Enantioselective nitroaldol (Henry) reaction catalyzed by chiral Schiff-base ligands

Article abstract of DOI:10.1016/j.tetasy.2008.02.005

Chiral Schiff-bases 2, 3, 4 and 5 were designed for the enantioselective nitroaldol (Henry) reaction. The highest enantioselectivity was observed for ligand 4 (82% ee) when CH₂Cl₂ was used as a solvent.

Full text of DOI:10.1016/j.tetasy.2008.02.005

Available online at www.sciencedirect.com
Tetrahedron: Asymmetry 19 (2008) 635–639
Enantioselective nitroaldol (Henry) reaction catalyzed by chiral
Schiff-base ligands
Mehmet C¸ olak and Nadir Demirel*
Department of Chemistry, Faculty of Science, University of Dicle, 21280 Diyarbakır, Turkey
Received 16 January 2008; accepted 5 February 2008
Abstract—Chiral Schiff-bases 2, 3, 4 and 5 were designed for the enantioselective nitroaldol (Henry) reaction. The highest enantioselec-
tivity was observed for ligand 4 (82% ee) when CH2Cl2 was used as a solvent.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Zhou.11 Two reviews have already been appeared in the lit-
erature on recent advances in the asymmetric nitroaldol
(Henry) reaction.12,13 In addition, various reports have
been continuously appearing in the literature.14–23 We have
previously reported the novel synthesis of a chiral Schiff-
base and enantioselective nitroaldol reactions.24 In the pre-
vious study, the observed enantioselectivities were low. As
a result, we decided to improve the catalyst and began with
the synthesis of a simple chiral Schiff-base with a phenol
group. Finally, to explore the effect of a spacer group, we
linked a simple chiral Schiff-base with a spacer group 1,3-
bis(bromomethyl) benzene.
The nitroaldol or Henry reaction is one of the classical
named reactions in organic synthesis. Essentially the cou-
pling of the nucleophile generated from a nitroalkane with
a carbonyl electrophile, it is a widely used transformation,
since its discovery in 1895.1 The resulting product of this
reaction is a b-nitroalcohol, which is a versatile intermedi-
ate in synthetic organic chemistry. However, until recently,
the wide applicability of this transformation was impaired
due to the non-availability of suitable catalysts for impart-
ing a definite stereochemistry onto the newly generated ste-
reogenic centres. The first asymmetric version of the Henry
reaction was reported by Shibasaki in 1992.2 Since then,
interest in this area has been expanded upon considerably
and various reports have been continuously appearing in
the literature with regard to the development of various
metal and non-metal based catalysts for the asymmetric
Henry reaction.
2. Results and discussion
To synthesize structurally simple, chiral Schiff-base, o-
hydroxybenzaldehyde was chosen as an aldehyde part of
the Schiff-base. To synthesize the desired simple chiral
Schiff-base ligands, the three commercially available chiral
amino alcohols (1S,2R)-2-amino-1,2-diphenylethanol, (S)-
(+)-phenylglycinol and (R)-(+)-2-amino-1,1,3-triphenyl-
propanol were used as chiral sources. The amino alcohols
were chosen to create a steric effect on the carbinol carbon.
The reaction of o-hydroxybenzaldehyde with three chiral
amino alcohols in EtOH gave chiral Schiff-base
compounds. To explore the effect of a spacer group, the
o-hydroxy benzaldehyde was reacted with 1,3-bis(bromo-
methyl) benzene in the presence of K2CO3 in CH3CN.
All reactions were simple and straightforward. All
In these processes, different chiral catalysts were developed,
such as those based upon BINOL by Shibasaki,2 bis(oxaz-
olines) by Evans and Jørgensen,3 cinchona alkaloid by
Corey,4 dinuclear zinc complexes by Trost,5 salen–cocom-
plexes by Yamada6 and amino alcohols by Palomo.7,8
A
chiral Schiff-base is one of the most frequently used cata-
lysts, especially in asymmetric cyclopropanation.9,10 The
first asymmetric Henry (nitroaldol) reaction catalyzed by
chiral copper Schiff-base complexes was first reported by
1
compounds were characterized with H NMR, 13C NMR
*
Corresponding author. Tel.: +90 412 2488550; fax: +90 412 2488039;
e-mail: demireln@dicle.edu.tr
and elemental analysis (Schemes 1–3).
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.02.005

