Dry media reactions

Most of the parallel reactions described in Schemes 4.23 and 4.24 were performed as dry-media reactions, in the absence of any solvent. In many cases, the starting materials and/or reagents were supported on an inorganic solid support, such as silica gel, alumina, or clay, that absorbs microwave energy or acts as a catalyst for the reaction (see also Section 4.1). In this context, an interesting method for the optimization of silica-supported reactions has been described [83], The reagents were co-spotted neat or in solution onto a thin-layer chromatographic (TLC) plate. 

Microwave-mediated reactions can also be easily carried out without solvents (see Section 4.1). The requirements for these dry media reactions are different to those for reactions in solution. As no solvent is involved, the pressure built-up is rather low, and in most instances such reactions are performed under open-vessel conditions. On the other hand, these mixtures can easily be locally overheated, even though the overall bulk temperature may be comparatively low (macroscopic hotspot formation). Stirring and accurate temperature measurement can prove rather difficult within such a matrix, impeding the investigation of certain reaction conditions. Thus, degradation or decomposition of reagents can be a severe problem for these kind of reactions. 

Scheme 12.25 Rapid dry-media reactions under microwave conditions.

Chemat and his coworkers [92] have proposed an innovative MW-UV combined reactor (Fig. 14.7) based on the construction of a commercially available MW reactor, the Synthewave 402 (Prolabo) [9[. It is a monomode microwave oven cavity operating at 2.45 GHz designed for both solvent and dry media reactions. A sample in the quartz reaction vessel could be magnetically stirred and its temperature was monitored by means of an IR pyrometer. The reaction systems were irradiated from an external source of UV radiation (a 240-W medium-pressure mercury lamp). Similar photochemical applications in a Synthewave reactor using either an external or internal UV source have been reported by Louerat and Loupy [93],... 

While studying the chemistry of 4,5-dichloro-l,2,3-dithiazolium chloride (Appel s salt) and its derivatives, Besson reported the synthesis of various benzothiazoles from N-arylimino-l,2,3-dithiazoles, which could be synthesised from commercially available aromatic amines28. In this work, the authors explored a variety of strategies to construct the benzothiazole ring and demonstrated that in all cases the focused microwave methodologies were more productive and under well-defined conditions provided convenient methods for scale-up (Scheme 3.17)28. Comparisons were also made between reactions performed under solvent-free conditions and in the presence of solvent29. It is noteworthy that there is no general rule and some reactions performed in the presence of solvent may sometimes be more convenient than the same dry media reaction. 

It is not always acknowledged that solvents are used for dry media reactions, both to load the reactants onto the support and to elute the products after reaction. If the supports are polar materials such as alumina or silica gel, which are commonly used in liquid chromatography, substantial quantities of solvent may be required to remove the organics. For clean processing, recycling of the solvent and the support would be essential. The latter does not appear to have been demonstrated and may prove difficult... 

Also, the susceptibility of a material to microwave energy can depend on variables including sample size, shape and dielectric properties as well as the location of the sample within the cavity [13]. Until recently, a lack of facilities for mixing reactions and for measuring temperature had affected the reproducibility of dry media reactions between microwave systems. That circumstance should be largely overcome through commercial reactors that enable sample mixing and temperature measurement. 

Fig. 18 Continuous microwave dry-media reactor (CMDR) for kg-scale dry-media reactions. Reproduced with permission from [71]...

Kidwai, M. Dry Media Reactions, Pure Appl. Chem. 2001, 73, 147-151. 

By mid-1995, only 220 papers had been published on microwave-assisted organic chemistry [15]. Until then the vast majority of the papers had covered dry media reactions and for approximately the next five years this trend continued. By the turn of the 21st Century, however, the applicability of dedicated microwave reactors and associated chemistry had stimulated other researchers and encouraged commercial manufacturers to construct systems for laboratory and pilot-scale studies [13, 14]. 

Although many interesting transformations with dry-media reactions, including cycloadditions, have been published in the literature [13c, 31], technical difficulties relating to nonuniform heating, mixing, and the precise determination of the reaction temperature remain unsolved, in particular when scale-up issues need to be addressed. In addition, phase-transfer catalysis (PTC) solvent-free conditions have rarely been applied to cycloaddition reactions [4c]. 

Kidwai M. 2001. Dry media reactions. Pure Appl Chem. 73 147-151. 

Examples of applied green chemistry are supercritical water oxidation, on water reactions, and dry media reactions. 

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