Ultrasound accelerated reductive coupling of imine or iminium ion generated in 5m lithium perchlorate solution by lithium metal

Article abstract of DOI:10.1081/SCC-100107005

A novel one-pot reductive coupling of imine or iminium ion generated in the concentrated ethereal lithium perchlorate solution was achieved in the presence of lithium metal. The yield of the reaction depends on the reactivity preformed imine or iminium ion. Diamines were formed with high diastereoselectivity in some cases in about 5 h. The reaction time was doubled without the use of ultrasound acceleration.

Full text of DOI:10.1081/SCC-100107005

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ULTRASOUND ACCELERATED REDUCTIVE COUPLING
OF IMINE OR IMINIUM ION GENERATED IN 5 M LITHIUM
PERCHLORATE SOLUTION BY LITHIUM METAL
Mohammad M. Mojtahedi a , Mohammad R. Saidi b , Jafar S. Shirzi a & Mohammad
Bolourtchian a
a Chemistry and Chemical Engineering Research of Iran, Tehran, Iran
b Department of Chemistry, Sharif University of Technology, Tehran, Iran
Published online: 16 Aug 2006.
To cite this article: Mohammad M. Mojtahedi , Mohammad R. Saidi , Jafar S. Shirzi & Mohammad Bolourtchian (2001)
ULTRASOUND ACCELERATED REDUCTIVE COUPLING OF IMINE OR IMINIUM ION GENERATED IN 5 M LITHIUM PERCHLORATE
SOLUTION BY LITHIUM METAL, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 31:23, 3587-3592, DOI: 10.1081/SCC-100107005
To link to this article: http://dx.doi.org/10.1081/SCC-100107005
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SYNTHETIC COMMUNICATIONS, 31(23), 3587–3592 (2001)
ULTRASOUND ACCELERATED
REDUCTIVE COUPLING OF
IMINE OR IMINIUM ION
GENERATED IN 5 M LITHIUM
PERCHLORATE SOLUTION BY
LITHIUM METAL
Mohammad M. Mojtahedi,2
Mohammad R. Saidi,1, Jafar S. Shirzi,2
*
and Mohammad Bolourtchian2
1Department of Chemistry, Sharif University
of Technology, Tehran, Iran
2Chemistry and Chemical Engineering Research
of Iran, Tehran, Iran
ABSTRACT
A novel one-pot reductive coupling of imine or iminium ion
generated in the concentrated ethereal lithium perchlorate
solution was achieved in the presence of lithium metal. The
yield of the reaction depends on the reactivity preformed
imine or iminium ion. Diamines were formed with high dia-
stereoselectivity in some cases in about 5 h. The reaction time
was doubled without the use of ultrasound acceleration.
*Corresponding author.
3587
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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MOJTAHEDI ET AL.
Vicinal diamines occur frequently in natural products and medicinal
compounds. The synthesis of aromatic vicinal diamines by various methods
has been an active area of research for many years. As a result many
methods are currently available in the literature.1,2 The reductive coupling
of imines can be used as a method for synthesis of these compounds. The
reductive coupling of aldimines catalyzed by titanium tetrachloride,2 chloro-
silane and zinc metal,3 samarium diiodide and nickel diiodide,4 and a great
variety of other reagents,5 are well-documented chemistry with wide appli-
cations.
Imines and iminium salts are important intermediates in organic syn-
thesis. These salts may be produced in situ by the reaction of lithium hexa-
methyldisilazane or (trimethylsilyl)dialkyl amines with various aromatic
aldehydes, promoted by a 5 M solution of LiClO4 in diethyl ether6–8
(Scheme 1).
Scheme 1.
In continuation of our research studies on Mannich type reactions, we
now report the one-pot reductive coupling of imine or iminium ion gener-
ated in 5 M lithium perchlorate solution by lithium metal. Aromatic alde-
hydes and lithium hexamethyldisilazane or (trimethylsilyl)dialkyl amines in
the concentrated ethereal lithium perchlorate solution produce the imine or
iminium salts very quickly as an intermediate at room temperature, which
can be detected in the solution by 13C NMR spectroscopy.9,10 Upon addi-
tion of lithium metal to the preformed imine or iminium ion in 5 M ethereal
LiClO4 solution under sonocation, followed by aqueous work-up, the coup-
ling product (diamine) was produced in about 5 h. By this method, variety of
diamines 4 and 5 could be synthesized within 5 h in moderate to good yields
with high diastereoselectivity in some cases (Scheme 2). From a series of
experiments, we found that 2.5 equivalent of lithium metal was required for
the maximum yields of aromatic diamines. Increasing the reaction time did
not change the yield of the products. Without sonocation, the reaction time
was more than 10 h. The vicinal diamines are obtained as a diastereomeric
(meso : dl ) mixture. The stereochemical assignment of the vicinal diamine

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IMINE/IMINIUM ION
3589
Scheme 2.
was made by comparison of the resonance of benzylic protons for the
meso and dl isomers as reported in the literature.11 The diastereomeric
ratio was determined by NMR from the non-equivalent benzylic protons
and carbon (in 1H and 13C NMR) with meso/dl ratio between 97 and 52. As
indicated in Tables 1 and 2, the isolated yield of vicinal diamines varies from
36% to 87%. The low yield of the diamine 4d is due to the low reactivity of
imine 2.
Table 1. Reductive Dimerization of Imine by Li Metal

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MOJTAHEDI ET AL.
Table 2. Reductive Dimerization of Iminium Ion by Li Metal
EXPERIMENTAL
CAUTION: Although we did not have any accident using lithium
perchlorate (LiClO4), it is advisable to dry lithium perchlorate in a hood
behind a lab-shield.
General Procedure for the Preparation of
Vicinal Diamine from an Aldehyde
Method A: The aldehyde (2 mmol) and 3 mL of 5 M LiClO4 in diethyl
ether were placed in a 50 mL flask under argon and stirred for 5 min.

