Microwave-assisted organic synthesis versus conventional heating. A comparative study for polyols condensation with ketones at heterogeneous catalysis SiO2-SO3H under free solvent

Ketals are well-known compounds with interesting industrial and commercial applications in a number of fields, especially as surfactants and emulsifiers.1i In organic synthesis, ketals are useful intermediates that protect carbonyl groups of the more reactive ketones or hydroxyl groups of diols, when performing organic transformations.2ii This work describes the synthesis of different ketals by polyols and ketones condensation, under conventional heating and microwave radiation, both at heterogeneous catalysis SiO2-SO3H. All of reactions were performed solvent-free conditions. Preparation of ketals was used the corresponding polyols (ethylene glycol, neopentylglycol, trimethylolpropane and glycerol) with ketones (propanone, cyclohexanone, acetophenone, 1-p-tolylethanone and benzophenone). The reactions were performed under microwave heating results in ketals compounds were obtained in higher yields and shorter reaction times (2-8 min) versus conventional heating (2-12h). An example involves the preparation of the cyclohexanone ketal in 90% yield by conventional heating at reflux in for 2 hours, the same reaction was performed under low microwave irradiation power (360W), the temperature of the slurry did not exceed 73 oC, and a 99.9% yield was obtained in only two minutes. However, these experimental conditions yielded only about 50% of the acetophenone ketal (microwave radiation), against 13% yield (12 hours reflux) at conventional heating. Interestingly, aromatics ketones they self did not give goods yields and many starting materials could be recovered.3 The microwave-assisted heterogeneous catalysis reaction was able to reduce ketal chemical reaction times from hours to minutes; increased yields also to decrease side reactions and improve reproducibility.