Resolution, Enantiomerization Kinetics, and Chiroptical Properties of 7,7‘-Dihydroxy-8,8‘-biquinolyl

(±)-7,7‘-Dihydroxy-8,8‘-biquinolyl (6) was resolved into its enantiomorphic atropisomers via reverse
phase (C18) chromatographic separation of epimeric bismenthyl carbonates, (−)-lk-9 and (+)-ul-9, derived
from 6 and (+)-menthyl chloroformate. The faster eluting diastereoisomer, (−)-lk-9, was revealed to
possess an (aS)-configurated biaryl axis by X-ray crystallographic analysis. Saponification of the separated
bismenthyl carbonates gave enantioenriched samples of biquinolyl 6, and absolute stereochemical
configurations were assigned to the two optical isomers as (−)-(aS)-6 and (+)-(aR)-6 by correlation
with their respective progenitors, (−)-lk-9 and (+)-ul-9. First-order rate constants for the enantiomerization
of 6 in water were obtained over the temperature range 316−366 K, and activation parameters were
determined as ΔH⧧ = 34.0 kcal mol-1 and ΔS ⧧ = 18.7 cal mol-1 K-1 by Eyring plot analysis. A low
level (AM1) computational study of the rotational dynamics of 6 showed excellent agreement with kinetic
experimental data and suggested that enantiomerization occurs preferentially via a syn pathway. In common
with (−)-(aS)-1,1‘-bi-2-naphthol (BINOL), (−)-(aS)-6 showed positive exciton chirality in its electronic
circular dichroism (CD) spectrum and gave a characteristic couplet composed of a positive maximum
Cotton effect at 250 nm and a negative minimum at 234 nm (ΔΔε = +40 M-1 cm-1 at 64% ee).