Absolute configuration of secondary alcohols by 1H NMR: in situ complexation of alpha-methoxyphenylacetic acid esters with barium(II).

A novel methodology that allows the assignment of the absolute configuration of chiral secondary alcohols by NMR using only one derivative is presented. All that is needed is (a) the derivatization of the alcohol of unknown configuration with one enantiomer--either the (R)- or the (S)--of alpha-methoxyphenylacetic acid (MPA), (b) the recording of the 1H NMR spectrum of the resulting ester in MeCN-d3, and (c) addition of a barium(II) salt [i.e. Ba(ClO4)2] to the NMR tube till saturation and recording of a second spectrum. The assignment of the R/S configuration to the alcohol takes a few minutes and consists on the comparison of the signs of the shifts (Deltadelta(Ba)) produced by addition of the barium(II) with those predicted for the (R) and the (S) enantiomers in accordance to a simplified model that reflects the conformational changes produced by the complexation with barium and their consequences in the chemical shifts. These conformational changes are based on experimental NMR and CD results showing that the formation of a barium(II) complex with the MPA ester moves the conformational equilibrium between syn- (sp) and anti-periplanar (ap) forms toward the most stable ones (sp), and that this leads to the increase of the shielding caused by the MPA phenyl group on a certain substituent of the alcohol. In addition, ab initio Hartree-Fock (HF) and density functional theory (DFT) calculations provide further evidence on the formation, structure, and stability of the complexes with Ba2+, Mg2+, and the influence of the solvent. The general applicability of this methodology and the reliability of the configurational assignment were assured by the study of about twenty alcohols of known configuration and diverse structural features. Its scope and limitations have also being established and other representative cations (i.e. Li+, Rb+, Cs+, Mg2+, Ca2+, Sc3+, V3+, Zn2+) were also evaluated. The procedure proposed is simple, fast, and cheap because it requires a very small amount of sample, only one derivatization, and the recording of only two 1H NMR spectra at rt. A graphical guide to facilitate the application of this new method is included at the end of the paper.