Regiodivergent Synthesis and π‐Stacking‐Induced Chiral Self‐Recognition of Hexabenzocoronene‐Based [6]Helicenes

Abstract

The lateral π-extension of helicenes would yield interesting physical properties derived from the enhanced π-stacking ability as well as intrinsic helical chirality. Here we report the regiodivergent synthesis of hexabenzocoronene (HBC)-based [6]helicenes via cationic rhodium(I)/segphos complex-mediated intramolecular [2 + 2 + 2] and [2 + 1 + 2 + 1] cycloadditions of triynes followed by Scholl reaction. The resulting HBC-based[6]helicenes possess stable helical chirality but more planar structure than previously reported HBC-based [7] and [9]helicenes. Therefore, these HBC-based [6]helicenes exhibit πstacking-induced chiral self-recognition in solution, allowing the measurement of enantiomeric ratios by 1 H NMR spectroscopy without any chiral additive.

Enantioselective Helicene Synthesis by Rhodium-Catalyzed [2+2+2] Cycloaddition

“…In the cycloaddition reaction of triyne 19 with a reduced number of fused rings, [2+2+2] and [2+1+2+1] cycloaddition products 20 and 21 were obtained in similar yields (44% and 48%, respectively, Scheme 10). 25 In the cycloaddition of triyne 22 containing a further reduced number of fused rings, dibenzo [5]helicene 23 was obtained as a single product, and no generation of the corresponding [2+1+2+1] cycloaddition product was observed (Scheme 10). 25 The Scholl reaction of the thus obtained dibenzo [6]helicene 20 gave hexabenzocoronene (HBC)─ based [6]helicene 24 with extended π ─ conjugation without decrease in ee (Scheme 11).…”

“…25 In the cycloaddition of triyne 22 containing a further reduced number of fused rings, dibenzo [5]helicene 23 was obtained as a single product, and no generation of the corresponding [2+1+2+1] cycloaddition product was observed (Scheme 10). 25 The Scholl reaction of the thus obtained dibenzo [6]helicene 20 gave hexabenzocoronene (HBC)─ based [6]helicene 24 with extended π ─ conjugation without decrease in ee (Scheme 11). 25 The Scholl reaction of [2+1+2+1] cycloaddition product 21 also gave an HBC─ based [6]helicene 25 with different substituent positions (Scheme 11).…”

“…25 The Scholl reaction of the thus obtained dibenzo [6]helicene 20 gave hexabenzocoronene (HBC)─ based [6]helicene 24 with extended π ─ conjugation without decrease in ee (Scheme 11). 25 The Scholl reaction of [2+1+2+1] cycloaddition product 21 also gave an HBC─ based [6]helicene 25 with different substituent positions (Scheme 11). 25 For these HBC─ based [6]helicenes 24 and 25, we observed π ─ stacking─ induced chiral self─ recognition in the solution state.…”

“…25 The Scholl reaction of [2+1+2+1] cycloaddition product 21 also gave an HBC─ based [6]helicene 25 with different substituent positions (Scheme 11). 25 For these HBC─ based [6]helicenes 24 and 25, we observed π ─ stacking─ induced chiral self─ recognition in the solution state. Thus, we could determine the enantiomeric ratios of 24 and 25 by 1 H NMR spectroscopy without any chiral additive.…”

“…Thus, we could determine the enantiomeric ratios of 24 and 25 by 1 H NMR spectroscopy without any chiral additive. 25 In the cationic rhodium(I) complex─ catalyzed intramolecular [2+2+2] cycloaddition reaction of 2─ phenylnaphthalene─ linked triyne 16 (Scheme 9), a signi cant ligand effect was observed on the selectivity between the [2+2+2] and [2+1+ 2+1] cycloaddition reactions. On the other hand, rather than the ligands, substituents on the carbonyl groups dominate the ratios of [2+2+2] and [2+1+2+1] cycloaddition products in the reactions of electron─ poor 2─ phenylnaphthalene─ linked triynes 26a─ d, possessing carbonyl groups at the alkyne ter-Scheme 10.…”