Thermal fragmentation of 1,2,5- and 1,2,4-oxadiazoles

Abstract

Thermolysis of 3,4-diphenyl-1,2,5-oxadiazole (diphenylfurazan) in tetradec-1-ene at 245 oC afforded 5-dodecyl-3-phenyl-2-isoxazoline resulting from cycloaddition of benzonitrile oxide to the alkene. Similarly, the tetramethylene and decamethylene furazans 13 and 15 in 4- methoxybenzonitrile at 240 oC fragmented to ω-cyanoalkanonitrile oxides, which were trapped as their 1,2,4-oxadiazole cycloadducts 17 and 19, respectively. The flash vacuum pyrolysis (FVP, 550-650 oC) technique was used to investigate the process in more detail using a range of mono and bicyclic furazans. In all cases the 1,2,5-oxadiazole ring cleaved cleanly at O(1)−N(2) and C(3)−C(4) to nitrile and nitrile oxide fragments. The nitrile oxides were trapped in high yield (81-100%) as their isoxazoline cycloadducts by reaction with hex-1-ene. The tetramethylene furazan 13 was converted into 4-cyanobutyl isocyanate by FVP and reaction of the condensate with sulfur dioxide. 3,5-Diphenyl-1,2,4-oxadiazole showed greater thermal stability under similar conditions (FVP, 600 oC), but at higher temperatures (700-800 oC) phenyl isocyanate was formed and trapped as its methyl urethane derivative. Attempts to trap the putative benzonitrile oxide intermediate were unsuccessful.