Synthesis and characterization of chiral nitrobenzaldehyde - Schiff base ligands

Article abstract of DOI:10.3390/81100788

Three chiral nitrobenzaldehyde/Schiff base ligands (2a-c) were prepared by the reaction of ortho, meta, and para-NO₂ substituted benzaldehydes (1a-c) with (1R,2R)-(-)-1,2-diaminocyclohexane. The structures of ligands 2a-c were characterized by

Full text of DOI:10.3390/81100788

Molecules 2003, 8, 788-792
molecules
ISSN 1420-3049
http://www.mdpi.org/
Synthesis and Characterization of Chiral Nitrobenzaldehyde -
Schiff Base Ligands
Wen Tao Gao 1,*and Zhuo Zheng2
1 Institute of Superfine Chemicals, Bohai University, Jinzhou, 121003, P.R. China
2
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, P. R. China;
Tel.: (+086) 411 4663087, Fax: (+086) 411 4686746
* Author to whom correspondence should be addressed; e-mail:gwt@jznu.edu.cn
Received: 10 June 2003; in revised form: 15 August 2003 / Accepted: 18 August 2003 / Published: 15
November 2003
Abstract: Three chiral nitrobenzaldehyde/Schiff base ligands (2a-c) were prepared by the
reaction of ortho, meta, and para-NO2 substituted benzaldehydes (1a-c) with
(1R,2R)-(-)-1,2-diaminocyclohexane. The structures of ligands 2a-c were characterized by
IR, 1H-NMR, 13C-NMR and elemental analysis.
Keywords: Synthesis, chiral, Schiff base, ligand, nitro-group, benzaldehyde.
Introduction
It is well known that the preparation of new ligands is perhaps the most important step in the
development of metal complexes which exhibit unique properties and novel reactivity. Changes in the
electronic, steric, and geometric properties of the ligand alter the orbitals at the metal center and thus
affect its properties. In asymmetric catalyst systems, small changes in the donating ability of the

Molecules 2003, 8
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ligand or the size of its substituents can have a dramatic effect a catalyst’s efficiency and
enantioselectivity [1-6]. The nitro group is a strong electron withdrawing group and, due to its steric
effects, has played an important role in affecting the reactivities and enantioselectivities in asymmetric
cyclopropanation and allylic alkylation reactions [7,8]. Jacobsen et al. have investigated the actions of
various chiral benzaldehyde/Schiff base ligands in the asymmetric azirination of olefins [9]. The best
result (75% yield and 98% e.e) was achieved with the 2,3-dichlorobenzaldehyde/Schiff base ligand.
To the best of our knowledge, however, nitrobenzaldehyde/Schiff base ligands have never been
applied in an asymmetric catalytic system, although some of them have been mentioned in recent
publications [10,11]. Thus, in order to investigate their catalytic properties, ortho, meta, and para-NO2
substituted benzaldehydes 1a-c were used to prepare the Schiff base ligands 2a-c by reaction with
chiral 1,2-diaminecyclohexane (Scheme 1).
Scheme 1
CHO
+
HC
N
N
CH
R
H2N
NH2
R
R
2a-c
1a-c
2a R=o-NO2
2b R=m-NO2
2c R=p-NO2
Results and Discussion
NO2 substituted benzaldehydes 1a-c were dissolved in absolute ethanol and then an ethanol
solution containing 0.5 molar equivalent of (1R,2R)-(-)-1,2-diaminocyclohexane was added to the
reaction mixture at room temperature. After 30~36 hours at reflux the reaction mixtures were cooled
to room temperature and diluted with water. As a result, ligands 2a-c were obtained in yields of 76.4%,
94.4%, and 74.5%, respectively. Their structures were characterized by spectroscopy and elemental
analysis. The absorption peak corresponding to the amino group disappeared in the IR, which showed
that the amino groups in 1,2-diaminocyclohexane had participated completely in the reactions.
1H-NMR, 13C-NMR and elemental analysis data for ligands 2a-c were also consistent with the
formulation indicated in Scheme 1.

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As a model reaction the ligands 2a-c were used in the asymmetric catalytic cyclopropanation of
2,5-dimethyl-2,4-hexadiene with L-menthyl diazoacetate. The results are shown in Table 1. It can be
seen from the data that the observed enantioselectivities were not satisfactory, although the activities
were moderate. The examination of these ligands in other catalytic asymmetric reactions is in
progress.
Table 1. Asymmetric cyclopropanation of 2,5-dimethyl-2,4-hexadiene with L-menthyl
diazoacetate using Cu(CH3CN)4ClO4/ ligands 2a-c catalysts in situ.
Entrya
Ligand
2a
trans:cis
72:28
e.e/%(trans/cis)
4.5/8.6
Yield/%
57.9
1
2
3
75:25
5.6/7.6
69.8
2b
73:27
1.3/8.9
74.0
2c
The reactions were carried out at 85ºC, Cu(CH3CN)4ClO4/ligand molar ratio 1/1.2,
catalyst/L-menthyl diazoacetate molar ratio 1/100.
Acknowledgements
We are grateful for the financial support provided by the National Natural Science Foundation of
China (No.29933050).
Experimental
General
Melting points were determined on a WRS-1B Digital Melting Point Apparatus and are uncorrected.
Infrared spectra were recorded as KBr pellets on an FTIR-8400 instrument. NMR spectra were
recorded on a Mercury 400 spectrometer and chemical shifts are expressed in ppm using TMS as
internal standard. Elemental analysis was performed on a PE-2400 elemental analyzer. The optical
rotations were recorded by means of Horiba Sepa-200 high sensitivity polarimeter.
Synthesis of ligand 2a
o-Nitrobenzaldehye
(1a,
2.5g,
16.56mmol)
was
added
to
a
solution
of
(1R,2R)-(-)-1,2-diaminocyclohexane (966mg, 8.47mmol) in absolute ethanol (8mL). The resulting
mixture was refluxed for 36 h. After cooling to room temperature, water (40mL) was added and the

