Origin Of The Acceleration Of Organic Reactions Under Microwave Irradiation

Since the beginning of the microwave chemistry, the researchers have found under microwave irradiation conditions, some organic reactions can be accelerated. In some cases, this powerful technique can produce higher yields, different selectivity (chemo-, region- and stereo-) and even make some reactions which cannot react under conventional heating happen. Such outstanding performances of microwave irradiation have sparked considerable speculations and discussions about the microwave effects. Though microwave-assisted organic synthesis has got considerable development, the origin of the acceleration of organic reactions under microwave irradiation is still not clear.Traditionally, the method to study the microwave effects is to compare the outcomes of the conventional heating reactions and the microwave-assisted reactions under identical experimental conditions, including the same temperature profiles. However, one of the main problems encountered when comparing reactions performed under conventional heating and with microwaves is to reproduce the reactions under the same conditions because microwaves couple directly with the molecules that are present in the reaction mixture, leading to a rapid rise in temperature. In this thesis, we use the 'molecular probe' in an effort to identify the microwave effects.(1)First, we use a two-substrate aromatic Claisen rearrangement of allyl 4-nitrophenyl ether (ANE) and allyl 4-methyl phenyl ether (AME) as a tool for probing the selective heating effects of microwave irradiation on intramolecular reactions. Aromatic Claisen rearrangements are suitable as model reactions because they are typical intramolecular reaction, the reactants can react directly without interchanging of energy. According to the mechanism of microwave dielectric heating, materials with large permanent dipole moment could strongly absorb microwave energy and, consequently, for efficient heating. With a significantly large dipole moment, ANE could effectively convert MW energy into heat, and the accumulated heat will create a high thermal energy "domain", whereas the weak-polar AME could slightly absorb the MW energy and only be heated by the surroundings through the convective heating. In the microwave transparent solvent tetradecane, the MW energy couple with the polar solute (ANE), creating high thermal energy "domains". The temperature inside the domains is higher than AME; while in the polar solvent NMP, temperature gradient is decreased. Microwave absorbing solvent NMP increases the overall microwave absorbing ability of the bulk and simultaneously heats the molecules of AME and ANE, resulting in the decrease of the temperature gradient.(2) Using the Hammett linear relationship of a'one-pot'saponification mixed with a series of substituted butyl benzoates as'temperature probe'to identify the selective heating effects of microwave irradiation on intermolecular reactions. If there is such phenomena in this saponification, the Hammett linear relationship will be distorted. In polar solvent butanol, the Hammett linear relationship doesn't change. Large amount of solvent absorb the microwave energy and shield the differences of the microwave absorbing ability between the substituted butyl benzoates. In the medium polar solvent THF, the strong polar reactant butyl 4-nitrobenzoate can play as'molecular radiators' which means that its instance temperature is higher than the surrounding. In this case, the Hammett linear relationship remains unchanged under microwave irradiation in both constant temperature mode and constant power mode.(3) Microwaves couple directly with the molecules that are present in the reaction mixture, leading to a rapid rise in temperature. For a specific reaction system, the level of this internal heating is depended on the microwave power. We selected acid-catalyzed esterification of benzoic acid with butanol as model reaction to study the influence of microwave power on the reaction rate. The result shown that the microwave power can affect the heating rate of the reaction mixture and cause the superheating phenomena at the initial reaction stage. The more microwave power, the more obvious superheating phenomena. When using an appropriate power with appropriate heating rate, the superheating at the initial reaction stage disappears. This means that the acceleration of the reactions is just thermal effects. The so-called non-thermal effects do not exist.