Visible light-induced diastereoselective E/Z-photoisomerization equilibrium of the C=C benzofuran-3-one-hydantoin dyad

The diastereoselective photodependent isomerization equilibrium of E/Z‐1,3‐ditolyl‐5‐[3‐oxobenzofuran‐2(3H)‐ylidene]imidazolidine‐2,4‐dione (5) is reported. Both diastereomers E-5 and Z-5 are stereochemically stable in solid state but show significant photosensibility in solutions of halogenated solvent. The photoisomerization equilibrium of E/Z‐5 is therefore deduced from the 1H NMR profile after visible‐light irradiation of both E-5 and Z-5 samples. The results of the kinetic study, monitored by UV‐HPLC, reveal that the E/Z equilibrium is diastereoselective and photodependent, being the transformation E ➔ Z proceeding faster than that of Z ➔ E, and the E/Z ratio at the equilibrium depends on the used solvent, light source, and temperature. Both diastereomers are visible‐light photosensitive tending to coexist together in equilibrium solutions at a determined ratio, which is always in favor of the Z‐product assuming a minimum thermodynamic energy and an increased entropy of the system. Time‐dependent density functional theory calculations suggest that the photoisomerization mechanism proceeds via a conical intersection involving the first‐excited state: Upon irradiation, the E-5 isomer is excited to the S1 potential energy surface, where it relaxes through rotation of the C=C bond and reaches a conical intersection with the ground‐state potential energy surface, thus yielding the Z-5 isomer. Copyright © 2014 John Wiley & Sons, Ltd. The visible‐light‐induced E/Z‐photoisomerization equilibrium of the bicyclic C=C benzofuran‐3‐one‐hydantoin olefin E/Z‐5 is studied experimentally and by electronic time‐dependent density functional theory calculations. The photoisomerization is mostly in favor of the Z‐product.