MoCl5 as an effective chlorinating agent towards α-amino acids: synthesis of α-ammonium-acylchloride salts and α-amino-acylchloride complexes

Abstract

The reactivity of MoCl₅ with α-amino acids was investigated for the first time by choosing CH₂Cl₂ as the reaction medium. The interaction of MoCl₅ with L-proline proceeded with Cl/O interchange and led to the formation of [NH₂(CH₂)₃CHC(O)Cl][MoOCl₄], 1, in 63% yield. The reactions of MoCl₅ with L-phenylalanine, sarcosine, N,N-dimethylglycine and N,N-dimethyl-L-phenylalanine afforded the α-amino-acylchloride complexes MoOCl₃[OC(Cl)CH(R)N(R′)(R′′)] (R = CH₂Ph, R′ = R′′ = H, 2a; R = R′ = H, R′′ = Me, 2b; R = H, R′ = R′′ = Me, 2c; R = CH₂Ph, R′ = R′′ = Me, 2d), in ca. 70% yields. According to DFT studies, 2a,d are mononuclear Mo(V) octahedral complexes bearing a N,O-coordinated α-amino-acylchloride ligand. The presumed species formed during the first stages of the MoCl₅/L-phenylalanine interaction were DFT-elucidated, thus the calculated Gibbs free energy profile for the multi-step reaction of MoCl₅ with L-phenylalanine was traced. [NH₃CH(CH₂Ph)C(O)Cl][MoOCl₄], 3, acting as an intermediate in the course of the formation of 2a, was isolated by combination of [NH₃CH(CH₂Ph)C(O)Cl][Cl] with MoOCl₃.