Phosphahelicenes with (thio)phosphinic acid and ester functions via the oxidative photocyclisation approach

Phosphahelicenes with thiophosphinic acid and ester functions have been obtained via the oxidative photocyclisation of olefins bearing both a benzophenanthrene and a benzophosphole unit. When the method has been extended to olefins bearing a partially saturated benzophospholene unit, a divergent regioselectivity of the photocyclisation step has been observed, leading to new helicenes where the phosphorus function is located on the external rim of the helical backbone. The observed regioselectivity correlates well with the free-valence numbers of the atoms involved in the photocyclisation reaction (DFT calculations).