RESUMO
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 {1 H, 6 Li, 13 C, 15 N, 19 F} 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 (5â 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)â a [dilithiated sulfoximine/(n-BuLi)] dimer solvated by four molecules of THF (Agg2, 39 %); ii)â a [dilithiated sulfoximine/(n-BuLi)3 ] tetramer solvated by six molecules of THF (Agg3, 39 %); iii)â 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.
RESUMO
Scalar coupling in organolithium systems can provide access to useful structural and dynamic informations. In this work, we propose a robust method for the accurate measurement of the effective 2JLi-Li coupling constant in tetramerics alkyllithium aggregates. This crucial information, unavalaible to date, gives a simple access to various structural factors, including the dynamics, solvation and the operative steric hindrance of alkyl chains.