Weak interaction alters the photochemical reactivity of 1,5-dienes within a supramolecular organic capsule

With the goal of testing the role of weak interactions and restricted space on photochemical reactions, intramolecular cross [2 + 2] addition of six 1,5-dienes (1–6, Scheme 1) was performed within the host octa acid (OA) capsule. Upon excitation of OA-diene complexes in water, photodimerization of the dienes occurred from their excited triplet states generated by energy transfer from the triplet of OA. OA encapsulation eliminates the need for diffusion required for triplet energy transfer and conformer exploration needed to bring the reactive orbitals of the diene closer. The 1,5-diene substituted with para-tolyl group on either side (6) fails to react within OA capsule, while in solution it does. Also, the reactivity order was found to depend on the number of para-methyl substituent on the diene, 4>5>>6. Based on molecular dynamics simulated structures and NOESY correlations we conclude that the para-methyl groups anchor the diene at the narrow ends of the capsule through weak C–H–π interaction. Such anchoring forbid the methyl group to relocate within the capsule during the conversion of the 1,5-diene to more compact [2.1.1] bicyclic product. 1D and 2D NMR spectra confirmed the inclusion of the dienes and their likely structure within the capsule.