Synthesis and X-ray Analysis of New [5]Helicenes – HMO Calculations on the Photocyclization of the Stilbene Precursors

Abstract

The syntheses of the new pentahelicenes 5, 11, 17, 21, and 28 with various substituents are described. In the case of 2,13‐dicyano‐[5]helicene (11) optical resolution was achieved by HPLC using a column packed with γ‐cyclodextrin. However, the enantiomers racemized within a few hours. On the other hand, the enantiomers of 28 turned out to be stable after separation on triacetylcellulose using MPLC. The crystal structures of 11, 17, and 21 were solved and indicated the typical distortions which are expected for helicenes. The model of the sum of free valence numbers was applied in order to rationalize the reactivity pattern of the photochemical phenanthrene cyclization.

Crystal structure of 3,13-dimethoxy-5,6,10,11-tetrahydrofuro[3,4-i][5]helicene-7,9-dione

“…The geometric parameters of the title molecule agree well with those reported for similar structures (McIntosh et al, 1954;Wang et al, 1997;Stammel et al, 1999;Rajapakse et al, 2011). The asymmetric unit of the title compound contains two independent molecules (A and B), as shown in Fig.…”

3,12-Dimethoxy-5,6,9,10-tetrahydro-[5]helicene-7,8-dicarbonitrile

“…The geometric parameters of the title molecule agree well with reported similar structures (McIntosh et al, 1954;Wang et al, 1997;Stammel et al, 1999;Rajapakse et al, 2011). Half a molecule of the title compound belongs to asymmetric unit and a molecule is completed by the crystallographic twofold axis as shown in Figure 1…”

Synthesis, characterization, and photophysical properties of a new pentacyclic helicene

“…This band exhibits lower extinction coefficient as compared to the absorption bands in the higher energy area. To our knowledge, the absorption in this region does not appear in the case of other pentahelicene derivatives [14,15], but it characterizes the dicyano-substituted ones [10]. This absorption is attributed to the intramolecular charge transfer given that 9 contains a strong electron-withdrawing cyano group [16].…”