Matthias Nüchter

Microwave assisted synthesis – a critical technology overview

Since 1986, when Gedye and Giguere published their first articles in Tetrahedron Letters on microwave assisted syntheses in household microwave ovens, there has been a steadily growing interest in this research field. Since the use of microwaves comprises more than the simple application of a goal-oriented, innovative tool, it is crucial to be aware of the fundamentals of chemistry in the microwave field before investigating challenging reaction mechanisms. Therefore, the following overview focuses mostly on reaction engineering in the microwave field. Parallel reactions and scale-up are also discussed. The last section of this review is dedicated to the development of experimentally sound protocols on microwave-assisted syntheses and separations, which are illustrated through experiments.

How hazardous are ionic liquids? Structure–activity relationships and biological testing as important elements for sustainability evaluationThis work was presented at the Green Solvents for Catalysis Meeting held in Bruchsal, Germany, 13–16th October 2002.

For ionic liquids only few toxicological and/or ecotoxicological data are available until now. A strategy is presented which aims at an environmental risk assessment of chemicals, using a combination of structure–activity relationships (SAR), toxicological and ecotoxicological tests and modelling. The parts “test-kit-concept” and “multidimensional risk analysis” are described in detail by means of selected imidazolium ionic liquids. The iterative process of this strategy offers a tool for sustainable product design.

Ionic liquids and their heating behaviour during microwave irradiation – a state of the art report and challenge to assessment

Ionic liquids of the 1,3-dialkylimidazolium-type reveal great potential for the innovative application of microwaves for synthesis as well as for separations. Ionic liquids efficiently absorb microwave energy and thus are employed as solvents and co-solvents. In this context, microwaves assist in carrying out syntheses or unit operations.

Organic processes initiated by non‐classical energy sources

Non-classical energy sources such as microwave energy, ultrasound and mechanoenergy and their combination with UV–VIS radiation are new tools in synthetic chemistry and chemical processing. Here we describe the application of microwave treatment for selected organic reactions such as (i) enzymatic transesterification of optically active alcohols, (ii) mercury-sensitized gas-phase photolysis of hydrocarbons in the microwave field, (iii) environmentally benign oxidations of olefins and (iv) the application of mechanoenergy separately and in combination with microwave irradiation for special oxidation reactions. Copyright

Oxidation of styrene and cyclohexene under microwave conditions

The coupling of microwave energy with solvent-free phase-transfer catalysis was studied for the oxidation of cyclohexene and styrene. The former Noyori oxidation system (Science, 1998, 281, 1646) was restudied under the aforementioned conditions and optimised. In addition, parallel oxidations were successfully employed by using a special microwave set-up.

MICROWAVE-ASSISTED SYNTHESIS OF ALKYL GLYCOSIDES

The reaction of sugars with alcohols and catalytic amounts of an acid in a high frequency field (2.45 GHz) leads to high yields of the corresponding alkyl glycosides. The ratio of the α/β-anomers is influenced by the reaction conditions and reaction controlling.

Tools for microwave-assisted parallel syntheses and combinatorial chemistry.

A protocol for transferring conventional reactions to the microwave field explores the effectiveness of parallel syntheses and combination approaches. Applicability to the reaction systems combiCHEM, multiPREP, MMR8 and HPR is defined. Model reactions have been performed in laboratory equipment.Emphasis is placed on similar reaction conditions in the individual vessels and the repeatability of results. Outcomes include esterifications and multi-center reactions exhibiting a great potential for combinatorial chemistry.

Mechanochemische Oxidation organischer Modellverbindungen mit Kaliumpermanganat

Mechanochemical Oxidation of Organic Model Compounds by Means of Potassium Permanganate Systematical investigation of solvent-free oxidations of organic model compounds with potassium permanganate on inorganic carriers leads to conclusion, that mechanochemical stimulation can induce oxidations. In case of the benzyl-type arenes these conversions are selective at good yields. Olefines react to carbonic acids. The presence of water enhances the yield of almost all educts.

From major to minor and back—a decisive assessment of C60H36 with respect to the Birch reduction of C60

Hydrogenated fullerenes derived from the Birch reduction and from hydrogen transfer reduction of [60]fullerene have been examined by matrix-assisted laser desorption ionization; applying 9-nitroanthracene as the matrix, it has been possible to desorb and ionise the samples with such a low degree of fragmentation as to enable the establishment of the amount of hydrogen bound to the fullerene; in contrast to the currently assumed polyhydrofullerene distribution featuring C60H36 only as a minor component, evidence has been obtained which re-establishes C60H36 as the major product of the Birch reduction.

Unimolecular ion dissociation and laser-induced coalescence of hydrogenated fullerenes

The ion-fragmentation behaviour of hydrogenated fullerenes has been investigated by tandem mass spectrometry applying sector instrumentation. In addition to the expected loss of hydrogen, unimolecular decay reactions are observed featuring cage rupture by the loss of hydrocarbon moieties. The findings are discussed in comparison with the behaviour of other fullerene derivatives. The gas-phase coalescence reactivity of C60H36 has been studied utilising laser desorption/ionization time-of-flight mass spectrometry. In contrast to pure fullerenes and certain organic fullerene derivatives, hydrogenated fullerenes do not readily undergo laser-induced fusion reactions.