Microwave irradiation in solvent-free conditions:: an eco-friendly methodology to prepare indazoles, pyrazolopyridines and bipyrazoles by cycloaddition reactions

A study on the behaviour of vinylpyrazoles, pyrazolyl imines and pyrazolyl hydrazones as dienes or dipoles is reported. Cycloaddition of the aforementioned compounds with electron-poor reagents under microwave irradiation in solvent-free conditions permits the preparation of indazoles, pyrazolopyridines and bipyrazoles in moderate to good yields. This represents an eco-friendly, green approach to these valuable pyrazole derivatives that avoids the harsh or highly contaminant conditions involving classical heating and offers a reduction or even elimination of solvent use and recovery, simplification of the work up procedures, facility of scale up and savings in energy consumption, in addition with higher yields.