Enantioselective Henry reaction catalyzed by a novel L-(+)-aspartic acid-derived Schiff base ligand and Cu(II) ion

Article abstract of DOI:10.3906/kim-1103-19

Mild and efficient enantioselective Henry reactions of nitromethane with various aldehydes were catalyzed by a novel L-(+)-aspartic acid-derived Schiff base ligand in the presence of Cu(II) ions, affording the corresponding adducts in high yields (up to 96%) and enantioselectivities (up to 92% ee). TUBITAK.

Full text of DOI:10.3906/kim-1103-19

Turk J Chem
35 (2011) , 553 – 560.
¨
˙
c
ꢀ TUBITAK
doi:10.3906/kim-1103-19
Enantioselective Henry reaction catalyzed by a novel
L-(+)-aspartic acid-derived Schiff base ligand and
Cu(II) ion
Gamze KOZ∗, Demet ASTLEY, Stephen Thomas ASTLEY
Department of Chemistry, Faculty of Science, Ege University,
˙
35100 Bornova, Izmir-TURKEY
e-mail: gamzedganer@yahoo.com
Received: 07.03.2011
Mild and efficient enantioselective Henry reactions of nitromethane with various aldehydes were catalyzed
by a novel L-(+)-aspartic acid-derived Schiff base ligand in the presence of Cu(II) ions, affording the
corresponding adducts in high yields (up to 96%) and enantioselectivities (up to 92% ee).
Key Words: Enantioselective Henry reaction, chiral Schiff base, L-(+)-aspartic acid
Introduction
The Henry (nitroaldol) reaction is a well-known tool for the building of a C-C bond.1 The resulting products
of this reaction, a coupling between nitroalkanes and carbonyl groups, can be converted into many valuable
building blocks depending on the different requirements in the synthesis of natural products and other useful
compounds.2−6 For the asymmetric Henry reaction,7 recent work focusing on the development of various metal-
based catalysts has been reported by the groups of Shibasaki,8 Trost, 9 Evans,10 and others,11,12 although
organocatalysts13 have also been employed.
Chiral amino acid-derived Schiff bases have frequently been used in catalytic asymmetric Henry reacti-
ons.14−19 For example, Wang 20 prepared tridentate amino alcohol-based Schiff base ligands starting from
phenylalanine (D and L) and obtained enantioselectivities of up to 96% ee for the Henry reaction in the presence
of Cu2+ ions. Although many similar chiral ligands derived from natural amino acids have found applications
as catalysts in asymmetric reactions, there are only a few examples of L-(+)-aspartic acid-derived ligands in the
∗Corresponding author
553

Enantioselective Henry reaction catalyzed by a novel..., G. KOZ, et al.
literature, and their application only includes the copper-catalyzed cyclopropanation of styrene.21 We herein
report a mild and efficient enantioselective Henry reaction catalyzed by Cu(II) ions and a novel chiral Schiff
base ligand (L) (Figure 1), which can be readily prepared from L-(+)-aspartic acid in a 3-step procedure.
Ph
HO
N
Ph
Ph
OH HO
Ph
L
Figure 1. L-(+)-aspartic acid-derived Schiff base ligand.
Experimental
All chemicals were purchased from Merck, Sigma-Aldrich, Alfa Aesar, and Fluka and were used without any
purification. Solvents were used as received from commercial suppliers. Silica gel F254 (Merck 5554) precoated
plates were used for thin layer chromatography. For column chromatography, silica gel 60 (Merck 7743) was
used. IR spectra were recorded using a PerkinElmer 100. 1 H-NMR and 13 C-NMR spectra were carried out
using a 400-MHz Varian NMR spectrometer at ambient temperature. Melting points were recorded with an
electrothermal digital melting point apparatus. Optical rotations were determined using a Rudolph Research
Analytical AUTOPOL I automatic polarimeter. HPLC analyses were performed using a Chiralcel OD-H column.
Preparation of (S)-dimethyl-2-aminosuccinate (1)
SOCl2 (12 mL) was added dropwise to a suspension of L-(+)-aspartic acid (354 mg, 2.66 mmol) in 60 mL of
methanol at 0 ˚C. The resulting colorless solution was refluxed until all L-(+)-aspartic acid had been consumed.
Methanol was evaporated in vacuo and water (5 mL) was added. Saturated NaHCO3 was then added dropwise
(pH = 8) and the mixture was extracted with ethyl acetate. The organic phase was dried with Na2 SO4 and
filtered. Ethyl acetate was evaporated to give the title compound as yellow oil (82% yield). IR (NaCl): 3385,
2956, 2851, 1738, 1438, 1366, 1203 cm−1
.
1 H-NMR (400 MHz, CDCl3) δ (ppm): 3.84 (dd, J = 7.6, 4.8 Hz,
1H), 3.76 (s, 3H, CH3), 3.71(s, 3H, CH3), 2.82 (dd, J = 16.4, 4.8 Hz, 1H), 2.71 (dd, J = 16.4, 7.6 Hz, 1H),
1.88 (bs, 2H, -NH2). 13 C-NMR (400 MHz, CDCl3) δ (ppm): 174.7, 171.8, 52.5, 52.0, 38.9. Anal. Calcd.
for C6 H11 O4 N (%): C 44.72, H 6.88, N 8.69; Found: C 43.86, H 6.04, N 9.01. [α]2D5 = +60.2 (c 0.156, ethyl
acetate).
Preparation of (S)-2-amino-1,1,4,4-tetraphenylbutane-1,4-diol (2)
To a solution of L-(+)-aspartic acid dimethyl ester (1) in dry ether was added excess freshly prepared PhMgBr
solution in dry ether. The resulting solution was refluxed until all L-(+)-aspartic acid dimethyl ester had been
consumed. The reaction was quenched with saturated NH4 Cl solution. The product was extracted with ether-
water and the organic phase was dried with Na2 SO4 and filtered, and then the ether fraction was evaporated in
554

