Directing an Elusive Photoredox [4+2] Cycloaddition Reaction in Water by Altering the Shape of Nano-confinement

The geometry of confinement and intermolecular packing of guest molecules inside supramolecular cavities can lead to product selectivity that is unattainable in bulk. Here, we report for the first time an unprecedented radical-mediated intermolecular [4+2] photocycloaddition reaction between a mixed ene-yne framework and an alkyne, to form biaryls inside the confinement of a square-pyramidal-shaped water-soluble nanocavity. Using a combination of multidimensional NMR, FT-IR and broadband femtosecond transient absorption spectroscopy, we characterize the reactive intermediates while providing mechanistic details of this unique host-guest charge-transfer-mediated photoreaction. Detailed electronic structure studies of the host-guest complex, along with deuterium-labelling experiments, reveal that both the reactive moieties are buried towards the smaller pore of the square-pyramidal-shaped nanocavity, thereby driving a novel proximity-induced [4+2] photocycloaddition product. Our results are in contrast to the typical alkyne hydration reaction via exposed solvent water molecules or the linear dimerization products observed in porous octahedral-shaped nanocavity, thereby highlighting the role of cavity shape and its confinement in driving product selectivity.