Greener media

Avalos M, Babiano R, Cintas P et al (2006) Greener media in chemical synthesis and processing. Angew Chem Int Ed 45(24) 3904-3908... 

While a number of supercritical fluids (SC) are known and have been studied as reaction media, probably only SC-CO2 and water are of practical use in the synthesis of pharmaceutical intermediates. The application of SC-CO2 as a greener eluent for chromatography has been discussed in Chapter 12. This medium is also used as an extraction solvent and in API isolation, although its use at any scale... 

Solvents are, perhaps, the greatest concern when developing greener processes. Solvents are necessary since they provide a medium for... 

This operationally simple PSSA-catalyzed aza-Michael protocol proceeded efficiently in aqueous medium without use of organic solvent. Also, the use of polymer-supported, relatively low toxicity, and inexpensive PSSA as a catalyst renders this method greener and eco-friendly. 

In conclusion, domino reactions under microwave irradiation, high pressure, and in water have become powerful and efHcient tools in organic synthesis with attractive, greener characteristics. The combination of the advantages of the domino concept, microwave irradiation, and use of an aqueous medium allows... 

Polshettiwar V, Varma RS (2007) Biginelli reaction in aqueous medium a greener and sustainable approach to substituted 3,4-dihydropyrimidin-2(17/)-ones. Tetrahedron Lett... 

In many examples of TH reactions performed under heterogeneous conditions, which are mentioned below, water was used as solvent, not only contributing to greener processes but, more importantly, the ketone reduction rates and selectivities can also be improved in water medium. 

Polshettiwar, V., and Varma, R. S. (2008). Greener and rapid access to bio-active heterocycles Room temperature synthesis of pyrazoles and diazepines in aqueous medium. Tetrahedron Lett., 49,397-400. 

RCM reaction had major impact in the field of medium ring-sized heterocycle synthesis as demonstrated by numerous publications in this area [60,61]. However, the reaction also suffers from many of the drawbacks of common organic transformations, and thus has drawn attention for greener applications. Grubbs and coworkers developed a highly active water-soluble olefin metathesis catalyst of the type 99 (Figure 5) for this purpose [62]. 

In recent years, the use of heterogeneous catalysis has received useful applications in various organic transformations due to several advantages over conventional homogeneous catalysis. Several solid acid catalysts [108] such as Amberlyst [109], solid-supported fluoroboric acid [110], polyaniline sulfate [111], polystyrene-supported sulfonic acids [112], sulfated zirconia [113], and silica [114] in conjunction with other greener techniques such as MW, US, environmentally benign solvents, solvent-free conditions, and so on have been screened, evaluated, and compared with respect to yields, reaction time, reaction temperature, ease of purification, reusability, toxicity, and other hazards for sustained applications. Many of these efforts have found applications in the medium ring-sized heterocycles such as oxepines, azepines, diazepines, oxazepines, thiazepines, and so on. 

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