Enantioselective copper(I/II)-catalyzed conjugate addition of nitro esters to β,γ-unsaturated α-ketoesters

Article abstract of DOI:10.1002/chem.201303512

A highly enantioselective Michael addition of nitroacetates to β,γ-unsaturated α-ketoesters was developed by using chiral copper catalysts. The Michael addition products can be obtained in high yields with up to 99 % ee. With these densely functionalized products, the chiral cyclic nitrones, which are important synthetic intermediates, can be obtained in one step. Copyright

Full text of DOI:10.1002/chem.201303512

DOI: 10.1002/chem.201303512
Communication
&
Asymmetric Catalysis
Enantioselective Copper(I/II)-Catalyzed Conjugate Addition of
Nitro Esters to b,g-Unsaturated a-Ketoesters
Sheng Zhang, Kun Xu, Fengfeng Guo, Yanbin Hu, Zhenggen Zha, and Zhiyong Wang*[a]
Henry reaction in our group, offered a high enantioselectivity
Abstract: A highly enantioselective Michael addition of ni-
(90% ee, Table 1, entry 5).[8] Encouraged by this result, other re-
troacetates to b,g-unsaturated a-ketoesters was devel-
action parameters were varied. Screening of Lewis acids
oped by using chiral copper catalysts. The Michael addi-
showed that [Cu(CH3CN)4]BF4 gave the best result (entries 1–5).
tion products can be obtained in high yields with up to
Subsequently, the effects of solvents and ligands on this re-
99% ee. With these densely functionalized products, the
action were examined. Toluene provided the best result
chiral cyclic nitrones, which are important synthetic inter-
among different solvents (Table 1, entries 5–8). Furthermore,
mediates, can be obtained in one step.
the Ar group of the chiral ligand (L1–4, Table 1) was varied. It
was found that L2 was the most efficient ligand, affording the
desired adduct with an excellent enantioselectivity (94% ee)
The conjugate addition of stabilized carbanions to Michael ac-
ceptors represents one of the most useful carbon–carbon
bond formation reactions in organic synthesis.[1] The develop-
ment of chiral catalysts for the asymmetric version of this reac-
tion constitutes an important research field and has been well-
explored in recent years.[2] Among various types of Michael
donors, nitro esters are active Michael donors with strong
acidic a-hydrogen atoms (pKa(H2O)=9.1).[3] Additionally, nitro
esters can be converted into a number of useful functionalized
compounds.[4] Therefore, the asymmetric Michael reactions em-
ploying nitroacetates as donors have received increased atten-
tion, and most of these reactions utilized organocatalysts.[5]
Meanwhile, b,g-unsaturated a-ketoesters are considered as ver-
satile synthons because of their dense functionalization.[6] In
this context, we chose b,g-unsaturated a-ketoesters as an elec-
trophile in the Michael reaction and realized a highly enantio-
selective conjugate addition of nitro esters to b,g-unsaturated
a-ketoesters in the presence of chiral copper complexes, af-
fording the desired adducts with excellent enantiomeric ex-
cesses and yields. This is an ongoing study of tridentate
ligand–metal-complex-catalyzed asymmetric reactions.[7,8]
and yield (99%) (entry 9). Reaction temperature had a little in-
fluence on this reaction. When the reaction temperature de-
creased from RT to 08C, a slightly higher ee value was achieved
without any loss in the reaction yield. Finally, the optimal reac-
tion conditions were obtained, as shown in entry 9 of Table 1.
These conditions were named as conditions I.
Interestingly, when K2CO3 was employed instead of Li2CO3 as
the base, the CuI ([Cu(CH3CN)4]BF4) could be replaced with CuII
(Cu(OTf)2), affording the addition product with a good result
(Supporting Information). Then we optimized the reaction con-
ditions on the basis of this case. Finally, it was found that L4
was the best ligand and Cs2CO3 was the best base for this reac-
tion. Under these conditions the highest reaction yield and an
excellent ee value were obtained at 08C (Table 1, entry 12);
these conditions were named as conditions II.
