Solid-State Photocycloaddition Reactions between Di-2-Pyrones Tethered by Arylalkyl Chain and Benzophenones

Solid-state photocycloaddition reactions between di-2-pyrones (1a-d) with benzophenone (2a) gave the corresponding oxetane derivatives (3a-d; 1:2 adducts) with high site- and regioselectivity across the C5-C6 and C5′-C6′ double bonds in 1 via the triplet excited state of 2a. The reactions were inferred by MO methods to be initiated by electrostatic interaction between the C6 position of 1a-d and the carbonyl oxygen of 2a at their ground states, and the solid-state interaction may be enhanced by electron density at carbonyl oxygen of the triplet 2a. The transition state (TS) analysis of the [2+2] cycloaddition reactions also suggested some triplet complex, and the high regioselectivity. The hydrogen-bonding interaction between 2a and 1a-d and the triplet reaction mechanism were also explained by the IR analyses and the quenching experiments, respectively.