Effects of an Electron-Withdrawing Group on Thermal Decomposition of 4-Alkylidene-1-pyrazolines:  A Novel Stereoselective Formation of Alkylidenecyclopropane Due to Participation of π-Electrons on the Methylene Carbon in Decomposition

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 °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 C3−N2 bond inducing the properties of intramolecular diazonium salt 8, in which the π-electrons on the methylene carbon become more nucleophilic and participate in the cleavage of the C5−N1 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.