Chiral amidophosphane-copper-catalyzed asymmetric conjugate addition of dialkylzinc reagents to nitroalkenes

Article abstract of DOI:10.1016/j.tet.2005.05.070

The copper-amidophosphane-catalyzed asymmetric addition reaction of dialkylzinc reagents with β-aryl and β-alkylnitroalkenes afforded the corresponding nitroalkanes with moderate to good enantioselectivities (54-80% ee). The performance was highly dependent on the reaction procedure where the addition of nitroalkene to the mixture of copper-amidophosphane and dialkylzinc gave higher ee than the addition of dialkylzinc to a mixture of copper-amidophosphane and nitroalkene.

Full text of DOI:10.1016/j.tet.2005.05.070

Tetrahedron 61 (2005) 7420–7424
Chiral amidophosphane–copper-catalyzed asymmetric conjugate
addition of dialkylzinc reagents to nitroalkenes
Fanny Valleix, Kazushige Nagai, Takahiro Soeta, Masami Kuriyama, Ken-ichi Yamada
*
and Kiyoshi Tomioka
Graduate School of Pharmaceutical Sciences, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
Received 26 April 2005; revised 20 May 2005; accepted 23 May 2005
Available online 15 June 2005
Abstract—The copper–amidophosphane-catalyzed asymmetric addition reaction of dialkylzinc reagents with b-aryl and b-alkylnitroalkenes
afforded the corresponding nitroalkanes with moderate to good enantioselectivities (54–80% ee). The performance was highly dependent on
the reaction procedure where the addition of nitroalkene to the mixture of copper–amidophosphane and dialkylzinc gave higher ee than the
addition of dialkylzinc to a mixture of copper–amidophosphane and nitroalkene.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
amidophosphane-catalyzed conjugate addition of dialkyl-
zinc reagents to nitroalkenes (Scheme 1). Moderate to good
enantioselectivities were obtained for b-aryl- and
b-alkylnitroalkenes.
The external chiral ligand-controlled asymmetric conjugate
addition reaction of organometallics with activated olefins
has been a challenge in carbon–carbon bond forming
chemistry.1,2 The dialkylzinc species as a source of
nucleophiles is a key to success of the recently reported
copper-catalyzed conjugate addition to enones.3 Among
good acceptors for conjugate addition, nitroalkenes are an
interesting class of compounds due to the strongly electron-
withdrawing nitro group,4,5 which can be transformed into a
wide range of functionalities.6 Enantioselective addition of
dialkylzinc reagents to nitroalkenes has been scarcely
studied and different types of catalytic copper–phosphorus
ligand were used. The structure of these ligands has an
important impact on the selectivity. It is possible to use
TADDOL,7 phosphoramidites,8 sulfonamides,9 binaphthol-
based thioethers,10 and dipeptide phosphanes11 to achieve
high performance. We have been involved in the develop-
ment of chiral amidophosphane ligands12 and their use in
the copper-catalyzed asymmetric reactions. The copper-
catalyzed asymmetric alkylation of N-sulfonylimines,13
b-aryl-a,b-unsaturated N-sulfonylimines14 and enones15
with dialkylzinc is the recent example of our success. In
our continuing effort to broaden the scope of the copper–
chiral amidophosphane catalyst, we turned our interest to
the asymmetric conjugate addition of dialkylzinc reagents
to nitroalkenes.16,17 We report herein, the copper–
Scheme 1. The chiral amidophosphane L-copper-catalyzed asymmetric
conjugate addition of dialkylzinc reagents to nitroalkenes 1.
2. Results and discussion
2.1. Chiral phosphane–copper(II) triflate catalyzed
asymmetric conjugate addition of diethylzinc to
b-nitrostyrene 1a
In the initial experiments, three different amidophosphanes
L1, L213c,17b and L3,18 and (S)-BINAP 6 were tested in the
reaction of trans-b-nitrostyrene 1a (R1ZPh) with diethyl-
zinc (Fig. 1). A mixture of 5 mol% of copper(II) triflate and
Keywords: Organocopper; Asymmetric reaction; Conjugate addition;
Nitroolefin.
*
Corresponding author. Tel.: C81 75 753 4553; fax: C81 75 753 4604;
e-mail: tomioka@pharm.kyoto-u.ac.jp
Figure 1. Chiral amidophosphanes L1–L3 and (S)-BINAP 6.
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.05.070

