A COBALT(III) CHIROPORPHYRIN AND ITS AMINE ADDUCTS - A POTENTIAL CHIRAL NMR SHIFT-REAGENT FOR AMINES

The chlorocobalt(III) complex of alpha beta alpha beta-tetramethylchir oporphyrin, CoCl(TMCP), has been prepared as a potential enantioselect ive host or chiral NMR shift reagent for optically active amines. The X-ray structure of CoCl(OHCH2CH3)(TMCP) shows the six-coordinate cobal t(III) ion at the center of a strongly ruffled porphyrin. The 2-fold-d isordered ethanol ligand interacts with the chiroporphyrin host by two C-H ... O hydrogen bonds to the carbonyl groups of two ester substitu ents. Primary amines bind to this diamagnetic cobalt(III) center to fo rm cationic 2:1 complexes in which the H-1 NMR resonances of the axial ligands are shifted upfield of tetramethylsilane by the porphyrin rin g current. Coordinated enantiopure 2-alkylamines exhibit NMR signals f or the protons of the amine group which are characteristic of their (R or S) absolute configuration. The bis-complexes of the same amines in racemic form exist as three different species, (R,R), (S,S), and (R,S ), in 1:1:2 relative ratios. Negligible enantioselection by the chiral host suggests kinetic control of bis(amine) complex formation on coba lt(III). The X-ray structure of the bis((S)-2-butylamine) complex [Co( (S)-NH2CH(CH3)CH2CH3)(2)(TMCP)][CoCL4](0.5) shows a 2-fold-disordered amine on one face of the porphyrin only. The unique amine on the Ether face is held within the porphyrin groove by a network of weak interac tions including N-H ... O and C-H ... O hydrogen bonds. With its abili ty to induce good resolution of axial ligand H-1 NMR resonances and sl ow dissociation kinetics of its bis-adducts, CoCl(TMCP) may be useful as a chiral NMR shift reagent for conformational studies of chiral ami nes and as an analytical reagent for the determination of their enanti omer composition.