Enantioselective synthesis and racemization dynamics of trithia[5]helicenes derived from the dithieno[2,3- <i>b</i> :3′,2′- <i>d</i> ]-thiophene unit

A series of configurationally stable trithia[5]helicenes having the dithieno[2,3-<i>b</i>:3',2'-<i>d</i>)-thiophene unit embedded in their skeletons has been prepared <i>via</i> a Au-catalyzed double alkyne hydroarylation reaction. The transformation proceeds with excellent yield, exquisite regioselectivity and high enantioselectivity. The absolute configuration of the newly prepared helicenes has been established by X-ray crystallography, and their inversion barriers were experimentally determined. Preliminary studies on the post-synthetic functionalization of the new structures prepared indicate that only the two peripheral thiophene units are oxidized to the corresponding sulfones, while their bromination occurs selectively at positions 12 and 15 of the bis-benzannulated bis(benzo[4,5]thieno)[2,3-<i>b</i>:3',2'-<i>d</i>]thiophene core.