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(O₂CCHNH)](O₃SCF₃)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(O₂CCHNH)](O₃SCF₃)₂ and diethyl (2-¹³C)malonate. The new N-methyliminoacetato complex, rac-[(en)₂Co(O₂CCHNMe)](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(O₂CCHNH)]Br₂·H₂O and rac-[(en)₂Co(O₂CCHNMe)]S₂O₆·1.5H₂O, and the diethyl carboxy-aspartate addition product, (ΛS,ΔR)-[(en)₂Co{O₂CCH(CH(CO₂Et)₂)NH₂}](ClO₄)₂, were determined by X-ray crystallography.