The enantiomers of N,N-dimethylthiobenzamides: Chromatographic behaviour and rotational barriers

Ruffling-Induced Chirality: Synthesis, Metalation, and Optical Resolution of Highly Nonplanar, Cyclic, Benzimidazole-Based Ligands

Expedient five-step syntheses of a cyclic bis(benzimidazole)-based amide 5 and two sterically more hindered analogues 23-24 have been developed. These amides are chiral due to the inherent ruffling of the macrocyclic plane. Racemization of the optical antipodes of these compounds has been studied using dynamic chiral stationary phase HPLC. These studies reveal that, while the parent amide 5 racemizes rapidly, for the sterically more hindered amides 23-24, the rate of racemization is significantly reduced. Bis(benzimidazole)-based amides 5 and 23-24 form stable Ni(II) complexes 25-27, respectively. Like their parent ligands, complexes 25-27 are chiral due to their highly ruffled geometry. Studies of these complexes by chiral stationary phase HPLC reveal that metalation leads to a much lower rate of racemization. Incorporation of a strap can slow racemization even further. A series of strapped cyclic amides 54-57, along with their corresponding dimers 58-61, have been prepared. The rate of racemization for amides 54-57 is strongly dependent on the length of the strap. X-ray single-crystal structure analysis of the Ni(II) complex of strapped amide 54 reveals that the bis(benzimidazole) core retains its highly ruffled shape, with the two phenyl rings of the macrocycle located anti to the strap. Chiral separation of strapped ligands 54-57 and their corresponding Ni(II) complexes is shown to be facile by chiral stationary phase HPLC.

Determination of the interconversion energy barrier of enantiomers by separation methods

Separation methods have become versatile tools for the determination of kinetic activation parameters and energy barriers to interconversion of isomers and enantiomers in the last 20 years. New computer-aided evaluation systems allow the on-line determination of these data after separating minute amount of pure compounds or mixture of isomers or enantiomers, respectively. Both dynamic interconversion during the separation process as well as static stopped-flow techniques have been applied to determine the kinetic activation parameters and interconversion energy barriers by separation methods. The use of (1) combinations of batchwise kinetic studies with enantioselective separations, (2) a continuous flow model, (3) a comparison of real chromatograms with simulated ones, (4) stopped-flow techniques, (5) stochastic methods, (6) approximation functions and (7) deconvolution methods, for the determination of interconversion energy barriers by separation methods is summarized in detail.

Evaluation of reversible and irreversible models for the determination of the enantiomerization energy barrier for N-(p-methoxybenzyl)-1,3,2-benzodithiazol-1-oxide by supercritical fluid chromatography

It has been found that the interconversion of enantiomers on a chromatographic column during the separation process can be studied by the first-order kinetic equations derived both for reversible and irreversible reactions in a stationary system if the extent of interconversion is not too high. The equation derived for irreversible reactions gives, however, results also for higher degrees of enantiomerization while that derived for reversible interconversion failed. The irreversible equation was used to determine the enantiomerization barrier of N-(p-methoxybenzyl)-l,3,2-benzodithiazol-l-oxide enantiomers by supercritical fluid chromatography. The racemate of N-(p-methoxybenzyl)-l,3,2-benzodithiazol-l-oxide was separated by supercritical fluid chromatography on the (R,R)-Whelk-Ol column with supercritical carbon dioxide containing 20% methanol as a mobile phase. Peak areas of enantiomers prior to and after the separation used for the calculation of the enantiomerization barrier were determined by computer-assisted peak deconvolution of peak clusters registered on chromatograms using commercial software.

Time-resolved cryogenic modulation reveals isomer interconversion profiles in dynamic chromatography

The dynamic chromatographic study of interconversion of E and Z forms of oximes has been investigated by using a novel cryogenic modulation method in a two-dimensional gas chromatographic array. The primary column is a conventional capillary GC column on which the molecular interconversion proceeds. In this case, the molecular dynamical process leads to a peak profile describing the kinetics and thermodynamics of the interconverting molecules during its chromatographic elution. Thus an interconversion region intercedes the elution of the individual stereoisomers of the reaction. Since the molecules are isomers, classical molecular identification methods such as gas chromatography-mass spectrometry are unable to study the individual instantaneous amounts of each of the compounds. Hence the infinitesimal profiles of interconversion along the entire column have never been experimentally observed; rather the total profile is normally subjected to mathematical modelling studies in order to match experiment with theory, and to gain the kinetic parameters of the process. In the present study, an instantaneous ratio of the individual isomers can be found during the chromatographic elution by direct measurement. This is achieved by using a cryogenic zone focussing process, with rapid longitudinal modulation of a cold trap and continual pulsing of collected zones into a fast-analysis high-resolution capillary column on which isomer interconversion is minimized. The data can be displayed as a two-dimensional contour plot to demonstrate the individual isomer profiles. The two-dimensional analysis also allows easy measurement of the peak ratios of the two isomers which is an indicator of the extent of interconversion that has taken place. Two model systems, acetaldoxime and butyraldoxime, were chosen to illustrate the use of the cryogenic modulation procedure. It is anticipated that the procedure could be applied to other molecules which exhibit gas-phase isomerizations or reactions.

Barriers to Stereoinversion of N-Aryl-1,3,2-benzodithiazole 1-Oxides Studied by Dynamic Enantioselective Liquid Chromatography

Free energies of activation for the enantiomerization of a series of racemic N-aryl-1,3,2-benzodithiazole 1-oxides have been determined by dynamic high-performance liquid chromatography (DHPLC) on a chiral stationary phase. From a comparison of experimental and computer-simulated chromatograms, the barriers to stereoinversion at sulfur were found to be around 80 kJ/mol and relatively insensitive to effects from substituents in the N-aryl group. Throughout the series the (+)-forms (436 nm) were found to be of (S)-configuration.

Polarimetric detection in high-performance liquid chromatography

Chiroptical detection for HPLC is particularly useful as a selective detection method for chiral molecules, and in enantiomeric purity determination with partial chiral separation or without chiral separation. The recent development of laser-based polarimeters with microdegree sensitivity has increased the applicability of optical rotation detection in HPLC. The detection limit of these instruments is submicrogram on-column for many chiral compounds in analytical HPLC. A variety of applications of the selective detection of optically active molecules are reviewed. The use of polarimetric detection with partial chiral separation is considered, both as an aid to method development and for enantiomeric purity determination. Finally applications to enantiomeric purity determination without chiral separation are reviewed, with the dual use of nonchirally selective and chiroptical detectors to determine the total amount and optical purity of the analyte. Determinations of chiral purity for samples of high enantiomeric excess are described, which with laser-based instrumentation may give accuracies of better than +/- 1% with sample loadings of 50 micrograms on an achiral column. Applications to the study of enantioselective reactions are also considered, with determination of enantiomeric excess in near-racemates to better than +/- 0.1%.