F. Valleix et al. / Tetrahedron 61 (2005) 7420–7424
7421
6 mol% of L in toluene was stirred at rt for 1 h and then
treated with 1a for additional 15 min. Then, a toluene
solution of 3 equiv of diethylzinc was added to the mixture
above at K30 8C and the whole was stirred at K30 8C until
consumption of 1a. Workup and column chromatography
(hexane–AcOEtZ9/1) gave 2a in 44–67% yield (Table 1).
procedure. Indeed, it was essential for the selectivity. Two
procedures A and B of the reaction were tested (Table 2).
Procedure A. A solution of diethylzinc was added to a
toluene solution of copper–amidophosphane L and nitro-
alkene 1 at K30 8C.
Procedure B. Nitroalkene 1 was added to a solution of
copper–amidophosphane L-diethylzinc at K30 8C.
Table 1. Asymmetric reaction of b-nitrostyrene 1a with diethylzinc
The reaction of a variety of b-arylnitroalkenes 1a–1d has
been examined under both procedures using L1. The
enantioselectivity was quite enhanced by the procedure B.
The reactions performed smoothly to afford 2 with higher ee
than the procedure A. An electron withdrawing such as
fluoro group (1b) (60% ee, entry 4) and an electron donating
as a methoxy group (1c) (60% ee, entry 6) at the para
position of the phenyl moiety of trans-b-styrene 1a (68% ee,
entry 2) did not improve the enantioselectivity. A steric
effect was observed in the case of naphthylnitroethene 1d
which afforded 2d with only 10% ee under procedure A and
54% ee under procedure B (entries 7 and 8).
Entry
Phosphane
Time (h)
Yield (%)
ee (%)
1
2
3
4
L1
L2
L3
6
1
3
3
3
65
67
56
44
54
26
7
0
The structure of the chiral amidophosphane L was critical
for the enantioselectivity. The phosphanes L2 and L3
bearing bulky substituents on the pyrrolidine ring or on the
nitrogen atom of pyrrolidine were detrimental to the rate of
conversion and to the enantioselectivity, giving (S)-2a with
26% and 7% ee (entries 2 and 3). BINAP 6 gave miserable
selectivity (entry 4). The best phosphane was L1 and
afforded (S)-2a with 54% ee in 65% yield (entry 1).
2.3. Chiral phosphane–copper(II) triflate catalyzed
asymmetric conjugate addition of dialkylzinc to
b-alkylnitroethene 1
In order to explore the breadth of the copper–amidophos-
phane L1 application, the reaction of b-alkylnitroalkenes
1e–1g were examined under the procedure B (Table 3).
2-Isopropylnitroethene 1e afforded the corresponding
alkane 2e with 80% ee in 50% yield (entry 2). It is also
possible to use diisopropylzinc in the reaction of
2-ethylnitroethene 1f to give ent-2e with 62% ee in 50%
yield (entry 4).
The use of other copper salts, (CuOTf)2$C6H6, CuCN–
2LiCl, Cu(MeCN)4BF4, Cu(OAc)2–H2O, gave poorer
selectivity than Cu(OTf)2. Other solvents such as THF or
ether were detrimental to the ee. Even the use of
Cu(OAc)2$2H2O in ether, which was very efficient with
Alexakis’ ligand,19 did not improve our results. Moreover,
running the reaction at higher temperature decreased the
enantioselectivity, whereas lower temperature did not
improve it.
However, with the use of less reactive trisubstituted
nitroalkene 1g, the reaction was slow, after 12 h, to give
2g with 74% ee as a cis-major 85:15 mixture in 52% yield
(entry 6). Isomerization of cis-major mixture with DBU in
ether at rt for 2 h gave a trans-major 95:5 mixture of 2g
without loss of enantioselectivity. It is important to note that
hydrolysis should be done, in this case, with acetic acid at
2.2. Reaction procedure dependency of
enantioselectivity
The main surprise came from the influence of reaction
Table 2. Asymmetric reaction of diethylzinc with b-arylnitroalkenesa
Entry
1
Ar
A/B
2
Yield (%)
ee (%)
1
2
3
4
5
6
7
8
1a
1a
1b
1b
1c
1c
1d
1d
Ph
Ph
A
B
A
B
A
B
A
B
2a
2a
2b
2b
2c
2c
2d
2d
65
67
46
50
65
60
57
54
54
68
54
60
46
60
10
54
4-FC6H4
4-FC6H4
4-MeOC6H4
4-MeOC6H4
1-Naphthyl
1-Naphthyl
a Nitroalkene 1 was consumed after 1 h.

