Density functional theory computational study on the thermal cycloreversion of 2-acetoxy-2-methoxy-5,5-dimethyl-Delta3-1,3,4-oxadiazoline: evidence for a carbonyl ylide intermediate.

Three [3 + 2] cycloreversions of 2-acetoxy-2-methoxy-5,5-dimethyl-Delta(3)-1,3,4-oxadiazolines were examined by computation at the density functional level of theory. The lowest activation energies are those for cycloreversion to 2-diazopropane and acetic methylcarbonic anhydride and for cycloreversion to N(2) and a carbonyl ylide. Those are the reactions that are observed experimentally. A third cycloreversion, to acetoxy(methoxy)diazomethane and acetone, has a much larger barrier. The carbonyl ylide is a real intermediate, but it fragments easily to acetone and acetoxy(methoxy)carbene. The lifetime of the ylide may be so short, in some cases, as to blur the distinction between a two-step cycloreversion of the oxadiazoline and a concerted process that generates three fragments in one step.