Highly enantioselective michael addition of acetone to nitro olefins catalyzed by chiral bifunctional primary amine-thiophosphoramide catalyst

Article abstract of DOI:10.1002/ejoc.201000945

A series of bifunctional primary amine-thiophosphoramides were synthesized, which proven to be effective organocatalysts for the asymmetric Michael reaction of acetone to both aryl and alkyl nitro olefins in the presence of phenol as a protic additive. The corresponding adducts were obtained in excellent chemical yields (up to >99%) with excellent enantioselectivities (up to 97% ee). A novel chiral phosphoramide functions well as an efficient bifunctional organocatalyst for the asymmetric Michael addition of acetone, one of the most challenge substrate in Michael addition, to nitro olefinsto afford the corresponding syntheticvaluable γ-nitro ketones in good to excellent yield with high levels of enantioselectivities (up to 97% ee, respectively).

Full text of DOI:10.1002/ejoc.201000945

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000945
Highly Enantioselective Michael Addition of Acetone to Nitro Olefins
Catalyzed by Chiral Bifunctional Primary Amine-Thiophosphoramide Catalyst
Aidang Lu,[a] Tao Liu,[a] Ronghua Wu,[a] Youming Wang,*[a] Zhenghong Zhou,*[a]
Guiping Wu,[a] Jianxin Fang,[a] and Chuchi Tang[a]
Keywords: Enantioselectivity / Michael addition / Nitro olefins / Chiral thiophosphoramide
A series of bifunctional primary amine-thiophosphoramides
were synthesized, which proven to be effective organocata-
lysts for the asymmetric Michael reaction of acetone to both
aryl and alkyl nitro olefins in the presence of phenol as a
protic additive. The corresponding adducts were obtained in
excellent chemical yields (up to Ͼ99%) with excellent
enantioselectivities (up to 97% ee).
antiomeric excesses of over 90% for the Michael product
with an acetone substrate.[5] Most recently, we have devel-
oped (S,aR)-pyrrolidine thiophosphoramide 1 as an ef-
ficient organocatalyst for promoting the asymmetric
Michael addition of cyclic ketones to nitro olefins.[6] It is
believed that its high catalytic efficiency is attributable to
its unique activation mode, arising from the thiophosphora-
mide moiety. This functionality serves as a hydrogen-bond-
ing donor for the activation of nitro olefin substrates and
formation of a well-controlled transition state. However,
when acetone was employed as the substrate, the reaction
is less enantioselective and only moderate stereocontrol was
observed (Scheme 1). Inspired by the proven ability of the
thiophosphoramide group in highly effective asymmetric
conjugate addition reactions, we envisioned that the incor-
poration of this moiety into a chiral trans-cyclohexanedi-
amine scaffold would result in a new bifunctional thiophos-
phoramide-primary amine organocatalyst 2, which could be
useful for effecting the Michael addition of challenging
acetone to nitro olefins.
Introduction
Asymmetric Michael addition reactions of different
carbon-centered nucleophiles to electron-deficient nitro ole-
fins represent a direct and most appealing approach to syn-
thetically valuable chiral nitro alkanes.[1] Among them, the
Michael additions of ketones to nitro olefins represent a
convenient access to γ-nitro ketones that are valuable build-
ing blocks in organic synthesis. Since the organocatalytic
asymmetric Michael addition of ketones to trans-β-nitro-
styrene was pioneered by Barbas[2] and List[3] indepen-
dently, great effort has been devoted to the development of
more selective and efficient catalytic systems for this syn-
thetically useful transformation. Impressive progress that
has been made recently in the development of bifunctional
organic catalysts for the enantioselective addition of cyclic
ketones, especially of cyclohexanone and its derivatives to
nitro alkenes.[4] Acetone is still one of the most problematic
substrates for the nitro-Michael addition. To the best of our
knowledge, only limited cases have been reported with en-
Scheme 1. (S,aR)-1-catalyzed asymmetric Michael addition of acetone to β-nitrostyrene.