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F. Valleix et al. / Tetrahedron 61 (2005) 7420–7424
Table 3. Asymmetric reaction of diethylzinc with b-alkylnitroalkenesa
Entry
1
R2
A/B
2
Yield (%)
ee (%)
1
2
3
4
1e
1e
1f
1f
Et
Et
i-Pr
i-Pr
A
B
A
B
2e
2e
ent-2e
ent-2e
60
50
45
50
74
80
48
62
5
6
1g
1g
Et
Et
A
B
2gb
64
52
58
74
2gb
a Nitroalkene 1 was consumed after 1 h except for entries 5 and 6 where the reaction required 12 h for completion.
b cis-Major 85:15 mixture was obtained as an initial product which was then isomerized to a trans-major 95:5 mixture. ee was determined for trans 2g after
isomerization.
K30 8C during 1 h to avoid the Nef reaction. However,
about 10% of ketone was obtained.
4.2. Typical procedure for the synthesis of racemic 2
CuCN (2.6 mmol, 230 mg) and LiCl (5.2 mmol, 220 mg)
was dissolved in 4 mL of THF and stirred for 15 min. To the
mixture 1.3 equiv of 1 M toluene solution of dialkylzinc
was added at K10 8C, and the mixture was stirred for
additional 10 min. Then, 2 mmol of nitroalkene in 2 mL of
THF was added at K78 8C, and the mixture was allowed to
warm to rt. After 2 h, the reaction was quenched with
saturated NH4Cl solution. The organic layers were washed
with brine, and then concentrated to afford nitroalkanes
2a–g with moderate to good yields after column
chromatography.
3. Conclusion
In conclusion, the catalytic asymmetric conjugate addition
of dialkylzinc reagent to different type of nitroalkenes was
achieved with moderate to good enantioselectivities and
reasonably good yields using copper–amidophosphane L1
catalyst.
4.3. Procedures A and B for the asymmetric conjugate
addition of dialkylzinc
4. Experimental
Procedure A. Copper(II) triflate (10 mg, 5 mol%) was dried
under heat and vacuum. After cooling down, L1 (12 mg,
6 mol%) and 10 mL of toluene were added. The solution
was stirred at rt for 1 h. Then, nitroalkene 1 (0.5 mmol) was
added and the reaction mixture was stirred for additional
15 min. The solution was then cooled to K30 8C and
diethylzinc in toluene (1.5 mL, 3 equiv) was added over
1 min. After that, the same procedure with B.
4.1. General
All reactions were carried out under argon atmosphere.
Column chromatography: Silicycle silica gel 230–400
mesh, TLC: Merck silica gel 60 F254. H and 13C NMR
1
spectra were recorded on a GEOL Unity 500 (500 and
125 MHz respectively) using CDCl3 as a solvent. Chemical
shift values are expressed in ppm relative to internal
tetramethylsilane. Coupling constants J values are presented
in Hz. Abbreviations are as follows: s, singlet; d, doublet; t,
triplet; m, multiplet. Diastereoisomeric excess was deter-
mined by 1H NMR. MS (EI) is presented in m/z.
Amidophosphane ligands L1, L2,13c,17b and L318 were
prepared according to the laboratory procedure. Nitroalk-
enes 1a, 1c and 1g are commercially available (Aldrich) and
1b, 1d5a and 1e, 1f5b were synthesized by the reported
procedure. Diethylzinc in toluene and diisopropylzinc in
toluene were purchased from Aldrich. Enantiomer sepa-
ration of compounds 2a–f was performed on a Hitachi
G-3900 gas chromatograph equipped with a Chirasil-Dex
CB column (25 m!0.25 mm) or b-Dex 225 column
(30 m!0.25 mm), helium and nitrogen as the carrier gas:
33 mL/min, detector temperature: 200 8C, injection tem-
perature: 200 8C, FID detector. Absolute configurations
were obtained by comparison of chiral stationary phase
HPLC data (OD-H column, hexane/2-propanolZ20:1, 0.5
or 1 mL/min) with literature data.
Procedure B. To copper(II) triflate (10 mg, 5 mol%) were
added L1 (6 mol%, 12 mg) and 10 mL of toluene. The
solution was stirred at rt for 1 h. To the solution was added a
toluene solution of diethylzinc (0.7 mL, 1.4 equiv) over
5 min at K35 8C. The mixture was stirred for 15 additional
min. Then, nitroalkene 1 (0.5 mmol) was added. The
reaction was monitored by TLC and GC. After completion
of the reaction, 5 mL of saturated NH4Cl solution was added
and the reaction mixture was allowed to warm to rt. The
mixture was extracted with 10 mL of AcOEt and the organic
layers were washed with brine, and dried over Na2SO4.
Concentration and silica gel column chromatography
(hexane–AcOEtZ9/1) gave 2 and less than 5% of an alkene
product arising from the substitution of the nitro group by
the alkyl group.
4.3.1. (K)-(S)-1-(1-Nitrobutan-2-yl)benzene (2a).20
Table 2, entry 2: a light yellow oil of [a]2D1 K31.7 (c 1.25,
CHCl3) ([a]2D3 C38.2 (c 1.1, CHCl3) for (R)-2a with 97%

