Studies on intramolecular trapping of N-nitrenes
The research described in this Thesis is an investigation of the intramolecular trapping of N-nitrenes by alkenes and alkynes. In the former case, the objective was to try to obtain a description of the transition state geometry for concerted addition of nitrenes to double bonds. This was attempted by a study of intramolecular nitrene additions since in the latter, some control over the approach geometry of the interacting components can be exercised by design of the molecular framework. From the effect of changes in this framework upon the characteristics of the cycloaddition, and in particular upon its concertedness, an ideal configuration for the participating atoms (i.e. transition state geometry) was definable. 2H-azirines are obtained in greatly improved yields in the intramolecular trapping of N-nitrenes by comparison with the analogous inter- molecular trapping. Analysis of the n.m.r. spectra of these azirines together with the X-ray crystal structure of one of them reveals that a preference for near-coplanarity of three of the bonds at the spiro-centre results in conformational anchoring of the five-membered ring. (Examination of all the azirine ring containing structures in the Cambridge crystallographic data file shows that all of these also show the same near-coplanarity of azirine C-C bond and substituent bonds at the spiro-centre). Attempts to trap a presumed 1H-azirine intermediate were unsuccessful. This is probably the result not only of their anti-aromatic character but also that migration of the N-N bond in the 1H-azirine delivers the 2H- azirines, in the cases studied, directly in their most stable conformations.