Lithiation of N‑Heterocyclic Olefins

Deprotonation of 1,3-dimethyl-2-methyleneimidazole (MeNHO) by nBuLi in hexane/benzene is fast and clean and gave a precipitate of the product in which the MeNHO ligand is cleanly lithiated in the alkene backbone (1). Lithiation of the much bulkier 1,3-bis­(2,6-di-iso-propylphenyl)-2-methyleneimidazole (DIPPNHO) needs the presence of TMEDA as a cosolvent to give backbone lithiation (2). Depending on the cosolvent, the following products could be crystallized and fully characterized by X-ray diffraction and NMR: 1-THF, 1-TMEDA, 1-(TMEDA)0.5, and 2-TMEDA. The MeNHO ligand with a saturated backbone is less easily lithiated by nBuLi and does not even react at elevated temperatures. Addition of TMEDA, however, already gave decomposition at room temperature by ring opening to give a complex aggregate consisting of a dilithiated ligand fragment and two lithiated TMEDA ligands. DFT calculations shed light on a possible mechanism for cleavage of one of the cyclic C–N bonds. This process, which most likely goes through prior lithiation of the saturated backbone, seems to be general for NHO ligands with saturated backbones. Convenient access to NHO ligands that have been deprotonated in the unsaturated backbone may promote this new ligand class for further exploration.