Why is monoalkylation versus bis-alkylation of the Ni(II) complex of the Schiff base of (S)-N-(2-benzoylphenyl)-1-benzylpyrrolidine-2-carboxamide and glycine so selective? MP2 modelling and topological QTAIM analysis of chiral metallocomplex synthons of α-amino acids used for the preparation of radiopharmaceuticals for positron emission tomography.

Abstract

Chiral Ni(II) complexes are used for the preparation of carbon-11 or fluorine-18 enantiomerically pure α-amino acids for positron emission tomography (PET). They enable the selective monoalkylation of a glycine synthon with high stereoselectivity and the preparation of enantiomerically pure α-amino acids with quarternary α-carbon. Molecular modelling of non-, mono- and di-substituted complexes using quantum theory of atoms-in-molecule (QTAIM) topological analysis of electron density allowed us to formulate a new theory explaining the reasons for highly selective monomethylation of the complexes. In the non-substituted complex (GK), the α-carbon atom exhibits a higher atomic volume and a more positive charge in comparison with mono- and di-substituted complexes. This unusual behaviour is accompanied by increasing the bond critical point (BCP) ellipticity of the iminic bond in GK explained by the higher mechanical strain. Both phenomena indicate the increased reactivity and probably originate in more compact core of GK where shorter distances in the internal coordination sphere result in the higher strain of its bonds.