Enantioselective Cu(II)-catalyzed Henry reactions with chiral cyclohexane-based amidophosphine ligands

Article abstract of DOI:10.1016/j.tetasy.2016.07.015

Copper-catalyzed asymmetric Henry reactions are described. Using a new chiral amidophosphine ligand, the Henry reaction of nitromethane and various aldehydes proceeded smoothly to provide chiral β-nitroalcohols in reasonable yields (up to 98%) with high e

Full text of DOI:10.1016/j.tetasy.2016.07.015

Tetrahedron: Asymmetry xxx (2016) xxx–xxx
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Enantioselective Cu(II)-catalyzed Henry reactions with chiral
cyclohexane-based amidophosphine ligands
⇑
⇑
Rukeya Rexiti, Jian Lu, Ge Wang, Feng Sha , Xin-Yan Wu
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper-catalyzed asymmetric Henry reactions are described. Using a new chiral amidophosphine ligand,
the Henry reaction of nitromethane and various aldehydes proceeded smoothly to provide chiral b-
nitroalcohols in reasonable yields (up to 98%) with high enantioselectivity (up to 97% ee).
Ó 2016 Elsevier Ltd. All rights reserved.
Received 10 May 2016
Accepted 26 July 2016
Available online xxxx
1. Introduction
reactions.8 To the best of our knowledge, the use of chiral cyclohex-
ane-based P,N-ligands in the copper-catalyzed reaction or Henry
The Henry reaction, which is
a
nucleophilic addition of
reaction has not been described.
nitroalkanes to carbonyl compounds, is a useful C–C bond forma-
tion reaction. The produced b-nitroalcohols can be easily trans-
formed to a variety of valuable synthetic intermediates such as
amino alcohols, carboxylic acids and aldehydes.1 Since Shibasaki
et al.2 reported the first example of an asymmetric Henry reaction
in 1992, impressive progress has been made on the exploration of
chiral organometallic and organic catalysts for this asymmetric
reaction.3 Among them, the copper-based catalyst systems have
received much attention due to the low toxicity, low cost and good
chelating properties. Although nitrogen- and phosphine-contain-
ing ligands are attractive in the field of asymmetric catalysis,4 to
our knowledge, there has been only one example of the chiral N,
P-ligand-copper catalyzed Henry reaction. In 2007, Shi et al.5
described a Cu(I)-catalyzed Henry reaction using chiral BINOL-
derived phosphine-salen type ligands, achieving up to 80% ee.
Therefore, the exploration of new and efficient chiral N,P-ligands
for the copper-catalyzed Henry reaction is still in demand.
It is well known that the trans-1,2-cyclohexane scaffold is one
of the most popular chiral backbones in chiral ligands and chiral
organocatalysts. Among the impressive advances in the develop-
ment of nitrogen- and phosphine-containing ligands based on
the trans-1,2-cyclohexane fragment, considerable effort has been
focused on the derivatization of chiral trans-1,2-diaminocyclohex-
ane, in particular the Trost ligands.6 On the other hand, the chiral
cyclohexane-based bidentate or tridentate P,N-ligands have been
poorly developed (Fig. 1), and these ligands were used in asymmet-
ric allylic alkylation and hygrogenation.7 Additionally, Toste et al.
have developed a monophosphine gold(I) catalyst based on the
chiral cyclohexane backbone for asymmetric three-component
Ph
N
NH
NR2
PPh2
PPh2
O
PAr2
NHBn
N
R
NH PPh2
Figure 1. The reported chiral cyclohexane-based bidentate and tridentate P,N-
ligands.
In our recent studies on asymmetric organocatalysis, we have
found that the chiral cyclohexane-based amide-phosphines are
efficient for [4+2] cycloaddition.9 Our new approach is to develop
novel amidophosphine ligands derived from chiral compound L1
and its enantiomer (Fig. 2). As mentioned above, the copper-cat-
alyzed Henry reaction is selected to evaluate the chiral ligands.
Herein, we report our preliminary results on the synthesis of novel
amidophosphine ligands and their application to the asymmetric
Henry reaction.
2. Results and discussion
2.1. Synthesis of the chiral ligands
Ligand L1 and its enantiomer were synthesized according to the
reported procedure.7b,10 Amidophosphine L2 was prepared by the
⇑
Corresponding authors.
http://dx.doi.org/10.1016/j.tetasy.2016.07.015
0957-4166/Ó 2016 Elsevier Ltd. All rights reserved.
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R. Rexiti et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
Table 1
O
O
Screening of the chiral ligands for the asymmetric Henry reactiona
N
H
N
H
Ph
NH2
CHO
OH
PPh2
N
PPh2
PPh2
L2
10 mol% Cu(OAc)2.H2O
NO2
10 mol% L1-L9
L1
L3
CH3NO2
+
CH3OH, 25 oC
O2N
O
O
O
NO2
N
H
N
H
N
H
Entry
Ligand
Time (d)
Yieldb (%)
eec (%)
PPh2
HN
PPh2
HN
PPh2
N
1
2
3
4
5
6
7
8
9
L1
L2
L3
L4
L5
L6
L7
L8
L9
2.5
2.5
1.5
2.5
1
0.5
1.5
1
91
93
98
81
96
96
98
91
95
0
1
78
49
33
ꢁ48
ꢁ29
ꢁ24
14
L4
L6
L5
O
O
N
H
N
H
N
PPh2
N
PPh2
NH2
PPh2
NHTs
L7
L8
L9
0.5
Figure 2. The structures of the chiral phosphine ligands.
