Configurationally stable dithia[7]helicene and dithia-quasi[8]circulene fused dithiolones

Abstract

Dithia-[7]helicene and quasi[8]circulene have been formed on the 1,3-dithiol-2-one ring by oxidative dehydrocyclisation of bis-naphthothiophenyl-1,3-dithiol-2-one. Quasi-circulenes are easily obtained at room temperature by successive dehydrocyclisations regardless the amount of oxidation reagent whereas selectivity towards helicene, involving a single dehydrocyclisation, has been achieved only at low temperature. DFT calculations suggest a mechanism via a dication intermediate rather than a mono-cation for the formation of the quasi-circulenes whereas the helicene should be formed via a radical cation intermediate. While dithia[7]helicene is known to give stable stereoisomers, here the unsubstituted dithia-quasi[8]circulene shows unprecedented configurational stability of the enantiomers, with a theoretically estimated enantiomerisation energy barrier of 30.4 kcal mol⁻¹, in excellent agreement with the experimentally determined value of 29.8 kcal mol⁻¹. The enantiomers of both helicene and quasi-circulene have been separated by chiral chromatography and their absolute configuration assigned from the calculated CD spectra.