The reactions of α-amino acids and α-amino acid esters with high valent transition metal halides: synthesis of coordination complexes, activation processes and stabilization of α-ammonium acylchloride cations

摘要

Titanium tetrachloride smoothly reacted with a selection of a-amino acids (aaH) in CH2Cl2 affording yellow\udto orange solid coordination compounds, 1a–d, in 70–78% yields. The salts [NHEt3][TiCl4(aa)], 2a–b, were\udobtained from TiCl4/aaH/NEt3 (aa ¼ L-phenylalanine, N,N-dimethylphenylalanine), in 60–65% yields. The\udcomplex , 3, was isolated from the reaction of L-proline with\udNbCl5/NHiPr2, performed in CH2Cl2 at room temperature. The X-ray structure of 3 features a bridging\ud(E)-1,2-bis(3,4-dihydro-2H-pyrrol-5-yl)ethene-1,2-diolate ligand, resulting from the unprecedented C–C\udcoupling between two proline units. Unusually stable a-ammonium acyl chlorides were prepared by the\udreactions of PCl5/MCln (MCln ¼ NbCl5, WCl6) with L-proline, N,N-dimethylphenylalanine, sarcosine and Lmethionine.\udMX5 (M ¼ Nb, Ta; X ¼ F, Cl) reacted with L-leucine methylester and L-proline ethylester to\udgive ionic coordination compounds, [MX4L2][MX6] (M ¼ Nb, L ¼ Me2CHCH2CH(NH2)CO2Me, X ¼ F, 9; Cl,\ud11a; M ¼ Nb, X ¼ Cl, , 11c; Ta, 11d), in moderate to good yields.\ud[NbCl5(Me2CHCH2CHNH3CO2Me)][NbCl6], 12, was isolated as a co-product of the reaction of NbCl5\udwith L-leucine isopropylester, and crystallographically characterized. The reaction of NbCl5 with L-serine\udisopropylester afforded NbCl3(OCH2CHNHCO2\udiPr), 13, in 66% yield. The activation of the ester O–R bond\udwas observed in the reactions of L-leucine methyl ester with NbF5 and L-proline ethyl ester with MBr5 (M ¼\udNb, Ta), these reactions proceeding with the release of EtF and EtBr, respectively. All the metal products\udwere characterized by analytical and spectroscopic methods, while DFT calculations were carried out in\udorder to provide insight into the structural and mechanistic aspects.