NMR detection of the strained metallacycles in organolithiums: theoretical study.

For the first time through quantum chemistry methods, the effective use of 1 J CLi spin-spin coupling constants as descriptors for assessing the formation of strained metallacycles is demonstrated. Both acyclic organolithiums and 3- to 7-membered metallacycles are examined. 80 organolithium compounds, including both monomeric and dimeric species, with ligands containing fluorine, nitrogen, oxygen, and carbon (in the form of carbanions), are tested. In general, the 1 J CLi values below 12 Hz for monomeric species and below 6 Hz for dimeric species serve as clear indicators of strained monomeric metallacycle formation (for 6 Li nuclei). The primary contributor to the overall 1 J CLi value is the Fermi-contact term, which correlates directly with the carbon-lithium interatomic distance and allows to distinguish between dimers and monomers.