Strong Asymmetry in the Perepoxide Bifurcation Mechanism: The Large‐Group Effect in the Singlet Oxygen Ene Reaction with Allylic Alcohols

Abstract

Abstract 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 ( k E ‐5 / k Z ‐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.