Thiophene-Based Double Helices: Syntheses, X‑ray Structures, and Chiroptical Properties

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 [7]­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.