title: Effects of an electron-withdrawing group on thermal decomposition of 4-alkylidene-1-pyrazolines: a novel stereoselective formation of alkylidenecyclopropane due to participation of pi-electrons on the methylene carbon in decomposition.
abstract: 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.
authors.affiliation: Department of Materials Chemistry and Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Toyonaka 560-0043, Japan. hamaro@ch.wani.osaka-u.ac.jp
authors.forename: Masashi
authors.initials: M
authors.lastname: Hamaguchi
authors.forename: Masahiro
authors.initials: M
authors.lastname: Nakaishi
authors.forename: Toshikazu
authors.initials: T
authors.lastname: Nagai
authors.forename: Hatsue
authors.initials: H
authors.lastname: Tamura