Theoretical Study on the Reactivity and Regioselectivity of the Ene Reaction of ¹Δ_g O₂ with α,β‐Unsaturated Carbonyl Compounds

Abstract

The ene reaction of singlet oxygen with α,β‐unsaturated carbonyl compounds gives unsaturated hydroperoxides and displays interesting features: Different reactivities of the s‐cis and s‐trans reactants and a marked regioselectivity, influenced to some extent by solvent polarity. All of these traits are accounted for by a polar diradical mechanism. A perepoxide intermediate is not a critical point on the reaction’s potential energy surface. Also, a trioxene intermediate is located too high in energy to be significantly populated. An alternative pathway leading to dioxetane is not as effective as the ene pathway giving the hydroperoxide. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Singlet oxygen and natural substrates: functional polyunsaturated models for the photooxidative degradation of carotenoids

“…preferred α-alkyl CH transfer) was already recognized by Foote et al as an indication for a two-step process involving asymmetric perepoxide intermediates [32]. Following a theoretical approach, Maranzana et al have postulated that α,β-unsaturated carbonyl compounds are prone to form biradical intermediates in Schenck ene-type reactions [33]. Whether this is relevant also to the singlet oxygen reactivity with highly conjugated systems that exist in carotenes and xanthophylls, has not yet been proven experimentally.…”

Recent Mechanistic Insights in the Singlet Oxygen Ene Reaction

“…In the mid-2000s, a stepwise pathway passing through a polar biradical intermediate was also suggested for the 1 O 2 ene reaction with E-2-methyl-but-2-enal; this substrate was chosen as a simple example model system of an a,b-unsaturated carbonyl compound. 73 Moreover, this mechanism was able to interpret the regioselectivity of the reactant E-2-methyl-but-2-enal. In 2008, studies using B3LYP/6-31G* and CASMP2 calculations were employed to study the 1 O 2 ene reaction with tetramethylethylene and trans-cyclooctene.…”

Unraveling the Mechanism of the Singlet Oxygen Ene Reaction: Recent Computational and Experimental Approaches

“…Later on, Tonachini and co‐workers34 carried out DFT calculations to examine the mechanism of the 1 O 2 addition to ( E )‐2‐methyl‐but‐2‐enal; this substrate was chosen as a simple example model system of an α,β‐unsaturated carbonyl compound. In agreement with their above‐mentioned results,33 a stepwise pathway passing through a polar biradical intermediate appears favorable.…”