Enantioselective Henry reaction catalyzed by a novel..., G. KOZ, et al.
vacuo.The crude product was purified with column chromatography (1:3 ethyl acetate:hexane) to give the title
compound as white crystals (78% yield). Mp 145.9-149.5 ˚C. IR (NaCl): 3376, 1491, 1596, 1447, 700 cm−1
.
1 H-NMR (400 MHz, CDCl3) δ (ppm): 7.12-7.41 (m, 20H, Ar-H), 3.65 (dd, J = 10.8, 1.2 Hz, 1H), 2.60 (bs,
-OH), 2.44 (dd, J = 14.4, 1.2 Hz, 1H), 2.05 (dd, J = 14, 10.8 Hz, 1H). 13 C-NMR (400 MHz, CDCl3) δ (ppm):
128.8, 128.6, 128.2, 128.1, 127.4, 127.2, 126.9, 126.8, 126.7, 125.96, 125.91, 125.8, 81.4, 78.2, 55.2, 40.1. Anal.
Calcd. for C28 H27 O2 N (%): C 82.12, H 6.65, N 3.42; Found: C 82.22, H 6.62, N 3.39. [α]2D5 = -3.15 (c 1.27,
ethyl acetate).
Preparation of (2S)-2-{(E)-[(2-hydroxynaphthalen-1-yl)methylidene]amino}-1,1,4,4-
tetraphenylbutane-1,4-diol (L)
A solution of (S)-2-amino-1,1,4,4-tetraphenyl butane-1,4-diol (200 mg, 0.49 mmol) (2) and 2-hydroxy-1-napht-
haldehyde (84 mg, 0.49 mmol) in 20 mL of ethanol was refluxed for 5 h. Ethanol was evaporated in vacuo.The
product was crystallized (dichloromethane-hexane) to give the title compound as yellow crystals (98% yield).
Mp 118-120 ˚C (subl.). IR (KBr): 3266, 3058, 3028, 1629, 1544, 1519, 1492, 1447, 1350, 1265, 1164, 1053, 838,
748, 669 cm−1 1 H-NMR (400 MHz, CDCl3) δ (ppm): 7.71 (s, 1H, -CH=N), 7.51-7.13 (m, 21H, Ar-H), 7.01
.
(t, J = 7.2 Hz, 1H, Ar-H), 6.94 (t, J = 7.2 Hz, 2H, Ar-H ), 6.75 (t, J = 7.2 Hz, 1H, Ar-H), 6.63 (d, J = 9.6 Hz,
1H, Ar-H), 4.38 (d, J = 7.2 Hz, 1H), 2.98 (dd, J = 14.8, 8.4 Hz, 1H), 2.94 (bs, 1H, -OH), 2.88 (dd, J = 14.8, 1.6
Hz, 1H), 1.73 (bs, 1H, -OH). 13 C-NMR (400 MHz, CDCl3) δ (ppm): 159.1, 146.8, 146.4, 144.2, 144.1, 137.2,
134.1, 129.0, 128.6, 128.5, 128.4, 128.2, 126.3, 126.19, 126.17, 124.9, 122.5, 118.3, 106.7, 81.4, 77.7, 68.2, 42.5.
Anal. Calcd. for C39 H33 O3 N (%): C 83.10, H 5.90, N 2.48; Found: C 82.65, H 5.89, N 2.38. [α]2D8 = +6.00
(c 1.00, CH2 Cl2).
General procedure for Henry reaction
The dark green solution of Cu(OAc)2 .nH2 O (0.06 mmol) and Schiff base ligand (L) (0.05 mmol) was allowed
to stir in 2 mL of solvent at room temperature for 2 h. At the end of 2 h, the appropriate aldehyde (0.5 mmol)
and nitromethane (2.5 mmol) were added. The reaction mixture was stirred at room temperature until most
of the aldehyde had been consumed. The solvent was evaporated in vacuo and the crude product was purified
with column chromatography.
(1S)-1-(2-Chlorophenyl)-2-nitroethanol (3a)
Colorless oil, 95% yield.1 H-NMR (400 MHz, CDCl3) δ (ppm): 7.56 (dd, J = 7.6, 2 Hz, 1H, Ar-H), 7.40-7.24
(m, 3H, Ar-H), 5.88-5.84 (m, 1H), 4.57 (dd, J = 13.6, 2.4 Hz, 1H), 4.36 (dd, J = 13.6, 9.6 Hz, 1H). HPLC
conditions: 93:7 hexane:i−PrOH, 0.8 mL/min, 267 nm, tminor = 14.4 min (R), tmajor = 15.3 min (S), 90%
ee, [α]2D8 = +44.0 (c 0.55, CH2 Cl2).
555

