NMR and DFT Studies with a Doubly Labelled¹⁵N/⁶LiS‐Trifluoromethyl Sulfoximine Reveal Why a Directedortho‐Lithiation Requires an Excess ofn‐BuLi

Abstract

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‐d8 using {1H, 6Li, 13C, 15N, 19F} 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.