Visible Light Photocatalyzed Oxidative Functionalization of Alkenes and Cyclic Thioacetals

Abstract

The revival of radical chemistry in organic synthesis has fuelled a renaissance in visible-light photoredox catalysis. This mild and versatile approach has rapidly gained traction across diverse fields, from biomedical research to materials science. While precious metal-based photocatalysts (e.g., Ir and Ru complexes) have been widely used, their high cost, scarcity, and toxicity limit broader application. In contrast, organic photocatalysts offer more than just “metal-free” alternatives—they often enable distinct reactivity profiles and expanded substrate scopes. Thus, developing organic photoredox systems is essential to unlock new transformations beyond the reach of traditional metal-based methods. Alkenes are among the most readily available organic molecules, derived from both petrochemical feedstocks and renewable resources. Their abundance and versatile reactivity have made alkene functionalization a cornerstone of modern organic synthesis. Depending on their substitution pattern, alkenes can exhibit diverse chemical behaviour. In our work, we focused on the oxidative difunctionalization of alkenes using visible-light photoredox catalysis. Initially, we developed a β-hydroxy trifluoromethylation method for the unactivated terminal and internal alkenes under mild, metal-free conditions using organic photocatalysts. We then demonstrated, for the first time, the oxidative functionalization of styrenes to synthesize α-alkoxylated carbonyl compounds, employing molecular oxygen (air) as the terminal oxidant—avoiding the need for pre-functionalized substrates like ketones or enolates. Furthermore, we achieved the challenging synthesis of 1,4-diketones from styrene derivatives, which is notably more difficult than accessing 1,3- or 1,5-diketones due to polarity mismatches in carbonyl intermediates. Careful screening of organophotocatalyst (redox potential) and optimizing reaction conditions (solvent and aerobic atmosphere) were key to these successful transformations. Cyclic thioacetals are widely used as protecting groups for carbonyl compounds and for enabling umpolung reactivity. We explored their selective single C–S bond cleavage under visible-light-driven oxidative organophotoredox catalysis to access rearranged thioether derivatives. Notably, phenyl thiocyanates were employed for the first time as organic cyanating agents under mild, metal-free conditions. The resulting diaryl disulfide by-product was efficiently reused for the sustainable synthesis of aryl thiocyanates, supporting the circular chemical economy.