Chimerical Pyrene-Based [7]Helicenes as Twisted Polycondensed Aromatics

• Published in: Chemistry : a European journal Vol. 21; no. 24; pp. 8910 - 8917
• Main Authors: Buchta, Michal, Rybáček, Jiří, Jančařík, Andrej, Kudale, Amit A., Buděšínský, Miloš, Chocholoušová, Jana Vacek, Vacek, Jaroslav, Bednárová, Lucie, Císařová, Ivana, Bodwell, Graham J., Starý, Ivo, Stará, Irena G.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 08.06.2015; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: alkynes; asymmetric synthesis; Chemistry; Chemistry, Multidisciplinary; chirality; Crystal structure; Cyclic compounds; cycloisomerisation; Fluorescence; helical structures; Helicity; Isomerization; Mathematical analysis; Physical Sciences; Pyrenes; Science & Technology; Synthesis; ASYMMETRIC-SYNTHESIS; chirality; ENANTIOSELECTIVE SYNTHESIS; HEPTA-HELICENE; alkynes; helical structures; PHOTOCHEMICAL-SYNTHESIS; CRYSTAL-STRUCTURES; STEREOSELECTIVE SYNTHESES; PHOTOPHYSICAL PROPERTIES; ONE HUNDRED YEARS; asymmetric synthesis; THERMAL RACEMIZATION; cycloisomerisation; 2+2+2 CYCLOISOMERIZATION
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201500826

Chimerical pyrene‐based dibenzo[7]helicene rac‐1 and 2H‐pyran[7]helicene (M,R,R)‐(−)‐2, in which two pyrene subunits are fused to the [7]helicene/[7]heterohelicene scaffold, were synthesised by means of Ni0‐ or CoI‐mediated [2+2+2] cycloisomerisation of dipyrenyl‐acetylene‐derived triynes. Pyrene‐based dibenzo[7]helicene 1 was obtained in enantioenriched form by enantioselective cycloisomerisation under Ni0/QUINAP catalysis (57 % ee) or in enantiopure form by racemate resolution by liquid chromatography on a chiral column. 1,3‐Allylic‐type strain‐controlled diastereoselective cycloisomerisation was employed in the synthesis of enantiopure (M,R,R)‐(−)‐2. Physicochemical properties of 1 and 2 encompassing the helicity assignment, stability to racemisation, X‐ray crystal structure, UV/Vis, experimental/calculated electronic circular dichroism and fluorescence spectra were studied. Accordingly, comparison of the X‐ray crystal structure of (M,R,R)‐(−)‐2 with calculated structures (DFT: B3LYP/cc‐pVDZ, B97D/cc‐pVDZ) indicated that its helical backbone is slightly over‐flattened owing to intramolecular dispersion forces between tert‐butylated pyrene subunits. Both 1 and 2 are fluorescent (with quantum yields in dichloromethane of ΦF=0.10 and 0.17, respectively) and are suggested to form intramolecular excimer states upon excitation, which are remarkably stabilised and exhibit large Stokes shifts (296 and 203 nm, respectively). Do the twist: Two pyrene units can be incorporated into dibenzo‐ or 2H‐pyran[7] helicene scaffolds to form a new class of twisted polycondensed aromatics (see figure). A key process employed in their synthesis is Ni0‐ or CoI‐mediated [2+2+2] cycloisomerisation of dipyrenyl triynes. The chimerical pyrene‐based [7]helicenes could be obtained in enantiopure form through racemate resolution or asymmetric synthesis. Their physicochemical properties have been studied.