Visible Light-Induced Ligation via o-Quinodimethane Thioethers

We introduce a photocaged diene system (o-quinodimethane thioethers) based on o-methylbenzaldehydes (o-MBAs) that can be activated with visible light. The pioneered system is accessible in a single step from commercially available starting materials in excellent yields. Variable synthetic handles can be attached to the photocaged diene, often without elaborate protecting group chemistry. Full conversion of various o-methylbenzaldehydes to the Diels-Alder adduct is achieved in the presence of maleimides under catalyst-free conditions triggered by visible light irradiation with LEDs under flow conditions. Unlike the previously reported UV-induced ligation of o-quinodimethanes, the reaction can be conducted both in organic solvents and in aqueous solution. We further demonstrate the ability of the photocaged dienes to ligate two polymer blocks by visible light. The [4+2] nature of the reaction makes it a powerful orthogonal ligation platform.