(Green) Photocatalytic Synthesis Employing Nitroaromatic Compounds
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(Green) Photocatalytic Synthesis Employing Nitroaromatic Compounds. Ralf Dillert, Amer Hakki, and Detlef W. Bahnemann Institut für Technische Chemie, Gottfried Wilhelm Leibniz Universität, Callinstrasse 3, 30167 Hannover, Germany. ABSTRACT The combination of a solid photocatalyst (TiO2) and a co-catalyst (p-toluenesulfonic acid) has been successfully applied for the light-induced conversion of nitroarenes in O2-free ethanolic suspensions yielding substituted quinolines and tetrahydroquinolines, while in the presence of TiO2 loaded with a noble metal (Pt, Pd) N-alkylarylamines and N,N-dialkylarylamines were formed. Depending on the compounds that have been detected by GC–MS the reaction mechanism is discussed comprising the formation of anilines and ethanal by a photocatalytic reaction step and their subsequent thermal reactions to quinolines, tetrahydroquinolines, and N-alkylated anilines via a Schiff base as an intermediate product. INTRODUCTION Organic photosynthesis in the presence of semiconducting metal oxides as heterogeneous photocatalysts has become an important area of research. Experimental results have been summarized in several review articles [1-5]. The light-induced charge separation occurring in TiO2 particles under irradiation with UV(A) light creates both, a reduction center and an oxidation center at the particles´ surface. These active centers can induce the reduction as well as the oxidation of adsorbed organic species by interfacial electron transfer (Scheme 1). In principle, this unique feature allows multistep reactions at the surface of a single photocatalyst particle: an intermediate generated at one type of the active centers can react with the intermediate generated at the other type of reaction center allowing a multi-step synthesis in a one-pot reaction.
Scheme 1. Basic mechanism of photocatalysis.
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It has been shown by several authors that light-induced reactions of nitroarenes in the presence of alcohols and TiO2 as the photocatalyst yield a variety of different products depending on the reaction conditions, i.e., the presence of a co-catalyst and / or metal deposits enabling the electron shuttle at the photocatalyst´s surface. In an early contribution Mahdavi et al. have reported that an amino compound and an aldehyde are formed in high yields by UV(A) irradiation of a suspension of TiO2 in a primary alcohol containing a nitro compound. [6]. Ferry and Glaze have investigated the mechanism of the photocatalytic reduction of some nitroarenes in suspensions of TiO2 in methanol and 2-propanol in the absence of molecular oxygen in more detail. They have shown that conduction band electrons (free or trapped as Ti(III)) are the principal reductive species driving the reactions of nitro compounds to the corresponding amino compounds. Secondary reductive radicals generated from alcohol oxidation by valence band holes were found to play no role in the formation of the amino compounds [7]. Valenzuela and co-workers have investigated the photocatalytic reactions of nitrobenzene in deaerate
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