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

Abstract

Isotopically 6Li-labeled lithioorganic cyclic dimers may not be distinguished from cyclic trimers or higher oligomers by conventional 13C-NMR spectroscopy: both a cyclic dimer and a trimer will exhibit a quintet due to coupling of 13C with two neighboring 6Li nuclei. However, a clear distinction can be made by means of a 2D 6Li{13C}-HMQC-TOCSY experiment without 13C decoupling during the acquisition period: in a 13C 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 13C,6Li magnetization transfer and subsequent homonuclear magnetization transfer between appropriate 6Li spins. Thus, a dimer reveals a pair of cross peaks located at the chemical shift of the 13C satellites in the f2 (6Li) domain, whereas a trimer leads to an extra cross peak at the chemical shift of the 6Li main signal. The usage of the novel method is exemplified for a lithioorganic trimer and a dimer model compound.