Photochemical generation and lifetimes in water of p-aryloxy- and p-alkoxyphenylnitrenium ions

Abstract

This paper describes product and flash photolysis studies following irradiation in aqueous solution of 4X–C₆H₄N₃ [X = MeO (12a), EtO (12b), PrⁱO (12c), Bu^tO (12d), C₆H₅O (12e), 4-MeOC₆H₄O (12f), F, Cl] and 4-methoxy-1-naphthyl azide (15). p-Benzoquinone (or 1,4-naphthoquinone) is observed as a product, in yields of 70–90% with 12a–d, 15, 40% with 12e, 26% with 4-F and 15% with 4-Cl. The quinone arises by a pathway whereby the initially-formed singlet arylnitrene is quenched by protonation by a solvent water molecule to form a nitrenium ion. Hydration of this cation at the para position leads through a hemiacetal (or halohydrin) to the quinone imine, whose hydrolysis results in the final quinone product. Three kinetic processes are observed, the nitrenium hydration on the µs time scale, the hemiacetal breakdown on the ms time scale, and the imine hydrolysis on the minutes time scale. The nitrenium ions have lifetimes in aqueous solution of 50 ns (4-PhO), 70 ns (4-MeOC₆H₄O), 370 ns (4-MeO), 550 ns (4-EtO), 1.25 µs (4-PrⁱO), 1.56 µs (4-Bu^tO) and 1.35 µs (4-methoxynaphthyl). A nitrenium transient is not observed with the 4-halophenyl azides, probably because the lifetime is too short for detection with ns laser flash photolysis (LFP). The alkoxyphenylnitrenium ions are argued to be better represented as oxocarbocations derived from O-alkylation of the quinone imine. The 4-ethoxyphenylnitrenium ion is not quenched by 0.01 mol dm^–3 2′-deoxyguanosine, so that k₂(dG) is less than 2 × 10⁷ mol ^–1 dm³ s^–1. This contrasts with the 4-biphenylylnitrenium ion, which has a similar solvent reactivity, but reacts with k₂(dG) = 2 × 10⁹ mol^–1 dm³ s^–1. The localization of the positive charge in the alkoxy system is a possible explanation behind this difference.