Synthesis of the Longest Carbohelicene by Multiple Oxidative Photocyclizations of Arylene–Vinylene Oligomers

Abstract

Oxidative photocyclization of stilbene-type precursors is still a powerful method for the synthesis of longer helicenes. Only the simplest subunits [1] and [2] (phenylene and naphthylene units) are arranged in the precursor to avoid unfavorable side reactions in photocyclization. Based on the synthetic guideline for such arylene–vinylene oligomers, [9]helicene was efficiently prepared by triple photocyclization of the precursor with a [2]+[1]+[1]+[2] sequence (where “+” denotes vinylene linkers). Moreover, the longest [16]helicene was prepared by sextuple photocyclization of the precursor with a [2]+[1]+[1]+[2]+[1]+[1]+[2] sequence. Other precursors with different sequences lowered the product yield in spite of the decreased number of cyclization. X-ray crystallographic analysis clearly showed the triple-layer structure of [16]helicene. The judiciously designed precursor has an intrinsic propensity to exhibit spontaneous helical folding under photoirradiation, which helps the formation of helicene skeletons.