a
The reactions were carried out with 4-nitrobenzaldehyde (0.2 mmol), nitro-
methane (0.6 mL), 10 mol% Cu(OAc)2ꢀH2O and 10 mol% chiral ligand in 0.8 mL
CH3OH at 25 °C.
b
Isolated yield.
c
condensation reaction between L1 and benzoyl chloride.11 Ami-
dophosphine L3–L8 were prepared via the condensation reaction
between L1 or its enantiomer and the corresponding carboxylic
acids,9 and a further deprotecting step was carried out to obtain
ligands L5–L7 (Scheme 1). Iminophosphine L9 was prepared by
the condensation of L1 with picolinaldehyde.
The ee values were determined by HPLC using a Chiralcel OD-H column.
Encouraged by these preliminary results, our attention was
then focused on the screening of copper salts (Table 2). The results
indicated that Cu(OAc)2ꢀH2O, Cu(OAc)2 and Cu(HCO2)2ꢀ4H2O were
good promoters for the Henry reaction, giving high yields with
78% ee (entries 1–3). Although Cu(acac)2, Cu(CH3CN)4ClO4 and Cu
(CH3CN)4BF4 provided excellent yields, the enantioselectivities
were poor (entries 4, 11 and 12). The Henry reaction in the pres-
ence of CuSO4 was sluggish, and both the conversion and the enan-
tioselectivity were unfavorable (entry 5). When Cu(OTf)2, Cu
(NO3)2ꢀ3H2O, CuCl2 and Cu(OTf)ꢀ0.5C6H6 were used as the pre-cat-
alyst, the acetal of 4-nitrobenzaldehyde was obtained instead of
the corresponding b-nitroalcohol (entries 6–9). These results indi-
cated that the copper salt played an important role in controlling
the reaction rate, reaction selectivity and stereoselectivity. In terms
of the yield and enantioselectivity, Cu(OAc)2ꢀH2O was chosen as
the copper source for subsequent reactions.
Table 2
Screening of copper salts for the asymmetric Henry reactiona
Scheme 1. Representative synthetic pathways for: (a) L3 and L4; (b) L5; (c) L9.
CHO
OH
10 mol% copper salt
NO2
10 mol% L3
CH3NO2
+
CH3OH, 25 o
C
2.2. Asymmetric Henry reaction
O2N
NO2
Initially, the enantioselective Henry reaction of nitromethane
with 4-nitrobenzaldehyde was selected as the model reaction to
test the catalytic performance of the new chiral ligands. The
reactions were performed with 10 mol% of Cu(OAc)2ꢀH2O and
10 mol% of ligands L1–L9 in CH3OH at 25 °C, and the results were
summarized in Table 1. The results indicated that all the ligands
gave good yields (81–98%) with different enantioselectivities.
While aminophosphine L1 and amidophosphine L2 provided race-
mic products (entries 1 and 2), the iminophosphine L9 displayed
poor stereoselectivity (entry 9). With 2-picolinic acid derivative
L3 as the chiral ligand, the Cu(II)-catalyzed Henry reaction was
achieved in 78% ee with 98% yield (entry 3). We observed a chiral
match between the cyclohexane backbone and the amino acid
scaffold, and ligand L6 was more efficient than its diastereomer
L5 (entry 6 vs entry 5). According to the results shown in Table 1,
ligand L3 was selected for further experiments, and the related
product was assigned the (R)-configuration by referring to the
specific rotations in literature.5
Entry
Copper salt
Time (d)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
Cu(OAc)2ꢀH2O
Cu(OAc)2
1.5
2
1.5
0.3
3
1
1
1
3
98
98
95
99
33
78
78
78
2
38
—
—
—
—
Cu(HCO2)2ꢀ4H2O
Cu(acac)2
CuSO4
Cu(OTf)2
ndd
ndd
ndd
ndd
76
Cu(NO3)2ꢀ3H2O
CuCl2
9
Cu(OTf)ꢀ0.5C6H6
CuI
Cu(CH3CN)4ClO4
Cu(CH3CN)4BF4
10
11
12
2.5
1
1.5
37
10
12
99
91
a
The reactions were carried out with 4-nitrobenzaldehyde (0.2 mmol), nitro-
methane (0.6 mL), 10 mol% copper salt and 10 mol% L3 in 0.8 mL CH3OH at 25 °C.
b
Isolated yield.
c
The ee values were determined by HPLC using a Chiralcel OD-H column.
The desired products were not detected, and the acetal of 4-nitrobenzaldehyde
d
was obtained.
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R. Rexiti et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
3
Table 4
Next, the reaction solvent was surveyed (Table 3). In all the sol-
Further optimization of the reaction conditionsa
vents screened except toluene and CHCl3, good yields were
achieved (86–99% ee, entries 2 and 4–15). The lower yields in
toluene and CHCl3 were a result of the lower conversion. The Henry
reaction in DMSO and CH3CN resulted in poor enantioselectivities
(entries 9 and 10). Among all the solvents in Table 3, t-BuOH pro-
vided the highest enantioselectivity. Therefore, t-BuOH was chosen
as the solvent to carry out additional experiments.
