Regioselective hydroxylation of phenols by simultaneous photochemical generation of phenol cation-radical and hydroxyl radical

Substituted phenols having pendant isoquinoline N-oxide were synthesized and their photochemical and luminiscent properties studied. Photolysis in an acid medium was found to yield the related photohydroxylation products, in a regioselective process, in addition to the isoquinoline deoxygenated precursor. Photoinduced electron transfer from the donor phenols to the protonated form of the first excited singlet state (S1) of the pendant isoquinoline N-oxide acting as acceptor leads to a red-shifted emissive charge transfer (CT) state that is in fact a radical/cation-radical pair. Homolysis of the N-OH bond restores the aromatic isoquinoline nucleus and produces a hydroxyl radical that can couple to the required ring carbon in the phenol cation-radical to give the photohydroxylation products in a regioselective process controlled by the spin density of the phenol cation-radical. These photohydroxylation processes efficiently compete with the reported tendency to deprotonation in phenol cation-radicals. The photohydroxylation process by itself, and its regioselectivity, exclude a proton-coupled electron transfer mechanism or a consecutive electron transfer/deprotonation reaction. By contrast, the phenol cation-radical exists long enough to undergo the hydroxyl radical coupling reaction that leads to the photohydroxylation products.

Graphical Abstract