Under the optimized conditions, the substrate scope for
asymmetric conjugate addition of nitro esters to various b,g-
unsaturated a-ketoesters was examined and the results were
summarized in Table 2. It was found that most reactions with
a-keto esters containing g-aryl or g-heteroaryl substituents
were carried out smoothly in good to excellent yields (80–
99%) with excellent enantioselectivities (Table 2, entries 1–14).
In terms of Table 2 the steric and electronic properties of sub-
stituents on aromatic rings had little effect on the efficiency of
this process. Notably, the substrate with a cinnamyl group also
gave an excellent ee value (Table 2, entry 14). Moreover, g-
alkyl-substituted a-keto esters also performed well to afford
the products in good yield with high enantioselectivity
(entry 15). Changing the ester groups on nitroacetate (R1) and
ketoesters (R3) has a little effect on the enantioselectivities and
yields (entries 16–21). The reaction of tert-butyl nitroacetate
was slow at 08C (entries 22); However, good yield and enantio-
selectivity could be obtained when the reactions were per-
formed at 108C (entries 16). Upon scaling up to gram quanti-
ties, the desired product 3a was still obtained with excellent
results (1.60 g, 91% yield, 94/89% ee) as shown in Scheme 1.
First of all, the reaction of ethyl nitroacetate 1a with b,g-un-
saturated a-ketoester 2a was chosen as a model reaction. A
preliminary result of 49% ee was obtained. Afterwards, the op-
timization of various ligands was performed for this reaction
(see the Supporting Information for details). To our delight, the
chiral ligand L1, which was developed for the asymmetric
[a] S. Zhang, K. Xu, F. Guo, Y. Hu, Prof. Z. Zha, Prof. Z. Wang
Hefei National Laboratory for Physical Sciences at Microscale
CAS Key Laboratory of Soft Matter Chemistry
and Department of Chemistry
University of Science and Technology of China
Hefei, Anhui 230026 (China)
Fax:(+86)551-3603185
E-mail: zwang3@ustc.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303512.
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Communication
The synthetic utility of this
asymmetric transformation was
demonstrated by the conversion
of adducts into 4a/4b, 5a as
shown in Scheme 2. Chiral cyclic
nitrones 4a/4b were prepared
from adduct 3a by a one-step
reduction procedure, which
could be readily converted to g-
lactam analogues of penicillanic
and carbapenicillanic acids.[9] The
adduct 3v underwent a facile
acidic hydrolysis to remove an
ester group, followed by in situ
Table 1. Optimization of the reaction conditions.[a]
Entry
Ligand
Metal
Solvent
Yield [%][b]
d.r.[c]
ee[d] [%]
1
2
3
4
5
6
7
8
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L4
Cu(OTf)2
Zn(OTf)2
CuBr2
toluene
toluene
toluene
toluene
toluene
CH2Cl2
CH3CN
Et2O
toluene
toluene
toluene
toluene
81
75
87
95
99
99
50
90
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
63/66
1/5
80/89
59/70
90/91
78/86
35/66
88/90
decarboxylation,
affording
[Cu(CH3CN)4]PF6
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]BF4
[Cu(CH3CN)4]BF4
Cu(OTf)2
methyl 5-nitro-2-oxo-4-phenyl-
pentanoate (5a) in a moderate
yield and with moderate enan-
tioselectivity. To the best of our
knowledge, there is no estab-
lished method to produce 5-
nitro-2-oxopentanoates from the
direct conjugate addition of ni-
troalkanes to b,g-unsaturated a-
ketoesters.[10]
9[e]
10
11
12[e,f]
99 (99)
89
95
1:1 (1:1)
1:1
1:1
93/94 (94/95)
84/86
91/93
99 (99)
1:1 (1:1)
93/94 (94/95)
[a] Reactions were carried out with 1a (0.2 mmol) and 2a (0.25 mmol) in toluene (2 mL) at room temperature
for 16 h, unless otherwise noted. [b] Isolated yield. [c] Determined by 1H NMR spectroscopic analysis of prod-
ucts. [d] The ee value was determined by chiral HPLC analysis. [e] The data in parentheses is the result of the
reaction conducted in 08C. [f] Cesium carbonate (7.5 mol%) was used as the base.