CHO
OH
L3
10 mol% Cu(OAc)2.H2O,
NO2
CH3NO2
+
t-BuOH, 25 oC
O2N
NO2
Entry
L3 (mol%)
Concn (M)
Time (d)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
10
10
10
10
5
15
20
5
0.35
0.25
0.15
0.05
0.25
0.25
0.25
0.25
0.25
0.25
0.25
1.5
1.5
2.5
4.5
3
1.1
0.8
3
98
98
96
98
87
99
99
98
38
99
99
73
83
84
84
83
22
13
85
87
58
53
Table 3
Screening of solvents in the asymmetric Henry reactiona
CHO
OH
10 mol% Cu(OAc)2.H2O
10 mol% L3
NO2
+
CH3NO2
8d
9e
10f
11g
solvent, 25 o
C
10
10
10
3.5
1
1
O2N
NO2
Entry
Solvent
Time (d)
Yieldb (%)
eec (%)
a
Unless stated otherwise, the reactions were carried out with 4-nitrobenzalde-
hyde (0.2 mmol), nitromethane (0.6 mL), 10 mol% Cu(OAc)2ꢀH2O in t-BuOH at 25 °C.
1
2
3
4
5
6
7
8
Toluene
CH2Cl2
CHCl3
Et2O
THF
EtOAc
Acetone
CH3NO2
DMSO
CH3CN
CH3OH
EtOH
i-PrOH
i-BuOH
t-BuOH
3
3
3
2.5
2
3
2.5
3
0.5
3
1.5
1.5
1.5
1.5
1.5
51
87
46
95
95
92
98
86
98
96
98
99
99
99
99
75
77
78
73
75
77
74
72
10
48
78
75
81
78
83
b
Isolated yield.
c
The ee values were determined by HPLC using a Chiralcel OD-H column.
d
The amount of (CuOAc)2ꢀH2O was 5 mol%.
e
The reaction was performed at 0 °C in i-PrOH.
f
The reaction was performed at 40 °C.
The reaction was performed at 40 °C in i-PrOH.
g
9
With the optimal reaction conditions in hand [5 mol% Cu
(OAc)2ꢀH2O and 5 mol% L3 in 0.8 mL t-BuOH (0.2 mmol scale) at
25 °C], the scope of substrates was examined (Table 5). A variety
of aromatic aldehydes were employed as electrophiles to react
with nitromethane, giving the corresponding b-nitroalcohols in
moderate-to-excellent yields and good enantioselectivities
(81–97% ee). Benzaldehydes containing electron-withdrawing
groups have been proven to be more reactive than benzaldehyde
10
11
12
13
14
15
a
The reactions were carried out with 4-nitrobenzaldehyde (0.2 mmol), nitro-
methane (0.6 mL), 10 mol% Cu(OAc)2ꢀH2O and 10 mol% L3 in 0.8 mL solvent at
25 °C.
b
Isolated yield.
c
Table 5
The ee values were determined by HPLC using a Chiralcel OD-H column.
Substrate scope of the enantioselective Henry reactiona
5 mol% Cu(OAc)2.H2O
O
OH
5 mol% L3
NO2
CH3NO2
+
With t-BuOH as solvent, the reaction conditions including the
substrate concentration, the catalyst loading and the reaction tem-
perature were investigated (Table 4). The results indicated that
similar yields were produced within a substrate concentration
range of 0.05–0.35 M (regarding the volume of t-BuOH), while an
obvious decrease of the enantioselectivity was observed with
0.35 M (entries 2–4 vs 1). Subsequently, the ratio of ligand L3 to
Cu(OAc)2ꢀH2O was examined, and the amount of Cu(OAc)2ꢀH2O
was kept consistent at 10 mol%. When the loading of ligand L3
was reduced to 5 mol%, a long reaction time was needed to
complete the Henry reaction (entry 5 vs entry 2). When the ratio
of L3 to Cu(OAc)2ꢀH2O was changed to 1.5:1 or 2:1, the reaction
rate was improved, but the enantioselectivity decreased
remarkably (entries 6 and 7). These results are in agreement with
Yudin’s observation on Pd-catalyzed allylic substitution using
cyclohexane-based iminophosphine ligands.7c When the catalyst
loading was reduced to 5 mol% (1:1 L3 to copper), similar results
were obtained with a longer reaction time (entry 8 vs entry 2).
Increasing the temperature to 40 °C accelerated the Henry
reaction rate, while the enantioselectivity was notably
decreased (entry 10 vs entry 2). Because of the melting point of
t-BuOH, the reaction at 0 °C was performed in i-PrOH. Although a
t-BuOH, 25 oC
Ar
H
Ar
1
2
Entry
Ar
Time (d)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
4-NO2C6H4
3-NO2C6H4
2-NO2C6H4
4-CF3C6H4
4-CNC6H4
4-BrC6H4
4-ClC6H4
4-FC6H4
3
4
4
4
4
5
4
4
4
5
5
5
5
6
6
6
5
5
6
98 2a
94 2b
93 2c
85 2d
68 2e
53 2f
71 2g
62 2h
27 2i
66 2i
54 2j
52 2k
60 2l
82 2m
77 2n
73 2o
97 2p
89 2q
64 2r
85
81
84
88
87
86
89
91
88
88
89
89
89
85
84
85
88
97
69
9
C6H5
C6H5
10d
11d
12d
13d
14d
15
16
17
18
19
4-MeC6H4
3-MeC6H4
2-MeC6H4
4-PhC6H4
Naphthalen-1-yl
Naphthalen-2-yl
2,3-Cl2C6H3
2,6-Cl2C6H3
(E)-C6H5CH = CH
a
Unless stated otherwise, the reactions were carried out with aldehyde
(0.2 mmol), nitromethane (0.6 mL), 5 mol% Cu(OAc)2ꢀH2O and 5 mol% L3 in 0.8 mL
slightly
higher
enantioselectivity
was
exhibited,
the
t-BuOH at 25 °C.
b
Isolated yield.
