Electrochemical reductive allylation of N-benzylideneethanolamine

Article abstract of DOI:10.1016/S0040-4039(02)01496-X

Electrochemical reductive allylation of N-benzylideneethanolamine by allyl bromide mediated by a Pb(II)/Pb(0) redox couple is reported.

Full text of DOI:10.1016/S0040-4039(02)01496-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6807–6809
Electrochemical reductive allylation of
N-benzylideneethanolamine
a
b
a,
F. Nawaz Khan, R. Jayakumar and C. N. Pillai *
a
Central Electrochemical Research Institute, Chennai Unit, CSIR Madras Complex, Tharamani, Chennai 113, India
b
Central Leather Research Institute, Bio-Organic Laboratory, Adayar, Chennai 20, India
Received 5 May 2002; revised 4 July 2002; accepted 19 July 2002
Abstract—Electrochemical reductive allylation of N-benzylideneethanolamine by allyl bromide mediated by a Pb(II)/Pb(0) redox
couple is reported. © 2002 Elsevier Science Ltd. All rights reserved.
‘
Barbier type’ allylation of imines using allylmetal
The presence of the hydroxyl group in the starting
material posed a problem in the use of usual
organometallic reagents. However, the organotin
reagents have been used successfully for reactions with
carbonyl compounds (but not imines) bearing hydroxyl
and other active hydrogen groups and, in fact, the
electrochemical reaction has been done in methanol–
reagents is a convenient route for the synthesis of
homoallylamines. Metals such as boron, tin, magne-
sium, zinc, lead, and titanium have been reported to
be effective as the partner in the organometallic
reagents. Torii’s group has reported the use of Pb(0)
and Ti(0) catalytically using Al as the chemical reduc-
ing agent for PbBr and TiCl , and Pb(0) generated
electrochemically from PbBr for the allylation of vari-
1
2
3
4
5
6
5b
6
7
2
4
water–acetic acid solutions. With this background
2
information to hand, the electrochemical allylation of
benzylideneethanolamine as a model compound was
attempted. Lead and tin were selected as promising
mediators. Some other metal ions were also tried. The
reaction scheme may be represented as outlined in
Scheme 1.
ous aldimines and ketimines. Catalysis by AlX gener-
3
ated in situ chemically in the first two cases and
electrochemically from the sacrificial aluminum anode
in the third case, was found to be essential for the
success of the reaction. In the present study our objec-
tive was to prepare N-alkylethanolamines as part of a
study on the electrochemical oxidation of aminoalco-
hols and model compounds of type I were required.
Reductive allylation of the Schiff’s base of aromatic
aldehydes with ethanolamine suggested itself as a possi-
ble synthetic route.
*
0
Corresponding author. Present address: 19 Srinivasapuram, I Street,
Tiruvanmiyur, Chennai-41, India.
Scheme 1. Electrochemical reductive amination of imines.
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01496-X

