Alumina and silica oxides as catalysts for the oxidation of benzoins to benzils under solvent-free conditions

Article abstract of DOI:10.3390/10111364

Alumina or silica gel are used as catalysts for a solvent-free oxidation of benzoins to the corresponding benzils. These catalysts are easily recovered after completion of the reactions, which are carried out either by heating in a sand bath or using micr

Full text of DOI:10.3390/10111364

Molecules 2005, 10, 1364-1368
molecules
ISSN 1420-3049
http://www.mdpi.org
Alumina and Silica Oxides as Catalysts for the Oxidation of
Benzoins to Benzils under Solvent-free Conditions
N. Noroozi –Pesyan 1,* and A. H. Dabbagh 2
1
Department of Chemistry, Faculty of Science, Urmia University, 57159, Urmia, Iran
2
College of Chemistry, Isfahan University of Technology, 84156, Isfahan, Iran. Tel.: (+98) 441
2780952; Fax: (+98) 441 2776707
* Author to whom correspondence should be addressed; e-mail: noroozi@alumni.iut.ac.ir or
noroozi304@yahoo.com
Received: 5 February 2004 / Accepted: 5 March 2004 / Published: 30 November 2005
Abstract: Alumina or silica gel are used as catalysts for a solvent-free oxidation of
benzoins to the corresponding benzils. These catalysts are easily recovered after
completion of the reactions, which are carried out either by heating in a sand bath or
using microwave irradiation. Comparison of the results obtained with both catalysts
indicates that all the reactants examined were oxidized faster on alumina than on silica
under these conditions.
Keywords: Benzoins; benzils; biologically active compounds
Introduction
α-Diketones are versatile compounds in organic chemistry that can be utilized for the preparation
of a variety of molecules, many of which are show a diversity of interesting biologically activities [1].
Oxidations of α-hydroxy ketones to α-diketones by a wide range of reagents have been reported,
including, among others, the dinitrogen tetroxide complex of iron (III) and copper (II) [2], dimethyl
sulfide ditriflate [3], peracetic acid in the presence of sodium bromide [4], dimethyl sulfoxide
(DMSO)-SbCl5 [5], thallium (III) nitrate (TTN) [6], a cerium (IV) ammonium nitrate (CAN)-charcoal
system [7], nickel acetate [8] and р-toluenesulfonic acid in boiling xylene [9]. Alumina and silica gel
are both important reagents in industry and in the laboratory and numerous reactions have carried out
using these substances as solid supports. In recent years Al2O3 and SiO2 based reagents have been

Molecules 2005, 10
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widely used for dehydration or dehydrogenation of alcohols [10-14] and in many oxidation reactions
carried out under microwave irradiation or under solid state conditions [15-19].
Benzoins are rapidly oxidized to benzils in high yield by the solid copper (II) sulfate-alumina
reagent system under the influence of microwaves [20]. The iodobenzene diacetate-alumina solid
reagent system expeditiously and selectively oxidizes sulfides to sulfoxides and alcohols to carbonyl
compounds in high yield under the influence of microwaves [21]. A simple and selective method for
the oxidation of alcohols to carbonyl compounds using chromium (VI) oxide supported on wet
alumina under solvent-free conditions has also been described, again accelerated by microwave
irradiation [22]. Other reactions that can be carried out using silica gel as a support include the
conversion of epoxides to β-halohydrins with silica gel supported lithium halide [23], the silica gel
supported hetero Diels-Alder reaction of quinolinetriones [24], oxidative glycol cleavage with silica
gel supported sodium metaperiodate [25], synthesis of benzophenones with Lewis acid catalysts using
silica gel [26] and so on. We now report the use of both Al2O3 and SiO2 separately as oxidizing agents
in the absence of an additional supported oxidant.
Results and Discussion
Benzoins are oxidized to benzils in high yield on alumina and silica under solvent-free conditions,
under both microwave irradiation assisted conditions and upon heating in a sand bath, without any
additional supported oxidant on the alumina or silica gel surface (Scheme 1).
Scheme 1
OH
O
Alumina or Silica gel
Ar'
Ar
Ar'
Ar
Microwave or Heat
a
b
Our results are summarized in Tables 1-4. They show that both alumina and silica gel are suitable
oxidants, but that alumina was consistently a better oxidation catalyst than silica gel under the
conditions used.
Table 1. Solid phase oxidation of benzoins on alumina a in a sand bath (140 oC)
Time
Yield
Compound
Substituents
(min)
10
(%) b
100
95
1
2
3
4
5
6
Ar′ = C6H5 , Ar =C6H5-CO
Ar′ = C6H5 , Ar = 4-Me2NC6H4-CO
Ar′ = 4-MeOC6H4 , Ar = 4-MeOC6H4-CO
Ar′ = C6H5 , Ar =4-MeOC6H4-CO
Ar′ = Ar = C6H5
60
180
65
N.R.c
N.R.
100
90
--
Ar′ = C6H5 , Ar = H
--
a) Activated alumina.
b) Yields based on the products isolated by column chromatography.
c) N.R. = no reaction.