[a] State Key Laboratory of Elemento-Organic Chemistry, Institute
Results and Discussion
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.201000945.
The newly designed primary amine-thiophosphoramides
2 were easily prepared as shown in Scheme 2. The conden-
sation of tert-butyl (1R,2R)-2-aminocyclohexylcarbamate 3
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A. Lu, T. Liu, R. Wu, Y. Wang, Z. Zhou, G. Wu, J. Fang, C. Tang
SHORT COMMUNICATION
Scheme 2. Synthesis of catalysts.
and thiophosphoryl chlorides 4 gave the corresponding N- Table 1. Catalyst screening.[a]
Boc-protected thiophosphoramides 5. Deprotection of 5
with trifluoacetic acid affords the target catalysts 2.
As shown in Table 1, the results of catalyst evaluation
employing the reaction between β-nitrostyrene (6a) and ace-
Entry
2
Time [h]
Yield [%][b]
ee[c]
tone as the model indicate that the electronic nature of the
substituents on the phosphorus atom of thiophosphor-
amides 2 have a prominent effect on both catalytic activity
and stereoselectivity. When O,O-diethyl thiophosphoramide
2a was employed as the catalyst, the reaction was completed
after 69 h, the corresponding adduct was obtained with ex-
cellent yield (98%) in good enantioselectivity (72% ee),
whereas catalyst 2c with 2,2,3,3-tetrafluoropropyl proved to
be much inefficient in terms of turnover frequency, a much
longer reaction time (120 h) was needed for total conversion
of the β-nitrostyrene 6a (Table 1, entry 3 vs. entry 1). Cata-
lyst 2b with a phenoxy group and 2f bearing a tropos bi-
phenyl skeleton gave results that were comparable to that
of catalyst 2a (Table 1, entry 2 and 7). An improvement
1
2
3
4
5
6
7
2a
69
96
120
50
96
192
69
98
95
92
72
69
65
88
61
68
70
2b
2c
2d
92
(R,R,aR)-2e
(R,R,aS)- 2e
2f
84
Ͻ10[d]
90
[a] All reactions were carried out using acetone (0.2 mL, 10 equiv.)
and 6a (44.7 mg, 0.3 mmol) in the presence of catalyst 2 (20 mol-
%) in toluene (1.0 mL). [b] Yield of the isolated product after
chromatography on silica gel. [c] Determined by chiral HPLC
analysis. [d] Most of 6a was recovered.
With the promising catalyst 2d in hand, other factors,
both on catalytic activity and stereoselectivtity was ob- such as cocatalyst, solvent, catalyst loading, and reaction
served when a phenyl group was introduced (2d), which temperature, influencing the reaction were thoroughly in-
provided the corresponding adduct in quantitative yield vestigated employing the reaction between β-nitrostyrene
with the highest ee value of 88% (Table 1, entry 4). In case (6a) and acetone as the model. The results are listed in
of catalyst 2e derived from axial chiral binaphthol, an obvi- Table 2.
ous matching and mismatching effects was observed be-
A survey of additives revealed that they play a certain
tween the chiralities of the (R,R)-1,2-diaminocyclohexane role in governing reaction yields and enantioselectivity. To
and binaphthol. For example, the (R,R,aR)-2e-catalyzed re- our delight, the addition of 10 mol-% of phenolic additive
action was complete within 96 h and afforded the Michael significantly accelerates the reaction as well as slightly im-
addition product in 84% yield with a enantioselectivity of proving the enantioselectivity relative to that with the cata-
61% ee. Under the otherwise identical conditions, catalyst lyst in the absence of additive (Table 2, entries 2–7 vs. entry
(R,R,aS)-2e demonstrated much lower catalytic activity. Al- 1). With respect to enantioselectitivity, the readily available
though a little better stereocontrol was realized, the reac- phenol gives the best result; the corresponding product is
tion became quite sluggish, and a dramatically decreased obtained in quantitative yield with 92% ee (Table 2, entry
yield of Ͻ10% was attained at a prolonged reaction time 2). The replacement of phenol with substituted phenols
(192 h) (Table 1, entry 6 vs. entry 5). This indicates that the bearing either electron-withdrawing or electron-donating
(R)-configuration of binaphthol matched (R,R)-1,2-di- group or 1-naphthol leads to a slight loss of stereocontrol
aminocyclohexane to enhance the catalytic activity of the (Table 2, entries 3–7 vs. entry 2). It is worth noting that
catalyst.
