First Direct Evidence of an ortho -Lithiated Aryloxetane: Solid and Solution Structure, and Dynamics
Résumé
Oxetanes are key synthons for asymmetric synthesis and also effective in directing ortho‐lithiation. This work first reports the solution and the solid‐state structure of an ortho‐lithiated aryloxetane (1‐Li) in the presence/absence of a bidentate ligand such as N,N,N′,N′‐tetramethylethylenediamine (TMEDA). Single crystal X‐ray diffraction analysis of 1‐Li revealed a singular crystallographic structure in which the asymmetric unit comprises a core where the lithium atom is coordinated to the nitrogen atom of half a molecule of TMEDA and intramolecularly stabilised by the oxetane ring oxygen. This aggregation state is unprecedented in ortho‐lithiated arenes. Variable temperatures multinuclear magnetic resonance (1H, 7Li, 13C) mono‐ and two‐dimensional NMR studies and DFT computations supported the coexistence in solution of three chelated bridged dimeric aggregates, in slow equilibration at 180 K. The major isomer is an heterochiral aggregate on the basis of 1H,7Li‐HOESY and 1H,1H‐NOESY experiments. Conclusions were supported by the preparation of enantiomerically enriched (S)‐1‐Li. The privileged formation of homochiral aggregates from racemic mixtures may also have implications for the development of chiral resolution processes.