Photolysis of chiral 1-pyrazolines to cyclopropanes: mechanism and stereospecificity.
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.
Muray
Elena
E
Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.
Illa
Ona
O
Castillo
José A
JA
Alvarez-Larena
Angel
A
Bourdelande
José L
JL
Branchadell
Vicenç
V
Ortuño
Rosa M
RM