Asymmetric crossed-conjugate addition of nitroalkenes to enones by a chiral bifunctional diamine organocatalyst

Article abstract of DOI:10.1002/chem.201002961

The crossed-conjugate addition involving the electronwithdrawing group (EWG) of activated alkenes is considered one of the most important C-C bond-forming reactions in organic synthesis. Among them, the β,β-disubstituted nitroolefins as powerful synthons have attracted considerable attention recently because the functionalized products can undergo transformations into useful skeletons. Asymmetric intermolecular crossed-conjugate addition between β-alkyl nitroalkenes and a,b-unsaturated carbonyl compounds to provide allylic nitro compounds with multiple stereogenic centers is still rare. The key challenges are to avoid the homo-crossed addition and realize perfect stereochemical control. Shi and co-workers developed the first asymmetric crossed-conjugate addition of β-alkyl nitroalkenes and enals catalyzed by a chiral diphenylprolinol though a dual activation strategy using a stoichiometric amount of (MeO)₃P as a Lewis base (LB) (Scheme 1, path A). Although the secondary amines proved to be effective catalysts in the crossed-Michael addition of nitro activated olefins to carbonyl compounds through the LB nucleophilic mechanism, the enantioselective process awaits further investigation.We envisioned that a chiral diamine catalyst would promote the enantioselective crossed-conjugate addition by a bifunctional activation approach (Scheme 1, path B). Herein, we report the chiral primary-secondary diamine Bronsted acid catalyzed asymmetric crossed-conjugate addition of β-methyl nitroalkenes with α, β-unsaturated acyclic and cyclic ketones with excellent enantioselectivities under mild conditions.

Full text of DOI:10.1002/chem.201002961

COMMUNICATION
DOI: 10.1002/chem.201002961
Asymmetric Crossed-Conjugate Addition of Nitroalkenes to Enones by a
Chiral Bifunctional Diamine Organocatalyst
Min Wang, Lili Lin, Jian Shi, Xiaohua Liu,* Yulong Kuang, and Xiaoming Feng*[a]
The crossed-conjugate addition involving the electron-
withdrawing group (EWG) of activated alkenes is consid-
tion by a bifunctional activation approach (Scheme 1, path
B). Herein, we report the chiral primary-secondary diamine
Brønsted acid catalyzed asymmetric crossed-conjugate addi-
tion of b-methyl nitroalkenes with a,b-unsaturated acyclic
and cyclic ketones with excellent enantioselectivities under
mild conditions.
Initially, nitroalkene 1a and cinnamone 2a were selected
for a model reaction with chiral 1,2-diphenylethane-1,2-di-
amine (DPEN) derivatives as catalysts in CH2Cl2. As ex-
pected, the desired product 3aa was obtained in 37% yield
with 56% enantiomeric excess (ee) of the syn isomer when
20 mol% of the commercially available (1S,2S)-DPEN L1
and benzoic acid were used as the catalyst (Table 1, entry 1).
À
ered one of the most important C C bond-forming reactions
in organic synthesis.[1] Among them, the b,b-disubstituted ni-
troolefins as powerful synthons[2] have attracted considera-
ble attention recently because the functionalized products
can undergo transformations into useful skeletons.[3,4] Asym-
metric intermolecular crossed-conjugate addition between
b-alkyl nitroalkenes and a,b-unsaturated carbonyl com-
pounds to provide allylic nitro compounds with multiple ste-
reogenic centers is still rare. The key challenges are to avoid
the homo-crossed addition and realize perfect stereochemi-
cal control. Shi and co-workers developed the first asymmet-
ric crossed-conjugate addition of b-alkyl nitroalkenes and
enals catalyzed by a chiral diphenylprolinol though a dual
Table 1. Optimization of the reaction conditions.[a]
activation strategy using
a stoichiometric amount of
(MeO)3P as a Lewis base (LB) (Scheme 1, path A).[2i] Al-
Entry Solvent Cat. Co-cat.
Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
CH2Cl2 L1
CH2Cl2 L2
CH2Cl2 L3
CH2Cl2 L4
CH2Cl2 L5
toluene L5
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
p-NO2PhCO2H
d-mandelic acid
–
d-mandelic acid, DIPEA 89
d-mandelic acid, DABCO 78
d-mandelic acid, DMAP
d-mandelic acid, DMAP
d-mandelic acid, DMAP
37
18
40
36
33
21
42
30
43
12
56
47
59
66
69
76
81
85
94
67
90
88
94
94
94
Scheme 1. Proposed activation methods for the enantioselective crossed-
conjugate addition reaction.
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
L5
L5
L5
L5
L5
L5
L5
L5
L5
9
though the secondary amines proved to be effective catalysts
in the crossed-Michael addition of nitro activated olefins to
carbonyl compounds through the LB nucleophilic mecha-
nism, the enantioselective process awaits further investiga-
tion.[2b] We envisioned that a chiral diamine catalyst[5,6]
would promote the enantioselective crossed-conjugate addi-
10
11
12
13
14[d]
87
87
88
15[d,e] Et2O
[a] Unless otherwise noted, all reactions were carried out with catalyst
(20 mol%), co-catalyst (20 mol%), 1a (0.1 mmol), and 2a (0.12 mmol) in
solvent (0.6 mL) at 258C for 24 h. [b] Total yield of two diastereomers.
[c] ee value of the major isomer determined by HPLC. The diastereomers
were separated and the low diastereomeric ratio (d.r.) (less than 2:1) in
all cases favored the syn isomer and similar ee values were obtained for
syn and anti products. [d] The catalyst loading was 10 mol% and Et2O
(0.3 mL) was used, the reaction time was 48 h. [e] L5 was treated with
one equivalent of d-mandelic acid to afford the salt, which was used di-
rectly as the catalyst. DIPEA=N,N-diisopropylethylamine, DABCO=
[a] M. Wang, Dr. L. Lin, J. Shi, Dr. X. Liu, Y. Kuang, Prof. Dr. X. Feng
Key Laboratory of Green Chemistry & Technology
Ministry of Education, College of Chemistry
Sichuan University, Chengdu 610064 (P.R. China)
Fax : (+86)28-8541-8249
E-mail: liuxh@scu.edu.cn
xmfeng@scu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002961.
1,4-diazabicycloACTHNUGRTNEUNG(2,2,2)octane.
Chem. Eur. J. 2011, 17, 2365 – 2368
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2365

Table 2. Substrate scope of the asymmetric conjugate addition of nitro-
olefins and acyclic enones.[a]
Next, a series of DPEN derivatives were synthesized and ex-
amined. Encouragingly, when the monoalkyl-protected pri-
mary-secondary diamines were used with PhCO2H as a co-
catalyst, the enantioselectivity increased slightly and the
yield was maintained (Table 1, entries 3–5). The enantiose-
lectivity also increased with an increase in steric hindrance
of the substituent on DPEN. The DPEN derivative L5, sub-
stituted with a monocyclohexyl group, which may offer suit-
able alkaline and steric hindrance, was found to be the best
candidate giving the desired allylic nitro compound in 33%
yield with 69% ee (Table 1, entry 5). However, the use of
disubstituted secondary diamine L2 resulted in a lower yield
than monoalkyl substituted diamine L3 (Table 1, entry 2 vs.
entry 3). Further investigation of the solvent showed that
toluene improves the enantioselectivity up to 76% ee, yet
the yield was low (Table 1, entry 6). Diethyl ether proved
more suitable and increased both the ee value and the yield
(42% yield, 81% ee of the syn product, Table 1, entry 7 vs.
entries 5 and 6).
