Microwave induced synthesis of hydrazones and Wolff-Kishner reduction of carbonyl compounds

Article abstract of DOI:10.1055/s-1999-2901

Carbonyl compounds 1 are converted to hydrazones 3 and then to hydrocarbons 4 by Wolff-Kishner reductions under microwave irradiations within 25-30 mins in excellent yields.

Full text of DOI:10.1055/s-1999-2901

LETTER
1573
Microwave Induced Synthesis of Hydrazones and Wolff-Kishner
Reduction of Carbonyl Compounds
Sunil Gadhwal, Mukulesh Baruah, Jagir S Sandhu*
Regional Research Laboratory, Jorhat 785006, Assam, India
Fax +91(037)6321158
Received 20 July 199
Table Characteristics of Hydrazones 3 and Hydrocarbons 4
Abstract: Carbonyl compounds 1 are converted to hydrazones 3
and then to hydrocarbons 4 by Wolff-Kishner reductions under mi-
crowave irradiations within 25-30 mins in excellent yields.
Key words: Wolff-Kishner reductions, carbonyl compounds, mi-
crowave irradiation, hydrazones and solid state reactions
The deoxygenation of aldehydes and ketones to methyl or
methylene derivatives, developed by Wolff and Kishner1
is one of the key functional group interconversions in the
2
synthesis of complex organic molecules. The original
protocols introduced involved the addition of preformed
hydrazone directly to hot potassium hydroxide with or
3
without added crushed platinized porous plate or palladi-
4
um/barium sulfate at atmospheric pressure or boiling the
hydrazone with sodium ethoxide in a sealed tube at 160-
o
2
00 C. Following these pioneering investigations, other
5
reduction systems, particularly catechol borane (hazard-
ous and expensive) have been introduced to effect reduc-6
tions. Other methods including catalytic hydrogenation,
7
reductions using Raney nickel and trichlorosilane-
8
9
trialkylamine and metal hydride reductions have been
applied more specifically to aryl aldehydes and ketones.
But most of these methods have some disadvantages in re-
lation to their general applicability, selectivity, operation-
al inconvenience or toxicity. Currently the use of
microwave energy to accelerate organic reactions has
been taken to a new dimension and is a matter of increas-
In a typical case, a mixture of benzophenone (1.82g, 10
mmol) and 80% hydrazine hydrate (1g, 20 mmol) in tolu-
ene (15ml) was taken in an Erlenmeyer flask and placed
in a commercial microwave oven operating at 2450 MHz
frequency. After irradiation of the mixture for 20 mins.,
(
monitored by tlc) it was cooled to room temperature, ex-
tracted with chloroform and dried over anhydrous
Na SO . Removal of solvent gave the benzophenone hy-
1
0
ing interest and offers several advantages. We wish to
1
1
2
4
report here the application of microwave energy for the
synthesis of hydrazone 3 and its subsequent conversion to
hydrocarbons 4 via one of the major reaction of organic
chemistry i.e Wolff-Kishner reaction under microwave ir-
radiations. The reaction proceeds efficiently in excellent
yields at ambient pressure within minutes and in the ab-
sence of solvent.
drazone in 95% yield. Similarly other hydrazones 3b-d
were prepared. These hydrazones were obtained much
faster compared to literature methods in excellent yields.
For the Wolff-Kishner reductions, a mixture of hydrazone
3
a (5 mmol) and KOH (2g) were taken in an Erlenmeyer
flask and placed in a microwave oven. After irradiation
for 30 mins. and usual work-up gave the corresponding
diphenylmethane in 95% yield. Similarly other hydra-
zones were reduced to hydrocarbons and their character-
istics are recorded in the table. The conditions do not
affect methoxyl, methyl, chloro and carbomethoxy groups
1
2
which have been reported to be problemsome on occa-
sion. Other conditions for the reaction were tried, but
were less effective. Thus with dimethylsulfoxide as
solvent under microwave
irradiations diphenyl-
methane was formed in 30% yield after 50 mins of
irradiations. When benzophenone hydrazone was pre-
pared in situ from benzophenone and hydrazine, and re-
Synlett 1999, No. 10, 1573–1574 ISSN 0936-5214 © Thieme Stuttgart · New York

1
574
S. Gadhwal et al.
LETTER
acted under microwave a mixture of benzophenone azine
and diphenylmethane was isolated. All the compounds
obtained were characterised by infrared and H NMR
spectroscopy and finally by comparison with an authentic
sample.
(8) Benkeser, R.A.; Acc. Chem. Res., 1971, 4, 94; Benkeser, R.A.;
Smith, W.E.; J. Am. Chem. Soc., 1969, 91, 1556.
(9) Conover, L.M.; Tarbell, D.S.; J. Am. Chem. Soc., 1950, 92,
1
3
586; Brown, B.R.; White, A.M.S.; J. Chem. Soc., 1957,3755;
Nystrom, R.F.; Berger, C.R.A.; J. Am. Chem. Soc., 1958, 80,
896.
2
(
(
10) Majdoub, M.; Loupy, A.; Peter, A.; Roudesli, M.S.;
In conclusion, this new method of Wolff-Kishner reduc-
tion under microwave irradiation offers significant im-
provements over the existing procedures and thus help
facile entry into a variety of hydrocarbons of potentially
high synthetic utility. Also this simple and easily repro-
Tetrahedron, 1996, 52, 617; Caddick, S.; Tetrahedron, 1995,
51, 10453.
1
3
11) For a recent report on microwave induced Cannizzaro reaction
see: Sharifi, A.; Mojtahedi, M.M.; Saidi, M.R.; Tetrahedron
Lett., 1999, 40, 1179.
ducible technique affords various hydrocarbons in shorter (12) Combic, R.C.; Hayward, R.C.; Missen, A.W.; Aust. J. Chem.,
reaction time with excellent yields without involvement
of toxic and expensive material and without the formation
of any undesirable side products.
1974, 27, 2413; Santesson, J.; Act. Chem. Scand., 1970, 24,
373.
3
(
13) For methods using Ni-Al alloy require long reflux time and
often give mixture of products see: Papa, D.; Schwenk, E.;
Whitman, B.; J. Org. Chem., 1942, 7, 587; Kleiderer, E.C.;
Kornfield, E.C.; J. Org. Chem., 1948, 13, 455; Cook, D.L.; J.
Org. Chem., 1962, 27, 3873; Tyman, J.H.P.; J. Appl. Chem.,
1970, 20A, 179.
References and Notes
(
(
(
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2890.; Org. Synthesis., 1970, Vol. 50, p-102.
(b) Barrett, J. W.; Linstead, R. P.; J. Chem. Soc., 1935, 436;
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Article Identifier:
1437-2096,E;1999,0,10,1573,1574,ftx,en;L11699ST.pdf
(
7) Cook, P.L.; J. Org. Chem., 1962, 27, 3873; Papa, D.;
Schwenk, E.; Whitman, B.; J. Org. Chem., 1942, 7, 587.
Synlett 1999, No. 10, 1573–1574 ISSN 0936-5214 © Thieme Stuttgart · New York