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

Abstract

AbstractOne of the most straightforward strategies nowadays for the synthesis of carbo‐ and hetero‐helicenes is the oxidative photocyclization of stilbene derivatives under Mallory conditions. In this study, the reactivity of a series of 3,4‐dihalostyryl‐naphthalenes (Hal=Br, F and Cl) has been investigated using 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 15–30 kcal/mol, was until now “hidden” in the non‐substituted series, it allows herein to explain its re‐aromatization through formal elimination of HX in order to provide the 2‐mono‐halogenated‐[4]helicene derivatives.