Different Brønsted acids that might facilitate the activa-
tion of the enone through the iminium intermediate were
evaluated to improve the results. A series of achiral and
chiral acids were tested, and d-mandelic acid was found to
increase the enantioselectivity (43% yield, 94% ee)
(Table 1, entry 9 vs. entries 7 and 8). Additionally, the prod-
uct was obtained in 12% yield in the absence of the acidic
co-catalyst, which also indicated that the acid was crucial in
the activation process (Table 1, entry 10). To improve the
yield, a mixture of diamine L5 and d-mandelic acid was
treated with an additional achiral organic base. The base ad-
ditive was found to have a remarkable effect on the catalytic
activity by releasing the amine catalyst from the nitroalkene
adduct, thereby accelerating the catalytic cycle (Table 1, en-
tries 11–13). When 20 mol% of 4-dimethylaminopyridine
(DMAP) was used as the additive, the yield was improved
without any decrease in enantioselectivity (87% yield,
94% ee; Table 1, entry 13 vs. entries 11 and 12). Further-
more, when the catalyst loading was decreased to 10 mol%,
excellent results could also be obtained by increasing the
substrate concentration and prolonging the reaction time
(87% yield, 94% ee, Table 1, entry 14). For convenience, the
same result could be obtained by treating L5 with one
equivalent of d-mandelic acid to give the salt, which was
used directly as the catalyst (Table 1, entry 15 vs. entry 14).[7]
After evaluating a variety of conditions, a combination of
L5-d-mandelic acid (10 mol%), DMAP (10 mol%) as the
additive, and diethyl ether as the solvent was found to be
optimal (Table 1, entry 15).[8]
Entry
R1
R2
Yield
[%][b]
d.r.[c]
syn/anti
ee [%][d]
syn(anti)
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3-ClC6H4
2-furnyl
4-ClC6H4
4-BrC6H4
Ph
88 (3aa)
91 (3ab)
87 (3ac)
92 (3ad)
75 (3ae)
78 (3af)
93 (3ag)
90 (3ah)
70 (3ai)
75 (3aj)
84 (3ca)
90 (3ha)
93 (3 fa)
85 (3ga)
1.7:1
1.3:1
3.0:1
2.7:1
1.6:1
1.5:1
2.6:1
1.3:1
3.0:1
1.3:1
1.5:1
2.6:1
24:1
93 (94)
96 (90)
90 (89)
91 (91)
92 (90)
92 (93)
93 (89)
93 (89)
87 (89)
89 (91)
85 (89)
94 (94)
97
3-MeOC6H4
4-MeOC6H4
3-MeC6H4
3-CF3C6H4
3-ClC6H4
4-FC6H4
2-furnyl
2-thiophenyl
2-naphthyl
Ph
9
10
11[e]
12[e]
13[e]
14[e]
Ph
Ph
Ph
24:1
96[g]
15[e]
4-ClC6H4
93 (3 fi)
33:1
95
16[e]
17[e]
18[f]
19[f]
4-ClC6H4
4-ClC6H4
Ph
2-naphthyl
4-PhC6H4
Ph
90 (3 fj)
91 (3 fh)
61 (3ak)
60 (3al)
25:1
5:1
1.1:1
1.5:1
90
96
80 (81)
80 (83)
Ph
3-MeC6H4
[a] All reactions were carried out by using the salt (10 mol%), which was
prepared beforehand,[7] DMAP (10 mol%),
(0.1 mmol), and
1
2
(0.12 mmol) in Et2O (0.3 mL) at 258C for 48 h. [b] Total yield of isolated
product of the diastereomeric mixture of 3. [c] Determined by 1H NMR
spectroscopy. [d] Determined by HPLC analysis; the data in parenthesis
is the ee value of the minor isomer. [e] The reaction time was 36 h.
[f] Catalyst loading was 20 mol% and the reaction time was 72 h. [g] The
absolute configuration was determined by X-ray analysis as (4S,5S)-6-(4-
bromophenyl)-5-nitro-4-phenylhept-6-en-2-one.
the catalyst was also efficient (Table 2, entries 8 and 9).
