-Michael–Henry Reaction
FULL PAPERS
Figure 2. proposed mechanism of aromatic iminium activation for the organocatalyzed asymmetric tandem oxa-Michael–
Henry reaction.
enamine, iminium ion, dienamine activation and the highly asymmetric Michael additions of ketones
SOMO (singly occupied molecular orbital)-enamine and nitroolefins in our previous study.[8] Previous
activation which was introduced by the research work has shown that carefully chosen acidic cocata-
groups of Sibi and Macmillan more recently.[7] How- lysts could enhance the efficiency and selectivity for
ever, to our best knowledge, the asymmetric reactions the Michael addition catalyzed by chiral secondary
reported through all these activation modes have not amines,[10] and we therefore examined the cocatalyst
involved the activation of aromatic aldehydes. In- effects on this domino reaction at the same time
spired by the previous study of organocatalytic mech- (Table 1). The reaction was fast when treated with 20
anisms, and based on our research interest in organo- mol% 1i (Figure 3) alone without any cocatalysts in
catalysis,[8] herein, we report the first example of aro- DMSO at room temperature, but the product was
matic iminium activation (AIA) in the field of asym- almost racemic (Table 1, entry 1). However, use of 20
metric organocatalysis.
mol% of 1i along with 10 mol%, 15 mol%, or 20
As shown in Figure 2, iminium-activated salicylal- mol% of hydrobromic acid (Table 1, entries 2–4) as
dehyde[9] A which is also activated by the Lewis base cocatalyst, respectively, gave the products in moderate
moiety X of the organocatalyst 1 would be attacked to high enantioselectivity (Table 1, entry 3, 78% ee),
through a domino oxa-Michael–Henry reaction by a although the yields of the product fell in the presence
b-nitrostyrene which is induced finely by the complex of the cocatalyst. These observations suggest that the
B. The intermediate C undergoes an elimination pro- acidic component is vitally important for constituting
cess to afford the enantioselective 3-nitro-2H-chro- the aromatic iminium intermediate and the Lewis
menes and recover the catalyst 1 and co-catalyst HY. base moiety of the organocatalyst 1i also enhances
the nucleophilicity by deprotonation of the OH
moiety of salicylaldehyde. Based on these initial re-
Results and Discussion
sults, we chose the conditions of entry 3 for the fur-
ther screening of various acids cocatalysts, including
Initially, to explore this possibility of the proposed HBF4, HBF6, TsOH and other organic acids. The best
domino oxa-Michael–Henry process, a model reaction result (67% yield, 85% ee) was obtained with 15
between 4-methoxysalicylaldehyde and b-nitrostyrene mol% salicylic acid as cocatalyst (Table 1, entry 13).
was performed in DMSO at room temperature in the The results were in accord with the results of Michael
presence of an organocatalyst. Firstly, a pyrrolidine- additions of ketones and nitroolefins, probably the
thioimidazole was chosed as an organocatalyst be- salicylic acid contributed a synergistic effect in the re-
cause its structure was suited for the proposed mecha- actions in which nitroolefins were involved.[8b,11]
nism and it exhibited an efficient catalytic activity for
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Dan-Qian Xu et al.
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Table 1. Tandem oxa-Michael–Henry reaction catalyzed by
organocatalyst 1i and different acids as cocatalysts.[a]
dine-thioimidazole was the most effective catalyst in
terms of stereocontrol, providing a relatively high
yield and good enantioselection. In contrast to results
obtained with diamine catalysts, except 1e, reactions
with l-proline and l-prolinol, as well as diphenylproli-
nol 1c and its derivative 1d only provided racemic
products (Table 2, entrie 1–4).
Entry Co-catalyst
Time
[h]
Yield[b]
[%]
ee[c]
[%]
Table 2. Tandem oxa-Michael–Henry reaction catalyzed by
organocatalyst 1 and salicylic acid.[a]
1
2
3
4
5
6
7
8
none
48
96
96
96
96
96
96
96
96
96
96
96
96
93
32
22
10
17
23
15
34
47
42
40
44
25
35
5
HBr (0.5 equiv. of 1i)
HBr (0.75 equiv. of 1i)
HBr (1.0 equiv. of 1i)
HBF4
HPF6
CH3COOH
CF3COOH
CH3SO3H
C6H5COOH
p-CH3-C6H4COOH
p-NO2-C6H4COOH
p-CH3-C6H4SO3H
2-(naphthalen-1-yl)ace- 96
tic acid
56
78
62
53
46
51
57
62
51
52
72
70
73
9
10
11
12
13
14
Entry
Catalyst
Time[h]
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
1a[d]
1b
1c
1d
1e
1f
96
96
96
96
96
96
96
96
13
21
17
14
<5
30
47
36
0
0
0
0
15
naphthalene-1-sulfonic
acid
96
33
62
nd[e]
70
76
72
16
salicylic acid
96
67
85
1g
1h
[a]
Unless otherwise specified, all reactions were carried out
with 4-methoxy salicylaldehyde (1 mmol), b-nitrostyrenes
(0.5 mmol), the catalyst 1i (0.1 mmol, 20 mol%) and the
specified co-catalyst (0.075 mmol, 15 mol%) in DMSO
(0.5 mL) at room temperature.
