Solvents microwave-assisted chemistry

Camptothecin was irradiated under solvent-free conditions for 7 min at the full power of the microwave oven (Scheme 28). The product, Mappicine ketone, was isolated in 96% yield without a trace of undesired side products, which clearly exhibits the potential of microwave-assisted chemistry. In comparison, when the reaction was run at rt in THF and in the presence of BF3 x Et20, Mappicine ketone was isolated in a mere 65% yield. 

The description of the association of heterocychc chemistry and microwave irradiation has also shown that performing microwave-assisted reactions should be considered with special attention. A few of these considerations can be applied generally for conducting microwave-assisted reactions and include the following (a) the ratio between the quantity of the material and the support (e.g., graphite) or the solvent is very important (b) for solid starting materials, the use of solid supports can offer operational, economical and environmental benefits over conventional methods. However, association of liquid/solid reactants on solid supports may lead to uncontrolled reactions which may result in worse results than the comparative conventional thermal reactions. In these cases, simple fusion of the products or addition of an appropriate solvent may lead to more convenient mixtures or solutions for microwave-assisted reactions. 

Principles and Characteristics Pare et al. [475] have patented another approach to extraction, the Microwave-Assisted Process (MAP ). In MAP the microwaves (2.45 GHz, 500 W) directly heat the material to be extracted, which is immersed in a microwave transparent solvent (such as hexane, benzene or iso-octane). MAP offers a radical change from conventional sample preparation work in the analytical laboratory. The technology was first introduced for liquid-phase extraction but has been extended to gas-phase extraction (headspace analysis). MAP constitutes a relatively new series of technologies that relate to novel methods of enhancing chemistry using microwave energy [476]. 

Recent advances in the use of Lawesson s reagent include its application in microwave assisted solvent-free syntheses, solid-phase synthesis and combinatorial chemistry.165 Despite the ubiquity of Lawesson s reagent for organic thionation reactions, there are still some classes of compounds that it does not... 

The 1,2,4-substituted triazine core is a versatile scaffold to access a wide range of condensed heterocyclic ring systems via intramolecular Diels-Alder reactions with a vast array of dienophiles. The triazine ring system is also a key component of commercial dyes, herbicides, insecticides and also recently appeared in medicinal chemistry. One way to synthesise triazines is to use a three-component reaction that has been described in the literature several times, both under traditional thermal heating and under solvent-free microwave-assisted conditions42. However, the previously described methods focussed only on simple aliphatic phenyl and ester substituents. 

Amajor difference among the respective methods for microwave-assisted organic chemistry is the presence or absence of solvents. Solvent-free conditions have been pursued... 

Microwave-assisted organic chemistry may be also coupled to inorganic-supported solvent-free conditions, thus allowing the simplification of work-up procedures (in many cases the pure expected products can be obtained directly by... 

Roberts and Strauss, 2005). As was described earlier, an added advantage to microwave chemistry is that often no solvent is required. In recent years, many commercial reactors have come on the market and some are amenable for scaling up reactions to the 10 kg scale. These new instruments allow direct control of reaction conditions, including temperature, pressure, stirring rate and microwave power, and therefore, more reproducible results can be obtained. For most successful microwave-assisted reactions, a polar solvent that is able to absorb the energy and efficiently convert it to heat is required, however, even solvents such as dioxane that are more or less microwave transparent can be used if a substrate, coreagent or catalyst absorbs microwaves well. In fact, ionic liquids have been exploited in this field as polar additives for low-absorbing reaction mixtures. 

---