Microwave assisted deprotection of N,N-dimethylhydrazones in water using palladium chloride-tin chloride as catalyst

Article abstract of DOI:10.1007/s007060200113

N,N-Dimethylhydrazones of ketones and aldehydes undergo facile cleavage to the corresponding carbonyl compounds upon exposure to microwaves in water containing a catalytic amount of PdCl₂-SnCl₂ in high yields.

Full text of DOI:10.1007/s007060200113

Monatshefte fur Chemie 133, 1413±1415 ꢀ2002)
Microwave Assisted Deprotection
of N,N-Dimethylhydrazones in Water
Using Palladium Chloride±Tin Chloride
as Catalyst
Rahman Hosseinzadeh1;Ã, Ali Shari®2;Ã, Kourosh Tabar-Heydar2,
and Farshid Mohsenzadeh2
1
Department of Chemistry, Mazandaran University, Babolsar, Iran
2
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
Summary. N,N-Dimethylhydrazones of ketones and aldehydes undergo facile cleavage to the corre-
sponding carbonyl compounds upon exposure to microwaves in water containing a catalytic amount of
PdCl2±SnCl2 in high yields.
Keywords. Dimethylhydrazones; Deprotection; Palladium; Microwave irradiation.
Introduction
The deprotection of hydrazones to carbonyl compounds is an important transfor-
mation in organic synthesis [1]. In recent years, many reagents compatible with a
wide range functionalities have been developed to regenerate carbonyl compounds
from the corresponding dialkylhydrazones, thus allowing the use of hydrazones in
the total synthesis of complex natural products [2]. Some of the procedures re-
ported earlier for the deprotection of hydrazones involve oxidative cleavage by
ozone [3], singlet oxygen [4], NaIO4 [5], or dimethyldioxirane [6] or hydrolytic
cleavage by oxalic acid [7], salts [8], or BF3 Á OEt2 [9]. These methods, however,
have certain limitations as e.g. the use of stoichiometric amounts of reagents and
organic solvents, which are often inherently toxic or require tedious work-up pro-
cedures. The catalytic method has only been reported using bismuth [10] and
palladium [11]. The cleavage of N,N-dimethylhydrazones using Pd requires long
reaction times ꢀ24±72 h) and DMF±H2O as solvent.
Microwave enhanced chemical reactions have gained popularity with respect to
usual homogeneous and heterogeneous reactions [12]. We report here a facile de-
protection of dimethylhydrazones to carbonyl compounds using PdCl2±SnCl2 as
the catalyst in water in a process that is accelerated by microwave irradiation.
Ã
Corresponding authors. E-mail: rahman@umz.ac.ir

1414
R. Hosseinzadeh et al.
Results and Discussion
Using PdCl2±SnCl2 under microwave irradiation, various dimethylhydrazones
were successfully deprotected to give the corresponding carbonyl compounds
ꢀTable 1). The data indicate that deprotection of a variety of dimethylhydrazones
readily occurs under these conditions.
When acetophenone N,N-dimethylhydrazone ꢀ1a; entry 1) was treated with a
catalytic amount of PdCl2±SnCl2 in water under microwave irradiation, acetophe-
none was obtained in excellent yield. However, when 1a was treated with PdꢀOAc)2
under the same conditions, 2a was obtained in 83% yield ꢀentry 2). When SnCl2,
PdCl2, or both of them were absent, the corresponding ketone was obtained in
lower yield ꢀentries 3±5). The cleavage of other acyclic or cyclic N,N-dimethylhy-
drazones proceeded easily with good yields ꢀentries 6±12). Octanal N,N-dimethyl-
hydrazone ꢀ1h; entry 12) afforded octanal in 88% yield when reacted employing
0.03 equiv. of this catalyst system.
In conclusion, the present methodology offers an ef®cient, fast, and environ-
mentally friendly procedure for the deprotection of N,N-dimethylhydrazones to the
corresponding carbonyl compounds. A particularly noteworthy advantage of this
method is the dramatically shortened reaction time ꢀfrom hours to seconds) without
the need to use organic solvents.
Table 1. Regeneration of carbonyl compounds from N,N-dimethylhydrazones with PdCl2±SnCl2
under microwave irradiation
Entry
Hydrazone
Conditions
Yield=%a
R
R0
b
1
2
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
Ph
CH3
98
83
18
32
10
97
79
89
88
82
78
88
Ph
CH3
PdꢀOAc)2 instead of PdCl2
without SnCl2
3
Ph
CH3
4
Ph
CH3
without PdCl2
5
Ph
CH3
without PdCl2 and SnCl2
b
6
Ph
CH3CH2
CH3
b
b
b
b
b
b
7
2-Naphtyl
PhCH2CH2
CH3ꢀCH2)5
CH2ꢀCH2)4CH2
CH2ꢀCH2)3CH2
CH3ꢀCH2)6
8
CH3
9
CH3
10
11
12
1g
1h
H
a
b
GLC yields; retention times compared with authentic samples; 1 equiv. substrate, 0.03 equiv.
PdCl2, 0.03 equiv. SnCl2 Á 2H2O, 2 cm3 H2O

Deprotection of Hydrazones with PdCl2±SnCl2
1415
Experimental
All reactions were carried out in a domestic microwave oven ꢀMoulinex Microchef, 900 W). N,N-
Dimethylhydrazones 1a±h were prepared from the corresponding carbonyl compounds according to
standard methods [13]. All isolated carbonyl compounds were identi®ed by GLC, IR, and NMR and
gave satisfactory results in comparison with authentic samples. GC: Perkin Elmer 8700, FID, He as a
carrier gas.
General procedure
A mixture of 1 mmol hydrazone, 0.03mmol PdCl2, 0.03 mmol SnCl2 Á 2H2O, and 2 cm3 H2O in a
25cm3 beaker was irradiated in a microwave oven for 90s. After irradiation ꢀ9Â 10s, 5 min intervals)
and cooling the mixture was extracted with 2 Â 20cm3 ethyl acetate. The organic layer was separated
and dried ꢀNa2SO4). After ®ltration, the solvent was evaporated, and the residue was analyzed by
GC and puri®ed on a short silica gel column ꢀeluent: light petroleum ether:ethyl acetateˆ 10:1).
Acknowledgments
We are grateful to the research council of Mazandaran University and the Chemistry and Chemical
Engineering Research Center of Iran for ®nancial support.
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Received December 27, 2001. Accepted ꢀrevised)February 6, 2002