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

We 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 Stary 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 pi-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.