Organolithium Chemistry: NMR-Based Distinction of Cyclic Dimers from Cyclic Trimers by 6Li{13C}-HMQC-TOCSY

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