Notable substituent and temperature effects on the regioselectivity and the efficiency in Paternò-Büchi reaction of 4,4′-disubstituted benzophenones with 1,3-dimethyluracil and 1,3-dimethylthymine

The photochemical [2 + 2] cycloadditions (the so-called Paternò-Büchi (PB) reaction) of 1,3-dimethyluracil (DMU) and 1,3-dimethylthymine (DMT) with benzophenone (1d) and its derivatives (BPs) with 4,4′-disubstituted groups including CH3O (1a), CH3 (1b), t-Bu (1c), F (1e), Cl (1f) and CN (1g) generate two series of regioisomeric oxetanes, A and B, and reveal notable substituent effects on the regioselectivity (A:B) and the photochemical efficiency, that is a higher ratio of A:B and lower efficiency for the BPs with electron-donating groups (EDGs), and a contrary effect for the BPs with electron-withdrawing groups (EWGs), such as A:B ≥ 95:5 for 1a to 32:68 for 1g in DMU photochemical system. The PB reactions of DMU with three BPs (1d, 1e and 1g) were performed at various temperatures, and temperature effects on the regioselectivity and the efficiency were observed. The ratio of photoproducts (A:B) in the DMU systems decreases with increasing the reaction temperature, such as DMU–1d system, ranging from 75:25 at −30 °C to 55:45 at 60 °C. On the basis of computational studies, the substituent effects were relationalized in terms of stability and charge distribution of intermediary triplet diradicals. The EWGs can stablize triplet diradicals especially the diradicals causing oxetanes B, and a destablization for EDGs. It was demonstrated that temperature effects originate from the conformational properties of intermediatary triplet diradicals.