636
M. C¸ olak, N. Demirel / Tetrahedron: Asymmetry 19 (2008) 635–639
Br
OH
K2CO3
+
O
O
CH3CN
CHO
CHO
CHO
Br
1
97%
Scheme 1.
to examine the solvent effect using solvents such as ethanol,
dichloromethane and toluene. Under the first experimental
conditions, 10 mol % triethyl amine was added as an
organic base in all the reactions (Table 1). The first experi-
mental results show that the reaction yields were higher
in all the cases when triethylamine was added to the reac-
tion medium as a promoter. Next, CH2Cl2 proved to be
the best solvent for the enantioselective nitroaldol reaction
catalyzed by chiral Schiff-base ligands. The organic base
triethylamine increased the reaction yield in all entries (Ta-
ble 1); however, it caused a slight decrease in ee. The effect
of organic base was shown dramatically in catalyst 4. The
enantioselectivity observed increased from 17 up to 82 ee
when triethylamine was not used as a promoter. In general,
the organic base increased the reaction yield, but decreased
enantioselectivity. The best catalyst proved to be 3 when
the organic base, triethylamine, was not added to the reac-
tion medium. Whenever triethylamine was added to the
reaction medium the best catalyst was 4. The spacer group
did not have a profound effect on selectivity but it did
change the configuration of adduct meaning that it may
be useful in applications for obtaining the desired configu-
ration (Table 2).
Ph
N
Ph
HO
OH
H2N
HO
80%
2
Ph
Ph
Ph
HO
Ph
N
OH
H2N
OH
CHO
HO
3
72%
Ph
Ph
Ph
Ph
Ph
Ph
OH
H2N
HO
N
HO
4
94 %
3. Conclusion
Scheme 2.
In conclusion we have synthesized simple chiral Schiff-base
ligands, which can be used in enantioselective metal-cata-
lyzed reactions. The experimental results show that simple
chiral Schiff-base ligands catalyze the enantioselective
nitroaldol reaction. The spacer group has the potential to
alter the configuration of the adduct which may be useful
for that application.
Under our previous reaction conditions,24 the reaction was
initially carried out at room temperature using 10 mol %
catalyst and triflate as the source for metal ion and trieth-
ylamine as a promoter for 40 h. We used chiral Schiff-base
ligands 2, 3, 4 and 5 under the above mentioned conditions
Ph
O
O
CHO
OH
EtOH
H2N
O
O
N
N
Ph
Ph
OH
HO
5
CHO
75 %
Scheme 3.

