(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, Kassoc = (1.0 ± 0.13) × 103 for F - and (4.6 ± 0.72) × 102 M-1 for Cl-) and cis-1 (host/guest = 1/1, Kassoc = (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 (Kassoc = (9.38 ± 2.67) × 10 M-1), whereas multistep equilibrium was expected for the trans-isomer.
All Science Journal Classification (ASJC) codes
- Organic Chemistry