Metal complex amino acid synthons: syntheses, structures and stereoselective reactions of (iminoacetato)cobalt(III) complexes.

Abstract

Amino acid anions (AAO-) chelated to cobalt(III) in [(en)2Co(AAO)](O3SCF3)2 (AA = Gly, Sar, Ala and Glu) were selectively oxidized to their imine derivatives by a new general procedure utilizing PBr3 and N-bromosuccinimide in dmf. The new iminoacetato complexes, Λ- and Δ-[(en)2Co(O2CCH=NH)](O3SCF 3)2, constitute chiral glycine equivalents which can serve as synthons for stereoselective α-amino acid synthesis. In alkaline EtOH, quantitative addition of CH2(COMe)2, CH2(CO2Et)2 or MeCOCH2CO2Et to the imine of the iminoacetate ligand initially produced both diastereomers of the product α-amino acid cobalt(III) complexes. However, subsequent crystallization-induced asymmetric transformations in the heterogeneous reaction mixtures led to better than 90% excess of a single diastereomer after five days, and the diastereopure product triflate salts were obtained after recrystallization. Both enantiomers of isotopically substituted (3-13C, 98%)aspartic acid were produced by facile synthesis from Δ-[(en)2Co(O2CCH=NH)](O3SCF 3)2 and diethyl (2-13C)malonate. The new N-methyliminoacetato complex, rac-[(en)2Co(O2CCH=NMe)](O3SCF3) 2, also yielded to imine addition reactions providing a route to the α-N-methylamino acid subclass. The molecular structures of the new imine complexes, Λ-(+)578-[(en)2Co(O2CCH=NH)]Br 2·H2O and rac-[(en)2Co(O2CCH=NMe)]S2O6 ·1.5H2O, and the diethyl carboxy-aspartate addition product, (ΛS,ΔR)-[(en)2Co{O2CCH(CH(CO2 Et)2)NH2}](ClO4)2, were determined by X-ray crystallography.