Indian Journal of Chemistry
Vol. 44B, December 2005, pp. 2598-2600
Note
Base induced carbon-nitrogen (C=N) double
bond migration in Schiff bases†
B Gangadasu, P Narender, B China Raju* &
V Jayathirtha Rao
Organic Chemistry Division-II, Indian Institute of Chemical
Technology, Hyderabad 500 007, India
E-mail: [email protected]
Received 20 November 2003; accepted (revised) 14 May 2005
Various Schiff bases have been prepared to study base induced
carbon-nitrogen double bond migrations. Schiff bases derived
from aliphatic aldehydes display highest selectivity. Hydrolysis of
the resulting rearranged Schiff base provides an entry to make
amines from aldehydes. The reaction has possible practical
application.
Keywords: Schiff bases, carbon-nitrogen double bond migrations,
aliphatic aldehydes
IPC: Int.Cl.7 C 07 C
The carbon-carbon double bond migrations
induced/catalyzed by bases1, KF-Alumina2, phasetransfer catalysts3 and noble metal catalysts4 are well
documented. But, the carbon-nitrogen double bond
(Schiff bases) migrations induced / catalyzed by bases
are not studied5. Schiff bases, carrying carbon-nitrogen
double bond, are very important class of compounds
and possess various biological activities. These Schiff
bases are known to be herbicides6, fungicides7,
neoplasm inhibitors8, antiviral9, anticonvulsants10,
antimicrobial11, anticancer12 and also as plant growth
regulators13. Our continued interest in the research on
Schiff bases14 made us to report this novel conversion
of one Schiff base to another Schiff base induced by
base, with high selectivity. Further, the hydrolysis of
Schiff base, after migration of carbon-nitrogen double
bond provides an entry to make amine from aldehyde.
Various Schiff bases were prepared15 according to
well known procedures (Scheme I, Table I). Thus
prepared Schiff bases were taken and heated with
base potassium hydroxide/potassium-t-butoxide (0.1
mole equivalent). The 1H NMR spectrum recorded
after the reaction clearly indicates the novel
conversion (Scheme II).
⎯⎯⎯⎯⎯⎯
†
IICT Communication No. 040506
The per cent of conversion was determined using
GC and 1H NMR. The products were characterized by
1
H NMR, GC-MS and also by preparing authentic
materials. The results obtained are given in Table I.
Perusal of Table I indicates that Schiff bases derived
from alkanal and benzyl amine (Entry No. 1 to 5)
gave highest selectivity compared to other Schiff
bases (Entry No. 6 to 9). Scheme II illustrates the
mechanism involved. Initially, the added base
abstracts a proton from the methylene group, sandwiched between aryl group and Schiff base nitrogen
atom. Thus generated carbanion undergoes double
bond migration leading to another form of carbanion
(resonance structure) and followed by proton
abstraction leads to a different Schiff base. Thus
obtained different Schiff bases can be hydrolyzed to
get amine and aldehyde. The excellent selectivity
observed in the case of alkanal derived Schiff base
(Entry 1 to 5), is due to the relative acidic nature of
the proton to be abstracted by the base.
In summary, a simple convenient and novel base
induced Schiff base C-N double bond migration is
reported. This C-N double bond migration procedure
provides a method to prepare amines from aldehydes.
The bases used and solvent-free condition of reaction
has the merit for the possible practical applications.
Experimental Section
General. Melting points were obtained on a
Toshniwal melting point apparatus and are uncorrected. IR spectra were recorded on a Nicolet
740 FT –IR spectrometer; 1H NMR on a Gemini 200
MHz spectrometer in CDCl3 with TMS as internal
standard; and mass spectra on a VG Micro Mass
7070H.
Typical Experimental procedure
(i) Schiff base preparation. The butyraldehyde
(1.34 g, 0.0186 mole) was added to a well cooled
(0°C) stirred solution of benzylamine (2 g, 0.0186
moles) over a period of 30 min. The reaction mixture
was further stirred for 30 min at the same
temperature. The reaction mixture was then allowed
to stand till the two layers separated completely. The
organic layer was separated and stored over KOH
NOTES
CH2
NH2
2599
KOH
+
R
CH2
CHO
N
CH
R
1a - 5a R=Pr; iPr; But;
i
But; n-Hexyl
CH2
NH2
+
1
OHC
CH2
R
N
1
R
CH
6a & 7a R1=OMe; Cl
R2
CH2
NH2
+
2
R
OHC
CH2
N
CH
8a & 9a R2=OMe; Cl
Scheme I
CH2
N
R
CH
+
Base(B-)
CH
N
CH
CH
N
CH
R
+
BH
1a - 9a
B
+
N
CH
CH2
R
R
>90% (R= alkyl)
CH
N
CH2
HOH
R
CHO
+
H2N
CH2 R
1b - 9b
Scheme II
Table I ⎯ C-N double bond migration using KOH and KOtBu
Entry
Schiff Base (a)
1
CH2 N CH
4
CH2 N CH
6
CH2 N CH
7
8
9
CH2 N CH
MeO
Cl
CH2 N CH
CH2 N CH
95
60
1
95
130
3
93
60
1
95
140
3.5
90
65
1
93
140
3.5
90
65
1
90
160
4
89
80
1.5
88
OCH3
160
4
65
100
2
68
Cl
160
4
60
100
2
65
165
6
62
110
2.5
60
165
6
62
110
2.5
60
CH N
CH N
CH2 N CH
5
3
CH N
CH2 N CH
3
130
CH N
CH2 N CH
2
KOH
KOtBu
Temp. Time Yield Temp. Time Yield
(hr) (%)
(hr) (%)
°C
°C
Product (b)
CH N
OCH3
CH N CH2
Cl
CH N CH2
MeO
Cl
CH N CH2
CH N CH2
2600
INDIAN J. CHEM., SEC B, DECEMBER 2005
pellets and the obtained yield is 95%; 1H NMR
(CDCl3): δ 0.9 (t, 3H), 1.4 (m, 2H), 2.3 (q, 2H), 4.5 (s,
2H), 7.18 (m, 5H), 7.74 (t, 1H).
(ii) (Base induced C=N double bond migration.
N-Benzyl butyraldimine (1a, 1 g, 0.006 mole) and
KOH (0.03 g, 0.0006 mole) were heated 130 ºC for
3 hr. Reaction mixture was cooled and diluted with
dry ether (2×50 mL). The organic layer was dried
over sodium sulphate and the solvent evaporated
under reduced pressure to give 1b in 95% yield; 1H
NMR (CDCl3): δ 0.9 (t, 3H), 1.38 (m, 2H), 1.64 (p,
2H), 3.58 (t, 2H), 7.28-7.40 (m, 3H), 7.62-7.76
(m, 2H).
(iii) Hydrolysis to amine and aldehyde. The
N-benzyl phenyl methanimine (1b, 0.5 g, 0.003 mole)
and H2O (5 mL) were stirred vigorously at room
temperature for a period of 10-15 min. The contents
were diluted with ether (2×25 mL) and the layers
separated. The organic layer was dried over sodium
sulphate and the solvent removed under reduced
pressure. The residue on column chromatography
afforded the benzaldehyde and butyl amine in
quantitative yield.
Acknowledgement
The authors thank the Director, IICT and Head,
Organic Chemistry Division-II for constant
encouragement. Authors (BG and PN) thank the UGC
and the CSIR, New Delhi for the award of
fellowships.
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