, 1646 (1984). 2-Aryl-2-methoxy-5.5-dimethyl-A" I ,3,4-oxadiazolines (4) and 5-aryl-2-methoxy-2,5-dimethyl-A3-1,3,4-oxadiazolines (5) were synthesized. Compounds 4 decompose in solution with first order kinetics. Rate constants are correlated with Hammett substituent constants (a-) with p(49.2"C) = 0.74 and 0.89 for CCI, and CD30D, respectively. The final products from 4 indicate that thermolysis involves the cleavage of both C-N bonds, to form Nz and, initially, a carbonyl ylide. Compounds 5, which were obtained as mixtures of cisltrans isomers containing several impurities, and which therefore gave poorer kinetic data, decomposed in CDCI, solution with ~( 4 5°C ) --1.1. Carbonyl ylide intermediates, similar to those from the closelyrelated compounds 4, were assumed on the basis of analogy and on the basis of partial identification of products. The effects of para substituents in the aryl groups of 4 and 5 show that the transition states have greater electron density at C-2 of 4 and at C-5 of 5 than do the starting materials. In spite of the increase in electron density at C-2 of 4, the transition state must be less polar, overall, than the ground state because rate constants for thermolysis of 4 in methanol are smaller than those for CCI, solvent. A plausible explanation for the substituent effects and the solvent effects is that the loss of N2 is concerted, with a transition state resembling more closely a spin paired 1,3-diradical than a I ,3-dipole. Alternative stepwise mechanisms, in which C2-N3 bond scission of 4 and C5-N4 bond scission of 5 are the rate-determining steps, leading to 1,5-diradical intermediates, can not be excluded on the basis of the evidence. intermtdiaires semblables a ceux observCs dans le cas des composts 4 qui leur sont trbs voisins. Les effets des substituants en position para des groupes aryles des composts 4 et 5 montrent que, par rapport aux produits de dtpart, les ttats de transition ont une plus grande densitt Clectronique en C-2 dans les composes 4 et en C-5 dans les composts 5. En dtpit d'une augmentation de la densit6 tlectronique en C-2, dans les composts 4, l'ttat de transition doit Ctre, dans I'ensemble, moins polaire que I'ttat fondamental puisque les constantes de vitesse de la thermolyse des composts 4 dans le mtthanol sont plus faibles que celles dans le CCI,. Une perte de N2 concertte, avec un Ctat de transition ressemblant de trbs prbs un diradical-1.3 de spin pairt plutbt qu'a un dipole, constitue une explication plausible des effets de substituant et de solvant. Les preuves obtenues n'excluent pas la possibilitt d'un micanisme par Ctapes dans lequel la scission de la liaison C2-N3 des composts 4 et de la liaison C5-N4 des composCs 5 constituerait l'ttape dtterminante et conduirait a la formation de diradicaux-1,5 intermtdiaires.[Traduit par le journal]