We demonstrate facile and efficient construction of conjugated double helical ladder oligomers from the saddle-shaped cyclooctatetrathiophene (COTh) building blocks. The key step involves deprotonation of tetra[3,4]thienylene (β,β-COTh) with n-BuLi which displays remarkably high ipsilateral selectivity. Three racemic double helical ladder oligomers, rac-DH-1, rac-DH-2, and rac-DH-3, containing two, three, and five COTh annelated moieties are efficiently synthesized by diastereoselective coupling of the racemic precursors. The X-ray crystallographic studies of rac-DH-1, rac-DH-2 and rac-DH-3 unambiguously revealed that each double helical scaffold has two single helices intertwined with each other via the C-C single bonds. Following removal of TMS groups, double helical ladder oligomer rac-DH-1-D had sufficient solubility to be resolved via chiral HPLC, thus enabling determination of its chirooptical properties such as CD spectra and optical rotation. (+)-DH-1-D has a large barrier for racemization, with lower limit of ΔG‡ > 48 kcal mol-1, which may be compared to DFT-computed barrier of 51 kcal mol-1. The enantiomers of DH-1-D show 1 order of magnitude stronger chirooptical properties than the carbon-sulfur helicene, as determined by the anisotropy factor g = Δϵ/ϵ = -0.039, based on Δϵmax = -11 and ϵ = 2.8 × 102 L mol-1 cm-1 in cyclohexane at 327 nm.
ASJC Scopus subject areas
- Colloid and Surface Chemistry