NMR spectrocopy of organolithium compounds, part XXXIV: Cyclopropyllithium and lithium bromide (1:1) in diethylether/tetrahydrofuran-Identification of a fluxional mixed tetramer.

Cyclopropyllithium, C3 H5 Li (1), was studied in the presence of one equivalent lithium bromide (LiBr) in diethylether (DEE)/tetrahydrofuran (THF) mixtures and in THF as solvents. Increasing the THF concentration in DEE/THF leads in the 6 Li NMR spectrum to a main signal (S1) at δ0.85 (rel. to ext. LiBr/THF) and a second resonance (S2) at δ0.26 aside from a minor component at δ0.07. In pure THF, the ratio of these signals was 66: 28:6. 6 Li and 13 C NMR allowed to identify the main signal as belonging to a mixed dimer, 1•LiBr, and the signal at 0.26 ppm to a fluxional mixed tetramer, 12 •(LiBr)2 . 1 J(13 C,6 Li) coupling constants of 11.0 and 9.8 Hz were measured at 168 K for S1 and S2, respectively, and chemical exchange between both signals was detected by 2D 6 Li,6 Li exchange spectroscopy and analyzed by temperature-dependent 1D 6 Li line-shape calculations. These yielded the equilibrium constants Keq for the chemical exchange Li4 (C3 H5 )2 Br2 ⇌ 2 Li2 C3 H5 Br. Their temperature dependence leads to van't Hoff parameters of ΔH° = 4.6 kJ/mol, ΔS° = 41.4 J/mol K, and ΔG°298 = -7.8 kJ/mol. From the rate constants k, Eyring parameters of ΔH* = 42.0 kJ/mol, ΔS* = 33.0 J/mol K, and ΔG*298 = 32.2 kJ/mol were calculated for the forward reaction Li4 (C3 H5 )2 Br2 → 2 Li2 C3 H5 Br and ΔH* = 37.5 kJ/mol, ΔS* = -8.4 J/mol K, and ΔG*238 = 40.0 kJ/mol for the reverse reaction 2Li2 C3 H5 Br → Li4 (C3 H5 )2 Br2 .

NMR studies of dilithiostyrenes: aggregation, NMR parameters, and DFT calculations for (E)-1-Lithio-2-(o-lithiophenyl)-1-trimethylsilylethene.

The dilithio compound (E)-1-lithio-2-(o-lithiophenyl)-1-trimethylsilylethene (5) was synthesized from 2-trimethylsilylbenzo-[b]tellurophene (6) with lithium-6 and a detailed analysis of its 1 H, 6 Li, 13 C, and 29 Si NMR spectra showed 5 to form a dimer 52 in tetrahydrofuran and diethylether, while addition of tetramethylethylenediamine stabilizes a monomer 51 . A monomer-dimer equilibrium exists with K at 230 K = 1.25 and ΔG230o = -0.43 kJ mol-1 . Homonuclear 6 Li,6 Li coupling of 0.25 ± 0.07 Hz in the dimer was detected by a 1D-6 Li,6 Li INADEQUATE experiment, and scalar 6 Li,13 C coupling constants were obtained from 13 C satellites in the 6 Li spectrum, from 13 C multiplet simulation and 6 Li,13 C-HMQC spectra. In addition, structures and coupling constants of 51 and 52 were calculated by density functional theory (DFT) methods. It was found that the magnitude of the 6 Li,13 C spin-spin interactions shows an inverse correlation with the C-Li bond lengths. The intra-aggregate exchange in the dimer, caused by 180° rotation of one monomer unit within the solvent cage, was studied by 6 Li DNMR and line shape analysis and yielded ΔG298≠ = 60 ± 3 kJ mol-1 ; ΔH≠ = 84 ± 3 kJ mol-1 ; ΔS≠ = 80 ± 3 J mol-1 K-1 for this process. Copyright © 2015 John Wiley & Sons, Ltd.

A critical evaluation of heteronuclear TOCSY (HEHAHA) experiments for 1H,6Li spin pairs.

Heteronuclear TOCSY (HEHAHA) experiments for (1) H,(6) Li spin pairs in organolithium compounds with adjacent strongly coupled (1) H,(1) H spin systems showed unexpected cross peak behaviour: for n-butyllithium (1) H,(6) Li cross peaks were completely missing, whereas for the dimer of (Z)-2-lithio-1-(o-lithiophenyl)ethane, a cross peak for remote protons was observed even at very short mixing times. It was assumed that strong magnetization transfer within the proton spin systems was responsible for these results, which prevented unambiguous chemical shift assignments. Selective experiments with the (6) Li,(1) H-HET-PLUSH-TACSY sequence then showed the expected (6) Li,(1) H cross peaks for the transfer via the directly coupled (1) H and (6) Li nuclei. For n-butyllithium transfer to H(Cα) via an unresolved heteronuclear coupling constant below 0.1 Hz is unambiguously observed. Cross peaks in the 2D (6) Li,(1) H-HET-PLUSH-TACSY spectra for the dimer of (Z)-2-lithio-1-(o-lithiophenyl)ethane are readily explained by the measured coupling network and the corresponding active mixing conditions.

7Li and 13C solid-state NMR spectra of lithium cuprates.

7Li and 13C solid-state MAS NMR spectra of three lithium cuprates with known X-ray structures--lithium([12]crown-4)2 dimethyl and diphenyl cuprate (1,2) and lithium(thf)4-[tris(trimethylsilyl) methyl]2 cuprate (3)--have been measured and analysed with respect to the quadrupolar coupling constants of lithium-7, chi(7Li), and the asymmetry parameters of the quadrupolar interactions, eta(7Li), as well as the 6, 7Li and 13C chemical shifts. The chi(7Li) values of 23, 30, and 18 kHz for 1, 2 and 3, respectively, are in line with the high symmetry around the lithium nucleus in the solvent-separated structures and may be used as reference data for this structural motif. Calculations based on charges derived from ab initio 6-31 G* HF computations using the point charge model (PCM) and the program GAMESS support the experimental findings.

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

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.