Chengyue Zhu

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.

A thermodynamic and structural study of the interactions of pyridino- and diketopyridino-18-crown-6 ligands with some primary organic ammonium cations

Equilibrium constant (K), enthalpy change (ΔH), and entropy change (ΔS) values have been determined calorimetrically at 25°C in 90%MeOH 10%H2O (v/v) for the interactions of pyridino-18-crown-6 (P18C6) and diketopyridino-18-crown-6 (K2P18C6) with perchlorate salts of ammonium, benzylammonium,α-phenylethylammonium,β-phenylethylammonium, andα-(1-naphthyl)ethyl-ammonium cations. The crystal structure of the complex of P18C6 with benzylammonium perchlorate was determined by X-ray crystallography. The1H 1D and 2D NMR spectra of some of these complexes were used to elucidate their structural features in solution. The logK values for the interaction of the ammonium cations with P18C6 are larger than those with K2P18C6, probably due to the higher degree of structural flexibility of P18C6. Ligand K2P18C6 displays appreciable π-π interaction with theα-(1-naphthyl)ethylammonium cation, but not with theα-phenylethylammonium cation.

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.

Evaluation of a Direct 1H NMR Method for Determining log K and ΔH Values for Crown Ether—Alkylammonium Cation Complexation

A direct 1H NMR method for determining log K and ΔH values for crown ether-ammonium cation complexation using milligrams of sample was tested and evaluated for accuracy and precision by comparing the results with those obtained using a titration calorimetric method. Log K values for the interactions of a non-chiral crown ether, diketopyridino-18-crown-6 (K2P18C6), with α-phenylethylammonium (PhEt+) perchlorate in 50%-50% and 90%-10% (v/v) mixtures of deuterated methanol (CD30D) and deuterated chloroform (CDCI3) at four temperatures and, with α-(I-naphthyl) ethylammonium (NapEt+) perchlorate in SO%CD3OD-50%CDCI3 (v/v) at 25°C were determined by a direct 1H NMR method. Values of ΔH for the interactions of KzPI8C6 with PhEt+ in the two solvents were calculated from the temperature dependence of log K. Log K values for the interactions of a chiral crown ether, dimethyldiketopyridino-18-crown-6 (M2K2P18C6), with (R) and (S) enantiomers of NapEt+ in pure CD30D at 2S.0°C were also determined by the NMR method. The results were compared with those determined by a calorimetric method at 2S.0°C in SO% - SO% and 90% -10% (v/v) mixtures of plain methanol and chloroform, in 100% plain methanol, and in a 50%-50% mixture of partially deuterated methanol (with deuterium substitution on the methanol OH group, CH30D) and deuterated chloroform. The log K values determined by both methods were found to be in good agreement, but the standard deviations associated with the NMR log K values were two to three times greater. The agreement of the ΔH values determined by the two methods was poor, differing by approximately 10 kJ/mol with the NMR method giving more negative values. The standard deviations associated with the NMR ΔH values were approximately ten times greater than those for the calorimetric values. Ion-pairing was observed for the interaction of perchlorate ion with both free and bound PhEt+ in SO%methanol-50%chloroform mixture. It is concluded that the NMR procedure is satisfactory for the determination of log K, but not ΔH values.

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.

Thermodynamics of the complexation of cerium-, europium- and erbium-(III) with 1,4,10-trioxa-7,13-diazapentadecane-N,N′-diacetic acid and 1,4,10,13-tetraoxa-7,16-diazaoxa-cyclooctadecane-N,N′-diacetic acid

Enthalpy changes for the complexation of Ce3+, Eu3+ and Er3+ with two cyclic diaza diacetates 1,4,10-trioxa-7,13-diazacyclopentadecane-N,N′-diacetic acid (H2L1) and 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-N,N′-diacetic acid (H2L2) have been obtained by isoperibol titration calorimetry. From these values and corresponding Gibbs energy data, the entropy changes for these reactions were derived. The results show that complex formation is stabilized in each case by entropy changes. There is a favourable enthalpy contribution for interactions of Ce3+ and Eu3+ with H2L2. There is a direct correlation between the entropy contributions and enthalpy terms for the complexes of both the ligands. The results are consistent with an encapsulated model for complexes of H2L2 and a cis-carboxylate model for complexes of H2L1.