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.

Pyrene bridged double[7]helicene embedded with a heptagonal ring

“…Overcoming the shortcomings of pristine [n]helicenes, helicene fused polycyclic aromatic hydrocarbons (PAHs) have shown a significant increase in the overall functional properties. [11][12][13][14][15][16][17][18][19][20][21][22][23][24] Moreover during the last few years multiple [n]helicene-embedded nanographenes with numerous aromatic cores have been developed. 23,[25][26][27][28][29][30] Preserving their core properties, multi[n]helicene showed unique characteristics due to an increased π-conjugation and molecular packing.…”

“…Several stable conformers are possible for multiple [n]heliceneembedded PAHs, hence the isolation and availability of enantiopure multi[n]helicenes are cumbersome. 22,23 To overcome these challenges, we follow a stereospecific synthetic pathway, which allows us to synthesize configurationally stable enantiopure helicene-containing functional molecules. 19,20,30,34 Recently, we reported stereospecific synthesis of C 2 and C 1 symmetric pyrene fused [5]helicenes 19 and [7]helicenes, 20 with excellent stereoselectivity.…”