The singlet oxygen reactions of sterically crowded allylic alcohols result in the preferential formation of γ‐hydroperoxy alcohols with regioselectivities up to 98:2. The kinetic cis effect still prevails in the first, rate‐determining step of this two‐step non‐intermediate ene mechanism (kE‐5/kZ‐5=1.9). The primary allylic alcohol 3 showed no regioselectivity at all, increasing the steric demands at the α carbon and directing the singlet oxygen addition towards the γ carbon up to 98:2 regioselectivity. DFT calculations rationalized these results and show the decisive role of the symmetry‐breaking bifurcation following the early transition states. The use of the products as substrates for the synthesis of five‐ and seven‐membered ring peroxides, namely 4‐methylene‐1,2‐dioxolanes and 6‐methylene‐1,2,4‐trioxepanes, by rare earth metal complexes as Lewis acids is also demonstrated.

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.

Enantioselective CuI-catalyzed 1,4-additions and DFT computations concerning the enantioselective mechanism.

Palladium-catalyzed allylic alkylations of Na(CH(CO2Me)2 with 1,3-diphenylallyl acetate, employing BIFOP-X (X = H, D, Cl, CN, N3) ligands, yield the C–C coupling product (up to 91% yield, 70% ee). A NBO effect reveals a change of stereochemistry.