The application of a recyclable organocatalytic system to the asymmetric domino Michael/Henry reaction in aqueous media

Article abstract of DOI:10.1002/adsc.201100395

A new type of pyrrolidine-based organocatalyst has been developed and found to be very effective for the domino Michael/Henry reaction in aqueous solvents. For the reaction involving pentane-1,5-dial and various nitroolefins, good yields (65-85%) and exce

Full text of DOI:10.1002/adsc.201100395

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DOI: 10.1002/adsc.201100395
The Application of a Recyclable Organocatalytic System to the
Asymmetric Domino Michael/Henry Reaction in Aqueous Media
Poornima Chintala,a Subrata K. Ghosh,a Elizabeth Long,a Allan D. Headley,a,*
and Bukuo Nia,*
a
Department of Chemistry, Texas A & M University – Commerce, Commerce, TX 75429-3011, USA
Fax : (+1)-903-468-6020; e-mail: allan_headley@tamu-commerce.edu or bukuo_ni@tamu-commerce.edu
Received: May 16, 2011; Published online: November 7, 2011; Accepted: August 7, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100395.
Abstract: A new type of pyrrolidine-based organo-
catalyst has been developed and found to be very
effective for the domino Michael/Henry reaction in
aqueous solvents. For the reaction involving pen-
tane-1,5-dial and various nitroolefins, good yields
(65–85%) and excellent enantioselectivities (99%)
were obtained using this catalyst. In addition, the
catalyst could be recycled up to four times resulting
in good yields and up to seven times with good
enantioselectivity.
ronmentally benign, compared to typical organic sol-
vents.[4] As a result, carrying out such reactions in
aqueous media has many advantages owing to the
unique properties of water.
In recent years, significant progress has been made
towards the development of water-compatible organo-
catalysts for asymmetric organic synthesis.[5,6] Most of
these organocatalysts are designed as water-insoluble
catalysts and, as a result, they are not recyclable. The
development of recyclable organocatalysts that can be
used in aqueous solvents has been a very challenging
task. One aspect of our research involves the develop-
ment of environmentally novel organocatalysts for
asymmetric organic reactions in water.[7] We recently
reported for the first time the use of diarylmethylpro-
linol trimethylsilyl ether as water-soluble and recycla-
ble organocatalyst for the highly asymmetric Michael
addition of aldehydes to nitroolefins in water with ex-
Keywords: asymmetric catalysis; catalyst recycling;
domino reactions; Henry reaction; Michael addi-
tion
The domino formation of multiple carbon-carbon cellent diastereo- and enantioselectivities [Scheme 1,
bonds in a single step represents an effective ap- Eq. (1)].[7b] The major advantages of these organoca-
proach for the synthesis of complex molecules.[1] talysts are that they require low catalyst loading, are
Domino reactions have tremendous advantages in non-toxic, easy to handle, environmentally benign,
that they are economical and time saving. This type and their synthesis is straightforward. In addition,
of reaction represents one of the most effective meth- they are water-soluble, which makes it easy for water-
odologies for the formation of complex molecules in insoluble products to be isolated from the reaction
a single process. In a one-pot reaction without the iso- mixture; as a result, they are also recyclable organo-
lation of the intermediates, they yield several bonds catalysts.
in addition to multiple stereocenters by using an ef-
The aspect of using a recyclable water-soluble orga-
fective organocatalyst.[2] Recently, Hayashiꢀs group nocatalyst for asymmetric reactions, which are capa-
has developed a very effective enantioselective cata- ble of producing desired products in a single opera-
lytic domino Michael/Henry reaction using pentane- tion in an aqueous media, represents a major break-
1,5-dial and nitroolefins, which provided cyclohexane through in the field of organic synthesis. In this com-
derivatives as products with four stereogenic centers munication, we report an expanded use of this new
in excellent stereoselectivities by the use of diphenyl- organocatalyst for the domino Michael/Henry reac-
prolinolsilyl ether as organocatalyst.[3] The reaction tion of pentane-1,5-dial with nitroolefins in aqueous i-
only proceeded in organic solvents, however, and was PrOH to form optical active cyclohexane derivatives
not successful in water as solvent. Performing organic with excellent diastereo- and enantioselectivities and
reactions in aqueous media has many advantages be- control of four stereogenic centers [Scheme 1, Eq.
cause water is safe, economical, non-toxic, and envi- (2)]. A major advantage of this catalytic system is
Adv. Synth. Catal. 2011, 353, 2905 – 2909
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COMMUNICATIONS
Scheme 1, and it is obvious that there is a hydrophilic
aspect to the catalyst, which makes it soluble in water
in the presence of benzoic acid. In addition, it also
provides steric bulk to act as an effective catalyst to
induce high stereoselectivities for these reactions.
To examine the effectiveness of catalyst 1 with ben-
zoic acid as additive, the domino Michael/Henry reac-
tion between trans-b-nitrostyrene and pentane-1,5-
dial to afford cyclohexane derivatives 3a–d was per-
formed and the results are shown in Table 1.