Henry reaction became sluggish at 0 °C (entry 9 vs entry 13 in
Table 3).
c
The ee values were determined by HPLC using a Daicel Chiralcel OD-H.
d
10 mol% (CuOAc)2ꢀH2O and 10 mol% L3 were used.
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R. Rexiti et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
and 4-methyl benzaldehyde, and better yields were obtained
(entries 1–8 vs 9–13). Notably, 2-nitrobenzaldehyde could provide
excellent yield and good enantioselectivity (entry 3). With 10 mol%
of (CuOAc)2ꢀH2O and L3, the Henry reaction between nitromethane
and 4-phenylbenzaldehyde provided 82% yield and 85% ee (entry
14). 1-Naphthaldehyde and 2-naphthaldehyde proceeded well to
afford the desired products in moderate yields and good enantios-
electivities (entries 15 and 16). In addition, dichloro-substituted
benzaldehydes gave high yields and stereoselectivities (entries
4. Experimental
4.1. General
All reactions were carried out under N2 atmosphere using stan-
dard Schlenk techniques with magnetic stirring. Anhydrous sol-
vents were distilled from CaH2 (dichloromethane, chloroform,
ethyl acetate, acetonitrile), sodium (CH3OH, EtOH, i-PrOH, t-BuOH),
or sodium-benzophenone (toluene, ether, THF). Anhydrous DMSO
was dried over CaH2 and distilled under reduced pressure. All alde-
hydes and nitroalkanes were commercially available and purified
by standard methods. Thin-layer chromatography (TLC) was per-
17 and 18). Moreover, the
a,b-unsaturated aldehyde such as
cinnamaldehyde gave the corresponding product in 64% yield with
69% ee (entry 19). Furthermore, different nitroalkanes were
tested with 4-nitrobenzaldehyde as the electrophile under the
typical reaction conditions (Fig. 3). Unfortunately, both the
diastereoselectivity and the enantioselectivity were unsatisfactory.
More efficient catalytic system should thus be explored for these
nitroalkanes.
formed on Silicycle 10–40 lm silica gel plates. Column chromatog-
raphy was performed using silica gel (300–400 mesh) eluting with
petroleum ether and ethyl acetate.
Melting points were taken without correction. Optical rotations
were measured on a WZZ-2A digital polarimeter at the wavelength
of the sodium D-line (589 nm). IR spectra were recorded on Nicolet
Magna-I 550 spectrometer. The NMR spectra were recorded on
Bruker 400 spectrometer. The chemical shifts of 1H NMR spectra
were referenced to tetramethylsilane (d 0.00) using CDCl3 as sol-
vent. The chemical shifts of 31P NMR spectra were referenced to
85% H3PO4 (0.0 ppm). High Resolution Mass spectra (HRMS) were
recorded on Micromass GCT with Electron Spray Ionization (ESI)
resource. HPLC analysis was performed on Waters equipment
using Daicel Chiralcel OD-H or Chiralpak IC-H column.
Ligands L17b,10 and L211 were prepared according to literature
procedures.
OH
OH
NO2
NO2
*
O2N
O2N
2t
2s
87% yield, 55: 45 dr
25% ee and 19% ee
92% yield, 7% ee
Figure 3. The Henry reaction of nitroalkanes.
We have tried but failed to get a single crystal of the Cu(II)–L3
complex, so the real active species is not yet fully understood in
this catalytic asymmetric Henry reaction. According to the litera-
ture concerning Cu(II)-catalyzed enantioselective Henry reac-
tions,12 a plausible transition state was proposed as shown in
Figure 4. The ligand L3 might be tridentate. The benzaldehyde
should be positioned in one of the Lewis acidic equatorial sites
for maximal activation. The Henry reaction does not need an exter-
nal base, suggesting that a weakly Lewis acidic metal complex
facilitates the deprotonation of nitroalkane. In the depicted transi-
tion state, the nitronate would attack the activated aldehyde from
the Si-face to form the Henry adduct with an (R)-configuration.
4.2. Synthesis of chiral ligands L3–L8
To a solution of L1 or its enantiomer (0.35 mmol) in dry CH2Cl2
(6 mL), dicyclohexylcarbodiimide (0.39 mmol), carboxylic acid or
N-protected amino acid (0.44 mmol) and DMAP (0.11 mmol) were
added, respectively. Then the reaction mixture was stirred at
25 °C under N2 atmosphere for 2–6 h (monitoring by TLC). After
the reaction was completed, the solvent was removed under
reduced pressure and the residue was purified by a flash column
chromatography on silica gel to afford the amidophosphine
compounds.
