Tingmin Wang

Factors influencing enantiomeric recognition of primary alkylammonium salts by pyridino-18-crown-6 type ligands

Equilibrium constant (K), enthalpy change (ΔH), and entropy change (ΔS) values were determined for the interactions of a series of chiral pyridino-18-crown-6 type ligands with enantiomers of several primary alkylammonium salts in various solvents. Good enantiomeric recognition in terms of Δ logK was observed in many systems with Δ logK values greater than 0.4. The extent of enantiomeric recognition and the stabilities of the chiral crown ether-ammonium salt complexes were found to depend on the rigidity of the macrocyclic frame of the ligand, the type and arrangement of the donor atoms on the ligand, the bulkiness of the substituents on the ligands chiral centers, the location of the chiral centers on the ligand, and the solvent. The effects of these factors on the extent of enantiomeric recognition and on the stabilities of the complexes were examined for the systems studied.

Enantiomeric recognition and separation of chiral organic ammonium salts by chiral pyridino-18-crown-6 ligands

Abstract Optically pure allyloxy and dimethyl-substituted pyridino-18-crown-6 (8) was attached to silica gel by the following reactions. 4-Allyloxy-2,6-pyridinedimethyl ditosylate (23) was first prepared from chelidamic acid. Ditosylate 23 was treated with (S,S)-dimethyl-substituted tetraethylene glycol to form 8. Ligand 8 was treated with triethoxysilane using a platinum catalyst. The resulting chiral crown-substituted triethoxysilane 32 was reacted with silica gel in toluene at 90 C to attach the crown to silica gel. Preliminary results of the separation of [α-(1-naphthyl)ethyl]ammonium perchlorate into its (R) and (S) forms using the bound chiral crown with acetone/methanol (7/3) (v/v) as the eluant are reported. The preparation of chiral dimethyl(allyloxyphenyl)pyridino-18-crown-6 (9) that could be attached to silica gel on the side opposite to the pyridine ring is also reported.

Enantiomeric recognition of chiral ammonium salts by chiral pyridino- and pyrimidino-18-crown-6 ligands: Effect of structure and solvents

Chiral pyridino- 18-crown-6 ligands interact with chiral primary organic ammonium salts by hydrogen bonding from the ammonium cation to the pyridino nitrogen and two alternate ring oxygen atoms. Enantiomeric recognition in these interactions are caused by the steric bulk of the substituents at chiral macrocycle ring positions. Recognition is best for the interaction of chiral pyridino-18-crown6 hosts with the enantiomers of a-( 1-naphthylethy1)ammonium perchlorate (NapEtHC10,) over (a-phenylethy1)ammonium perchlorate (PhEtHClO,) possibly because of a greater x-x overlap between the naphthalene ring of the guest and pyridine ring of the host. Solvents play an important role in the degree of recognition. A binary solvent composed of 7/3 &H4C12/CH,0H (v/v) gave an enhanced degree of recognition. A new chiral pyrimidino- 18-crown-6 ligand exhibited recognition for the enantiomers of NapEtHC10,.

A novel two-step method to prepare new unsymmetrical cryptands

Abstract A novel two-step method to prepare unsymmetrical cryptands from available diethanolamine, oligoethylene glycol dichloride and other dihalides is reported. A new cryptand with two bridges each containing one methylene unit exhibited a sodium over potassium selectivity of more than 1,000.

