Computational Investigation of the Photochemical Reaction Path of Some Synthesized and Experimentally Analyzed Small-Chain Conjugated Nitrones

Abstract

This combined theoretical and experimental study has revealed the photochemistry of two small open-chain conjugated N-methylnitrone systems with phenyl substitutions at the C-terminal positions. The UV spectra of these synthesized nitrones have shown intense peaks around 330 nm while the new bands formed near 260 nm after their photoirradiation are predicted to be arising from the photoproduct oxaziridine. Photoexcitation of α-styryl N-methylnitrone populates the first excited singlet state which relaxes by 8 kcal/mol from the vertically excited state and subsequently goes toward the lowest-energy conical intersection (CI) geometry (situated 27-30 kcal/mol below) with a terminal CNO-kink. Following the gradient difference vectors of this CI, we have located the oxaziridine structure with its characteristic geometry at roughly 14 kcal/mol above the ground state. This whole process is triggered by a transfer of electronic cloud from oxygen to the conjugated chain side. On the other hand, the photoexcitation of the nonplanar 3,3-diphenylethylene N-methylnitrone has two strong singlet-singlet absorptions with almost 5 D transition moment values. Here the initial S2-S1 relaxation is followed by oxaziridine formation through the terminally twisted CI. However, the initially photoexcited S1 state in this nitrone is found to head toward some other direction with transfer of huge amount of nonbonding electron cloud of oxygen to the π* orbital, creating a stable excited state geometry with an elongated N-O bond which gets involved in a sloped CI with the ground state.

Oxidation of 5-methylaminomethyl uridine (mnm5U) by Oxone Leads to Aldonitrone Derivatives

“…The observed strong π−π* band at 319 nm on UV-visible spectrum of 1 (Figure 2) is compatible with an N -alkyl aldonitrone function as shown in Scheme 1 [34,41,42]. The aldonitrone structure of 1 was unambiguously determined by NMR.…”

“…Usual oxidants are oxygen atom donors, such as hydroperoxides (including H 2 O 2 ) activated by metal ions, peracids, and dioxirane. However, oxaziridines are the most common oxidation products of imines with a variety of reagents [26,34,35,36,37]. On the other hand, oxidation of primary amines by oxone has been reported to afford nitrones [38,39,40].…”

“…Oxaziridines (Scheme 1) are often reported in the literature as the products of imine oxidation by peracids [34,35,37]. During the oxidation of mnm 5 U with oxone, N -methyl-oxaziridine derivative did not form as no NMR signal was observed at the typical chemical shift of the CH proton of oxaziridines, namely around δ 4–5 ppm [45].…”

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

“…Photochemical reactions of nitrones have been thoroughly investigated by our group in the last few years . Various categories of acyclic and cyclic nitrones have been analyzed through quantum mechanical studies.…”

“…These two cyclic nitrones were found to differ in their subsequent oxaziridine‐lactam conversion processes. Oxaziridine obtained from 5,5‐dimethyl‐1‐pyrroline‐1‐oxide (DMPO) was found to overcome a small barrier to form lactam through–H‐shift. On the other hand, its 2‐methyl‐substituted analogue was found to form highly stable oxaziridine.…”