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

Abstract

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 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.

Helical Threads: Enantiomerically Pure Carbo[6]Helicene Oligomers

“…The synthesis of monomeric (AE)-2,15-bis[(triisopropylsilyl)ethynyl]carbo[6]helicenew as achieved in 25 %y ield over six steps. [6] The unexpectedly intense chiroptical properties displayed by thesed imeric systems were found to originate from an increase in the p-conjugated chromophore surface. The dimeric (+ +)-(P) 2 -a nd (À)-(M) 2 -configured and the tetrameric (+ +)-(P) 4 -a nd (À)-(M) 4configured oligomers were obtainedb ys equential oxidative acetylenic coupling.…”

One‐Step Synthesis of [16]Helicene

“…Crystal data of TIPSO- [16]helicene 2:M onoclinic space group P2 1 /c, T = 90(2) K, a = 20.518 (5), b = 20.834(5), c = 14.490(4) , b = 94.050(4)8 8, V = 6179(3) 3 , Z = 4, 1 calcd = 1.262 Mg m À3 , F(000) = 2496, reflections collected/unique 65 733/12 525 (R int = 0.1579). Thes tructure was solved by direct methods (SHELXS-97) and refined by full-matrix leastsquares methods (SHELXL-2014/7) on F 2 with 805 parameters.…”

“…13) have failed. [5] Thek ey to the success of the [16]helicene synthesis in this study is the rational design of precursor 1,i nw hich the phenylene and naphthylene units are arranged such that unfavorable side reactions are avoided.…”

“…Larger [n]subunits were not included in our precursor design because of increasing synthetic difficulty,and because the [6] + [6] precursor failed to furnish [13]helicene under any conditions,d espite favorable theoretical predictions. [5] We, however, avoided the simplest [1] + [1] + [1] and [2] + [2] sequences in the precursor to exclude the formation of an unfavorable [5]helicene framework that easily cyclizesi nto benzoperylene (Scheme 2a). [7] The[ 2] + [1] + [2] sequence was also avoided because it has been reported that this sequence predominantly gives planar dinaphthanthracene rather than the expected [7]helicene (Scheme 2b).…”

Enantiomerically Pure 5,13‐Dicyano‐9‐oxa[7]helicene: Synthesis and Study

“…Compounds with the rigid shape and twisted π-condensed ring systems, with the presence of different functionalities, tunable planarity are potential candidates for the study of their distinct photophysical and chiroptical properties. Several types of helical systems have been investigated with diverse photophysical properties such as luminescence, [2] circularly polarized luminescence, [3] fluorescence, [4] circular dichroism, [1a,5] etc., and the optical properties are discussed in few reviews. [1h,6] The helical compounds have also been used in asymmetric catalysis, self-assembly and biomolecular recognition.…”