Microsolvation and ¹³C−Li NMR Coupling

Abstract

The empirical expression (1)J(CLi) = L[n(a + d)](-1) is proposed; it claims a reciprocal dependence of the NMR coupling constant (1)J((13)C, Li) in a C-Li compound on two factors: (i) the number n of lithium nuclei in bonding contact with the observed carbanion center and (ii) the sum (a + d) of the numbers a of anions and d of donor ligands coordinated at the Li nucleus that generates the observed (1)J(CLi) value. The expression was derived from integrations of separate NMR resonances of coordinated and free monodentate donor ligands (t-BuOMe, Et2O, or THF) in toluene solutions of dimeric and monomeric 2-(alpha-aryl-alpha-lithiomethylidene)-1,1,3,3-tetramethylindan at moderately low temperatures. This unusually slow ligand interchange is ascribed to steric congestion in these compounds, which is further characterized by measurements of nuclear Overhauser correlations and by solid-state structures of the dimers bearing only one donor per lithium atom (d = 1). Increasing microsolvation numbers d are also accompanied by typical changes of the NMR chemical shifts delta (positive for the carbanionic (13)C(alpha), negative for C(para) and p-H). The aforementioned empirical expression for (1)J(CLi) appears to be applicable to other cases of solvated monomeric, dimeric, or tetrameric C-Li compounds (alkyl, alkenyl, alkynyl, and aryl) and even to unsolvated (d approximately 0) trimeric, tetrameric, or hexameric organolithium aggregates, indicating that (1)J(CLi) might serve as a tool for assessing unknown microsolvation numbers. The importance of obtaining evidence about the (13)C NMR C-Li multiplet splitting of both the nonfluxional and fluxional aggregates is emphasized.

Microsolvation and sp2-stereoinversion of monomeric α-(2,6-di-tert-butylphenyl)vinyllithium as measured by NMR

“…Among the rare opposite cases are the very strong donor ligand (Me 2 N) 3 PO (HMPA) [6], or THF at the endocyclic Li of dimeric Me 2 CuLi&LiCN [7], or intramolecular (chelating) donor functions [8], whereas nonchelating monodentate ethereal donor ligands such as THF, Et 2 O, and tert -butyl methyl ether ( t- BuOMe) normally show only the averaged signals. However, our sterically congested model system [9] 1 exhibited separate resonances of the latter three donor ligands (“Don” in Scheme 1) above the melting points of the solvent mixtures, so that the microsolvation numbers d could be measured by simple NMR integrations. On this basis, dimerization equilibria of 1a and 1b were analyzed [10] with a proper allowance for the differing microsolvation of the monomeric and dimeric components.…”

“…The alkenyllithiums 1a–e and 2 are exclusively trisolvated ( d = 3) monomers [9,11] in THF as the solvent, whereas 1a forms purely disolvated ( d = 2) monomers in Et 2 O, t- BuOMe, and 1,2-bis(dimethylamino)ethane (TMEDA). All dimeric species of 1a and 1b are disolvated ( d = 1 at each Li) by the above three donor ligands in toluene solution and in the solid state [9].…”

“…All dimeric species of 1a and 1b are disolvated ( d = 1 at each Li) by the above three donor ligands in toluene solution and in the solid state [9]. These d values were found [9] to change inversely with the magnitude of the scalar one-bond 13 C, 6 Li NMR coupling constants 1 J C,Li as quantified by the empirical Eq.…”

“…All dimeric species of 1a and 1b are disolvated ( d = 1 at each Li) by the above three donor ligands in toluene solution and in the solid state [9]. These d values were found [9] to change inversely with the magnitude of the scalar one-bond 13 C, 6 Li NMR coupling constants 1 J C,Li as quantified by the empirical Eq. 1, where n is the number of Li cations coordinated to the carbanionic center 13 C-α under consideration, while a is the number of C-α centers in direct contact with a certain Li cation; obviously, n = a = 1 for monomeric species such as 1 or 2 .…”

“…1, where n is the number of Li cations coordinated to the carbanionic center 13 C-α under consideration, while a is the number of C-α centers in direct contact with a certain Li cation; obviously, n = a = 1 for monomeric species such as 1 or 2 . Synthesized with the experimentally most convenient (because least line-broadening [12]) isotope 6 Li, almost [9] all species of 1a and 2 were found to have a common sensitivity factor of L = 42.8 Hz in Eq. 1, whereas L decreased slightly in a nonlinear fashion with increasing Hammett parameters σ p – for R in 1a–e [13].…”