Thermoracemization Of Chloroacetanilide Herbicides And Supermolecular Recognition Of Cyclodextrins

Chloroacetanilide herbicides have been frequently detected in groundwater, surface waters, and soils. Due to the asymmetrical substitution on CAr-N bond, the pesticides consist at least one pair of enantiomers, which exhibit differernt behavior in biological activity, toxicology and environmental fate. Therefore, it is of much significance to set up method for quantitative and residual ananylsis of chloroacetanilide herbicides enantiomers. In this study, enantiomeric resolution, thermoracemiztation of the enantiomers and supermolecular recognition mechnism of cyclodextrins have been carried out.Enantiomers of acetochlor and propisochlor were separated using normal phase HPLC with a Chiralpak AS-H column. The optimal separate conditions were as follows:n-hexane/ 2-propanol (97/3, V/V),25℃,1.0 mL/min,220 nm. The stereoconfiguration of the eluting enantiomers of acetochlor and propisochlor were designated by the octant rule and their cotton effect. The order of the enantiomers for both compounds were aR-(-) and aS-(+). Moreover, the enantiomeric resolution was controlled by enthalpy term, in which the N-alkoxyalkyl substituents played important roles.Successful stereoseparation of metolachlor by normal phse HPLC using chiral Chiralcel OD-H column was performed, and the elution orders of the eluting enantiomers were detemined by ORD and CD detector. Validation parameters including linearity, precision, and limit of detection (LOD) were confirmed. The method was reliable for residual analysis of the compound. The separation of the mixed stereoisomers on chiral Chiralpak AS-H column were enthalpy controlled and axil-chiral enantiomers were more difficultly resolved than C-chiral enantiomers.Combine kinetic experiment with density fuctional theory, the thermoracemization of aisx-enantiomers have been studied. At elevated temperatures, the atropisomers rapidly interconvert through internal rotation about the phenyl-nitrogen bond. The recemiztion reaction obeyed first-order kinetics, which was enthalpy-driven and controlled by the steric effects of N-alkoxyalkyl substituted group. The influence of cyclodextrins on the racemization was determined kinetically. CDs caused inhibition of racemization and resuted in enantio-differentiating isomerztion, which depended on its cavity size, property and the place of the substituted group. CDOCKER module in Discovery Studio 2.5 was used to study the chiral recognition of CDs. The results showed that the calculated complex stability constants were in good agreement with the experimental data.