Electronically Connected [n]Helicenes: Synthesis and Chiroptical Properties of Enantiomerically Pure (E)‐1,2‐Di([6]helicen‐2‐yl)ethenes

Abstract

AbstractWe synthesized stilbenoid (E)‐(P,P)‐ and (E)‐(M,M)‐[6]helicene dimers in enantiomerically pure form as part of a program aimed at the exploration of new strategies for the synthesis of large helicenes. The [2+2+2] cyclotrimerization of suitable triynes, reported by Starý and co‐workers, was applied for the preparation of a racemic 2‐hydroxymethylated [6]helicene precursor, which was conveniently resolved by HPLC on a chiral stationary phase. Two optically active helicenes were subsequently connected to the stilbenoid dimer by olefin metathesis. Electronic circular dichroism (ECD) studies of the stilbenoid ethen‐1,2‐diyl‐linked dimer revealed bathochromically shifted ECD bands with exceptionally large Cotton effects relative to those of the monomeric [6]helicenes or to that of a dimeric [6]helicene with a saturated ethan‐1,2‐diyl linker. These comparative studies, complemented by computational investigations, show that the pronounced chiroptical properties of the stilbenoid dimer originate from π‐conjugation between the two [6]helicene moieties and from the rigidity that results from conjugation. All attempts to form (P)‐[13]helicene by photocyclodehydrogenation of the stilbenoid (P,P)‐dimer were unsuccessful.