Zinc-promoted reduction of 1,2-diketones: A simple, efficient, and mild approach to α-hydroxy ketones

Article abstract of DOI:10.1007/s706-002-8250-y

1,2-Diketones were readily converted to α-hydroxy ketones in high to excellent yields using zinc in saturated ammonium chloride solution.

Full text of DOI:10.1007/s706-002-8250-y

Monatshefte f uÈ r Chemie 133, 195±197 ꢀ2002)
Zinc-Promoted Reduction of 1,2-Diketones:
A Simple, Ef®cient, and Mild Approach
to a-Hydroxy Ketones
Ã
and Farnoosh Faridbod
Rahim Hekmatshoar , Majid M. Heravi, Yahya S. Beheshtiha,
Department of Chemistry, School of Science, Azzahra University, Vanak, Tehran, Iran
Summary. 1,2-Diketones were readily converted to ꢀ-hydroxy ketones in high to excellent yields
using zincin saturated ammonium ch loride solution.
Keywords. Zinc; 1,2-Diketones; ꢀ-Hydroxy ketones.
Introduction
ꢀ
-Hydroxy Ketones are very useful synthons in organic ch emistry [1]. Although
the oxidation of enolates or their equivalents is a method of choice [2], the reduc-
tion of 1,2-diketones [3] also offers a useful and convenient approach. Problem
arise from overreduction of diketones to diols [4] and sometimes to monoketones
[
5].
Conversions of 1,2-diketones to ꢀ-hydroxy ketones employing Grignard
reagents [6], titaniumꢀIV) chloride [7], active uranium [8], and titaniumꢀIV) iodide
9] have been reported. The electrochemical reduction of benzil to benzoin in DMF
[
has been studied as a function of pH [10]. Each of these methods has its merits and
drawbacks.
In spite of several contributions using zinc as a reducing agent [11], for
example in the Clemmensen reduction [12], literature lacks reports using zinc
under mild and non-acidic conditions. Therefore, research in this ®eld is still in
demand.
Results and Discussion
In this communication we report that zinc in aqueous ammonium chloride can
ef®ciently reduce 1,2-diketones to the corresponding ꢀ-hydroxy ketones. Under
optimum conditions, benzoin was obtained in excellent yield when benzil was
treated with zincpowder in THF-saturated aqueous ammonium chloride. Regard-
ing the choice of solvent, THF proved to be the best. To assess the generality of
à Corresponding author. E-mail: rhekmat@azzahra.ac.ir

1
96
R. Hekmatshoar et al.
Table 1. Reduction of 1,2-diketones to ꢀ-hydroxy ketones using zincin THF-saturated aqueous
ammonium chloride
a,b
c
R
Reaction time
min)
Yield=%
ꢀ
Ph
40
50
60
45
70
45
97
92
94
90
95
92
4
4
4
-MePh
-MeOPh
-BrPh
CH3
ÀCH CH CH CH À
2
2
2
2
a
b
All substrates were purchased from commercial sources; all products were characterized by
c
comparison of their physical and spectroscopic data with those of known compounds; yields refer to
isolated products
the method, various 1,2-diketones were tested ꢀTable 1). The reactions were
in most cases completed within 10±60 minutes, as evidenced by the disappear-
ance of the metal.
In conclusion, the zinc promoted reduction of 1,2-dicarbonyl compounds in
aqueous media is remarkably ef®cient, rapid, selective, and affords high yields
methodology. Compared to other methods [6±9], the present protocol is ex-
perimentally simple and inexpensive.
Experimental
Melting points were measured with an Electrothermal 9100 apparatus and are uncorrected. IR spectra
1
13
were recorded on a Philips Pu 9800 FT-IR instrument. H and C NMR spectra were recorded on a
JEOL EX-90A spectrometer at 90 and 22.6 MHz. All products are known compounds; their physical
and spectroscopic data were compared with those of authentic samples and found to be identical.
General procedure
3
Zn powder ꢀ4 mmol, 0.260 g) and 2 mmol of an appropriate 1,2-diketone were slurried in 5 cm
3
saturated aqueous NH Cl and 5 cm THF. The mixture was stirred at ambient temperature for the
4
indicated time during which the Zn powder disappeared. The solutions were evaporated to dryness
under reduced pressure. The crude product was extracted with diethyl ether. The solvent was
evaporated; solid products were recrystallized from EtOH, liquid products were chromatographed on
silica gel ꢀAcOEt:hexane ˆ 1:4) to afford the ꢀ-hydroxy ketones ꢀTable 1).
References
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Reduction of 1,2-Diketones
197
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[
[
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[
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Received June 7, 2001. Accepted !revised) August 7, 2001