[a]
Unless otherwise specified, all reactions were carried out
with 4-methoxy salicylaldehyde (1 mmol), b-nitrostyrene
(0.5 mmol), the catalyst 1 (0.1 mmol, 20 mol%) and sali-
cylic acid (0.075 mmol, 15 mol%) in DMSO (0.5 mL) at
room temperature.
[b]
[c]
Isolated yield.
Determined by chiral HPLC analysis (Daicel Chiralpak
AS-H, hexane/i-PrOH=90/10).
[b]
[c]
Isolated yield.
Determined by chiral HPLC analysis (Daicel Chiralpak
AS-H, hexane/i.PrOH=90/10).
With no acid added.
[d]
[e]
To search for more optimal catalysts, we synthe-
sized and screened a number of structurally related
amines (Figure 3). However, we found that pyrroli-
nd means not determined.
Having established the best catalyst/cocatalyst
system for the domino oxa-Michael–Henry reaction,
we next investigated the impact of different reaction
media on the efficiency of this process. The results in-
dicated that the solvents played a significant role in
this reaction (Table 3). Reactions in polar solvents
such as CH3OH, EtOH, i-PrOH, DMF, DMSO
(Table 3, entries 1–5), generally proceeded in relative-
ly high yields, whereas those in less polar solvents
(Et2O, CH2Cl-CH2Cl, CH3CN, THF and toluene) took
place in low yields or even did not react at all
(Table 3, entries 6–10). This is presumably due to the
polar solventsꢀ ability to stabilize the intermediates
possessing partial charges as well as its dissolving
power for the reaction system. However, the reaction
in more polar medium (such as ionic liquids, Table 3,
entries 11 and 12) did not perform as well as expect-
ed.
Figure 3. Screened organocatalysts.
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A Novel Enantioselective Catalytic Tandem Oxa-Michael–Henry Reaction
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Table 3. Screening of the solvents for the tandem oxa-Mi-
chael–Henry reaction catalyzed by organocatalyst 1i and sal-
icylic acid.[a]
Entry
Solvent
Time[h]
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
CH3OH
EtOH
i-PrOH
DMF
DMSO
Et2O
CH2ClCH2Cl
CH3CN
THF
96
96
96
96
96
96
96
96
96
96
96
96
25
14
10
20
70
76
82
72
85
nd
nd
nd
nd
nd
nd
nd
Figure 4. The theoretical simulated ECD spectrum (dotted
trace) for the R-configuration of compound 4f by the means
of the TD-DFT/B3LYP/6-31+G* method, compared to the
experimental spectrum (full trace).
67
<10
<10
<10
<10
0
9
10
11
12
Toluene
G
ACHTUNGTRENNUNG
<10
<10
spectra of 4f matched with the experimental data
very well when it was assumed as R.
[a]
All reactions were carried out with 4-methoxy salicylal-
dehyde (1 mmol), b-nitrostyrene (0.5 mmol), the catalyst
1i (0.1 mmol, 20 mol%) and salicylic acid (0.075 mmol,
15 mol%) in the specified solvent (0.5 mL) at room tem-
perature.
Based on the observed absolute configurations of
the products and the discussion above, a mechanism
as shown in Figure 2 was proposed to explain the ob-
served results. For the proof of its rationality, the exis-
tence of intermediates A and C in the reaction mix-
ture was confirmed using the ESI-MS method
(Figure 5).
[b]
[c]
Isolated yield.
Determined by chiral HPLC analysis (Daicel Chiralpak
AS-H, hexane/i-PrOH=90/10).
