Utilizing Visible Light to Enable Contra-Thermodynamic Skeletal Rearrangement for Dearomative meta-Cycloadditions of 2-Acetonaphthalenes via Triplet Energy Transfer Cascade

Abstract

With the renaissance of visible light-mediated triplet-triplet energy transfer (VLEnT) catalysis as a greener and milder catalytic regime, dearomative cycloadditions (DAC) have emerged as a powerful arsenal in arriving at sophisticated three-dimensional molecular scaffolds. With ortho- and para-variants having been well documented under VLEnT catalysis and dearomative meta-cycloadditions being known to be symmetry allowed in the excited singlet potential energy surface under harsher UV irradiations, the prospective [3 + 2] dearomative cycloadditions propelled via a VLEnT catalysis remains elusive. Herein, we report a formal dearomative meta-cycloaddition of 2-acetonaphthalenes propagated via a two-step VLEnT cascade circumventing the attainment of energetically higher singlet excited states. The work showcases the judicious selection of photosensitizer backed by DFT calculations to selectively promote the [4 + 2] DAC followed by a contra-thermodynamic stepwise skeleton rearrangement cascade. The detailed DFT studies in conjugation with electrochemical, photoluminescence, kinetic, quadratic dependency, and control experiments support the VLEnT cascade. The observed meta-selectivity over ortho- and para-DAC was rationalized by the reactivity difference in the singlet and triplet spin surfaces. The described protocol delivers highly sp3-rich polycyclic frameworks in high yields and moderate selectivities with wide functional group tolerance. The inclusion of bioactive agents and the establishment of a wide array of post-synthetic derivatizations further climaxes the efficiency of the designed protocol.