The photo‐oxidation of hexamethylbenzene and pentamethylbenzene by singlet oxygen

Abstract

Abstract. In a two step reaction, hexamethylbenzene consumes two molecules of singlet oxygen to form epidioxy hydroperoxide 3. The first step involves a [4 + 2]-cycloaddition, which is followed by an ene-reaction. The structure of the intermediary endoperoxide 2 has been established spectroscopically. Pentamethylbenzene reacts in the same way with singlet oxygen, albeit much more slowly. Two epidioxy hydroperoxides are formed in this case.

Reaction of Aniline with Singlet Oxygen (O2 1Δg)

“…When the amine group is added to the benzene ring, it decreases the energy gap between the HOMO (−5.084 eV) and LUMO (−1.055 eV) orbitals in comparison with that of the HOMO (−5.388 eV) and LUMO (−0.168 eV) of benzene. 66,67 This change means that the ring becomes more reactive with singlet oxygen in the presence of an amine substituent. Figure 1 indicates the Fukui indices and Hirshfeld electronic charges of aniline with respect to benzene.…”

Singlet Oxygen as a Reagent in Organic Synthesis

“…Compounds 15,98,101,103, and 107 led to the formation of bisperoxides. Growing experimental data has been accumulated for singlet oxygen reactions, which give rise to spiro compounds [82,83,[127][128][129][130], or yield bis-or polyperoxides, or polyepoxides [42,58,[114][115][116][117][118][119][120]131].…”

“…Although, benzene, toluene, and xylenes do not react chemically with singlet oxygen, hexamethylbenzene (34) is sufficiently electron-rich to react with singlet oxygen [42]. Hexamethylbenzene-1,4-endoperoxide (43) formed in a MB-sensitized reaction in CD 3 NO 2 at 15 C and, over time, also endoperoxy-hydroperoxide 43 0 , arose from a secondary singlet oxygenation in 90% yield (not isolated, quantitated from the NMR spectrum with an added internal standard) (Scheme 11.8).…”

“…Heating the primary endoperoxide product 43 at 40 C for 1 h led to oxygen evolution and the regeneration of hexamethylbenzene 34. Singlet oxygen also reacted with pentamethylbenzene, but much more slowly than did hexamethylbenzene [42,43]. The reaction of phenol with singlet oxygen led to p-catechol and p-quinone products in near-quantitative yield, although the proposed endoperoxide intermediate was not directly observed (Scheme 11.9) [44,45].…”

“…The 1 O 2 ene reaction of alkenes gives allylic hydroperoxides, where the double bond has shifted; an example was presented in Scheme 11.8 (vide supra; Section 11.3.2.1) [42,43]. Schemes 11.16 to 11.19 demonstrate several reported examples of hydroperoxides formed in good to high yields, where singlet oxygen reacts rapidly with allylic alkenes in solution or confined within zeolites.…”