Consecutively fused single‐, double‐, and triple‐expanded helicenes

Abstract

We report the quantitative, chemoselective, and diastereospecific formation of the defect-free C-shaped single and S-and M-shaped consecutively fused doubleand triple-expanded homochiral helicenes (all-right (P) or all-left (M) handed), in which two and three expanded helicene subunits were exclusively fused together in a helically winding manner, respectively, through the trifluoroacetic acid-promoted intramolecular multistep cascade alkyne benzannulations of anthracene-based cyclization precursors composed of a large number of complicated dynamic stereoisomers due to axial chirality. The resulting racemic expanded helicenes were all separated into enantiomers by chiral chromatography and showed intense circular dichroism and circularly polarized luminescence (CPL) with the absorption and luminescence dissymmetry factors of up to 1.5 × 10 −2 and 0.94 × 10 −2 , respectively. Moreover, the triple-expanded helicene showed the highest CPL brightness (255 M −1 ⋅cm −1 ) among all the reported CPL-active carbo-and heterohelicenes and -helicenoids. The helicity inversion barriers remarkably increased with the increasing number of incorporated helicene subunits, and the racemization of the triple-expanded helicene was completely suppressed even at 80 • C, providing the first example of an optically active expanded helicene with a static helical chirality.

A π-extended phenanthrene-fused aza[7]helicenium as a novel chiroptically-active architecture in organic and aqueous media

“…The modulation of helicene opto-electronic properties can be envisaged by two means: i. via the introduction of one or more heteroatoms in the π-conjugated structure, resulting in heterohelicenes such as phosphahelicenes, azahelicenes, borahelicenes and silicahelicenes with strongly modified chiroptical response compared to carbohelicenes; 10 ii. by enhancing their specific chiroptical properties through the design of multiple helicenes 11 or π-extended systems, 12 thereby promoting large π-electron delocalization and affording unique optoelectronic properties. However, the increase in the number of fused aromatic rings may give rise to solubility issues and limit their study to organic media exclusively.…”

Expanded [23]-Helicene with Exceptional Chiroptical Properties via an Iterative Ring-Fusion Strategy

“…Yashima and co-workers have shown that fusing multiple helical anthracenes together also effectively enables the separation of enantiomers, but these multiple helicenes display inferior |g abs | of ≤0.015. 32 Notably, the original expanded helicenes illustrated in Figure 1a have larger diameters than helical anthracenes (∼13.5 vs ∼11.5 Å), suggesting that they should show further improvement in |g abs |; however, a similar noncovalent locking strategy was attempted on such an expanded helicene that failed to provide configurational stability. 33 Lengthening the helicene backbone also increases ΔG ‡ e , but this poses a considerable synthetic challenge.…”

“…This compound displayed a maximum CD dissymmetry factor, | g abs |, of 0.024 at 352 nm, supporting the theory that expanded helicenes should display superior chiroptical properties. Yashima and co-workers have shown that fusing multiple helical anthracenes together also effectively enables the separation of enantiomers, but these multiple helicenes display inferior | g abs | of ≤0.015 . Notably, the original expanded helicenes illustrated in Figure a have larger diameters than helical anthracenes (∼13.5 vs ∼11.5 Å), suggesting that they should show further improvement in | g abs |; however, a similar noncovalent locking strategy was attempted on such an expanded helicene that failed to provide configurational stability …”

Facile Synthesis and Chiral Resolution of Expanded Helicenes with up to 35 cata-Fused Benzene Rings

“…Later, Yashima et al reported the facile synthesis of several consecutively fused expanded helicenes (D) through the trifluoroacetic acid-promoted intramolecular multistep cascade alkyne benzannulations. 8 Notably, the triple-expanded helicene (n=3) showed persistent chirality with the absorption and luminescence dissymmetry factors of up to 1.5 × 10 −2 (|gabs|) and 0.94 × 10 −2 (|glum|), respectively. These studies imply promising chiroptical properties of expanded helicenes but the synthesis of even longer expanded helicenes remains very challenging.…”