Selective oxidation of tertiary amines on gold catalysts Cristina Della Pina, Ermelinda Falletta and Michele Rossi* Dipartimento di Chimica Inorganica, Metallorganica e Analitica, CIMAINA and ISTM, University of Milano, Via Venezian, 21 20133 Milano, Italy Gold-based mono-and bi-metallic catalysts have been evaluated in the aerobic oxidation of aqueous solutions of tertiary amines to produce the corresponding N-oxides obtaining 100% yield in the case of triethylamine, pyridine and 3-dimethylaminopropan-1- ol under mild conditions. 1. Introduction In recent years, the application of gold in catalysis has become an important research area owing to the discovery of the surprising activity of gold in form of nano particulates [1], thus opening new horizons for its application. Therefore, several years after the first report on the challenging perspective for ethyne hydrochlori- nation [2] gold has been evaluated in fundamental reactions for organic synthesis such as oxidation and reduction, and the use of gold catalysts for industrial application for fine chemicals has been explored by academic and industrial research groups [3–6]. Particu- lar interest has been devoted by researchers at Milan University, to the selective oxidation of the alcoholic and aldehydic groups with molecular oxygen, discover- ing for the first time the use of gold catalysis for liquid phase oxidation in organic synthesis [7]. According to many different studies, the catalytic activity and selec- tivity of supported gold depend upon the nature of the support and the preparation method [8–10]. Among the various techniques for obtaining supported gold cata- lysts, co-precipitation [11], deposition-precipitation [12], vapour-phase deposition [13], co-sputtering [14], and impregnation of phosphine complexes or clusters [15] have been proposed as methods for preparing active catalysts mainly for CO abatement but also for organic synthesis. In many applications, the deposition of col- loidal gold on activated carbons and oxides has been proposed as a suitable method for preparing gold cata- lysts showing high selectivity and allowing discrimina- tion among chemical groups and geometrical positions, thus favouring high yields of the desired product [16]. For instance, glycols can be converted to monocarb- oxylates [7] and saturated and unsaturated alcohols to unsaturated aldehydes [17], with selectivity approaching to 100%. In the case of aminoalcohols oxidation, the use of gold catalysis gave rather encouraging results because, unlike Pd and Pt catalysts, which were relatively unre- active, conversion to the corresponding aminoacids was observed, although this is somewhat dependent on the nature of substrate [18]. In this paper we describe the application of gold catalysis in the oxidation of tertiary amines using molecular oxygen in aqueous solution under mild conditions producing mainly the corresponding N- oxides. 2. Experimental 2.1. Materials Gaseous oxygen from SIAD was 99.99% pure. All the other reagents were of the highest purity from Fluka and were used without any further purification. The Carbon X40S (S = 1300 m 2 /g, V P = 0.37 cm 3 /g) supplied by Camel chemicals S.p.A., alumina (S = 150 m 2 /g) supplied by Condea, titania P 25 (V P = 0.3 cm 3 /g) supplied by Degussa. Gold sponge of 99.999% purity supplied by Fluka, K 2 PtCl 4 (46.0 % w/w Pt) by Alfa Aesar-Johnson Mat- they GmbH, RhCl 3 (40% w/w Rh) by Engelhard. 2.2. Catalyst preparation The catalysts 1%Au/C, 1%Pt/C, 1%Rh/C, 0.5%Au- 0.5%Pt/C, 0.5%Au-0.5%Rh/C, 1%Au/Al 2 O 3 , 1%Au/ TiO 2 , were prepared by deposition of a preformed gold solution on carbon, Al 2 O 3 , and TiO 2 powders. Thus, aqueous solutions of HAuCl 4 , K 2 PtCl 4 and RhCl 3 (100 mg/l) were prepared, PVA was added as a pro- tecting agent (metal:PVA = 50:1 w/w) and, under stir- ring, a freshly prepared solution of NaBH 4 (0.1 M, NaBH 4 /M = 1 w/w) was added to obtain a dark yellow (Au) and dark brown (Pt and Rh) colloidal dispersion. * To whom correspondence should be addressed. E-mail: michele.rossi@unimi.it Topics in Catalysis Vol. 44, Nos. 1–2, June 2007 (Ó 2007) 325 DOI: 10.1007/s11244-007-0306-8 1022-5528/07/0600-0325/0 Ó 2007 Springer Science+Business Media, LLC