Preparation and characterization of d6 tungsten compounds with amino acid derivatized diimine ligands and preparation of dipeptide derivatives using peptide coupling agents

Attempts to prepare dipeptide compounds from organometallic N-substituted amino acids are reported. Condensation of pyridine-2-carboxaldehyde, α-amino acids (H-l-Ala-OH or H-l-Asp-OH) and W(CO)4(pip)2 leads to formation of W(CO)4(pyca-Et) (1) (pyca refers to the α-diimine fragment, C5H4NCHdouble bond; length as m-dashN) following decarboxylation of one or two equivalents of CO2. This decarboxylation does not occur for β-alanine or GABA (γ-aminobutyric acid). Coupling of [Hpip][W(CO)4(pyca-β-Ala-O)] (2) or [Hpip][W(CO)4(pyca-GABA-O)] (3) to amino acid esters, H-l-Ala-OEt, or H-l-Val-OMe, using the standard 1,3-dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt) procedure produced four new dipeptide compounds, 4–7. The reactions proceed in good yield and compounds were characterized spectroscopically. The dipeptide complex, W(CO)4(pyca-Ala-Ala-OMe) (8), was prepared by reaction of W(CO)4(pip)2 with H-l-Ala-l-Ala-OMe and pyridine-2-carboxaldehyde. In addition the molecular structure of W(CO)4(pyca-β-Ala-Val-OMe) (5) is reported.