Directing an elusive photoredox [4+2] cycloaddition reaction in water by altering the shape of nano-confinement

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The geometry of confinement and intermolecular packing of guest molecules inside supramolecular cavities can lead to product selectivity that is challenging to achieve in a bulk organic medium. Here, we report a 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 nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), Fourier transform infrared (FTIR) spectroscopy, and broadband femtosecond transient absorption (fs-TA) 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-labeling experiments, reveal that both of the reactive moieties are buried toward the smaller pore of the nanocavity, thereby driving a proximity-induced [4+2] photocycloaddition. Our results are in contrast to the typical alkyne hydration reaction or the linear dimerization products observed in the octahedral-shaped nanocavity, thereby highlighting the role of cavity shape in driving product selectivity.