Enantioselective Henry reaction catalyzed by a novel..., G. KOZ, et al.
(1S)-1-(2-Nitrophenyl)-2-nitroethanol (3b)
Brown crystals, 81% yield.1 H-NMR (400 MHz, CDCl3) δ (ppm): 8.06 (dd, J = 8, 1.2 Hz, 1H, Ar-H), 7.95 (d,
J = 8 Hz, 1H, Ar-H), 7.75 (td, J = 7.6, 0.8 Hz, 1H, Ar-H), 7.55 (td, J = 8.4, 1.6 Hz, 1H, Ar-H), 6.03 (d, J = 8
Hz, 1H), 4.85 (dd, J = 14, 2.4 Hz, 1H), 4.56 (dd, J = 13.6, 9.2 Hz, 1H), 3.35 (bs, 1H, -OH). HPLC conditions:
90:10 hexane:i−PrOH, 1 mL/min, 267 nm, tminor = 15.9 min (R), tmajor = 18.3 min (S), 88% ee, [α]D28
+235 (c 0.89, CH2 Cl2).
=
(1S)-1-(3-Nitrophenyl)-2-nitroethanol (3c)
Yellow oil, 90% yield.1 H-NMR (400 MHz, CDCl3) δ (ppm): 8.31-8.30 (m, 1H, Ar-H), 8.21-8.19 (m, 1H, Ar-H),
7.79-7.77 (m, 1H, Ar-H), 7.61 (t, J = 7.6 Hz, 1H, Ar-H), 5.61 (dd, J = 7.6, 4.4 Hz, 1H), 4.64-4.62 (m, 2H), 3.51
(bs, 1H, -OH). HPLC conditions: 90:10 hexane:i−PrOH, 1 mL/min, 267 nm, tminor = 25.9 min (R), tmajor
= 28.6 min (S), 70% ee, [α]2D8 = +28.8 (c 1.04, CH2 Cl2).
(1S)-1-(4-Nitrophenyl)-2-nitroethanol (3d)
White crystals, 71% yield.1 H-NMR (400 MHz, CDCl3) δ (ppm): 8.28-8.26 (m, 2H, Ar-H), 7.64-7.62 (m, 2H,
Ar-H), 5.59-5.63 (m, 1H), 4.60 (d, J = 6 Hz, 1H), 4.58 (d, J = 2 Hz, 1H), 3.17 (bs, 1H, -OH). HPLC conditions:
90:10 hexane:i−PrOH, 1 mL/min, 267 nm, tminor = 28.7 min (R), tmajor = 35.40 min (S), 76% ee, [α]D28
+29.3 (c 0.75, CH2 Cl2).
=
(1S)-1-Phenyl-2-nitroethanol (3e)
Yellow oil, 96% yield.1 H-NMR (400 MHz, CDCl3) δ (ppm): 7.39-7.34 (m, 5H, Ar-H), 5.43 (dd, J = 9.6, 2.8
Hz, 1H), 4.59 (dd, J = 13.6, 9.6 Hz, 1H), 4.49 (dd, J = 13.2, 2.8 Hz, 1H), 3.08 (bs, 1H, -OH). HPLC conditions:
90:10 hexane:i−PrOH, 1 mL/min, 267 nm, tminor = 13.8 min (R), tmajor = 15.0 min (S), 78% ee, [α]D28
+35.3 (c 1.36, CH2 Cl2).
=
(1S)-1-(4-Methylphenyl)-2-nitroethanol (3f)
Yellow crystals, 88% yield.1 H-NMR (400 MHz, CDCl3) δ (ppm): 7.29-7.20 (m, 4H, Ar-H), 5.42 (d, J = 9.2
Hz, 1H), 4.60 (dd, J = 13.6, 10.4 Hz, 1H), 4.48 (dd, J = 13.2, 2.8 Hz, 1H), 2.74 (bs, 1H, -OH), 2.36 (s, 3H,
CH3). HPLC conditions: 85:15 hexane:i−PrOH, 0.5 mL/min, 267 nm, tminor = 19.8 min (R), tmajor = 24.5
min (S), 74% ee, [α]2D8 = +17.3 (c 0.81, CH2 Cl2).
(+)-1-(4-Ethylphenyl)-2-nitroethanol (3g)
Yellow oil, 60% yield.1 H-NMR (400 MHz, CDCl3) δ (ppm): 7.31-7.29 (m, 2H, Ar-H), 7.23-7.21 (m, 2H, Ar-
H), 5.43-5.39 (m, 1H), 4.50 (dd, J = 13.2, 9.6 Hz, 1H), 4.48 (dd, J = 13.2, 3.2 Hz, 1H), 2.86 (d, J = 3.6 Hz,
1H), 2.65 (q, J = 7.6 Hz, 2H, -CH2), 1.23 (t, J = 7.6 Hz, 3H, CH3). HPLC conditions: 90:10 hexane:i−PrOH,
1 mL/min, 267 nm, tminor = 12.2 min (R), tmajor = 15.7 min (S), 76% ee, [α]2D8 = +32.0 (c 0.75, CH2 Cl2).
556

