title: In situ assembly of octahedral Fe(II) complexes for the enantiomeric excess determination of chiral amines using circular dichroism spectroscopy.
abstract: A method for discriminating between α-chiral primary amine enantiomers is reported. The method utilizes circular dichroism (CD) spectroscopy and a sensing ensemble composed of 2-formyl-3-hydroxypyridine (4) and Fe(II)(TfO)(2). Aldehyde 4 reacts rapidly with chiral amines to form chiral imines, which complex Fe(II) to form a series of diastereomeric octahedral complexes that are CD-active in both the UV and visible regions of the spectrum. NMR studies showed that for enantiomerically pure imine complexes, the Δ-fac isomer is preferred. A statistical analysis of the distribution of stereoisomers accurately modeled the calibration curves for enantiomeric excess (ee). CD signals appearing in the UV region were bisignate, and the nulls of the CD signals were coincident with maxima in the UV spectrum, consistent with exciton coupling. Time-dependent density functional theory and semiempirical calculations confirmed that the CD signals in the UV region arise from coupling of the π-π* transitions in the imine chromophores and that they can be used to describe the signs and magnitudes of the curves accurately. The CD signals in the visible region arise from metal-to-ligand charge-transfer bands, and these signals can be used to determine the ee values of chiral amines with an average absolute error of ±5%. Overall, the strategy presented herein represents a facile in situ assembly process that uses commercially available simple reagents to create large optical signals indicative of ee values.
authors.affiliation: Department of Chemistry and Biochemistry, The University of Texas, Austin, Texas 78712, USA.
authors.forename: Justin M
authors.initials: JM
authors.lastname: Dragna
authors.forename: Gennaro
authors.initials: G
authors.lastname: Pescitelli
authors.forename: Lee
authors.initials: L
authors.lastname: Tran
authors.forename: Vincent M
authors.initials: VM
authors.lastname: Lynch
authors.forename: Eric V
authors.initials: EV
authors.lastname: Anslyn
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Di Bari
mesh_terms: D000588:Amines / Q000737:chemistry*; D002942:Circular Dichroism; D005296:Ferrous Compounds / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D000588:Amines; D005296:Ferrous Compounds