A Facile Reduction Procedure for N,N′-bis[5-Substituted Salicylidene]-m/p-phenylenediamines with Sodium Borohydride-Silica Gel System

Article abstract of DOI:10.1081/SCC-120028641

An easy approach involving sodium borohydride/silica gel system offers a fast method of reduction for N,N′-bis[5-substituted salicylidene] m/p-phenylenediamines to the corresponding amines with excellent yield. The observed formation of boron chelates in

Full text of DOI:10.1081/SCC-120028641

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A Facile Reduction Procedure for
N,N′‐bis[5‐Substituted Salicylidene]‐m/
p‐phenylenediamines with Sodium Borohydride–Silica
Gel System
V. Sridharan a , S. Muthusubramanian a & S. Sivasubramanian a
a Department of Organic Chemistry , Madurai Kamaraj University , Madurai, 625 021,
India
Published online: 21 Aug 2006.
To cite this article: V. Sridharan , S. Muthusubramanian & S. Sivasubramanian (2004) A Facile Reduction Procedure for
N,N′‐bis[5‐Substituted Salicylidene]‐m/p‐phenylenediamines with Sodium Borohydride–Silica Gel System, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 34:6, 1087-1096, DOI:
10.1081/SCC-120028641
To link to this article: http://dx.doi.org/10.1081/SCC-120028641
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 6, pp. 1087–1096, 2004
A Facile Reduction Procedure for
N,N0-bis[5-Substituted Salicylidene]-
m/p-phenylenediamines with Sodium
Borohydride–Silica Gel System
*
V. Sridharan, S. Muthusubramanian,
and S. Sivasubramanian
Department of Organic Chemistry, Madurai Kamaraj University,
Madurai, India
ABSTRACT
An easy approach involving sodium borohydride/silica gel system offers
a fast method of reduction for N,N0-bis[5-substituted salicylidene] m/p-
phenylenediamines to the corresponding amines with excellent yield. The
observed formation of boron chelates in the borohydride reduction of
some related Schiff bases has been avoided in this method.
Key Words: Sodium borohydride; Silica gel; Schiff base; Diimines.
*
Correspondence: S. Muthusubramanian, Department of Organic Chemistry, Madurai
Kamaraj University, Madurai-625 021, India.
1087
DOI: 10.1081/SCC-120028641
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Sridharan, Muthusubramanian, and Sivasubramanian
Sodium borohydride is a powerful reducing agent and has been employed
in the reduction of a range of functional groups. Imines can be effectively
reduced to amines by several reducing agents,[1–3] sodium borohydride is one
among them.[4,5] However, the reduction of o,o0-dihydroxy Schiff bases by
this reagent has been found to be difficult.[6] Though Tanaka et al.[7] proposed
a method to reduce these hydroxy Schiff bases, it has led to the formation of
boron complexes, rather than the expected amine, in some cases.
This article describes a very simple methodology for effective reduction
of related N,N0-bis[5-substituted salicylidene] m/p-phenylenediamines to
amines involving silica based sodium borohydride. It should be mentioned
that a similar methodology has been found to effect reduction of ketone to
alcohol in an aprotic solvent,[8] which has been extended to imines with

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modification in the present approach improving the yield. Incidentally all the
diamines obtained and some of the imine precursors are new compounds.
A set of N,N0-bis[5-substituted salicylidene] m/p-phenylenediamines
(1 & 2) have been prepared from the respective aldehydes and m/p-phenyl-
enediamine by refluxing a mixture of m/p phenylenediamine with 2-hydroxy-
5-substituted benzaldehydes in alcohol. They have been fully characterized by
NMR (Table 1). The reduction was carried by mixing the diimines with
sodium borohydride in silica gel with few drops of chloroform and water at
room temperature. The diamine (3/4) was extracted repeatedly with hot ethyl
acetate and finally recrystallized from ethyl acetate with excellent yield. The

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Sridharan, Muthusubramanian, and Sivasubramanian

