Studies on the configuration of nitrogenous stereogenic centres in adducts of rhodium(II) tetraacylates with chiral amines: the application of 1H and 13C NMR spectroscopy

Abstract

The 1H and 13C NMR spectra of enantiomerically pure amines ( S)- N, N-dimethyl-1-phenylethylamine, ( S)- N-methyl-1-phenylethylamine, ( S)- N-ethyl-1-phenylethylamine and ( S)- N-ethyl- N-methyl-1-phenylethylamine in the presence of a twofold molar excess of dirhodium(II) tetratrifluoroacetate and dirhodium(II) Mosher’s acid derivatives [(4 S) and (4 R)] were measured in CDCl 3 as a solvent. The amines having various substituents at the nitrogen atom (H, CH 3 and CH 2CH 3) formed in such conditions as an equilibrium mixture of C S N R and C S N S 1:1 adducts. The signals of both diastereoisomers were observed in NMR spectra at either room temperature (303 K) or moderately decreased temperatures (263–273 K). The rates of mutual diastereoisomer conversion were estimated by selective inversion recovery experiments and varied from less than 0.1 to ca. 10 s −1, depending on the ligand and temperature. Analysis of 13C NMR data and NOE experimental data resulted in the unambiguous determination of the configuration at the nitrogen atom with respect to the carbon stereogenic centre. Modelling of adduct structures and calculations of molecular energy and NMR parameters (GIAO) using Density Functional Theory (DFT) were performed in order to support the experimental findings. The calculations were carried out using 3-21G//B3LYP (structure optimizing) and 311G(2d,p)/LanL2DZ//B3LYP theory levels (molecular energy and NMR shielding).