VCD analysis of 16 diastereomeric pairs of NHC precursors containing two isolated chirality centers and different substitution patterns identified VCD transitions characteristic of the chirality center in the imidazolium ring or in the side chain, which, in contrast to ECD and OR, could be utilized to assign the two chirality centers separately by simple comparison, regardless of the type and position of achiral aromatic substituents. While the ECD and OR data showed great dependence on the position of an achiral substituent such as a methoxy group, characteristic experimental VCD transitions remained consistent and they could be used to determine the absolute configuration of all the regio- and stereoisomers and substituted analogues. VCD, ECD and OR approaches were evaluated, and several carbene precursors were found, for which only the VCD method could distinguish the four stereoisomers. With t-butyl, phenyl or 2-naphthyl substituents at the C-1' chirality center, the ECD spectra of the C-1' epimers were near-identical, and hence it was only the VCD approach that showed distinct differences suitable for the configurational assignment. The chiroptical characterization of our diastereomeric pairs of NHC precursors enables the future application of related derivatives having different substitution patterns in stereoselective transformations.
Keywords: DFT calculations; NHC precursors; characteristic VCD transitions; conformational analysis; electronic circular dichroism.