Thermolysis of a spiro-fused oxadiazoline: The carbonyl ylide-cyclic carbene-diradical sequence

Thermolysis of spiro-fused oxadiazoline 1 in benzene led to loss of N-2 to form a carbonyl ylide intermediate. Most of the ylide fragmented to acetone and 4-phenyl-1,3-dioxane-2-ylidene, which could be trapped with tert-butyl alcohol. In the absence of the trapping agent, the major pathway followed by the carbene was fragmentation to a diradical, 5-phenyl-2-oxa-1-oxo-1,5-pentanediyl. The latter diradical coupled to alpha-phenyl-gamma-butyrolactone and decarboxylated to afford cyclopropylbenzene. Other products from the reaction were those of apparent insertion of the carbene into a C-H bond of the benzene solvent and into a C-H bond of acetone. Such reactions appear to be without precedent - alternative, non-carbene mechanisms are proposed.