an obvious rate acceleration was also observed when using
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Highly Enantioselective Michael Addition Reaction
Table 2. Optimization of reaction conditions.[a]
stereoselectivity of the reaction was further slightly im-
proved to 93% ee when variation the reaction temperature
from 5 to 0 °C. Further lowering the reaction temperature
resulted in a quite sluggish reaction.
With optimal catalyst (20 mol-% of 2d) and reaction con-
ditions established (phenol as an additive, in toluene, at
0 °C), a variety of nitro olefins were then evaluated as sub-
strates and the results are summarized in Table 3.
Entry Cocatalyst [10 mol-%]
Solvent Time [h] Yield [%][b] ee (%)[c]
1
–
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
Et2O
132
60
66
72
84
84
60
72
90
Ͼ99
Ͼ99
Ͼ99
97
Ͼ99
Ͼ99
Ͼ99
75
89
92
91
90
90
90
91
89
92
92
88
83
33
77
91
93
2
PhOH
3
4
5
6
7
8
2-NO2C6H4OH
4-NO2C6H4OH
2-MeC6H4OH
4-Me-2-tBuC6H3OH
1-naphthol
H2O[d]
Table 3. Substrate scope of 2d catalyzed asymmetric Michael ad-
dition of acetone to nitro olefins.[a]
9
n-C3H7CO2H
PhCO2H
PhOH
PhOH
PhOH
PhOH
PhOH
PhOH
120
120
96
10
11
12
13
14
15
16
85
Ͼ99
95
Entry
R
Time [h]
Yield [%][b]
ee [%][c]
CHCl3
MeOH
acetone
toluene
toluene
96
120
96
120
120
1
2
3
4
5
6
7
8
Ph (a)
72
72
120
96
120
144
108
84
69
96
96
120
84
42
108
42
168
96
120
72
Ͼ99
90
82
95
80
91
93
99
95
98
99
96
72
99
25[e]
97
90
86
96
85
82
81
93
90
96
94
93
93
94
96
93
97
97
94
96
95
97
93
97
94
95
93
87
83
20[e]
98
Ͼ99
Ͼ99
Ph (a)[d]
2-MeOC6H4 (b)
3-MeOC6H4 (c)
4-MeOC6H4 (d)
4-MeC6H4 (e)
benzo[d][1,3]dioxol-5-yl (f)
2-ClC6H4 (g)
4-ClC6H4 (h)
2,4-Cl2C6H3 (i)
2-BrC6H4 (j)
4-BrC6H4 (k)
2-CF3C6H4 (l)
3-CF3C6H4 (m)
2-NO2C6H4 (n)
4-NO2C6H4 (o)
1-naphthyl (p)
2-furyl (q)
[a] Reactioin conditions: solvent (1.0 mL); acetone (0.2 mL,
10 equiv.); 6a (44.7 mg, 0.3 mmol); catalyst 2d (20 mol-%) except
for entry 15 (15 mol-%); T = 5 °C except for entry 16 (0 °C). [b]
Yield of the isolated product after chromatography on silica gel. [c]
Determined by chiral HPLC analysis. [d] 1 equiv. of water was
added. [e] Most of 6a was recovered.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
1 equiv. of water as an additive. This observation is consis-
tent with the fact that water can increase the catalyst turn-
over, presumably due to facilitating the release of the pri-
mary catalyst from the imine.[7] Surprisingly, the use of
more acidic carboxylic acids, such as benzoic acid and but-
anoic acid, as an additive was inefficient in this catalytic
system. Although a slight improvement on enantioselecti-
tivity was observed, the addition of carboxylic acid resulted
in reducing of the reactivity and a relatively poor yield
2-thienyl (r)
(E)-cinnamyl (s)[f]
phenylethyl (t)[f]
C2H5 (u)[g]
72
72
[a] All reactions were carried out using acetone (0.2 mL, 10 equiv.)
