title: Photolysis of chiral 1-pyrazolines to cyclopropanes: mechanism and stereospecificity.
abstract: The photodenitrogenation of chiral trisubstituted 1-pyrazolines has been studied by laser flash photolysis. These experiments have permitted the detection of two transients that have been assigned, for each pyrazoline, to the trimethylene-type diradical resultant from the extrusion of nitrogen (lifetime tau = 0.1-0.8 micros) and to the pyrazoline triplet (tau < 9 ns), respectively. The efficiency of the photosensitization process has been evaluated by determination of the corresponding quenching rate constant in each instance. Theoretical calculations support a mechanistic pathway involving a trimethylene radical as intermediate that rapidly evolves to the corresponding cyclopropane derivative. The cyclopropane ring-closure is predicted to be faster than rotation around the C-C bond, thus accounting for the observed stereospecificity.
authors.affiliation: Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.
authors.forename: Elena
authors.initials: E
authors.lastname: Muray
authors.forename: Ona
authors.initials: O
authors.lastname: Illa
authors.forename: José A
authors.initials: JA
authors.lastname: Castillo
authors.forename: Angel
authors.initials: A
authors.lastname: Alvarez-Larena
authors.forename: José L
authors.initials: JL
authors.lastname: Bourdelande
authors.forename: Vicenç
authors.initials: V
authors.lastname: Branchadell
authors.forename: Rosa M
authors.initials: RM
authors.lastname: Ortuño