For preparing ligands L5–L7, a mixture of 1:1 CH2Cl2/TFA
(12 mL) was added to the N-Boc amidophosphine, and the solution
was stirred at 25 °C for 3 h under N2 atmosphere. Then the reaction
mixture was neutralized with aqueous NaHCO3 to pH ca. 9, and it
was extracted with CH2Cl2 (3 ꢂ 10 mL). The combined organic
layers were dried over Na2SO4 and concentrated under reduced
pressure. The residue was purified by a flash column chromatogra-
phy on silica gel to afford the deprotected products.
O
N
H
N
P
OAc
O
CuII
Ar
H
Ph
Ph
O
O
N
4.2.1. N-((1S,2S)-2-(Diphenylphosphino)cyclohexyl)picolinamide
Si-attack
L3
White solid, 91% yield, mp 52.9–53.7 °C, [a]
25 = + 89.1 (c 0.74,
D
CH2Cl2); 1H NMR (CDCl3, 400 MHz) d: 8.35 (d, 1H, J = 4.4 Hz), 8.14
(d, 1H, J = 8.0 Hz), 8.05 (d, 1H, J = 8.8 Hz), 7.73 (t, 1H, J = 7.6 Hz),
7.56–7.48 (m, 4H), 7.31–7.26 (m, 4H), 7.23–7.19 (m, 2H), 7.15–
7.12 (m, 1H), 4.11–4.03 (m, 1H), 2.55–2.51 (m, 1H), 2.21–2.18 (m,
1H), 1.82–1.74 (m, 3H), 1.51–1.26 (m, 3H), 1.16–1.08 (m, 1H); 13C
NMR (CDCl3, 100 MHz) d: 162.9, 149.8, 147.7, 137.0, 136.8 (d,
J = 13.2 Hz), 135.9 (d, J = 16.1 Hz), 134.3 (d, J = 20.7 Hz), 132.8 (d,
J = 18.7 Hz), 128.7, 128.2 (d, J = 6.4 Hz), 128.1 (d, J = 7.5 Hz), 127.9,
125.7, 122.0, 50.2 (d, J = 16.6 Hz), 40.1 (d, J = 15.3 Hz), 33.3 (d,
J = 7.0 Hz), 27.3 (d, J = 5.5 Hz), 25.3 (d, J = 5.5 Hz), 24.4; 31P NMR
Figure 4. A plausible transition state of the asymmetric Henry reaction.
3. Conclusion
In conclusion, seven new phosphine- and nitrogen-containing
ligands L3–L9 were successfully synthesized and employed in the
asymmetric Henry reaction. In the presence of the copper(II) com-
plex with chiral ligand L3, the Henry reaction was achieved in
moderate to excellent yields (up to 98%) and good to excellent
enantioselectivities (up to 97% ee) under mild conditions. X-ray
crystallographic analysis of the copper complexes and the exten-
sion of these chiral ligands to other asymmetric reactions are
currently underway in our laboratory.
(CDCl3, 122 MHz, 85% H3PO4): ꢁ8.94; IR (KBr, cmꢁ1):
m 2933,
2859, 1677, 1519, 1430, 749, 697, 659, 614, 510; HRMS (ESI) calcd
for C24H26N2OP ([M+H]+): 389.1777, found: 389.1788.
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R. Rexiti et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
5
4.2.2. N-((1S,2S)-2-(Diphenylphosphino)cyclohexyl)quinoline-2-
122 MHz, 85% H3PO4): ꢁ7.64; IR (KBr, cmꢁ1):
m
3323, 2941, 2859,
carboxamide L4
1662, 1512, 1437, 1079, 741, 697, 502; HRMS (ESI) calcd for
White solid, 94% yield, mp 64.1–65.2 °C, [
a
]
D
25 = +204.4 (c 0.77,
C
23H32N2OP ([M+H]+): 383.2247, found: 383.2249.
CH2Cl2); 1H NMR (CDCl3, 400 MHz) d: 8.26 (d, 1H, J = 8.4 Hz),
8.23 (d, 1H, J = 8.4 Hz), 8.17 (d, 1H, J = 8.8 Hz), 7.98 (d, 1H,
J = 8.4 Hz), 7.83 (d, 1H, J = 8.0 Hz), 7.72 (t, 1H, J = 7.6 Hz), 7.60–
7.50 (m, 5H), 7.33–7.25 (m, 3H), 7.12 (t, 1H, J = 7.6 Hz), 6.94–6.90
(m, 1H), 4.20–4.11 (m, 1H), 2.64–2.59 (m, 1H), 2.28–2.17 (m,