6
808
F. Nawaz Khan et al. / Tetrahedron Letters 43 (2002) 6807–6809
Table 1. Allylation of N-benzylidineethanolamine
a
The reactions were carried out in a beaker-type undi-
vided cell of 250 ml capacity. A platinum wire (15 cm
long) coiled around a glass rod or a platinum gauze was
used as the cathode. The anode was a perforated alu-
minum sheet (5×7 cm) rolled into a cylindrical shape.
The cathode was positioned inside the cylindrical anode
and properly fixed to prevent contact between the two.
Dry THF was used as the solvent. (Analar THF was
S. No.
Yield (%)b
1
2
3
4
80
25
20
10–12
c
d
e
a
Al anode, Pt cathode (unless otherwise stated). 20 mmol of Schiff’s
base (C H -CHꢀNH-CH -CH -OH) in 100 ml THF, 3–10 mmol of
dried over precalcined 3 A molecular sieves). Tetra-
,
6
5
2
2
butylammonium bromide (6.6 mmol/100 ml of solvent)
was used as the electrolyte. The Schiff’s base formed
from benzaldehyde and ethanolamine was purified by
distillation under vacuum. Allyl bromide was prepared
tetrabutylammonium bromide, 1 mmol of PbBr2 (unless otherwise
stated); 60 mmol of allyl bromide (unless otherwise stated), constant
current (1 A) 10 h.
Yield (based on Schiff’s base) of homoallylamine. Other products
isolated and identified: benzaldehyde, tributylamine, N,N-diallyl-
ethanolamine. Also unidentified higher boiling components were
present.
Lead cathode instead of platinum.
Allyl chloride instead of allyl bromide.
Bi(III), Cd(II), Sn(II) and Zn(II) salts as mediators instead of PbBr2.
b
8
from allyl alcohol by a reported procedure. In a typical
experiment, a solution of 2.98 g (20 mmol) of the
Schiff’s base, 7.26 g (60 mmol) of allyl bromide, 0.367
g (1 mmol) of PbBr and 2.13 g (6.6 mmol) of tetra-
butyl-ammonium bromide in 100 ml of THF were
placed in the cell.
c
d
e
2
on literature information, the following scheme is pre-
sented for the reaction.
The electrolysis was carried out under constant current
conditions (0.1 A for 10 h) with magnetic stirring.
Variations were done by (i) replacing the platinum
cathode by lead, (ii) replacing the allyl bromide by allyl
chloride and (iii) increasing the applied potential and
(
iv) replacing the lead bromide by Bi(NO) , CdSO ,
3 4
SnCl ·2H O or Zn(Ac) ·2H O. After the electrolysis the
2
2
2
2
solvent was removed by distillation and a small portion
of the residue was dissolved in dichloromethane,
washed with water to remove the electrolytes and
directly analyzed by gas chromatography. (Chemito
GC 8610 HR. SE 30 column; temperature programmed
from 100 to 250°C, flame ionization detector). The bulk
of the product mixture was treated with dilute HCl,
washed with dichloromethane to remove neutral com-
ponents. The HCl extract was neutralized with alkali
and extracted with dichloromethane to recover the
basic components. This mixture was once again ana-
lyzed by GC and then subjected to column chromato-
graphy by successively eluting with hexane, hexane/
ethyl acetate with an increasing proportion of ethyl
acetate and finally with ethyl acetate/methanol mix-
tures. All the fractions were analyzed by GC and those
fractions which gave single peaks in the GC were
appropriately combined to give four fractions which
were homogeneous. These were analyzed by proton
The observed by-products are (i) tributylamine, (ii)
N,N-diallylethanolamine, (iii) benzaldehyde and higher
boiling amine components. Tributylamine arises from
the electrolyte, tetrabutylammonium bromide by Hoff-
mann elimination induced by electrogenerated bases.
These by-products can be decreased by working at
lower electrode potentials. Benzaldehyde and diallyl-
ethanolamine arise by hydrolysis of the starting mate-
rial (Schiff’s base) brought about by traces of water
being present in the solvent (THF). The Schiff’s base
itself is not readily hydrolyzed by water at room tem-
perature. However, the presence of acid (Lewis acid–
9a,b,c
NMR and mass spectroscopy.
Some of the reaction
products (total mixture) were subjected to GC–MS
analysis. Some components which could not be sepa-
rated by column chromatography were identified by
GC retention time comparison with suspected authentic
samples. The results are presented in Table 1.
In our study the yields of the homoallylamine are
invariably less than those reported in the literature for
the benzylamine–benzaldehyde Schiff’s bases. The yield
of the expected product is very much dependent on the
various experimental parameters. PbBr as mediator,
AlBr ) catalyses the hydrolysis. This was verified by an
2
3
THF as solvent, aluminum as sacrificial anode, Pt as
cathode and tetrabutylammonium bromide as elec-
trolyte were found to be the best in these studies. Based
independent experiment where a 0.01 M solution of
Schiff’s base in THF containing traces of water was
kept for 5 h with and without the addition of 0.01 M