M. C¸ olak, N. Demirel / Tetrahedron: Asymmetry 19 (2008) 635–639
637
Table 1. Asymmetric nitroaldol (Henry) reaction between nitromethane
and p-nitrobenzaldehyde in the presence of triethylamine as the base
catalyzed by a chiral Schiff-basea
Table 2. Asymmetric nitroaldol (Henry) reaction between nitromethane
and p-nitrobenzaldehyde in the absence of triethylamine catalyzed by a
chiral Schiff-basea
OH
OH
CHO
CHO
NO2
NO2
2-5
2-5
*
*
10% mol
10% mol
CH NO
CH NO
3 2
+
+
3
2
room temp.
room temp.
NO2
NO2
NO2
NO2
7
6
7
6
Entry Cat. M(OTf)2 Time Solvent Yieldb eec Config.d
Entry Cat. M(OTf)2 Time Solvent Yieldb eec Config.d
(h)
(%)
(h)
(%)
1
2
3
4
5
6
7
8
9
2
2
2
3
3
3
4
4
4
5
5
5
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
EtOH
PhMe
CH2Cl2 61
EtOH
PhMe
CH2Cl2 52
EtOH
PhMe
CH2Cl2 53
EtOH
PhMe
CH2Cl2 56
53
55
4
15
24
14
44
62
73
36
82
54
24
65
(S)
(S)
(S)
(R)
(R)
(R)
(R)
(R)
(R)
(R)
(R)
(R)
1
2
3
4
5
6
7
8
9
2
2
2
3
3
3
4
4
4
5
5
5
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
Cu(OTf)2 40
EtOH
PhMe
CH2Cl2 73
EtOH
PhMe
CH2Cl2 72
EtOH
PhMe
CH2Cl2 67
EtOH
PhMe
CH2Cl2 64
62
68
8
17
16
5
(R)
(R)
(R)
(S)
(S)
(S)
(S)
(S)
(S)
(S)
(S)
(S)
46
49
61
65
4
58
17
11
22
45
15
57
48
50
56
69
10
11
12
54
51
10
11
12
63
66
a All reactions were performed on a 0.2 mmol scale of 4-nitrobenzalde-
hyde, 0.6 mL of nitromethane.
a All reactions were performed on a 0.2 mmol scale of 4-nitrobenzalde-
hyde, 0.6 mL of nitromethane and 10 mol % triethylamine as a base.
b After purification with TLC ethylacetate/petroleum ether (30:70),
Rf: 0.36, lit.:26 0.34.
b After purification with TLC ethyl acetate/petroleum ether (30:70),
Rf: 0.36, lit.:25 0.34.
c Determined by chiral HPLC using an OD column.
d Determined by comparing the HPLC elution order of the enantiomers
with an authentic sample according to the literature24 and also deter-
mined by HP Chiral Detector.
c Determined by chiral HPLC using an OD column.
d Determined by comparing the HPLC elution order of the enantiomers
with an authentic sample according to the literature25 and also deter-
mined by HP Chiral Detector.
4. Experimental
H2O. The layers were separated and the organic phase
was washed sequentially with H2O, 1 M aq NaOH solu-
tion, H2O and brine. After drying over MgSO4, concentra-
tion in vacuo gave a white solid (8.7 g, 97%), mp = 117–
118 °C; 1H NMR (400 MHz, CDCl3) d 5.243 (s, 4H),
7.058–7.093 (q, 4H, J 4 Hz), 7.469–7.580 (m, 6H), 7.867–
7.891 (q, 2H, J 4 Hz), 10.567 (s, 2H); 13C NMR
(100 MHz, CDCl3), d 70.25, 113.04, 121.18, 125.23,
126.06, 127.18, 128.64, 129.23, 135.94, 136.77, 160.88,
189.61; IR: m 3078 3043, 3012, 2935, 2877, 2765, 1666,
1601, 1481, 1458, 1400, 1369, 1304, 1288, 1234, 1165,
1103, 1007, 895, 845, 822, 791, 752, 710, 660, 509. Anal.
Calcd for C22H18O4: C, 76.29; H, 5.24. Found: C, 76.22;
H, 5.31.
4.1. General information
All chemicals were reagent grade unless otherwise specified.
Melting points were determined with a GALLENKAMP
Model apparatus with open capillaries and are uncor-
rected. Infrared spectra were recorded on a MATTSON
Model 1000 spectrophotometer. The elemental analyses
were obtained with CARLO-ERBA Model 1108 appara-
tus. H (400 MHz) and 13C (100 MHz) NMR spectra were
1
recorded on a Bruker DPX-400 high performance digital
FT-NMR spectrometer, with tetramethylsilane as the inter-
nal standard for solutions in deuteriochloroform. J values
are given in Hertz. Optical rotations were recorded using
an PERKIN ELMER Model 341 polarimeter. HPLC mea-
surements were performed with BIORAD instrument. Sep-
aration was carried out on Chiralcel OD column
(250 ꢀ 4.60 mm) with hexane/2-propanol (85:15) as an elu-
ent. TLC plates were purchased from Fluka.
4.1.2. Synthesis of chiral Schiff-base ligands 2–5
4.1.2.1. Ligand 2. To a solution of o-hydroxybenzalde-
hyde (1.83 g, 15 mmol) in 100 mL of EtOH was added
(S)-(+)-phenylglycinol (2.47 g, 18 mmol). The reaction
was then heated at reflux for 16 h. The EtOH was removed
and the residue was washed with ether three times and then
4.1.1. Synthesis of dialdehyde 1
crystallized from ethylacetate to give a solid (2.89 g, 80%),
20
4.1.1.1. Compound 1. To a solution of o-hydroxybenz-
aldehyde (8 g, 0.07 mol) in 200 mL of CH3CN were added
anhydrous K2CO3 (38.4 g, 0.28 mol) and 1,3-bis(bromo-
methyl) benzene (6.86 g, 0.026 mol). The reaction was
heated at reflux for 24 h. Then, CH3CN was removed
and the residue was partitioned between CH2Cl2 and
mp: 87–88 °C; ½aꢁD ¼ þ99:3 (c 2, CH2Cl2); 1H NMR
(400 MHz, CDCl3) d 2.69 (br s, 1H), 3.911–3.927 (d, 2H,
J 4 Hz), 4.462–4.495 (t, 1H, J 4 Hz), 6.889–7.425 (m,
9H), 8.47 (s, 1H), 13.45 (br s, 1H); 13C NMR (100 MHz,
CDCl3), d 67.61, 75.79, 117.05, 118.74, 118.91, 127.13,
127.90, 128.86, 131.79, 132.71, 139.35, 161.05, 166.28; IR:

