NMR and DFT Studies with a Doubly Labelled 15N/6Li S-Trifluoromethyl Sulfoximine Reveal Why a Directed ortho-Lithiation Requires an Excess of n-BuLi

This work shows why it is imperious to use an excess of butyllithium for a directed ortho-lithiation of a trifluoromethyl sulfoximine. The analysis of mixtures of n-BuLi and sulfoximine 1 in THF-d(8) using {H-1, Li-6, C-13, N-15, F-19} NMR experiments at low temperatures reveal that a first deprotonation occurs that leads to dimeric and tetrameric N-lithiated sulfoximine (93 : 7). Using an excess n-BuLi (5 equivalents), the second deprotonation on the ortho-position of the aromatic occurs. Six species were observed and characterized on the way. It includes three aggregates involving a sulfoximine: i) a [dilithiated sulfoximine/(n-BuLi)] dimer solvated by four molecules of THF (Agg2, 39 %); ii) a [dilithiated sulfoximine/(n-BuLi)(3)] tetramer solvated by six molecules of THF (Agg3, 39 %); iii) a [dilithiated sulfoximine/(n-BuOLi)(3)] tetramer solvated by four molecules of THF (Agg1, 22 %). A DFT study afforded optimized solvated structures for all these aggregates, fully consistent with the NMR data.

Automated summary

This work explains the importance of using an excess of butyllithium in the directed ortho-lithiation of a trifluoromethyl sulfoximine. The study shows that the presence of excess n-BuLi leads to deprotonation on the ortho-position of the aromatic, resulting in the formation of six different aggregates. DFT calculations provide optimized solvated structures for these aggregates that are consistent with the experimental NMR data.