An efficient method for the synthesis of 1,5-benzodiazepine derivatives under microwave irradiation without solvent

Article abstract of DOI:10.1016/S0040-4039(02)00115-6

2,3-Dihydro-1H-1,5-benzodiazepines have been synthesized in solvent-free conditions from o-phenylenediamines and ketones in the presence of a catalytic amount of acetic acid, under microwave irradiation. This method is a very easy, rapid and high yielding

Full text of DOI:10.1016/S0040-4039(02)00115-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1755–1758
An efficient method for the synthesis of 1,5-benzodiazepine
derivatives under microwave irradiation without solvent
Minothora Pozarentzi, Julia Stephanidou-Stephanatou* and Constantinos A. Tsoleridis
Department of Chemistry, Laboratory of Organic Chemistry, University of Thessaloniki, Thessaloniki 54006, Macedonia, Greece
Received 21 November 2001; revised 2 January 2002; accepted 14 January 2002
Abstract—2,3-Dihydro-1H-1,5-benzodiazepines have been synthesized in solvent-free conditions from o-phenylenediamines and
ketones in the presence of a catalytic amount of acetic acid, under microwave irradiation. This method is a very easy, rapid and
high yielding reaction for the synthesis of 1,5-benzodiazepine derivatives. © 2002 Elsevier Science Ltd. All rights reserved.
Benzodiazepines are very important compounds
because of their pharmacological properties.1,2 Many
members of the family are nowadays used as anticon-
vulsant, antianxiety and hypnotic agents. In addition,
1,5-benzodiazepines are used as starting materials for
the preparation of fused ring compounds such as tria-
zolo-,3–5 oxadiazolo-,5 oxazino-4 or furano-benzo-
diazepines.4,6
yields, tedious work-up procedures and the occurrence
of several side reactions. The application of microwave
energy for conducting organic reactions at highly accel-
erated rates is an emerging technique. In recent years,
microwaves have become popular among synthetic
organic chemists both to improve classical organic reac-
tions, shortening reaction times and/or improving
yields, as well as to promote new reactions.16
Despite their importance from a pharmacological,
industrial and synthetic point of view, comparatively
few methods for their preparation are reported in the
literature,7–10 a great number of which have appeared
only very recently.11–15 These include condensation
reactions of o-phenylenediamines with a,b-unsaturated
carbonyl compounds,7 b-haloketones8 or ketones in the
presence of BF3–etherate,9 NaBH4,10 polyphosphoric
acid,11 SiO2,11 MgO and POCl3,12 Yb(OTf)3,13 and
Al2O3/P2O5 under MW.14 Unfortunately, many of these
processes suffer major or minor limitations, such as
drastic reaction conditions, expensive reagents, low
Moreover, often when carrying out a reaction in a
microwave oven, the use of a solvent can be avoided,
which is important in order to make the synthesis more
environmentally friendly (‘green chemistry’). These
observations led us to investigate the possibility of
improving the methods used for the synthesis of the
1,5-benzodiazepine scaffold.
This paper describes a facile synthesis of 2,3-dihydro-
1H-1,5-benzodiazepines by condensation of ketones
with o-phenylenediamines (Scheme 1) by application of
microwave irradiation without solvent. The syntheses
Scheme 1.
Keywords: benzodiazepines; o-phenylenediamines; microwave irradiation; green chemistry.
* Corresponding author. Fax: 0310-997679; e-mail: ioulia@chem.auth.gr
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00115-6

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M. Pozarentzi et al. / Tetrahedron Letters 43 (2002) 1755–1758
Table 1. Synthesis of 2,3-dihydro-1H-1,5-benzodiazepines
Reactants
Product 3
Yield (%)* Irrad. time (min) Power (W) Ref.
1,2-Phenylenediamine
Acetone
Acetone
97
2
80
11
4-Methyl-1,2-phenylenediamine
97
4
240
17
4,5-Dimethyl-1,2-phenylenediamine Acetone
96
93
2
2
80
17
4-Chloro-1,2-phenylenediamine
Acetone
560
New
4-Benzoyl-1,2-phenylenediamine
Acetone
95
7
400
New
2,3-Naphthalenediamine
Acetone
97
2
80
New

M. Pozarentzi et al. / Tetrahedron Letters 43 (2002) 1755–1758
1757
Table 1. (Continued)
Reactants
Product 3
Yield (%)* Irrad. time (min) Power (W) Ref.
1,2-Phenylenediamine
Acetophenone
98
2
80
11
4,5-Dimethyl-1,2-phenylenediamine Acetophenone
99
2
80
New
1,2-Phenylenediamine
2-Butanone
98
99
98
90
2
7
2
2
80
560
560
560
13
4,5-Dimethyl-1,2-phenylenediamine 2-Butanone
New
New
13
2,3-Naphthalenediamine
2-Butanone
1,2-Phenylenediamine
3-Pentanone
* Yields of pure isolated products characterized by IR, MS, 1H and 13C NMR spectra.18
** Inseparable regioisomers.
were carried out simply by mixing the o-phenylenedi-
amine (1 mmol) with the ketone (2.1 mmol) in the
presence of a catalytic amount of acetic acid and irradi-
ating in a domestic microwave oven, whereupon the
benzodiazepine derivatives were obtained in almost
quantitative yield.
purified either by recrystallization from n-hexane or by
preparative thick layer chromatography using ethyl ace-
tate and cyclohexane (1:6) as eluant.
In conclusion, we have described a rapid and efficient
(90–99%) synthesis of 2,3-dihydro-1H-1,5-benzodi-
azepines using a microwave oven and provided a fur-
ther example of the utility of microwave irradiation in
environmentally friendly (absence of solvent) organic
synthesis.
The desired parameters (microwave power, time) were
set as reported in Table 1. The crude products were

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1941–1951.
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