Asymmetric michael reaction of acetaldehyde with nitroolefins catalyzed by highly water-compatible organocatalysts in aqueous media

Article abstract of DOI:10.1002/adsc.201200215

A novel category of diarylprolinol silyl ether catalysts which contain different lengths of alkylamine tags, was designed and synthesized. These catalysts were used, along with benzoic acid as the co-catalyst, to catalyze the asymmetric Michael reaction of the highly reactive acetaldehyde with nitroolefins. For the reactions studied, this catalytic system exhibited high reactivity in brine without any organic solvents; good yields (30-61%) and high enantioselectivities (80-97% ee) for a wide variety of nitrostyrenes were obtained.

Full text of DOI:10.1002/adsc.201200215

UPDATES
DOI: 10.1002/adsc.201200215
Asymmetric Michael Reaction of Acetaldehyde with Nitroolefins
Catalyzed by Highly Water-Compatible Organocatalysts in
Aqueous Media
Yupu Qiao,a Junpeng He,a Bukuo Ni,a,* and Allan D. Headleya,*
a
Department of Chemistry, Texas A&M University – Commerce, Commerce, TX 75429-3011, USA
Fax : (+1)-903-468-6020; e-mail: bukuo.ni@tamuc.edu or allan.headley@tamuc.edu
Received: March 30, 2012; Revised: June 19, 2012; Published online: September 28, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200215.
Abstract: A novel category of diarylprolinol silyl
ether catalysts which contain different lengths of al-
kylamine tags, was designed and synthesized. These
catalysts were used, along with benzoic acid as the
co-catalyst, to catalyze the asymmetric Michael re-
action of the highly reactive acetaldehyde with ni-
troolefins. For the reactions studied, this catalytic
system exhibited high reactivity in brine without
any organic solvents; good yields (30–61%) and
high enantioselectivities (80–97% ee) for a wide va-
riety of nitrostyrenes were obtained.
a nucleophile and electrophile and, as a result, has
the potential to generate various side products such
as multi/poly-aldolization, dehydration, and polymeri-
zation type products.[5]
In 2008, the List and Hayashi groups independently
reported for the first time the asymmetric Michael ad-
dition of acetaldehyde in which diarylprolinol silyl
ether was used as the catalyst.[6] Subsequently, re-
newed interest was generated in Michael reactions in
which acetaldehyde is utilized. Several reactions in-
volving acetaldehyde and nitroolefins were also
achieved by the application of other types of secon-
dary amine organocatalysts, which are typically car-
ried out in organic solvents and with enzymes.[7] The
development of water-compatible asymmetric organo-
catalysts is a growing area of research and many have
been developed and applied to a wide range of organ-
ic transformations, in which the asymmetric products
Keywords: acetaldehyde; asymmetric synthesis; Mi-
chael reaction; water-compatible organocatalysts
The use of organocatalysts to catalyze asymmetric re- are obtained with high stereoselectivities.[8–9]
actions has received much attention in the past Recently, our research efforts have focused on the
decade. Organocatalysts are highly effective, relative- development of water-compatible organocatalysts that
ly non-toxic, environmentally friendly, and they re- are effective in aqueous media.[10] In addition to the
quire mild reaction conditions. As a result, they are obvious advantages of the ease of product isolation
now an integral component of asymmetric catalysis.[1] from such media, water is an environmentally benign
For the broad spectrum of reactions that are essential solvent and economical. Based on its unique physical
for organic synthesis, the catalyzed asymmetric Mi- properties, it is a desirable medium in which to carry
chael reaction of aldehydes with nitroolefins is espe- out asymmetric organocatalysis.[11] In 2010, our re-
cially important.[2] This type of reaction is especially search group reported for the first time the design
important because it is one of the most powerful tools and synthesis of a new category of highly active orga-
for carbon-carbon bond formation and affords syn- nocatalysts diarylprolinol silyl ether
2 and 3a
thetically useful g-nitro carbonyl compounds with ex- (Table 1), which were shown to be highly water-com-
cellent diastereoselectivities both in nature and in patible organocatalysts and effective for the asymmet-
modern organic synthesis.[3] Recently, different types ric Michael addition of aldehydes to nitroolefins in
of organocatalysts have been developed and this has which high diastereo- and enantioselectivities were
resulted in high reactivities and enantioselectivities obtained.[10a]
for this type of transformation.[4] Nevertheless, the
In this paper, we report the results of further inves-
direct Michael reaction involving acetaldehyde, which tigation of these catalysts and their applications to
is the simplest enolizable carbonyl compound, is a wider scope of reactions. This category of diarylpro-
known to be a difficult reaction. Acetaldehyde is linol silyl ether organocatalysts was applied to the Mi-
a highly reactive compound since it can act both as chael reaction involving the highly reactive acetalde-
Adv. Synth. Catal. 2012, 354, 2849 – 2853
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Yupu Qiao et al.
