Hydrogen‐Bonding Effects on Excited States of Para ‐Hydroxyphenacyl Compounds

Photorelease of phosphates: Mild methods for protecting phosphate derivatives

“…The role of H 2 O and hydroxylic solvents observed in this series is in complete accord with solvent effects on the parent pHP photoreactions [ 6 , 13 , 15 , 40 – 46 ]. The photo-Favorskii reaction ( Scheme 7 ) is favored by aqueous-based solvents that serve both as a proton donor and an acceptor to the conjugate base generated from the chromophore’s triplet state heterolytic cleavage of the leaving group.…”

“…1,4-HNA DEP ( 14a ) and 1,4-MNA DEP ( 14b ) upon irradiation at 350 nm in 1% aq MeOH quantitatively release diethyl phosphate concomitant with a photo-Favorskii rearrangement with quantum yields of 0.028 for 1,4-HNA DEP and 0.0076 for 1,4-MNA DEP. The lower quantum efficiency of 1,4-MNA in MeOH is fully in accord with a similar methylation of the hydroxy group of pHP DEP, emphasizing the role of the phenolic OH on the reaction and rearrangement [ 6 , 15 , 46 , 48 ]. In anhydrous, non hydroxylic media, the mechanism of the reaction changes from heterolysis and reverts to the more traditional photochemically induced homolysis mechanism, leading to more complicated product mixtures and poor conversions.…”

“…For pHP, the reactive excited state is a π,π* triplet that is favored only in hydroxylic solvents, especially H 2 O [15,45–46 48]. In non-hydroxylic solvents, the (n,π*) 3 dominates which leads to the classical α- and β-homolytic cleavage and H-abstraction reactions.…”

“…The controlling feature leading to the change in mechanism for these two examples is more reasonably imbedded in the route to the reactive triplet excited state configuration [ 10 – 11 ]. For pHP, the reactive excited state is a π,π* triplet that is favored only in hydroxylic solvents, especially H 2 O [ 15 , 45 – 46 48 ]. In non-hydroxylic solvents, the (n,π*) 3 dominates which leads to the classical α- and β-homolytic cleavage and H-abstraction reactions.…”