Evaluation of cobalt(III) tetramethylchiroporphyrin as an analytical reagent for the determination of enantiomer composition of primary amines and aziridines by H-1 NMR spectroscopy

The irreversible reaction of chlorocobalt(iii) tetramethylchiroporphyrin Co Cl(TMCP), 1, with nitrogen donor ligands L, such as primary amines and azir idines, in deuterated chloroform solution at room temperature gives cationi c bis-adducts of the type [Co(L)(2)(TMCP)]Cl-+(-) in quantitative yields. U pon binding to the chiral host 1, the protons of L experience a shielding b y the porphyrin, and several of their H-1 NMR signals are shifted to a vaca nt spectral window upfield of tetramethylsilane (0 > delta > -6.5). The coo rdinated (R) and (S) enantiomers show well-resolved signatures at 200 MHz, allowing their relative concentrations to be readily determined by peak int egration. Racemic amine derivatives such as 2-butylamine, 1-aminoindane, al pha -amino-gamma -butyrolactone, 1-phenylethylamine, 2-aminomethoxypropane and 1-(1-naphthyl)ethylamine bind to 1 in CDCl3 solution without detectable kinetic resolution, affording a 1:1 concentration ratio of the two cobalt- bound enantiomers (R-Co) and (S-Co), as measured by H-1 NMR. Aziridines suc h as ethyl 2-aziridinecarboxylate and 2-aziridinemethanol behave in an anal ogous manner. Standard solutions of 1-(1-naphthyl)ethylamine covering a ran ge of enantiomer compositions x=(R)/[(R)+(S)] (x=0, 0.1, 0.25, 0.5, 0.75, 0 .9, 1) were analyzed using 1 as a chiral NMR shift reagent. The bound enant iomer composition values y=(R-Co)/[(R-Co)+(S-Co)] measured by NMR were in g ood agreement with those of the standard solutions, and the calibration cur ve y=f(x) was linear in the range 0 less than or equal tox less than or equ al to1, within experimental error. These features of 1 as a chiral analytic al reagent may prove suitable for the accurate determination of a wide rang e of enantiomeric excesses of amines and aziridines. The crystal structures of {Co[(R)-1-(1-naphthyl)ethylamine](2)(TMCP)}Cl-+(-) and {Co[(R)-2-butyla mine](2)(TMCP)}Cl-+(-) are described.