Abstract
A facile synthesis of carbo- and heterohelicenes was achieved via tandem cycloaromatization of bisacetal precursors, which were readily prepared through C–C bond formation by Suzuki–Miyaura coupling. This cyclization was efficiently realized by a catalytic amount of trifluoromethanesulfonic acid (TfOH) in a cation-stabilizing solvent, 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP), which readily allowed gram-scale syntheses of higher-order helicenes, double helical helicenes, and heterohelicenes.
Highlights
Helicenes are a class of polycyclic aromatic hydrocarbons (PAH) that consist of ortho-fused aromatic rings arranged in a helical manner [1,2,3,4,5,6]
It was noted that the gram-scale synthesis for both [5]- and [6]helicenes was possible, in which no serious decrease in yield was observed in Suzuki–Miyaura coupling and cycloaromatization [30]
We demonstrated the synthesis of unsymmetrical thia[6]helicene as a hetero[6]helicene synthesis (Scheme 6c). 3-Bromobenzothiophene bearing a (1,3-dioxolan-2yl)methyl group at the 2-position, which was readily prepared from benzothiophene, underwent Suzuki–Miyaura coupling with boronic acid ester with a biphenyl structure and an acetal moiety, to afford bisacetal in 92% yield
Summary
Helicenes are a class of polycyclic aromatic hydrocarbons (PAH) that consist of ortho-fused aromatic rings arranged in a helical manner [1,2,3,4,5,6]. Carbohelicenes such as [5]helicene (1a) and [6]helicene (1b) were synthesized via Suzuki–Miyaura coupling of readily available dihalogenated arenes 2 with phenylboronic acid ester 3 bearing a (1,3-dioxolan-2-yl)methyl group [29], followed by triflic acid-catalyzed cycloaromatization (Scheme 3, route a and Scheme 4a). The obtained bisacetal 4a successfully underwent cycloaromatization in the presence of 17 mol % of TfOH in HFIP to afford [5]helicene (1a) in 90% yield.
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