Next, the mechanism of the
reaction was studied. We found
that similar results were ob-
tained under conditions I and
Table 2. Conjugate addition of nitroacetates to b,g-unsaturated a-ketoesters.[a,b]
conditions II.
A
weak base,
Li2CO3 and CuI, was used in con-
ditions I, while a stronger base,
Cs2CO3 and CuII, was used in
conditions II.[11] To illustrate the
role of Li2CO3, tBuOK was used
to replace Li2CO3 and the load-
ing was varied. As shown in
Scheme 3, when one equivalent
of tBuOK was employed, almost
the same result was obtained as
using Li2CO3. When the amount
of tBuOK was increased to two
Entry
R1
R2
R3
Product
Yield[c] [%]
d.r[d]
ee[e] [%]
1
2
3
4
5
6
7
8
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
Et (1a)
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
Bn
Bn
iPr
iPr
iPr
iPr
iPr
iPr
Me
Bn
Me
Ph (2a)
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
99 (99)
99 (99)
99 (98)
99 (99)
99 (99)
99 (97)
97 (95)
97 (99)
92 (90)
88 (86)
99 (95)
90 (91)
80 (80)
81 (81)
90 (87)
95 (99)
94 (96)
92 (94)
87 (90)
99 (99)
99 (99)
79 (77)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
1: 5 (1.5:1)
1: 1 (1:1)
1: 1 (1:1)
1: 1 (1:1)
94/95 (94/95)
93/94 (95/97)
96/97 (95/97)
95/87 (94/91)
95/96 (96/91)
94/97 (95/96)
93/93 (96/90)
94/85 (95/93)
95/95 (92/92)
92/91 (85/85)
93/95 (91/95)
91/95 (96/91)
96/89 (95/95)
92/91 (91/90)
92/93 (93/94)
92/92 (95/92)
94/93 (89/99)
94/93 (98/99)
94/96 (94/99)
96/90 (94/89)
96/95 (99/96)
90/91 (89/89)
4-NO2C6H5 (2b)
4-CF3C6H5 (2c)
4-FC6H5 (2d)
4-ClC6H5 (2e)
4-BrC6H5 (2 f)
4-MeOC6H5 (2g)
4-MeC6H5 (2h)
3-ClC6H5 (2i)
2-ClC6H5 (2j)
2-naphthyl (2k)
2-thienyl (2l)
2-furyl (2m)
PhCH=CH (2n)
n-pentyl (2o)
Ph (2a)
9
10
11
12
13
14
15
16[f]
17
18
19
20
21
22
equivalents,
a significant de-
crease in enantioselectivity was
observed. As we know, one
equivalent of HBF4 was generat-
ed when L2 coordinated with
CuI. The HBF4 generated in situ
was consumed by one equiva-
lent of base and the excess of
base would lead to a background
reaction, which has a negative
effect on the enantioselectivity.
Therefore, one equivalent of
Li2CO3 functioned as one equiva-
lent of tBuOK in this transforma-
tion.
tBu (1b)
Me (1c)
c-C6H11 (1d)
l-menthyl (1e)
Et (1a)
Ph (2a)
Ph (2a)
Ph (2a)
Ph (2p)
Ph (2q)
Ph (2p)
3s
3t
3u
3v
Et (1a)
tBu (1b)
[a] Reactions were carried out with 1a (0.2 mmol) and 2a (0.25 mmol), ligand L2 (7.5 mol%), Cu[(CH3CN)4BF4]
(7.5 mol%), Li2CO3 (7.5 mol%) in toluene (2 mL) at 08C, unless otherwise noted. [b] The data in parentheses is
the result of the reaction conducted under condition II. [c] Isolated yield. [d] Determined by 1H NMR spectro-
scopic analysis of products. [e] The ee value was determined by chiral HPLC analysis. [f] The reaction were per-
formed at 108C.