A thermodynamic study of enantiomeric recognition of organic ammonium cations by pyridino-18-crown-6 type ligands in methanol and a 1: 1 methanol-1,2-dichloroethane mixture at 25.0°C

LogK, ΔH, andTΔS values for interactions of (R) and (S) enantiomers of α-(1-naphthyl)ethylammonium perchlorate (NapEt), α-phenylethylammonium perchlorate (PhEt), and the hydrogen perchlorate salt of 2-amino-2-phenylethanol (PhEtOH) with a series of chiral and achiral pyridino-18-crown-6 type ligands and 18-crown-6 (18C6) were determined from calorimetric titration data valid in methanol and in a 1: 1 (v/v) methanol-1,2-dichloroethane (MeOH-1,2-DCE) mixture at 25.0°C. In the MeOH-1,2-DCE solvent mixture, the chiral macrocyclic ligands exhibit excellent recognition for enantiomers of the three organic ammonium cations as shown by large differences in logK values (Δ logK) which range from 0.4 to 0.6 (2.5- to 4.0-fold difference in binding constants). The Δ logK values in the solvent mixture MeOH-1,2-DCE are increased by 0.1–0.5 logK units over those in absolute methanol, indicating a favorable effect of 1,2-dichloroethane on enantiomeric recognition. In 1,2-dichloroethane, however, the interactions are too strong (logK>6) to observe the degree of recognition by a direct calorimetric method. Complexation of organic ammonium cations by these macrocyclic ligands is driven by favorable enthalpy changes. The entropy changes ure unfavorable in all cases. The thermodynamic origin of enantiomeric recognition for NapEt in 1:1 (v/v) MeOH-1,2-DCE is enthalpic, but those for PhEt and PhEtOH are entropic. Effects of the ligand structure and flexibility and of the organic cation structure on recognition and complex stability are discussed on the basis of the thermodynamic quantities. Different thermodynamic behaviors of achiral 5 and 18C6 from those of chiral macrocyclic ligands indicate a difference between chiral and achiral macrocycle interactions with the chiral organic ammonium cations. The different thermodynamic behavior of NapEt enantiomers compared to those of PhEt and PhEtOH enantiomers supports the idea that the solution complex conformation of NapEt is different from those of PhEt and PhEtOH. π-π interaction is absent for the PhEt and PhEtOH complexes with diesterpyridino-18-crown-6 ligands in solution. Therefore, the higher degree of enantiomeric recognition for NapEt than for either PhEt or PhEtOH by these chiral macrocyclic ligands is a result of the presence of π-π interaction in the NapEt system.

A structural analysis of the complexes of (S, S)-dimethylpyridino-18-crown-6 with (R) and (S)-[α-(1-naphthyl)ethyl]ammonium perchlorate by NMR techniques and molecular modeling

Significant π-π interaction is found in the complexes of (S, S)-dimethylpyridino-18-crown-6 with (R)- and (S)-[α-(1-naphthyl)ethyl]ammonium perchlorate. This finding is supported by the1H NOESY NMR spectral technique, greater chemical shift changes of aromatic protons in both host and guest molecules upon complexation, and by molecular mechanics calculations. Because of the flexibility of the ligand, the tripod hydrogen bonding causes13C relaxation times of all periphery carbons to decrease without significant selectivity. Rotational energy barrier calculations of the methyl groups of the complexed ligand also show that the (S, S)-host-(R)-guest is the more stable complex.

Thermodynamic and structural studies of cation-macrocycle interaction

The rapid progress in host-guest chemistry involving macrocycles has been due in no small part to the availability of reliable and extensive thermodynamic data for a large number of host-guest systems. Although valuable information can be derived from AH and AS values, few calorimetric data have been reported for host-guest systems. The procedures required to obtain reliable thermodynamic data are discussed and some of the kinds of information which these data can provide are given together with an example. Recent studies have demonstrated the power of combined thermodynamic and spectroscopic data in understanding host-guest interactions. Examples of these studies are presented.

Various aspects of enantiomeric recognition of (S,S)-dimethylpyridino-18-crown-6 by several organic ammonium salts

Abstract Factors responsible for complex stability and enantiomeric recognition for the interactions of (S,S)-dimethylpyridino-18-crown-6 with several organic ammonium salts were examined using an 1H NMR technique. The results indicate that cation structures have a significant effect on enantiomeric recognition; solvents play a very important role in the stability of the complexes, and anions can compete with ligands for the ammonium cations.