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physical constants, yields, and NMR data for the diamines are given in Table 2.
The presence of silica gel seems to be specific, because replacement of this by
alumina or any other support did not accelerate the reduction. Chloroform also
plays a vital role in the reduction step, because absence or replacement of this
solvent affects the complete reduction. The addition of water at the final stage
is again important as the reaction occurs very slowly otherwise. Though the
exact role of silica gel and chloroform in this reduction is not clear, it can be
suggested that the complex formation is prevented here as the polar solvents
like methanol used in the conventional reduction, which could stabilize the
complex, is avoided in the present method. Attempts to carry out the reaction
in the pure solid state or under microwave irradiation did not lead to total
reduction.
The method reported by Tanaka et al.[7] for the reduction of a-(2-
hydroxyaryl)-N-2-hydroxyarylimines by sodium borohydride in methanol led
to the formation of boron chelates of the imines rather than the expected
amines directly, though the expected amines can be generated from the
complex. But in the present method even this system gives the amine
directly, not through borate complex. The scope of this methodology for the
general reduction of imines and nitrones is also explored. The imines of type
5 also give the respective amines without any complex formation in excellent
yield. Simple and substituted a,N-diaryl nitrones do not undergo effective
reduction under the same reaction condition. Similarly the diimines derived
from the reaction of o-phenylenediamine and 2-hydroxy-5-substituted
benzaldehydes are reluctant to this method of reduction, probably due to
steric inhibition.

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Melting points were uncorrected. 1H and 13C NMR spectra were recorded
at Bruker 300 MHz instrument. Chemical shifts are given in ppm downfield
from TMS in CDCl3 (in the case of diimines) and DMSO-d6 (in the case of
diamines). Coupling constants are given in Hz. New compounds show
satisfactory C, H, N analysis and spectral features.
Preparation of Diimines
A mixture of 0.01 mol of m/p-phenylenediamine and 0.01 mol of
2-hydroxy-5-substituted benzaldehyde was refluxed in 50 mL of alcohol for an
hour. The pure diimine (1/2) gets solidified on cooling after concentrating the
solvent.
Reduction of Diimines
A mixture of 0.01 mol of the diimine and 0.02 mol of sodium borohydride
was ground well in a mortar and taken in a beaker. To this mixture about 5 g of
silica gel (60–120 mesh) was added and mixed thoroughly. After that few
drops of chloroform was added, the reaction is initiated at this stage, as
evidenced by the liberation of heat and finally few drops of water was added
and stirred vigorously by means of a glass rod. The total disappearance of the
orange color indicates the completion of the reaction, which happen within a
minute. The reduction product was extracted repeatedly with hot ethyl acetate
and finally recrystallized from ethyl acetate.
ACKNOWLEDGMENT
The authors thank DST, New Delhi for assistance under the IRHPA
program for the sNMR facility. One of the authors (V.S.) thanks CSIR, New
Delhi for a Senior Research Fellowship.
REFERENCES
1. Banik, B.K.; Hackfeld, L.; Becker, F.F. Synth. Commun. 2001, 31, 1581.
2. Tsukinoki, T.; Mitoma, Y.; Nagashima, S.; Kawaji, T.; Hashimoto, I.;
Tashiro, M. Tetrahedron Lett. 1998, 39, 8873.

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3. Fujii, M.; Aida, T.; Yoshihara, M.; Ohno, A. Bull. Chem. Soc. Japan 1989,
62, 3845.
4. Periasamy, M.; Devasagayaraj, A.; Satyanarayana, N.; Narayana, C. Synth.
Commun. 1989, 19, 565.
5. Csaszar, J. Acta. Phys. Chem. 1988, 34, 17.
6. Billman, J.H.; Diesink, A.C. J. Org. Chem. 1957, 22, 1068.
7. Tanaka, T.; Koyama, M.; Ikegami, S.; Koga, M. Bull. Chem. Soc. Japan.
1972, 45, 630.
8. Yakabe, S.; Hirano, M.; Morimoto, T. Synth. Commun. 1999, 29, 295.
Received in India September 10, 2003

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