Uncatalyzed gas phase aziridination of alkenes by organic azides. Part 2. Whole azide reaction with alkene

The B3LYP/6-31G(d,p) DFT method was used to study alkene aziridination by azides through uncatalyzed thermal gas phase routes which involve the whole azide reactant molecule without dissociation. Two mechanisms were studied – Route I involving concerted azide addition to alkene with the elimination of N 2 , and the multi-step Route II involving 1,3-dipolar cycloaddition between azide and alkene. Three azides R N 3 ( R = H, Me, Ac) are reacted with alkene substrates forming aziridine products. The concerted addition–elimination step of Route I is exothermic with an appreciable barrier, where the facility order Ac > Me > H points to electrophilicity of the azide reactant. The initial 1,3-dipolar cycloaddition step of Route II involves smaller barriers than Route I, while thermal decomposition of the triazoline intermediate to aziridine and N 2 involves two more steps with an N-alkylimine intermediate. The very high barrier for N-alkylimine cyclization to aziridine could be offset by the high exothermicity of the previous step. Geometries of the transition states for various reaction steps studied here are described as ‘early’ or ‘late’ in good accordance with the Hammond postulate. Two other mechanisms (Routes A and B) studied earlier (involving discrete nitrene intermediates) are compared with Routes I and II, where Route II involving 1,3-dipolar cycloaddition is predicted to be energetically the most favored of all the four mechanisms for thermal gas-phase aziridination of alkenes by azides. Graphical Abstract SYNOPSIS This DFT study examines various routes for alkene aziridination by whole azides ( R N 3 ) . Route I involves concerted addition-elimination of R N 3 to alkene. The multi-step Route II involves 1,3-dipolar cycloaddition. Including two other routes involving discrete nitrenes which were studied earlier, Route II is predicted as the most feasible.