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

The H-1 and C-13 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 [(4S) and (4R)] were measured in CDCl3 as a solvent. The amines having various substituents at the nitrogen atom (H, CH3 and CH2CH3) formed in such conditions as an equilibrium mixture of CSNR and CSNS 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 C-13 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). (C) 2009 Elsevier Ltd. All rights reserved.