F. Nawaz Khan et al. / Tetrahedron Letters 43 (2002) 6807–6809
6809
HBr acid. An internal standard (naphthalene) for GC
analysis was also added. In the former experiment
2. Keck, G. E.; Enholn, E. J. J. Org. Chem. 1985, 50, 146.
3. Uneyama, K.; Matsuda, H.; Torii, S. Tetrahedron Lett.
1984, 25, 6017.
4. (a) Breton, G. W.; Shugart, J. H.; Hughey, C. A.; Con-
rad, B. P.; Perala, S. M. Molecules 2001, 6, 655; (b) Li, C.
J.; Chan, T. H. Organic Reactions in Aqueous Media;
John Wiley & Sons: New York, 1997; p. 65.
(
(
without acid) the Schiff’s base was 95%. In the latter
acid added) the Schiff’s base had undergone hydrolysis
to the extent of 40%. The ethanolamine acid formed by
hydrolysis is allylated by allylbromide forming diallyl–
ethanolamine (triallylethanolammonium bromide may
also be formed but is lost during work-up, being water
soluble. Unallylated ethanolamine itself is also lost
during work-up due to the same reason). The higher
boiling basic components present in the reaction mix-
ture may include products of reductive dimerization of
the imine mediated by the Pb(0)/Pb(II) redox couple
catalyzed by Lewis acid. Such reductive dimerization of
imines brought about by a Pb/Al–bimetal redox system
5
. (a) Tanaka, H.; Nakahara, T.; Dhimane, H.; Torii, S.
Tetrahedron Lett. 1989, 30, 4161; (b) Tanaka, H.;
Yamashita, S.; Ikemoto, Y.; Torii, S. Chem. Lett. 1987,
673; (c) Tanaka, H.; Yamshita, S.; Hamatani, T.; Naka-
hara, T.; Torii, S. Stud. Org. Chem. 1987, 30, 307; (d)
Tanaka, H.; Yamashita, S.; Katayama, Y.; Torii, S.
Chem. Lett. 1986, 2043; (e) Tanaka, H.; Yamashita, S.;
Hamatani, T.; Ikemoto, Y.; Torii, S. Synth. Commun.
1
0
is known.
1989, 17, 789; (f) Tanaka, H.; Dhimane, H.; Ikemoto, Y.;
Torii, S. Chem. Express 1987, 2, 487.
The observation that the reaction is successful, albeit
with lower yields, even with substrates bearing a
hydroxyl group was promising and further studies are
in progress to optimize the reaction conditions and to
access other substrates.
6
. Tanaka, H.; Inoue, K.; Pokorski, U.; Taniguchi, M.;
Torii, S. Tetrahedron Lett. 1990, 31, 3023 and references
cited therein.
7
. Uneyama, K.; Matsuda, H.; Torii, S. Tetrahedron Lett.
1984, 25, 6017.
8
. Furniss, B. S.; Hannaford, A. J.; Rogers, V.; Smith, P.
W. G.; Tatchell, A. R. Vogel’s Textbook of Practical
Organic Chemistry, 1976, ELBS 4th ed., p. 389.
Acknowledgements
9
. (a) Product I: 1-phenyl-N-(hydroxyethyl)-but-3-enyl-
+
’
amine. Mass spectrum: m/z 191 (M 3%), 160 (50%), 150
4%), 131 (5%), 120 (2%) 105 (5%), 91 (100%), 77 (4%)
5(12%) NMR spectrum: As expected; (b) Product II:
This work was supported by CSIR Emeritus Scientist
Schemes to C.N.P. and JRF to F.N.K. The authors are
grateful to the SPIC Science Foundation and IIT,
Madras for the NMR and mass spectroscopy.
(
6
+
’
tributylamine mass spectrum: m/z 185 (M 5%), 142
100%), 100 (95%), 58 (30%) NMR spectrum: As
expected; (c) Product III: N,N-diallylethanolamine mass
(
+
’
spectrum: m/z 141 (M , 5%), 110 (100%), 86 (26%), 60
30%) NMR spectrum: As expected.
References
(
1
0. Tanaka, H.; Dhimane, H.; Fujita, H.; Ikemoto, Y; Torii,
1
. Yamamoto, Y.; Nishii, S.; Maniyama, K.; Komatsu, T.;
Ito, W. J. Am. Chem. Soc. 1986, 108, 7778.
S. Tetrahedron Lett. 1988, 29, 3811.