NMR spectroscopy of organolithium compounds. XXVI—The aggregation behaviour of methyllithium in the presence of LiBr and LiI in diethyl ether and tetrahydrofuran

Abstract

1H, 6Li and 13C NMR spectroscopy were used to determine the structure of aggregates formed in mixtures of methyllithium, H3CLi, and lithium bromide and iodide in diethyl ether and tetrahydrofuran. From the chemical shifts, the signal intensity distribution and the isotope shifts observed for partially deuterated systems, it was shown that generally tetrameric structures with different halogen contents dominate. For methyllithium-lithium bromide (1:1) in THF a considerable concentration of an H3CLi-LiBr dimer was found. For the first time, deuterium-induced 6Li isotope shifts over four bonds were observed.

Understanding the directed ortho lithiation of (R)-Ph2P(NCO2Me)NHCH(Me)Ph. NMR spectroscopic and computational study of the structure of the N-lithiated species

“…To validate 31 Signals Mγ and Mδ are tentatively assigned as mixed aggregates formed between monomers Mα-Mβ and the LiBr present in the organolithium base (they persist in the most diluted sample). 32 Mixed aggregates of N-lithium phosphazenes and lithium bromide are known in the literature. 33 Support for the assignments made was obtained based on 31 The 7 Li NMR spectrum of the most diluted sample showed similar features to those found for 31 P (Fig.…”

NMR spectroscopy of organolithium compounds, part XXXIII: trans‐2,3‐dimethylcyclopropyllithium aggregation in diethylether: An equilibrium between three different complexes of comparable energy and the influence of LiBr on aggregate structure

“…On the other hand, THF as the stronger donor as compared with DEE may stabilize these dimers to the extent that detection by NMR is possible. In contrast to our results obtained for methyl‐ and n ‐butyllithium, [8e,9a] the number of different mixed complexes with LiBr is drastically reduced for the cyclopropyllithium systems 1 to 3 .…”

“…Further measurements ( 1 H, 13 C NMR, or 2D spectroscopy) were prevented by rapid crystallization, and recording the 6 Li spectrum was only possible when the sample prepared at room temperature was inserted in the pre‐cooled magnet. On the basis of the X‐ray results, we can tentatively assign the 2 smaller singlets to a static mixed tetramer ( 1 •LiBr) 2 that has 2 different lithium environments, Li1(C,Br,Br) and Li2(C,C,Br), also found for CH 3 Li/LiBr (1:1) in DEE [9a] . The larger signal comes than forms a mixed dimer 1 •LiBr where only 1 lithium environment exists (Scheme ).…”

“…Sometimes, as for example for n ‐butyllithium in diethylether (DEE) or phenyllithium in tetrahydrofuran (THF), a particular structure dominates to the extent that other structures are not detectable, while quite often two different complexes are in fast and reversible equilibrium as in the case of n ‐butyllithium in THF a tetramer and a dimer or in the case of iso ‐propyllithium in hydrocarbon solvents a tetramer and a hexamer . In addition, equilibria between larger numbers of complexes have also been found, as for instance, for n ‐propyllithium in cyclopentane and for mixed aggregates of methyllithium with LiI [8a] or LiBr and butyllithium with LiBr [8e] as well as for cyclopentadienyl systems . At low temperature, mostly below −50°C, the lifetime of these aggregates is long enough that the individual structures can be characterized by NMR spectroscopy, using nuclei like 1 H, 6/7 Li, 13 C, and 31 P…”

Transition‐Metal‐Free Heterobiaryl Synthesis via Aryne Coupling

“…The previous investigation evaluating the effect of ligands clearly underlined influences of such compounds on the reactivity of organolithiums . In this context, “salt effects” have attracted significant interest in recent years, leading to comprehensive spectroscopic and theoretical investigations, more specifically in organolithium chemistry where the effects of lithium halides are commonly encountered . Lithium salts are undoubtedly among the most widely studied salts, being ubiquitous in a large number of lithiation reactions – either because they are generated in the reaction, or because they are involuntarily brought by a contaminated commercial source.…”