For this reaction, it was observed that 3a is the
major diastereomer. When THF was used as solvent,
10 mol% of catalyst 1 in combination with 100 mol%
of benzoic acid as additive was used as the catalytic
system, the reaction proceeded smoothly to give the
product in 85% yield and 99% ee (entry 1). This
result is comparable with that found in the litera-
ture,[3] in which THF was used as the solvent, but the
diastereomeric ratio is lower. Since one of the main
goals of the research is to recycle the catalyst, the re-
action was carried out using water as a solvent with
Scheme 1. Water-soluble and recyclable organocatalyst 1-
catalyzed asymmetric reactions.
that in the presence of an acidic additive,[8] such as 10 mol% of catalyst and (100 mol%) of benzoic as ad-
benzoic acid, it forms an ammonium ionic liquid and ditive which gave moderate yield and selectivity in
this type of ionic liquid is known to have widespread 19 h (entry 2). A slight reduction in the polarity of the
applications.[9] In addition, it contains a tunable arm, solvent system was accomplished by using methanol,
which can be modified to give various properties to ethanol and i-PrOH, respectively (entries 3–5), and
the catalytic system. The catalytic system that is the solvent in which the reaction was completed
formed upon protonation of catalyst 1 is shown in within 5 h with an increased yield (76%) was i-PrOH
Table 1. Optimization of the Michael/Henry reaction conditions of pentane-1,5-dial to trans-b-nitrostyrene.[a]
Entry Solvent
Time [h] Pentane-1,5-dial [equiv.] PhCO2H [mol%] Yield [%][b] ee [%][b] 3a:3b:3c:3d[d]
1
2
3
4
5
6
7
8
THF
water
methanol
ethanol
4
3
3
3
3
3
3
3
2
1.3
1.3
1.1
1.3
100
100
100
100
100
100
100
100
100
60
85
67
57
65
76
82
81
80
81
82
57
80
99
99
97
98
99
99
99
98
99
98
98
99
41:30:25:4
56:24:14:6
59:21:16:4
47:30:19:4
52:24:18:6
69:15:10:7
69:16:10:6
74:15:6:5
72:15:8:5
75:11:10:4
68:11:16:5
63:16:17:4
19
12
9
5
4
5
4
4
4
i-PrOH
i-PrOH:H2O (1:3)
i-PrOH:H2O (1:4)
i-PrOH:H2O (1:4)
i-PrOH:H2O (1:4)
i-PrOH:H2O (1:4)
i-PrOH:H2O (1:4)
i-PrOH:H2O (1:4)
9
10
11
12
6
5
60
30
[a]
[b]
[c]
[d]
Reactions performed on a 0.4-mmol scale using catalyst 1, benzoic acid, pentane-1,5-dial, and solvent (0.5 mL).
Isolated yield.
Determined by chiral HPLC of the product.
1
Determined by H NMR.
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The Application of a Recyclable Organocatalytic System to the Asymmetric Domino Michael/Henry Reaction
Table 2. Reaction of pentane-1,5-dial with nitroolefines catalyzed by 1.[a]
Entry
R1
Time [h]
Product
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
Ph
4
3a
4a
5a
6a
7a
8a
9a
10a
11a
82
68
70
80
85
65
64
65
70
98
99
99
99
99
99
99
99
99
4-MeO-C6H4
4-Br-C6H4
3-MeO-C6H4
2-furyl
4-NO2-C6H4
3-Br-C6H4
3,4-methylenedioxy-C6H4
4-CH3-C6H4
18
12
9
7
27
48
24
7
[a]
Reactions performed on a 0.4-mmol scale using catalyst 1, benzoic acid, pentane-1,5-dial, and solvent (0.5 mL).
Isolated yield of major isomer.
Determined by chiral HPLC.
[b]
[c]
(entry 5). The reaction was next carried out in a tioselectivities only slightly. For nitroolefins with elec-
water/i-PrOH mixture (3:1) and it was completed tron-rich groups, the reaction proceeded smoothly to
within 4 h with an increased yield and selectivity afford the products 4a, 6a, and 10a–11a in excellent
(entry 6). Furthermore, a change in the solvent ratio enantioselectivity (99% ee) (entries 2, 4, 8 and 9). For
did not improve the yield and selectivity (entry 7). nitroolefins with electron-deficient groups, the prod-
When the amount of Michael donor pentane-1,5-dial ucts 5a, 8a and 9a were also obtained in moderate
was decreased from 3 equivalents to 2 and 1.3 equiva- yields (64–70%) with excellent enantioselectivity (up
lents, respectively, there is an increase in diastereosec- to 99% ee) (entries 3, 6 and 7). This catalytic system
tivity (entries 8 and 9). In addition, a decrease in alde- is also suitable for a heterocyclic substrate to give the
hyde resulted in a decreased yield and selectivity corresponding product 7a with excellent yield and
(entry 11). When the amount of benzoic acid is re- enantioselectivity (entry 5). However, when the ali-
duced from 100 mol% to 60 mol%, comparable re- phatic nitroolefins 1-nitro-1-hexene and 2-nitroeth-
sults were obtained (entry 10). Further decrease in
ACHTUNGTRENeNGNU nylcyclohexane were used as substrates; the reac-
the acid resulted in decreased diastereoselectivity tions appeared to work sluggishly and no products
(entry 12). On the basis of the results summarized in were isolated.
