Chiral thiophosphoramidate-catalyzed asymmetric michael addition of ketones to nitro olefins

Article abstract of DOI:10.1002/ejoc.201000069

A novel type of pyrrolidine-based chiral (thio)phosphoramidates was synthesized. Among them, compound (S,aR)-3d was proven to be an effective bifunctional organocatalyst for the asymmetric Michael addition of ketones to nitro olefins. The corresponding adducts were obtained in good to excellent chemical yields with high levels of diastereo- and enantioselectivities (up to >99:1 dr and 99%ee).

Full text of DOI:10.1002/ejoc.201000069

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000069
Chiral Thiophosphoramidate-Catalyzed Asymmetric Michael Addition of
Ketones to Nitro Olefins
Aidang Lu,[a] Ronghua Wu,[a] Youming Wang,*[a] Zhenghong Zhou,*[a] Guiping Wu,[a]
Jianxin Fang,[a] and Chuchi Tang[a]
Keywords: Diastereoselectivity / Enantioselectivity / Michael addition / Asymmetric catalysis / Organocatalysis
A novel type of pyrrolidine-based chiral (thio)phosphoramid-
ates was synthesized. Among them, compound (S,aR)-3d was
proven to be an effective bifunctional organocatalyst for the
asymmetric Michael addition of ketones to nitro olefins. The
corresponding adducts were obtained in good to excellent
chemical yields with high levels of diastereo- and enantio-
selectivities (up to Ͼ99:1dr and 99%ee).
“privileged” pyrrolidine unit could result in a potential bi-
functional organocatalyst. In this type of catalyst, the pyr-
rolidine backbone can serve as a catalytic site and the
(thio)phosphoramide moiety can function as a hydrogen-
bonding donor to activate the nitro olefins. Generally, very
subtle changes in the catalyst structure can often lead to
large and unpredictable differences in the performance of
the catalyst, especially in terms of enantioselectivity. The
great advantage of this type of organocatalyst is that the
diversity of phosphorus compounds offers an opportunity
to finely tune the catalytic activity by changing the substitu-
ents on the phosphorus atom. Herein, we report the asym-
metric organocatalytic Michael addition of cyclic ketones
to nitro olefins, which is promoted by a simple bifunctional
organocatalyst bearing a simple amino thiophosphoramide.
Introduction
The Michael reaction of carbon-centered nucleophiles to
nitro olefins represents a direct and most appealing ap-
proach to nitroalkanes, which are versatile synthetic inter-
mediates owing to the various possible transformations of
the nitro group into other useful functional groups. The de-
velopment of organocatalytic asymmetric versions of such
processes has been extensively investigated in recent years.[1]
Among them, the direct Michael addition of ketones to ni-
troalkenes offers particularly attractive access to syntheti-
cally valuable γ-nitro ketones in an atom-economical man-
ner. Inspired by the pioneering work of List and Barbas at
the beginning of this century,[2] a great number of pyrrol-
idine-type organocatalysts have been reported for this type
of transformation.[3] The cyclic five-membered secondary
amine structure of these compounds is now regarded as one
of the “privileged” backbones for asymmetric organocata-
lysis. Among them, organocatalysts that consist of an ap-
propriate combination of a hydrogen-bonding donor moi-
ety and the “privileged” pyrrolidine unit have been proven
to be effective in the Michael addition of ketones to nitro
olefins, which is due to their strong activation of the nitro
groups through efficient hydrogen-bonding interac-
tions.[3c,3d,3g,3i,3k,3y,3z,4] Notably, N-triflyl phosphoramides,
which contain an acidic hydrogen, also represent a novel
fine-tuning class of strong Brønsted acid organocatalysts
for organic transformations.[5] Therefore, it is anticipated
that the acidic hydrogen of simple (thio)phosphoramides
can also function as a hydrogen-bonding donor, and the
combination of the N-(thio)phosphoramide moiety with the
Results and Discussion
The trifluoroacetic acid salts of newly designed pyrrol-
idine–(thio)phosphoramidates 3 were easily prepared by the
coupling of (S)-tert-butyl 2-(aminomethyl)pyrrolidine-1-
carboxylate (1)[3w] with the corresponding phosphoryl chlo-
rides 2 as shown in Scheme 1.
In control reactions, it was found that the in situ gener-
ated catalyst gave results that were comparable to those of
the free-base catalyst (Table 1, Entry 2 vs. 1). So we kept
these catalysts in their trifluoroacetate form and employed
the in situ generated free base as the catalyst with the ad-
dition of an equivalent of organic base, such as triethyl-
amine. Then, rough examination of the effect of the substit-
uents on the phosphorus atom of 3 was carried out by em-
ploying the reaction of β-nitrostyrene (4a) with cyclohex-
anone as a model. As shown in Table 1, it is noteworthy that
not only the steric demand of the substituents but also their
electronic nature had a prominent effect on both the cata-
lytic activity and the stereoselectivity. When O,O-diethyl
[a] State Key Laboratory of Elemento-Organic Chemistry, Institute
of Elemento-Organic Chemistry, Nankai University,
Tianjin 300071, P. R. China
Fax: +86-22-23508939
E-mail: z.h.zhou@nankai.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000069.
