High Diastereoselection of a Dissymmetric Capsule by Chiral Guest Complexation

Abstract

Encapsulation of chiral guests in the dissymmetric capsule 1⋅4 BF4 formed diastereomeric supramolecular complexes G⊂1⋅4 BF4 (G: guest). When chiral guests 2 a-q were encapsulated within the dissymmetric space of the self-assembled capsule 1⋅4 BF4 , circular dichroism (CD) was observed at the absorption bands that are characteristic of the π-π* 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 BF4 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 van't Hoff plot. The enthalpic components were linearly correlated with the substituent Hammett parameters (σp (+) ). The electron-rich benzyl ester moieties generated donor-acceptor π-π 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 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).…”

Frozen Dissymmetric Cavities in Resorcinarene‐Based Coordination Capsules

“…The induced chirality of 2 a (BF 4 ) 4 was confirmed by circular dichroism spectroscopy. The CD spectrum displayed bisignate‐induced CD signals with positive and negative cotton effects at λ max =334 and 302 nm, demonstrating that the ( M )‐enantiomeric form of the capsule was primarily produced through encapsulation of the guest compound ( R )‐ 3 . Upon encapsulation of ( S )‐ 3 , the mirror image CD spectrum was obtained, indicating the formation of the ( P )‐enantiomeric form.…”

Supramolecular Graft Copolymerization of a Polyester by Guest‐Selective Encapsulation of a Self‐Assembled Capsule

“…[16] Thedissymmetric nature of the capsule generates P and M enantiomers, and ad ynamic interconversion of these forms is permitted due to the labile Ag + -N coordination (Figure 1b). [17] The encapsulation of ac hiral biphenyl guest (R)-G1 biases the interconversion, which leads to the M form with an extremely high diastereoselectivity of 98 % de.W eenvisioned that such ad iastereoselective guest encapsulation could be applied in the synthesis of ag raft copolymer. Here,w ereport an ovel synthesis approach to introduce graft side chains onto polymers poly-2a-d by site-selective encapsulation (Scheme 1a nd Figure 1d).…”

“…Supramolecular grafting of poly-(R)-2awas carried out by encapsulation in capsule 1(BF 4 ) 4 .The encapsulation of aguest unit was monitored by 1 HNMR spectroscopy.Figure 2shows the 1 HNMR spectra of poly-(R)-2a in the presence of two equivalents of 1(BF 4 ) 4 .T he acetoxymethyl resonance at d = 2.3 ppm moved to ac lear window upfield of d = 0ppm ( Figure 2b). [17] Guest units of polymers poly-(R)-2a-d were also encapsulated by the capsule in ad iastereoselective fashion. Thed ominant acetoxymethyl signal of the bound guest unit appeared at d = À1.22 ppm together with minor signals,a nd this demonstrates the diastereoselective formation of diastereomeric complexes.…”