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

Amino acid anions (AAO(-)) chelated to cobalt(III) in [(en)(2)Co(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, Lambda- and Delta-[(en)(2)Co(O2CCH=NH)](O3SCF3)(2), constitute chiral glycine equivalents which can serve as synthons for stereoselective alpha-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 alpha-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-C-13, 98%) aspartic acid were produced by facile synthesis from Delta-[(en)(2)Co(O2CCH=NH)](O3SCF3)(2) and diethyl (2-C-13) malonate. The new N-methyliminoacetato complex, rac-[(en)(2)Co((OCCH)-C-2=NMe)](O3SCF3)(2), also yielded to imine addition reactions providing a route to the alpha-N-methylamino acid subclass. The molecular structures of the new imine complexes, Lambda-(+)578-[(en)(2)Co(O2CCH=NH)] Br-2.H2O and rac-[(en)(2)Co(O2CCH=NMe)]S2O6.1.5H(2)O, and the diethyl carboxy-aspartate addition product, (LambdaS,DeltaR)-[(en)(2)Co{O2CCH(CH(CO2Et)(2))NH2}](ClO4)(2), were determined by X-ray crystallography.