Photoreactions of 1,4-Naphthoquinones: effects of substituents and water on the intermediates and reactivity

The photochemistry of lapachol and other 1,4-naphthoquinone (NQ) derivatives, e.g. 2-methoxy-1,4-naphthoquinone (MeONQ), 2-hydroxy-1,4-naphthoquinone (2-HONQ) or 5-hydroxy-1,4-naphthoquinone (5-HONQ) and 2-methyl-5-hydroxy-1,4-naphthoquinone (P-NQ) in solution at room temperature was studied by ultraviolet-visible spectroscopy after nanosecond laser pulses at 248 nm. The triplet state and semiquinone radicals were observed for MeONQ, HONQ and P-NQ, whereas for lapachol, intramolecular H-atom and charge transfer processes take place, as in the case of vitamin K1. The photoinduced reaction of NQ into HONQ is initiated by nucleophilic water addition to the triplet state, and for the secondary reactions, a modified mechanism is proposed.