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L4-CuII-1, as a Brønsted base, was able to regain a hy-
drogen from the nitroalkanes.[8b] Therefore, the reac-
tion could also proceed well with L4-CuII-1. Since the
presence of the alcoholic hydroxy group was crucial
to the asymmetric Michael addition, we envisioned
that the reaction should yield a similar result when
one equivalent of tBuOK was utilized under condi-
tions II, which was verified by the further experiment.
To further demonstrate the role of the alcoholic hy-
droxyl group, the ligand L2’ was synthesized, in
which the hydroxyl group was replaced by a methoxy
group. In the presence of L2’, whether the reaction
was under conditions I or II, almost racemic mixtures
and low yields were obtained (Scheme 3). Based on
these results and the absolute configuration of the
product (see the Supporting Information for details),
two similar transition-state models (TS I/TS II), under
conditions I and II, were postulated (Scheme 4).[12]
In conclusion, the highly enantioselective copper-
catalyzed Michael addition of nitroacetates to b,g-un-
saturated a-ketoesters was developed under mild re-
action conditions. A series of versatile chiral adducts
Scheme 1. Asymmetric Michael reaction on a gram scale.
Scheme 2. Synthetic manipulation of the Michael addition products.
Scheme 4. Proposed transition-state models for the Michael addition.
were obtained with excellent enantioselectivity and yields.
Meanwhile, two transition-state models that attributed to the
equal effects of CuI and CuII salts on the reaction were pro-
posed. Efforts to apply this catalyst system to other asymmet-
ric transformations are underway in our group.
Scheme 3. Control experiments to gain insight into the reaction mechanism.
Experimental Section
General procedure (conditions I)
As for Cs2CO3, it was considered as a base which could con-
sume two equivalents of acid to form L4-CuII-1.[8b] The employ-
ment of two equivalents of tBuOK could afford a similar result
as that of one equivalent of Cs2CO3, which illustrated the role
of Cs2CO3, as shown in Scheme 3. The structure of L4-CuII-1, in
which the hydrogen of the alcoholic hydroxy group was ex-
tracted, was different from L2-CuI. In our previous work, it was
found that the oxygen atom of the alcoholic hydroxy group in
A mixture of [Cu(CH3CN)4]BF4 (4.7 mg, 0.015 mmol) and the ligand
(L2, 6.8 mg, 0.015 mmol) in toluene (2 mL) with Li2CO3 (1.1 mg,
0.015 mmol) was stirred at room temperature for 1 h under nitro-
gen. Nitroacetate 1a (22 mL, 0.2 mmol) was then added and the re-
sulting mixture cooled to 08C. After stirring the mixture for 1 h,
the b,g-unsaturated a-ketoester 2a (55 mg, 0.25 mmol) was added
in one portion. After the reaction was complete (monitored by TLC
analysis), the reaction was quenched by HCl (2 mL, 2m) and then
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washed with saturated NaHCO3 and brine, dried with Na2SO4, and
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matography afforded the desired Michael product 3a (70 mg), as
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AS-H, iPrOH/hexanes=10:90, 0.5 mLminÀ1
,
l=215 nm: tR =
47.4 min (major), tR =53.8 min (minor); tR =59 min (major), tR =
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[11] The concentration of CuI did not change from the beginning to the
end of the reaction, which showed the complex of CuI was stabilized
under the reaction conditions (see the Supporting Information).
[12] The kinetic study and deuterium-labeling experiment of the reaction
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the Supporting Information).
50 min (minor)).
Acknowledgements
We are grateful to National Nature Science Foundation of
China (20932002, 20972144, 90813008, 21172205, 20772188,
J1030412).
Keywords: asymmetric catalysis · copper · ketoesters · Michael
addition · nitroacetate
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Received: September 6, 2013
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