Moreover, the conjugate addition of a fused-ring aromatic
enone with 1a proceeded well to give the desired product in
good yield with an excellent ee value (Table 2, entry 10). In
these cases, low diastereoselectivity was obtained due to a
poor spatial arrangement and the acidic deprotonation on
the nitro carbon atom (1.3:1–3:1 syn/anti, Table 2, entries 1–
10). This was a minor problem because the two diastereo-
mers could be separated with high optical purity. Subse-
quently, substituted nitroalkenes were also tested with (E)-
4-aryl-but-3-en-2-ones (Table 2, entries 11–17). Notably, the
diastereoselectivity greatly depended on the substituted po-
sition of the nitroalkenes; p-Br- and p-Cl-substituted nitroal-
kenes gave the corresponding products with high diastereo-
selectivities (up to 33:1) in good yields and with excellent
enantioselectivities (Table 2, entries 13–17 vs. entries 11 and
12). In addition, chalcones also performed well in the reac-
tion with good enantioselectivities in moderate yields
(Table 2, entries 18 and 19).
Under the optimized conditions (Table 1, entry 15), the
substrate scope of the crossed-conjugate addition of differ-
ent a,b-enones with b-methyl nitrostyrene (1a) was exam-
ined. In general, regardless of the electronic property or
steric hindrance of the substituent on the aromatic ring of
the enones, chiral allylic nitro compound 3 could be ob-
tained in good to excellent yields with excellent enantiose-
lectivities for both isomers (75–93% yield, 89–96% ee,
Table 2, entries 1–7). In the case of heteroaromatic enones,
Next, we expanded our attention to investigate the sub-
strate scope of the crossed-conjugate addition of cyclo-
2366
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Chem. Eur. J. 2011, 17, 2365 – 2368

Asymmetric Crossed-Conjugate Addition of Nitroalkenes
COMMUNICATION
Table 3. Substrate scope of asymmetric conjugate addition of nitroolefins
and cycloenones.[a]
On the basis of X-ray structural analysis, we proposed a
bifunctional transition state for the intermolecular crossed-
conjugate addition reaction between nitroalkene 1 and
enone 2. As shown in Scheme 2, the primary moiety associ-
Entry
R1
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
Ph
95 (5aa)
96 (5ab)
70 (5ac)
87 (5ad)
93 (5ae)
94 (5af)
89 (5ag)
90 (5ah)
60 (5ba)
1.1:1
1.1:1
1.2:1
1.5:1
1.1:1
1.2:1
1.1:1
1.2:1
1.1:1
99 (99)
97 (99)
99 (99)
99 (98)
96 (93)
98 (97)
98 (97)
99 (99)
87 (87)
2-FC6H4
3-ClC6H4
3,4-Cl2C6H3
4-FC6H4
4-ClC6H4
4-BrC6H4
2-furyl
Scheme 2. Proposed transition state for the crossed-conjugate addition of
nitroalkene 1 with enone 2 (the anion was omitted).
8
9[e]
Ph
[a] Unless otherwise noted, all reactions were carried out with the salt
(10 mol%) prepared beforehand,[7] DMAP (10 mol%), 1 (0.1 mmol), and
4a (0.12 mmol) in Et2O (0.3 mL) at 258C for 24 h. [b] Total yield of the
two diastereomers. [c] Determined by 1H NMR spectroscopy. [d] Deter-
mined by HPLC analysis; data in parenthesis is the ee value of the minor
isomer. [e] Cyclopent-2-enone (4b) was introduced, and the reaction
time was 48 h.
ating with the Brønsted acid could activate the enone by
forming the iminium ion intermediate.[10] On the other hand,
the secondary amine moiety served as the Lewis base to ac-
tivate the nitroalkene by adding in a 1,4-fashion to the nitro-
alkene, activating it for addition to the activated enone.