1H), 1.84–1.79 (m, 3H), 1.61–1.32 (m, 3H), 1.21–1.13 (m, 1H);
13C NMR (CDCl3, 75 MHz) d: 163.0, 149.5, 146.1, 137.0, 136.8 (d,
J = 13.1 Hz), 136.0 (d, J = 15.8 Hz), 134.2 (d, J = 20.6 Hz), 132.8 (d,
J = 19.1 Hz), 129.7 (d, J = 14.5 Hz), 129.0, 128.7, 128.2 (ꢂ2), 128.1,
128.1, 127.9, 127.5, 118.7, 50.7 (d, J = 17.0 Hz), 40.3 (d,
J = 15.5 Hz), 33.5 (d, J = 6.9 Hz), 27.6 (d, J = 5.7 Hz), 25.4 (d,
J = 5.5 Hz), 24.6; 31P NMR (CDCl3, 122 MHz, 85% H3PO4): ꢁ8.80;
4.2.6. (S)-N-((1R,2R)-2-(Diphenylphosphino)cyclohexyl)-3-methyl-
2-(4-methylphenylsulfonamido)butan-amide L8
White solid, 92% yield, mp 96.1–98.3 °C, [
a]
25 = ꢁ19.1 (c 0.98,
D
CH2Cl2); 1H NMR (CDCl3, 400 MHz) d: 7.74 (d, 2H, J = 8.0 Hz),
7.46 (t, 2H, J = 6.8 Hz), 7.39–7.27 (m, 8H), 7.22 (d, 2H, J = 8.4 Hz),
6.16 (d, 1H, J = 8.4 Hz), 5.29 (d, 1H, J = 7.6 Hz), 3.72–3.64 (m, 1H),
3.47 (dd, 1H, J1 = 7.6 Hz, J2 = 4.4 Hz), 2.26 (s, 3H), 2.20 (td, 1H,
J1 = 10.4 Hz, J2 = 2.4 Hz,), 2.04–1.98 (m, 2H), 1.70–1.55 (m, 3H),
1.26–1.14 (m, 3H), 0.94–0.85 (m, 1H), 0.79 (d, 3H, J = 6.8 Hz),
0.72 (d, 3H, J = 6.8 Hz); 13C NMR (CDCl3, 100 MHz) d: 169.0,
143.4, 136.8 (d, J = 13.7 Hz), 136.6, 135.4 (d, J = 17.0 Hz), 134.2 (d,
J = 20.5 Hz), 132.5 (d, J = 18.1 Hz), 129.6, 128.8, 128.6 (d,
J = 5.9 Hz), 128.2, 128.0 (d, J = 7.4 Hz), 127.1, 61.7, 51.0 (d,
J = 16.8 Hz), 39.1 (d, J = 16.1 Hz), 33.0 (d, J = 6.1 Hz), 31.2, 27.1,
25.2, 24.1, 21.3, 19.0, 16.8; 31P NMR (CDCl3, 122 MHz, 85%
IR (KBr, cmꢁ1):
m 2933, 2851, 1669, 1527, 1505, 1430, 1273, 847,
741, 697; HRMS (ESI) calcd for C28H28N2OP ([M+H]+): 439.1934,
found: 439.1944.
4.2.3. (S)-N-((1S,2S)-2-(Diphenylphosphino)cyclohexyl)pyrrolidi-
H3PO4): ꢁ8.61; IR (KBr, cmꢁ1):
m 3277, 2933, 1662, 1535, 1445,
1333, 1153, 1093, 741, 697, 659, 555; HRMS (ESI) calcd for C30H38
ne-2-carboxamide L5
-
White solid, 83% total yield, mp 166.2–167.1 °C, [
a]
25 = ꢁ10.4 (c
N2O3PS ([M+H]+): 537.2335, found: 537.2342.
D
0.70, CH2Cl2); 1H NMR (CDCl3, 400 MHz) d: 7.78 (d, 1H, J = 9.2 Hz),
7.52 (t, 2H, J = 6.8 Hz), 7.42–7.33 (m, 4H), 7.31–7.25 (m, 4H), 3.73–
3.67 (m, 1H), 3.63 (dd, 1H, J1 = 9.2 Hz, J2 = 5.6 Hz), 2.89–2.84 (m,
1H), 2.74–2.68 (m, 1H), 2.34–2.30 (m, 1H), 2.12–2.03 (m, 2H),
1.94–1.86 (m, 2H), 1.70–1.52 (m, 5H), 1.38–1.23 (m, 3H), 1.00–
0.91 (m, 1H); 13C NMR (CDCl3, 100 MHz) d: 173.9, 137.1 (d,
J = 13.6 Hz), 135.5 (d, J = 17.6 Hz), 134.6 (d, J = 20.7 Hz), 132.2 (d,
J = 17.3 Hz), 128.6, 128.2 (d, J = 5.6 Hz), 127.9 (d, J = 7.6 Hz),
127.7, 60.6, 49.3 (d, J = 16.6 Hz), 47.0, 39.8 (d, J = 15.6 Hz), 33.5
(d, J = 6.8 Hz), 30.5, 27.1, 26.0, 25.4 (d, J = 3.3 Hz), 24.4; 31P NMR
4.3. Synthesis of chiral ligand L9
To a solution of L1 (0.35 mmol) in 3 mL dry THF, the picolinalde-
hyde (0.39 mmol) was added dropwise. Then the reaction mixture
was stirred at 25 °C under N2 atmosphere for 2 h (monitoring by
TLC). After the reaction was completed, the solvent was removed
under reduced pressure and the residue was purified by a flash col-
umn chromatography on alumina to afford compound L9.
