Ketonization of enols in aqueous solution: is carbon protonation always rate-determining?

Excited-state antiaromaticity relief drives facile photoprotonation of carbons in aminobiphenyls

A combined computational and experimental study reveals that ortho-, meta- and para-aminobiphenyl isomers undergo distinctly different photochemical reactions involving proton transfer. Deuterium exchange experiments show that the ortho-isomer undergoes a facile photoprotonation at a carbon atom via excited-state intramolecular proton transfer (ESIPT). The meta-isomer undergoes water-assisted excited-state proton transfer (ESPT) and a photoredox reaction via proton-coupled electron transfer (PCET). The para-isomer undergoes a water-assisted ESPT reaction. All three reactions take place in the singlet excited-state, except for the photoredox process of the meta-isomer, which involves a triplet excited-state. Computations illustrate the important role of excited-state antiaromaticity relief in these photoreactions.

A new bi-radical species formed during the photochemical degradation of synthetic musk tonalide in water: Study of in-situ laser flash photolysis and validation of synthesized standard sample

The ubiquitous presence of synthetic musks is causing serious concern due to the species produced from their transformation and environmental impacts. In this study, tonalide was selected as a representative synthetic musk to evaluate the transformation mechanism and pathway in water under ultraviolet (UV) irradiation. The results showed that tonalide could undergo rapid photochemical degradation through a new pivotal bi-radical, which acts as the initial active species. The bi-radicals with a typical absorption peak at 340 nm was observed by in-situ laser flash photolysis technology, and the absolute decay rate constant was obtained as 3.61 ± 0.01 × 10⁹ M^-1 s^-1 with the life-time of 83.3 ns. The photochemical degradation by-products of tonalide were also identified by high-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry, and the precise structures of key by-products have been validated by our preparative synthesized standard samples confirmed by nuclear magnetic resonance. Thus, the mechanism of tonalide photochemical degradation, continuous photoenolization of the bi-radicals and followed cycloaddition reaction with O₂, was proposed as the predominant pathway. The main degradation by-product, photoenol which has a higher bioconcentration than that of tonalide, was found to form from the bi-radicals photoenolization. This study is the first work to propose a new bi-radical as the photoenol precursors during photochemical degradation of tonalide in water.