Enantioselective Henry reaction catalyzed by a novel..., G. KOZ, et al.
(1S)-1-(4-Methoxyphenyl)-2-nitroethanol (3h)
Yellow oil, 68% yield.1 H-NMR (400 MHz, CDCl3) δ (ppm): 7.30 (d, J = 8.8 Hz, 2H, Ar-H), 6.91 (d, J = 8.8
Hz, 2H, Ar-H), 5.40-5.38 (m, 1H), 4.59 (dd, J = 13.2, 9.6 Hz, 1H), 4.46 (dd, J = 12.8, 2.8 Hz, 1H), 3.81 (s, 3H,
-OCH3), 2.84 (bs, 1H, -OH). HPLC conditions: 90:10 hexane:i−PrOH, 1 mL/min, 267 nm, tminor = 20.4 min
(R), tmajor = 25.5 min (S), 70% ee, [α]2D8 = +28.0 (c 0.50, CH2 Cl2).
Results and discussion
The desired amino alcohol (2) derived from the chiral L-(+)-aspartic acid dimethyl ester was obtained using
a Grignard reaction and used in the condensation reaction with 2-hydroxy-1-naphthaldehyde to synthesize the
Schiff base ligand (L) (Figure 2).
OH NH2
O
NH2
O
NH2
Ph
Ph
OH
Ph
OH
OCH3
SOCl2, MeOH
reflux
PhMgBr
HO
H3CO
ether, reflux
Ph
O
O
L-(+)-Aspartic Acid
1
2
Ph
HO
N
Ph
CHO
OH
OH NH2
Ph
Ph
EtOH
reflux
OH
Ph
Ph
OH HO
Ph
Ph
2-Hydroxy-1-naphthaldehyde
2
L
Figure 2. Synthesis of the chiral amino alcohol derivative (2) and the Schiff base ligand.
Initial studies of this ligand were focused on the reaction of nitromethane with 2-chlorobenzaldehyde, as
shown in Table 1.
First, the effect of changing the solvent was investigated for reactions carried out in the presence of
10 mol% catalyst at room temperature (Table 1, Entries 1-15). When ethanol was used as the solvent, the
nitroalcohol product was obtained in a high yield and high ee (Table 1, Entry 1). The reaction solvent was
found to have a very significant influence on the reaction. For example, when the reaction solvent was changed
from ethanol to THF or toluene, the reaction yield and ee were reduced considerably (Table 1, Entries 6 and 9)
despite these solvents being commonly used for Henry reactions.23,24 Since the catalyst is also stable in water,
we attempted to carry out this asymmetric reaction in aqueous media. However, the addition of water disfavored
the reaction, resulting in a decrease in both ee and yield (Table 1, Entry 4). Thus, all the experimental data
demonstrated that ethanol is a superior solvent in terms of yield and enantiomeric excess.
Under the optimized reaction conditions, different aldehydes were tested in order to extend the substrate
scope, as shown in Table 2.
The L-(+)-aspartic acid-derived Schiff base catalyst worked well for various aromatic aldehydes, regardless
of the presence of electron-withdrawing or electron-donating substituents on the benzaldehyde ring. It appears
that aromatic aldehydes with ortho-substituted groups (Table 2, Entries 1 and 2) increase the stereoselectivity.
557

