Variable microwave effects in the synthesis of ureidopyrimidinones: the role of heterogeneity

Microwave-irradiated and conventionally heated nucleophilic additions of various C-6 substituted isocytosines (methyl, ethyl, isopropyl, and phenyl) to (di)isocyanates have been compared. As compared to conventional heating, the heterogeneous reaction mixtures showed higher reaction rates on using microwave heating. Variation of C-6 substituent, temperature, and amount of cosolvent influenced significantly the magnitude of these microwave effects. The magnitude of the microwave effect was governed by the solubility and the intrinsic reactivity of the variation in C-6 substitution on isocytosine. Presumably, the liquid layer near the solid surface is the area where selective heating by microwaves is occurring. As a consequence of locally higher temperatures, the solubility of the reactant as well as the rate coefficient of the reaction increase. Thus, higher reaction rates are observed than those corresponding with the measured bulk temperature. The observed microwave effects have a thermal rationale based on direct, fast, and selective heating, and the local heating effect is not found during conventional heating. This finding is crucial for a justified process scale-up scenario.