SET and HAT/PCET acid‐mediated oxidation processes in helical shaped fused bis‐phenothiazines

Helical shaped fused bis‐phenothiazines 1–9 have been prepared and their red‐ox behaviour quantitatively studied. Helicene radical cations (Hel(.+)) can be obtained either by UV‐irradiation in the presence of PhCl or by chemical oxidation. The latter process is extremely sensitive to the presence of acids in the medium with molecular oxygen becoming a good single electron transfer (SET) oxidant. The reaction of hydroxy substituted helicenes 5–9 with peroxyl radicals (ROO(.)) occurs with a ‘classical’ HAT process giving HelO(.) radicals with kinetics depending upon the substitution pattern of the aromatic rings. In the presence of acetic acid, a fast medium‐promoted proton‐coupled electron transfer (PCET) process takes place with formation of HelO(.) radicals possibly also via a helicene radical cation intermediate. Remarkably, also helicenes 1–4, lacking phenoxyl groups, in the presence of acetic acid react with peroxyl radicals through a medium‐promoted PCET mechanism with formation of the radical cations Hel(.+). Along with the synthesis, EPR studies of radicals and radical cations, BDE of Hel‐OH group (BDE(OH)), and kinetic constants (k(inh) ) of the reactions with ROO(.) species of helicenes 1–9 have been measured and calculated to afford a complete rationalization of the redox behaviour of these appealing chiral compounds.