638
M. C¸ olak, N. Demirel / Tetrahedron: Asymmetry 19 (2008) 635–639
m 3203, 3029, 2914, 2852, 1624, 1586, 1497, 1466, 1412,
1389, 1316, 1278, 1216, 1158, 1124, 1066, 1047, 1035,
985, 927, 854, 812, 758, 700, 642. Anal. Calcd for
C15H15NO2: C, 74.67; H, 6.27; N, 5.80. Found: C, 74.61;
H, 6.33; N, 5.74.
4.1.2.5. Typical procedure for the asymmetric Henry
reaction. Asymmetric Henry reaction was performed
according to our previous method.24
Acknowledgements
4.1.2.2. Ligand 3. To a solution of o-hydroxybenzalde-
hyde (1.83 g, 15 mmol) in 100 mL of EtOH was added
(1S,2R)-2-amino-1,2-diphenylethanol (3.84 g, 18 mmol).
The reaction was then heated at reflux for 16 h. The EtOH
was removed and the residue was washed with ether three
We are thankful for the financial support of the Turkish
¨
Scientific and Technical Research Council (TUBITAK)
under Grant No. 106T394.
_
times and then crystallized from ethyl acetate to give a solid
20
(3.531 g, 72%), mp: 144–145 °C; ½aꢁD ¼ ꢂ11:4 (c 2,
1
CH2Cl2); H NMR (400 MHz, CDCl3) d 2.19 (br s, 1H),
References
4.536–4.553 (d, 1H, J 8 Hz), 5.056–5.072 (d, 1H, J 8 Hz),
6.824–7.412 (m, 14H), 8.097 (s, 1H), 13.190 (br s, 1H);
13C NMR (100 MHz, CDCl3), d 78.35, 80.16, 116.92,
118.69, 127.21, 128.06, 128.08, 128.11, 128.17, 128.42,
128.78, 131.68, 132.55, 139.49, 140.18, 160.88, 165.94; IR:
m 3949, 3084, 3064, 2872, 1632, 1582, 1497, 1459, 1428,
1351, 1278, 1201, 1158, 1120, 1093, 1062, 1027, 989, 908,
854, 816, 766, 700, 643, 585, 535, 465. Anal. Calcd for
C21H19NO2: C, 79.47; H, 6.03; N, 4.41. Found: C, 79.53;
H, 6.11; N, 4.36.
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4.1.2.3. Ligand 4. To a solution of o-hydroxybenzalde-
hyde (0.16 g, 0.13 mmol) in 100 mL of EtOH was added
1,1,3-triphenyl-(R)-(+)-2-amino propanol (0.4 g, 0.13 mmol).
The reaction was heated to reflux for 16 h. The EtOH
was removed and the residue was washed with ether
three times and then crystallized from ethylacetate to
20
give a solid (0.51 g, 94%), mp: 159–161 °C, ½aꢁD
¼
þ169:4 (c 2, CH2Cl2); 1H NMR (400 MHz, CDCl3) d
2.868–2.902 (q, 1H, J 8 Hz), 3.071–3.105 (d, 2H, J
12 Hz), 4.307–4.422 (d, 1H, J 8 Hz), 6.763–7.678 (m,
19H), 7.697 (s, 1H), 12.651 (br s, 1H); 13C NMR
(100 MHz, CDCl3), d 37.40, 78.54, 79.77, 116.85, 118.16,
118.73, 126.01, 126.19, 126.42, 127.03, 127.18, 128.35,
128.42, 128.53, 129.76, 131.69, 132.73, 138.81, 143.99,
145.27, 160.68, 166.64; IR: m 3571, 3062, 3029, 2890,
2374, 1959, 1628, 1588, 1496, 1450, 1158, 1053, 954, 755,
696, 558. Anal. Calcd. for C28H25NO2 C, 82.53; H, 6.18;
N, 3.44. Found: C, 82.28; H, 6.26; N, 3.36.
4.1.2.4. Ligand 5. To a solution of 1 (1.384 g, 4 mmol)
in 100 mL of EtOH was added S-(+)-phenylglycinol
(1.207 g, 8 mmol). The reaction was heated at reflux for
16 h. The EtOH was removed and the residue was washed
with ether three times and then hexane to give a yellow oil
20
(1.752 g, 75%); ½aꢁD ¼ þ40:2 (c 2, CH2Cl2); 1H NMR
(400 MHz, CDCl3) d 2.997 (br s, 2H), 3.863–3.991 (m,
4H), 4.433–4.465 (q, 2H, J 4 Hz), 5.148 (s, 4H), 6.963–
6.984 (d, 2H, J 8 Hz), 7.036–7.491 (m, 18H), 8.128–8.147
(d, 2H, J 8 Hz), 8.902 (s, 2H); 13C NMR (100 MHz,
CDCl3), d 67.73, 70.25, 76.72, 112.69, 121.27, 124.89,
126.03, 126.90, 127.35, 127.43, 127.86, 128.52, 129.03,
132.28, 137.25, 141.10, 158.13, 158.70; IR: m 3375, 3066,
3031, 2931, 2877, 1639, 1604, 1489, 1458, 1388, 1295,
1245, 1160, 1118, 1052, 910, 755, 736, 705, 647, 532. Anal.
Calcd for C38H36N2O4: C, 78.06; H, 6.21; N, 4.79. Found:
C, 78.01; H, 6.31; N, 4.73.
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