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

摘要

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.