Table 1, the optimal set of conditions was obtained by
The recyclability of the catalytic system (10 mol%
using a mixture of i-PrOH:H2O (1:4) as solvent, of catalyst 1 was used) was studied in a reaction of
1.3 equivalents of pentane-1,5-dial, and 60 mol% of pentane-1,5-dial with trans-b-nitrostyrene under stan-
benzoic acid as additive with good yield (82% of 3a– dard reaction conditions. As soon as the reaction was
3d) and it was found that 3a was obtained as major completed, the product was extracted with
diastereomer (64%) with excellent enantioselectivity Et2O:hexane mixture (1:6). The recovered aqueous
(98% ee) and high diastereoselectivity (3a:3b:3c:3d= phase was reused for the next cycle by adding benzoic
75:11:10:4) (entry 10).
acid and the two substrates, pentane-1,5-dial and
These optimized reaction conditions were chosen to trans-b-nitrostyrene, and the results are shown in
study the scope of the domino Michael/Henry reac- Table 3. The catalyst could be recycled at least four
tion using different substituted nitroolefins in the times with slight reductions in yields; for cycles 3–6,
presence of 10 mol% of catalyst 1 with 60 mol% of there was a reduction in diastereoselectivity. For the
additive benzoic acid in i-PrOH:H2O (1:4) at room reaction, there was a slight decrease in enantioselec-
temperature to give the cyclohexyl derivative (3a– tivity fter the 4th cycle; and the conversion rate fell off
11a) in moderate to high yields with excellent enan- after the 6th cycle. From the results, it is observed that
tioselectivities (98–99% ee), the results are summar- catalyst 1 is an effective organocatalyst that can be re-
ized in Table 2. Good yields and excellent stereoselec- cycled in domino Michael/Henry reactions.
tivity were obtained for all substituted nitroolefins
The absolute stereochemistry of product 3a was de-
studied. It is obvious that the nature of the substitu- termined to be 1S,2S,3S,4R by comparing its optical
ents on the aryl group influences the yields and enan- rotation with literature values.[3] The absolute stereo-
Adv. Synth. Catal. 2011, 353, 2905 – 2909
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Table 3. Recycling studies of water-soluble 1-catalyzed reac-
tion of pentane-1,5-dial and trans-b-nitrostyrene.[a]
Typical Procedure for the Synthesis of 3–11
To a mixture of H2O:i-PrOH=4:1(v/v, 0.5 mL) was added
pentane-1,5-dialsolution (50 wt% in water, 0.52 mmol), ni-
troolefin (0.4 mmol), catalyst (0.04 mmol) and benzoic acid
(0.24 mmol). The reaction mixture was stirred at room tem-
perature for the time indicated in Table 2. The products
were extracted with Et2O:hexane=1:5 (v/v). The combined
organic phase was evaporated under reduced pressure. The
residue was purified by column chromatography on silica
gel to yield the desired products 3–11.
Entry Conversion Time Yield
ee
dr
[h]
[%][b]
[%][c] (3a:3b:3c:3d)[d]
1
Data for 3a (82% yield): H NMR (CDCl3, 400 MHz): d=
1.74–1.81 (m, 2H), 1.91–2.04 (m, 1H), 2.21–2.28 (m, 1H),
2.65–2.80 (m, 2H), 3.90 (t, J=12.0 Hz, 1H), 4.57 (br-s, 1H),
4.82 (dd, J=12.0 and 2.4 Hz, 1H), 7.21–7.35 (m, 5H), 9.40
(d, J=2.8 Hz, 1H); 13C NMR (CDCl3, 100 MHz): d=19.3,
30.2, 41.1, 54.0, 67.1, 92.6, 127.0, 127.9, 128.2, 133.7, 200.9;
HPLC: (Chiralpak AD-H, i-PrOH/hexanes=4/96, flow
1
2
3
4
5
6
7
100%
100%
100%
100%
100%
100%
70%
3
3
5
6
8
18
38
82
82
80
79
67
52
40
98
98
98
98
97
97
95
75:10:11:4
74:11:11:4
74:12:11:3
67:12:10:5
60: 24:11:5
53: 32:9:6
52:25:11:12
rate=1.0 mLminÀ1, l=205 nm):
135.1 min, ee=98%.
tmajor =118.9 min, tminor =
[a]
Reactions performed on a 0.4-mmol scale using catalyst
1, benzoic acid, pentane-1,5-dial, and solvent (0.5 mL).
Isolated yield of major isomer.
Determined by chiral HPLC.
Determined by H NMR.
[b]
[c]
[d]
Acknowledgements
1
We are grateful to the Robert A. Welch Foundation (T-1460)
for financial support for this research.
chemical results can be explained by a related transi-
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In conclusion, a new type of pyrrolidine-based or-
ganocatalyst has been developed and found to be
very effective for the domino Michael/Henry reaction
in aqueous solvents. For the reaction involving various
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enantioselectivities were obtained, and the catalyst
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