BITHIENOLs: new promising C2-symmetric bis-heteroaromatic diols for organic transformation

2,2',5,5'-Tetraphenyl-3,3'-bithiophene-4,4'-diol, the first member of a new class of chiral C2-symmetric atropoisomeric diols based on a biheteroaromatic scaffold, was synthesized. Dynamic enantioselective HPLC experiments provided a racemization barrier of 22.9 kcal/mol at 25 °C which was high enough to permit the chromatographic collection of both the enantiomers at semipreparative scale, but at the borders for application as promoter in asymmetric catalysis. The ground and transition states involved in the enantiomerization process have been identified through DFT calculations, which suggested the intermediacy of a tautomeric species, where a thiophene-3-ol unit and a 2,3-dihydro-thiophene-3-one ring are present. The racemization mechanism was supported by dynamic HPLC experiments. Calculations suggested also which possible substituent groups in position 2,2' of the thiophene rings might lead to an increase in the enantiomerization barriers sufficient to confer full configurational stability to the 3,3'-bithiophene framework.