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Reactivity and Mechanistic Issues in the Photocyclization of Dihalostyryl‐Naphthalenes towards Halo‐[4]helicenes: A Transposition on a Mallory Theme

Abstract : One of the most straightforward strategies nowadays for the synthesis of carbo-and heterohelicenes is the oxidative photocyclisation of stilbene derivatives in the Mallory conditions. In this study, the reactivity of a series of 3,4-dihalostyryl-naphthalenes (Hal = Br, F and Cl) has been investigated in the Mallory conditions towards the corresponding halogenated [4]helicene compounds. The difluoro precursor afforded the two isomeric 2,3-and 1,2-difluoro-[4]helicenes, resulting from the ring closure on the two possible positions of the substituted benzene ring, the dichloro compound led to the formation of a mixture of the two isomeric 2,3 and 1,2-dichloro-[4]helicenes together with the 2-mono-chloro derivative, while the dibromo precursor provided, unexpectedly, only 2,3-dibromo-[4]helicene and the 2-mono-bromo derivative. DFT calculations performed on the entire series of precursors, final helicenes and intermediate dihydrohelicenes, including the non-halogenated congeners, reveal the highly energetically favorable formation of a second dihydrohelicene intermediate following a [1,9]hydrogen sigmatropic transposition. Whereas the existence of this transposed dihydrohelicene intermediate, more stable than the initially formed dihydrohelicene by fifteen to thirty kcal/mol, was hitherto "hidden" in the non-substituted series, it allows here to explain its re-aromatization through formal elimination of HX in order to provide the 2-mono-halogenated-[4]helicene derivatives.
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https://hal.univ-angers.fr/hal-03452633
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Soumis le : samedi 27 novembre 2021 - 10:21:04
Dernière modification le : dimanche 26 juin 2022 - 03:21:49
Archivage à long terme le : : lundi 28 février 2022 - 18:18:10

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Kévin Martin, Caroline Melan, Thomas Cauchy, Narcis Avarvari. Reactivity and Mechanistic Issues in the Photocyclization of Dihalostyryl‐Naphthalenes towards Halo‐[4]helicenes: A Transposition on a Mallory Theme. ChemPhotoChem, Wiley, 2021, ⟨10.1002/cptc.202100215⟩. ⟨hal-03452633⟩

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