Synthesis and structural and photoswitchable properties of novel chiral host molecules: Axis chiral 2,2′-dihydroxy-1,1′-binaphthyl-appended stiff-stilbene

(Chemical Equation Presented) Novel photoswitchable chiral hosts having an axis chiral 2,2′-dihydroxy-1,1′-binaphthyl (BINOL)-appended stiff-stilbene, trans-(R,R)- and -(S,S)-1, were synthesized by palladium-catalyzed Suzuki-Miyaura coupling and low-valence titanium-catalyzed McMurry coupling as key steps, and they were fully characterized by various NMR spectral techniques. The enantiomers of trans-1 showed almost complete mirror images in the CD spectra, where two split Cotton effects (exciton coupling) were observed in the β-transitions of the naphthyl chromophore at 222 and 235 nm, but no Cotton effect was observed in the stiff-stilbene chromophore at 365 nm. The structures of (R)-10 and trans-(R,R)-1 were confirmed by X-ray structural analysis. The optimized structure of cis-1 by MO calculations has a wide chiral cavity of 7-8 Å in diameter, whereas trans-1 cannot form an intramolecular cavity based on the X-ray data. Irradiation of (R,R)-trans-1 with black light (λ = 365 nm) in CH₃CN or benzene at 23°C led to the conversion to the corresponding cis-isomer, as was monitored by ¹H NMR, UV - vis, and CD spectra. At the photostationary state, the cis-1/ trans-1 ratio was 86/14 in benzene or 75/25 in CH₃CN. On the other hand, irradiation of the cis-1/trans-1 (75/25) mixture in CH₃CN with an ultra-high-pressure Hg lamp at 23°C (λ = 410 nm) led to the photostationary state, where the cis-1/trans-1 ratio was estimated to be 9/91 on the basis of the ¹H NMR spectra. The cis-trans and trans-cis interconversions could be repeated 10 times without decomposition of the C=C double bond. Thus, a new type of photoswitchable molecule has been developed, and trans-1 and cis-1 were quite durable under irradiation conditions. The guest binding properties of the BINOL moieties of trans- and cis-(R,R)-1 with F ^-, Cl^-, and H₂PO₄^- were examined by ¹H NMR titration in CDCl₃. Similar interaction with F^- and Cl^- was observed in trans-1 (host/guest = 1/1, K_assoc = (1.0 ± 0.13) × 10³ for F ^- and (4.6 ± 0.72) × 10² M^-1 for Cl^-) and cis-1 (host/guest = 1/1, K_assoc = (1.0 ± 0.13) × 10³ for F^- and (5.9 ± 0.69) × 10 M^-1 for Cl^-), but H₂PO₄ ^- interacted differently: the cis-isomer formed the 1/1 complex (K_assoc = (9.38 ± 2.67) × 10 M^-1), whereas multistep equilibrium was expected for the trans-isomer.