Mechanistic studies of intramolecular CH insertion reaction of arylnitrenes: Isotope effect, configurational purity and radical clock studies

Article abstract of DOI:10.1002/poc.837

In order to reveal the mechanism of the intramolecular CH insertion of arylnitrenes, three experiments were carried out: measurement of isotope effects, determination of the extent of configurational retention and radical clock studies. Irradiation of the deuterium-substituted azide 4-d in an inert solvent exclusively afforded the indolines 5-h and 5-d, in which the kinetic isotope effect k_H/k_D on the intramolecular CH insertion of the nitrene was evaluated as 12.6-14.7 at room temperature. A chiral Chromatographic analysis of the indoline 11 obtained from the optically active azide (S)-6 revealed that the enantiomeric purity of the starting azide was almost completely lost during the intramolecular CH insertion of the photolytically generated nitrene (enantiomeric excess <10%). The thermolysis of the azide 7 at 180°C mainly gave a mixture of the cyclopropyl ring-opened products 20-22, together with the intramolecular CH insertion product with an intact cyclopropyl ring 19. On the basis of these observations, we concluded that the intramolecular CH insertion of the nitrene proceeds primarily by the hydrogen abstraction-recombination mechanism. We propose, however, a small contribution of the concerted mechanism to the intramolecular CH insertion, based on the solvent dependence of the isotope effect and the extent of the configurational retention. Copyright