Wavelength‐Gated Photochemical Synthesis of Phenalene Diimides

Herein, we pioneer a wavelength-gated synthesis route to phenalene diimides. Consecutive Diels-Alder reactions of methylisophthalaldehydes and maleimides afford hexahydro-phenalene-1,6-diol diimides via 5-formyl-hexahydro-benzo[f]isoindoles as the intermediate. Both photoreactions are efficient (82-99 % yield) and exhibit excellent diastereoselectivity (62-98 % d.r.). The wavelength-gated nature of the stepwise reaction enables the modular construction of phenalene diimide scaffolds by choice of substrate and wavelength. Importantly, this synthetic methodology opens a facile avenue to a new class of persistent phenalenyl diimide neutral radicals, constituting a versatile route to spin-active molecules.