Kinetic Control in the Chiral Recognition of Three-Bladed Propellers

✍ Scribed by Clément Bonnot; Emmanuel Aubert; Natalie Banerji; Jérôme Lacour; Enrique Espinosa; Jean-Claude Chambron

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 414 KB
• Volume: 16
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.200903058

✦ Synopsis

The ion pair of the stereolabile C(3)-symmetric, i(+)o proton complex 1H of diaza-macropentacycle 1 and the configurationally stable Delta-TRISPHAT (Delta-3) anion exists in the form of two diastereomers, namely, [Delta-(1.H)][Delta-3] and [Lambda-(1.H)][Delta-3], the ratio of which, in terms of diastereomeric excess (de) decreases in the order [D(8)]THF (28%)>CD(2)Cl(2) (22%)>CDCl(3) (20%)>[D(8)]toluene (16%)>C(6)D(6) (7%)>[D(6)]acetone (0%) at thermodynamic equilibrium. Except in the case of [D(6)]acetone, the latter is reached after a period of time that increases from 1 h ([D(8)]THF) to 24 h (CDCl(3)). Moreover, the initial value of the de of [1.H][Delta-3] in CDCl(3), before the thermodynamic equilibrium is reached, depends on the solvent in which the sample has been previously equilibrated (sample "history"). This property has been used to show that the crystals of [1.H][Delta-3] formed by slow evaporation of CH(2)Cl(2)/CH(3)OH mixtures had 100% de, which indicates that [1.H][Delta-3] has enjoyed a crystallization-induced asymmetric transformation. Structural studies in solution (NMR spectroscopy) and in the gas phase by calculations at the semiempirical PM6 level of theory suggest that the optically active anion is docked on the i(+) (endo) external side of the proton complex such that one of the aromatic rings of Delta-3 is inserted into a groove of 1.H, a second aromatic ring being placed astride the outside i(+) pocket. Solvent polarity controls the thermodynamics of inversion of the 1.H propeller. However, both polarity and basicity control its kinetics. Therefore, the rate-limiting steps correspond to the ion-pair separation/recombination and 1.H/1 deprotonation/protonation processes, rather than the inversion of 1H, the latter being likely to take place in the deprotonated form (1).