UPDATES
Table 1. Screening and optimization of reaction media and organocatalysts for the Michael reaction.[a]
Entry
Catalyst
Solvent
Yield [%][b]
ee [%][c]
1[d]
2
3
4
5
6
7
8
9
10
11
12
13
14
15[e]
16[e]
1
2
brine
brine
brine
brine
brine
H2O
CH3CN
DMF
23
16
52
22
25
20
trace
10
20
41
19
42
23
24
57
29
90
90
90
89
79
90
–
93
91
79
90
90
90
90
91
90
3a
3b
3c
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
1,4-dioxane
H2O:1-propanol (1:30)
H2O:1-propanol (1:20)
brine: 1,4-dioxane (1:30)
brine:2-propanol (1:40)
brine:2-propanol (1:10)
brine
no solvent
[a]
The reactions were carried out with nitroalkene (0.5 mmol), acetaldehyde (5 mmol), benzoic acid (0.05 mmol) and cata-
lyst (0.05 mmol) in brine (500 mL) at 48C for 22 h.
Isolated yield.
Determined by chiral HPLC.
No acid was added.
[b]
[c]
[d]
[e]
10 mmol of acetaldehyde was used.
hyde and nitroolefins in various aqueous media and entry 2). To our delight, the catalyst 3a that contains
the results are shown in Table 1. Catalysts 2 and 3 the dimethylamine functionality was found to give the
(structures shown in Table 1) were synthesized from best results for this Michael reaction, providing the
the “chiral pool” using l-proline as a starting material desired product in 52% yield and 90% ee (Table 1,
using established procedures.[10a]
entry 3). As shown in Table 1, catalysts 3b and 3c can
The reaction of acetaldehyde with b-nitrostyrene also accomplish this transformation, but gave poor
was selected as our initial focus, and catalyst screen- yields (Table 1, entries 4 and 5), this may be due to
ing and optimization of reaction conditions were per- the longer alkyl groups, which resulted in poor solu-
formed; the results for these highly water-compatible bility in the reaction medium studied.
diarylprolinol silyl ethers when used as catalysts for
With the optimum catalyst in hand, 3a, other sol-
this asymmetric Michael reaction are shown in vents were examined in order to further improve the
Table 1. When diarylprolinol silyl ether 1 which was reactivity and enantioselectivity. When the reaction
the best catalyst in previous reports was used as the was performed in pure water, only 20% yield was ob-
catalyst, the reaction proceeded slowly in brine, with tained with a high enantioselectivity 90% (Table 1,
23% yield and 90% ee (Table 1, entry 1). When the entry 6). Other organic solvents, such as acetonitrile,
catalysts 2 and 3 were used for the screening, along 1,4-dioxane and DMF, which are typically excellent
with the co-catalyst benzoic acid (10 mol%) in brine, solvents for these type reactions all gave poor yields
catalyst 2 gave the final product in even lower yield, (Table 1, entries 7–9). In addition, different types and
16%, but high enantioselectivity 90% (Table 1, ratios of solvent mixtures were also examined, but no
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Asymmetric Michael Reaction of Acetaldehyde with Nitroolefins
real improvement in the yield or selectivity was ob- tions with those of known compounds. The high enan-
served (Table 1, entries 10–14). It was observed, how- tioselecitivities of this water-soluble diarylprolinol
ever, that when the amount of acetaldehyde was in- silyl ether catalyst for the Michael additions can be
creased to 10 mmol and the reaction carried out in explained by the formation of a polar transition state,
brine, the yield was increased to 57% with a slightly similar to that described previously.[6b]
increased enantioselectivity 91% (Table 1, entry 15).