Eur. J. Org. Chem. 2010, 2057–2061
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Lu, R. Wu, Y. Wang, Z. Zhou, G. Wu, J. Fang, C. Tang
Table 1. Catalyst screening.[a]
SHORT COMMUNICATION
Entry
Catalyst
Time [h] Yield [%][b] dr [syn/anti][c] ee [%][d]
1
2
3
4
5
6
7
3a/CF3CO2H
6
8
72
12
1
95
92
48
88
Ͼ99
82
99:1
94:6
78:22
98:2
99:1
91:9
92:8
77
79
77
81
90
63
74
3a
3b
3c
(S,aR)-3d
(S,aS)-3d
3e
48
20
95
[a] All reactions were carried out by using cyclohexanone (226 mg,
2.3 mol) and 4a (0.23 mmol) in the presence of catalyst 3. [b] Yield
of the isolated product after chromatography on silica gel. [c] De-
termined by 1H NMR spectroscopy. [d] Determined by chiral
HPLC analysis.
Scheme 1. Synthesis of catalysts.
reaction. Almost the same level of enantioselectivity was
thiophosphoramidate 3a was employed as the catalyst, the observed for benzoic acid (Table 2, Entry 4; 90%ee), bulky
reaction was complete in 8 h, and the corresponding adduct pivalic acid (Table 2, Entry 5; 89%ee), and chiral mandelic
was obtained with excellent diastereoselectivity (syn/anti = acid [Table 2, Entries 6 and 7; 90%ee for (S)-enantiomer
94:6) and good enantioselectivity (79%ee), whereas catalyst and 89%ee for (R)-enantiomer]. Considering both the reac-
3b with phenoxy groups proved to be poor in terms of turn- tion rate and diastereoselectivity, benzoic acid is the best
over frequency and diastereoselectivity (Table 1, Entry 2 vs. choice. The reaction was complete within only 1 h with ex-
3). An improvement in both the catalytic activity and the cellent diastereoselectivity (Table 2, Entry 4). Moreover, the
stereoselectivity was observed when the phenoxy groups reaction temperature was found to be an essential factor to
(3b) were replaced with phenyl groups (3c) (Table 1, Entry 3 the enantioselectivity of this reaction. The stereoselectivity
vs. 4). With respect to selectivity, the best result was ob- gradually increased with a decrease in the reaction tempera-
tained with bulky catalyst (S,aR)-3d bearing an (R)-bi- ture from 25 to –30 °C (Table 2, Entries 4, 9, 10, and 11;
naphthyl skeleton (Table 1, Entry 5; 99:1dr, 90%ee). How- 90–96%ee). Although the same excellent selectivity was ob-
ever, much lower catalytic activity and stereocontrol were tained upon lowering the temperature further to –40 °C, the
observed for the corresponding oxo analogue 3e. This may reaction became quite sluggish to afford the product in
be attributed to the decrease in the acidity of the hydrogen rather low yield even with a prolonged reaction time
upon substitution of the sulfur in the P=S bond with oxy- (Table 1, Entry 12). In addition, reducing the amount of
gen.[6] Under otherwise identical conditions, bifunctional (S,aR)-3d from 20 mol-% to 10 mol-% resulted in an obvi-
thiophosphoramidate (S,aS)-3d derived from (S)-bi- ous decrease in both the turnover frequency and the stereo-
naphthol demonstrated much lower catalytic activity. The selectivity (Table 2, entry 8).
reaction became quite sluggish, and a decreased yield of
With optimized reaction conditions in hand, a variety of
82% was attained at a prolonged reaction time. Moreover, nitro olefins with different structures were investigated, and
a dramatic decrease in both the diastereoselectivity and the the results are summarized in Table 3. Various styrene-type
enantioselectivity were recorded (Table 1, Entry 6 vs. 5). nitro olefins reacted smoothly with cyclohexanone to pro-
This indicates that the (R)-configuration of binaphthol vide the corresponding adducts in good to excellent yields
matched (S)-2-(aminomethyl)pyrrolidine to enhance the with excellent diastereoselectivities and enantioselectivities
catalytic activity of the catalyst.
(Table 3, Entries 1–12). Generally, the nature of the substit-
Having identified bulky (S,aR)-3d to be the best catalyst uent on the benzene ring exhibited a slight influence on the
for the reaction, other factors influencing the reaction were reaction. Nitrostyrene bearing either electron-withdrawing
further thoroughly investigated. The results are summarized or electron-donating substituents on the benzene ring pro-
in Table 2. A survey of six acidic cocatalysts revealed that vided high syn selectivity (up to Ͼ99:1) as well as enantio-
the acidic cocatalyst has an important influence on the re- selectivity (up to Ͼ99%ee). The excellent enantioselectivity
action. The addition of 10 mol-% of carboxylic acid as co- achieved for heteroaryl nitro olefin 4l (Table 3, Entry 12)
catalyst significantly accelerated the reaction rate and en- indicates that it is also a good Michael acceptor for cyclo-
hanced the selectivity relative to that with the catalyst in hexanone. Furthermore, alkenyl-substituted nitro olefin 4m
the absence of an acidic cocatalyst (Table 2, Entries 2–7 vs. can also be employed, albeit with a slight decrease in dia-
entry 1). In terms of enantioselectivity, a variety of carbox- stereo- and enantioselectivity (Table 3, Entry 13). Notably,
ylic acid cocatalysts were tolerated by this Michael addition aliphatic aldehyde derived nitro olefin 4n also appeared to
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Eur. J. Org. Chem. 2010, 2057–2061

