Abstract
The title compounds thermolyse at 50–100 °C in a variety of solvents via two (and only two) competing unimolecular pathways, one producing a diazoalkane and carbon dioxide, the other a ketone, carbon monoxide, and dinitrogen. Diazoalkane formation is favoured by polar solvents and electron-donating substituents, with a Hammett ρ of −1.93 for seven p-substituted 5-methyl-5-phenyloxadiazolinones. Moderate solvent polarity effects, secondary kinetic isotope effects (2.1 ± 0.3% per D for 2b-d6), the Hammett ρ, and Frontier Molecular Orbital Theory, all support concerted diazoalkane formation through a transition state with nonsynchronous bond rupture.Neither decomposition mode shows any correlation with Taft steric parameters, Es, and ketone formation does not correlate with inductive parameters. The enthalpy change during the exothermic three piece fragmentation was calculated to be only −30 ± 2 kcal mol−1, precluding chemiluminescence. A vigorous search for intermediates by esr, ms, CIDNP, trapping, and racemization experiments leads to the conclusion that ketone formation must also be concerted.A new oxidation procedure using lead tetraacetate in the presence of five equivalents of trifluoroacetic acid was useful in the synthesis of oxadiazolinones from ketone semicarbazones which would not cyclize under normal LTA oxidation conditions.
Published Version
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