Then the formation of the Michael adduct by attack of the
nucleophile from the Si face of the cinnamone was favored
to form the corresponding (4S,5S)-3ga as the major product,
in accordance with the experiment results.[8] However, there
is strong repulsion interaction between the two substrates in
TS2, which would afford the disfavored enantiomer. Initial-
ly, DMAP might react as a nucleophilic catalyst to activate
the nitroalkene,[11] then the secondary amine moiety of the
catalyst would substitute DMAP to form the real nucleo-
phile as shown in Scheme 2. In addition, DMAP might also
accelerate the elimination process, and release the secon-
dary amine moiety.
In summary, we have developed the first highly enantiose-
lective crossed-conjugate addition of b,b-dialkyl nitroalkenes
to a,b-unsaturated acyclic or cyclic ketones. An easily avail-
able new type of primary-secondary diamine catalyst was
synthesized and showed great capability in the reaction. No-
tably, a catalytic amount of amine rather than stoichiometric
Lewis base was added to this system.[2b,i] The wide substrate
scope, excellent yield (up to 96%) and enantioselectivity
(up to 99% ee), and mild reaction conditions provide a po-
tential method for the formation of allylic nitro compounds.
The bifunctional activation approach, which creates a per-
fect steric environment, was considered to be crucial for the
asymmetric reaction. Further applications of the catalyst
system and of b,b-disubstituted nitroalkenes in organic syn-
thesis are ongoing.
enones 4 with substituted nitroolefins under the optimal
conditions (Table 3). Regardless of the substituted position
on the aromatic ring, nitroalkenes containing an EWG per-
formed well with cyclohex-2-enone (4a) in good to excellent
yields (70–96%) with excellent enantioselectivities (93–
99% ee) (Table 3, entries 1–7). Heteroaromatic nitroolefins
proved to be competent candidates for the crossed-conju-
gate addition in up to 90% yield and with 99% ee for both
isomers (Table 3, entry 8). Moreover, this transformation
could also tolerate cyclopent-2-enone (4b) to give a moder-
ate yield and good ee value (Table 3, entry 9). Low diaste-
reoselectivities were obtained even using para-substituted
nitroolefins, which implied that the diastereoselectivity was
determined by the steric effect between the aryl group of
the nitroalkene and the b substituent of the enone.
The optically pure product 3ga was obtained after recrys-
tallization from n-hexane and dichloromethane. The abso-
lute configuration of the major enantiomer was established
by X-ray crystallography as 4S,5S (Figure 1).[9]
Experimental Section
General procedure: A mixture of L5 (294.2 mg, 1 mmol) and d-mandelic
acid (152.0 mg, 1 mmol) was stirred in CH2Cl2 (10 mL) for 30 min at
258C, then the solvent was evaporated to afford the corresponding
Figure 1. X-ray crystal structure of 3ga.
Chem. Eur. J. 2011, 17, 2365 – 2368
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2367

X. Feng, X. Liu et al.
DPEN salt, which was used directly as the catalyst. DPEN salt (4.5 mg,
0.01 mmol), DMAP (1.2 mg, 0.01 mmol), and enones 2 or 4 (0.12 mmol)
were dissolved in Et2O (0.3 mL) and stirred at 258C for 40 min. The ni-
troalkenes (0.1 mmol) were then added and the mixture was stirred at
258C for 24–72 h. The residue was purified by flash chromatography on
silica gel to afford the desired product directly.
Wang, Y. Zhu, D. J. Shang, B. Gao, X. H. Liu, X. M. Feng, Z. S. Su,
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Acknowledgements
We appreciate the National Natural Science Foundation of China (nos.
20732003, 21021001, and 20902060), PCSIRT (no. IRT0846) and the Na-
tional Basic Research Program of China (973 program: no.
2010CB833300) for financial support. We also thank Sichuan University
Analytical & Testing Center for the NMR spectroscopic analysis.
Keywords: asymmetric
catalysis
·
crossed-conjugate
addition · enones · nitroalkenes · organocatalysis
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Received: October 14, 2010
Published online: January 17, 2011
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