Organolithium chemistry: NMR-based distinction of cyclic dimers from cyclic trimers by 6Li{C-13}-HMQC-TOCSY

Isotopically Li-6-labeled lithioorganic cyclic dimers may not be distinguished from cyclic trimers or higher oligomers by conventional C-13-NMR spectroscopy: both a cyclic dimer and trimer will exhibit a quintet due to coupling of C-13 with two neighboring Li-6 nuclei. However, a clear distinction can be made by means of a 2D Li-6{C-13}-HMQC-TOCSY experiment without C-13 decoupling during the acquisition period: in a C-13 isotopomer, the existence of two chemically equivalent (dimer) or nonequivalent (trimer) Li nucleus sites gives rise to specific cross peak patterns. The distinction is based on C-13,Li-6 magnetization transfer and subsequent homonuclear magnetization transfer between appropriate Li-6 spins. Thus, a dimer reveals a pair of cross peaks located at the chemical shift of the C-13 satellites in the f(2) (Li-6) domain, whereas a trimer leads to an extra cross peak at the chemical shift of the Li-6 main signal. The usage of the novel method is exemplified for a lithioorganic trimer and a dimer model compound.