Yellow oil, 89% yield, [a]
25 = +189.5 (c 1.08, CH2Cl2); 1H NMR
D
(CDCl3, 162 MHz, 85% H3PO4): ꢁ8.32; IR (KBr, cmꢁ1):
m
3307,
(CDCl3, 400 MHz) d: 8.43 (d, 1H, J = 4.4 Hz), 8.19 (s, 1H), 7.42–
7.35 (m, 5H), 7.30 (d, 1H, J = 7.6 Hz), 7.17 (br s, 3H), 7.10–6.99
(m, 4H), 3.30–3.23 (m, 1H), 2.66–2.61 (m, 1H), 1.73–1.55 (m,
5H), 1.28–1.17 (m, 2H), 1.10–1.01 (m, 1H); 13C NMR (CDCl3,
100 MHz) d: 160.3, 154.3, 148.7, 137.5 (d, J = 12.4 Hz), 136.4 (d,
J = 14.7 Hz), 135.9, 134.5 (d, J = 20.8 Hz), 133.0 (d, J = 18.8 Hz),
128.6, 127.9 (d, J = 7.7 Hz), 127.7 (d, J = 6.6 Hz), 127.5, 124.2,
121.1, 73.1 (d, J = 15.6 Hz), 39.9 (d, J = 13.2 Hz), 34.8 (d,
J = 8.4 Hz), 27.2 (d, J = 7.0 Hz), 25.8 (d, J = 6.5 Hz), 24.3; 31P NMR
2941, 2851, 1669, 1430, 1101, 749, 697, 487; HRMS (ESI) calcd
for C23H30N2OP ([M+H]+): 381.2090, found: 381.2101.
4.2.4. (S)-N-((1R,2R)-2-(Diphenylphosphino)cyclohexyl)pyrroli-
dine-2-carboxamide L6
White solid, 89% total yield, mp 164.8–165.3 °C, [
a
]
25 = ꢁ78.2 (c
D
0.75, CH2Cl2); 1H NMR (CDCl3, 400 MHz) d: 7.76 (d, 1H, J = 8.8 Hz),
7.52 (t, 2H, J = 6.8 Hz), 7.43–7.34 (m, 4H), 7.32–7.27 (m, 4H), 3.74–
3.67 (m, 1H), 3.64 (dd, 1H, J1 = 9.2 Hz, J2 = 5.6 Hz), 2.91–2.86 (m,
1H), 2.75–2.69 (m, 1H), 2.32 (t, 1H, J = 10.0 Hz), 2.14–2.05 (m,
2H), 1.94–1.86 (m, 2H), 1.71–1.54 (m, 5H), 1.39–1.24 (m, 3H),
1.00–0.91 (m, 1H); 13C NMR (CDCl3, 100 MHz) d: 174.0, 137.2 (d,
J = 13.5 Hz), 135.6 (d, J = 17.6 Hz), 134.6 (d, J = 20.6 Hz), 132.3 (d,
J = 17.4 Hz), 128.7, 128.3 (d, J = 5.6 Hz), 128.0 (d, J = 7.5 Hz),
127.8, 60.7, 49.4 (d, J = 16.7 Hz), 47.1, 39.9 (d, J = 15.6 Hz), 33.6
(d, J = 7.0 Hz), 30.6, 27.2, 26.1 (d, J = 1.4 Hz), 25.5 (d, J = 3.8 Hz),
24.5; 31P NMR (CDCl3, 162 MHz, 85% H3PO4): ꢁ7.71; IR (KBr,
(162 MHz, CDCl3, 85% H3PO4): ꢁ5.01; IR (KBr, cmꢁ1):
m 3052,
2930, 2853, 1644, 1586, 1467, 1434, 740, 690, 511; HRMS (ESI)
calcd for C24H26N2P ([M+H]+): 373.1828, found: 373.1821.
4.4. General procedure for the asymmetric Henry reaction
A flame-dried Schlenk tube was charged with L3 (0.01 mmol)
in dry t-BuOH (0.8 mL), Cu(OAc)2ꢀH2O (0.01 mmol) was added
and the mixture was stirred at 25 °C under N2 atmosphere for
30 min. To the resulting solution was added nitromethane
(0.6 mL) by a syringe. Then the reaction mixture was stirred for
an additional 10 min, and the aldehyde (0.2 mmol) was added.
The reaction mixture was stirred at 25 °C (monitoring by TLC).
After the reaction was completed, the reaction mixture was fil-
tered through silica gel pad and rinsed with EtOAc. After concen-
trating under reduced pressure, the residue was purified by
column chromatography on silica gel to afford the b-nitroalcohols.
The ee values were determined by HPLC analysis with a chiral
column.
cmꢁ1):
m 3309, 2935, 2851, 1669, 1430, 1101, 751, 697, 487; HRMS
(ESI) calcd for C23H30N2OP ([M+H]+): 381.2090, found: 381.2096.
4.2.5. (S)-2-Amino-N-((1R,2R)-2-(diphenylphosphino)cyclohexyl)-
3-methylbutanamide L7
White solid, 79% total yield, mp 51.2–51.9 °C, [
a
]
25 = ꢁ38.9 (c
D
0.90, CH2Cl2); 1H NMR (CDCl3, 300 MHz) d: 7.60 (t, 2H,
J = 6.9 Hz), 7.52–7.42 (m, 2H), 7.39–7.26 (m, 6H), 7.19 (d, 1H,
J = 9.0 Hz), 4.01–3.88 (m, 1H), 3.00 (d, 1H, J = 3.0 Hz), 2.37–2.27
(m, 1H), 2.27–2.17 (m 1H), 2.05–2.00 (m, 1H), 1.71–1.64 (m, 3H),
1.43–1.00 (m, 7H), 0.92 (d, 3H, J = 6.9 Hz), 0.75 (d, 3H, J = 6.6 Hz);
13C NMR (CDCl3, 100 MHz) d: 172.6, 137.8 (d, J = 13.9 Hz), 135.9
(d, J = 15.8 Hz), 134.2 (d, J = 20.6 Hz), 133.0 (d, J = 19.1 Hz), 128.7,
128.3 (d, J = 6.5 Hz), 128.1 (d, J = 7.6 Hz), 59.6, 50.1 (d,
J = 16.8 Hz), 40.4 (d, J = 15.3 Hz), 34.5 (d, J = 7.6 Hz), 30.6, 27.9 (d,
J = 5.8 Hz), 25.6 (d, J = 5.7 Hz), 24.7, 19.6, 15.7; 31P NMR (CDCl3,
4.4.1. (R)-2-Nitro-1-(4-nitrophenyl)ethanol 2a
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
19.77 min (major), 24.70 min (minor), 85% ee.