Molecules 2005, 10
Table 2. Solid phase oxidation of benzoins on alumina under microwave irradiation.
1366
Time
Yield
Compound
(min)
(%)
100
95
90
70
--
1
2
3
4
5
6
5
5
5
5
N.R.
N.R.
--
Table 3. Solid phase oxidation of benzoins on silica gel a in a sand bath (140 oC).
Time
Yield
Compound
(min)
180
180
150
b
(%)
100
70
1
2
3
4
5
6
90
100
--
N.R.
N.R.
--
a) HF 254(type 60).
b) The reaction time was more than 20 hours.
Table 4. Solid phase oxidation of benzoins on silica gel under microwave irradiation.
Time
Yield
Compound
(min)
5
(%)
95
95
85
70
--
1
2
3
4
5
6
10
15
15
N.R.
N.R.
--
All of benzoins examined were readily oxidized but neither diphenyl carbinol (5a) nor benzyl
alcohol (6a) reacted. It seems that an adjacent carbonyl group is necessary for the oxidation of the
hydroxyl groups to take place. Naturally, substrates such as 1a- 6a can be partially oxidized in air after
extended exposure to oxygen, but if O2 played an important role in this oxidation we would expect
compounds 5a and 6a to have been oxidized too. We attribute the observation that alumina is a better
oxidation catalyst than silica to the fact that it is a stronger Lewis acid than silica, although so far we
have not proposed a mechanism for this process and at this time we are simply reporting our
experimental observations. The reaction times also appear to depend on the temperature, as they are
seen to decrease as the temperature increases.

Molecules 2005, 10
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Conclusions
We report that benzoins are oxidized to benzils in the solid state in good yields under microwave
irradiation or in a heated sand bath, using alumina or silica gel as catalysts without added oxidants.
The main advantages of these reactions are: (1) easy recovery of the alumina or the silica gel upon
completion of the reactions; (2) yields are high and the products contain minimum by-products; (3) no
environmental pollution; (4) low cost as no reaction solvents are required.
Acknowledgements
We thank the Urmia University Research Council for the partial support of this work.
Experimental
General
Melting points were determined on a Gallenkamp apparatus and are uncorrected. All IR spectra
1
were recorded on a Shimadzu IR-470 spectrometer and H-NMR spectra were recorded on a Varian
EM390 instrument. Chemical shifts (δ-scale) are quoted in parts per million and following
abbreviations are used: s = singlet; d = doublet; m = multiplet. All the benzoin starting materials were
synthesized in our laboratory and were characterized by comparison of their physical data (melting
points) with those of authentic samples [27]. All the reported yields refer to isolated products after
1
purification. All products were characterized by comparison of their spectral (IR, H-NMR and TLC)
and physical data (melting points) with those of authentic samples [2, 28]. The apparatus used for the
oxidations was either a Samsung domestic microwave oven (2450MHz, 900w) or a sand-bath mortar
oven.
Typical procedure for the oxidation of benzoins to benzils on alumina or silica gel.
Benzoins (0.05 g) was added to Al2O3 or SiO2 (0.5 g, 1:10 w/w), mixed and homogenized in a
mortar. The reactions were then allowed to proceed for the times given in Tables 1-4, respectively,
o
either in a sand-bath at 140 C or under microwave irradiation. Reaction times were monitored and
optimized by TLC (checking every 5 minutes under microwave irradiation and every 10 minutes under
sand-bath conditions). Upon completion of the reactions the mixtures were washed with a few mL of
CH2Cl2, filtered, evaporated and the products purified by silica gel column chromatography. The
analytical data of two products, 1b and 3b, are given as examples: Benzil (1b): light green needle-like
crystals, mp 95 °C, (lit. [2,28] 95 °C); IR (KBr): 3100 (C–H, Ar), 1680 (C=O), 1595 (C=C); 1H-NMR
δH (90 MHz, CCl4), 7.3 – 7.9 (m, 3H, m and p-PhH), 8.0 – 8.3 (m, 2H, o-PhH); 4,4'-Dimethoxybenzil
(3b): green solid, mp 133 °C, (lit. [2, 28] 132-134 °C); IR (KBr): 3050(C–H, ar.), 2900(s), 2800 (C–H,
aliph.), 1670 (C=O), 1595 (C=C), 1160 (C–O); 1H-NMR δH (90 MHz, CDCl3, CCl4); 4.0 (s, 3H, CH3),
7.2 (d, J = 9Hz, 2H, m-PhH), 8.2 (d, J = 9Hz, 2H, o-PhH).

Molecules 2005, 10
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