Enantioselective Henry reaction catalyzed by a novel..., G. KOZ, et al.
Table 1. Effects of solvent, temperature, and catalyst loading on the Henry reaction of 2-chlorobenzaldehyde and
nitromethane under the catalysis of L.
O
OH
NO2
L, Cu(OAc)2.nH2O
H
CH3NO2
solvent
Cl
Solvent
Cl
Entry
1
Temp. (˚C) Time (h) Yielda (%) eeb (%) Config.c
Ethanol
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
0
48
48
48
48
48
48
48
48
48
48
48
48
48
48
48
120
72
72
95
87
86
78
81
47
57
59
39
12
16
21
42
43
79
69
82
84
90
90
86
84
72
70
68
60
58
52
48
38
38
28
14
92
88
88
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
2
t-Butanol
3
i-Propanol
4
Ethanol:Water (10:1)
Methanol
5
6
Tetrahydrofuran
Diethyl ether
Hexane
7
8
9
Toluene
10
11
12
13
14
15
16
17d
18e
Ethyl acetate
Acetone
Dimethylformamide
Acetonitrile
Dichloromethane
n-Propanol
Ethanol
Ethanol
RT
RT
Ethanol
aIsolated yields after column chromatography.
bDetermined by HPLC analysis using a Chiralcel OD-H column.
cThe absolute configuration of the major product was assigned by comparison with the literature values.22
dWith 5 mol% catalyst loading.
eWith 20 mol% catalyst loading.
It is likely that the mechanism involves initial coordination of the aromatic aldehyde to a copper atom,
followed by nucleophilic attack of –CH2 NO2 onto the less sterically hindered face of the carbonyl group,
affording products with high enantioselectivity.26
558

Enantioselective Henry reaction catalyzed by a novel..., G. KOZ, et al.
Table 2. Henry reaction of nitromethane with various aldehydes.
O
OH
10 mol% L, Cu(OAc)2.nH2O
CH3NO2
NO2
Ar
H
Ar
EtOH, rt
Entry Product
ArCHO
Time (h) Yield (%)a ee (%)b Config.c
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
2-Chlorobenzaldehyde
2-Nitrobenzaldehyde
3-Nitrobenzaldehyde
4-Nitrobenzaldehyde
Benzaldehyde
48
24
24
24
96
96
96
96
95
81
90
71
96
88
60
68
90
88
70
76
78
74
76
70
S
S
S
S
S
4-Methylbenzaldehyde
4-Ethylbenzaldehyde
4-Methoxybenzaldehyde
S
3g
3h
n.d.
S
aIsolated yields after column chromatography.
bDetermined by HPLC analysis using a Chiralcel OD-H column.
cThe absolute configuration of the major product was assigned by comparison with the literature values.13,23,25
n.d.: Not determined.
Conclusion
The enantioselective Henry reaction was performed at room temperature by the employment of a novel L-
(+)-aspartic acid-derived Schiff base ligand, yielding β-nitroalkanols with good yields and ee values. The
L-(+)-aspartic acid-derived Schiff base ligand is an advantageous catalyst for the Henry reaction as it can be
prepared from cheap and easily accessible starting materials in an easy 3-step procedure. The novel catalyst
described here and analog compounds with readily tunable sterics and hydrogen-bonding attributes are likely
to further expand the scope of various other asymmetric reactions.
Acknowledgements
¨
˙
We are grateful to the Scientific and Technological Research Council of Turkey (TUBITAK) for its financial
¨
˙
support (Project Number 107T778). Gamze Koz thanks TUBITAK for graduate scholarships.
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