Phenol-Rich Carbon Dots as Metal-Free Nano-Photocatalysts for [3 + 2] Cycloaddition Reactions

Carbon dots (CDs) are unique carbon-based nanoparticles with potential applications in the field of photocatalysis. In this context, the proper selection of precursors and synthetic conditions is of paramount importance when tailoring the photocatalytic features of the resulting nanomaterials. Herein, a novel bottom-up methodology has been developed for the preparation of phenol-rich CDs (Ph-CDs) that allowed us to capitalize on the excellent photoredox properties of surface phenolate anions that can be obtained upon deprotonation of Ph-CDs. Specifically, in this study, Ph-CDs are used in combination with a suitable base and an organocatalyst, namely the Schreiner thiourea (A), to drive valuable [3 + 2] photocycloaddition reactions between suitable cyclopropyl ketones 1 and unsaturated hydrocarbons 2. Mechanistic studies show the key role played by A in the transformation. In fact, this organocatalyst may not only activate the carbonyl moiety of 1 but also stabilize the formation and enhance the photocatalytic performances of phenolate anions on the nanoparticle surfaces. Remarkably, this photocatalytic transformation provides a wide variety of densely functionalized five-membered rings 3 (17 examples, up to 99% yield) under mild operative conditions. Lastly, it is demonstrated that Ph-CDs can be easily recovered and reused up to three times without any significant drop in yield.