In conclusion, we have developed a novel highly
Interestingly, in the absence of brine, the yield was water-soluble diarylprolinol silyl ether asymmetric
greatly reduced to 29%, highlighting the importance catalytic system. This catalytic system is very effective
of a polar medium, such as brine, for the reaction to for the enamine-based Michael additions of the very
proceed using this highly water-compatible catalytic reactive acetaldehyde with nitroolefins. These reac-
system (Table 1, entry 16).
tions occur readily in brine with no need of any or-
Encouraged by these results, the scope of the reac- ganic solvents, to give good yields and high enantiose-
tion with different substituted nitroolefins was exam- lectivities for a broad range of nitroolefins. Further
ined using catalyst 3a in brine. As shown in Table 2, development of other types of water-cpmpatible orga-
relatively good yields and high enantioselectivities nocatalysts and their application to a wider scope of
were obtained when the reactions were carried out at reactions in aqueous media are currently underway in
room temperature using brine as the solvent. Also, our laboratory.
both electron-deficient and electron-rich nitrostyrenes
afforded the good results (Table 2, entries 2–8). More-
over, a heteroatom-containing aryl-substituted nitroal-
kene could also be used as a suitable substrate to give
Experimental Section
the Michael product in moderate yield and high enan-
tioselectivity (Table 2, entry 9). Furthermore from en-
Typical Procedure for the Michael Addition
To the solution of brine (0.5 mL) was added nitrostyrene
(0.5 mmol), acetaldehyde (10 mmol), catalyst 3a
tries 10 and 11, Table 2, it is noted that this catalytic
system is also highly effective for conjugate addition
of acetaldehyde to alkyl-substituted nitroalkenes pro-
viding the desired products 5j and 5k in 30–34% yield
and high to excellent enantioselectivities 88–97%.
These results demonstrate that this catalytic system is
highly efficient and versatile for a broad scope of Mi-
chael reaction.
(0.05 mmol) and benzoic acid (0.05 mmol) at room tempera-
ture. The resulting mixture was stirred at the indicated tem-
perature for the time indicated in Table 2 and then
quenched with 1M HCl. The products were extracted three
times with CH2Cl2 (3ꢁ10 mL). The combined organic phase
was dried over anhydrous Na2SO4 and concentrated under
reduced pressure to give the crude Michael product. To this
was added MeOH (3 mL) and NaBH4 (4 mmol), and the re-
The absolute configurations of the Michael adducts
were determined by comparison of the optical rota- sulting mixture was stirred at 08C for an additional 20 min
Table 2. Scope of catalyst 3a for the asymmetric Michael reaction involving acetaldehyde and various nitroalkenes.[a]
Entry
R
Time [h]
Product 5/Yield [%][b]
ee [%][c]
1[d]
2
3
4
5
6
7
8
9
C6H5
4-NO2-C6H4
2-Br-C6H4
22
60
36
60
60
72
96
96
72
72
72
5a/57
5b/42
5c/41
5d/40
5e/42
5f/56
5g/61
5h/40
5i/43
5j/30
5k/34
91
88
82
85
84
84
80
90
84
97
88
4-Cl-C6H4
4-CH3-C6H4
3-CH3O-C6H4
4-CH3O-C6H4
3,4-methylenedioxy-C6H3
2-furyl
10
11
C6H5CH2CH2
n-Bu
[a]
Reactions were carried out with nitroalkene (0.5 mmol), acetaldehyde (10 mmol), catalyst 3a (0.05 mmol), in brine
(0.5 mL) at room temperature.
Isolated yield.
Determined by chiral HPLC.
Reaction proceeded at 48C.
[b]
[c]
[d]
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Yupu Qiao et al.
UPDATES
before quenching by addition of water. The products were
extracted with CH2Cl2 (3ꢁ10 mL), and the combined organ-
ic phase was dried over anhydrous Na2SO4 and concentrated
under vacuum. The residue was purified by flash chromato-
graph on silica gel to afford the desired products 5a–k.
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Acknowledgements
We are grateful to the Robert A. Welch Foundation (T-1460)
for financial support of this research.
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