Thiophosphoramidate-Catalyzed Asymmetric Michael Additions
Table 2. Optimization of reaction conditions.[a]
Entry
(S,aR)-3d [mol-%]
Cocatalyst [10 mol-%]
Temp. [°C]
Time [h]
Yield [%][b]
dr [syn/anti][c]
ee [%][d]
1
2
3
4
5
6
7
8
20
20
20
20
20
20
20
10
20
20
20
20
–
25
25
25
25
25
25
25
25
0
48
2
2
43
92
98
Ͼ99
Ͼ99
91
92
Ͼ99
96
95
96
76:24
97:3
86:14
99:1
98:2
98:2
98:2
90:10
97:3
Ͼ99:1
Ͼ99:1
Ͼ99:1
37
79
82
90
89
90
89
85
92
93
96
96
CH3CO2H
PrCO2H
PhCO2H
1
tBuCO2H
(S)-mandelic acid
(R)-mandelic acid
PhCO2H
4.5
4.5
1
9
4
10
22
63
9
PhCO2H
PhCO2H
PhCO2H
PhCO2H
10
11
12
–15
–30
–40
60
[a] All reactions were carried out by using cyclohexanone (226 mg, 2.3 mol) and 4a (0.23 mmol) in the presence of catalyst 3. [b] Yield of
the isolated product after chromatography on silica gel. [c] Determined by 1H NMR spectroscopy. [d] Determined by chiral HPLC
analysis.
be a good candidate at an elevated temperature (Table 3, ing cyclopentanone worked well to provide desired product
Entry 14; syn/anti = 73:27, 91%ee for syn isomer), which 6 in 85% yield with excellent enantioselectivity (syn/anti =
clearly demonstrated the broad generality of this asymmet- 86:14, 92%ee for syn-6 and 99%ee for anti-6) at room tem-
ric Michael addition reaction.
perature. Other cyclic ketones such as cyclohexane-1,4-di-
one monoethylene acetal also reacted smoothly with 4a in
THF at –30 °C and excellent stereoselectivities were main-
tained. Moreover, the desymmetrization of prochiral 4-
methylcyclohexanone could also be realized under identical
conditions to give the desired Michael adducts bearing
three carbon stereocenters. In this case, the reaction went
to completion in 68 h, affording 90% yield, 79:21dr, and
96%ee for the major isomer.
Table 3. Substrate scope of (S,aR)-3d-catalyzed asymmetric
Michael addition of cyclohexanone to nitro olefins.[a]
Entry
R
Time Yield
[h]
dr
ee
[%][b] [syn/anti][c] [%][d]
1
2
3
4
5
6
7
8
9
10
Ph (a)
22
21
23
19
21
21
21
19
19
Ͼ99
85
87
95
85
98
83
77
Ͼ99
82
Ͼ99:1
98:2
Ͼ99:1
99:1
96
96
99
97
98
97
98
96
97
98
3-CF3C6H4 (b)
4-CF3C6H4 (c)
4-FC6H4 (d)
4-ClC6H4 (e)
2-BrC6H4 (f)
4-BrC6H4 (g)
2-MeOC6H4 (h)
4-MeC6H4 (i)
98:2
Ͼ99:1
Ͼ99:1
99:1
99:1
Ͼ99:1
benzo[d][1,3]dioxol-5- 40
yl (j)
11
12
13
14
1-naphthyl (k)
2-furyl (l)
(E)-cinnamyl (m)
phenylethyl (n)
19
16
97
Ͼ99
92
Ͼ99:1
98:2
93:7
97
95
89
91
17[e]
4[e]
89
73:27
[a] All reactions were carried out by using cyclohexanone (226 mg,
2.3 mol) and 4a (0.23 mmol) in the presence of catalyst 3. [b] Yield
of the isolated product after chromatography on silica gel. [c] De-
termined by 1H NMR spectroscopy. [d] Determined by chiral
HPLC analysis. [e] The reaction was carried out at room tempera-
ture (25 °C).
The asymmetric additions of other ketones to nitrosty-
rene (4a) with the use of (S,aR)-3d as a catalyst were also
preliminarily investigated. As shown in Scheme 2, challeng- Scheme 2. Reaction of other ketones.
Eur. J. Org. Chem. 2010, 2057–2061
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2059

