Selective C–H Activation of Molecular Nanodiamonds via Photoredox Catalysis

While substituted
adamantanes have widespread use in medicinal
chemistry, materials science, and ligand design, the use of diamantanes
and higher diamondoids is limited to a much smaller number. Selective
functionalization beyond adamantane is challenging, as the number
of very similar types of C–H bonds (secondary, 2°, and
tertiary, 3°) increases rapidly, and H atom transfer does not
provide a general solution for site selectivity. We report a method
using pyrylium photocatalysts that is effective for nanodiamond functionalization
in up to 84% yield with exclusive 3° selectivity and moderate
levels of regioselectivity between 3° sites. The proposed mechanism
involving photooxidation, deprotonation, and radical C–C bond
formation is corroborated through Stern–Volmer luminescence
quenching, cyclic voltammetry, and EPR studies. Our photoredox strategy
offers a versatile approach for the streamlined synthesis of diamondoid
building blocks.