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

Carbon dots 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. Here, we have developed a novel bottom-up methodology for the preparation of phenol-rich carbon dots (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 were 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. Interestingly, mechanistic studies showed 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, we demonstrate that Ph-CDs could be easily recovered and reused up to three times without any significant drop in yield.