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IMINE/IMINIUM ION
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(Trimethylsilyl)dialkyl amines (3 mmol) was then added via syringe. After
10 min, lithium metal (5 mmol, 35 mg) was added and the mixture stirred for
5 h at room temperature under sonocation. Then, ether (30 mL) was added
and excess lithium metal removed by filtration. About 30 mL of water was
added to the filtrate and the organic phase was separated, dried over
MgSO4, and the solvent was removed by the means of rotary evaporator.
The crude material was further purified by chromatography on basic alu-
mina or aqueous acid extraction, if needed.
Method B: Hexamethyldisilazane (3.3 mmol, 0.54 g) was placed in
two-necked flask fitted with a condenser, and a stirring bar under argon,
and 3.5 mmol of sodium hydride (60–65%, after washing with light pet.
ether) or 3.4 mmol of methyllithium in dry diethyl ether was added, and
the mixture was stirred for about 5 min. Then 3 mL solution of 5 M lithium
perchlorate in diethyl ether and 2 mmol of aldehyde were added via syringe.
After stirring for 30 min, lithium metal (5 mmol, 35 mg) was added and the
mixture stirred for 5 h at room temperature under sonocation. Then, ether
(30 mL) was added and excess lithium metal removed by filtration. About
30 mL of water was added to the filtrate and the organic phase was sepa-
rated, dried over MgSO4, and the solvent was removed by the means of
rotary evaporator. The crude material was further purified by chromatogra-
phy on basic alumina or aqueous acid extraction, if needed.
In conclusion, we have described one-pot reductive coupling of
aromatic imine or iminium ion derivatives generated in 5 M lithium perchlo-
rate solution by lithium metal, with formation of 1,2-diarylethylendiamines
or N, N,N 0,N 0-tetraalkyl-1,2-diarylethylendiamines which was accelerated
by ultrasound. 1,2-Diarylethylendiamines were formed with high diastereo-
selectivity in some cases in about 5 h. The reaction time was doubled without
the use of ultrasound acceleration.
SELECTED SPECTROSCOPIC DATA
1
5a (dl isomer); IR (neat): n ¼ 2930, 1630, 1453, 707 cmÀ1. H NMR
(CDCl3): d ¼ 2.04 (s, 12H), 4.25 (s, 2H), 7.10–7.30 (m, 10H). 13C NMR
(CDCl3): d ¼ 40.4 (CH3), 67.5 (CH), 126.8 (CH), 128.1 (CH), 129.0 (CH),
132.5 (C). MS, m/z: 268 (Mþ), 134 (base peak), 91 and 77.
5d (meso isomer); IR (neat): n ¼ 2953, 2815, 1600, 1453, 1253, 1130,
1
846, 707 cmÀ1. H NMR (CDCl3): d ¼ 0.81 (t, 12H, 7.1 Hz), 1.85–2.34 (m,
8H), 4.48 (s, 2H), 6.60–7.25 (m, 6H). 13C NMR (CDCl3): d ¼ 13.3 (CH3),
44.1 (CH2), 61.9 (CH), 123.0 (CH), 125.1 (CH), 126.2 (CH), 140.9 (C). MS,
m/z: 336 (Mþ), 168 (base peak), 97, 85 and 56.

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MOJTAHEDI ET AL.
5e (dl isomer); IR (neat): n ¼ 2969, 2807, 1615, 1515, 1453, 1384, 1115,
1
807 cmÀ1. H NMR (CDCl3): d ¼ 0.75 (t, 12H, 6.8 Hz), 1.85–2.15 (m, 8H),
2.32 (s, 6H), 4.32 (s, 2H), 6.90–7.20 (m, 8H). 13C NMR (CDCl3): d ¼ 13.7
(CH3), 20.9 (CH3), 43.4 (CH2), 63.6 (CH), 127.5 (CH), 129.2 (CH),
134.7 (C), 135.3 (C). MS, m/z: 352 (Mþ), 176 (base peak), and 105.
4b(mesoisomer); 1H NMR (CDCl3): d ¼ 2.06 (broad s, 4H), 3.72 (s, 6H),
4.51 (s, 2H), 6.79 (d, 4H, J ¼ 8.8 Hz), 7.20 (d, 4H, 8.8 Hz). 13C NMR
(CDCl3): d ¼ 55.0 (CH3), 65.1 (CH), 123.7 (CH), 128.3 (CH), 130.2 (C),
134.8 (C). MS, m/z: 272 (Mþ), 136, 121 (base peak), 97, 95 and 57.
ACKNOWLEDGMENTS
We thank ‘‘Volkswagen-Stiftung, Federal Republic of Germany’’ for
financial support towards the purchase chemicals. M. R. Saidi thanks
Iranian National Research Council for partial financial support.
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Received in Japan November 7, 2000

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