2,4,5,7‐Tetranitrofluorenone Oximate for the Naked‐Eye Detection of H‐Bond Donors and the Chiroptical Sensing of Enantiopure Reagents

Abstract

Hydrogen bonding greatly influences rates and equilibrium positions of chemical reactions, conformations, and sometimes even stereochemistry. Herein, we report 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.

Pfeiffer effect on configurationally labile dyes within ternary complexes with metal ions and enantiopure macrocycles

“…In the case of divalent alkaline earth metal ions bearing higher electrostatic factors, 16 the low energy band of 1 was blue-shifted, reminiscent of the interaction of 1 with hydrogen-bond donors. 9 However and of importance, alkali metal ions and Bu 4 N + did not cause any spectral shift as absorptions remained centered at 590 nm; this opening the door to ECD studies of chiral induction at rather low energy. Then, enantiopure macrocycle 2 was titrated with inert Bu 4 N + as well as alkali and alkaline earth metal perchlorate salts.…”

“… 9 Thanks to low energy visible light absorption and effective complexation properties with hydrogen bond donors, a Pfeiffer effect could be detected in the presence of chiral bis-thiourea derivatives and characterized by ECD. 9 Of note, oximate 1 is related to Newman 's 2-(((2,4,5,7-tetranitro-9 H -fluoren-9-ylidene)amino) oxy)propanoic acid (Fig. S1 † ), 10 a π-acceptor reagent used prominently for the resolution of electron-rich polyaromatic carbohelicenes.…”