Molecular Orbitals and Thermolysis and Hydrolysis Mechanisms for Δ³-1,3,4-Oxadiazolin-2-ones

Abstract

<p>The title compounds were first synthesized in 1970, and because of their highly functionalized nature, undergo a medley of quite distinctive reactions.</p> <p>Interaction between the two main functional groups (azo N==N and carboxylate) was investigated theoretically. CNDO/2 frontier molecular orbital densities, ionization potentials (IPs), and electronic transition energies were (calculated for 5,5-dimethyloxadiazolinone (1-Me₂) and the 5,5-diprotio (l-H₂), and 5,5-difluoro (l-F₂) analogs. Bond lengths and angles were not optimized but rather taken from a crystal structure of a closely related system, with an assumed carbonyl bond length of Rⅽo = 1.19 Å. This value is supported by a high infrared stretching frequency (1835 cmˉ¹), X-ray crystal data, and the effect of Rⅽo variation on calculated IPs. The photoelectron spectrum of 1-Me₂ was obtained, and the first three bands assigned as nɴ- (10.20 eV), πɴɴ (11.52 eV), and nⅽo (13.26 eV). This unusually large nⅽo IP is traced to an inductive effect of the neighbouring strongly electron withdrawing azo group. The IPs obtained by CNDO/2 calculation (12.81, 13.37, and 15.15) are 1.5 to 2.5 eV higher than the experimental values, which is well within the predictive capability of the method. Thus, the theoretical treatment gives a good representation of the MOs of these compounds and may be employed with confidence to provide insight to the mechanistic studies.</p> <p>The title compounds thermolyse at 50-100°C in a variety of solvents via two unique competing unimolecular pathways: one furnishing synthetically useful diazoalkane with carbon dioxide, the other, a theoretically interesting 3-piece fragmentation, spawns a molecular each of ketone, carbon monoxide, and dinitrogen. Diazoalkane formation is favoured by polar solvents and electron donating substituents, with a Hammett rho of -1.93 for seven ᴅ-substituted 5-methyl-5-phenyloxadiazolinones. Moderate solvent polarity effects, significant secondary kinetic isotope effects (2.1 ± 0.3% per D for 1-(CD₃)₂), the Hammett rho, and Frontier Molecular Orbital Theory, all support a concerted mechanism for diazoalkane formation through a transition state with non-synchronous bond rupture.</p> <p>Neither thermal decomposition mode correlations with Taft steric parameters, Es, and ketone formation does not correlate with inductive paramerers, σ*. Interestingly, the enthalpy change during the exothermic three piece fragmentation was calculated to be only -30 ± 2 kcal molˉ¹, precluding chemiluminescence. A vigorous search for intermediates by esr, ms, CIDNP, trapping, and racemization experiments leads to the conclusion that ketone formation is also concerted.</p> <p>Synthetic procedures to make oxadiazolinones were augmented with a new oxidation procedure developed for synthesizing oxadiazolinones from ketone semicarbazones which would not cyclize under normal lead tetraacetate oxidation conditions. In the presence of five equivalents of trifluoroacetic acid, a rapid reaction occurred at lower temperature.</p> <p>Hydrolysis of dimethyloxadiazolinone was investigated at 20° over 30 orders of magnitude of acidity. Over the range H。= -6 to pH = 14, the products are carbon dioxide, ketone, and diazene, as expected on the basis of strong kinetic parallels to neutral ester hydrolysis. Large C5 substituent electronic effects seem most consistent with rate limiting breakdown of the tetrahedral intermediate, but ¹⁷O carbonyl oxygen isotope exchange was not detected. This is rationalized in terms of Deslongchamps' recent theory of stereoelectronic orbital control of the cleavage of tetrahedral intermediates, and supported by a number of related observations. The observed high reactivity of oxadiazolinones toward nucleophiles is in accord with the theoretical calculations, which predict a very electropositive carbonyl carbon.</p> <p>Preliminary studies on acid catalysed decomposition of dimethyloxadiazolinone suggest that the thermodynamically favourable N4-protonated form may not be the kinetically active one leading to product 2-propanol over the acid range -16 </p>