Uncatalyzed thermal gas phase aziridination of alkenes by organic azides. Part I: Mechanisms with discrete nitrene species

Abstract

Alkene aziridination by azides through uncatalyzed thermal gas phase routes has been studied using the DFT B3LYP/6-31G(d,p) method, where the possible role of discrete nitrene intermediates is emphasized. The thermal decomposition of azides is studied using the MP2/aug-cc-pVDZ strategy as well. The MP2 (but not the B3LYP) results discount the existence of singlet alkylnitrenes where the alkyl group has an α-hydrogen. Addition of the lowest lying singlet and triplet nitrenes R-N (R = H, Me, Ac) to four different alkene substrates leading to aziridine formation was studied by the B3LYP method. Singlet nitrenes with alkenes can yield aziridines via a concerted mechanism, where H-N insertion takes place without a barrier, whereas Me-N shows larger barriers than Ac-N. Methyl substitution in the alkene favors this reaction. Triplet nitrene addition to alkenes is studied as a two-step process, where the initially formed diradical intermediates cyclize to form aziridines by ISC (intersystem crossing) and collapse. Scope for C-C bond rotation in the diradical leads to loss of stereochemical integrity for triplet nitrene addition to cis- and trans-2-butenes. Geometries of the transition states in the various reaction steps studied here are described as “early” or “late” in good accordance with the Hammond postulate. The addition of free nitrenes to alkene substrates is studied using DFT and MP2 methods. The B3LYP results predict that concerted azide decomposition can yield singlet nitrenes, unlike the MP2 results. Singlet nitrenes insert into alkenes with a modest or no barrier. Triplet nitrenes employ a step-wise pathway to give aziridines.