An efficient pathway to high persistence length helicenes from scalable [4]-helicene synthons

Abstract

A convergent synthetic strategy to high-persistence length helicenes via a regioselective and scalable menthyloxycarbonato-[4]-helicene synthon accessed in 84% yield is reported. To demonstrate the utility of the [4]-helicene building block, bis(menthyloxycarbonato)-[11]-helicene diastereomers were prepared via palladium-mediated cross-couplings, followed by a Mallory-type photo-induced annulation reaction. Our synthetic strategy leverages Stille and Heck couplings to synthesize bis(aryl)ethene precursors that exhibit preferential formation of helical products via 6π-electrocyclization. [11]-Helicene product yields are enhanced from 7 to 42% by maintaining photoreaction temperatures above 40 °C limiting linear and cyclobutane byproduct formation while recrystallization yielded diastereo-enrichment. This work enables the end goal of embedding enantiopure hydroxy-terminated helicenes into polymer backbones to create chiroptical responsive strain sensors.