and nitro olefin (0.3 mmol) in the presence of catalyst 2d (20 mol-
(Table 2, entries 9, 10). Finally, phenol was selected as the %) in toluene (1.0 mL). [b] Yield of the isolated product after
chromatography on silica gel. [c] Determined by chiral HPLC
additive for further investigation.
analysis. [d] The reaction was performed in gram scale. [e] 70% of
Solvent evaluation revealed that the solvent also has a
6n was recovered. [f] The reaction was performed at 20 °C. [g] The
significant effect on the rate and the enantioselectivity of
the reaction. The reaction conducted best in a nonpoplar
reaction was performed at 25 °C.
solvent like toluene, which provided the highest ee value
As shown in Table 3, the reaction has broad applicability
(92%, see Table 2, entry 2). By contrast, the use of polar with respect to the nitro olefins. The corresponding adducts
solvent, such as diethyl ether, CHCl3, and acetone led, in were obtained in high enantioselectivity in all the cases ex-
general, to an obvious loss of stereocontrol with respect to amined. Generally, nitro olefins with electron-donating
toluene (Table 2, entry 2 vs. entries 11, 12, 14). Notably, group on the benzene ring demonstrated slightly lower reac-
when protic solvent, such as methanol, was employed as tivity relative to unsubstituted or with an electron-with-
the solvent, a significant drop of both the reactivity and drawing group substituted β-nitrostyrenes. At the same
enantioselectivity was found (Table 2, Entries 13), probably time, the nature of the substituents of the nitro olefin ap-
because the protic solvent weaken or retard the formation pears to have very little influence on the enantioselectivities
of hydrogen bonds between trans-β-nitrostyrene and the of the reaction, which range from 93% to 97%. Surpris-
thiophosphoramide moiety of catalyst 2d. These results ingly, in case of nitro-substituted substrates, the position of
strongly demonstrate that hydrogen-bond formation has an the nitro group plays an important role on the reaction. For
important effect on both the reactivity and the enantio- example, the reaction of para-nitro-substituted nitro olefin
selectivity of the reaction.
6o ran smoothly to afford the addition product 7o in excel-
In addition, reducing the catalyst loading to 15 mol-% lent yield with high ee value within 42 h (Table 3, entry 16).
led to a little loss of enantioselectivity with a prolonged Although an enhanced enantioselectivity was observed for
reaction time (Table 2, entry 15). On the other hand, the ortho-nitro-substituted nitro alkene 6n, the reaction became
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A. Lu, T. Liu, R. Wu, Y. Wang, Z. Zhou, G. Wu, J. Fang, C. Tang
SHORT COMMUNICATION
quite sluggish, and the corresponding adduct 7n was ob- nitrostyrene was carried out. To avoiding interference of the
tained with a significantly decrease in yield (Table 3, entry hydrogen-bonding interaction between the amino and nitro
15). This may be attributed to the competitive hydrogen- group, the corresponding N-dimethylated compounds 9 was
bonding interaction of the ortho-position nitro group with selected instead of catalyst 2d for NMR investigation (see
1
the thiophosphoramide moiety of the catalyst, which to Supporting Information). In the H NMR of a 1:1 (molar
some extent prohibits the activation of the electrophilic ni- ratio) mixture of thiophosphoramide 9 and β-nitrostyrene
tro olefin. Moreover, alkenyl-substituted nitro olefin 6s can (6a), the signal relative to the thiophosphoramide proton
also be employed successfully, a same high level of enantio- moved downfield from 4.45 to 4.46 ppm. At the same time,
selectivity was observed as those found in the aryl-substi- about 0.01 ppm downfield was also observed in the 31P
tuted ones. Perhaps more significant, nitro alkene bearing NMR spectra. This observation indicates that there may
aliphatic substituent also proved to be viable electrophilic exist a weak hydrogen-bonding interaction between the ni-
reacting partner in this catalytic system. For example, 3- tro group and the thiophosphoramide moiety.
