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

This 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.

Revealing the Unexplored Mechanism of Photochemical Oxaziridine Conversion Process of 2H‐imidazole 1‐oxides

“…In the recent years, our group has been actively involved in the investigations on the photochemistry of nitrones. In addition to the acyclic nitrone systems, in recent years, we have also proposed [10][11][12][13] the possible photo-isomerization mechanisms for most of the above-mentioned cyclic nitrone systems through quantum mechanical investigations. These studies have indicated that the nitrone to oxaziridine conversion process goes through a low-lying S 0 /S 1 conical intersection (CI).…”

“…It can be added here that in the last few decades, several photochemical reactions have been reported where the involvement of CIs have been found. In our earlier studies on the cyclic nitrone systems, [11][12][13] we have studied various pyrroline 1-oxide and 2H-imidazole-1,3-dioxide systems. There remains huge scope to explore the mechanism of the photochemistry of the substituted 2H-imidazole 1-oxide systems.…”

The mechanism of photoconversion of cyclic dinitrone to oxaziridine and dioxaziridine: A computational investigation of an experimentally reported photochemical reaction

“…In recent years, our group has proposed [ 10–13 ] the possible mechanism of photo‐isomerization for most of these above‐mentioned systems through high‐level computational studies. These investigations have revealed that the conversion of nitrone to oxaziridine goes through a low‐lying conical intersection (CI).…”

“…[7] Nature of this cyclic nitrone is quite different in comparison with 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) [4] and its 2-methyl-substituted analogue (2-Me-DMPO). [5] In recent years, our group has proposed [10][11][12][13] the possible mechanism of photo-isomerization for most of these above-mentioned systems through high-level computational studies. These investigations have revealed that the conversion of nitrone to oxaziridine goes through a lowlying conical intersection (CI).…”

“…In our previous studies on the cyclic N-oxides, [12,13] we have only focused on the pyrroline 1-oxide systems. The photochemistry of another similar cyclic system, namely, 2H-imidazole N-oxide, has been investigated by the experimental chemists nearly three decades back.…”