Determination of the Absolute Configuration of Pentacoordinate Chiral Phosphorus Compounds in Solution by Using Vibrational Circular Dichroism Spectroscopy and Density Functional Theory

Vibrational circular dichroism (VCD) spectroscopic measurements and density functional theory (DFT) calculations have been used to obtain the absolute structural information about four sets of diastereomers of pentacoordinate spirophosphoranes derived separately from L- (or D-) valine and L- (or D-) leucine for the first time. Each compound contains three stereogenic centers: one at the phosphorus center and two at the amino acid ligands. Extensive conformational searches for the compounds have been carried out and their vibrational absorption (VA) and VCD spectra have been simulated at the B3LYP/6-311 ++ G** level. Although both VA and VCD spectra are highly sensitive to the structural variation of the apical axis, that is, the O-P-O or N-P-O arrangement, the rotamers generated by the aliphatic amino side chains show little effect on both. The dominant experimental VCD features in the 1100-1500 cm(-1) region were found to be controlled by the chirality at the phosphorus center, whereas those at the C=O stretching region are determined by the chirality of the amino acid ligands. The good agreement between the experimental VA and VCD spectra in CDCl3 solution and the simulated ones allows us to assign the absolute configurations of these pentacoordinate phosphorus compounds with high confidence. This study shows that the VCD spectroscopy complemented with DFT calculations is a powerful and reliable method for determining the absolute configurations and dominating conformers of synthetic phosphorus coordination complexes in solution.