A. Lu, R. Wu, Y. Wang, Z. Zhou, G. Wu, J. Fang, C. Tang
SHORT COMMUNICATION
We envisioned that the acidic hydrogen of the (thio)phos- a bifunctional organocatalyst to promote the asymmetric
phoramide can function as a hydrogen-bonding donor in Michael reaction of ketones to nitro olefins. In the presence
the reaction. To testify to this hypothesis, the corresponding of the newly prepared catalyst, the reaction takes place
N-methylated compound 10 of catalyst (S,aR)-3d was syn- smoothly with excellent diastereo- (up to Ͼ99:1dr) and
thesized and examined under the same conditions for the enantioselectivity (up to 99%ee), which may provide a po-
Michael addition of cyclohexanone to the best suitable ni- tentially useful method for the preparation of enantiomer-
tro olefin 4c (Scheme 3). As shown in Scheme 3, obvious ically enriched γ-nitro ketones. Further investigations on
decreases in both the diastereo- and enantioselectivity were the application of this catalyst in asymmetric catalysis are
observed in this case; especially, it is worth noting that the in progress.
use of N-methylated catalyst 10 led to a dramatic decrease
in the reaction rate (reaction time: 23 vs. 80 h; only 25%
conversion of 4c was achieved after stirring for 23 h). In
Experimental Section
comparison to the results of catalyst 10, the observed sig-
General Procedure for Asymmetric Michael Addition of Ketones to
nificant rate acceleration for catalyst (S,aR)-3d may be at-
Nitro Olefins Catalyzed by (S,aR)-3d: A mixture of catalyst (S,aR)-
3d (0.046 mmol) and triethylamine (0.046 mmol) in cyclohexanone
(226 mg, 2.3 mmol) was stirred at room temperature for 30 min.
tributed to the capability of the acidic hydrogen to form a
hydrogen bond with the nitro group of the nitro olefin.
Then, benzoic acid (2.8 mg, 0.023 mmol) was added, and the reac-
tion mixture was stirred for 15 min. To the resulting mixture was
added nitro olefin (0.23 mmol) at the required temperature. After
the reaction was complete (monitored by TLC), the mixture was
purified by column chromatography on silica gel (200–300 mesh,
petroleum ether/EtOAc, 1:5) to afford the product.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, characterization of the prepared
compounds, copies of the NMR spectra, chiral HPLC spectra of
the Michael addition products.
Acknowledgments
Scheme 3. Michael addition catalyzed by catalyst 10.
We are grateful to the National Natural Science Foundation of
On the basis of the experimental results, a possible tran-
China (No. 20772058, 20972070) and the Key laboratory of Ele-
sition state for this reaction was proposed to account for
mento-Organic Chemistry for generous financial support of our
programs.
the observed high diastereo- and enantioselectivity. As
shown in Figure 1, the free base of (S,aR)-3d functioned as
a bifunctional catalyst. The pyrrolidine ring will first react
with a carbonyl compound to form an enamine with the aid
of an acidic cocatalyst. Subsequently, the acidic hydrogen as
well as the Brønsted acid additive will orientate the nitro
[2] a) B. List, P. Pojarliev, H. J. Martin, Org. Lett. 2001, 3, 2423–
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amine will act as a nucleophile and attack the nitro olefin
from the Re face to give the highly enantio- and diastereo-
selective product. This explanation is consistent with the
experimental results.
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In conclusion, we have developed a novel pyrrolidine-
based thiophosphoramidate catalyst, which worked well as
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Eur. J. Org. Chem. 2010, 2057–2061

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Published Online: March 2, 2010
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Eur. J. Org. Chem. 2010, 2057–2061
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