Short-lived radical characterisation: novel radical trap synthesis, application and methodology development

Radical intermediates play a key role in many chemical processes. However, existing methods for their characterisation have flaws that limit mechanistic and kinetic understanding of these processes, especially for short-lived radicals. A new radical characterisation technique was developed which used novel radical traps, consisting of an allyl group attached to a leaving group, which formed a stable radical upon cleavage. Reaction of a radical with novel radical trap formed a stable radical and non-radical product containing the reactant radical, which was then characterised by conventional techniques, such as NMR spectroscopy and MS. Novel radical trapping was used to successfully detect and characterise a diverse array of short-lived and long-lived radical intermediates across a wide range of radical reactions, including synthetic, biochemical and atmospheric radical reactions, offering valuable mechanistic and kinetic insights. Experiments indicated that novel radical trapping did not lead to false positives, in contrast to most existing short-lived radical characterisation techniques. Full characterisation of an isolated trapped phenylthiyl radical confirmed the trapping mechanism occurred as expected. Novel radical trapping indicated the radical resting state for different substrates in Ru-photocatalysed radical thiol-ene addition and enabled an initiation mechanism to be hypothesised for catalyst-free photoinitiated radical dearomative spirocyclisation. The antioxidant activity of ascorbic acid was probed using aqueous novel radical trapping. Observations from novel radical trapping of gaseous α-pinene ozonolysis offered validation to mechanisms hypothesised but not widely accepted in literature. Detection limits of gaseous [RO2●] using novel radical trapping were estimated to be >1×10^9 molec. cm^-3 (S/N = ~2, 10 min), which would be suitable for some atmospheric field measurements. These investigations demonstrated the viability of novel radical trapping as a tool to investigate any radical reaction. It is hoped that chemists will widely adopt this technique to improve understanding and aid development of reactions involving radical intermediates.