phenylpropanal-derived nitro olefin 6t worked well to give
Based on the experimental results, a possible transition
the desired product in good yield with a slightly loss of state for this reaction was proposed to account for the ob-
stereocontrol (Table 3, entry 21, 87% ee), the reaction of served high enantioselectivity. As shown in Figure 1, thi-
(E)-1-nitrobut-1-ene (6u) also ran smoothly to afford the ophosphoramide 2d functioned as a bifunctional catalyst.
corresponding adduct in satisfactory yield with good The primary amine will first react with a carbonyl com-
enantioselectivity (Tabe 3. entry 22, 83% ee), which clearly pound to form an enamine with the aid of acidic co-cata-
demonstrated the broad generality of this asymmetric lyst. Subsequently, the acidic hydrogen will orientate the ni-
Michael addition reaction. To demonstrate the potential of tro group via hydrogen-bonding interaction so that the en-
this method for preparative purposes, the reaction is also amine will nucleophilic attack the nitro olefin from the Si-
carried out in gram scale giving the isolated product with face to give the highly enantioselective product. This expla-
comparable ee value albeit with a slightly decrease in chemi- nation is consistent with the experimental results.
cal yield (Table 3, entry 2).
The asymmetric addition of cyclic ketones to nitro-
styrene 6a using 2d as a catalyst was also preliminarily in-
vestigated under the identical reaction conditions
(Scheme 3). Unfortunately, this catalytic system is ineffec-
tive to cyclohexanone. The reaction is very sluggish even at
room temperature, and failed to provide the corresponding
adduct. However, the Michael addition of cyclopentanone
Figure 1. Possible transition state.
to nitro olefin 6a ran smoothly to give the desired product
8a in quantitative yield with excellent enantioselectivity
(92% ee for syn-8a and 98% ee for anti-8a, respectively)
albeit with relatively low diastereoselectivity (syn/anti: 40/
60). It is interesting that the observed anti-diastereoselecti-
vity is just opposite to the result obtained for the secondary
amine-thiophosphoramide catalyst (S,aR)-1.
Conclusions
In conclusion, we have developed a novel chiral trans-
cyclohexanediamine-based thiophosphoramdie primary
amine catalyst 2d, which worked well as a bifunctional or-
ganocatalyst to promote the asymmetric Michael reaction
of acetone to both aryl and alkyl nitro olefins. The corre-
sponding adducts were obtained in high yields (up to
Ͼ99%) with excellent enantioselectivity (up to 97% ee).
Moreover, this catalytic system can be applied to challeng-
ing cyclopentanone. Further efforts on the application of
this catalyst to other valuable organic transformations are
under active investigation.
Experimental Section
Thiophosphoramide 2d Catalyzed Asymmetric Michael Addition to
Nitro olefins (General Procedure): A mixture the catalyst 2d
(19.8 mg, 0.06 mmol), phenol (2.8 mg, 0.03 mmol) and acetone
(0.2 mL, 10 equiv.) in toluene (1.0 mL) was stirred at room tem-
perature to form a clear solution. Then, to the resulting solution
was added nitro olefin (0.3 mmol) at the required temperature. Af-
ter the reaction is complete (monitored by TLC), the mixture was
purified by column chromatography on silica gel (200–300 mesh,
Scheme 3. Reaction of cyclic ketones.
We envisioned that the acidic hydrogen of the thiophos-
phoramide can function as hydrogen-bonding donor in the
reaction. To testify this hypothesis, an NMR study on the
interaction between the thiophosphoramide catalyst and β- PE/EtOAc = 5:1) to afford the desired product. The enantiomeric
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Highly Enantioselective Michael Addition Reaction
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We are grateful to the National Natural Science Foundation of
China (No. 20772058, 20972070), the National Basic Research Pro-
gram of China (973 program 2010CB833301) and the Key labora-
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Received: July 5, 2010
Published Online: September 21, 2010
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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