Stereospecific Synthesis of Enantiopure [6]Helicene Containing a Seven‐Membered Ring and [7]Helicene by Acid‐Promoted Stepwise Alkyne Annulations of Doubly Axial‐Chiral Precursors**

Abstract

Enantiopure [6]helicene containing an embedded seven‐membered ring and carbo[7]helicene (>99 % ee) with opposite helicity were simultaneously and quantitatively (>99 %) synthesized with a perfect stereospecificity through stepwise acid‐promoted intramolecular alkyne annulations of doubly axial‐chiral cyclization precursors. The helical handedness of the [6]‐ and [7]helicenes was fully stereocontrolled by the doubly axial chirality of the precursors as a result of complete axial‐to‐helical chirality transfer. The cyclizations proceeded in a stepwise manner; the first six‐membered ring formation was followed by the kinetically controlled seven‐ or six‐membered ring formation with or without helix‐inversion of a [4]helicene intermediate generated during the first cyclization step, thus quantitatively producing enantiopure circularly polarized luminescent [6]‐ and [7]helicenes with opposite helicity.

All Carbon Helicenes and π‐Extended Helicene Derivatives

“…In 2023, Yashima's group enantioselectively synthesized [6]helicenes 15 (>99 % ee ) containing a seven‐numbered ring and [7]helicenes 16 (>99 % ee ) from doubly axial‐chiral precursors in totally 99 % yield (Figure 6). [11] The results indicated that the acid‐promoted intramolecular alkyne annulations was stepwise and one hexagon was formed at first. This study once again proved that certain chiral precursors have the capability to produce helical molecules with a high enantiopurity.…”

“…In 2023, Martín's group generated bilayer [9]helical nanographene 51 and [11]helical nanographene 52 (Figure 22) with 4 step reactions in overall yield as 11.2 % and 16.1 %, respectively. [27] The UV-vis absorption spectra of 50-52 show peaks at 569, 557 and 518 nm, respectively.…”

Unravelling the Superiority of Nonbenzenoid Acepleiadylene as a Building Block for Organic Semiconducting Materials**

“…In recent years, a number of novel nonbenzenoid PAHs have been synthesized either in solutions [2] or on metal surfaces [3] . The remarkable synthetic achievements have enabled extensive investigations of the properties of these novel compounds, such as (anti)aromaticity, [4] open‐shell characters, [2g,5] and chiroptical responses [2b,u,6] . Furthermore, as exemplified by azulene derivatives, [7] the optoelectronic applications of nonbenzenoid PAHs have also been demonstrated, e.g.…”

Gold‐Catalyzed 1,2‐Aryl Shift and Double Alkyne Benzannulation

“…[21] This reaction has also been used to assemble S-doped [22,23] and Ndoped nanographenes, [24] and, more recently, for the construction of helicenes. [25][26][27][28] Alkyne benzannulation occurs at the free position adjacent to the biaryl linkage leading to the formation of new rings of various sizes. Depending on the conditions and the catalyst used in the reaction, 5-, 6-and 7membered rings can be formed through 5-exo-dig, 6-endo/ exo-dig and 7-endo-dig ring closure mechanisms, respectively (Figure 1a).…”

“…Among several emblematic examples, a spectacular illustration concerns the synthesis of a polymeric graphene nanoribbon by Chalifoux and co‐workers [21] . This reaction has also been used to assemble S‐doped [22,23] and N‐doped nanographenes, [24] and, more recently, for the construction of helicenes [25–28] . Alkyne benzannulation occurs at the free position adjacent to the biaryl linkage leading to the formation of new rings of various sizes.…”