Photosensitized selective semi-oxidation of tetrahydroisoquinoline: a singlet oxygen path

Abstract

Selective semi-oxidation of tetrahydroisoquinoline (THIQ) leads to a valuable dihydroisoquinoline (DHIQ) derivative via singlet oxygen photooxidation process. Typical photosensitisers (i.e., Ru complexes) can activate the reaction even under heterogeneous conditions that facilitate catalyst separation and reusability. In contrast to DHIQ, THIQ acts as an efficient singlet oxygen quencher driving the reaction selectivity. The reaction can also be facilitated by semiconductor catalysts such as MoCo@GW, a glass wool-based catalyst that is easy to separate and reuse and compatible with flow photochemistry. Its role is to mediate the formation of isoquinoline (IQ) and thus an in situ-generated singlet oxygen catalyst. Laser flash photolysis with NIR detection provides proof of the singlet oxygen mechanism proposed and rate constants for the key steps that mediate the oxidation.