A detailed study of the thermal denitrogenation of 3-carbomethoxy-substituted 4-spirocyclopropane-1-pyrazolines 6 was conducted. Alkylidenecyclobutane derivatives 7 were selectively formed in a stereospecific manner. Unrestricted density functional calculations for a 1-pyrazoline 10a indicated that the concerted cleavage of two C-N bonds is the energetically favored process for the denitrogenation reaction to give the 2-spirocyclopropyl 1,3-diyl, followed by a conrotatory ring-closure process, which was calculated to be the energy minimum pathway, to afford a spiropentane derivative. The calculated energy minimum pathway is largely inconsistent with the experimental results observed for the denitrogenation of 6 and 10a. The contradiction between the experimental and standard computational results was solved by considering nonstatistical dynamics effects in the concerted denitrogenation reactions. Although the energy minimum pathway from the transition states of the concerted denitrogenation of the 3-carboalkoxy-substituted 1-pyrazolines involves generation of the corresponding 1,3-diradicals, many trajectory calculations using the Bohn-Oppenheimer molecular dynamics model from the transition state for the concerted denitrogenation led directly to the formation of alkylidenecyclobutanes at the UB3LYP/6-31G(d) level of theory.

Thermal decomposition of 4-alkylidenepyrazolines 14 bearing a methoxycarbonyl group at C-3, prepared by 1,3-dipolar cycloaddition between allenecarboxylates 12 and diazoalkanes 13, was carried out. Unlike normal 4-alkylidenepyrazolines, which decompose in stepwise mechanisms at high temperatures, 14 decomposed concertedly at moderately low temperatures (45-110 degrees C), resulting in selective formation of the two isomeric alkylidenecyclopropanes 7 arising from the bond formation between the exo-methylene carbon and the 5-carbon. The selective formation and the configurations of the products are rationalized in terms of the concerted process via the folded conformation of the pyrazolines. Introduction of an electron-withdrawing group at the 3-position of the 4-alkylidenepyrazoline system causes the polarization of the C(3)-N(2) bond inducing the properties of intramolecular diazonium salt 8, in which the pi-electrons on the methylene carbon become more nucleophilic and participate in the cleavage of the C(5)-N(1) bond. The X-ray crystal structure of the typical normal alkylidenepyrazoline 14a with only small steric interactions between the substituents was determined to be a nearly planar ring structure.