Influence of the Product Polarity on Temperature Profiles in the Microwave-assisted Claisen Rearrangement

Background: The influence of the microwave effect in organic reactions is still not a completely clear issue. Higher temperature region (hot spots) in organic reactions have been observed and attributed to the selective heating mechanism of microwave irradiation. The accumulated heat in the polar reactants results in the acceleration of certain reactions. However, whether lower temperature region (cool spots) exists is not clear. Methods: On the basis of the microwave selective heating mechanism, less polar reactant allyl 4-methylphenyl ether was selected as a molecular probe and conducted Claisen rearrangement in more polar solvent N-methylpyrolinone in different concentrations. The temperatures of reactions under different conditions were deduced from Arrhenius equation. The dielectric constants of starting material allyl 4-methylphenyl ether, product 2-allyl-4-methylphenol, and solvent N-methylpyrolinone were determined under 2.54 GHz microwave irradiation conditions by use of the capillary method. Results: Lower temperature regions were observed in the Claisen rearrangement of allyl 4-methylphenyl ether in N-methylpyrolinone only in low concentration. However, they were not in high concentrations. The results of determination indicate that product 2-allyl-4-methylphenol shows higher polarity than reactant allyl 4-methylphenyl ether. This is a possible reason why lower temperature regions were not observed in reactions performed in higher concentrations because much more polar product was generated. Stirring speed does not affect temperature difference possibly due to the reason that microwave frequency is huge as compared with the stirring speed. Conclusion: Lower temperature regions were observed in the Claisen rearrangement of allyl 4-methylphenyl ether in N-methylpyrolidinone, an intramolecular reaction with a less polar reactant in a more polar solvent, followed by the first order kinetic. The results reveal the reason why some organic reactions cannot be accelerated because their reactants are located in lower reaction temperature regions (cool spots).