F. Valleix et al. / Tetrahedron 61 (2005) 7420–7424
7423
ee). 68% ee by Chirasil DEX at 120 8C: 15.5 and 16.2 min
for major (S) and minor (R), and HPLC (0.5 mL/min): 20.5
and 29.7 min for minor (R) and major (S). Rf 0.5 (hexane–
Infrastructure for Genome Science’ and a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. V. F. and M. K.
thank to financial support provided by the Japanese Society
for the Promotion of Science (JSPS).
1
AcOEtZ9/1). H: 1.60 (t, 3H, JZ7.4 Hz), 1.75 (m, 2H),
3.30 (m, 1H), 4.55 (m, 2H), 7.05–7.40 (m, 6H). 13C: 11.4,
26.0, 46.0, 80.7, 127.6, 128.9, 139.3. m/z: 179 (MC).
4.3.2. (K)-(S)-1-Fluoro-4-(1-nitrobutan-2-yl)benzene
(2b).8a Table 2, entry 4: a light yellow oil of [a]2D1 K22.9
(c 1.00, CHCl3). 60% ee by Chirasil DEX at 110 8C: 36.8
and 39.0 min for major (S) and minor (R), and HPLC (1 mL/
min): 18.6 and 20.5 min for minor (R) and major (S). Rf 0.4
References and notes
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1
(hexane/AcOEt Z9:1). H: 0.80 (t, 3H, JZ7.4 Hz), 1.65
(m, 2H), 3.60 (m, 1H), 4.50 (m, 2H), 7.00–7.60 (m, 4H).
13C: 11.4, 26.2, 45.3, 80.7, 115.8 (d, JZ21.7 Hz), 129.1 (d,
JZ8.3 Hz), 135.0, 163.1 (d, JZ247.3 Hz). m/z: 197 (MC).
4.3.3. (K)-(S)-1-Methoxy-4-(1-nitrobutan-2-yl)benzene
(2b).21 Table 2, entry 6: a light yellow oil of [a]2D1 K25.7
(c 1.00, CHCl3). 60% ee by Chirasil DEX at 110 8C: 47.2
and 48.7 min for major (S) and minor (R), and HPLC (1 mL/
min): 23.5 and 36.8 min for minor (R) and major (S). Rf 0.55
¨
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1
(hexane/AcOEt Z9:1). H: 0.80 (t, 3H, JZ7.4 Hz), 1.65
(m, 2H), 3.25 (m, 1H), 3.75 (s, 3H), 4.50 (m, 2H), 6.80 (m,
2H), 7.20 (m, 2H). 13C: 11.5, 26.1, 45.3, 55.2, 81.0, 114.3,
128.6, 131.2, 159.0. m/z: 209 (MC).
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1
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13C: 11.3, 25.9, 80.3, 122.5, 125.4, 125.9, 126.7, 128.1,
129.2, 131.9, 134.2, 135.2. m/z: 229 (MC). HRMS: calcd for
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1
for major and minor. Rf 0.55 (hexane–AcOEtZ9/1). H:
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10H), 2.11–2.21 (m, 1H), 4.21 (td, 1H, JZ11.3, 3.4 Hz for
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This research was supported by the 21st Century COE
(Center of Excellence) Program ‘Knowledge Information

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