Among the two possible orientations of the aromat-
ic iminium, the TS-B is favored (Figure 6), as the un-
The scope and limitation of this catalytic system favorable steric interaction between the phenyl group
were explored next by using a wide range of salicylal- of salicylaldehyde and the catalyst backbone is avoid-
dehydes and b-nitrostyrenes. Use of salicylaldehyde ed, which constitutes the chiral environment for the
with no substitution gave the chromene derivatives in oxa-Michael addition to the re face of the b-nitrostyr-
relatively high yields and moderate enantioselectivi- ene, leading to the observed R products, while the dis-
ties (Table 4, entries 1–4). When introducing the sub- favored TS-B’ may lead to the S products.
stituted salicylaldehydes (Table 4, entries 5–8), the
The involvement of both the organocatalyst and the
domino reactions afforded the corresponding prod- cocatalyst in this asymmetric tandem oxa-Michael–
ucts with higher enantioselectivites, which demon- Henry reaction was quite crucial as neither 1i nor sali-
strated the reactions were more selective with the cylic acid alone could catalyze the formation of chiral
steric interaction increasing. For example, 4-methoxy- 3-nitro-2H-chromenes efficiently. The organocatalyst
salicylaldehyde was found to be a competent substrate 1i may not only serve in the role of asymmetric imini-
for this transformation to give the products in a good um activation, but also in the role of a Lewis base to
selectivity. The corresponding products were in fact facilitate the deprotonation of the salicylaldehyde so
obtained in good enantioselectivites (61–91%), irre- as to promote the Michael addition to the b-nitrostyr-
spective of the electronic properties of the substitu- ene. The salicylic acid may also have a dual role in
ents at the aromatic ring of the b-nitrostyrene the process, which might favor the formation of the
(Table 4, entries 9–14).
aromatic iminium as well as serve for a hydrogen-
To determine the absolute configuration, TD-DFT bonding interaction between the nitro oxygen of 3
calculations of the electronic circular dichroism and its hydroxy group (Figure 6, TS-B).
(ECD) spectra were implemented in the Gaussian 03
program.[12] As show in Figure 4, the theoretical ECD
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Dan-Qian Xu et al.
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Table 4. Scope of tandem oxa-Michael–Henry reactions.[a]
Entry
R1
R2
Product 4
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
H
H
H
H
Ph
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
72
81
87
51
78
67
32
72
57
46
37
52
47
35
51
45
52
53
77
85
48
62
69
61
91
71
72
70
4-MeC6H4
4-MeOC6H4
naphthalen-2-yl
Ph
Ph
Ph
3-MeO
4-MeO
5-MeO
5-Cl
4-MeO
4-MeO
4-MeO
4-MeO
4-MeO
4-MeO
Ph
9
4-ClC6H4
4-BrC6H4
2-MeOC6H4
3-MeOC6H4
4-MeOC6H4
naphthalen-2-yl
10
11
12
13
14
[a]
All reactions were carried out with 2 (1 mmol), 3 (0.5 mmol), the catalyst 1i (0.1 mmol, 20 mol%) and salicylic acid
(0.075 mmol, 15 mol%) in DMSO (0.5 mL) at room temperature for 96 h.
[b]
Isolated yield.[c] Determined by chiral HPLC analysis (Daicel Chiralpak AS-H or OD-H).
Figure 5. ESI-mass spectra: the existence of intermediates A and C in the reaction mixture was confirmed.
Conclusions
and the cocatalyst salicylic acid through the first re-
ported aromatic iminium activation, serves as an effi-
In summary, the new organocatalytic tandem oxa-Mi- cient method for the preparation of chiral 3-nitro-2H-
chael–Henry reaction, promoted by organocatalyst 1i chromenes with moderate to good enantioselectivites.
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A Novel Enantioselective Catalytic Tandem Oxa-Michael–Henry Reaction
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Figure 6. The proposed transition states in the tandem oxa-
Michael–Henry reaction.
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Experimental Section
Typical Procedure for the Organocatalytic
Asymmetric Oxa-Michael–Henry Reaction
After stirring a solution of catalyst 1i (0.1 mmol, 20 mol%)
and salicylic acid (0.075 mmol, 15 mol%) in DMSO
(0.5 mL) at 258C for 1 h, 4-methoxysalicylaldehyde (0.152 g,
1 mmol) and b-nitrostyrene (0.075 g, 0.5 mmol) were added
sequentially. After being stirred at 258C for 96 h, the reac-
tion mixture was quenched with distilled water (10 mL), and
extracted with EtOAc (3ꢃ10 mL). The organic layer was
dried over anhydrous Na2SO4, and evaporated. The crude
product was purified by flash chromatography to furnish a
yellow solid in 67% yield. The enantiomeric excess was de-
termined by HPLC with a Chiralpak AS-H column (hex-
ACHTUNGTRENNUNG ; tR major
ane:i-PrOH=90:10; flow rate 1.0 mLminÀ1
isomer=14.2 min, tR minor isomer=21.1 min).
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 20772110), the National Basic Re-
search Program of China (2003CB114400) and the Program
for Changjiang Scholars and Innovative Research Team in
University of China.
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Dan-Qian Xu et al.
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Adv. Synth. Catal. 2008, 350, 2610 – 2616