High Diastereoselection of a Dissymmetric Capsule by Chiral Guest Complexation

Abstract

Encapsulation of chiral guests in the dissymmetric capsule 1·4 BF 4 formed diastereomeric supramolecular complexes G&1·4 BF 4 (G: guest). When chiral guests 2 a-q were encapsulated within the dissymmetric space of the selfassembled capsule 1·4 BF 4 , circular dichroism (CD) was observed at the absorption bands that are characteristic of the p-p* transition of the bipyridine moiety of the capsule, which suggests that the P and M helicities of the capsule are biased by the chiral guest complexation. The P helicity of diastereomeric complex (S)-2 l&1·4 BF 4 was determined to be predominant, based on CD exciton coupling theory and DFT calculations. The diastereoselectivity was highly influenced by the ester substituents, such that benzyl ester moieties were good for improving the diastereoselectivity. A diastereomeric excess of 98 % was achieved upon the complexation of 2 j. The relative enthalpic and entropic components for the distereoselectivity were obtained from a vant Hoff plot. The enthalpic components were linearly correlated with the substituent Hammett parameters (s p + ). The electron-rich benzyl ester moieties generated donor-acceptor p-p stacking interactions with the bipyridine moiety, which resulted in a significant difference in energy between the predominant and subordinate diastereomeric complexes.

Supramolecular Chirality in Self-Assembled Systems

“…The energy between the diastereomeric complexes would be different as a result of the chiral interior recognizing the shape of the chiral guest when a chiral guest is captured. The group of Haino developed a calixarene-based capsule, which can encapsulate a variety of guest molecules and heterodimeric hydrogen-bonded pairs of carboxylic acids . When the chiral guest was encapsulated, two diastereomeric isomers were formed, suggesting that the P and M helicities of the capsule can be biased by the chiral guest encapsulation.…”

“…The group of Haino developed a calixarene-based capsule, which can encapsulate a variety of guest molecules and heterodimeric hydrogen-bonded pairs of carboxylic acids. 161 When the chiral guest was encapsulated, two diastereomeric isomers were formed, suggesting that the P and M helicities of the capsule can be biased by the chiral guest encapsulation.…”

“…Ooi et al developed supramolecular assemblies containing ion pairs through intermolecular hydrogen bonding. This system was formed by the coassembly of a chiral tetraaminophosphonium cation, two phenols, and a phenoxide anion and was found to have chiral catalytic activity (161). In solution, this ion pair complex promotes a highly stereoselective conjugate addition of acyl anion equivalents to α,β-unsaturated ester surrogates with a broad substrate scope (Figure 144).…”

Chirality Induction on a Coordination Capsule for Circularly Polarized Luminescence

“…Our group has developed coordination capsules [29–33] . Capsule 1 has ( P )‐ and ( M )‐enantiomeric forms (Figure 1).…”

“…A problem that remains is the poor accessibility of chiral templates that bias the relative population of the enantiomeric forms. The optical resolution of a precursor, 4,4′‐diacetoxy‐6,6′‐dimethylbiphenyl‐2,2′‐dicarboxylic acid, relies on cocrystallization with brucine, [29] requiring time‐consuming repeated crystallization. A possible alternative procedure is to organize chiral templates inside 1 through the self‐assembly of small components.…”