Hydrogen bonding greatly influences rates and equilibrium positions of chemical reactions, conformations, and sometimes even stereochemistry. This study reports on tetranitrofluorenone oximate, a novel dye capable of naked-eye detection of hydrogen-bond donating species (HBDs) and of rapid determination of H-bond donation strength by hypsochromic shift monitoring. In addition, the molecule possesses atropisomeric conformations, of M and P configuration, as evidenced in solid and solution state studies by X-ray diffraction and electronic circular dichroism (ECD), respectively. In the latter case, enantiopure bis-thioureas were the most effective HBDs to promote a chiral induction (diastereoselective recognition, Pfeiffer effect); the ECD results being rationalized by time-dependent density functional theory (TDDFT) calculations. Based on these experiments, bis-thioureas were used as chiral reagents in asymmetric 1,3-dipolar cycloadditions of structurally-related nitrones; the ECD sensing of the stereoinduction between bis-thioureas and the oximate serving as an indirect method of selection of the most effective HBD for asymmetric synthesis.
Keywords: Pfeiffer effect; asymmetric amplification; circular dichroism; hydrogen bonds; solvatochromism.
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