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General Papers

ARKIVOC 2006 (x) 102-106

A very simple and chemoselective air oxidation of benzoins to benzils using alumina
Konstantinos Skobridis,*a Vassiliki Theodorou,a and Edwin Weber*b
a b

Department of Chemistry, University of Ioannina, GR-451 10 Ioannina, Greece Institut für Organische Chemie, TU Bergakademie Freiberg, Leipziger Straße 29, D-09596 Freiberg/Sachs., GermanyE-mail: kskobrid@cc.uoi.gr

Abstract An efficient and facile air oxidation of benzoins in the presence of the heterogeneous catalyst aluminum oxide, avoiding the use of noxious metal compounds as oxidizing agents has been developed. It thus constitutes a very simple, environmentally friendly, clean, economical and selective method for the aerobic preparation of benzils from the corresponding benzoins.Keywords: Benzoins, benzils, air oxidation, alumina, chemoselectivity

Introduction
Benzils or α-dicarbonyl compounds, in general, are important materials due to their practical applications, i.e. as starting materials for the synthesis of heterocycles1 and as photosensitive agents.2 The oxidation of benzoins has been accomplished by several oxidative reagents such as nitric acid, thalliumnitrate, copper sulfate, copper acetate, ammonium nitrate, bismuth oxide and bismuth nitrate, iron (II) thiolate and oxone, trichlorooxyvanadium and vanadium oxide, titanium (IV) chloride and triethylamine 3 or by molecular oxygen using microwave irradiation on zeolite A or clayfen,4 or by nickel hydrotalcite activation.5 However, while all these methods can generate the desired products efficiently,many of the above oxidants are toxic and hazardous metal compounds, generating waste disposal problems. An indirect electrochemical method was, also, recently used for the oxidative transformation of benzoins to benzils.6 The cleanest and the most ideal reagent for oxidation reactions would be air. Thus, oxidative transformations of organic compounds effected by air oxygen are attractive from aneconomic and environmental point of view, and have received much attention in recent years.7 On the other hand, catalytic processes induced by heterogeneous catalysts8 have a distinct advantage in view of ease of handling, simple work-up and regenerability.

ISSN 1424-6376

Page 102

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ARKAT

General Papers

ARKIVOC 2006 (x) 102-106

Herein, we describe such an extremely simple andefficient transformation process which is the chemoselective aerobic oxidation of benzoins to benzils, supported by commercially available aluminum oxide as a solid catalyst, without the use of any metal salts as oxidizing agents or other reagents (Scheme 1). The oxidation is performed heterogeneously within the micropores of the solid alumina. The role of alumina as a heterogeneous catalyst onseveral organic reactions, such as oxidation, reduction and displacement reactions, is known from the previous work of Posner.9 These reactions are heterogeneous, taking place at the alumina surfaces.
OH Ar CH C O Ar alumina, air solvent, r.t. Ar O C C Ar ArCHO

O main product

Aryl = phenyl, 4-anisyl, 2-furanyl, 4-tolyl, 4-chlorophenyl

Scheme 1. Oxidation reaction of aryloins with airoxygen in the presence of alumina.

Results and Discussion
In order to study the optimization of the reaction conditions and to evaluate the parameters that may affect the reaction, we carried out the oxidation experiments under an air atmosphere using acidic, neutral and basic alumina, i.e. pH 4.5, 7.0 and 9.5, respectively. The reaction rate was found to be strongly influenced by the pH of thealumina, and the best result was obtained with basic aluminum oxide (pH 9.5). The higher reaction rates with basic aluminum oxide may be attributed to the easier abstraction of an α-proton by a basic site of the catalyst,10 leading to a stabilized enediolate anion which reacts further (Figure 1). The reaction rate was also influenced significantly by the amount of alumina, being retarded with the...
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