Abstract
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.
Highlights
In domino reactions, the product formed in one step undergoes a subsequent transformation under identical reaction conditions
Photochemical electrocyclisation of stilbene derivatives followed by thermal in situ dehydrogenation with iodine is the standard protocol for synthesising a variety of helicene homologues10–14
We have demonstrated that an elongated stilbene derivative with six vinylene spacers undergoes oxidative photocyclisation at six separate positions to afford [16]helicene, the longest known carbohelicene15
Summary
The product formed in one step undergoes a subsequent transformation under identical reaction conditions. A 1,3-diyne with a tethered alkynyl diynophile undergoes an intramolecular hexadehydro-Diels–Alder reaction to form a highly reactive benzyne intermediate, which is quickly trapped by a cycloalkane solvent. A 1,3-diyne with a tethered alkynyl diynophile undergoes an intramolecular hexadehydro-Diels–Alder reaction to form a highly reactive benzyne intermediate, which is quickly trapped by a cycloalkane solvent6,7 This trap reaction is an intermolecular double hydrogen atom transfer from the cycloalkane 2H-donor to the benzyne 2H-acceptor, and produces the corresponding cycloalkene and substituted benzene. The four neighbouring fluorine atoms on the same benzene ring of 1 are separated into two fluorine pairs in a single photochemical operation In this triple photochemical domino process, intermediates 2 and 3 are thermally stable and can be characterised by X-ray crystallography, which allows the elucidation of the otherwise putative domino process
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