LAUSR

In domino reactions, the product formed in one step undergoes a subsequent transformation under identical reaction conditions. Owing to the spontaneous nature of these reactions, it is difficult to isolate the key intermediates, and these are thus usually presumed. Here we perform a photoinduced domino reaction consisting of three photochemical steps. First, oxidative photocyclisation of a tetrafluorostilbene derivative generates tetrafluoro[7]helicene, which readily undergoes a photoinduced intramolecular Diels–Alder reaction. The resulting product then undergoes a double fluorine atom transfer under the same photochemical conditions. As a result, the four originally adjacent fluorine atoms are separated into two pairs in the final product. One advantage of a photochemical domino process over a thermal one is that the process can be suspended and restarted. Hence, precise control of the irradiation time allows us to isolate the thermally stable intermediates and characterise them using X-ray crystallography, thus confirming the until-now putative domino process.Domino reactions can offer efficient synthetic access to complex products, but characterisation of unstable intermediates is often not possible. Here the photochemical domino cyclisation of tetrafluorostilbene is characterised by NMR and X-ray crystallography of each intermediate, and shown to terminate in an unusual double fluorine atom transfer reaction.