Molecules 2003, 8
791
resulting mixture was stirred for 14 hours. The white precipitate formed was filtered off, washed with
water and dried to afford the title compound as white needles after recrystallization from absolute
ethanol (yield: 2.460g, 76.4%); mp: 78~79ºC; [α]D27= -3.56° (C=1 in EtOH); IR νmax (cm-1): 1640
(C=N), 1530, 1345 (NO2), 2930, 2860, 750(Ar-H ring); 1H-NMR, δH: 8.67 (s, 2H, CH=N), 7.96 (d, 2H,
J=7.76 Hz, Ar-H3,3’), 7.93 (d, 2H, J=10.68 Hz, Ar-H6,6’), 7.60 (t, 2H, J=7.64 Hz, 7.48 Hz, Ar-H5,5’),
13
7.47 (t, 2H, J=7.80 Hz, 7.76 Hz, Ar-H4,4’), 3.56 (t, 2H, CH), 1.90~1.51 (m, 8H, CH2CH2); C-NMR,
δC: 170.72 (C=N), 163.54 (Ar-C2), 132.24 (Ar-C4), 128.27 (Ar-C6), 119.11 (Ar-C1), 118.36 (Ar-C5),
117.00 (Ar-C3), 62.84 (NCH), 32.23 (CH2), 24.06 (CH2); Anal. Cald. for C20H20N4O4: C, 63.15; H,
5.30; N, 14.73; Found: C, 62.98; H, 5.16; N, 14.55.
Synthesis of ligand 2b
Compound 2b was synthesized by the method described above in 94.4% yield. White needles, mp:
92~93ºC; [α]D29= -255.00° (C=1 in EtOH); IR νmax (cm-1): 1652 (C=N), 1534, 1352 (NO2), 2928,
1
2862, 748(Ar-H ring); H-NMR, δH: 8.47 (s, 2H, CH=N), 8.28 (s, 2H, Ar-H3,3’), 8.17 (dd, 2H, J=6.12
Hz, J=2.00 Hz Ar-H4,4’), 7.91 (d, 2H, J=7.68 Hz, Ar-H6,6’), 7.50 (t, 2H, J=7.92 Hz, Ar-H5,5’), 3.49 (dd,
13
2H, J=5.00 Hz, J=2.28 Hz, CH), 1.92~1.51 (m, 8H, CH2CH2); C-NMR, δC: 158.23 (C=N), 148.43
(Ar-C3), 137.76 (Ar-C1), 133.57 (Ar-C6), 129.46 (Ar-C5), 124.75 (Ar-C4), 122.38 (Ar-C2), 73.69
(NCH), 32.62 (CH2), 24.19 (CH2); Anal. Cald. for C20H20N4O4: C, 63.15; H, 5.30; N, 14.73; Found: C,
63.01; H, 5.13; N, 14.57.
Synthesis of ligand 2c
Compound 2c was synthesized by the above method in 74.5% yield. Yellow needles, mp:
110~112ºC; [α]D29= -384.10° (C=1 in CH2Cl2); IR νmax (cm-1): 1648 (C=N), 1526, 1350 cm (NO2),
1
2921, 2858, 752 (Ar-H ring); H-NMR, δH: 8.28 (s, 2H, CH=N), 8.15 (d, 4H, J=8.56 Hz, Ar-H3,3’,5,5’),
7.76 (d, 4H, J=8.72 Hz, Ar-H2,2’,6.6’), 3.50 (t, 2H, J=4.96 Hz, J=4.88 Hz, CH), 1.92~1.52 (m, 8H,
CH2CH2); 13C-NMR, δC: 158.55 (C=N), 148.73 (Ar-C4), 141.46 (Ar-C1), 128.44 (Ar-C2,6), 123.66
(Ar-C3,5), 73.88 (NCH), 32.48 (CH2), 24.10 (CH2); Anal. Cald. for C20H20N4O4: C, 63.15; H, 5.30; N,
14.73; Found: C, 63.02; H, 5.10; N, 14.26.
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Sample Availability: Samples of the ligands 2a, 2b and 2c are available from MDPI.
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