Flash vacuum pyrolysis of 1,2,5-oxadiazole 2-oxides and 1,2,3-triazole 1-oxides

The flash vacuum pyrolysis (FVP, 450-600 degrees C/10(-3) mmHg) of 3,4-diaryl- and 3,4-dialkyl-1,2,5-oxadiazole 2-oxides (furoxans) has been investigated. In all cases the 1,2,5-oxadiazole ring cleaved cleanly at O(1)-N(2) and C(3)-C(4) to afford two nitrile oxide fragments, which were trapped in high yield (75-97%) as their isoxazoline cycloadducts by reaction with alk-1-enes. At higher temperatures (700-800 degrees C) isocyanates were formed as by-products. The dimerisation of acetonitrile oxide to dimethylfuroxan was followed by H-1 NMR spectroscopy, and the rate constant for the 2(nd) order reaction determined. The furoxans were converted into isocyanates in good yield (61-95%) by FVP, followed by sulfur dioxide-mediated isomerisation of the resulting nitrile oxides. 2,4,5-Trisubstituted-1,2,3-triazole 1-oxides showed greater thermal stability, but at 700-800 degrees C decomposition of the 4,5-dimethyl compound 25b lead to 1,2-di(5-methyl-2-phenyl-1,2,3-triazol-2-yl) ethane as the major product; attempts to trap acetonitrile oxide were unsuccessful.