Solvent-free route for microwave-assisted organic synthesis

Abstract

Microwaves offer a number of advantages over conventional healing such as non-contact heating, energy transfer instead of heat transfer and material selectivity. One particular advantage they can offer is efficiency in solventless reactions thus adding another green chemistry benefit. Ullmann type C-N couplings between N-heterocycles such as imidazole and pyrazole and aryl bromides with additive L-lysine or L-glutamine have been conducted in solvent free condition by microwave irradiation. Interestingly, this microwave assisted cross coupling can be promoted by different L-amino acids to different extents under solvent free conditions. The functionalized N-arylimidazoles thus obtained were further used in the synthesis of the corresponding task-specific ionic liquids (TSILs) by quaternization with excess alkyl halide under solvent free condition. The reactions were also conducted by microwave heating and they proceeded cleanly with high yields. The conversion of the functionality of TSIL l-butyl-3-(3-acetophenyl)imidazlium bromide was also studied. Besides, we have demonstrated the application of our solvent free microwave-assisted Ullmann coupling system toward complex molecules. The C-N coupling system between imidazole and bromo-flavone was successfully developed and the same reaction between imidazole and a more complex flavone dimer was also investigated.