Abstract
The α-methylated chalcones 7a–7e behave as P-type photochromic substances with photo-stationary states (PSS) as high as 15:85 when irradiated at 350 nm. These compounds are easily accessible in pure E-configuration by aldol condensation or by oxidative coupling/elimination. The α-methyl groups make these compounds potentially reactive with singlet oxygen following the gem-rule that predicts 1O2 regioselectivity. Even after long irradiations times in the presence of the singlet oxygen sensitizer tetraphenylporphyrin (TPP) and oxygen, however, no oxygenation products were detected. Under these conditions, all substrates were converted into 9:1 E/Z-mixtures despite the use of low-energy light that does not allow direct or sensitized excitation of the substrates 7. Additionally, chalcone 7a reduced the singlet oxygen reactivity of the tiglic ketone 3a by about a factor of two, indicating substantial physical quenching of singlet oxygen by the α-methylated chalcones 7a–7e. Thus, a singlet oxygen-induced E/Z-isomerization involving 1,2-dioxatetra-methylene biradicals that leads to triplet oxygen and thermodynamic E/Z-mixtures is postulated and supported by quantum chemical (DFT)-calculations.
Highlights
Singlet oxygen (1 ∆g − 1 O2 ) is a highly reactive and structurally simple oxygenation reagent
In order to become applicable for singlet oxygen Schenck-type ene reaction, α-methylated chalcones were chosen for the investigation
A large family of corresponding α-methylated α,β-unsaturated ketones, aldehydes, esters, amides, and carboxylic acids have already been described in the literature as reactive in 1 O2 ene reactions with high hydrogen transfer selectivity from these α-methyl groups (Scheme 1, termed the 1 O2 “gem effect” in the literature)
Summary
Singlet oxygen (1 ∆g − 1 O2 ) is a highly reactive and structurally simple oxygenation reagent. It is generated from air oxygen (ground-state electronic triplet state) by energy transfer from a vast number of electronically excited state dye molecules [1,2,3,4]. Because it is such an available oxidation and oxygenation (O versus O2 -transfer) reagent,. We were especially interested in chalcones in the course of our investigations of naturally occurring antioxidants [6,25,26] and designed new potentially reactive and chemically modified chalcones
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
