Environmental friendly methods ionic liquids

Initially, sulfuric acid was used to initate the Borsche—Drechsel cyclization. However, Perkin found that acetic acid provided cleaner products. Recently, other reagents—such as cerium ammonium nitrate (CAN) and the more environmentally friendly acidic ionic liquids —have been used to catalyze the ring formation. It should be noted that people have found it difficult to promote the cyclization when there are substituents at the ortho-position of the phenylhydrazine. This difficulty becomes more pronounced when both the hydrazine and the cyclohexanone have a-substitution This difficulty usually leads to low yields compared with other methods of forming the carbazole. 

Nevertheless, alternative or nontraditional methods are interesting fields for new applications, due to the need for more selective synthetic and environmentally friendly methods, including the aspect of reaction media (solvent) and energy transfer into a reaction system. The use of supercritical fluids (SCFs) or ionic liquids (IL) as process solvents is well established. From the view point of green chemistry or sustainable chemisfiy, it is SCFs, especially supercritical carbon dioxide (SCCO2), that are the main focus [2, 3]. It is currently under discussion whether ioiuc liquids as alternative process solvents are beneficial. In general, more fundamental problems, like recovery, reuse, purification and ecology (eco-toxicity), have to be solved. 

Advances toward more ecologically safe processes for the synthesis of compounds containing the pyridazine core were made in 2013.The Yadav group devised an efficient and more environmentally friendly method for the synthesis of polysubstituted pyridazines 30 in ionic liquids, such as l- -butyl-3-methylimidazolium hydroxide ([BMIMJOH), under micro-wave irradiation conditions (13JHC1165).The products were obtained in good yields. A general example for this protocol is shown in Scheme 12. 

Supercritical carbon dioxide (scCOa) can be combined with ionic liquids providing an efficient, simple, and environmentally friendly separation method. For example, methanol is miscible with the ionic liquids and forms one phase. When this phase is saturated with scCOa, the formation of two phases could be observed and the upper phase contains most of the methanol. It should be noted that SCCO2 reduces the viscosity of ionic liquids and could facilitate mass transfer. The scCOa-philic, less polar products are easy to extract from reaction media. 

Most of the methods for the Hosomi-Sakurai allylation using silicon and other Lewis acids require the use of chlorinated organic solvents such as CH2CI2 and often inconveniently low temperatures. A recent study has demonstrated that the (la)-catalyzed allylation with allyltrimethylsilane (10) in ionic liquids proceeds smoothly at room temperature to afford the corresponding homoallyl ethers in good yield [55]. Since ionic liquids can easily be recovered and recycled, they are environmentally friendly alternatives to CH2CI2 (Scheme 9.13). 

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