A Computational Study on the Photochemistry of 2,4,4‐Trimethyl‐1‐pyrroline 1‐Oxide and Investigation on the Reaction Paths of Its Photoproduct Oxaziridine

Abstract

AbstractThis computational study reveals the reaction pathways of the experimentally reported photo‐conversion of 2,4,4‐trimethyl‐1‐pyrroline 1‐oxide to oxaziridine and the subsequent reactions (thermal and photochemical) of this photo‐product. The latter involve the oxaziridine to N‐acetyl azetidine, pyrrolidone and pyrroline formation paths. The photo‐excitation of the cyclic nitrone gives weakly allowed S0‐S1 and strongly allowed S0‐S2 transitions with transition moment values of 0.12 D and 3.16 D, respectively. The low‐lying conical intersection (S0/S1) situated at 80 kcal / mol (at CASSCF level) above the ground state nitrone geometry is found to be responsible for the oxaziridine formation. This bicyclic compound has to overcome a barrier (∼ 50 kcal / mol) to form N‐acetyl azetidine. The transition state involved in this process has an imaginary frequency of 909i cm−1. A parallel photochemical path has been also predicted for the same reaction. The thermal oxaziridine‐pyrrolidone conversion involves a transition state with an imaginary frequency of 1136i cm−1 which indicates the breaking of the C–CH3 bond adjacent to the nitrogen atom. This cyclic amide compound is almost 20–25 kcal/mol more stable than the 4‐membered cyclic azetidine and both are lower in energy than oxaziridine.