Please cite this article in press as: Rexiti, R.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.07.015

6
R. Rexiti et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
4.4.2. (R)-2-Nitro-1-(3-nitrophenyl)ethanol 2b
4.4.13. (R)-1-([1,10-Biphenyl]-4-yl)-2-nitroethanol 2m
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
16.63 min (major), 20.10 min (minor), 85% ee.
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
21.35 min (major), 24.56 min (minor), 81% ee.
4.4.3. (R)-2-Nitro-1-(2-nitrophenyl)ethanol 2c
4.4.14. (R)-1-(Naphthalen-1-yl)-2-nitroethanol 2n
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
12.83 min (major), 14.16 min (minor), 84% ee.
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
14.43 min (major), 17.55 min (minor), 84% ee.
4.4.4. (R)-2-Nitro-1-(4-(trifluoromethyl)phenyl)ethanol 2d
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 90/10, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
14.35 min (major), 18.47 min (minor), 88% ee.
4.4.15. (R)-1-(Naphthalen-2-yl)-2-nitroethanol 2o
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
26.19 min (major), 36.23 min (minor), 85% ee.
4.4.5. (R)-4-(1-Hydroxy-2-nitroethyl)benzonitrile 2e
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
23.36 min (major), 27.77 min (minor), 87% ee.
4.4.16. (R)-1-(2,3-Dichlorophenyl)-2-nitroethanol 2p
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 90/10, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
14.76 min (major), 20.14 min (minor), 88% ee.
4.4.6. (R)-1-(4-Bromophenyl)-2-nitroethanol 2f
4.4.17. (R)-1-(2,6-Dichlorophenyl)-2-nitroethanol 2q
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 90/10, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
12.44 min (major), 14.24 min (minor), 97% ee.
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 90/10, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
22.67 min (major), 30.69 min (minor), 86% ee.
4.4.7. (R)-1-(4-Chlorophenyl)-2-nitroethanol 2g
4.4.18. (R,E)-1-Nitro-4-phenylbut-3-en-2-ol 2r
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 90/10, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
18.77 min (major), 23.85 min (minor), 89% ee.
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.6 mL/
min, and detected at a UV wave length of 215 nm. Retention times:
46.07 min (minor), 48.76 min (major), 69% ee.
4.4.8. (R)-1-(4-Fluorophenyl)-2-nitroethanol 2h
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 90/10, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
15.71 min (major), 18.99 min (minor), 91% ee.
4.4.19. (1R)-2-Nitro-1-(4-nitrophenyl)propan-1-ol 2s
The enantiomeric excess was determined on a Daicel Chiralpak
IC–H column with hexane/2-propanol = 90/10, flow = 0.9 mL/min,
and detected at a UV wave length of 220 nm. Retention times:
12.20 min (major), 14.34 min (minor) for one diastereoisomer,
19% ee; 19.72 min (minor), 21.58 min (major) for other
diastereoisomer, 25% ee.
4.4.9. (R)-2-Nitro-1-phenylethanol 2i
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 90/10, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
21.01 min (major), 26.38 min (minor), 88% ee.
4.4.20. (R)-2-Methyl-2-nitro-1-phenylpropan-1-ol 2t
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 90/10, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
18.10 min (major), 22.28 min (minor), 7% ee.
4.4.10. (R)-2-Nitro-1-(p-tolyl)ethanol 2j
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
12.27 min (major), 15.43 min (minor), 89% ee.
Acknowledgements
We are grateful for the financial support from Science and Tech-
4.4.11. (R)-2-Nitro-1-(m-tolyl)ethanol 2k
nology
Commission
of
Shanghai
Municipality,
China
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
11.45 min (major), 13.22 min (minor), 89% ee.
(15ZR1409200), and the Fundamental Research Funds for the Cen-
tral Universities, China.
Supplementary data
4.4.12. (R)-2-Nitro-1-(o-tolyl)ethanol 2l
Supplementary data (the copies of NMR spectra of chiral ligands
L3–L9 and HPLC spectra of the Henry products) associated with
this article can be found, in the online version, at http://dx.doi.
org/10.1016/j.tetasy.2016.07.015.
The enantiomeric excess was determined on a Daicel Chiralcel
OD-H column with hexane/2-propanol = 85/15, flow = 0.9 mL/
min, and detected at a UV wave length of 220 nm. Retention times:
11.24 min (major), 17.54 min (minor), 89% ee.
Please cite this article in press as: Rexiti, R.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.07.015

R. Rexiti et al. / Tetrahedron: Asymmetry xxx (2016) xxx–xxx
7
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Please cite this article in press as: Rexiti, R.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.07.015