Outer-Sphere Effects in Visible-Light Photochemical Oxidations with Immobilized and Recyclable Ruthenium Bipyridyl Salts

Abstract : The ability to accelerate visible-light photochemical reactions in a simple setup and with little photocatalyst modification is an important challenge. We report that the adsorption of a widely used organometallic photocatalyst ([Ru(bpy)3]Cl2) on unmodified silica particles provides opportunities in the intensification of photochemical oxidations with an almost 10-fold increase in reactivity. This outstanding performance is attributed to noncovalent outer-sphere interactions between the substrate and the solid particles, because higher concentrations of reactive species are produced at the interface. This simple catalytic system is efficiently recycled and shows an up to 4-fold increase in stability, compared to its homogeneous counterpart. As a proof of concept, we apply this straightforward immobilization strategy to the semisynthesis of the antimalarial drug artemisinin from dihydroartemisinic acid. Our results demonstrate that the surface has a cooperative and bifunctional role, which avoids the use of hazardous acid reagents and can potentially afford a more efficient and lower cost access to this important pharmaceutical compound.